Title:
Method of Production of Hot Dipped Hot Rolled Steel Strip
Kind Code:
A1


Abstract:
Production of hot dipped hot rolled steel strip by producing steel strip by casting by a thin slab continuous casting process and hot rolling, said steel strip containing, by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by heating it to a peak maximum steel strip temperature of 550° C. to less than 650° C. by a temperature elevation rate of 25° C./sec or more for 15 sec or more for oxidation, heating it to a peak maximum steel strip temperature of 700° C. to 760° C. so that the time when the steel strip temperature is 570° C. or more is 25 sec to 45 sec for reduction, then hot dipping it.



Inventors:
Katsube, Makoto (Fukuoka, JP)
Miyake, Masayuki (Fukuoka, JP)
Application Number:
11/887176
Publication Date:
11/20/2008
Filing Date:
03/27/2006
Primary Class:
Other Classes:
266/249
International Classes:
C21D8/02; C21D9/54
View Patent Images:



Primary Examiner:
ZHU, WEIPING
Attorney, Agent or Firm:
Hunton Andrews Kurth LLP/HAK NY (Washington, DC, US)
Claims:
1. A method of production of hot dipped hot rolled steel strip characterized by producing steel strip by casting by a thin slab continuous casting process and hot rolling, said steel strip containing, by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more, heating it to a peak maximum steel strip temperature of 550° C. to less than 650° C. by a temperature elevation rate of 25° C./sec or more for 15 sec or more for oxidation, heating it to a peak maximum steel strip temperature of 700° C. to 760° C. so that the time when the steel strip temperature is 570° C. or more is 25 sec to 45 sec for reduction, then hot dipping it.

2. A method of production of hot dipped hot rolled steel strip as set forth in claim 1 characterized in that said hot dipping is hot dip galvanization.

3. A facility for production of hot dipped hot rolled steel strip which hot dips steel strip produced by casting and by the thin slab continuous casting process and by hot rolling, which facility for production of hot dipped hot rolled steel strip is characterized by having a furnace used for oxidation and a furnace used for reduction and by a ratio of length between said furnace used for oxidation and said furnace used for reduction along a conveyance direction of said steel strip being 0.5 to 0.9.

4. A facility for production of hot dipped hot rolled steel strip as set forth in claim 3, characterized in that said steel strip passes through said furnace used for oxidation in a time of 15 sec to 25 sec.

Description:

TECHNICAL FIELD

The present invention relates to a method of production of hot dipped plating hot rolled steel strip produced by the thin slab continuous casting process, hot rolling process and hot dipping plating process.

BACKGROUND ART

In recent years, due to the need to save energy and cut costs, technology for production of steel plate using the thin slab continuous casting process (thin slab casting process) such as described in Japanese Patent Publication (A) No. 2-197358 has come under the spotlight in the world. This thin slab continuous casting process is characterized by the point of the thin strip being directly sent from the continuous casting process to the rolling process. For this reason, compared with a conventional continuous casting machine requiring cooling of the steel slab, fault detection, fault removal, heating, and numerous other processes between the continuous casting process and rolling process, the energy efficiency is extremely good and the capital costs can be kept low. Further, the fact that this thin slab continuous casting machine can be utilized together with electric furnaces using scrap as raw materials is another major reason why attention is being gathered.

However, there is the problem that hot rolled steel strip produced by the thin slab continuous casting process is harder to improve in surface quality compared with hot rolled steel strip produced by a conventional continuous casting machine. For this reason, up until recently, the thin slab continuous casting process has not spread in use that widely. Further, there is very little information on hot rolled steel strip produced by the thin slab continuous casting process. When hot dip galvanizing this hot rolled steel strip, the method used for hot rolled steel strip obtained by a conventional continuous casting machine has been used as it is.

As the method for hot dip galvanization of hot rolled steel strip, in general a “non-oxidizing furnace method” is used. With this method, hot rolled steel strip is continuously run through a non-oxidizing furnace, reduction furnace (annealing furnace), and cooling furnace to heat it to a high temperature and oxidize and reduce it. By oxidizing hot rolled steel strip in the non-oxidizing furnace, then reducing it in the reduction furnace in this way, an Fe layer can be formed on the hot rolled steel strip surface. The FeO or other oxide film on the hot rolled steel strip surface is resistant to adherence by the hot melt, so removing this from the surface of the hot rolled steel strip has the effect of improving the plating wettability for hot dipping.

Such a conventional hot dipping facility is designed mainly for the purpose of processing cold rolled steel sheet, so the temperature elevation rate in the heating zone was about 10° C./s to 20° C./s in range. Further, when using this hot dipping facility to plate hot rolled steel strip, since general steel does not require recrystallization annealing, the maximum temperature at the time of annealing was usually adjusted to 640° C. to 660° C. or so.

Note that, as another method, the “flux method” is also known. With this method, the hot rolled steel strip surface is coated with a flux of zinc chloride, ammonium chloride, etc. to activate the hot rolled steel strip surface and improve the wettability for the hot dipping. However, this method is not generally used for the production of hot dipped steel strip in view of the difficulty of continuous production and plating adhesion.

If the hot rolled steel strip produced using the thin slab continuous casting process is hot dip galvanized by the method of production of hot dipped steel strip using the above-mentioned “non-oxidizing furnace type plating facility”, nonplating defects are formed on the surface of the hot dip galvanized steel strip. This is believed to be partially due to the addition of Ca specific to the thin slab continuous casting process.

A thin slab continuous casting machine has a much narrower casting mold than a conventional continuous casting machine and has an injection nozzle of a special structure as well, so alumina easily clogs the nozzle. Therefore, to prevent this, in a thin slab continuous casting machine, Ca is added to the ladle to lower the melting point of the alumina.

In the thin slab continuous casting process, a cast 50 mm to 80 mm or so thickness slab is sent directly to the rolling process while held at a high temperature and rolled. This hot rolling mill is a hot rolling mill corresponding to a final rolling machine of a conventional hot rolling process and rolls a slab to a thickness of 1.2 mm to 4 mm or so to produce hot rolled steel strip. In this case, to keep the thin slab warm, a tunnel furnace with a long residence time is used, so a large amount of scale is formed on the slab surface before rolling.

The Ca added as explained above and remaining in the thin slab oxidizes in the scale and remains in the form of CaO. As a result, the oxide CaO formed by this addition of Ca causes unevenness and pitting in the oxide film on the surface of the hot rolled steel strip when oxidized in the non-oxidizing furnace in the plating process, causes partial degradation of the plating wettability with the hot dip galvanization, and causes plating defects.

Further, the hot rolled steel strip produced using the thin slab continuous casting process exhibits a greater amount of smut compared with a conventional continuous casting machine. This is because with the thin slab continuous casting process, the cast steel thin slab is directly sent to the rolling process and rolled while keeping it at a high temperature, so Fe3C and C easily remain on the steel strip surface in the separated state. If a lot of these Fe3C etc. remain on the surface of the hot rolled steel strip, when oxidized in the non-oxidizing furnace, the C reacts with the oxygen, the formation of an Fe oxide film is partially delayed, and unevenness and pitting are formed on the oxide film. These unevenness and pitting are considered to lower the plating wettability with zinc and cause plating defects.

Further, it was learned that if the hot rolled steel strip produced using the thin slab continuous casting process is produced by a conventional hot dipping line, coil breakage will occur. In particular, remarkable coil breakage similar to “fluting” occurs with hot rolled steel strip of a thickness of 2 mm or more. The reason is that if produced by a conventional hot dipping line, the yield point falls more than necessary at the heating and annealing stage, so in particular if processing thick-gauge hot rolled steel strip of a thickness of 2 mm or more, coil breakage occurs on the processing line after the plating.

To prevent coil breakage, in the past the technology of heating the hot rolled steel strip after plating to adjust the yield point and the technology of increasing the roll diameter of the processing line after plating to reduce the bending strain have been proposed, but the former technology is complicated in operation. The latter technology requires precision processing of the roll profile etc. to produce large diameter rolls and therefore sophisticated technology and processing facilities, so as a result requires considerably high cost for production of the rolls.

DISCLOSURE OF THE INVENTION

The present invention was made in consideration of the above problems and in particular provides a means for preventing nonplating defects formed on a plate surface when hot dipping hot rolled steel strip produced by the thin slab continuous casting process.

To solve the above problems, according to the present invention, there is provided a method of production of hot dipped hot rolled steel strip characterized by producing steel strip by casting by a thin slab continuous casting process and hot rolling, said steel strip containing, by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more, heating it to a peak maximum steel strip temperature of 550° C. to less than 650° C. by a temperature elevation rate of 25° C./sec or more for 15 sec or more for oxidation, heating it to a peak maximum steel strip temperature of 700° C. to 760° C. so that the time when the steel strip temperature is 570° C. or more is 25 sec to 45 sec for reduction, then hot dipping it.

Note that, in the method of production of hot dipped hot rolled steel strip, the hot dipping may be made hot dip galvanization.

Further, according to the present invention, there is provided a facility for production of hot dipped hot rolled steel strip which hot dips steel strip produced by casting by the thin slab continuous casting process and by hot rolling, which facility for production of hot dipped hot rolled steel strip is characterized by having a furnace used for oxidation and a furnace used for reduction and by a ratio of length between the furnace used for oxidation and the furnace used for reduction along a conveyance direction of the hot rolled steel strip being 0.5 to 0.9.

Note that, in the facility for production of hot dipping hot rolled steel strip, the steel strip can pass through the furnace used for oxidation in a time of 15 sec to 25 sec.

According to the present invention, it is possible to prevent nonplating defects formed on the plated surface when hot dipping hot rolled steel strip produced by the thin slab continuous casting process. Further, it is also possible to perform the hot dipping without coil breakage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the configuration of a suitable hot dip galvanized hot rolled steel strip production facility according to the present invention.

FIG. 2 is a view explaining the temperature changes at a non-oxidizing furnace and annealing furnace of a suitable hot dip galvanized hot rolled steel strip production facility according to the present invention.

FIG. 3 gives views before and after oxidation of hot rolled steel strip produced by the thin slab continuous casting process. (a) shows hot rolled steel strip before oxidation, (b) shows hot rolled steel strip after oxidation by the present invention, and (c) shows hot rolled steel strip after oxidation by the prior art.

FIG. 4 gives views of the hot rolled steel strip oxidized in a non-oxidizing furnace before and after reduction. (d) shows hot rolled steel strip before reduction, (e) shows hot rolled steel strip reduced without excess or shortage, (f) shows hot rolled steel strip which is insufficiently reduced, and (g) shows hot rolled steel strip which is excessively reduced.

FIG. 5 is a view of the configuration of a washing apparatus in front of the hot dipping apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, preferred embodiments of the present invention will be explained while referring to the drawings. Note that in this Description and the drawings, elements having substantially the same functional configurations are assigned the same reference numerals.

In the present invention, as the hot dipping steel strip produced by the method of production of hot dip galvanized hot rolled steel strip, hot dip galvanized steel strip SGHC, SGH340, SGH400, SGH440, SGH540, etc. defined by JIS G 3302 are covered. Hot rolled steel strip produced by casting and rolling steel containing, by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by the thin slab continuous casting process is used.

If Ca is less than 0.001%, nozzle clogging sometimes cannot be prevented, so at least that amount is contained. Ca is usually added in the steelmaking process by adding CaAl, CaSi, FeCa, or metallic Ca to the molten steel after deoxidation.

FIG. 1 is a view of the configuration of a suitable facility for production 1 of hot dip galvanized hot rolled steel strip according to the present invention. This facility for production of hot dip galvanized hot rolled steel strip is comprised of a feeding reel 10 serving as the starting point of the hot dip galvanization process line, a coiling reel 11 serving as the end point, a preheating furnace (not shown) arranged between the reels 10, 11, a non-oxidizing furnace 12, an annealing furnace 15 including a reduction zone 13 and cooling zone 14, a hot dip galvanization tank 16, a wiping apparatus 17, and a cooling furnace 18.

The feeding reel 10 is a reel on which is coiled hot rolled steel strip produced by casting steel containing, by mass %, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by the thin slab continuous casting process, then rolling it as is without lowering the temperature.

The non-oxidizing furnace 12 is known as a furnace for “slight” oxidation of the hot rolled steel strip fed out from the feeding reel and has a length in the conveyance direction of the steel strip of for example 15 m to 25 m. In the case of this embodiment, the processing rate is 120 m/min, so the oxidation time of the hot rolled steel strip in the non-oxidizing furnace 12 is 7 sec to 12 sec. The fuel-air ratio in the non-oxidizing furnace 12 is set to 0.9 to 0.98 or so. Further, the length in the conveyance direction of the non-oxidizing furnace 12 plus the preheating furnace is set to for example 30 m to 50 m. The overall oxidation time (passage time) in the non-oxidizing furnace 12 and preheating furnace becomes 15 sec to 25 sec.

The annealing furnace 15 arranged right after the non-oxidizing furnace 12 is comprised of the reduction zone 13 for reducing the oxidized hot rolled steel strip and the cooling zone 14 for cooling the hot rolled steel strip and has a length in the conveyance direction of for example 70 m to 100 m. In the case of this embodiment, the processing rate is 120 m/min, so the reduction time of the hot rolled steel strip in the annealing furnace 15 becomes for example 25 sec to 45 sec in the region of 570° C. or more where the reduction is relatively fast. Further, H2 and N2 etc. are made the atmosphere in the annealing furnace 15. Note that the reduction zone in which reduction is mainly performed is comprised of a reduction furnace and soaking furnace or just a reduction furnace. Its length in the conveyance direction is for example set to 50 m to 70 m.

The hot dip galvanization tank 16 is a tank for treating hot rolled steel strip for deposition by hot dipping. The wiping apparatus 17 is an apparatus for wiping off excessive molten melt adhered to the hot rolled steel strip by a gas. The cooling furnace 18 is a furnace for then cooling the hot rolled steel strip.

Next, the method of production of hot dip galvanized hot rolled steel strip using the hot dip galvanized hot rolled steel strip production facility 1 will be explained while using FIG. 2 to FIG. 4.

FIG. 2 is a view showing the change in temperature of the steel strip surface when the hot rolled steel strip passes through the non-oxidizing furnace 12, the reduction zone 13, and the cooling zone 14 of the facility for production 1 of hot dip galvanized hot rolled steel strip. In FIG. 2, the temperature point where the hot rolled steel strip enters the non-oxidizing furnace 12 is O, the temperature point where it leaves the non-oxidizing furnace 12 is P, the temperature point where it enters the reduction furnace of the reduction zone 13 is Q, the temperature point where it leaves the reduction furnace of the reduction zone 13 and enters the soaking furnace of the reduction zone 13 is S, the temperature point where it leaves the soaking furnace of the reduction zone 13 and enters the cooling zone 14 is T, and the temperature point where it leaves the cooling zone 14 is V.

First, the hot rolled steel strip produced by the thin slab continuous casting process is fed out from the feeding reel 10, proceeds on the line, passes through the preheating furnace, and enters the non-oxidizing furnace 12.

The hot rolled steel strip entering the non-oxidizing furnace 12, as shown by the section I in FIG. 2, is heated so that the peak maximum steel strip temperature becomes 550° C. to less than 600° C. at a temperature elevation rate of 25° C./sec or more for a period of 15 sec to 25 sec, whereby the surface of the hot rolled steel strip is oxidized. Here, the “oxidation time” means the time of passage through the preheating zone and non-oxidizing furnace.

The states of the hot rolled steel strip surface before and after this oxidation are shown in FIG. 3. FIG. 3(a) shows the hot rolled steel strip before oxidation, FIG. 3(b) shows the hot rolled steel strip after oxidation by the present invention, and FIG. 3(c) shows the hot rolled steel strip after oxidation by the prior art.

By setting the temperature elevation rate in the section I of FIG. 2 to 25° C./sec or more, which is higher than the above-mentioned conventional temperature elevation rate, the effect of preventing the formation of nonplating defects is obtained. As opposed to this, if setting the temperature elevation rate in the section I at less than 25° C./sec, the oxide CaO and calcium aluminate formed by the addition of Ca and the smut Fe3C etc. cause the formation of nonplating defects. The reason why setting the temperature elevation rate at 25° C./sec or more prevents nonplating defects will be explained below.

As shown in FIG. 3(a), Fe oxide film on the hot rolled steel strip surface is formed by the Fe atoms of the Fe layer moving to the surface layer and reacting with oxygen. Further, when an Fe oxide film is formed, the Si and Mn present in the steel strip are oxidized in the same way as Fe, so SiO2 and MnO and other secondary oxide films are formed under the Fe oxide film. Here, when an Fe oxide film is formed, if the CaO, Fe3C, etc. shown in FIG. 3(a) adhere to the steel strip surface, the formation of an Fe oxide film will be inhibited and the pits 19 shown in FIG. 3(c) will end up being formed. In the case of Fe3C, this breaks down into C which reacts with oxygen whereby, as shown in FIG. 3(c), the formation of an Fe oxide film is inhibited. In the above way, if pits 19 are formed, as shown in FIG. 3(c), SiO2 and MnO and other secondary oxide films end up appearing on the surface. These SiO2 and MnO and other secondary oxide films degrade the wettability with the hot dip galvanization, so nonplating defects end up being caused at the time of hot dip galvanization.

Therefore, in the present invention, the temperature elevation rate is set to a high value of 25° C./sec or more and the rate of formation of an Fe oxide film is made large.

If the heating temperature becomes high, formation of an oxide film will be promoted, so the larger the heating rate, the greater the rate of formation of an oxide film. An oxide film is formed mainly by movement to the Fe surface, so if the rate of formation of an oxide film is large, in the end CaO, Fe3C, etc. will be pushed out at the steel strip surface. Even if CaO, Fe3C, etc. result in pits, Fe oxide film will also be formed at their bottom.

The action is believed to occur since the concentration of oxygen at the steel strip surface is high at the time of heating, so Fe2O3 (hematite) is formed at the extreme surface of the hot rolled steel strip. The formation of Fe2O3 is said to proceed due to the diffusion of oxygen to the inside of the steel strip. From this, it may be considered that as a result CaO, Fe3C, etc. are pushed out at the steel strip surface.

The concentration of oxygen inside the Fe oxide film of the surface becomes smaller the more to the inside from the surface, so under the Fe2O3, Fe3O4 (magnetite) is formed at 570° C. or less while FeO (wüstite) is formed at 570° C. or more. These Fe3O4 and FeO grow due to outward diffusion of Fe ions. Therefore, at 570° C. or more, Fe2O3 is formed at the extreme surface of the hot rolled steel strip, Fe3O4 is formed below that, and FeO is formed below that. At less than 570° C., Fe2O3 is formed at the extreme surface and Fe3O4 is formed below that.

Below these FeO and Fe2O3, when the concentration of Si or Mn in the steel is high, secondary oxide films comprised of Si or Mn oxides or Si and Mn composite oxides are formed.

If CaO, Fe3C, etc. adhere to the hot rolled steel strip surface and are not pushed out at the surface, the CaO, Fe3C, etc. will block the supply of oxygen from the surface layer, so secondary oxide films comprised of Si or Mn oxides or Si and Mn composite oxides will be directly formed below the CaO, Fe3C, etc. In this case, in the succeeding reduction process, if the surface CaO, Fe3C, etc. drop off, pits of Si or Mn oxides or Si and Mn composite oxides exposed at the surface will be formed and as a result nonplating defects will be detected after plating.

However, as explained above, when setting the temperature elevation rate to a high value of 25° C./sec or more, the CaO, Fe3C, etc. adhered to the steel strip surface are pushed out at the surface, the concentration of oxygen at the pits after they are pushed out becomes high, and Fe3O4 and FeO are formed at these parts, so Si or Mn oxides or Si and Mn composite oxides will never be exposed at the surface.

Due to this, even if the pits 19 shown in FIG. 3(b) are formed at the Fe oxide film due to the inhibitory actions of CaO, Fe3C, etc., an Fe oxide film is formed at the bottom of this pitting 19. Therefore, the SiO2, MnO, and other secondary oxide films are covered by the Fe oxide film and will not appear at the steel strip surface.

That is, the properties of the steel strip surface after the end of the temperature elevation process become as follows: As shown in FIG. 3(b), from the inside, the surface is comprised of Fe (hot rolled steel strip), a secondary oxide film comprised of Si or Mn oxides or Si and Mn composite oxides, and an oxide film comprised of Fe3O4 and FeO or FeO over that. CaO, Fe3C are present at the surface. There are pits under the CaO, Fe3C, but there is the FeO layer.

As opposed to this, if setting the temperature elevation rate at less than 25° C./sec, CaO, Fe3C, etc. will be hard to push out at the surface, so as shown in FIG. 3(c), a secondary oxide film comprised of Si or Mn oxides or Si and Mn composite oxides will end up appearing at the surface.

Note that, the secondary oxide films comprised of Si or Mn oxides or Si and Mn composite oxides on Fe (hot rolled steel strip) will be simply described in FIGS. 3(b), (c) as “SiO2, MnO”.

Further, by setting the peak maximum steel strip temperature in the non-oxidizing furnace to 550° C. or more, the effect is obtained that an oxide layer is uniformly formed and the CaO, Fe3C, etc. present at the surface part of the oxide film can be easily removed. This effect is not obtained if the peak maximum steel strip temperature is made less than 550° C.

Further, by setting the peak maximum steel strip temperature in the non-oxidizing furnace to less than 600° C., excessive formation of an oxide film is prevented. If the peak maximum steel strip temperature inside the non-oxidizing furnace is made 600° C. or more, the oxide film will be excessively produced and oxide film will end up remaining in the subsequent reduction.

In this case, the time for holding the temperature elevation rate at 25° C./sec or more is made 15 sec or more. If less than 15 sec, a sufficient oxide film thickness is not possessed, so as a result, the secondary oxide films comprised of Si or Mn oxides or Si and Mn composite oxides will end up being exposed at the surface without being covered by the FeO film.

Next, as shown by the section II of FIG. 2, the oxidized hot rolled steel strip proceeds on the line and enters the reduction zone 13 of the annealing furnace 15. In the annealing furnace 15, first, the strip is heated in the reduction zone 13 to give a peak maximum steel strip temperature of 700° C. to 760° C., then proceeds to the cooling zone 14 where it is cooled. The hot rolled steel strip is reduced in the reduction zone 13 and the cooling zone 14 in the annealing furnace in a state holding the steel strip temperature at 570° C. or more for a period of 25 sec to 45 sec. That is, in FIG. 2, the time from the temperature point R where the steel strip temperature is 570° C. to the temperature point U is set to 25 sec to 45 sec.

Here, the reason for limiting the temperature of the reduction to the region of a temperature of 570° C. or more is as follows: That is, above 570° C., FeO becomes the main Fe oxide and is reduced, while at less than 570° C., Fe3O4 becomes the main Fe oxide and is reduced. FeO, compared with Fe3O4, is easier to reduce due in part to the high processing temperature. Therefore, the method of reducing FeO is easier to control than reduction of Fe3O4.

The hot rolled steel strip surfaces before and after the above reduction are shown in FIG. 4. The hot rolled steel strip before reduction is (d), the hot rolled steel strip reduced without excess or shortage is (e), the hot rolled steel strip which is insufficiently reduced is (f), and the hot rolled steel strip which is excessively reduced is (g). Note that, in FIG. 4, the CaO and Fe3C shown in FIG. 3 are not shown, but these CaO and Fe3C are blown away from the steel strip surface by the flow or the reduction atmosphere H2, N2, and the like when passing through the annealing furnace 13 etc.

Note that, the secondary oxide films comprised of Si or Mn oxides or Si and Mn composite oxides formed on the Fe (steel strip) are described simply as “SiO2, MnO” in FIG. 4 as well.

As a result, the oxide film in the state of FIG. 3(b) is suitably reduced and, as shown in FIG. 4(e), the structure becomes, from the inside, Fe (steel strip), a secondary oxide film comprised of Si or Mn oxides or Si and Mn composite oxides, and a film of Fe above that. Pits where CaO and Fe3C had been present remain on the surface, but there is an Fe layer at the bottom.

By reducing the hot rolled steel strip to give a peak maximum steel strip temperature of 700° C. to 760° C. in a state holding the steel strip temperature at 570° C. or more for a period of 25 sec to 45 sec, the surface of the hot rolled steel strip shown in FIG. 4(d) is reduced without excess or shortage in the annealing furnace 15.

That is, as shown in FIG. 4(e), the Fe oxide film formed by the non-oxide film is reduced and becomes a completely Fe layer. Further, this Fe layer completely covers the SiO2, MnO, and other secondary oxide films formed by the oxidation and reduction as well. The SiO2, MnO, and other secondary oxide films degrading the plating wettability with the hot dip galvanization are completely covered, so the plating wettability becomes extremely good, and nonplating defects do not occur.

As opposed to this, when the peak maximum steel strip temperature is less than 700° C. or when the time for holding the steel strip temperature at 570° C. or more is less than 25 sec, the reduction at the annealing furnace 15 becomes insufficient and, as shown in FIG. 4(f), Fe oxide film ends up remaining. Therefore, this Fe oxide film degrades the plating wettability for hot dipping, so nonplating defects end up occurring.

Further, when the peak maximum steel strip temperature exceeds 760° C. or the time for holding the steel strip temperature at 570° C. or more exceeds 45 sec, the reduction in the annealing furnace 15 becomes excessive. In this case, as shown in FIG. 4(g), the Fe oxide film is sufficiently reduced and an Fe layer is formed. However, Si and Mn have a stronger oxidizing power than Fe, so even when the Fe oxide film is reduced by the annealing furnace 15, secondary oxide layers of SiO2 and MnO excessively grow and end up appearing at the steel strip surface. As explained above, SiO2 and MnO degrade the plating wettability of hot rolled steel strip, so nonplating defects end up being formed.

Next, the reduced hot rolled steel strip proceeds on the line from the annealing furnace 15 to a hot dip galvanization tank 16 heated to a predetermined temperature where it is dipped and hot dip galvanized.

Next, the hot dip galvanized hot rolled steel strip proceeds on the line and the deposition of the hot dip galvanization on the hot rolled steel strip is adjusted to a predetermined amount by a wiping apparatus 17.

Next, the hot rolled steel strip proceeds on the line and is cooled in the cooling furnace 18.

In the above embodiment, the hot rolled steel strip entering the non-oxidizing furnace 12 is heated to give a peak maximum steel strip temperature of 550° C. to less than 600° C. at a temperature elevation rate of 25° C./sec or more over a period of 15 sec to 25 sec for oxidation. When an Fe oxide film is formed, even if the Fe3C and other smut and Ca-based oxides form pits 19, the bottom of the pits 19 are covered by the Fe oxide film.

Further, in the above embodiment, the oxidized hot rolled steel strip is heated to give a peak maximum steel strip temperature of 700° C. to 760° C. while holding the steel strip temperature at 570° C. or more for 25 sec to 45 sec to reduce it, whereby the Fe oxide film on the hot rolled steel strip surface is reduced without excess or shortage. Further, no secondary oxide layers of SiO2 and MnO appear on the surface either. Therefore, the formation of nonplating defects can be prevented.

Further, in the above embodiment, the length in the conveyance direction of the furnace used for oxidation (preheating furnace and non-oxidizing furnace 12) was set to 30 m to 50 m, while the length in the conveyance direction of the furnace used for reduction (reduction zone 13) was set to 50 m to 70 m. From experiments, it reveals that if the ratio of lengths along the conveyance direction of the furnace used for oxidation and the furnace used for reduction is 0.5 to 0.9, a good plating state can be obtained. In the present embodiment, by setting the ratio of lengths along the conveyance direction of the furnace used for oxidation and the furnace used for reduction to be 0.5 to 0.9, the formation of nonplating defects can be prevented. Further, the furnace used for oxidation and the furnace used for reduction are set to suitable lengths without excess or shortage, so the investment in capital cost are optimized.

Above, a preferred embodiment of the present invention was explained while referring to the attached drawings, but the present invention is not limited to these examples. A person skilled in the art could clearly conceive of various modifications or changes within the scope of the technical ideas described in the claims. These should naturally also be understood as falling under the technical scope of the present invention.

Further, in the above embodiment, the hot rolled steel strip was fed out from a feeding reel, but it is also possible to directly connect it to a line performing thin slab continuous casting.

Further, in the above embodiment, the hot rolled steel strip was fed out from a feeding reel to the non-oxidizing furnace, but it may also be treated by pickling, surface scrubbing, etc. before being fed out to the non-oxidizing furnace.

Further, in the above embodiment, the hot rolled steel strip was fed out from a feeding reel to the inside of the non-oxidizing furnace, but it is also possible to provide an apparatus for pickling, surface scrubbing, and other processing before oxidation.

Further, in the above embodiment, an annealing furnace including a reduction zone and cooling zone was used, but it is also possible to use separate furnaces such as a reduction furnace and a cooling furnace.

Further, in the above embodiment, as the hot dipping, hot dip galvanization was used, but aluminum, lead, tin, etc. may also be used other than zinc.

Further, in the above embodiment, the present invention is particularly effective in hot rolled steel strip. The reason is believed to be that the surface of hot rolled steel strip has coarser grain boundaries, larger surface areas, easier oxidation and reduction, and larger growth rate of the oxide layer.

Here, to compare the amount of oxidation and amount of reduction under the hot dip galvanization conditions of cold rolled steel sheet, the conventional formulas for estimating the amount of oxidation and amount of reduction of cold rolled steel sheet are applied to hot rolled steel strip giving a good plating state under the oxidation and reduction conditions of the present invention so as to calculate the amount of oxidation and amount of reduction of hot rolled steel strip.

The formula for estimating the amount of oxidation of cold rolled steel sheet estimates the amount of oxidation from the two variables of time stayed in the preheating furnace and non-oxidizing furnace and the peak temperature of the cold rolled steel sheet. The formula for estimating the amount of reduction of cold rolled steel sheet estimates the amount of reduction from the two variables of time stayed in the reduction furnace and the peak temperature of the cold rolled steel sheet. When estimating this amount of reduction, the amount of reduction in the case of a temperature of the reduction furnace of 570° C. or more and the amount of reduction in the case of less than 570° C. are separately calculated and the sum of the two is estimated as the amount of reduction. The specific forms of the formulas for estimation of the amount of oxidation and amount of reduction are not shown, but can be derived from experiments.

Hot rolled steel strips obtained by hot rolling cast slabs obtained by a thin slab casting machine were oxidized and reduced under suitable oxidation and reduction conditions defined by the present invention. The values of the amounts of oxidation and amounts of reduction were found by the above formulas for estimating the amount of oxidation and amount of reduction. As a result, the amounts of oxidation were 0.12 to 0.2 mg/m2 or so, and the amounts of reduction were 0.2 to 0.35 mg/m2 or so. These values are smaller compared with the amounts of oxidation of 0.1 to 0.8 mg/m2 and amounts of reduction of 0.45 to 1 mg/m2 of cold rolled steel sheet obtained by the same formulas.

From the above results, the oxidation rate and the reduction rate are faster than the case of cold rolled steel sheet, so it can be estimated that the calculated values of the suitable amount of oxidation and amount of reduction when hot dip galvanizing hot rolled steel strip would give smaller values than the values in the case of cold rolled steel sheet.

By applying the present invention to hot dip galvanization of hot rolled steel strip, compared with the case of application to cold rolled steel sheet, the oxidation time and reduction time can be shortened. Further, the length of the furnaces for the oxidation and reduction can be shortened and therefore the hot dip galvanization facility can be reduced in size.

However, in front of the hot dipping facility of the present invention, as shown in FIG. 5, an alkali washing system comprised of an alkali spray tank 20, alkali scrubber tank 21, warm water rinse tank 22, and hot air drier 23 and not using electrolytic washing and an alkali scrubber using nylon brushes 24 are arranged. The reason why the generally used electrolytic washing is not used is that when using a thin slab continuous casting machine and a hot rolling mill connected with it to produce hot rolled steel strip, the thin slab is hot rolled, then the hot rolled steel strip surface is pickled and coated with a rust preventative. The time from the pickling to the hot dipping is 2 days or less or so, therefore the amount of the rust preventative coated may be made smaller from the usual circumstances.

However, after pickling, the steel strip surface has a small amount of rust preventative and rust preventative and Fe3C etc. present on it, so the alkali washing system not using electrolytic washing is used to wash off the rust preventative, Fe3C, etc. adhering to the surface, then an alkali scrubber using nylon brushes is used to remove the rust preventative, Fe3C, etc.

This washing removes the rust preventative usually burned off in a heating furnace. In the heating furnace, the oxygen in the atmosphere is used to stabilize the oxidation of the hot rolled steel strip surface. Therefore, the amount of formation of oxide film is stable, so this is a good condition for preventing nonplating defects.

Note that the suitable ratio of the amount of oxidation and amount of reduction when dealing with hot rolled steel strip obtained by hot rolling a cast slab obtained by a thin slab casting machine was found by experiments to be 0.4 to 0.55 or so. As opposed to this, in the case of conventional cold rolled steel sheet, it was 0.2 to 1.2 or so, i.e., the values fluctuated.

Further, if using an oxidation process and reduction process like in the present invention, it was confirmed that even with hot rolled steel strip produced by directly hot rolling a slab produced in a thin slab continuous casting machine and having a thickness of 2 mm or more, no coil breakage occurs even if using the usual conveyance rolls of a diameter of 1500 mm in the processes after plating.

The reason is believed to be that by setting the temperature elevation rate at the oxidation process to 25° C./s and making the reduction time shorter than that of the reduction process of conventional cold rolled steel sheet, the yield point of the hot rolled steel strip becomes higher and processing becomes possible at less than the strain where yield elongation occurs, so coil breakage no longer occurs.

Note that the usual processing rate in the current art is 90 mpm to 180 mpm, so it is possible to apply the present invention to newly establish or modify hot dipping facilities having this range of rates. The upper limit of the processing rate of a hot dipping facility is, in the current art, 180 mpm or so. However, even if a hot dipping facility with an even higher processing rate is developed, the present technology can be applied. Further, the lower limit of the processing rate may be any rate so long as the conditions of the present invention can be realized.

Some hot dip galvanization facilities are limited in terms of economic ton/hr of the furnaces. In such a case, if the strip becomes thicker, the processing rate is reduced, so the time for passage through the oxidation furnace becomes longer and as a result the average temperature elevation rate becomes smaller. In this case, the facility may also be operated so that part of the temperature elevation process satisfies the temperature elevation rate of the present invention.

EXAMPLE 1

The ingredients of four types of hot rolled steel strips A, B, C, and D produced using the thin slab continuous casting process are shown in Table 1 expressed by mass %.

TABLE 1
SteelCSiMnPSAlNCa
type(mass %)(mass %)(mass %)(mass %)(mass %)(mass %)(mass %)(mass %)
ASGHC0.0540.060.250.020.010.0290.00810.0017
BSGHC0.0430.0460.240.250.0120.050.0750.0023
CSGH4400.160.051.10.020.0120.040.070.002
DSGH5400.110.031.510.0250.0050.050.060.0025

The various conditions and results when using the method of production of hot dip galvanized hot rolled steel strip according to the present invention to produce hot dip galvanized hot rolled steel strips from these four types of hot rolled steel strip are shown in Table 2. For the production of the hot dip galvanized hot rolled steel strips, four types of hot rolled steel strips were passed through a preheating furnace, non-oxidizing furnace, reduction furnace, soaking furnace, and cooling furnace for oxidation, reduction, and cooling, then were hot dip galvanized.

The amount of coating of the hot dip galvanization was in the range of 80 to 120 g/m2 (one side).

TABLE 2
Peak
max.Preheating
(oxid)furnace +
OverallTemp.steelPeak max.non-ReductionPlating
HotLineoxidationelevationstripReduction(reduction)oxidizingzonestate G:
Datarolledspeedtimeratetemp.timesteel stripfurnacelengthgood, P:
no.steel(m/min)(sec)(° C./S)(° C.)(sec)temp. (° C.)length (m)(m)poor)
1A1002028550397103352G
2B1201929560367303862G
3C1002029570397503352G
4D1401634550367003862G
5A1201731510397303352P
6B1201932600367503852P
7C772621550507103352P
8D1801344560217303862P
9B1201929550366803852P

As shown in Table 2, the Data Nos. 1 to 4 are examples satisfying all of the conditions defined in the present invention. The surfaces of the produced hot dip galvanized hot rolled steel strips were extremely good in terms of plating state.

On the other hand, the Data Nos. 5 to 9 shown in Table 2 are comparative examples where some of the conditions defined in the present invention are not satisfied. The surfaces of the produced hot dip galvanized hot rolled steel strips had nonplating defects or residual scale or other plating defects.

EXAMPLE 2

The ingredients of two types of hot rolled steel strips A and B produced using the thin slab continuous casting process are shown in Table 3 expressed by mass %.

TABLE 3
SteelCSiMnPSAlNCa
type(mass %)(mass %)(mass %)(mass %)(mass %)(mass %)(mass %)(mass %)
ASGHC0.0540.060.250.020.010.0290.00810.0017
BSGHC0.0430.0460.240.250.0120.050.0750.0023

The various conditions and results when using the method of production of hot dip galvanized hot rolled steel strip according to the present invention to produce hot dip galvanized hot rolled steel strips from these two types of hot rolled steel strip are shown in Table 4. For the production of the hot dip galvanized hot rolled steel strips, the two types of hot rolled steel strips were oxidized by a preheating furnace and non-oxidizing furnace, reduced by a reduction zone (reduction furnace and soaking furnace), then were hot dip galvanized. Note that in this experiment, the preheating furnace and non-oxidizing furnace correspond to the “furnace used for oxidation”, while the reduction zone corresponds to the “furnace used for reduction”.

TABLE 4
PeakPeak
max.max.Preheating
(oxid)(red)furnace +Ratio of
Temp.steelsteelnon-ReductionPlatinglength
HotLineOverallelevationstripReductionstripoxidizingzonestate G:used for
Datarolledspeedoxidationratetemp.timetemp.furnacelengthgood, P:oxidation/
no.steel(m/min)time (sec)(° C./S)(° C.)(sec)(° C.)length (m)(m)poor)reduction
1A1002028550397103353G0.63
2B1201929560367303862G0.61
3B1201929560247303841P0.93
4B1201929560467303878P0.49

The Data No. 3 and 4 shown in Table 4 had lengths of preheating furnaces fixed at 17 m and lengths of non-oxidizing furnaces fixed at 21 m, had different cooling conditions, and had lengths of reduction zones adjusted to become pseudo 41 m and 78 m. The reduction time is the value calculated from a processing rate of 120 m/min.

As shown in Table 4, Data No. 1 and 2 are examples where the ratio of the total length of the preheating furnace and non-oxidizing furnace and the length of the reduction zone satisfies the condition of being in the range of 0.5 to 0.9 defined in the present invention. The surfaces of the produced hot dip galvanized hot rolled steel strips were extremely good in terms of plating state.

On the other hand, the Data No. 3 and 4 shown in Table 4 are comparative examples where the ratio of the total length of the preheating furnace and non-oxidizing furnace and the length of the reduction zone is outside of the range of 0.5 to 0.9 defined in the present invention. The surfaces of the produced hot dip galvanized hot rolled steel strips had nonplating defects and other plating defects.

Note that, the present invention is worked in the range of processing rate shown in the examples. In this case, the upper limit of the processing rate is, with current technology, 180 mpm or so. However, even if a hot dipping facility with a further greater processing rate is built, the present technology can be applied.

Further, the lower limit of the processing rate may be any rate so long as the conditions of the present invention can be realized. The usual processing rate in current technology is 90 mpm to 180 mpm, so some hot dip galvanization facilities are limited in terms of economic ton/hr of the furnaces. In such a case, if the hot rolled steal strip becomes thicker, the processing rate is reduced, so the time for passage through the oxidation furnace becomes longer and as a result the temperature elevation rate becomes smaller. In this case, the facility may also be operated so that part of the temperature elevation process satisfies the temperature elevation rate of the present invention.

INDUSTRIAL APPLICABILITY

The present invention is effective for preventing nonplating defects from occurring on plated surfaces when hot dip galvanizing hot rolled steel strip produced by the thin slab continuous casting process.