JPS61204325A - Production of as-rolled thin steel sheet for working having excellent ridging resistance and strength-elongation balance - Google Patents

Abstract

PURPOSE:To obtain titled thin steel sheet having good workability with a new process without including cold rolling and recrystallization annealing by specifying rolling conditions including a relation between a strain rate and rolling roll diameter in a stage for rolling a low carbon steel to a prescribed sheet thickness. CONSTITUTION:The low-carbon steel 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 epsilon and under the conditions satisfying equation [R: radius (mm) of the rolling roll] 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 the excellent ridging resistance and strength-elongation balance 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 strength-elongation balance is based on cold rolling and recrystallization by regulating rolling conditions. The purpose is to disclose the development results of a new process that can omit the annealing process.

Processing thin steel sheets 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 are required to have the following properties.

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

Additionally, the steel plate to be processed is basically required to have an excellent strength-elongation balance.

This is because a steel plate with poor strength-elongation balance causes problems such as wall cracking during processing.

In particular, when aiming to increase the strength for the purpose of lower plate thickness, the strength -
Elongation balance is an important characteristic.

Here, TS (kg/mm”) X E l (%)≧
1500 is a rough guide for a steel plate with excellent strength-elongation balance.

(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.

Low-carbon steel is mainly used as the steel material, and after being made into slabs with a thickness of about 200 mm by ingot-forming and blooming rolling, the slabs are heated and soaked in a heating furnace, and then the thickness is reduced by a rough hot rolling process. Approximately 30
The sheet bar is made into a hot-rolled steel strip with a predetermined thickness in a final hot rolling process at a finishing temperature of Ar2 transformation point or higher, and then, after pickling, it is 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 does the amount of energy, personnel and time required to produce the product increase, but additional disadvantages are added during these long steps which can cause problems with the quality of the product, particularly its surface properties. For example, unavoidable problems include the occurrence of surface defects in the cold rolling process, deterioration in surface beauty due to surface concentration and surface oxidation of impurity elements in the recrystallization annealing process, and further deterioration in surface treatment properties.

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 Ar=transformation, so the texture becomes random during the γ-α transformation.

In addition, it is extremely difficult to manufacture thin steel sheets with a thickness of 2.0 mm or less using only a hot rolling process. Moreover, in addition to the problem of dimensional accuracy, the drop in steel plate temperature due to thinning forces the rolling of low carbon steel below the Ar=transformation point, and the material (
ductility, drawability). Matatae A
Even if the quality of the material can be secured by rolling at or below the r3 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. , which generally tends to occur when processing is performed at relatively high temperatures in the ferrite (α) region (this tendency is particularly strong when the rolling reduction 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.

However, the manufacturing process for 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 been lowered from the conventional 1200°C to around 1100°C or lower in order to improve material quality and save energy. There is a tendency. Furthermore, a process is being put into practical use in which a copper strip having a thickness of 50 mm or less is produced immediately from molten steel, thereby omitting the hot rolling heat treatment and rough rolling steps.

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 a strong casting texture with <uVW> as the main orientation.

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 at a relatively low temperature below the Arl transformation point, and then do not undergo cold rolling or recrystallization annealing steps. . For example, Japanese Patent Application Laid-open No. 48-4329 discloses that the yield point of 4 mm thick copper strip is 26.1 kg/mm when low carbon rimmed steel is rolled at 90% in Ar at a temperature below the transformation point.
, tensile strength 37.3 kg/mm'', elongation 49.7%.

A production example with a characteristic of r=1.29 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 (Ar, below the transformation point) and coiled at a temperature of 600 to 730°C.
℃ has been shown to produce a low yield point steel sheet with a yield point of 20 kg/am" or less. However, the conical cup value, which is an index of drawability, is 60.
．． It is about 60 to 62.1 hm, which is compared to the conventional example of 60
．． Compared to 58 to 60.61, the drawability is the same or lower. Furthermore, Japanese Patent Application Laid-open No. 53-22850 discloses that low carbon rimmed steel is also hot-rolled at a finishing temperature of 710 to 750°C.
．． A method for producing a low carbon hot rolled steel sheet is shown in which the thickness is 8 to 2.31°C and the coiling temperature is 530 to 600°C.

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, and the drawing property is inferior. Furthermore, in JP-A-54-109022, low carbon aluminum killed steel is hot-rolled at a finishing temperature of 760 to 820°C.
Yield point: 14.9-18.8 kg/mm2° Tensile strength: 27.7-29.8 kg/mm2. Elongation 39.0-4
An example of manufacturing a low strength mild steel plate having a property of 4.8% is disclosed. In addition, JP-A-59-226149 has C10,002°5i10.02. MnO,23,Pl
o, 009. S10.008. A It 10.025
°N10.0021. By rolling low carbon A1 killed steel with Ti10.10 at 500 to 900°C with lubricating oil at 76% to form a steel strip with a thickness of 1.6 mm, r = 1.
．． An example of manufacturing a thin steel sheet having 21 properties 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) An object of the present invention is to provide a method for manufacturing thin steel sheets with excellent ridging resistance and strength-elongation balance by a new process that does not include not only cold rolling but also recrystallization annealing. It is a purpose.

(Means for Solving the Problems) This invention provides a method for rolling low carbon steel to a predetermined thickness in a process of rolling low carbon steel to a predetermined thickness by reducing the rolling reduction in a small number of passes (at least one pass) in a temperature range of Ar=transformation point or lower and 500°C or higher. = 35% or more, strain rate (; ): 300 (s
-1) A method for manufacturing an as-rolled thin steel sheet for processing, which is characterized by rolling under the relationship of 2/R≧2.0 and having excellent ridging resistance and strength-elongation balance.

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

The test materials are hot-rolled steel sheets of two types of low-order aluminium-killed steel shown in Table 1. These test materials A and B were heated to 700°C, and after soaking, 20%, 40%, and 60% were heated in one pass. % of each rolling reduction.

Table 1 [ [ Figure 1 shows the relationship between the strain rate (ε) at this time, the lower value of the steel plate after rolling, and the ridging index.

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.

The strain rate) was calculated according to the following formula.

Here, n: Rolling speed (rpm) of the rolling roll R: Rolling ratio (χ)/100 R: Radius of the rolling roll (mm) Ho: Thickness of the plate before rolling (mm) Also, using steel B in Table 1, Strength of Azurold thin steel plate -
Figure 2 shows the results of an investigation into the relationship of g/R on elongation balance (rsxEl).

As is clear from the figure, by setting a/R≧2.0, an excellent strength-elongation balance of TSXEl≧1500 was obtained.

As a result of repeated research based on these basic data, the inventors have confirmed that a thin steel sheet with excellent ridging resistance and strength-elongation balance can be manufactured by regulating the manufacturing conditions as described below.

+1) WjII! The effect of high strain rate rolling is essentially independent of 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 the content of 0 in steel by adding 81 is advantageous for improving the material quality, especially the 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. Of course, the so-called CC-DR (continuous casting-direct rolling) method, in which rolling is started without reheating the steel billet after continuous casting, 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, and when rolling low carbon steel to a predetermined thickness, at least one bath is carried out in the finish rolling at a temperature range of Ar = below the transformation point and above 500°C. The reduction rate is 35% or more, the strain rate is 300 s-' or more, and g
It is essential to roll under the condition that /R≧2.0.

In a high temperature range where the finish rolling temperature exceeds the Ar + transformation point, even if rolling is performed at a reduction rate of 35% or more and a strain rate of 300 s-' or more, and then coiled at 400°C or less,
Only inferior workability and ridging resistance can be obtained, while temperatures below 500°C result in a significant increase in deformation resistance.
Since the cold rolling method poses a particular problem, the finish rolling temperature was limited to the range of Ar+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 s-' or more, particularly 500 to 2500 s-', is suitable.

Furthermore, if /R is less than 2.0, as shown in Figure 2 above, an excellent strength-elongation balance cannot be obtained, so it is important that /R≧2.0. .

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, 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 steel 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) The reason why rolling at a high reduction rate and high strain rate according to the present invention significantly improves ridging resistance, workability, and strength-elongation balance has not yet been clearly elucidated. However, it is thought to be closely related to changes in the texture and working strain of the rolled material.

(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 thin steel plate with a thickness of 0.8 to 1.2 mm was produced using a rolling mill consisting of 6 rows. At this time, high strain rate rolling was performed on the stand in the last row. At this time, by changing the roll diameter of the last row,
ε/R was changed.

Table 3 shows the material properties of the thus obtained thin steel sheet 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 virtually no shearing occurred using the conventional manufacturing method of 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 sheet manufactured according to the present invention exhibits a lower value, ridging resistance, and strength-elongation balance that are superior to those of the comparative example.

(Effects of the Invention) Thus, according to the present invention, by high rolling reduction and high strain rate rolling in the temperature range from Ar3 transformation point to 500°C,
It is possible to obtain thin steel sheets with good workability, excellent ridging resistance, and strength-elongation balance by using as-rolled steel sheets that omit not only conventional cold rolling but also recrystallization annealing. It is compatible with the sheet bar caster method, strip caster method, etc., and can significantly simplify the manufacturing process of thin steel sheets for processing.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the effect of strain rate on the lower value and ridging index using rolling reduction as a parameter. FIG. 2 is a graph showing the effect of ε/R on the strength-elongation balance. Figure 1: Diagonal Speed (5-L Figure 2J, 7B Procedure Amendment) March 1, 1985 Director General of the Patent Office Uga Michibu 1, Indication of the Case 1985 Patent Application No. No. 43976 No. 2, Title of the Invention Method for manufacturing an azu-rolled thin steel sheet for processing with excellent ridging resistance and strength-elongation balance 3, Relationship with the amended case Patent applicant (125) Kawasaki Steel Corporation 4, Attorney 5 , subject to correction

Claims (1)

[Claims] 1. In the process of rolling low carbon steel to a predetermined thickness, at least one pass is carried out at a temperature range of below the Ar_3 transformation point and above 500°C, with a rolling reduction rate of 35% or more and a strain rate (■ ):300(s^-^
1) and above, and rolling under conditions that satisfy the following formula: ■/R≧2.0, where R: radius of rolling roll (mm). A manufacturing method for excellent as-rolled thin steel sheets for processing.