JPS61204337A - Manufacture of steel sheet for working having superior ridging resistance and bulgeability - Google Patents

Abstract

PURPOSE:To obtain the titled steel sheet having satisfactory workability in shortened stages including no cold rolling stage by specifying rolling conditions including the relation between the rate of strain and rolling temp. in a stage for rolling a low carbon steel to a prescribed thickness and by subjecting the resulting steel strip to recrystallization annealing. CONSTITUTION:In a stage for rolling a low carbon steel to a prescribed thickness, rolling is finished at 300-800 deg.C and >=300 s<-1> rate of strain (epsilon) under conditions which satisfy the formula [where T is the rolling temp. ( deg.C)] in at least one pass. The resulting steel strip is subjected to recrystallization annealing. Thus, a steel sheet having superior ridging resistance as well as a high n-value (work hardening index) and a high r-value is obtd. by warm rolling at such a high rate of strain without carrying out conventional cold rolling.

Description

[Detailed Description of the Invention] (Industrial Application Field) The description in this specification regarding the production of thin steel sheets with excellent ridging resistance and stretch formability does not include the cold rolling process due to the regulation of rolling conditions. It is related to the evolving results of development research based on experimental knowledge that process saving is possible.

W1 steel sheets with a thickness of approximately 211III+ or less, which are used for applications such as building materials, automobile body materials, can stock, and various surface-treated original sheets, have good mechanical properties such as bending workability, stretch formability, and drawing workability. In order to obtain high ductility and high Lankford value (r value), high ductility and high Lankford value (r value) are required.

In recent years, an increasing number of parts have been subjected to stretch forming as a forming method in order to improve the walking performance of steel plates during processing and forming. This is because stretch molding can reduce the flow of material from the wrinkle suppressing portion during processing.

This application requires a particularly high n value (0,23
above) (work hardening index) is required.

Furthermore, since these materials are mainly used on the outermost side of the final processed product, the surface condition after processing has become particularly important.

The general manufacturing procedure for these thin steel sheets for processing is as follows.

First, low-carbon steel is mainly used as the steel cable material, and steel slabs with a thickness of about 200 mm are made by continuous casting or ingot-blowing and rolling, and then by hot rolling process to a plate thickness of about 3 mm.
0 hot-rolled steel strip, followed by pickling and cold rolling to a predetermined thickness, followed by recrystallization treatment by box annealing or continuous annealing to produce the final product.

The biggest drawback of this practice is that it is a long process; not only does it take a lot of energy, manpower, and time to produce a product, but during this long process, there are some problems with the quality of the product, especially its surface properties. There is also the added disadvantage of causing

As mentioned above, it has been essential to include a cold rolling process (rolling temperature less than 300° C.) in the manufacturing procedure of thin steel sheets for processing.

This cold rolling process not only aims to reduce the desired thickness, but also utilizes the plastic strain introduced by cold working to produce crystals with (111) orientation, which is advantageous for deep drawability, in the final annealing process. Helps promote grain growth.

However, in cold working, the deformation resistance of the steel strip is significantly higher than in hot working, so the energy required for rolling is also significant, the rolling rolls are severely worn, and rolling problems such as slipping occur. Easy to occur.

On the other hand, if rolling can be done in a relatively high temperature range of 300°C or higher and 800°C or lower (so-called warm range), and particularly good workability can be obtained, the above problems can be eliminated and there are great manufacturing advantages. You could say that.

However, there are major problems with manufacturing by warm rolling. That is Rising. Rigging is a defect in surface irregularities that occurs during processing of products, and is a fatal defect for this type of steel plate, which is mainly used on the outermost side of processed products.

In terms of metallurgy, ridging is caused by crystallographically oriented grain groups (e.g. (100) oriented grains) that are not easily divided even after undergoing the annealing recrystallization process and remain stretched in the rolling direction. , generally like warm rolling, ferrite (
This tends to occur when processing is carried out at relatively high temperatures in the α) region, and is particularly noticeable when the reduction rate in the warm region is high (i.e., when manufacturing thin steel sheets).

Recently, as processed products have become more complex and sophisticated, these processed steel plates are often subjected to severe processing, and excellent ridging resistance is required.

By the way, the manufacturing process of steel materials has changed significantly in recent years, and the case of processed FiIEfa plates is no exception.

That is, the molten steel is made into a steel billet with a thickness of about 250 mm by ingot-making and blooming rolling, then heated and soaked in a heating furnace, processed into a sheet bar with a thickness of about 30 mm by a rough hot rolling process, and then processed into a sheet bar with a thickness of about 30 mm by a finishing hot rolling process. In contrast to the conventional practice of hot-rolled steel strips, the introduction of continuous casting processes in recent years has made it possible to omit the blooming process, and in order to improve material quality and save energy, the heating temperature of steel slabs has been reduced from the conventional 1200 mm. There is a tendency for the temperature to decrease from around 1100°C to around 1100°C or lower.

On the other hand, a new process is being put into practical use that allows the heat treatment and rough rolling process of hot rolling to be omitted by immediately producing a steel strip with a thickness of 50 mm or less from molten steel.

However, all of these new manufacturing processes are disadvantageous in that they destroy the cast structure formed by solidifying molten steel. Especially formed during solidification (100)
It is extremely difficult to destroy the strong casting texture whose main orientation is <u v w).

As a result, the final thin steel sheet tends to suffer from gluing, and the warm rolling process particularly promotes this.

(Prior Art) Several methods for producing deep drawing steel sheets by warm rolling have been disclosed, for example, Japanese Patent Publication No. 47-30809 and Japanese Patent Application Laid-Open No. 49-1989.
-86214, JP-A No. 59-93835, JP-A-Sho 5
9-133325, JP 59-136425, JP 59-185729, and JP 59-226
No. 149 and No. 8 is an example. Both methods are characterized by recrystallization treatment immediately after rolling in the warm region, and are innovative technologies that can omit the cold rolling step.

However, these known techniques do not give any consideration to improving the above-mentioned ridging resistance.
In this respect, warm rolling is generally more disadvantageous than cold rolling when it comes to the ridging resistance of thin steel sheets.

(Problems to be Solved by the Invention) It is an object of the present invention to provide a method for manufacturing a thin steel sheet with excellent ridging resistance and stretch formability by a process saving process that does not include a cold rolling process.

(Means for Solving the Problems) This invention provides at least one pass in the process of warm rolling low carbon steel to a predetermined thickness at a strain rate (;) of 300 s - in a temperature range of 800 to 300°C. This is a method for producing a thin steel sheet for processing which has excellent ridging resistance and stretch formability, which is characterized by finishing with the above conditions and i≧0.8T+60, followed by recrystallization annealing.

First, the research results that formed the basis of this invention will be explained.

The test materials were two types of hot-rolled low carbon aluminum killed steel sheets shown in Table 1. The test materials are both (A) and (B) 600
It was heated and soaked at a temperature of 0.degree. C. and rolled for one pass at a rolling reduction of 30%.

The strain rate (j) at this time and after annealing (soaked water temperature 800
FIG. 1 shows the relationship between the r value (°C) and the ridging index.

r value and ridging resistance strongly depend on strain rate60
By setting the strain rate to a high strain rate of 300 s-' or more at a rolling temperature of 0°C, the r value and ridging resistance were significantly improved.

FIG. 2 shows the relationship between strain rate and rolling temperature on the work hardening index n value after adding 1.0% skin pass after annealing using fiB shown in Table 1.

i≧0. In the region of BT + 60, n≧0.230, and a steel plate with excellent stretch formability is obtained.

As a result of repeated research based on this basic data, the inventors confirmed that a thin steel plate with excellent stretch formability and ridging resistance can be manufactured by regulating the manufacturing conditions as described below.

(1) Steel composition The effects of high strain rate warm rolling essentially do not depend on the steel composition. However, in order to ensure workability above a certain level, the interstitial solid solution elements C and N should each be 0.10%.
It is preferably 0.01% or less. Also, 0 in steel is A
Reduction by adding 1 is advantageous for improving material quality, especially ductility.

A special element that can precipitate and fix C and N as stable carbonitrides in order to obtain even better workability.

For example, addition of Ti, Nb, Zr, B, etc. is also effective.

Further, in order to obtain high strength, P, Si, Mn, etc. can be added depending on the strength.

(2) Manufacturing method of rolled material Steel slabs obtained by conventional methods, ie, ingot-blowing rolling or continuous casting methods, can naturally be applied.

The heating temperature of the steel piece is suitably 800 to 1250°C, and preferably less than 1100°C from the viewpoint of energy saving.

Of course, the so-called CC-DR (continuous casting-direct rolling) method, in which rolling of a steel billet from continuous casting is started without reheating, is also applicable.

On the other hand, the methods of directly casting rolled material of about 5 (hm) or less from molten steel (sheet bar caster method and strip caster method) are also particularly advantageous as methods for manufacturing rolled material because they have large economic effects from the viewpoint of energy saving and process saving. be.

(3) Warm rolling This process is the most important, and in the process of rolling low carbon steel to a predetermined thickness, at least one pass is
It is essential to finish at a strain rate of 300 g -1 or higher in a temperature range of 300°C.

Regarding the rolling temperature, it is difficult to obtain good workability and ridging resistance by controlling the strain rate when rolling at a high temperature of over 800℃, while a temperature lower than 300℃ results in a significant increase in deformation resistance, which is unique to the cold rolling method. Since the same problems as mentioned above are involved, temperatures of 800 to 300℃, especially 700 to
400°C is particularly suitable.

The target material quality cannot be secured unless the strain rate is 300 s −' or higher.

This strain rate range is particularly from 500 to 2500 s.
-' is preferred.

The number of rolling passes and the distribution of the rolling reduction ratio may be arbitrary as long as the above conditions are satisfied.

The arrangement, structure, roll diameter, tension, presence or absence of lubrication of the rolling mill, etc. have no essential influence.

Note that the strain rate (j) is calculated according to the following formula.

n: Roll rotation speed (rpm) r: Reduction rate (%) / 100 R: Roll radius (n) Ho: Plate thickness before rolling (4) Annealing Steel strips that have undergone rolling must be recrystallized and annealed.

The annealing method may be either a box annealing method or a continuous annealing method, but the latter is advantageous from the viewpoint of homogeneity and productivity.

The heating temperature is suitably in the range from the recrystallization temperature to 950°C. For steel plates with a carbon content of 0.01 wt% or more, it is advantageous to perform an overaging treatment after soaking to improve the material quality.

The annealing treatment can also be carried out by holding the wound coil after rolling.

Here, since the rolling temperature is much lower than that of conventional hot rolling, the scale on the belt surface is thin and easily removed. Therefore, descaling can be done mechanically or by controlling the annealing atmosphere, in addition to the conventional removal with acid.

After annealing, temper rolling of 10% or less can be applied to the belt for shape correction, adjustment of surface roughness, etc.

The steel sheet obtained as described above can be used as an original sheet for a surface-treated steel sheet for processing. Surface treatments include zinc plating (including alloys), tin plating, and coating.

(Function) Regarding the behavior of high strain rate warm rolling according to the present invention,
The mechanism that brings about the ridging resistance and stretch formability is not necessarily clear, but it is thought to be closely related to changes in the texture and processing strain of the rolled material.

(Example) Steel slabs having the chemical composition shown in Table 2 were manufactured by a converter continuous casting method and a sheet bar caster method.

In the converter continuous casting method, a sheet bar having a thickness of 20 to 301 m was obtained by heating and soaking at 1100 to 950°C and then rough rolling.

These sheet bars are continuously turned into thin steel strips with a thickness of 1.0 to 0.7 using a finishing mill consisting of 6 rows, and then high strain rate rolling is performed using the last 2 rows of rolling mills. Ta. Table 3 shows rolling conditions and material properties after continuous annealing (soaking temperature 750-810°C). Steel (C) was subjected to overaging treatment at 400° C. for 2 minutes after soaking as continuous annealing conditions. .

The tensile properties were determined using a JIS No. 5 test piece.

The ridging property was evaluated using a JIS No. 5 test piece cut out from the rolling direction and subjected to a tensile strain of 15%, and the surface unevenness was visually evaluated on a scale of 1 (good) to 5 (poor). Evaluation criteria have not been established for this evaluation because the conventional manufacturing method of low-carbon cold-rolled steel sheets made it impossible to notice any shearing.

Therefore, in the present invention, the index evaluation criteria based on the visual method for conventional stainless steels are applied as they are.

An evaluation of 1.2 indicates ridging properties that pose no problem in practical use.

(Effects of the Invention) According to the present invention, Ti1w exhibits high n value and r value in high strain rate warm rolling and has excellent ridging resistance.
Not only can the conventional cold rolling process be omitted by obtaining an I plate, but also the rolling material is compatible with sheet bar caster method, strip caster method, etc., which simplifies the manufacturing process of thin steel sheets for processing. can be realized.

[Brief explanation of drawings]

FIG. 1 is a graph showing the effect of rolling strain rate on the r value and ridging property, and FIG. 2 is a graph showing the effect of rolling temperature and strain rate on the n value. Patent Applicant Kawasaki Steel Corporation Figure 2 Rolling Section 5/i ('C) Procedural Amendment Written March 1, 1985 Director General of the Patent Office Michibu Uga 1, Indication of Case Patent Application No. 1985 43988 No. 2, Title of Invention Method for manufacturing thin steel sheets for processing with excellent ridging resistance and stretch formability 3, Relationship with the amended case Patent applicant (125) Kawasaki Steel Corporation 4, Attorney 5, Amendment The scope of claims in Column 1 of ``Detailed Description of the Invention'' and Lines 5 to 14 of page 1 of the specification are corrected as follows. 2. Claim 1: In the process of rolling low carbon steel to a predetermined thickness, a small (1 bath) is applied at a strain rate (t) of 300 g -' or more in a temperature range of 300 to 800°C. "A method for producing a thin steel sheet for processing which has excellent ridging resistance and stretch formability, the method comprising finishing at a rolling temperature ('C) and subsequent recrystallization annealing." 2. Specification No. 15 Correct "Table 3" on the page as shown in the attached sheet.
Correct S (kg/m'') J and rTS (kg/day2)'' to rYS (kg/m'') J, and set the 1.2nd ro in the n-value tank, 205 J. rO, 205°, ro, 210 J to ro, 210
'Correct each to J)

Claims (1)

[Claims] 1. In the process of rolling low carbon steel to a predetermined thickness, at least one pass is performed at a temperature range of 300 to 800°C at a strain rate (■) of 300 s^-^1 or more, and ■≧ 0.
8T+60 where T is a rolling temperature (°C) A method for producing a thin steel sheet for processing which has excellent ridging resistance and stretch formability, characterized by finishing at a rolling temperature (°C) and subsequent recrystallization annealing.