JP2000054031A - Production of cold-rolled steel sheet having excellent press-formability and little variation of press- formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel sheet excellent in deep-drawability and uniformity of the mechanical property in a coil width direction by specifying the component composition of blank steel and adapting various conditions in a hot rolling process. SOLUTION: The composition (wt.%) of the blank steel is made to have <=0.02% C, <=0.6% Si, <=2.5% Mn, <=0.10% P, <=0.05% S, <=0.004% O, 0.01-0.10% sol.Al and 0.01-0.40% at least one kind of Ti, Nb, V and Zr. Further, in order to regulate the average crystal grain diameter to 5-50 μm after hot-rolling and the fluctuation degree Dmax/Dmin of the average crystal grain diameter in the coil width direction to <=2.0, a part or all of rough-rolling bar in the longitudinal direction is heated and the finish temp. is regulated in the range of (Ar3)-(Ar3+30 deg.C) from the leading part to the trailing part of the hot-rolled coil. The cooling at a run-out is started within 15 sec after completing the finish-rolling, and after cooling to 750 deg.C at >=20 deg.C/sec average cooling speed, the hot-rolling is executed under condition of <=700 deg.C coiling temp.

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 cold-rolled steel sheet having excellent deep drawability suitable for use as an outer panel of an automobile.

[0002]

2. Description of the Related Art Cold rolled steel sheets are widely used for automobiles and the like. In automotive applications, many parts are press-molded, so various workability is required depending on the shape of the parts.
In particular, cold rolled steel sheets for press working which are excellent in deep drawability are required for applications such as outer panels of automobiles. Recently, demands for rationalization from automobile manufacturers have been severe, and in particular, there has been an increasing demand for lowering the cost of materials and improving the yield in manufacturing products. Therefore, in terms of the material of the cold-rolled steel sheet, it is particularly important that the uniformity of the material is high.

[0003] Against this background, for example,
No. 45692 discloses that, in a continuous casting-direct rolling process, hot rolling is started in a temperature range of less than 900 ° C. and 600 ° C. or more, where the surface temperature at the center of the width of 0.015 wt% or less ultra-low carbon steel slab, Further, there is disclosed a technique for improving the surface properties and deep drawability of a steel sheet by performing a holding treatment for 30 minutes or less in the middle of a hot rolling step.

[0004]

However, although this technique improves the surface properties and deep drawability of a steel sheet to a satisfactory level, there is a major problem in the uniformity of mechanical properties in the coil. In other words, in this technique, the heating temperature in hot rolling is set to a low temperature ferrite region, so that the temperature distribution in the width direction of the material during rolling (temperature distribution due to a remarkable temperature drop at the edge and its vicinity) during hot rolling A difference occurs in the texture in the coil width direction, and as a result, the mechanical properties in the coil width direction after cold rolling and final annealing vary.

[0005] When the mechanical properties vary in the width direction of the coil as described above, the workability in the material (coil) becomes non-uniform, and in particular, the deep drawability excellent in applications such as automobile outer panels is obtained. If required, there is a major problem in quality after press molding (for example, occurrence of cracks, wrinkles, etc.).
As a result, the automobile manufacturer has to perform the stripping in the coil under low yield conditions (for example, the stripping direction is set to an unreasonable direction such as 45 degrees, or the stripping is not performed near the coil edge). No longer.

Accordingly, an object of the present invention is to solve such problems of the prior art, and to have excellent press formability (deep drawability and the like) suitable for applications such as outer panels of automobiles, and furthermore, it is possible to use a coil in a coil. It is an object of the present invention to provide a method for producing a cold-rolled steel sheet which has less variation in press formability.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have excellent press moldability suitable for applications such as outer panels of automobiles, and have excellent coil formability. In order to obtain a cold-rolled steel sheet with less variation in press formability within the steel, first, after optimizing the component composition of the base steel,
It has been found that the average crystal grain size [D] after hot rolling and the variation [Dmax / Dmin] of the average crystal grain size in the coil width direction need to be within specific ranges.

In such a cold-rolled steel sheet, the raw steel composition is adjusted to specific conditions, and (1) rough rolling is performed by specific heating means in finish rolling of the rough rolling bar in the hot rolling step. By controlling the temperature by heating a part or all of the bar in the longitudinal direction, the finishing temperature from the leading end to the trailing end of the hot-rolled coil is controlled to a specific temperature range. (2) Hot rolling Hot rolling is performed under the limited conditions of optimizing the cooling start time in the later runout and the average cooling rate up to a specific temperature range, and (3) optimizing the hot rolling winding temperature. It was found that the hot-rolled steel strip obtained as described above could be manufactured by sequentially performing pickling, cold rolling, final annealing, and temper rolling. Furthermore, it has also been found that in order to obtain the above-mentioned cold rolled steel sheet more rationally, it is effective to perform accelerated rolling of the rough rolling bar under predetermined conditions in finish rolling.

The present invention has been made based on such findings, and the features thereof are as follows. [1] C: 0.02 wt% or less, Si: 0.6 wt% or less, Mn: 2.5 wt% or less, P: 0.10 wt% or less, S: 0.05 wt% or less, O: 0.004 wt% or less , Sol. Al: 0.01 to 0.10 wt%, and 1 selected from Ti, Nb, V, and Zr
A slab made of steel containing a total of 0.01 to 0.40 wt% of one or more types is roughly rolled by a rough rolling mill in a hot rolling step, and the rough rolling bar is continuously subjected to a continuous hot finishing rolling mill. When finishing rolling is performed, a part or all of the rough rolling bar in the longitudinal direction is heated by an induction heating device provided on the entrance side of the finishing rolling mill and / or between stands of the finishing rolling mill. Rolling from the leading end to the trailing end of the rolled coil so as to be in the range of Ar ₃ point to Ar ₃ point + 30 ° C., and cooling with the subsequent run-out is started within 1.5 seconds after finishing rolling, After cooling from the finishing temperature to 750 ° C at an average cooling rate of 20 ° C / sec or more, winding at a winding temperature of 700 ° C or less,
Pickling, cold rolling, final annealing, and temper rolling are sequentially performed on the obtained hot-rolled steel strip. The cold-rolled steel sheet is excellent in press formability and has little change in press formability in the coil. Production method.

[2] The method of the above-mentioned [1], wherein the slab further contains B: 0.0001 to 0.005 wt%, and is excellent in press formability and press formability in a coil. A method for producing cold-rolled steel sheets with little fluctuation [3] In the manufacturing method of the above [1] or [2], the widthwise edge of the rough rolling bar is heated by an induction heating device between the entrance of the finishing mill and / or between stands of the finishing mill. A method for producing a cold-rolled steel sheet that has excellent press formability and has little variation in press formability in a coil.

[4] In the manufacturing method according to any one of the above [1] to [3], the rolling speed of the rough rolling bar to be finish-rolled is controlled to one of the following (A) to (C). A method for producing a cold-rolled steel sheet that has excellent press formability and has little variation in press formability in a coil. (A) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finishing rolling mill, and then rolling is performed at a constant speed. (B) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finish rolling mill, and thereafter, rolling is performed at a constant speed, and then the rolling speed is further accelerated. (C) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finishing mill, and thereafter the rolling speed is further accelerated.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention and the reasons for limiting the same will be described below. As described above, in order to produce a cold-rolled steel sheet having excellent press formability (deep drawability, etc.) suitable for use as an outer panel of an automobile, and having little change (variation) in press formability in a coil. After optimizing the composition of the raw steel, the average crystal grain size [D] after hot rolling and the variation [Dmax / Dmin] of the average crystal grain size in the coil width direction,
That is, the maximum value of the average crystal grain size in the coil width direction [Dm
ax] and the minimum value [Dmin] [Dmax / Dmi]
n] must be within a specific range.

First, the reasons for limiting the component composition of the raw steel in the present invention will be described. Since C is an element that has an adverse effect on the deep drawability of a steel sheet, the content thereof is preferably small. If the C content exceeds 0.02 wt%, the deep drawability targeted by the present invention cannot be obtained, so the content is set to 0.02 wt% or less (including the case of no addition). Further, in order to further improve the deep drawability, the C content is set to 0.0020 wt% or less, and in order to further improve the workability, the C content is set to 0.0015 w%.
It is preferably set to t% or less.

Although Si has the effect of strengthening the solid solution of a steel sheet, it is an element that has an adverse effect on the deep drawability, so that its content is preferably as small as possible. If the Si content exceeds 0.6 wt%, the plating property and the deep drawability deteriorate, so the content is set to 0.6 wt% or less (including the case of no addition). Further, in order to further improve the plating property, the amount of Si is set to 0.1 wt% or less, and in order to further improve the workability, the amount of Si is set to 0.03%.
It is preferable that the content be not more than wt%.

Mn has the effect of improving the toughness and strengthening the solid solution of the steel sheet, but it is also an element that has an adverse effect on the workability. If the Mn content exceeds 2.5 wt%, the strength increases and the deep drawability deteriorates remarkably, so the content is 2.5 wt% or less (including the case of no addition).
And Further, in order to improve the deep drawability, the amount of Mn is set to 2.
The Mn content is preferably set to 0 wt% or less, and in order to further enhance the workability, the Mn content is preferably set to 0.5 wt% or less.

P has the effect of solid solution strengthening the steel sheet.
If the P content exceeds 0.10 wt%, grain boundary embrittlement due to grain boundary segregation is likely to occur, and ductility also deteriorates. For this reason, the content of P is set to 0.10 wt% or less (including the case of no addition). Further, in order to further improve the ductility, the P content is preferably set to 0.05 wt% or less, and in order to further increase the ductility, the P content is preferably set to 0.02 wt% or less.

If the S content exceeds 0.05% by weight, the precipitation amount of sulfide increases, and the deep drawability and ductility deteriorate. Therefore, the content of S is 0.05 wt% or less (however,
(Including the case of no addition). Further, in order to further improve the workability, the amount of S is set to 0.02 wt% or less, and in order to further increase the workability, the S content is set to be less than 0.02 wt%.
It is preferable that the amount is 0.01 wt% or less.

Sol. Al is used as a deoxidizing material for steel, and is an indispensable additive element for improving the addition yield of Ti, Nb, Zr, and V described later. So
l. If the Al content is less than 0.01 wt%, the above effects cannot be obtained. On the other hand, if the Al content exceeds 0.10 wt%, the effects are saturated, which is rather uneconomical. For this reason, Sol. Al
Is 0.01 to 0.10 wt%.

The smaller the content of O, the better the workability. If the O content exceeds 0.004 wt%, a decrease in workability of the steel sheet cannot be avoided. Therefore, the content of O is 0.004 wt% or less (including the case of no addition).
And In addition, such an O content is determined by the above Sol.
This is achieved by adjusting the amount of Al.

The base steel further contains one or more selected from Ti, Nb, V, and Zr in addition to the above-described components in a total amount of 0.01 to 0.40 wt%. These components form carbonitrides and sulfides to form C, N, S in steel.
And improve workability. However, if the total content is less than 0.01 wt%, the desired effect cannot be obtained. On the other hand, if the total content exceeds 0.40 wt%, the strength is excessively increased and the workability is deteriorated. 0
1 to 0.40 wt%.

Further, the steel material of the present invention contains B in an amount of 0.0001 to 0.005 wt.
% May be added. B content is 0.0001wt%
If it is less than 0.005% by weight, the effect of improving the vertical cracking resistance cannot be sufficiently obtained. Therefore, the addition amount of B is 0.000
1 to 0.005 wt%.

Next, the manufacturing conditions of the present invention will be described. Given the conditions of use of cold-rolled steel sheets by automobile manufacturers, etc., in order to perform high-yield stripping of products from cold-rolled coils, in addition to optimizing the material steel components described above, the average crystal It is necessary to optimize the variation [Dmax / Dmin] of the grain size [D] and the average crystal grain size in the coil width direction, and specifically, the average crystal grain size after hot rolling [D]. And the variation [Dmax / Dmin] of the average crystal grain size in the coil width direction needs to be 2.0 or less.

The average grain size [D] after hot rolling is 5 μm
If less than the excellent deep drawability intended in the present invention is not obtained,
On the other hand, when the average crystal grain size [D] exceeds 50 μm, roughening during processing becomes a problem. On the other hand, when the average crystal grain size [D] is 5 to 50 μm, excellent deep drawability is obtained, and no rough surface occurs. Further, [D] is set to 15 to 30 μm in order to obtain more excellent deep drawability, and [D] is set to 5 to 15 μm in order to increase strength while securing excellent deep drawability. It is preferable that

When the variation [Dmax / Dmin] of the average crystal grain size in the width direction of the hot-rolled coil exceeds 2.0,
The uniformity of mechanical properties (mean-r value, elongation at break) in the coil width direction after cold rolling and final annealing becomes poor, and [Dm
[ax / Dmin] is set to 2.0 or less, so that the mechanical properties in the coil width direction can be kept uniform.

FIG. 1 shows an embodiment described later (Tables 2 and 3).
Of the present invention (Material No. 1, [Dmax / Dm
in]: 1.50) and the comparative material (material No. 2, [Dma]
x / Dmin]: 2.35) shows the distribution of mean-r value and elongation at break (L direction) in the coil width direction after cold rolling and final annealing, and [Dmax / Dmin] is 2.35. The material of the present invention having a value of 0 or less has a small variation (variation) in mechanical properties in the coil width direction, whereas [Dmax /
[Dmin] exceeds 2.0, the mechanical properties in the coil width direction fluctuate greatly.

In the manufacturing method of the present invention, the average crystal grain size [D] of the hot-rolled coil and the variation of the average crystal grain size in the width direction of the hot-rolled coil [Dmax / Dmi]
n], the following manufacturing conditions are employed. First, a slab made of steel having the above component composition,
In the hot rolling step, rough rolling is performed by a rough rolling mill, and the rough rolling bar is subsequently subjected to finish rolling by a continuous hot finishing rolling mill.
Alternatively, by heating a part or the entire length of the rough rolling bar in the longitudinal direction by an induction heating device provided between the stands of the finishing rolling mill, the finishing temperature (the temperature at the final stand of the finishing rolling mill) is increased to the tip of the hot-rolled coil. From the part to the rear end, rolling is performed so as to be in the range of Ar ₃ point to Ar ₃ point + 30 ° C.

Only by performing hot rolling under such conditions, the mean-r value and ductility (elongation at break) in the coil including the coil width direction and the longitudinal direction can be set to the levels intended by the present invention. It becomes possible. The finishing temperature from the leading end to the trailing end of the hot-rolled coil is 30 ° C as described above.
In order to keep the temperature within the above range, it is indispensable to heat a part or the whole of the rough rolling bar in the longitudinal direction to adjust the temperature. The heating of the rough rolling bar is performed in an appropriate range from the leading end to the trailing end of the rough rolling bar, and as a result,
The finishing temperature is controlled in the above range.

Here, if the finishing temperature is lower than the Ar ₃ point, hot rolling after ferrite transformation occurs, so that [D] and [Dmax / Dmin] exceed the range intended by the present invention. On the other hand, if the finishing temperature exceeds the _three points of Ar + 30 ° C., even after finish rolling, austenite crystal grains grow remarkably, and there is a temperature difference in the coil width direction.
[D] and [Dmax / Dmin] exceed the range intended by the present invention.

The heating of the rough rolling bar by the induction heating device is as follows.
It can be carried out at any position between the entrance of the finish rolling mill and the stand of the finish rolling mill, and a plurality of positions among these (for example, one or more places between the entrance of the finish rolling mill and the stand of the finish rolling mill) ). The reason why the induction heating device is used as a heating device for adjusting the temperature of the rough rolling bar is that good controllability (temperature fluctuation) in the longitudinal direction of the rough rolling bar is obtained by using the induction heating device. This is because temperature control can be performed with the following characteristic.

FIG. 2 shows the material of the present invention (material No.
1, [D]: 15 μm, [Dmax / Dmin]: 1.
50) and a comparative material (material No. 2, [D]: 16 μm,
[Dmax / Dmin]: 2.35), the hot-rolling finishing temperature from the coil front end to the rear end and mechanical properties in the coil after cold rolling and final annealing (coil width direction and longitudinal direction) This shows the relationship with the change in properties. According to this, [Dmax / Dmin] exceeds 2.0 in the comparative material in which the hot-rolling finishing temperature from the leading end to the trailing end of the coil is from Ar ₃ point −10 ° C. to Ar ₃ point + 35 ° C. Therefore, cold rolling in the coil (coil width direction and longitudinal direction)
The mean-r value and elongation at break after the final annealing fluctuate greatly. On the other hand, [Dmax / Dmin] is suppressed to 2.0 or less in the present invention material in which the hot-rolling finishing temperature from the leading end to the trailing end of the coil is from Ar ₃ point to Ar ₃ point + 22 ° C. Therefore, fluctuations in the mean-r value and the elongation at break after cold rolling and final annealing in the coil (in the coil width direction and the longitudinal direction) are appropriately suppressed.

In the manufacturing method of the present invention, in addition to the control of the finishing temperature (rolling temperature at the final stand of the finishing mill), the rolling temperature at each stand upstream of the final stand of the finishing mill is also controlled. By rolling so that the temperature from the leading end portion to the trailing end portion of the rough rolling bar becomes Ar ₃ point to Ar ₃ point + 30 ° C.,
It is possible to manufacture a steel sheet having more excellent deep drawability and having less variation in mechanical properties in the coil (in the longitudinal direction and the width direction of the coil). Such temperature adjustment of the rough rolling bar is also performed by heating a part or all of the rough rolling bar in the longitudinal direction by the induction heating device.

In the production method of the present invention, the subsequent cooling in the runout is started within 1.5 seconds after the finish rolling, and after cooling from the finishing temperature to 750 ° C. at an average cooling rate of 20 ° C./sec or more. At a winding temperature of 700 ° C. or lower. By starting the cooling in the run-out within 1.5 seconds after the finish rolling, the grain growth of the austenite crystal grains before the transformation after the finish rolling can be suppressed, and the present invention contemplates [D] and [D]. Dmax / Dm
in]. If the time from the end of the finish rolling to the start of cooling exceeds 1.5 seconds, the above-described grain growth of the crystal grains cannot be suppressed, and [D] and [Dmax / Dmin] intended in the present invention cannot be obtained.

By setting the average cooling rate from the finishing temperature to 750 ° C. at 20 ° C./sec or more, the frequency of ferrite nucleation during austenite-ferrite transformation increases, and the ferrite grain size is intended in the present invention. [D]
And [Dmax / Dmin]. If the average cooling rate is less than 20 ° C./sec, the nucleation frequency of the ferrite described above is low, and [D] and [Dmax / Dmin] intended in the present invention cannot be obtained.

By setting the winding temperature after hot rolling to 700 ° C. or less, [D] and [Dmax / Dmi
In the steel strip in which [n] is controlled within the range intended by the present invention, coarsening due to ferrite grain growth can be suppressed. When the winding temperature exceeds 700 ° C., [D] and [Dmax / Dm
in] exceeds the range intended by the present invention.

Further, in order to obtain a target steel sheet more rationally in the production method of the present invention, the rolling speed of the rough rolling bar to be finish-rolled is adjusted by setting the leading end of the rough rolling bar to the finishing mill. Preferably, the rolling speed is controlled to one of the following (A) to (C). (A) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finishing rolling mill, and then rolling is performed at a constant speed. (B) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finish rolling mill, and thereafter, rolling is performed at a constant speed, and then the rolling speed is accelerated. (C) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finishing mill, and thereafter the rolling speed is further accelerated.

By performing the finish rolling accompanied by such accelerated rolling, the temperature drop of the material can be suppressed as much as possible.
For this reason, the amount of heating of the rough rolling bar by the induction heating device provided on the entrance side of the finishing mill and / or between the stands of the finishing mill can be minimized, and energy for heating can be saved. In the rolling patterns (A) to (C), the degree of accelerated rolling in the finish rolling mill is (C)>(B)> (A). ,
A higher effect is obtained with (C) than with (B).

Further, in the manufacturing method of the present invention, a step of heating the widthwise edge portion of the rough rolling bar by the induction heating device between the entrance of the finishing rolling mill and / or between the stands of the finishing rolling mill may be added. preferable. By such heating of the edge portion, the variation in the temperature in the width direction of the rough rolling bar is reduced, and [Dmax / Dmin] can be set to a smaller value. In addition, heating of the rough rolling bar edge portion by the induction heating device can be performed at any position between the entrance of the finishing rolling mill and the stand of the finishing rolling mill, and a plurality of positions (for example, finishing) (One or more places between the entrance of the rolling mill and the stand of the finishing mill).

In the production method of the present invention, the hot-rolled steel strip obtained by the hot rolling as described above is subjected to pickling, cold rolling, final annealing and temper rolling in order to improve press formability. It is possible to obtain a cold-rolled steel sheet which is excellent and has little change in press formability in the coil. Cold rolling is carried out in order to reduce the thickness of the hot-rolled steel strip to a predetermined thickness, develop a rolling texture, and develop a favorable texture for improving workability in the subsequent final annealing (recrystallization annealing) step. You. For this purpose, it is preferable to perform cold rolling to a final sheet thickness at a reduction of 50% or more, preferably 76% or more.

The final annealing (recrystallization annealing) is 550 to 550.
The annealing is preferably performed in a temperature range of 900 ° C., and the ferrite is recrystallized in the final annealing. Annealing temperature is 5
When the temperature is lower than 50 ° C., recrystallization does not sufficiently occur even in a long-time box annealing. On the other hand, at an annealing temperature exceeding 900 ° C., austenitization proceeds even in continuous annealing, and workability deteriorates. As a method of performing the final annealing, any of continuous annealing, box annealing, and continuous heat treatment preceding the hot-dip galvanizing treatment may be used. Temper rolling is performed mainly to improve the shape of the cold-rolled steel strip.

The raw steel used in the production method of the present invention is produced by, for example, a converter or an electric furnace. The production of slabs may be any of ingot-bulking rolling, continuous casting, thin slab casting, and strip casting. For slabs obtained by continuous casting, a method of hot rolling immediately with continuous casting (a so-called direct-feed rolling method), a method of holding or heating continuously cast slabs without cooling them to room temperature, How to roll,
Any method of cooling the continuously cast slab to room temperature, then reheating and hot rolling may be employed.

In the production method of the present invention, a slab obtained by continuous casting or ingot-bulking rolling is used.
In the case of hot rolling by heating, the slab that has been continuously cast or ingot-bulk rolled is cooled to an arbitrary temperature equal to or higher than room temperature, and then charged into a hot-rolling heating furnace to reduce the slab to a predetermined temperature. Temperature, in which case the slab charging temperature to the hot rolling furnace is Ar
It is preferable that the number is ₃ or less in controlling the hot-rolled structure.

In the manufacturing method of the present invention, the shape may be corrected by a straightening device such as a leveler as a process before or after the process of heating the rough rolling bar by the induction heating device. If this shape correction is performed as a step before the step of heating the rough rolling bar with an induction heating device, the rough rolling bar can be uniformly heated by the induction heating device because the shape of the rough rolling bar becomes good, As a result, the uniformity of the structure in the rough rolling bar increases. In addition, since the shape of the rough rolling bar inserted into the finishing mill is improved, the uniformity during plastic deformation by the finish rolling is improved, and as a result, the structure of the obtained steel sheet is also uniform. In addition, when the above-described shape correction is performed as a step after the step of heating the rough rolling bar with the induction heating device, the shape of the rough rolling bar inserted into the finishing rolling machine becomes good, The uniformity during plastic deformation is increased, and as a result, the structure of the steel sheet becomes uniform.

The cold-rolled steel sheet obtained by the production method of the present invention may be subjected to a surface treatment (hot-dip galvanizing, galvannealing, electroplating, organic coating, etc.) as necessary.
After being subjected to press processing, for example, automobiles, home appliances (frame materials for TV, various container materials, etc.), industrial equipment,
It is used for various applications including steel structures, enameled products, etc., and has both high workability and strength required for these applications.

[0044]

EXAMPLES Steel having the chemical composition shown in Table 1 (material no.
1 to 11) are melted and cast into slabs by continuous casting.
This slab was hot-rolled under the conditions shown in Table 2 to form a hot-rolled coil, and the average crystal grain size [D] of the hot-rolled coil and the variation of the average crystal grain size in the coil width direction [Dmax / Dmin]. Was measured. After pickling the above hot rolled coil, the rolling reduction was 75%.
%, Then recrystallization annealing (800 ° C. × 40
Second) and temper rolling were sequentially performed to obtain a product steel strip, and the mechanical properties of the obtained steel strip coil were examined (refer to the footnote in Table 3 for the conditions for collecting the sample). Table 2 shows the hot rolling conditions of each material and the measured values of [D] and [Dmax / Dmin], and Table 3 shows the measured mechanical properties of each product steel strip.

According to Table 3, in the material of the present invention, the mean-r value and the level of elongation at break are high, and the fluctuation of mechanical properties in the coil is small. It can be seen that the press formability is also uniform.
In addition, it can be seen that by performing the accelerated rolling in the finish rolling, the finishing temperature can be controlled within the range of the present invention without excessively increasing the heating temperature of the entire rough rolling bar. Also,
The above-mentioned effects of the accelerated rolling are obtained by the above-described patterns (A) to
It can be seen that (C)>(B)> (A) in (C). Further, the material No. in which the edge portion of the rough rolling bar was heated.
1, No. 3, No. 5, no. No. 8 has a particularly small value of [Dmax / Dmin] among the examples of the present invention, and therefore has a particularly small variation in the characteristic value (mean-r value, elongation at break) in the coil width direction and is an excellent material. I understand. Also,
Material No. as a comparative example. It can also be seen that when the entire rough rolling bar is not heated as in 2, even if accelerated rolling is performed, finish rolling cannot be controlled within the scope of the present invention.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

As described above, according to the manufacturing method of the present invention, excellent press formability suitable for use as an outer panel of an automobile and the like, and variation in press formability in a coil is small. Cold rolled steel sheets can be manufactured stably.

[Brief description of the drawings]

FIG. 1 shows the material of the present invention ([Dmax / Dm
in]: 1.50) and a comparative material ([Dmax / Dmi
n]: 2.35) is a graph showing mean-r value and elongation at break in the coil width direction after cold rolling and final annealing.

FIG. 2 shows the material of the present invention ([D]: 15 μ) shown in the examples.
m, [Dmax / Dmin]: 1.50) and a comparative material ([D]: 16 μm, [Dmax / Dmin]: 2.3)
Regarding 5), a graph showing the relationship between the finishing temperature from the leading end to the trailing end of the hot-rolled coil and the mechanical properties in the coil after cold rolling and final annealing.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoichi Honashiki 1-2-2 Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4K037 EA01 EA02 EA04 EA05 EA15 EA16 EA19 EA22 EA23 EA25 EA27 EA31 EA32 EA35 EB02 EB03 EB09 FB04 FB06 FC07 FD03 FD04 FE01 FE02 FE03

Claims (4)

[Claims] 1. C: 0.02 wt% or less, Si: 0.6
wt% or less, Mn: 2.5 wt% or less, P: 0.10 w
t% or less, S: 0.05 wt% or less, O: 0.004 w
t% or less, Sol. Al: 0.01 to 0.10 wt%, and one or more selected from Ti, Nb, V, and Zr in total of 0.01 to 0.40 wt%
The slabs containing steel are roughly rolled by a rough rolling mill in a hot rolling step, and when this rough rolling bar is subsequently rolled by a continuous hot finishing rolling mill, the entry side of the finishing rolling mill and / or finishing is performed. By heating some or all of the rough rolling bar in the longitudinal direction with an induction heating device provided between the stands of the rolling mill, the finishing temperature is Ar ₃ points from the leading end to the trailing end of the hot rolled coil. Ar ₃ points + 30 ° C
Rolling so as to be in the range, and cooling with the subsequent run-out is started within 1.5 seconds after the finish rolling, and after cooling from the finishing temperature to 750 ° C at an average cooling rate of 20 ° C / sec or more, Winding is performed at a winding temperature of 700 ° C. or less, and the resulting hot-rolled steel strip is sequentially subjected to pickling, cold rolling, final annealing, and temper rolling, and is excellent in press formability and is pressed in a coil. A method for producing a cold-rolled steel sheet with less change in formability. 2. The slab is B: 0.0001 to 0.005.
The method for producing a cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet is excellent in press formability and has little change in press formability in a coil. 3. The induction heating device according to claim 1, wherein an edge of the rough rolling bar in the width direction is heated by an induction heating device between an entrance of the finishing mill and / or a stand of the finishing mill. A method for producing a cold-rolled steel sheet which has excellent press formability and has little variation in press formability in a coil. 4. The press formability according to claim 1, wherein the rolling speed of the rough rolling bar to be finish-rolled is controlled to one of the following (A) to (C). A method for producing a cold-rolled steel sheet which is excellent and has little variation in press formability in a coil. (A) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finishing rolling mill, and then rolling is performed at a constant speed. (B) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finish rolling mill, and thereafter, rolling is performed at a constant speed, and then the rolling speed is further accelerated. (C) The rolling speed is accelerated after the leading end of the rough rolling bar enters the finishing mill, and thereafter the rolling speed is further accelerated.