JPH10121145A - Production of hot-rolled steel sheet for working excellent in deep drawability by using continuous hot-rolling process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformize the quality of a hot-rolled steel sheet over the whole length, to avoid the defective quality at the front end part, to improve the cost merit by saving energy in a heating furnace and to produce the hot-rolled steel sheet for working excellent in the deep drawability. SOLUTION: A steel slab having <=0.1% carbon content is heated to <=1150 deg.C extraction temp. in the heating furnace and the roughly rolled steel sheet is coiled at >=0.5% bending strain and >=0.05s<-1> bending strain speed and held at <=1 deg.C/s cooling speed for >=3sec. Thereafter, uncoiling is executed and successively, the front end part of this steel sheet is joined with the rear end part of the steel sheet roughly rolled theretofore and preceding the rolling line, and the hot finish-rolling is continuously executed in the temp. range of the αzone at <=750 deg.C and after completing the rolling, the recoiling is executed at 600-750 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 hot-rolled steel sheet by continuous hot-rolling, and particularly to a method for continuously hot-rolling a hot-rolled steel sheet excellent in deep drawability used for automobiles and industrial machines. The present invention relates to a method for manufacturing by rolling.

[0002]

2. Description of the Related Art Conventionally, in the technical field of steel sheets for processing of automobiles and the like, cold-rolled steel sheets having good workability have been used. Is being used.

[0003] In a method for producing a hot-rolled steel sheet for processing used in automobiles, industrial machines, and the like, a continuously cast steel slab is heated in a heating furnace, then subjected to rough rolling and finish rolling, and then cooled and wound into a coil. It is common.

FIG. 1 is a diagram showing a distribution of a finishing temperature from a front end to a rear end of a hot-rolled steel sheet. The finishing temperature at the front end of the steel sheet to be hot-rolled is the lowest, and the temperature increases toward the rear end. The reason why the temperature at the rear end is high is due to the heat generated during processing due to the increase in the rolling speed.

[0005] As described above, since the finishing temperature of the steel sheet is not uniform over the entire length of the steel sheet, the material of the steel sheet is different at the front end portion, the middle portion, and the rear end portion, which is problematic. In particular, in the case of the leading end, if the temperature is extremely low, the material is deteriorated, and the yield may be reduced, for example, it may be cut off before shipment.

[0006] In order to prevent such deterioration of the material, it is essential to secure the temperature at the tip end of the steel plate, and for that purpose, it is necessary to increase the heating temperature of the heating furnace. From the viewpoint of energy saving cost balance, it is apparent that excessive heating is performed at the temperature other than at the tip of the steel sheet, and there is a problem that the cost balance is poor.

[0007] A method for producing a hot-rolled steel sheet for processing in which heat consumption is extremely reduced to reduce the unit consumption of a heating furnace is disclosed in
No. 4-11695. In this method, the temperature of the hot slab obtained by continuous casting is reduced to 5 to 30% at a temperature of Ar _{3 to} 1200 ° C. without lowering the temperature to the Ar ₃ transformation point or lower.
The hot rolling is performed after inserting and heating in a heating furnace maintained at 950 to 1150 ° C.
In this method, even though the hot rolling conditions for the temperature of the intermediate portion of the hot rolled steel sheet are appropriate, no consideration has been given to solving the above-described problem of the decrease in the temperature of the front end portion, and heating is not performed. Even if the unit energy consumption is reduced,
It is technically difficult to finish roll the entire steel sheet without deteriorating workability and obtain a homogeneous steel sheet.

On the other hand, hot-rolled steel sheets for processing are required to have deep drawability with a high r-value for their applications.
A method for manufacturing a hot-rolled steel sheet having excellent deep drawability is disclosed in Japanese Patent Application Laid-Open No. Hei 3-13528. Although this method discloses performing hot finish rolling in a low temperature range of the α region, in order to ensure the workability of the steel sheet, a device such as controlling a reduction ratio of rough rolling and finish rolling is used. Is required, or a method of performing lubricating rolling with an increase in cost to obtain a high r value is described. However, since these methods are not continuous hot rolling, the material cannot be made constant over the entire length of the steel sheet, and these methods cannot be directly applied to continuous hot rolling.

On the other hand, in the conventional manufacturing method in which the heating temperature cannot be lowered in order to prevent the deterioration of the material at the tip of the steel plate, precipitation of precipitates such as MnS and AlN is not sufficient, and there is a problem from the viewpoint of ductility and aging. there were. As a technique for promoting the precipitation of MnS and AlN, for example, Japanese Patent Publication No. 7-74376 discloses that a steel plate after rough rolling is subjected to bending at 1100 ° C. or less at _three points of Ar and kept in the above temperature range for 10 seconds or more. A technique for performing this is disclosed.

On the other hand, the inventor of the present invention further promotes precipitation by adding conditions of low-temperature heating and bending strain and bending strain rate, and controlling the steel sheet by controlling the combination of the finish rolling in the α region and the winding temperature condition. Has been found to improve the deep drawability. Further, as described above, the low-temperature heating is a condition that can be achieved only by joining the steel sheets after the rough rolling and continuously rolling them. This continuous rolling method is particularly effective when the thickness of the steel sheet is as thin as 2 mm or less.

[0011]

SUMMARY OF THE INVENTION Accordingly, the present invention is to provide a hot rolled steel sheet having a uniform material over its entire length, avoiding a defective material at the tip end, and improving the cost merit by saving energy in a heating furnace. Another object of the present invention is to produce a hot-rolled steel sheet for processing excellent in deep drawability.

[0012]

The solution of the present invention is as follows.

(1) An extraction temperature of a steel slab having a carbon content of 0.1% or less in a heating furnace is set to 1150 ° C. or less, and a roughly-rolled steel sheet has a bending strain of 0.5% or more and a bending strain rate of 0.1%. 05
Winding at s ^-1 or more, holding at a cooling rate of 1 ° C./s or less for 3 seconds to 1 ° C./s, and then rewinding. Joining, performing hot finish rolling continuously in a temperature range of α range of 750 ° C. or less, and winding at a temperature of 600 to 750 ° C. after the end of rolling, which is excellent in deep drawability. Manufacturing method of hot rolled steel sheet for processing.

Hereinafter, the present invention will be described in detail.

The hot-rolled steel sheet for processing excellent in deep drawability produced by the present invention can be used for 280 to 280 steels used in automobiles and industrial machines.
It is intended for 380 MPa class soft hot rolled steel sheets, and the specific ranges of the chemical components of these steel sheets are as follows. That is, C: 0.1% or less, Mn: 0.
1 to 1.0%, Si: 0.1% or less, P: 0.025%
Hereinafter, S: 0.025% or less, Al: 0.1% or less, and the balance is Fe excluding unavoidable impurities. In some cases, Ti or Nb alone or both of them may be added in an amount of 0.1% or less.

[0016] C contained in the hot-rolled steel sheet for processing is hardened when the C content is increased, and the workability is deteriorated.
% Or less.

Mn is an element necessary for imparting toughness, but when it is increased, it hardens and deteriorates workability. Si and Al are added as deoxidizing agents, but when they are increased, the workability is deteriorated. P and S are inevitably contained, but if they are increased, the workability is adversely affected. Ti and Nb combine with C and N to produce a softening effect, but when they increase, workability deteriorates.

For these reasons, the chemical components are adjusted to the above range. However, the present invention does not exclude inclusion of other components according to the purpose.

FIG. 2 is a diagram schematically showing the relationship between the heating temperature and the finishing temperature of a hot-rolled steel sheet. The temperature drop from the heating furnace to the end of finish rolling is different between the steel sheet middle part and the tip part, and there is a remarkable temperature drop particularly at the tip part of the steel sheet.

However, in the present invention, since the leading end of the rough-rolled hot-rolled steel sheet is joined to the rear end of the preceding steel sheet by rough-rolling before it, continuous hot rolling can be performed. Becomes possible, and the hot rolling can be performed at high speed and constant speed rolling, so that the rolling conditions are the same over the entire length of the steel sheet, and unlike the conventional batch type hot rolling accelerated rolling, There is no variation in the finishing temperature. In addition, since high-speed constant-speed rolling is possible, a decrease in temperature during finish rolling is small and the heating temperature of the heating furnace can be lowered. That is, in the method of the present invention, the temperature pattern is as shown by the marks ● and ▲ in FIG. In particular, when producing a thin steel plate having a thickness of 2 mm or less, the effect of the present method is very remarkable.

For this reason, in the present invention, the heating temperature can be reduced as compared with the prior art, and MnS
Since the precipitation of AlN or the like can be promoted, the workability can be improved. According to the experiment, the effect was large in the region where the heating temperature was 1150 ° C. or lower.

In order to improve the workability, in addition to the above-mentioned low-temperature heating, a bending strain of 0.5% or more after the rough rolling is completed.
It has been found that it is only necessary to apply a bending strain rate of not less than 05 s ^{-1 and} to hold the cooling rate of not more than 1 ° C./s for not less than 3 seconds.
It is considered that the precipitates such as MnS and Al precipitated during the first low-temperature heating act as nuclei of the precipitates precipitated by the subsequent bending strain, and further promote the precipitation. The effect is larger as the bending strain is larger, and 0.5% or more is required. Further, in order to apply bending strain at a high temperature, a bending strain rate of 0.05 s ^-1 or more is required to reduce a loss due to thermal extinction. Further, in order for the precipitate to further precipitate and grow, it is necessary to maintain the cooling rate of 1 ° C./s or less for 3 seconds or more.

In the present invention, the finish rolling in the temperature range of the α region of 750 ° C. or less is performed by setting the temperature range of the α region of 750 ° C. or less to recrystallize a texture in which preferred orientations are accumulated in deep drawing. This is because the distortion can be imparted. That is, in order to increase the r value, that is, to cause recrystallization in a texture in which preferred orientations are accumulated in deep drawing,
It is necessary to apply a strain which is a driving force for performing the recrystallization, and the strain can be achieved by finish rolling in a temperature range of α range of 750 ° C. or less.

It is preferable that the finish rolling temperature at 750 ° C. or lower is lower, but the lower limit is determined by the capacity of the rolling mill. Usually, finish rolling is performed at about 700 ° C.

Also, at 600 to 750 ° C., preferably at 65
The winding at 0 to 750 ° C. is for raising the temperature to a temperature higher than the recrystallization temperature to recrystallize into a texture in which preferred orientations are accumulated in deep drawing during winding. That is, 600
When the temperature is higher than ℃, it is possible to effectively recrystallize a texture in which preferred orientations for deep drawing are accumulated. Therefore, in the present invention, recrystallization is performed in a winding temperature range of 600 to 750 ° C.

Further, in the present invention, since the finish rolling temperature is low, it is not always necessary to cool the steel sheet to the winding temperature after the rolling is completed. However, when the finish rolling temperature is low and the temperature of the steel sheet is equal to or lower than the winding temperature range of 600 to 750 ° C, a heating device is installed between the finish rolling mill outlet side and the winding machine to reduce the temperature of the steel sheet. What is necessary is just to raise a temperature so that it may be set to 600-750 degreeC winding temperature range.

As a specific method for raising the temperature, for example, a heating device for heating the steel plate by directly supplying power to the steel plate from a roll can be provided on the run-out table.

In this manufacturing method, in order to save energy of the heating furnace, inserting a warm piece into the heating furnace (so-called hot charge or warm charge) is an important factor for realizing the effects of the present invention. No obstacles. That is, the present invention is established regardless of whether a cold piece or a hot piece is inserted into the heating furnace.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.

FIG. 3 is a diagram showing an outline of a method for producing a hot-rolled steel sheet for processing excellent in deep drawability in the hot-rolling continuous method.

As shown in FIG.
A steel slab having a carbon content of 0.1% or less heated below is rolled by a rough rolling mill 2, and is rolled at a bending strain of 0.5% or more and a bending strain rate of 0.05 s ^-1 or more. The rolled coil 3 is used. The coil box method (Stahl / u / Eisen, 198) is used for winding and rewinding the steel sheet after the rough rolling.
3, vol. 103, no. 7, p. 295-305 and Iron and Steel Engineer,
1981, No. 11, p. 452) can be used. This method has a heat retaining effect since the steel sheet is bent and wound into a coil at the same time. Coarse rolled coil 3 is 1 ° C / s for 3 seconds or more
Hold at the following cooling rate and then rewind. The leading end of the unwound coarsely-rolled coil 3 is cut by a welding cutting machine 4 to form a leading edge groove suitable for welding. The preceding rough-rolled steel sheet is conveyed to a finish rolling mill and finish-rolled. The trailing end is similarly cut by the welding cutting machine 4 to form a trailing edge suitable for welding. The rear end of the preceding rough rolled steel sheet and the front end of the subsequent rough rolled steel sheet are welded and joined by the welding device 5.

The welding device 5 is integrated with the moving trolley and is controlled so as to be able to move in synchronization with the moving speed of the trailing end of the preceding rough rolled steel sheet. The rear end of the preceding rough rolled steel sheet is welded to the front end of the subsequent rough rolled steel sheet. As a welding method, a laser beam welding method is suitable, but other known welding methods can also be applied.

The rough-rolled steel sheet integrally joined by the welding device 5 is finish-rolled continuously in a finish rolling mill 6 in a temperature range of α range of 750 ° C. or less, and then 600 to 750 ° C. which is a recrystallization temperature. , Preferably at a winding temperature of 650 to 750 ° C as a coil by the winder 9. When a predetermined length of the steel sheet is wound, the steel sheet is cut by the cutting machine 8 and wound as another coil by the winding machine 9.

In the present invention, the front end of the rough-rolled steel sheet is joined to the rear end of the preceding rough-rolled steel sheet in the rolling line, and there is no temperature variation throughout the finish-rolled steel sheet under constant conditions. Can be reduced to 1150 ° C. or lower, which is lower than before. Moreover, ductility can be improved by low-temperature heating and appropriate bending strain imparting and holding after rolling.
The deep drawability can be improved by controlling the finish rolling in the temperature range of the α range of not more than ° C. and the winding temperature after rolling.

[0035]

EXAMPLES Examples of the present invention and comparative examples will be described below.

Examples of the present invention shown in Table 1 (Nos.
All 4) show the results that the ductility is high over the entire length of the steel sheet and the average r value is high (average r ≧ 1.0).

On the other hand, in Comparative Example No. In No. 5, since the steel sheet (sheet bar) after the rough rolling was not joined, the rolling speed at the front part of the steel sheet was low, and the low temperature extraction (10
(80 ° C.), the finishing temperature decreases and the ductility of the front part deteriorates. In addition, No. No. 6 is an example of the same component No. 6 because the extraction temperature is high. Compared with No. 1, the ductility is reduced. No. No. 7 has a small amount of bending strain. No. 8 has a low bending strain rate. 9 has a short retention time,
No. In No. 10, the ductility was not sufficiently improved because the cooling rate during holding was high. No. No. 11 is because the finishing temperature is higher than the transformation point. 12 and No. In No. 13, the average r value and ductility are deteriorated because the temperature is outside the winding temperature. As can be seen from the above, no. 5-No. No. 13 has a portion lacking the specific requirements of the present invention as compared with the present invention, and thus two ductility and an average r value cannot be secured.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

According to the method of the present invention for producing a hot-rolled steel sheet for processing excellent in deep drawability using the hot-rolling continuation method, the product yield due to defective material at the tip of the hot-rolled steel sheet is improved. In addition, since the heating and extracting temperature of the steel slab can be reduced, not only can the cost merit due to the energy saving of the heating furnace be enjoyed, but also the improvement in the deep drawability of the steel sheet can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a finishing temperature distribution of a hot-rolled steel sheet.

FIG. 2 is a diagram schematically showing a relationship between a heating temperature and a finishing temperature of a hot-rolled steel sheet.

FIG. 3 is a diagram showing an outline of a method for manufacturing a hot-rolled steel sheet for processing excellent in deep drawability using the hot-rolling continuous process of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Rough rolling machine 3 Rough rolling coil 4 Welding cutting machine 5 Welding device 6 Finish rolling machine 7 Cooling device 8 Cutting machine 9 Winding machine

Claims (1)

[Claims] 1. A steel slab having a carbon content of 0.1% or less,
The extraction temperature in the heating furnace was set to 1150 ° C. or less, and the roughly rolled steel sheet was subjected to a bending strain of 0.5% or more and a bending strain rate of 0.05 s ^−1.
Winding is performed at a cooling rate of not less than 3 seconds and not more than 1 ° C./s, and then rewinding. Then, the front end of the steel sheet is joined to the rear end of the preceding steel sheet which has been subjected to rough rolling before the rolling line. Characterized in that hot finish rolling is continuously performed in a temperature range of α range of 750 ° C. or less, and after rolling is completed, winding is performed at a temperature of 600 to 750 ° C .; Manufacturing method of rolled steel sheet.