JPS61204323A - Production of as-rolled thin steel sheet for working having small plane anisotropy and excellent ridging resistance - Google Patents

Abstract

PURPOSE:To obtain the titled thin steel sheet having good workability in new process without including cold rolling and recrystallization annealing by executing rolling under application of tension under specific conditions 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 rate and under the application of tension in at least one pass in finish rolling in the stage of rolling the low-carbon steel to the prescribed sheet thickness. The tension to be applied is preferably >=1 kg/mm<2>. The thin steel sheet having the small in plane anisotropy, good workability and exellent 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 under the application of the tension 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 small in-plane anisotropy and excellent ridging resistance and workability is The present invention discloses the development results of a new process that can omit the inter-rolling and recrystallization annealing steps.

Processing thin steel sheets with a thickness of approximately 21 mm or less and 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.

Furthermore, even if the workability in a particular direction is good, since the actual processing is planar, if the in-plane anisotropy is large, wrinkles may occur after processing. In this respect, if the anisotropy is small, the amount of edge cutting after molding is small and the blank area can be reduced.
Steel plate yield is significantly improved. The anisotropy of such mechanical properties is ΔE1 (anisotropy parameter of elongation) and Ar (
r value anisotropy parameter), ΔE1≦5%,
A steel plate with excellent anisotropy is required to have Δr≦0.5.

(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 200mm 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 200mm. Approximately 30m
The sheet bar is made into a hot-rolled steel strip with a predetermined thickness in a finishing hot rolling process at a finishing temperature of Ar3 transformation point or above, and then, after pickling, it is cold-rolled to a predetermined thickness (2 , 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, manpower and time required to produce the product increase, but during these long processes there are also additional disadvantages that can cause various problems in product quality, especially surface properties. Unavoidable troubles include the occurrence of surface defects during the recrystallization annealing process, deterioration of surface beauty due to surface concentration and surface oxidation of impurity elements during the recrystallization annealing process, and further deterioration of 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 that required for Ars transformation, so the texture becomes random during T→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 decrease in temperature of the steel sheet due to thinning requires rolling below the Arz transformation point of low carbon steel, and the material quality (
ductility, drawability). Again analogy A
Even if the quality of the material can be secured by rolling at or below the rs 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, the thickness of the plate is 50IIIl immediately from molten steel.
A process that can omit the hot rolling heat treatment and rough rolling process by melting the following copper strip is also being put into practical use.

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 <uvH> 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 a thin steel sheet of a predetermined thickness in a relatively low temperature range below Ar=transformation point, and then are not subjected to cold rolling or recrystallization annealing steps. There is. 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% at a temperature below Ar=transformation point.
, tensile strength 37.3ksr/mm", elongation 49.7%r
A manufacturing example with a characteristic of =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 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++uw, and in this respect, the drawability is the same or lower than that of the conventional example, which is 60.58 to 60.61. Furthermore, JP-A-53-22850 discloses that low carbon rimmed steel is also hot-rolled at a finishing temperature of 710 to 750.
The plate thickness was 1.8 to 2.3 mm at ℃, and the winding temperature was 530 to 6.
A method for producing a low carbon hot rolled steel sheet by heating the steel sheet to 00°C 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,
Hot rolling finishing temperature of low carbon aluminum killed steel: 760-820℃
The yield point is 14.9 to 18.8 kg/mm" when the plate thickness is 1.6 mm and the winding temperature is 650 to 690°C.

Tensile strength 27.7-29.8kg/mm", 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. Mn0.2
3. Plo, 009. S10.008. Al
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, 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) To provide a method for manufacturing thin steel sheets with small in-plane anisotropy and excellent ridging resistance and workability by a new process that does not include not only cold rolling but also recrystallization annealing. is the purpose of this invention.

(Means for Solving the Problems) This invention provides at least one pass in the process of rolling low carbon steel to a predetermined thickness in a temperature range of Ar = below the transformation point and above 500°C, with a rolling reduction rate of 235%. This is a method for manufacturing an as-rolled thin steel sheet for processing, which is characterized by rolling at a strain rate of 300 (s") or higher and under tension, and which has small in-plane anisotropy and excellent ridging resistance.

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

The test materials were hot-rolled steel sheets of two types of low-order aluminum killed steel shown in Table 1, and 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%.

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 Jr value and the ridging resistance were significantly improved by increasing the rolling reduction rate to 35% or higher and the high strain rate to 300 s-' or higher.

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

Where, n: Rolling speed (rpm) of rolling rolls R: Rolling reduction ratio (χ)/100 R: Radius of rolling rolls (mm) Ho: Thickness of plate before rolling (mm) In addition, composition steel C shown in Table 2 The following experiment was conducted using a rolling mill consisting of six rows.

High strain and high pressure rolling was performed on the 6th stand, and rolling was performed with a tension of 3 kg/mm'' between the 5th and 6th stands. The final rolling temperature was 700°C. Figure 2 shows the surface of the sample after annealing. Shows internal anisotropy.

Note that the anisotropy is Δr = (r L+ r c-2ro)
/2゜ΔE 1 = (E I L+E It c-2
It was calculated as Bio)/2.

As is clear from the figure, the in-plane anisotropy of the tension-rolled sample is significantly reduced at a strain rate of 300 s-' or higher.

As a result of repeated research based on these basic data, the inventors discovered that it is possible to produce thin steel sheets with small in-plane anisotropy and excellent ridging resistance and workability by regulating the production conditions as shown below. confirmed.

(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. In addition, reducing O in steel by adding Al is advantageous for improving material quality and ductility. Furthermore, in order to obtain even better workability, C and N can be precipitated and fixed as stable carbonitrides. It is also effective to add special elements such as Tt+Nb+Zr and B.

Furthermore, 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 501iII+ 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 producing 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 at a strain rate of 35% or more, a strain rate of 300 s-' or more, and under tension.

In the high temperature range where the finish rolling temperature is Ar and exceeds the 1 transformation point, even if rolling is performed at a reduction rate of 35% or more, a strain rate of 300 s-' or more, and under tension, the workability,
Only poor ridging resistance was obtained; on the other hand, at 500℃
If the temperature is lower than that, the deformation resistance will significantly increase and problems specific to the cold rolling method will occur.

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.

Further, the additional tension is preferably 1 kg/mm2 or more.

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, and presence or absence of lubrication of the rolling mill 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 strip obtained by the above-mentioned 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) The reason why rolling at a high reduction rate and high strain rate under tension according to the present invention reduces in-plane anisotropy and significantly improves ridging resistance and bottom value is as follows. Although it has not yet been clearly elucidated, it is thought to be closely related to changes in the texture and processing strain of the rolled material.

(Example) Each of the composition steels shown in Table 3 was processed to a thickness of 2 by the method shown in Table 4.
After forming the sheet bar into a sheet bar having a thickness of 0 to 40+ sm, a thin steel plate having a thickness of 0.8 to 1.2 - sm was obtained using a rolling mill consisting of 6 rows. At this time, high strain rate rolling was performed on the stand in the last row. Also, tension was applied between the 5th stand and the 6th stand. The thin steel sheet thus obtained is subjected to pickling, temper rolling (
Table 4 shows the material properties after rolling reduction 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). . This evaluation is based on the fact that no ridging occurs when conventional low-carbon cold-rolled steel sheets are manufactured, so the evaluation criteria cannot be established. Therefore, in the present invention, the index evaluation criteria based on the visual method for conventional stainless steels are applied as they are. Ratings 1 and 2 indicate ridging properties that pose no problem in practical use.

Table 3 Steel sheets manufactured according to the present invention have smaller in-plane anisotropy than the comparative example, and also exhibit excellent lower values and ridging resistance.

(Effects of the Invention) Thus, according to the present invention, by rolling at a high reduction rate and high strain rate under tension in the temperature range from the transformation point to 500°C, it is possible to perform not only conventional cold rolling but also recrystallization annealing. It is possible to obtain a thin steel plate with small in-plane anisotropy, good workability, and excellent ridging resistance using as-rolled steel sheets, which also omit the process of rolling. The manufacturing process for thin steel sheets for processing can be greatly simplified, such as by complying with laws and regulations.

[Brief explanation of drawings]

Figure 1 is a graph showing the effect of strain rate on lower value and ridging index using rolling reduction as a parameter. Figure 2 is a graph showing the effect of strain rate and tension on r value and elongation anisotropy. It is a graph. Fig. 1 Taz' η speed cliff (S-ri Fig. 2 Taz'2 - speed 1 Its ~ procedure amendment book (color %) 1986
February 13th, Michibe Uga, Commissioner of the Patent Office1, Indication of the case, 1985 Patent Application No. 439742, Title of the invention: Method for producing az-rolled thin steel sheet for processing with low in-plane anisotropy and excellent ridging resistance 3. Relationship with the case of the person making the amendment Patent applicant (125) Kawasaki Steel Corporation 4. Agent 5. Subject of amendment ``Detailed description of the invention'' column of the specification■
, "perverted" in line 15 of the fourth sentence of the specification is corrected to "perverted point." 2. Table 3 on page 17 of the same is corrected as follows. Table 3 Procedural amendments March 1, 1986 Michibe Uga, Commissioner of the Patent Office 1, Indication of the case Patent Application No. 43974/2, 1985, Name of the invention Low in-plane anisotropy and resistance to ridging Manufacturing method of azu-rolled thin steel sheet for processing with excellent properties 3, relationship with the amended case Patent applicant (125) Kawasaki Steel Co., Ltd. 4, attorney 1, “After annealing” on page 11, line 4 of the specification is corrected to "after rolling". 1. "Table 4" on page 18 of the same page is corrected as shown in the attached sheet. (7th r41 in the column for finishing temperature (°C) of rolling conditions)
9 J is corrected to r542 J, and the material property El(χ)
Correct the third “51” in the column for “52” and correct the tenth “1” in the Rising Index column to “5゛”)

Claims (1)

[Claims] 1. In the step of rolling low carbon steel to a predetermined thickness, at least one pass is carried out at a temperature range of below Ar_3 transformation point and above 500°C, rolling reduction: 35% or above, strain rate: 300 s. A method for producing an as-rolled thin steel sheet for processing, which has small in-plane anisotropy and excellent ridging resistance, characterized by rolling under tension of ^-^1 or more.