JP2016084506A - Hot rolled steel sheet excellent in cold workability and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformize micro structure and soften material quality of a hot rolled steel sheet before being cold-rolled and containing relatively large C content.SOLUTION: There is provided a hot rolled steel sheet that has been hot-rolled in the temperature range of Ar3 point °C to 600°C and that has micro structure containing ferrite of 20% or more by volume fraction and is excellent in cold workability. The hot rolled steel sheet is produced by (i) cooling the hot rolled steel sheet that has been hot-rolled in the temperature range of Ar3 point °C to 600°C to 300°C or less, heating the hot rolled steel sheet to 400°C to 700°C just before winding it and winding it as it is or (ii) cooling the hot rolled steel sheet that has been hot-rolled in the temperature range of Ar3 point °C to 600°C to 300°C or less and winding it, then rewinding, heating it to 400°C to 700°C and winding it.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a hot-rolled steel sheet excellent in cold workability for manufacturing a hot stamping steel sheet and a method for manufacturing the hot-rolled steel sheet.

  In recent years, hot stamping has become widespread as a technique for manufacturing high-strength parts of 1000 to 2000 MPa used for automobiles or the like with high dimensional accuracy. Hot stamping is a technology for improving the dimensional accuracy and increasing the strength of a molded product by press-molding a steel sheet in a state of being heated to an austenite region and simultaneously performing molding and quenching.

  Generally, steel sheets for hot stamping contain a relatively large amount of C to ensure the strength of the molded product, and require Mn and B (austenite stabilizing elements) to ensure the hardenability when cooling the mold. Although this amount is contained, this point is disadvantageous in the production of the steel sheet.

  For example, in a coil wound with a hot-rolled steel sheet containing the required amounts of Mn and B in order to ensure hardenability, the cooling rate is different at the coil part, and the microstructure becomes non-uniform. The material tends to be hard. That is, it is considered that the non-uniformity of the microstructure in the hot rolled steel sheet leads to deterioration of cold workability.

  In addition, when a large amount of C is contained to ensure the strength of the molded product, the hardness of the hot-rolled steel sheet is excessively increased, and the next cold rolling becomes difficult.

  Tempering by batch annealing can be considered as a means for eliminating the unevenness of the microstructure and ensuring the cold workability, but usually 3 to 4 days are required, which is not preferable from the viewpoint of productivity. Since the agglomeration occurs, there is a problem in that carbides are not easily dissolved during heating when hot stamping is performed, and the strength after quenching cannot be obtained sufficiently.

  Furthermore, from the viewpoint of productivity, a method by continuous annealing is conceivable. However, since the annealing time is short, it is difficult to sufficiently soften the steel plate and make the microstructure uniform.

  If the hot-rolled steel sheet or the cold-rolled steel sheet is not sufficiently softened and the microstructure is not uniform, the dimensional accuracy of the hot stamped product is lowered, and the hardness and strength are varied. So far, several techniques for improving the dimensional accuracy of hot stamped articles and eliminating variations in hardness and strength have been proposed (see, for example, Patent Documents 1 to 6).

  For example, in Patent Document 6, in a method of manufacturing a steel sheet for hot pressing by hot rolling, cold rolling, and continuous annealing of a steel sheet having a predetermined component composition, the steel sheet is (Ac3-10 ° C.) to a continuous annealing step. After heating to (Ac3-60 ° C), cooling at 10 ° C / second or less and holding at 550-660 ° C for 1-10 minutes, temper rolling is performed, and there is a non-heated part in the hot stamping process In this method, a hot stamping product with a small hardness variation is obtained, in which the hot stamping is performed when the maximum heating temperature of the quenching portion is Ac3 or higher.

JP 2007-169679 A International Publication No. 2012/053637 International Publication No. 2012/120692 JP 2012-237048 A JP 2013-122076 A JP 2013-151708 A

  According to the aforementioned prior art, softening of the hot-rolled steel sheet and uniforming of the microstructure facilitates the next cold rolling, improving the dimensional accuracy of hot stamped molded products and eliminating variations in hardness and strength. Can be planned. Therefore, the present invention aims to homogenize the microstructure of the hot-rolled steel sheet before cold rolling and soften the material in the steel sheet containing a relatively large amount of C, and a hot-rolled steel sheet that solves the problem It aims at providing the manufacturing method.

  The present inventors diligently studied a method for solving the above problems. As a result, the present inventors perform finish rolling in a temperature range lower than the Ar3 point, and after cooling, if the hot-rolled steel sheet is reheated for a short time, the microstructure of the hot-rolled steel sheet is made uniform and the material is made uniform. I found that it can be softened.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

  (1) Cold workability, a hot-rolled steel sheet that has been hot-rolled in a temperature range of not more than Ar3 and not less than 600 ° C., and has a microstructure containing ferrite with a volume fraction of 20% or more. Excellent hot-rolled steel sheet.

  (2) The hot-rolled steel sheet contains, in mass%, C: 0.07 to 0.50%, Si: 0.10 to 1.00%, and Mn: 0.70 to 3.00%. The hot rolled steel sheet having excellent cold workability as described in (1) above.

  (3) The hot-rolled steel sheet is further in mass%, P: 0.010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010% or less, O: The hot-rolled steel sheet having excellent cold workability as described in (2) above, containing 0.010% or less.

  (4) A method for producing a hot-rolled steel sheet having excellent cold workability according to any one of (1) to (3) in a continuous hot-rolling step, and a temperature range of Ar3 point or lower and 600 ° C or higher. The hot-rolled steel sheet that has been hot-rolled is cooled to 300 ° C. or lower, and immediately before winding, heated to 400 ° C. or higher and 700 ° C. or lower, and rolled as it is. A method of manufacturing a steel sheet.

  (5) A method for producing a hot-rolled steel sheet having excellent cold workability according to any one of (1) to (3) in a continuous hot-rolling step, and a temperature range of Ar3 point or lower and 600 ° C or higher. The hot-rolled steel sheet that has been hot-rolled is cooled to 300 ° C. or lower and wound up, and then rewinded and heated to 400 ° C. or higher and 700 ° C. or lower, and is excellent in cold workability. A method for producing a hot-rolled steel sheet.

  According to the present invention, since the microstructure of the hot-rolled steel sheet is uniformized and the cold workability is improved, it is possible to increase the rolling reduction and manufacture a thin cold-rolled steel sheet having a uniform microstructure. As a result, the dimensional accuracy of the hot stamped product is improved, and variations in hardness and strength are eliminated.

It is a figure which shows the process of the manufacturing method of the hot rolled sheet steel of this invention. It is a figure which shows another process of the manufacturing method of the hot rolled sheet steel of this invention. (A) shows the process until it winds up a hot-rolled steel plate once, (b) shows the process of unwinding and heating the wound hot-rolled steel plate, and winding up again.

  The hot-rolled steel sheet excellent in cold workability of the present invention (hereinafter sometimes referred to as “the present-invention hot-rolled steel sheet”) is a hot-rolled steel sheet that has been hot-rolled in a temperature range of 600 ° C. or higher below the Ar3 point. And having a microstructure containing ferrite of 20% or more in volume fraction.

  The method for producing a hot-rolled steel sheet excellent in cold workability of the present invention (hereinafter sometimes referred to as “the present invention production method”) is a method for producing the hot-rolled steel sheet of the present invention in a continuous hot rolling step. The hot-rolled steel sheet, which has been hot-rolled in a temperature range of Ar3 or lower, is cooled to 300 ° C or lower, heated immediately before winding to 400 ° C or lower and 700 ° C or lower, and wound as it is.

  Further, the production method of the present invention is a method of producing the hot-rolled steel sheet of the present invention in a continuous hot-rolling step, and once the hot-rolled steel sheet that has been hot-rolled in a temperature range of not more than Ar3 point and 600 ° C., It is characterized in that it is cooled to 300 ° C. or lower and wound up, and then rewound and heated to 400 ° C. or higher and 700 ° C. or lower and rolled up.

  First, the manufacturing method of the present invention will be described.

  In FIG. 1, the process of this invention manufacturing method is shown. Usually, the rough bar S ′ obtained after the rough rolling is subjected to finish rolling with a finish rolling mill to form a hot-rolled steel sheet, cooled by a cooling means, and then wound by a winder. A heating means is disposed in front of the take-up machine, the hot-rolled steel sheet S immediately before winding is heated to a required temperature, and the material in the thickness direction and the longitudinal direction is softened.

  FIG. 2 shows another step of the production method of the present invention. FIG. 2A shows a process until the hot-rolled steel sheet is wound up once, and FIG. 2B shows a process of rewinding and heating the wound hot-rolled steel sheet and winding it again.

  In the manufacturing method of the present invention shown in FIG. 2, the rough bar S ′ after the rough rolling is subjected to finish rolling with a finish rolling mill to form a hot rolled steel sheet, cooled with a cooling means, and then wound up with a winder. (See FIG. 2 (a)). The hot-rolled steel sheet S wound up and cooled to room temperature is rewound, heated to a required temperature by a heating means, and the heated hot-rolled steel sheet S1 is again wound up by a winder.

  Finish rolling is completed at a temperature range of not more than Ar3 and not less than 600 ° C. By carrying out finish rolling in the temperature range below the Ar3 point to promote ferrite transformation. The material of the hot-rolled steel sheet becomes soft, and it becomes possible to increase the rolling reduction in the next cold rolling.

  The hot-rolled steel sheet after the hot rolling is finished is cooled to 300 ° C. or lower by a normal cooling means. When the cooling temperature exceeds 300 ° C., the degree of material softening effect developed in cooperation with the next heat treatment (400 ° C. or higher and 700 ° C.) becomes insufficient. The cooling temperature is preferably 270 ° C. or lower.

  Next, the hot rolled steel sheet cooled to 300 ° C. or lower is heated to 400 ° C. or higher and 700 ° C. or lower immediately before winding. Since it is necessary to immediately heat the hot-rolled steel sheet immediately after cooling to 400 ° C. or more and 700 ° C. or less, the heating means is preferably a heating means capable of rapid heating. Electric heating with fast response is preferable, for example, energization heating or induction heating is preferable. The heating rate is not particularly specified.

  If the heating temperature is less than 400 ° C., the material softening effect due to heating is insufficient, so the temperature is set to 400 ° C. or higher. Preferably it is 450 degreeC or more. When the heating temperature exceeds 700 ° C., the temperature fluctuation in the coil caused by winding becomes large, so the temperature is set to 700 ° C. or less. Preferably it is 650 degrees C or less. The hot rolled steel sheet heated to 400 ° C. or more and 700 ° C. or less is wound as it is at that temperature.

  In the production method of the present invention, immediately after the hot-rolled steel sheet is cooled to 300 ° C. or lower, the present inventors consider the reason why the cold workability of the hot-rolled steel sheet is remarkably improved by the heat treatment heating to 400 ° C. or higher and 700 ° C. or lower. Guesses as follows.

  During finish rolling, ferrite transformation is promoted, strain introduced by rolling remains, recrystallization is promoted by reheating, the material of the hot-rolled steel sheet is softened, and cold workability is remarkably improved. To do. In the hot-rolled steel sheet of the present invention, the Vickers hardness Hv is set to 350 or less as the strength that can sufficiently ensure the cold rollability.

  The hot-rolled steel sheet of the present invention manufactured by the manufacturing method of the present invention has a hot-rolled steel sheet that has been hot-rolled in a temperature range of Ar3 point or lower and 600 ° C or higher, cooled to 300 ° C or lower, and immediately before winding up to 400 ° C or higher. It is characterized by containing recovered or recrystallized ferrite as a microstructure formed in a hot-rolled steel sheet by heating to 700 ° C. or lower.

  In addition, the hot-rolled steel sheet of the present invention is preferably a hot-rolled steel sheet that has been hot-rolled in a two-phase region (γ + α) in a temperature range of not more than the Ar3 point and 600 ° C. By hot rolling in the two-phase region (γ + α), a ferrite-based microstructure with a volume fraction of 20% or more can be reliably obtained.

  Since the effect of the heat treatment is an effect that does not directly depend on the component composition of the hot-rolled steel sheet, the hot-rolled steel sheet of the present invention is basically not limited to the hot-rolled steel sheet having a specific component composition, but in mass%, C: 0.07 to 0.50%, Si: 0.10 to 1.00%, and Mn: 0.70 to 3.00% are preferable.

  C is an element necessary for ensuring the strength of the molded product, and in the hot-rolled steel sheet of the present invention, in order to ensure a strength after hot stamping of 980 MPa or more, mass% (hereinafter, simply referred to as “%”). Therefore, 0.07% or more is preferable. More preferably, it is 0.10% or more. On the other hand, if it exceeds 0.50%, it becomes too hard and the cold workability deteriorates, so the content is made 0.50% or less. Preferably it is 0.40% or less.

  Since Si is an element that contributes to improvement in strength, it is preferably 0.10% or more. More preferably, it is 0.30% or more. On the other hand, if it exceeds 1.00%, it becomes too hard and the cold workability deteriorates, so 1.00% or less is preferable. More preferably, it is 0.50% or less.

  Since Mn is an element that enhances hardenability and contributes to improvement in strength, it is preferably 0.70% or more. More preferably, it is 1.00% or more. On the other hand, if it exceeds 3.00%, it becomes too hard and the cold workability deteriorates, so 3.00% or less is preferable. More preferably, it is 2.70% or less.

  The hot-rolled steel sheet of the present invention is C: Si, Mn, P: 0.010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N :: 0.010% or less. , O: You may contain 0.010% or less.

  Since P and S are impurity elements, they are preferably as small as possible, and both are preferably 0.010% or less. More preferably, both are 0.005% or less.

  Al is a deoxidizing element, and 0.001% or more is preferable for obtaining a deoxidizing effect. More preferably, it is 0.005% or more. On the other hand, if it exceeds 0.100%, coarse oxides are generated and the cold workability of the hot-rolled steel sheet is hindered, so 0.100% or less is preferable. More preferably, it is 0.050% or less.

  N is an element inevitably mixed from the iron raw material, so 0.010% or less is preferable. More preferably, it is 0.005% or less.

  O is an element that inevitably remains after deoxidation, so 0.010% or less is preferable. More preferably, it is 0.005% or less.

  In addition to the above elements, the hot-rolled steel sheet of the present invention may contain Ca, REM, Mo, Nb, V, Ti, Zr, Cr, Ni, B, Cu, etc., as long as the properties of the hot-rolled steel sheet of the present invention are not impaired. May be included.

  In the hot-rolled steel sheet of the present invention, the balance other than the elements constituting the component composition is Fe and inevitable impurities.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on these one example conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
A slab having the composition shown in Table 1 is manufactured by continuous casting, hot-rolled into a hot-rolled steel sheet under the conditions shown in Table 2, cooled and reheated under the conditions shown in the same table, and wound up as it is. It was. That is, after cooling to 300 ° C. or less in a cooling zone, the sample was heated to 400 ° C. or more and 750 ° C. or less, wound up as it was, and measured for hardness Hv (load: 1 kg) with a Vickers hardness meter.

  The structure was observed using SEM and EBSD, and the ferrite fraction was calculated. The cold workability of the hot-rolled steel sheet of the present invention was evaluated by Vickers hardness. Vickers hardness: Hv of 350 or less was regarded as an excellent cold workability range.

  Furthermore, each hot-rolled steel sheet is cold-rolled to a thickness of 1.4 mm, cut to a size of 100 mm × 200 mm, and then inserted into an electric furnace simulating the heat history of the hot stamping process. Thereafter, a heat treatment was performed at 900 ° C. for 3 minutes, and then quenching with a mold was performed using a press. In order to evaluate the strength of the steel plate after quenching, Vickers hardness was measured. The obtained results are shown in the same table.

  In the hot-rolled steel sheet according to the invention method, a value of 350 Hv or less is obtained in Vickers hardness, and it is understood that cold workability is ensured. Also, the hardness after hot stamping corresponds to the C amount. However, a hot-rolled steel sheet that has been hot-rolled with a finishing temperature exceeding the Ar3 point has a hardness exceeding 350 Hv, so that it is difficult to cold-roll.

  When the end-point temperature of cooling deviates from the range of the present invention (300 ° C. or less), or when the reheating temperature and the coiling temperature deviate from the range of the present invention (from 400 ° C. to 700 ° C.), hot rolling The carbide formed in the stage becomes coarse, does not dissolve sufficiently during heating at the time of hot stamping, and the hardness after martensitic transformation is insufficient.

  In steel J where C is out of the range of the present invention (0.07 to 0.50%), sufficient hardness has not been obtained, but in steel K, where C is out of the range of the present invention, The hardness is too high. In L steel in which Si deviates greatly from the range of the present invention (0.10 to 1.00%), sufficient strength is not obtained due to the promotion of ferrite transformation that occurs during cooling and the formation of retained austenite. .

  Moreover, in M steel in which Mn is outside the range of the present invention (0.70 to 3.00%), sufficient strength is still not obtained due to the formation of retained austenite. On the other hand, when the cooling after finish rolling is finished at a temperature higher than 300 ° C., and when the reheating temperature exceeds the range of the present invention, the carbide precipitated in the hot-rolled steel sheet is coarsened, and the hot stamping is performed. In the heating stage at the time of carrying out, it does not fully re-dissolve, and the hardness corresponding to the added amount of C is not obtained.

  Also, as in the past, when the finishing temperature is higher than the Ar3 point, the ferrite fraction of the hot-rolled steel sheet is low, so the hardness of the hot-rolled steel sheet becomes too high and the load during cold rolling increases too much. Is concerned.

(Example 2)
Using steels D, F, and G shown in Table 1, a hot-rolled steel sheet having a thickness of 2 mm is manufactured, and then the hot-rolled steel sheet is cooled under cooling conditions within the scope of the present invention, and is cooled as it is to room temperature. Rolled up. Subsequently, the wound hot-rolled steel sheet was rewound and subjected to a reheating treatment, and when rewinding, the reheating treatment was performed with the heat history shown in Table 5.

  About the obtained hot-rolled steel sheet, the hardness Hv (load: 1 kg) was measured with a Vickers hardness meter in the same manner as in Example 1. The results are also shown in Table 5.

  In the hot-rolled steel sheet according to the invention method, ferrite is sufficiently secured and a hardness of 350 Hv or less is obtained.

  In addition, a hot-rolled steel sheet (test piece) was heated to 900 ° C. at 100 ° C./second by conducting heating to simulate a hot stamping process, kept for 3 seconds, and immediately cooled to room temperature after keeping the heat. After cooling, the hardness of the test piece was measured with a Vickers hardness tester. The results are also shown in Table 5.

  In the hot-rolled steel sheet according to the invention method, a hardness of 350 Hv or less is obtained, and good cold workability is secured. Moreover, the hardness of the hot-rolled steel sheet after hot stamping has a hardness corresponding to the C amount.

  According to the invention method, ferrite is sufficiently secured in the hot-rolled steel sheet, so that a hardness of 350 Hv or less is obtained and cold workability is secured. Moreover, the hardness of the hot-rolled steel sheet after hot stamping is suitable for the C amount. On the other hand, when the temperature of the reheating treatment exceeds the range of the present invention, the hardness after hot stamping is remarkably lowered.

  As described above, according to the present invention, the microstructure of the hot-rolled steel sheet is made uniform and the cold workability is improved, so that the rolling ratio is increased to produce a thin cold-rolled steel sheet having a uniform microstructure. Is possible. As a result, the dimensional accuracy of the hot stamped product is improved, and variations in hardness and strength are eliminated. Therefore, the present invention has high applicability in the steel plate manufacturing industry.

S 'Coarse bar S, S1 Hot rolled steel sheet

Claims (5)

  A hot-rolled steel sheet that has been hot-rolled in a temperature range of Ar3 point ° C or lower and 600 ° C or higher, and has a cold workability characterized by having a microstructure containing ferrite with a volume fraction of 20% or higher. Hot rolled steel sheet.   The hot-rolled steel sheet contains C: 0.07 to 0.50%, Si: 0.10 to 1.00%, and Mn: 0.70 to 3.00% in mass%. Item 2. A hot-rolled steel sheet excellent in cold workability according to Item 1.   The hot-rolled steel sheet is further mass%, P: 0.010% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.010% or less, O: 0. The hot-rolled steel sheet having excellent cold workability according to claim 2, comprising 0.010% or less.   It is a method of manufacturing the hot-rolled steel plate excellent in cold workability of any one of Claims 1-3 by a continuous hot rolling process, Comprising: It hot-rolls in the temperature range below 600 degreeC above Ar3 point. The hot-rolled steel sheet having excellent cold workability is characterized in that the hot-rolled steel sheet having been finished is cooled to 300 ° C. or lower, and immediately before winding, heated to 400 ° C. or higher and 700 ° C. or lower and wound as it is. .   It is a method of manufacturing the hot-rolled steel plate excellent in cold workability of any one of Claims 1-3 by a continuous hot-rolling process, Comprising: Temperature range hot rolling below Ar3 point 600 degreeC or more is performed. The finished hot-rolled steel sheet is cooled to 300 ° C. or lower and wound up, and then rewound and heated to 400 ° C. or higher and 700 ° C. or lower and wound up to produce a hot-rolled steel sheet excellent in cold workability Method.

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