JPS61204320A - Production of as-rolled thin steel sheet for working having excellent ridging resistnace - Google Patents

Abstract

PURPOSE:To obtain the titled thin steel sheet having excellent ridging resistance and workability with a new process without including cold rolling and recrystallization annealing by specifying rolling conditions in a stage for rolling a low carbon steel to a prescribed sheet thickness. CONSTITUTION:The low-carbon steel is is rolled in the temp. range of the Ar3 transformation point or below and >=500 deg.C and at 35% draft and >=300 s<-1> strain rate in at least one pass in finish rolling in the stage of rolling the low-carbon steel to the prescribed sheet thickness. The thin steel sheet having the good workability and excellent ridging resistance is thus obtd. as-rolled without executing not only the conventional cold rolling but also recrystallization annealing by the high draft and high strain rate rolling in the above-mentioned temp. range.

Description

[Detailed Description of the Invention] (Industrial Application Field) The technical content described in this specification regarding the production of thin steel sheets with excellent ridging resistance and workability is limited to cold rolling and recrystallization annealing processes by regulating rolling conditions. The goal is to disclose the development results of a new process that can eliminate the above.

Processing thin steel sheets with a thickness of approximately 21III or less and used for applications such as building materials, automobile body materials, can stock, or various surface-treated original sheets are required to have the following properties.

(11) Mechanical properties In order to obtain good bending, stretching and drawing properties, high ductility and high Lankford value (r value) are mainly required.

(2) Surface properties Since these materials are mainly used on the outermost side of the final product, not only the shape and surface beauty of the material but also various surface treatments are important.

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

First, low-carbon steel is mainly used as the steel material, and after being made into a steel billet with a thickness of about 200 mm by ingot-making and blooming rolling, it is heated and soaked in a heating furnace, and then subjected to a rough hot rolling process to achieve a thickness of about 200 mm. Approximately 30
After forming a sheet bar into a hot-rolled steel strip with a predetermined thickness in a final hot rolling process at a finishing temperature of Ar3 transformation point or higher, the strip is then pickled and cold-rolled to a predetermined thickness (2 mm). , 0 mm or less), and then subjected to recrystallization annealing to produce the final product.

The biggest drawback of this practice is the extremely long process required to reach the final product. As a result, not only is the amount of energy, personnel and time required to produce the product, but also the disadvantages that arise during these long steps are various problems in product quality, particularly surface properties. For example, unavoidable troubles include the occurrence of surface defects in the cold rolling process, deterioration in surface beauty due to surface concentration of impurity elements and surface oxidation in the recrystallization annealing process, and deterioration in surface treatability.

By the way, as a method of manufacturing thin steel sheets for processing, a method of producing the final product through a hot rolling process is also considered. According to this method, cold rolling and recrystallization annealing steps can be omitted, which is a great advantage.

However, the mechanical properties of a hot-rolled thin steel sheet are far inferior to those obtained through a cold rolling-annealing process. In particular, press-formed materials used for automobile bodies require excellent deep drawability, but hot-rolled steel sheets have a low r value of around 1.0, so their processing applications are extremely limited. Become something. This is because in the conventional hot rolling method, the finishing temperature is higher than the Arl transformation temperature, so the texture becomes random during the γ-α transformation.

In addition 2. It is extremely difficult to manufacture thin steel sheets with a thickness of less than 100 mm using only a hot rolling process. Moreover, in addition to the problem of dimensional accuracy, a decrease in the temperature of the steel sheet due to thinning necessitates rolling of low carbon steel below the Ar transformation point, resulting in significant deterioration of the material quality (ductility, drawability). Again analogy Ar
Even if the quality of the material can be secured by rolling at or below the 3 transformation point, a new problem arises in that steel sheets rolled in the ferrite region are more likely to suffer from cringing.

Here, ridging is a defect in surface irregularities that occurs during the processing of a product, 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 crystal orientation groups (for example, (100) oriented grain groups) that are not easily divided even after undergoing the annealing recrystallization process and remain stretched in the rolling direction. , generally tends to occur when processing is performed at a relatively high temperature in the ferrite (α) region, and this tendency is particularly strong when the reduction rate in the ferrite region is high, that is, when manufacturing thin steel sheets.

Recently, these thin steel sheets for processing have become more complex,
As materials become more sophisticated, they are often subject to more severe processing, and excellent ridging resistance is now 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.

In other words, in recent years, the introduction of a continuous casting process has made it possible to omit the blooming process, and the heating temperature of steel slabs has tended to be lowered from the conventional 1200°C to around 1100°C or lower in order to improve material quality and save energy. It is in. Furthermore, a process is being put into practical use that can omit the heat treatment and rough rolling process of hot rolling by immediately producing a strip of K 50 mm or less from molten steel.

However, all of these new manufacturing processes are disadvantageous in that they destroy the structure (cast structure) formed when molten steel solidifies. especially formed during solidification (100)
It is extremely difficult to destroy the strong casting texture whose main orientation is <uvw>.

As a result, the final thin steel sheet was prone to the aforementioned jinging.

(Prior Art) Several methods have been proposed for manufacturing thin steel sheets for processing, which are formed into thin steel sheets of a predetermined thickness in a relatively low temperature range below the Ars transformation point, and then do not undergo cold rolling or recrystallization annealing steps. . For example, JP-A-48-4329 discloses that low carbon rimmed steel is rolled into a 4 mm thick copper strip by 90% rolling at a temperature below the Ar3 transformation point, yielding a yield point of 26.1 kg/mmζ.
Tensile strength: 37.3 kg/mm", elongation: 49.7%.

A production example with the characteristic r=129 is shown.

Furthermore, in JP-A-52-44718, low carbon rimmed steel is hot-rolled to a thickness of 2.0 mm at a finishing temperature of 800 to 860°C (below the Arz transformation point) and coiled at a temperature of 600 to 730°C.
℃ has been shown to produce a low yield point steel plate with a yield point of 20 kg/mm or less. However, the conical cup value, which is an index of drawability, is 60.
．． It is about 60 to 62.18 mm, which is different from the conventional example 6.
Compared to 0.58 to 60.61, the drawability is the same or lower. Furthermore, in JP-A-53-22850, a low carbon rimmed steel is hot-rolled to a thickness of 1.8 to 2.3 mm at a finishing temperature of 710 to 750°C, and a coiling temperature of 530 to 600° C.
A method for producing a low carbon hot rolled steel sheet is shown. However, the conical cup value of the product obtained by this method is also higher than that of the conventional example, as in the case of JP-A-52-44718 cited above, and the drawability is inferior.

Furthermore, in Japanese Patent Application Laid-Open No. 54-109022, low carbon aluminum killed steel is rolled at a hot rolling finishing temperature of 760 to 820°C.
．． The yield point is 14.9 to 18.8 kg/mmz when the plate thickness is 6 mm and the winding temperature is 650 to 690°C.

Tensile strength 27.7-29.8kg/nap”, elongation 39
．． An example of manufacturing a low strength mild steel plate having properties of 0 to 44.8% is disclosed. In addition, JP-A-59-226149 has C10,002°5i10.02. Mn0.
23. Plo, 009. S10.00B, AI
10.025°N10.0021. By rolling low carbon Af killed steel with Ti 10.10 at 500 to 900°C with lubricating oil at 76% to form a copper strip with a thickness of 1.6 mm, a thin steel plate having a property of r = 1.21 is produced. A manufacturing example is shown.

However, none of the above-mentioned known techniques give any consideration to improving the above-mentioned ridging resistance.

(Problems to be Solved by the Invention) It is an object of the present invention to provide a method for manufacturing thin steel sheets with excellent ridging resistance and workability by a new process that does not include not only cold rolling but also recrystallization annealing. be.

(Means for Solving the Problems) This invention provides at least one pass in the process of rolling low carbon steel to a predetermined thickness at a temperature range of below the transformation point and above 500°C, with a rolling reduction rate of 35°C. % or more and strain rate: 300 (s-') or more.

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

The test materials are two types of hot-rolled low carbon aluminum killed steel sheets shown in Table 1. These test materials ^ and B were heated to 700°C,
After soaking, rolling was performed in one pass at rolling reductions of 20%, 40%, and 60%.

Table 1 The relationship between the strain rate (=) at this time, the lower value of the steel plate after rolling, and the ridging index is shown in FIG.

The lower value and the ridging index strongly depend on the strain rate and the rolling reduction rate, and the lower value and the ridging resistance were significantly improved by setting the rolling rate to 35% or higher and a high strain rate of 300 s-' or higher.

Note that the strain rate (i) was calculated according to the following formula.

Here, n; rotation speed (rpm) of the rolling roll r: rolling reduction ratio (χ)/100 R: radius of the rolling roll (mm) Ho: plate thickness before rolling (mm) The inventors based on this basic data As a result of repeated research based on this, it was confirmed that thin steel sheets with excellent ridging resistance and workability can be manufactured by regulating the manufacturing conditions as shown below.

(1) Steel composition The effects of high strain rate rolling do not essentially depend on the steel composition. However, in order to ensure workability above a certain level, the amount of interstitial solid solution elements C and N must be 0.10% each.
Hereinafter, it is preferably 0.01% or less. Further, reducing O in steel by adding Al is advantageous for improving material quality, especially ductility. Furthermore, in order to obtain even better workability, it is also effective to add special elements such as Ti, Nb, Zr, and B that can precipitate and fix C and N as stable carbonitrides.

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 of course 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. So-called CC-DI, which starts rolling without reheating the billet from continuous casting? Of course, the (continuous casting-direct rolling) method is also applicable.

On the other hand, the method of immediately casting a rolled material of 50 mm or less from molten steel (sheet bar caster method and trip caster method) also has great economic merit from the viewpoint of energy saving and process saving, so it is particularly advantageous as a method for manufacturing rolled material.

(3) Rolling process This process is the most important. When rolling low carbon steel to a predetermined thickness, at least one pass is performed in finish rolling at a temperature range of below Ar3 transformation point and above 500°C. It is essential to roll under conditions where the rolling rate is 35% or more and the strain rate is 300 s-' or more.

When the finish rolling temperature is Ar, in a high temperature range exceeding the transformation point, even if rolling is performed at a reduction rate of 35% or more and a strain rate of 300 s-' or more, only poor workability and ridging resistance can be obtained; If it is less than 500°C, the deformation resistance will significantly increase, causing problems specific to the cold rolling method, so the finish rolling temperature was limited to a range from the Ar3 transformation point to 500°C.

Regarding the strain rate, if the strain rate is less than 300 s-', the target material quality cannot be secured, so a strain rate of 300 g-' or more, particularly 500 to 2500 s-', is suitable.

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.

In principle, recrystallization annealing treatment is not necessary, but due to material requirements, heat retention and soaking treatment must be performed on the runout table after rolling and during the winding process, and if necessary, some heat treatment must be performed after rolling. Heat treatment is not prohibited.

(4) Pickling and temper rolling The copper strip obtained by the above procedure is rolled at a lower temperature than conventional methods, so the oxidation layer is thinner and the pickling property is extremely good, so no pickling is required. It can also be used for a wide range of purposes. In addition to conventional acid removal, mechanical removal can also be used for descaling. Furthermore, for the purpose of shape correction, surface roughness adjustment, etc.
Temper rolling of 10% or less can be added.

(5) Surface treatment The copper strip thus obtained is galvanized (including alloy-based)
It has excellent surface treatment properties such as tin plating and enameling, so it can be used as various surface-treated base plates.

(Function) Although it has not yet been clearly elucidated why the ridging resistance and T value are significantly improved by rolling at a high reduction rate and high strain rate according to the present invention, This is thought to be closely related to changes in the texture and processing strain.

(Example) Each of the composition steels shown in Table 2 was processed to a thickness of 2 by the method shown in Table 3.
After forming a sheet bar with a thickness of 0 to 40 mm, a rolling mill consisting of 6 rows was used to form a thin steel plate with a thickness of 0.8 to 1.2 mm.

At this time, high strain rate rolling was performed on the stand in the last row.

Table 3 shows the material properties of the thus obtained thin steel sheets after pickling and temper rolling (reduction ratio of 0.5 to 1χ).

The tensile properties were determined using a JIS No. 5 test piece. In addition, the ridging property was evaluated using a JISS No. test piece cut out from the rolling direction and subjected to 15% tensile strain by visually observing the surface unevenness from 1 (good) to 5 (poor). . No evaluation criteria have been established for this evaluation because there was virtually no shearing when using the conventional manufacturing method for low carbon cold rolled steel sheets.

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.

The steel sheets produced in accordance with the present invention exhibit superior lowering and ridging resistance than the comparative examples, and are
There is no resemblance to those manufactured through the recrystallization annealing process.

(Effects of the Invention) Thus, according to the present invention, by high rolling reduction and high strain rate rolling in the temperature range from Arz transformation point to 500°C,
It is possible to obtain a thin steel plate with good workability and excellent ridging resistance by using as-rolled, which omits not only conventional cold rolling but also recrystallization annealing. It is compatible with the strip caster method, etc., and can significantly simplify the manufacturing process of thin steel sheets for processing.

[Brief explanation of drawings]

FIG. 1 is a graph showing the influence of strain rate on the lowering value and the ridging index using the rolling reduction as a parameter. Figure 1: Amendment of the S-Regular Procedures, March 1, 1986, Michibe Uga, Commissioner of the Patent Office, 1, Indication of the Case, Patent Application No. 43971, 1985, 2, Invention Name: Manufacturing method of as-rolled thin steel sheet for processing with excellent ridging resistance 3, relationship with the amended case Patent applicant (125) Kawasaki Steel Corporation 4, agent

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 below Ar_3 transformation point and above 500°C, with a rolling reduction rate of 35% or above and a strain rate of: 300(s^-^1
) A method for manufacturing an azu-rolled thin steel sheet for processing which has excellent ridging resistance, the method comprising rolling at a temperature of at least 1.