JPH0849016A - Production of high carbon steel with fine pearlitic structure - Google Patents

Abstract

PURPOSE:To produce a steel with a fine structure high in toughness and ductility by subjecting an intermediate forming material, obtd. by subjecting a carbon steel or a low allay steel having a specified carbon content to rough rolling, to finish rolling in which the surface temp., working temp. and working strains are specified. CONSTITUTION:An intermediate forming material obtd. by subjecting the slab of a carbon steel contg. 0.6 to 10% C to hot rolling is heated or cooled to 850 to 1050 deg.C surface temp. and to the working temp. T ( deg.C) at which the final target gamma grain size in the curve of the working temp.-the size number of boundary recrystallized grains is found by the formula I:T50XN+1380+ or -30; where N denotes the target gamma grain diameter. Successively, it is subjected to finish rolling into a final product shape by working strains (e) readable from the finish rolling temp. in the curve of the working temp.-boundary recrystallized strains found by the formula II: e=-0.002XT+2.15+ or -0.04. Thus, the high carbon steel with a target fine pearlitic structure can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high carbon steel having high toughness and ductility which is widely used in rails, shaped steel, steel bars and other industrial machines.

[0002]

2. Description of the Related Art Steel with a high carbon and pearlite metallographic structure is used as a structural material because it has high strength and good wear resistance. Corresponding to the trend, it is especially often used for rails.

As a method of manufacturing such a steel material, for example, Japanese Patent Laid-Open No. 58-212229 discloses "C: 0.6.
A high-temperature Mn steel rail containing 5 to 0.85% and Mn: 0.5% to 2.5% is rapidly cooled from the austenite region, and the structure of the rail or rail head is used as fine pearlite to improve wear resistance. Heat treatment method ", JP-A-59
JP-A-133322 discloses, "A rolling rail of a specific component capable of stably obtaining a pearlite structure is immersed in a molten salt bath at a temperature of Ar ₃ or higher to a specific temperature and Hv> about 10 mm below the surface of the top of the rail head. Heat treatment method for rails having a fine pearlite structure having a hardness of 350 ", JP-A-63-
JP-A-277721 discloses, "As a manufacturing method combining controlled rolling and heat treatment and a low-temperature heat treatment method after rolling, rolling with a cross-section reduction rate of 5% or more at 800 ° C. or less is performed.
Many techniques are known, as is disclosed in "A manufacturing method of rail steel in which heating is performed at 50 to 900 ° C and accelerated cooling is performed at 1 to 15 ° C / sec".

However, although the strength and wear resistance of pearlite steel can be easily obtained by adding an alloying element to the rail of the required standard product, the toughness is remarkably lower than that of steel mainly composed of ferrite structure, For example, for pearlite rail steel, the room temperature test value in the JIS No. 3 U notch Charpy test is about 1 to ² kgfm / cm ² . When such a steel having low toughness is used as a structural member in a field where repeated load or vibration is applied, there is a problem in that low stress brittle fracture is caused by minute initial defects and fatigue cracks.

In general, the means for improving the toughness of high carbon steel is achieved by refining the metal structure, that is, refining the austenite structure and intragranular transformation. Of these, intragranular transformation is difficult to control, and the refinement of the austenite structure is the key to improving toughness. In particular, if the size of γ grain exceeds 7 and preferably 8 or more, JIS3
No. U Notch Charpy test at room temperature 2.5kgfm
It was found from the experiments conducted by the present inventors that about / cm ² appears.

Japanese Unexamined Patent Publication (Kokai) No. 63-277721 discloses a technique for refining the metal structure by utilizing pearlite transformation, which includes a finishing rolling temperature range of 850 to 1050. The amount of processing strain in the range of ° C has not been examined. Further, no other study has examined the optimum processing temperature and processing amount for obtaining fine austenite in this temperature range.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a target γ grain size with an appropriate temperature and working amount in finish rolling, efficiently and with a fine pearlite structure which is a target structure. The purpose is to produce carbon steel.

[0008]

SUMMARY OF THE INVENTION The gist of the present invention is C:
An intermediate formed material obtained by hot rough rolling a billet of carbon steel or low alloy steel containing 0.6 to 1.0% has a surface temperature of 850 to
At 1050 ° C., heating or cooling is performed at a processing temperature at which the final target γ grain size of the processing temperature-grain boundary recrystallization grain size number curve shown in FIG.
Subsequently, the processing temperature can be read from the finishing rolling temperature of the processing temperature-grain boundary recrystallization strain curve shown in FIG. It is a method for producing a high carbon steel exhibiting a perfect pearlite structure. T = −50 × N + 1380 ± 30 ... T: processing temperature C, N: target γ grain size e = −0.002 × T + 2.15 ± 0.04 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ e: Working strain The present inventors conducted many experiments in order to efficiently obtain a steel having a fine grained pearlite structure and improved toughness. Tried in a laboratory and a real production facility. The present invention will be described in detail below based on the experimental results and consideration.

First, the reason why the steel composition is limited as described above in the present invention will be explained. Carbon steel slab produced by continuous casting method or ingot-agglomeration method of molten steel melted in an ordinary melting furnace, or further Cr, Mo, V, N
In a low alloy steel slab containing a small amount of an element for improving strength and toughness such as i, C is an effective component for producing a pearlite metallographic structure and particularly improving the wear resistance as steel for rails, and this structure is obtained. Therefore, it is necessary to contain 0.60% or more. However, if an excessive amount exceeding 1.00% is contained, a large amount of cementite metal structure precipitates and the hardness increases, but the ductility decreases. Therefore,
The C content in the present invention is limited to 0.6% to 1.0%.

Next, rough rolling will be described. The steel slab produced as described above is roughly rolled into a rail shape after heating.
However, even if a high pressure is applied in the finish rolling, a fine structure cannot be finally obtained with crystal grains that have become extremely coarse after rough rolling,
In addition, it is necessary to have a degree of freedom of temperature in the finish rolling process. Therefore, it is preferable that the temperature of the final pass of the rough rolling is 1000 ° C. or higher and 1100 ° C. or lower.

Next, finish rolling will be described. The rough rolled hot billet is subsequently subjected to finish rolling. The reason why the temperature is limited by the surface temperature is that the internal temperature cannot be measured in actual processing or experiments, and the toughness that is most important is not necessarily the deep inside. The reason for limiting the temperature range to 850 to 1050 ° C. is that recrystallization occurs near the grain boundaries, but grain growth is not a problem.

Further, a method of determining finishing processing conditions will be described. In order to obtain a fine-grained γ structure of high carbon steel, processing and subsequent recrystallization must be repeated. . The present inventors have experimentally found that the grain size of static recrystallization of high carbon steel is almost dependent on the processing temperature, and that most of the recrystallization sites at that time are at the old grain boundaries. It was

First, the processing temperature is determined. FIG. 1A shows a relationship curve between the processing temperature and the recrystallized grain size at the grain boundary obtained from the results of the above experiment. This curve is the processing temperature-grain boundary recrystallization particle size curve, and since this line was almost linear, a regression equation was obtained and the following equation was obtained. Using this formula, the processing temperature for obtaining the target γ grain size near the grain boundary before recrystallization can be obtained. T = −50 × N + 1380 ± 30 ... T: processing temperature C, N: target γ grain size

Next, the processing amount at this time is determined. We conducted an experiment to change the working amount and temperature, and clarified the region where most of the grain boundaries recrystallized, and defined the boundary line between this region and the region where most of the grain boundaries did not recrystallize as the working temperature-working strain relationship curve. To do. FIG. 1B is a processing temperature-grain boundary recrystallization strain curve thus obtained. Since this curve is also almost linear, a regression equation was obtained and the following equation was obtained. Since the processing temperature has already been determined, the processing strain is obtained using this formula. e = −0.002 × T + 2.15 ± 0.04 ... e: Processing strain

Since only the grain boundaries are recrystallized, if the old crystal grain size before processing is larger than twice the recrystallized new crystal grain size, mixed grains are formed. On the contrary, if the diameter of the old crystal grains is smaller than twice the diameter of the new crystal grains formed by recrystallization, a grain-sized structure can be obtained. The difference in the grain size which is twice as large is the second difference in grain size. Therefore, when it is desired to make the final target γ grain size finer than the grain size of the intermediate molded material after rough rolling by 2 or more, a grain size control structure with an intermediate grain size is obtained. You just need to get the organization one more step.
This intermediate grain size is defined as the required grain size before processing. Here, the required grain size before processing is one to two coarser than the target γ grain size for the above reason. Since it is difficult to actually make the grain size finer by the growth of crystal grains, it is preferable that the grain size be 1
It is desirable and realistic that the number is about 1.5. In order to obtain the required grain size before processing, the processing temperature and processing strain are determined and processed in the same manner as in obtaining the target particle size.

Therefore, preferably, when the difference between the final target grain size number and the grain size number of the intermediate molding material is 2 or more, before the final finish rolling, the final target of the processing temperature-grain boundary recrystallization grain size number curve. The steel slab is heated or cooled to a processing temperature at which a grain size 1 to 2 is coarser than the grain size number, and then the pre-final finishing rolling is performed at a working strain determined by the finishing rolling temperature of the processing temperature-recrystallization strain curve. It is desirable to do.

Further, preferably, when the difference between the final target grain size number and the grain size number of the intermediate molding material is 4 or more, the processing temperature-final target grain size number relationship curve is obtained before the final finish rolling and the pre-finish rolling. Heating or cooling to a processing temperature to obtain a grain size 2 to 4 coarser than the final target γ grain size, and then at the finishing rolling temperature-the processing strain of the recrystallization strain curve,
It is desirable to manufacture by rolling.

In the actual rolling process, the work strain must be converted into a cross-section reduction rate. The cross-section reduction rate is merely a conversion between the nominal strain and the true strain, and can be easily obtained by the following formula. [gamma] = {1-exp (-e)} * 100 [gamma]: area reduction rate%, e: processing strain Conversely, the processing strain is defined from the area reduction rate in the above equation.

In the finish rolling, the final shape is required to be precise, and it is assumed that the processing strain is too large to be removed in one pass. When the processing strain is larger than 0.2 (cross-section reduction rate 18%), it is difficult to manufacture a complicated shape accurately.
If it is not more than 0.15 (14% reduction in area), a precise shape cannot be expected. However, it was experimentally found that the processing strain can be recrystallized even if it is divided into multiple passes. However, the time between passes is limited to 10 seconds, and if it is longer than that, recovery occurs, so that recrystallization does not occur. Preferably, when the time is 6 seconds or less, recovery is not sufficiently performed and recrystallization is easy. Therefore, preferably, when the amount of finish rolling has to be greater than 0.15, it is desirable to manufacture by dividing into multiple passes with a time between passes of 10 seconds or less. In order to suppress grain growth, no time is taken, and it is preferable that the cooling between passes is rapid cooling such as water cooling.

Finally, a method of determining the processing temperature and the processing amount using FIG. 2 will be shown. FIG. 2 shows a case where the target grain size is obtained by three passes. Determine the target γ grain size immediately after processing. A grain size 1 to 2 coarser than the target grain size is determined as a required grain size before processing, and a further required grain size before processing is determined if necessary. The target γ grain size and the required grain size before processing are plotted on the horizontal axis of FIG. Figure 1 shows the processing temperature to obtain the target γ particle size and the required particle size before processing.
Read from (a). The processing temperature obtained in step (b) is plotted on the vertical axis. The required processing strain is read from the horizontal axis of (b). Calculate the area reduction rate from the work strain.

[0021]

【Example】

[Example-1] When it is desired to obtain γ crystal grain size 9 FIGS. 3A and 3B are obtained by explaining FIGS. 1A and 1B for Example 1. At this time, the γ grain size of the intermediate molded material was 4.6. Since the target grain size is 9 and there is a grain size difference of 4 or more, the required grain size before processing was set to 7.8, and the required grain size before processing was set to 6.6. From these, Fig. 3 (a)
If you read the processing temperature more, 930 ℃, 990 respectively
C., 1050.degree. From these, further, FIG.
If you read the processing temperature more, 0.29 and 0.1 respectively
It becomes 7, 0.05. Since a strain of 0.29 cannot be taken in the final pass, tandem rolling is performed in two passes of 0.15. Converting this into a cross-section reduction rate results in 13.9% 2-pass, 15.6% and 4.9%, respectively.

Therefore, the finish rolling in this case has the following schedule. Final: 930 ° C, processing strain 0.29, tandem 2 pass 1
Second pass from the end by 3.9%: 990 ° C., processing strain 0.17, 15.
6% 3rd pass from the end: 1050 ° C., processing strain 0.05, 4.
9%

Rolling according to this schedule was carried out as a hole rolling for rail production. As the cooling means, high-pressure water was sprayed up to near the target temperature, and air cooling was performed after reaching the target temperature. The time between tandem passes was 5 seconds. The composition of the steel material used at this time is C: 0.79, Si: 0.5 in the unit of wt%.
6, Mn: 1.07, P: 0.017, S: 0.01
1, V: 0.09, Cr: 0.22, balance Fe and unavoidable impurities (the same applies to Examples 2 to 4). As a result of rolling on such a schedule, rapid cooling on the rolling-out side, and measurement of γ grain size, a γ grain size of 9.0 was obtained.

[Example 2] When it is desired to obtain a γ crystal grain size of 9.5, FIGS. 4 (a) and 4 (b) are obtained by explaining FIGS. 1 (a) and 1 (b) in Example 2. is there. At this time, the γ grain size of the intermediate molded material was 4.6. Since the target grain size is 9.5 and there is a grain size difference of 4 or more, the required grain size before processing was set to 8.1, and the required grain size before processing was set to 6.6. When the pass schedule is read from FIG. 4 as in the first embodiment, the result is as follows, and it is decided that the final pass is performed in tandem of 3 passes. Further, as in Example 1, the cooling means sprayed high-pressure water up to near the target temperature and air-cooled after reaching the target temperature. Final: 905 ° C, processing strain 0.34, tandem 3 pass 1
3.9%, 13.9%, 3.9% Second pass from the end: 975 ° C, work strain 0.2, 18.1
% 3rd pass from the last: 1050 ° C., processing strain 0.05, 4.
9% Rolling was carried out on such a schedule, quenching was performed on the rolling-out side, and γ grain size was measured. As a result, a γ grain size of 9.5 was obtained.

[Embodiment 3] When it is desired to obtain a γ crystal grain size of 8.5, FIGS. 5 (a) and 5 (b) are obtained by explaining FIGS. 1 (a) and 1 (b) to explain Embodiment 3. is there. At this time, the γ grain size of the intermediate molded material was 4.6. Since the target grain size is 8.5 and there is a grain size difference of 2 or more, the required grain size before processing was set to 7.0.
When the pass schedule was read from FIG. 5 as in Example 1, the result was as follows. This is also done by dividing the final pass into two pass tandems. Further, as in Example 1, the cooling means sprayed high-pressure water up to near the target temperature and air-cooled after reaching the target temperature. Final: 955 ° C, processing strain 0.24, tandem 11.3
%, 2 passes each, 2nd pass from the last: 1030 ° C., processing strain 0.09, 8.
6% Rolling was carried out on such a schedule, quenching was performed on the rolling-out side, and γ grain size was measured. As a result, a γ grain size of 8.4 was obtained.

[Embodiment 4] When it is desired to obtain a gamma grain size of 8.0, FIGS. 6 (a) and 6 (b) are obtained by explaining FIGS. 1 (a) and 1 (b) to explain Embodiment 4. is there. At this time, the γ grain size of the intermediate molded material was 4.6. Since the target grain size is 8.0 and there is a grain size difference of 2 or more, the required grain size before processing was set to 6.6. When the pass schedule was read from FIG. 6 as in Example 1, the result was as follows. Further, as in Example 1, the cooling means sprayed high-pressure water up to near the target temperature and air-cooled after reaching the target temperature. Final: 980 ° C., processing strain 0.19, 17.3% Second pass from the end: 1050 ° C., processing strain 0.05, 4.
9% Rolling was performed on such a schedule, rapid cooling was performed on the rolling-out side, and γ grain size was measured. As a result, a γ grain size of 7.8 was obtained.

[0027]

As described above, by carrying out the present invention, it becomes possible to obtain a high carbon steel having a target fine pearlite structure while suppressing the processing cost. Further, as a result, it is possible to stably obtain a steel having a γ grain size of more than 7, and to obtain a carbon steel having high toughness and ductility.

[Brief description of drawings]

FIG. 1 is a diagram for determining a pass schedule, in which (a) is a processing temperature-final target grain size number relationship curve,
(B) shows a processing temperature-processing strain relationship curve.

FIG. 2 is a diagram for determining a pass schedule (when a target grain size is obtained by three passes), (a) is a machining temperature-final target grain size number relation curve, and (b) is a machining temperature-machining strain. The relationship curve is shown.

FIG. 3 is a diagram for determining a pass schedule of Example 1 (when it is desired to obtain γ grain size 9), (a) is a machining temperature-final target grain size number relation curve, and (b) is machining. The temperature-working strain relationship curve is shown.

FIG. 4 is a diagram for determining a pass schedule according to the second embodiment (when it is desired to obtain a γ grain size of 9.5);
Shows a processing temperature-final target grain size number relationship curve, and (b) shows a processing temperature-processing strain relationship curve.

FIG. 5 is a diagram for determining a path schedule of Example 3 (when it is desired to obtain a γ grain size of 8.5), (a)
Shows a processing temperature-final target grain size number relationship curve, and (b) shows a processing temperature-processing strain relationship curve.

FIG. 6 is a diagram for determining a path schedule of Example 4 (when it is desired to obtain a γ grain size of 8.0);
Shows a processing temperature-final target grain size number relationship curve, and (b) shows a processing temperature-processing strain relationship curve.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B21B 1/16 B 37/00 BBG // C21D 9/00 102 A 9352-4K 9/04 A

Claims (1)

[Claims] 1. An intermediate formed material obtained by hot rough rolling a carbon steel or low alloy steel billet containing C: 0.6 to 1.0%,
A surface temperature is 850 to 1050 ° C., and heating or cooling is performed at a processing temperature obtained by the following regression equation-grain boundary recrystallization grain size number curve to a processing temperature at which a final target γ grain size is obtained,
Subsequently, the processing temperature obtained by the following regression equation-grain boundary recrystallization strain curve, the processing strain read from the finish rolling temperature,
A method for producing a high carbon steel having a fine pearlite structure, characterized by finish rolling into a final product shape. T = −50 × N + 1380 ± 30 ... T: processing temperature C, N: target γ grain size e = −0.002 × T + 2.15 ± 0.04 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ e: Processing strain