Kozasu, Isao (Yokohama, JA)
Shimizu, Teruhiko (Yokohama, JA)

05/340942

10/21/1975

03/14/1973

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Nippon Kokan Kabushiki Kaisha (Tokyo, JA)

148/660

148/122, 266/114, 148/622, 148/121, 148/337, 148/332, 266/113, 148/334

C21D1/00; C21D1/18; C21D1/18

148/144,122,121,31.55,12.1,120,111,112 75/123L,123N

3620856	PROCESS TO IMPROVE MAGNETIC CHARACTERISTICS OF CARBON STEEL	November 1971	Hiraoka
3632456	METHOD FOR PRODUCING AN ELECTROMAGNETIC STEEL SHEET OF A THIN SHEET THICKNESS HAVING A HIGH-MAGNETIC INDUCTION	January 1972	Sakakura et al.
3657022	PROCESS FOR THE MANUFACTURE OF COLD-ROLLED STEEL STRIP WITH SUPERIOR MECHANICAL WORKABILITY, ESPECIALLY DEEP FORMING PROPERTIES	April 1972	Kubotera et al.
3661656	CASE-HARDENED STEEL PRODUCT AND PROCESS FOR ITS MANUFACTURE	May 1972	Jarleborg
3671337		June 1972	Kumai et al.

Sachs, G. et al.; Practical Metallurgy, ASM, Cleveland, 1940 pp. 466-467. .
Young, J., Materials & Processes, New York, 1954, pp. 251, 278-279, 290-291, 293-295..

Satterfield, Walter R.

Drucker, William Anthony

We claim

1. The method of preparing an improved steel material which includes the steps of:

2. - 0.25% C

3. 6 - 2.0% mn

4. 01 - 0.10% Sol.Al

5. 01 - 0.20% Nb or less than 0.20% (Nb+V)

6. The method of preparing an improved steel material which includes the steps of:

7. - 0.25% C

8. 6-2.0% mn

9. 01 - 0.10% Sol.Al

10. 01- 0.20% Nb or less than 0.20% (Nb+V)

11. - 1.0% Ni & Cu

12. - 0.5% Mo & Cr

13. - 0.2% Ti and Zr.

14. The method of preparing an improved steel material, as claimed in claim 1, wherein said steel has a thickness of about 40 mm.

15. The method of preparing an imrpoved steel material, as claimed in claim 2, wherein said steel has a thickness of about 40 mm.

16. The method of preparing an improved steel material, as claimed in claim 1, wherein said cooling step for a steel of about 14.3 mm thickness is carried out at a rate within the range 0.6° to 2.0°C per second.

17. The method of preparing an improved steel material, as claimed in claim 2, wherein said cooling step for a steel of about 14.3 mm thickness is carried out at a rate within the range 0.6° to 2.0°C per second.

18. The method of preparing an improved steel material, as claimed in claim 1, wherein said cooling step for a steel of about 10 to 12 mm thickness is carried out at a rate within the range 0.8° to 2.0°C per second.

19. The method of preparing an improved steel material, as claimed in claim 2, wherein said cooling step for a steel of about 10 to 12 mm thickness is carried out at a rate within the range 0.8° to 2.0°C per second.

BACKGROUND OF THE INVENTION

This invention relates to a method of improving the properties of steel material and more particularly, by producing fine ferrite-pearlite structure in said steel without adding special elements or using quench-temper treatment, and with only accelerate cooling at the most suitable rate.

In general, to improve hot rolled structure, and the refining of crystal grain size, there is employed a known normalizing process (in which steel is air-cooled after heating up to austenite region and then kept for a required time. In such a case, there is no problem when the thickness of said steel is relatively thin, e.g. less than 6mm, since the cooling rate, even if by air-cooling, is still high enough. However, as said thickness becomes thicker, e.g. more than 10mm, and especially beyond 30mm, many difficulties tend to appear. That is, there are limits on improving said properties, and especially toughness, because said rate of air-cooling is lowered as said thickness increases. Therefore, an alloying element, e.g. Ni, is further added to said steel or a known quench-temper treatment is employed, e.g. as applied to an Al-killed steel for low temperature services. Needless to say, such action brings about a high cost. At the same time, in the latter case, it is wellknown that the shape of said materials is often limited and increasing of quench distortion is brought about, because all of said defects have been considerably improved by subjecting said steel to a known platen type quench or a known roller quench process. However, when accelerated cooling at a relatively slow rate is required, there are produced other defects. That is, firstly, said cooling rate is difficult to control. Secondly, unevenness of cooling or a known hard spot appear. In particular, said hard spot is produced in a location of said steel where many drop permit cooling water to strike at early cooling stage of cooling. Needless to say such phenomena bring about non-uniformity of the mechanical properties and especially deterioration of toughness. In a different manner from the above process, sometimes an air-blast cooling is also employed on said steel. It is obvious however obvious that a cooling rate capable of influencing on said properties of steel is difficult to obtained and, when it is employed without adding an alloying element, there is naturally a limit in improvement of yield-strength and toughness.

Thus, it is a fact that a steel having high yield strength and excellent toughness is very difficult to obtain at a low cost. Therefore, many attempts have been made to do so. For example, an improvement based on the publication of Japanese Patent Application, Showa 35-4111 or Showa 45-31058 is a typical instance. According to Showa 35-4111, a steel having high notch toughness can be produced without adding any special alloying element. The feature of said steel lies in producing ferrite-pearlite structure whose fraction is at least more than 50% by the special heat-treatment. It is, however, confirmed that the yield strength of said steel is still insufficient while notch toughness is considerably improved. On the other hand, in Showa 45-31058. Nb is added to the steel as a strengthening element and fine ferrite-pearlite structure, whose fraction is at least more than 60% is formed. The feature of the above Showa 45-31058 lies in forced cooling, whereas such forced cooling should be avoided in Showa 35-4111 since undesirable bainite or partial martensite transformation tends to be produced. When such low temperature transformation products as bainite are formed in steel even partially, it is certain that said yield strength is lowered and fracture appearance transition temperature deteriorates. It is, in fact, confirmed that the above tendency appears in a steel based on Showa 45-31058, which is cooled by force. The chemical composition of materials tested in the experiments is shown in Table I and the changes of physical properties are shown in FIG. 2.

TABLE I ____________________________________________________________ ______________ Chemical Composition (Wt.%) Steel C Si Mn P S Cu Cr Nb V Sol.Al ____________________________________________________________ ______________ A 0.08 0.40 1.26 0.015 0.014 -- -- -- -- 0.043 B 0.09 0.41 1.31 0.016 0.014 -- -- -- 0.058 0.069 C 0.09 0.40 1.29 0.015 0.016 -- -- 0.022 -- 0.036 D 0.13 0.33 1.28 0.013 0.013 0.20 0.08 0.008 -- 0.023 E 0.14 0.21 1.33 0.006 0.009 0.09 0.31 0.025 0.048 0.008 F 0.17 0.41 1.36 0.017 0.017 -- -- -- -- 0.031 ____________________________________________________________ ______________

Steels A and F in the above Table I are steels based on Showa 35-4111, and Steel C based on Showa 45-31058. Referring now to FIG. 2, it will be well understood that said yield strength and 50% fracture appearance transition temperature are worsened as cooling rate increases. It should be noted that this tendency appears on the steel based on Showa 45-31058, which is forced cooled in contrast with Showa 35-4111. In other words, the producing of the bainitic structure, and/or martensite in the Showa 45-31058 steel seems to be unavoidable. Thus, a feasible manufacturing process for steel having high yield strength and excellent toughness without adding a special alloying element or heat-treating e.g. the known quench-temper (or normalizing) is not yet in existence.

SUMMARY OF THE INVENTION

This invention has been developed to overcome the present situation. The features of this invention lie in subjecting a steel consisting of less than 0.25% C, 0.6 to 2.0%, Mn, 0.01 to 0.10% Sol.A1 and 0.01 to 0.2% Nb or less than 0.2% (Nb+V) to accelerate cooling at 0.8° to 2.0°C/sec until the transformation is completed after heating up from Ac ₃ point to 1,000°C. Based on this method, a steel exhibiting very stable properties, i.e. high yield strength and excellent toughness can be obtained. Undesirable bainitic structure or martensite does not form in the least.

An object of this invention is to provide a method of improving properties of steel material without adding many special alloying elements or using heat-treating such as quench-temper or normalizing.

Another object of this invention is to provide an improved steel in which no bainitic structure and no martensite is formed but in which fine ferrite-pearlite structure is fully formed.

A further object of this invention is to provide an improved steel exhibiting a high yeld strength and excellent toughness.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages will be apparent from the following description with reference to the accompanying drawings in which:

FIG. 1 is an explanatory view of an accelerate cooling step with two-phase flow gas (mist).

FIG. 2 is a graph showing a relation between cooling rate and fracture appearance transition temperature and yield strength, and

FIG. 3 shows variation of the state of hardness through plate thickness depending upon cooling rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the above-mentioned Table I and FIG. 2, it is evident that the physical properties of steels are caused to deteriorate as cooling rate increases, especially at more than 2°C sec.; regardless of the chemical composition of the steels. Experiments were carried out with the following conditions.

Thicnkess:

Steels A, B, C and F: 40mm

Steels D and E: 14.3mm

Heating temperature:

Every steel: 900°C

Cooling rate: varied (average rate at 850°C - 450°C)

Cooling method:

Blowing of mist jet consisting of water and air, or water srpay.

Test of toughness: 2mm V notch Charpy test

According to said Table I and FIG. 2, it is clearly shown that both said yield strength and said fracture appearance transition temperature are remarkably improved at a cooling rate of 0.3°C to 2°C/sec. in the case of Steel C of 40mm thickness and of 0.6°C to 2.0°C/sec. in the case of Steels D and E of 14.3mm thickness (Nb is contained in each of these steels). In the above mentioned cases, it was confirmed that fine ferrite-pearlite structure is fully formed with no bainitic structure. it should be noted that when the cooling rate is beyond 2°C/sec. said properties are worsened, and furthermore that those of steels A, B and F containing no Nb are little improved. That is, the improvement of yield stress of steels A and F is very and the yield stress of steel B, to which only V is added is little improved and its fracture appearance transition temperature rapidly changes for the worse at a cooling rate of more than 1.5°C/sec.

Thus, the reasons that the properties of steel are remarkably improved with accelerate cooling at 0.3°C to 2°C/sec as mentioned above lie in the following behavior of Nb. That is, firstly, when said steel is heated up, the coarsening of the austenite grain is prevented by fine dispersion of Nb-carbonitride. Secondly, the ferrite-pearlite structure formed in steel is more refined than that of the air-cooled case, where the cooling rate is 0.3°C/sec. in case of said 40mm thickness or 0.6°C/sec in the case of said 14.3mm, due to the facts that the Y-a transformation temperature is lowered to some extent and that the grain growth after ferrite transformation is restrained with the presence of Nb. Nb is a very effective element to produce said fine ferrite-pearlite structure under the above-mentioned accelerated cooling rate. On the other hand, in a steel containing no Nb, there is shown little grain refining effect with said accelerate cooling. In a steel containing only V, said accelerated cooling is of no utility, because precipitation behavior of V-carbonitride in the cooling stage changes widely depending upon its cooling rate. As mentioned above, the reason that said properties of steel are lowered by degrees as said accelerated cooling rate increases and is beyond 2°C/sec, is based on the fact that said as bainite is formed or that dislocation density in ferrite increases even if said bainitic structure does not appear. It is confirmed by many experiments that the above phenomena are pronounced in the steel containing no Nb.

Influence of heating rate on said properties of steel D in Table was investigated. The results are shown in FIG. 2 as tests D-1 and D-2. That is, in test D-1 heating was done by an ordinary furnace and in test D-2, by high frequency induction. In such a case, it is needless to say that the heating rate of the high-frequency furnace is higher than that of the ordinary furnace. In FIG. 2, it will be understood that said properties are markedly improved with the above-mentioned accelerate cooling. Especially improved effects with the high-frequency heating are noticed in addition to those of accelerated cooling. The reason for this is based on the facts that the coarsening of austenite grain and the coalescence and the coarsening of Nb carbonitride are prevented, and accordingly the structure after transformation is more refined.

Thus, in the above-mentioned experiments, it became clear that said cooling rate has a large effect on said properties of steel and the most suitable range of said cooling rate is rather narrow. This fact shows that said cooling rate should be closely measured. Therefore, a metal sheathed chromel-Alumel thermo couple was inserted in to the steel body directly to measure temperature during cooling instead of using radiation pyrometry or an unsheathed thermocouple.

When the above-mentioned cooling rate is applied to a steel consisting of the following chemical composition, the highest effect appears. That is:

C: less than 0.25% Mn: 0.6 to 2.0% Sol.Al: 0.01 to 0.10% Nb: 0.01 to 0.20%

or

Nb+V: less than 0.2%

if necessary.

one or more selected from group consisting of less than 1.0% Ni and Cu, less than 0.5% Mo and Cr, less than 0.2% Ti and Zr

Each of said steels C, D and E shown in Table I is one of steels based on this invention. The soaking temperature range for the above steel is from more than Ac ₃ point to 1,000°C and the cooling rate is limited within the range of 0.8°C to 2.0°C/sec in view of the above-mentioned experiments.

The reasons that chemical composition of a steel based on this invention is limited as mentioned above, are as follows: C and MN: When the C content is beyond 0.25% and the Mn content is beyond 2.0%, abnormal micro structure tends to appear and said toughness changes for the worse as if a cooling rate of less than 2.0°C/sec is employed. When said Mn content is less than 0.6%, the required strength cannot be obtained. Nb and Nb+V: Even if Nb or Nb+V of more than 0.2% is added, no additional effect appears. When said content is less than 0.01%, no effect appears and it is meaningless. Sol.Al: When Sol.Al content is less than 0.01%, deoxidizing effect and fixing effect of nitrogen does not appear. More than 0.10% Sol.Al brings about little incremental effect and cleanliness of steel become worse.

Heating temperature is within ordinary normalizing temperature range, i.e. more than Ac ₃ point and is limited to less than 1,000°C. If said temperature is beyond 1,000°C. said austenite grain tends to coarsen, and Nb-carbide tends to dissolve into the matrix, consequently, undesirable bainitic structure tends to be formed during cooling.

cooling rate from the above-mentioned heating temperature is closely limited within the range of 0.8°to 2.0°C/sec. The lower limit, i.e. 0.8°C/sec corresponds to an air-cooling rate for steel plate of 10 to 12mm thickness and, accordingly, a noticeable effect has not yet been exhibited. When said rate is beyond 2.0°C/sec, undesirable bainite begins to form even if martensite does not appear. If once said structure is formed, the discontinuous yield phenomenon disappears and the lowering of the yield stress is brought about. At the same time, the 50% fracture appearance transition temperature is raised. Consequently, efforts to improve said properties of steel will be brought to naught.

Such accelerate cooling as mentioned above may be carried out with water-cooling by the common spray nozzle. It is, however, recommended in this invention that a two-phase gas jet in which liquid is atomized is employed. The features of said cooling system with said two-phase gas jet lie in that said cooling is thereby very uniform and is controllable with accuracy. An example of two-phase gas jet system e.g. mist cooling system is shown in the accompanying drawing, wherein numeral 1 is a cooled steel material, 2 a spray nozzle, 3, a gas reservoir 4, a roller table 5 a feed pipe of gas, and 6 a feeding pipe for cooling water. A typical single nozzle arrangement of the above basic configuration is shown in FIG. 1-(a), a double nozzle arrangement in FIG. 1-(c), and a three nozzle arrangement in FIG. 1-(b). Moreover, said FIG. 1-(c) also is an example of a reversing mechanism for said cooled material as an arrow shows. These configurations are selected as occasion demands.

The embodiments based on the above-mentinoned process of this invention are as follows. The chemical composition of embodiments is shown in Table 2 and, the obtained physical properties, in Table 3 and FIG. 3 respectively.

TABLE 2 ____________________________________________________________ ______________ Chemical Composition (Wt. %) Steel C Si Mn P S Cu Cr Nb V Sol.Al ____________________________________________________________ ______________ G 0.08 0.40 1.26 0.015 0.014 -- -- -- -- 0.043 H 0.09 0.40 1.29 0.015 0.016 -- -- 0.022 -- 0.036 I 0.13 0.33 1.28 0.013 0.013 0.20 0.08 0.008 -- 0.023 J 0.14 0.21 1.33 0.006 0.009 0.09 0.31 0.025 0.048 0.008 ____________________________________________________________ ______________

The details of steels in the above Table 3 are as follows.

Steel (G) is a comparative steel.

Steels (H), (I) and (J) are based on this invention.

Heating requirements:

Steels (G), (H) and (J): 900°C × 40min, in an ordinary heating furnace.

Steel (I): 900°C. in a high frequency furnace after controlled rolling.

Cooling requirements:

air-cooling and mist jet cooling (as shown in the following Table 3).

Results:

cooling requirements and obtained physical properties are shown in Table 3, and variation of hardness through thickness, in FIG. 3.

TABLE 3 ______________________________________ Physical Properties <1> <2> <3> <4> <5> <6> <7> <8> <9> mm °C/sec 1/Kg Kg/mm ² Kg/mm ² % Kg-m °C ______________________________________ G air- cooling 0.3 -- 31.0 44.6 45.4 29.0 -54 G 40 1.2 0.06 32.9 46.3 42.1 29.1 -90 1.8 0.35 33.1 47.0 42.7 29.6 -70 air- cooling 0.3 -- 32.1 47.1 40.1 29.3 -66 H 40 0.9 0.06 36.2 49.0 41.9 29.1 -92 1.8 0.34 40.7 49.5 42.0 29.6 -93 air- cooling 0.6 -- 43.0 54.9 47.0 4.3 -75 I 14.3 1.9 0.12 47.5 57.3 40.0 4.8 -82 air- cooling 0.6 -- 41.8 54.4 41.3 8.8 -45 J 14.3 1.7 0.12 44.9 55.7 40.7 9.4 -48 ______________________________________ <1> Steel <2> Thickness <3> Cooling rate <4> Water : Air <5> Yield Stress <6> Tensile Strength <7> Elongation <8> Impact energy (vE.) at 0°C <9> 50% fracture appearance transition temperature (vTrs).

According to the above Table 3 and FIG. 2 (wherein are given results of the above-mentioned basic experiments, i.e. Table I steel), it will be well understood that the properties based on this process are far more excellent than those of the comparative steel (including comparative process, i.e. air-cooling). That is, in the case of steel of 40mm thickness, said yield stress with accelerate cooling increases by 4 to 8 Kg/mm ² in comparison with the comparative process in which the cooling rate is 0.3°C/sec, i.e. air-cooling. At the same time, 50% fracture appearance transition temperature (vTrs) is improved by about 30°C. Also, in the case of steel of 14.3 mm thickness, said steel shows a similar tendency. An example of variation of hardness through section is shown FIG. 3. In FIG. 3, the upppr is Steel (G) and the lower is Steel (H). Referring to FIG. 3, it is understood that not only said hardness of steel G (not containing Nb) is influenced sensitively by the increase of said cooling rate, but also said yield stress is little improved. On the other hand, in the case of Steel (H) (containing Nb), said hardness is little changed in comparison with that of air-cooled, i.e. normalized, steel. It should be noted that this fact shows that said stable ferrite-pearite structure is fully produced. In FIG. 3, it is understood that the variation of hardness through at section is within the range of ±1 (Vicker Hardness). There is no precedent for such uniformity.

Thus, to obtain a steel having high yield stress and excellent toughness, the cooling rate, i.e. accelerate cooling at 0.8°C to 2.0°C/sec. should be closely retained. In this way the distortion of steel can be minimized. Needless to say said steel material includes slab, tube, pipe, bar, section steel or the like.