JPH1161270A - Manufacture of hot rolled steel sheet minimal in plastic anisotropy and excellent in workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method capable of stably manufacturing a hot rolled steel sheet minimal in plastic anisotropy and excellent in workability, more particularly in ductility. SOLUTION: A steel, which has a composition containing, by weight, 0.015-0.040% C, <=0.02% Si, 0.05-0.30% Mn, <0.020% P, <=0.0030% S, 0.005-0.030% Al, <=0.0025% N, and 0.0008-0.0020% B and satisfying 0.0002<B%-0.47N% <=0.0010 and P%+S%<=0.020, is used. Immediately after casting of this steel or after reheating up to a temp. exceeding 1,100 deg.C, hot rolling is finished at a finishing temp. not lower than the Ar3 point. The resultant hot rolled steel sheet is coiled at 600-700 deg.C.

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 a thin hot-rolled steel sheet having a small plastic anisotropy and being excellent in workability, particularly excellent ductility, which can be used as a substitute for a cold-rolled steel sheet.

[0002]

2. Description of the Related Art In recent years, in the field of thin steel sheets for high working, represented by automotive materials, in place of conventional cold-rolled steel sheets,
There is a movement to apply a thin hot-rolled steel sheet with a small number of manufacturing steps, and a thin hot-rolled steel sheet having excellent workability has been demanded.

[0003] The thickness of a thin hot-rolled steel sheet competing with such a cold-rolled steel sheet is as thin as 1.0 to 2.0 mm. Since the decrease is remarkable, the temperature falls below the Ar ₃ point of the steel sheet with a decrease in the finishing end temperature. Steel sheet thus finished with Ar ₃ point or less of the ferrite presence zone, compared to steel sheet finished with Ar ₃ point or more austenite region, since the ferrite structure is coarsened, a problem that significantly ductility decreases There is.

[0004] On the other hand, in order to improve the workability, particularly ductility, of a steel sheet, it is effective to reduce solid solution strengthening elements such as C and Mn, which increase the strength of the steel sheet. an element, for causing an increase of the Ar ₃ point by reducing them, act disadvantageous for discussion of ductility derived from the above Ar ₃ point and finishing temperature. That is, when the solid solution strengthening elements such as C and Mn are reduced to improve the ductility, the Ar ₃ point rises, so that the finish tends to be finished at the Ar ₃ point or less, the ferrite structure is coarsened, and the ductility is rather reduced. Will be reduced.

To cope with such a problem and secure a finishing temperature, a hot-rolled steel sheet containing 0.06% or less of C using the lowering of the Ar ₃ point due to the addition of B is disclosed in No. 139849. However, this method does not aim to positively improve the ductility, and although the decrease in ductility is suppressed by the formation of a mixed grain structure even at a finishing temperature of about 830 ° C., the conventional production without considering the improvement in ductility is considered. In the method, since the C content is high, ductility as required by the consumer cannot be obtained.

Further, a production method for improving the material properties of B-added steel is disclosed in, for example, JP-A-2-104614 and JP-A-2-209423.
However, these are all aimed at improving the anisotropy, but not at improving the ductility.

[0007] Japanese Patent Application Laid-Open No. 7-90384 discloses a technique for reducing C by using B or Cr, or a steel to which B and Cr are added in combination, mainly for improving the ductility of a thin hot-rolled steel sheet. In this technique, the range of the amount of B added is wide, being 0.0002 to 0.0050%. In this technique, the Ar ₃ point is measured from a change in thermal expansion when the test piece is heated to 950 ° C. in an austenite region and then cooled at a cooling rate of 5 ° C./s. For the evaluation of the Ar ₃ point, an accurate measurement of the Ar ₃ point cannot be performed unless the measurement is performed under the condition of processing. That is, B has a large recrystallization delay effect in the hot working stage in the austenite region, and completes recrystallization of the worked structure between passes in high-speed passing in a low-temperature austenite region such as a thin material. On the contrary, work-induced transformation is accelerated, and the Ar ₃ point rises. Therefore, the actual Ar ₃ point is higher than the Ar ₃ point shown in the embodiment of Japanese Patent Laid-Open No. 7-90384, and the finishing temperature is lower than the Ar ₃ point in that embodiment. In the examples, the elongation of the sample in the rolling direction is evaluated by a tensile test, and thus shows good ductility.
Since the plastic anisotropy of the steel sheet that has been rolled at a finishing temperature lower than the Ar ₃ point becomes large, there is a high risk of breaking if severe processing is applied in a direction other than the rolling direction in actual press forming.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and has a small plastic anisotropy.
An object of the present invention is to provide a manufacturing method capable of stably manufacturing a thin hot-rolled steel sheet having good workability, particularly good ductility.

The present inventors have conducted various studies to obtain a method for producing a hot-rolled steel sheet having low plastic anisotropy and excellent workability. As a result, the following (1) and (2) Obtained knowledge.

(1) The effect of lowering the Ar ₃ point in the B-added steel can be summarized as B% −0.47N% in relation to the N content in the steel. Here, using the steel A series of Table 1, in the laboratory, as shown in FIG.
After heating at 0 ° C. for 10 minutes and then cooling at a cooling rate of 10 ° C./S, the Ar ₃ point measured from the thermal expansion change was B% −0.47N% and the Ar ₃ temperature shown in the unprocessed state in FIG. The relationship is shown below.

It is well known that B reacts with nitrogen in steel in the low temperature region of austenite to precipitate BN. The effect of lowering the Ar ₃ point by B is due to segregation at austenite grain boundaries, which are ferrite nucleation sites during γ-α transformation, and exhibits its effect in a solid solution state. Therefore, in the continuous cooling stage in the austenite region, the remaining solid solution B in which some B is precipitated as BN contributes to lowering the Ar ₃ point. However, when the amount of solid solution B increases and segregates at all austenite grain boundaries, A
The r ₃ point drop saturates, and even if the amount of solute B increases further, A
r ₃ points is not reduced.

(2) When the B-added steel is continuously cooled while being worked in the austenite region, the effect of lowering the Ar ₃ point is significantly reduced as compared with the case of no working. B% -0.4 which can obtain the effect of the degree
There is a 7N% region.

As shown in FIG. 1 (b), after heating the steel A series in Table 1 at 1250 ° C. for 10 minutes, the cooling rate was 10 ° C. /
When cooling at S, cooling is performed in the austenitic low-temperature range while processing is performed, and the Ar ₃ point is measured.
47N% up to about 0.0012% A compared to B-free steel
Although the r ₃ point is lowered, the amount of the reduction is extremely small as compared with the case of no processing and cannot be lowered below 850 ° C. Further, when B% −0.47N% exceeds 0.0012%, the Ar ₃ point becomes a temperature equal to or higher than that of the steel without B, and the effect opposite to the reduction of the Ar ₃ point due to the desired B addition is obtained. turn into. Therefore, Ar _{3 in the} unprocessed state
When the rolling is completed at a temperature not lower than the point and not higher than the Ar ₃ point during processing, a ferrite region or an austenitic-ferrite two-phase region is finished. In In such ferrite high temperature range, since the recrystallization retardation effect by solid solution B is applied in the same manner as when austenite processing, not the case as the coarsening of the B-free steels, only evaluate ductility Ar ₃ point or more Difficult to distinguish from the finished case. However, when recrystallized after being rolled in the ferrite region, the plastic anisotropy increases significantly, which is disadvantageous for actual press forming.

However, since the tendency of the change of the Ar ₃ point due to the addition of B during such hot working is significantly different from that during the non-working, the same effect of reducing the Ar ₃ point in the hot rolling as in the non-working can be obtained. It was found that by reducing the amounts of P and S in the steel, the same Ar ₃
It was found that there was a region of B% -0.47N% where the point lowering effect could be obtained.

The steel B series shown in Table 1 has a P content of 0.00.
5 to 0.017%, S content 0.0004 to 0.0027
% In the range of P% + S% is 0.0200% or less, and using these steels, the Ar in the processing shown in FIG.
The behavior of _three points was investigated. As a result, as shown in FIG. 2, the Ar ₃ point at the time of machining the steel B series was B% −0.4.
Unprocessed Ar of steel A series until 7N% is 6ppm
It is about the same as ₃ points. Bp-0.47N% is 6p
pm, the Ar ₃ point starts to rise again due to the strain-induced transformation promoting effect due to the recrystallization delay effect of solid solution B.
ppm is 810 ° C. or less, which is lower than 820 ° C. which is the limit finishing temperature of the thin hot rolled material, and the austenitic finish rolling can be completed. When rolling is completed in the austenite region, γ
The crystal orientation is randomized by the -α transformation, and plastic anisotropy can be reduced.

Although the detailed mechanism of maintaining the effect of lowering the Ar ₃ point of solid solution B by reducing the amounts of P and S in the steel is unknown, P and S, which have a slower diffusion rate than B, are previously formed of austenite grains. In a hot rolling process in which processing is performed during continuous cooling when B is segregated at the grain boundaries, even though B has a high grain boundary segregation coefficient, the B
It is considered that the transition to the deformation zone introduced at the time of processing becomes easy without overcoming the tension, which further promotes the recrystallization delay effect and raises the Ar ₃ point.

[0017]

[Table 1]

Based on such findings, the present invention reduces the C and Mn levels to improve ductility and optimizes the steel composition to reduce the effect of lowering the Ar ₃ point by adding B by hot working. It is intended to reduce the plastic anisotropy so that it can be obtained effectively even at the time.

That is, in the present invention, C: 0.0
15 to 0.040%, Si: 0.02% or less, Mn:
0.05 to 0.30%, P: less than 0.020%, S:
0.0030% or less, Al: 0.005 to 0.030
%, N: 0.0025% or less, B: in the range of 8 to 20 ppm, and 0.0002 ≦ B% −0.47N% ≦ 0.0
010, P% + the steel satisfies S% ≦ 0.020, was reheated beyond or 1100 ° C. Immediately after the casting, Ar ₃
Finish hot rolling at a finishing temperature above the point, 600-70
An object of the present invention is to provide a method for producing a hot-rolled steel sheet having low plastic anisotropy and excellent workability, characterized by winding at 0 ° C.

The present invention also provides a method for producing a hot-rolled steel sheet having low plastic anisotropy and excellent workability, characterized in that the entire rough bar is reheated online before finish rolling in the above method. Is provided.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail by dividing into components and compositions of steel and manufacturing conditions. 1. Steel composition and composition In the present invention, C: 0.015 to 0.0% by weight.
40%, Si: 0.02% or less, Mn: 0.05-0.
30%, P: less than 0.020%, S: 0.0030% or less, Al: 0.005 to 0.030%, N: 0.002
5% or less, B: contained in the range of 0.0008 to 0.0020%, and 0.0002 ≦ B% −0.47N% ≦ 0.
0010, P% + S% ≦ 0.020 are satisfied.

C: 0.015 to 0.040% C precipitates as carbide in the steel sheet, and this carbide increases the strength of the steel sheet and lowers the ductility. When the amount of C increases, the amount of carbide precipitation increases. In particular, when the amount exceeds 0.040%, workability deteriorates due to an increase in strength. On the other hand, in the hot rolling stage in the steel sheet manufacturing process, C forms a solid solution in austenite to stabilize austenite and has an effect of lowering the Ar ₃ point. This is the main object of the present invention,
This is preferable in the production of a thin hot-rolled steel sheet in which the temperature is significantly lowered during hot rolling and it is difficult to finish hot rolling in the austenite region. Thus, from the viewpoint of lowering the Ar ₃ point of steel, the C content is 0.0
15% or more is required. Further, when the C content is less than 0.015%, the frequency of nucleation of carbides precipitated in the winding stage after the end of hot rolling decreases, the amount of solid solution C in steel increases,
It deteriorates the aging of the steel sheet. Therefore, the C content is in the range of 0.015 to 0.040%.

Si: 0.02% or less Si forms a solid solution in a steel sheet to increase the strength and reduce the ductility. Si also raises the Ar ₃ point, making it difficult to secure the hot rolling finish temperature. Therefore, the Si content is set to 0.02
% Or less.

Mn: 0.05 to 0.30% Mn has an action of suppressing cracks generated during hot rolling as Sn present as an impurity in steel as MnS. If the content is less than 0.05%, the effect of suppressing cracking cannot be obtained. On the other hand, if the content exceeds 0.30%, the effect is not only saturated but also the workability is deteriorated due to the increase in strength. Occurs. Therefore, the content of Mn is adjusted to 0.05 to 0.3.
The range is 0%.

P: less than 0.020% P is a solid solution strengthening element like Si, and increases the strength and deteriorates the workability. Therefore, the P content is 0.020%
Defined as less than.

S: 0.0030% or less S combines with Fe or Mn to form a sulfide, and when a steel sheet is processed, it tends to be a starting point of void generation in a severely processed part, thereby deteriorating the workability. Therefore, the S content is specified to be 0.0030% or less.

Al: 0.005 to 0.030% In addition to being added as a deoxidizing agent, Al is combined with N to form Al.
Form N to improve aging. In order to exert such effects effectively, 0.005% or more of addition is required. On the other hand, Al is a strong ferrite stabilizing element. If it exceeds 0.030%, it not only raises the Ar ₃ point, but also has a solid solution strengthening effect and causes a decrease in ductility due to an increase in strength. Therefore, the Al content is 0.005 to 0.0
The range is 30%.

N: 0.0025% or less N deteriorates ductility by aging in a solid solution state. Also,
When present as a precipitate, it is BN or AlN. If the content exceeds 0.0025%, the ductility decreases in any of the existing forms with the increase in strength due to aging or precipitation strengthening. Therefore, the N content is specified to be 0.0025% or less.

B: 0.0008-0.0020%;
0002 ≦ B% −0.47N% ≦ 0.0010, P% +
S% ≦ 0.020 B has the effect of lowering the Ar ₃ point during hot rolling, and is an important element in the present invention. In order to effectively exhibit the effect, the B content needs to be 0.0008 to 0.0020%. However, as described above, when processing is involved, the Ar _three- point lowering effect of B is effectively exerted under certain specific conditions. In other words, in order to effectively exert the effect of lowering the Ar ₃ point of B even when processing is involved,
It is necessary to properly control the amounts of N, P and S in steel,
Therefore, 0.0002 ≦ B% −0.47N% ≦ 0.0
010 and P% + S% ≦ 0.020.

2. Manufacturing Conditions In the present invention, for steel having the above-described components and compositions, immediately after casting or after reheating above 1100 ° C.,
Ar Finish hot rolling at a finishing temperature of ₃ points or more, 600
Wind at ~ 700 ° C.

When the finishing temperature is lower than the Ar ₃ point,
By processing in the ferrite region, a crystal orientation unfavorable for plastic anisotropy is generated from the surface layer portion of the steel sheet, and not only the r value decreases, but also the ductility may decrease. Therefore, the finishing temperature is set to _three or more Ar points.

Further, in producing a thin hot-rolled steel sheet having the above-described component system, in order to end hot rolling at a temperature higher than the Ar ₃ point, the slab once cooled is heated to 1100 ° C.
It is necessary to heat over. Further, if such a heating temperature is ensured, it is possible to directly perform hot rolling immediately after continuous casting.

At the time of finish rolling, in order to prevent the temperature of the rough bar having the intermediate thickness once formed by rough rolling from dropping on the side of the finishing rolling mill, the entire rough bar is re-used online using a heat retaining cover or a rough bar heating device. Heating is also possible.

The winding temperature must be in the range of 600 to 700 ° C. from the viewpoint of reducing solid solution C and N which are harmful to the ductility of the steel sheet. If the temperature is lower than 600 ° C., the precipitation of solid solution N becomes insufficient. On the other hand, if the temperature exceeds 700 ° C., the number of precipitation nuclei of carbide decreases, and the solid solution C is not sufficiently precipitated in the coil cooling stage. , N are not sufficiently reduced, resulting in reduced ductility.

[0035]

Embodiments of the present invention will be described below. After heating steel having a component composition shown in Table 2 to 1200 to 1230 ° C., it was finished to a sheet thickness of 1.2 mm under the hot rolling conditions shown in Table 3. JIS No. 5 test pieces were taken from the rolling direction of these hot-rolled steel sheets, and the yield point (YP), tensile strength (TS) and elongation (El) were evaluated by a tensile test, and JIS No. 5 in the rolling direction and 45 ° direction were evaluated. A test piece was sampled, and the plastic anisotropy r value and the plate surface anisotropy | Δr | value were evaluated. In addition, in order to evaluate the aging characteristics of the steel sheet, after increasing the tensile test specimen in the rolling direction by 8% prestrain, the aging treatment at 100 ° C. × 1 hour, the YP increase (AI value), and the steel sheet To 3
After the treatment at 00 ° C. × 1 hour, the amount of increase in YP (AI ′ value) measured after fixing treatment of solid solution C by furnace cooling was evaluated. The Ar ₃ point in Table 2 is the temperature at which the change in thermal expansion at a cooling rate of 10 ° C./s after the hot working shown in FIG. 1 was measured.

[0036]

[Table 2]

[0037]

[Table 3]

As shown in Table 3, in the examples of the present invention, it was confirmed that elongation was large, plastic anisotropy was small, aging was small, and good workability was obtained.

As described above, according to the present invention,
When a thin hot rolled steel sheet is manufactured from steel with reduced C and Mn, Ar ₃ becomes a problem due to a decrease in the finishing temperature.
In order to prevent the decrease in ductility of the steel sheet due to finishing below the point, B is added to the steel, and the amounts of N, P, and S are adjusted appropriately so that the effect of lowering the Ar ₃ point by B can be maximized. The hot-rolled thin steel sheet with excellent press-formability combines excellent plastic anisotropy and excellent ductility because the steel composition is such that even if the finishing temperature is lowered, the temperature becomes Ar ₃ points or more. Can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat pattern for measuring Ar ₃ points in a non-machining state and a machining state.

FIG. 2 is a view showing the relationship between the value of B% −0.47% and the value of the Ar ₃ point measured by the method shown in FIG. 1 in the steel shown in Table 1.

Claims (2)

[Claims] 1. C: 0.015 to 0.040 by weight%
%, Si: 0.02% or less, Mn: 0.05 to 0.30
%, P: less than 0.020%, S: 0.0030% or less,
Al: 0.005 to 0.030%, N: 0.0025%
Hereinafter, B: contained in the range of 0.0008 to 0.0020%, and 0.0002 ≦ B% −0.47N% ≦ 0.00
10. For a steel satisfying P% + S% ≦ 0.020, immediately after casting or after reheating above 1100 ° C., Ar ₃
Finish hot rolling at a finishing temperature above the point, 600-70
Characterized by winding at 0 ° C., with low plastic anisotropy,
A method for manufacturing hot-rolled steel sheets with excellent workability. 2. The method for producing a hot-rolled steel sheet having low plastic anisotropy and excellent workability according to claim 1, wherein the entire rough bar is reheated online before finish rolling.