JPS61204334A - Manufacture of steel sheet for working having superior ridging resistance and suitability to chemical conversion treatment - 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 and coiling temp. in a stage for rolling a low carbon steel to a prescribed thick ness and by subjecting the resulting steel strip to recrystallization annealing. CONSTITUTION:In a stage for rolling a low carbon steel to a prescribed thick ness, rolling is carried out at 800-300 deg.C and >=300 s<-1> rate of strain in at least one pass and the rolling is followed by coiling at <=400 deg.C. The resulting steel strip is subjected to recrystallization annealing. Thus, a steel sheet having supe rior ridging resistance and suitability to chemical conversion treatment as well as high ductility 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) This specification describes the production of thin steel sheets with excellent ridging resistance, chemical conversion treatment properties, and workability, including 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 it is possible to reduce the production process.

Thin steel plates with a thickness of approximately 2 mm or less 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. 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.

Further, steel sheets for automobiles require pre-painting treatment, that is, chemical conversion treatment. Therefore, chemical conversion treatability becomes an important characteristic of steel sheets. If this chemical conversion treatment property is not good, the subsequent baking coating will not be successful.

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

First, low carbon steel is mainly used as the steel material, and approximately 20,011II1
A steel billet with a thickness of 1 plate is made, which is then hot-rolled into a hot-rolled steel strip with a thickness of approximately 3IIIm, then pickled and cold-rolled into a copper strip with a predetermined thickness, and then box-annealed or The final product is recrystallized using a continuous annealing method.

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 copper strip is significantly higher than in hot working, so the energy required for rolling is much more, the rolling rolls are severely worn, and rolling troubles 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. Ridging is a defect in surface irregularities that occurs during the processing of products, and is a fatal defect for this type of steel sheet, 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.

Incidentally, the manufacturing process of steel materials has changed significantly in recent years, and the case of thin steel sheets for processing is no exception.

That is, molten steel is made into a steel billet with a thickness of about 250 nm 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 strip with a thickness of 50 mm or less from molten steel.

However, these new manufacturing processes are disadvantageous in that they destroy the structure formed by solidifying molten steel (cast structure). Especially formed during solidification (100)
It is extremely difficult to destroy a 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 59-135425, 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 an 81M plate that is excellent in ridging resistance, chemical conversion treatment property, and workability 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 of rolling at a temperature range of 800 to 300°C and a strain rate of 3oos-' or more in the process of warm rolling low carbon steel to a predetermined thickness. And 40
This is a method for producing a thin steel sheet for processing, which has excellent ridging resistance and chemical conversion treatment properties, and is characterized by winding the steel sheet at 0° C. or lower and then 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 at this time (■) and after annealing (soaking 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 rate, 6
By setting the strain rate to a high strain rate of 3003-' or more at a rolling temperature of 00°C, the r value and ridging resistance were significantly improved.

” (wt%) Next, the steel with the composition shown in Table 2 was continuously cast and rough hot rolled.
A sheet bar with a thickness of 5.5 mm was used and rolled at a high strain rate (573 s-') in the 6th stand of a finishing mill consisting of 6 rows. The finishing temperature is 652°C, and the plate thickness is 1.2 mm.

This copper strip was wound at various winding temperatures, and its chemical conversion properties after annealing were investigated.

FIG. 2 shows the relationship between winding temperature and chemical conversion treatment properties.

When the winding temperature was 400°C or lower, the chemical conversion treatment properties were significantly improved.

In addition, chemical conversion treatment properties were determined by degreasing the steel plate, washing with water, phosphate treatment, and conducting the pinhole test described below.

Evaluation was made based on the pinhole area ratio.

In addition, the phosphoric acid treatment is made by Nippon Parkerizing BT31.
12, total acidity 14.3, free acidity 0.5 at 55℃
and sprayed for 120 seconds.

In other words, in the pinhole test, a filter paper soaked with a reagent that reacts with iron ions to form a color is brought into close contact with the test surface to detect areas where phosphoric acid crystals remain on the surface of the steel plate, and this is image-analyzed to detect pinholes. It was quantified as an area ratio. The evaluation criteria for chemical conversion treatment is that the pinhole area ratio is 0.5% or less.
o, 5-2% is 2.2-9% is 3.9-15% or more is 5
I asked for it as. 1 and 2 indicate pinhole area ratios that pose no problem in practice.

As a result of repeated research based on these basic data, the inventors confirmed that by regulating the manufacturing conditions as shown below, it is possible to manufacture thin steel sheets with excellent workability, ridging resistance, and chemical conversion treatment properties.

(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 steel middle O ^
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, St, 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 a steel billet is continuously cast and then rolled without being reheated, is also applicable.

On the other hand, the methods of directly casting rolled materials of approximately 50 cranes or less from molten steel (sheet bar caster method and strip caster method) are also particularly advantageous as methods for manufacturing rolled materials because they have great economic effects from the viewpoint of energy saving and process saving. .

(3) Warm rolling This process is the most important, and in the process of warm rolling low carbon steel to a predetermined thickness, at least one pass is
It is essential to roll at a strain rate of 300 s-4 or higher in a temperature range of 0 to 300°C and to wind at a temperature of 400°C or lower.

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 seconds or higher.

This strain rate range is particularly from 500 to 2500 g-'
is suitable.

The number of rolling buses 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 (λ) is calculated according to the following formula.

n: Roll rotation speed (rpm) r: Roll reduction rate (%) / 100 R: Roll radius (fin) Ho: Thickness of the plate before rolling The coiling temperature after rolling at this high strain rate determines the chemical conversion treatment property. As already mentioned, excellent chemical conversion treatment properties can be obtained by setting this temperature to 400° C. or higher.

(4) Annealing The rolled strip needs to 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.

In any case, the scale on the surface of the copper strip is thin and easily removed because the rolling temperature is much lower than that of conventional hot rolling and occurs at a low temperature. Therefore, descaling can be done mechanically or by controlling the annealing atmosphere, in addition to the conventional removal with acid.

The steel strip after annealing may be subjected to temper rolling of 10% or less in order to correct the shape and adjust the surface roughness.

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 mechanisms that provide ridging resistance and workability, as well as the causes of the behavior that allows excellent chemical conversion treatment properties to be obtained at high strain rates and at a coiling temperature of 400°C or higher after rolling, are not necessarily clear; It is thought that there is a close relationship with changes in the texture and processing strain of the rolled material.

(Example) Steel slabs having the chemical compositions shown in Table 3 were manufactured by a converter-continuous casting method and a converter-seed per carrier method. In the converter continuous casting method, a sheet bar having a thickness of 20 to 3011 mm was obtained by heating and soaking at 1100 to 950° C. and then rough rolling.

Table 3 (wt%) These sheet bars are continuously made into thin steel strips with a thickness of 0.2 to 0.8 by using a finishing mill consisting of 6 rows, and at this time, the rolling mill in the last row is used to Strain rate rolling was performed. Table 4 shows rolling conditions and material properties after continuous annealing (soaking temperature: 750 to 810°C). Regarding steel (D), as continuous annealing conditions, an overaging treatment was performed at 400° C. for 2 minutes after soaking.

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

The ridging property was evaluated using a JIS No. S test piece cut out from the rolling direction and subjected to 15% tensile strain, and the surface unevenness was visually evaluated on a scale of 1 (good) to 5 (poor). No evaluation criteria have been established for this evaluation since ridging did not appear when conventional low-carbon cold-rolled steel sheets were produced. 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, a thin II plate that exhibits high ductility and r value in high strain rate warm rolling, as well as excellent chemical conversion treatability and ridging resistance, can be obtained, and can be obtained in a conventional cold rolling process. Not only can this be omitted, but the rolling material can also be applied to the sheet bar caster method, strip caster method, etc., thereby simplifying the manufacturing process of thin steel sheets for processing.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of rolling strain rate on r value and ridging property. FIG. 2 is a graph showing the influence of winding temperature on chemical conversion treatment properties.

Claims (1)

[Claims] 1. In the step of warm rolling low carbon steel to a predetermined thickness, at least one pass is rolled at a temperature range of 800 to 300°C and at a strain rate (■) of 300 s^-^1 or more. , and 4
A method for producing a thin steel sheet for processing, which has excellent ridging resistance and chemical conversion treatment properties, characterized by winding at 00° C. or lower, followed by recrystallization annealing.