JPS6082615A - Production of steel sheet having high drawability - Google Patents

Abstract

PURPOSE:To produce the titled steel sheet having high Lankford value with a simple process in the production of steel sheet including a continuous annealing stage by controlling the annealing stage by specifying the relation between the carbon content of steel sheet and the rate of transformation to austenite. CONSTITUTION:An annealing stage is controlld so as to satisfy the formula, wherein Xc is wt% of carbon content of the steel sheet and Ygamma(%) is the rate of max. transformation rate of austenite when the steel sheet is heated to above the Ac1 transformation point and then cooled to Ar1 transformation point. By the formula, preferred relation between Xc(%) and Lankford value is obtd. Accordingly, in the continuous annealing of a steel sheet having a constant value for Xc(%), the control is to be performed so as to let the value Yalpha to satisfy the formula. By this method, the control is performed more easily because the variable to be controlled is only Yalpha, therefore, the control is executed easily and accuracy of the control is high.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a thin steel sheet with good drawing workability,
In particular, the present invention relates to a manufacturing method for controlling the j value based on the relationship between the carbon content of steel and the austenite transformation rate.

When continuously annealing mild steel or high-strength steel sheets using a regular continuous annealing line or continuous hot-dip galvanizing line, there may be some disadvantages in terms of material quality, workability, high tensile strength, and aging properties compared to conventional methods. This has become a major problem.

The workability of a steel plate has a strong correlation with the 7 values of the material properties of the steel plate (Lankford value), In (elongation), yp (yield point), etc. A value is required.

Various methods have been used in the past to increase the 7 value, but the main ones are as follows.

(b) Adjust the structure of steel, Naka) Control hot rolling conditions including slab heating conditions.

C-1 Optimize cold rolling conditions.

(-1 Control the annealing conditions.

Regarding the above-mentioned conventional methods, are there any concrete methods disclosed that have achieved a certain degree of success?These conventional methods generally have the disadvantage of being complicated and difficult to control ((, therefore, resulting in high manufacturing costs). could not pass away.

The purpose of the present invention is to eliminate the drawbacks of the above conventional methods for increasing the i value, and to provide an effective method that can accurately increase the i value and obtain a thin steel plate with good drawing workability. K provides a method.

The gist of the present invention is as follows.

That is, in a method for manufacturing a thin steel sheet including a continuous annealing process, the carbon content of the thin steel sheet is Xc (heavily %), and the maximum carbon content of the steel sheet after heating to the Ac+ transformation point or higher and cooling to the Ar+ transformation point is The austenite transformation rate is Yr(
%), a method for producing a thin steel plate with good drawability, characterized in that the annealing process is controlled so as to satisfy the following relational expression: 5≦Yγ(Xc)≦50;
It is.

The present inventor is currently studying a continuous annealing process for various cold-rolled steel sheets, and found that when a steel sheet is heated above the Ac+ transformation point, the relationship between the carbon content of the steel and the austenite transformation rate after annealing is 7.
It was discovered that there is a close relationship with the value, and as a result of further investigation, the present invention was obtained.

First, the experimental results of the present inventor will be explained. Seven types of test steels having chemical compositions as shown in Table 1 were melted in a converter.

After continuous casting, the molten steel produced in the above converter is hot rolled to 640°C.
High temperature winding was carried out at the above temperature. A cold-rolled steel plate obtained by cold-rolling a hot-rolled steel plate is continuously annealed, and the austenite fraction transformed during this annealing is defined as the transformation rate Yγ, and the transformation −$
Yγ was continuously measured and its correlation with the 7 values of the steel plate after annealing was investigated.

The relationship between the carbon content Xc (%) and the maximum austenite transformation rate Yr of each sample material in this experiment is as shown in FIG.

As is clear from Fig. 1, the C content Xc (%
) is smaller, the 7 value is higher, but the austenite transformation rate Yr
It can be seen that if the T value is too small or too large, the T value tends to become small, and the range of transformation rates in which a small C content Xc (%) can be obtained is wide. It is known that the minimum 〒 value that must be secured for normal deep drawing is approximately 1.5. Therefore, according to the present experimental results shown in Figure 1, the C50.08% steel If so, it can be seen that a steel plate with 〒≧1.5 can be obtained by holding the temperature at an appropriate temperature above AC+transformation point for an appropriate time.

However, steel with a high C content inevitably has other carbon content.
24 The material must have good properties, such as elongation properties, but also be able to withstand deformation of 11 degrees. Therefore, the following relationship between C content and 7 values is desirable.

o, in the range of i≧C>0.06, 7≧1.50.06
≧C> 0.02 r≧1.8 0.02≧C> 0.
OO7r≧2.00, 007≧CP≧2.2 To achieve this, the carbon content: Xc (wt%), the austenite ratio: Yr.
0゛08-0.08, and the B line is represented by Yγ=5・(X), so the chain acid shown by the diagonal I, which is sandwiched by the A line and the B line, that is, s s Yr ・(XC)s 50 = lIJ (1
) It has been found that this can be achieved by annealing at the soaking temperature or holding time during annealing so that the Kfl ratio Yγ satisfies the equation. More preferably, it has been found that it is most effective to perform annealing under conditions such that the amount of Yr is 2/3 from the lower limit of the range shown by the above formula (1).

In other words, it has been found that as long as the transformation rate Yγ that satisfies the formula (1) is obtained, a suitable relationship between the carbon content X c (%) and the 7 value is inevitably obtained.

Therefore, by managing the carbon content Xc (%) - the austenite transformation rate Yγ during continuous annealing of the copper plate of the foot to satisfy the IJ equation, the lower value of the steel plate can always be kept at 1.5 or more. By controlling the material quality based on the transformation rate Yγ, the following effects can be expected in addition to the above.

(b) The material properties of the steel sheet, especially the r value, are not directly related to the annealing temperature or time (basically, they are related to changing the various structures of the steel sheet, such as crystal grains, texture, precipitates, transformed phases, etc.) However, the material itself is directly related to the resulting structure.Therefore, the orstenite transformation rate Yγ
The method of the present invention, which manages the organization, directly manages the organization, so it is more direct and has higher accuracy.

(b) Since the annealing process using a continuous annealing line for steel plates is a short-time treatment, the phenomenon of changes in the structure occurring inside the steel plate is non-equilibrium at any point in time.

Therefore, in some cases, it is possible to compensate for the temperature deficiency during °C annealing by increasing the time, but the control parameters are difficult to control, and therefore the stacking capacity is also reduced.In contrast, the present invention In the case of V drilling at an austenite transformation rate of Yγ, the number of management parameters is reduced, control is easy, and the accuracy is therefore significantly improved.

As shown in Table 2 of Examples, the amount of C is from 0.0008% to 0, Q
Seven types of test steels with chemical compositions that changed by up to 87% were melted in a converter, and then continuously cast and directly hot rolled or heated under various slab heating conditions of 1000 to 1280°C.
and various finish rolling temperatures of 700-890°C, 350-890°C.
After hot rolling at various coiling temperatures of 700° C., cold rolling was performed at a reduction ratio of 50% or more, and nine types of test materials obtained therefrom were all annealed in a continuous annealing furnace. The maximum value of the austenite transformation rate Yγ measured during annealing, the manufacturing conditions, and the 7 values of the annealed material are shown in Table 3.

As is clear from Tables 2 and 3, it is clear that as long as the limiting requirements based on the 91 studies are satisfied, the less C-containing steel sheets have, the higher the r value will be. As is clear from the test results of Comparative Example Sample Materials IA and 6A, even if the chemical composition of the steel is the same as Sample Materials 1 and 6, respectively, if the above formula (1) is not satisfied, , Sample materials /461-7 that have a significantly low r value and satisfy the requirements of the present invention are:
All of them showed a high value of 7, indicating good drawing and machinability.

As is clear from the above examples (, austenite transformation rate Y
A high r value is exhibited in a range where γ satisfies formula (1), and the reason for this is as follows. In other words, the +11.11 million-position texture due to the growth of untransformed ferrite grains basically develops the more it is annealed in Sangang, but on the other hand, the higher the annealing temperature, the more austenitic transformation progresses and the texture becomes untransformed.
This is because as the ferrite fraction decreases, the transformation rate Yr is balanced within a certain appropriate range and the i value increases. Therefore, the optimum range of the transformation rate Yγ becomes narrower as the C content increases. The optimum range of Yr shifts to the lower side. Therefore, even in the range of formula (1) that satisfies the present invention, the lower the C content Xc (%), the wider the optimum range of the variable M4Yγ for obtaining a high 7 value.

As described above, in the method of manufacturing a thin steel sheet including a continuous annealing process, the present invention allows the maximum austenite transformation rate Yγ in cooling to the ArI transformation point after heating to the Ac + j dust point in the annealing process to the ArI transformation point to be calculated using the above C11 formula. to satisfy you? By using this method, it has become possible to produce a thin copper plate with high drawability and good drawing workability. Since the only control parameter in the invention is the austenite transformation rate, it is easy to control, and therefore the control accuracy is high (and in turn, an effect that can greatly contribute to reducing manufacturing costs8)

[Brief explanation of drawings]

Figure 1 shows the C content tXc, maximum austenite transformation rate Yγ and ? of a steel plate during continuous annealing in an experiment to obtain non-invention. FIG. 3 is a diagram showing the relationship between values. Representative Patent Attorney Takeo Nakaji

Claims (1)

[Claims] (1) In a method for manufacturing a thin steel sheet including a continuous annealing process,
The carbon content of the thin steel sheet is Xc (wt%), and the maximum austenite transformation rate when the steel sheet is heated to the AcIf-9i point or higher and then cooled to the Ar+ transformation point is Yγ (
%), a method for producing a thin steel sheet with good drawing work, comprising controlling the annealing process so as to satisfy the following relational expression: 5≦Yr(Xc)≦50.