JPS59190330A - Method for improving drawability of steel sheet - Google Patents

Abstract

PURPOSE:To increase considerably the r-value of a steel sheet and to improve the drawability by annealing the sheet having a regulated C content by heating to a temp. between the Ac1 and Ac3 transformation points and slow cooling at a cooling rate corresponding to the C content. CONSTITUTION:A cold or hot rolled steel sheet contg. <=0.15wt% C is annealed by heating or soaking to a temp. between the Ac1 and Ac3 transformation points and controlled cooling to 600 deg.C under conditions satisfying an equation C.V<=0.2 [where C is the carbon content (wt%) of the steel sheet, and V is the average cooling rate ( deg.C/sec)]. The r-value of the sheet is considerably increased, and superior drawability can be obtd.

Description

[Detailed Description of the Invention] (Technical Field) Regarding the improvement of the drawing strength of thin steel sheets, this article describes the improvement of thin steel sheets with a carbon content of up to 0.15% by weight (hereinafter simply expressed in %) by annealing. This is a development result aimed at improving drawability, and this annealing is particularly advantageously suited for continuous annealing of cold-rolled steel sheets, but it may also be applied to hot-rolled steel sheets, and furthermore, it can be applied to hot-rolled steel sheets. It is also effective in such batch annealing, and is generally positioned in the field of technology related to annealing thin steel sheets.

(problem) The material properties required for thin steel sheets that are subjected to drawing processing are: ■ Low yield point ■ Excellent ductility; ■ And especially high Lankford value (r value) It is.

All of these three properties are closely related to the softness of the steel sheet; in general, the softer the steel sheet, the higher the degree of drawing possible. , it can be said that the drawability is inferior.

Conventionally, the workability of hard steel can be improved by lowering the yield point by creating a composite structure, but this alone cannot improve the so-called drawability.

In other words, the r value of high-strength steel sheets is generally low, and the reason for this is that M, which is an additive component to increase the tensile strength,
This is because the development of [1t) texture, which is advantageous for improving drawing workability of thin steel sheets, is inhibited by N, Si+P+O, N, and the like. However, in order to ensure the desired high tensile strength, the amount of the reinforcing component added can be reduced, which is a trade-off.

In addition to limiting the amount and type of reinforcing components, the main means to improve the r value is to fully control hot rolling conditions, cold rolling conditions, and annealing conditions. .

However, despite all efforts, among the conventional high tensile strength steel plates,
For box-fired sweetened materials, P-added aluminum-killed low carbon steel, and for continuously annealed materials, aluminum-killed ultra-low carbon steel with Nb or Ti mira addition have only slightly satisfactory drawing workability.

The former P-added steel cannot obtain a high r-value through continuous annealing, and the latter Nb- or Ti-added steel not only has the disadvantage of high manufacturing costs but also has a high r-value. The disadvantage that it is difficult to obtain a tensile strength of 40 kgf/- or more cannot be overlooked as a steel plate.

Therefore, if it is possible to obtain a high r-value regardless of the composition of the thin steel sheet or the conditions of hot rolling or cold rolling, the uses of the thin steel sheet can be significantly increased, and the benefits are enormous. be.

(Start of development research) Regarding the above-mentioned problems, the inventor heated a cold-rolled thin steel sheet to various temperatures above its AC□ point, held it for a certain period of time, and then cooled it. i.e. 600″
When experiments were conducted with various cooling rates during the process of reaching C], it was found that the r value of the material was strongly influenced by the cooling rate.

In this experiment, 0: 0.15%, Si: 0.3
5%, In: O, Sa%, P: o, oa%,
Al: 0.04%, Hb: 0.02%, N:
A cold-rolled thin steel plate made of aluminum-killed steel containing 0.006% and made into a thickness of 0.7 mm by hot-rolling and coating-rolling processes according to the normal process was used as a test material.

This sample material was heated to 780°C, which corresponds to the temperature range between the AC process and Ac8 transformation points, which is the so-called α and γ two-phase structure region, and after holding for 1 minute, it was determined that the temperature was below the Ar process transformation point temperature. When the r value was measured after cooling to 600°C at various cooling rates and then air cooling to room temperature, it was found to be the curve shown in Figure 1, where the horizontal axis is the average cooling rate V up to 600°C. As shown, it has been found that the lower the value of the average cooling rate V, the more remarkable the improvement in the r value is brought about.

In view of this unexpected result, a cold-rolled thin steel sheet with a carbon content of 0.7 mm and having the components shown in Table 1 below, which has a lower carbon content than the above-mentioned test steel, was used as the test steel CB), ((3) Experiments similar to those described above were conducted, except that the heating temperature was changed to 800"C and 890°C, respectively.
, CO) The results shown in each curve were obtained.

Table 1 Here, in particular, the sample steel (0) is a high-strength steel plate with P addition, and due to ultra-low carbonization, even though it has an inherently high r value, the r value exceeds the r value by about 30%. is worthy of special recognition.

As is clear from Figure 1, the reason why the r value is uniformly high and low in all the sample steels regardless of the carbon content range is that the rate of transformation from austenite to ferrite during cooling is slow. , and there have been no reported cases of this behavior to date.

(Purpose of the invention) Therefore, based on the above basic experiment, carbon content of 0.15%
It is an object of the present invention to improve drawability by improving the r value of a thin steel sheet as described below.

(Structure of the Invention) The above-mentioned test steels (AJ, CB) and CG) have r values exceeding approximately 1.2°, 1.4° and 1.7°, respectively, depending on their carbon content ranges. , de facto r
According to FIG. 1, the above cooling rate at which these r values can be guaranteed is [%0) 0.
1.6 °C/s at 15% ((A) steel), 5 °C/s at < 0.04% ((B) steel 9, and the same (4IO at 0,005%) ″C/sec or less ((C
) steel), therefore, when C indicates the carbon content (%) and V indicates the cooling rate (℃/sec), the product of both C
By setting −V to 0.2 or less, the above r value can be improved.

In other words, this invention requires heating or soaking to a temperature between AO and Aca liquid points when annealing a thin steel sheet with a carbon content of 0.15% or less, and During cooling, cooling control is performed to satisfy the conditions shown in the following formula (1) % formula % (1) C: Carbon content of thin copper plate (%) ■: Average cooling rate (07 seconds) This is a method for improving drawing workability of thin steel sheets that combines the following steps, and can advantageously solve the above-mentioned problems.

The reason why the carbon content is limited to 0.15% or less is that the absolute value of the r value is low due to the high tensile strength derived from the carbon content exceeding this, and the ductility is poor, and This is because it is not suitable as a material that requires drawability, and not only that, but also because its weldability deteriorates and its range of use is restricted.

On the other hand, when the carbon content is 0.15% or less, the r value of a thin steel sheet is about 1.0 because it has a certain tensile strength in the vicinity. It can also be used for drawing processing.
It is possible to increase the r value more than a certain degree, and in this way, the utility value of thin steel sheets with a carbon content of 0.15% or less can be expanded.

Even though ultra-low carbon thin steel sheets such as 10,005% have originally a high absolute value of r value, it is possible to reduce the reduction by further improving the r value as already mentioned for (C) steel. Adaptations advantageous to improving the situation can be achieved.

Furthermore, even in low carbon steels such as CB) steels, it is possible to bring about an increase in r value that is almost comparable to the lowest value of CO)@4, which is an extremely low carbon steel.

The improvement of r value by this invention requires 0.15% or less of carbon, but the other alloy components are 811.20%,
In 1.20%, PO, 11%, A#
0.05%.

Nb O, 05%, T soil 0.08%,
BO,005%, NO,006%, V
It can be provided according to the required properties such as tensile strength by entraining O, 01% or less.

Next, during annealing, heating or soaking to a temperature between the Acm and Ac8 transformation points of the steel is done to reduce the risk of ILrz transformation from the α and γ structural regions, according to the results of the above experiments. By reducing the ferrite transformation rate by slow cooling directly below, especially by specifying the cooling rate that depends on the carbon content, the above-mentioned noteworthy r The value can be improved,
Therefore, the heating or soaking temperature range for annealing must be within the above range.

Next, the end temperature of slow cooling after annealing under solar heat can bring about the r-value improvement mentioned above only by slow cooling in the above two coexisting temperature ranges, and slow cooling to just below the Ar□ transformation point. The temperature is limited to 600°C, taking into account changes in the Ar□ transformation point depending on the steel composition and cooling rate.

The relationship between the cooling rate and carbon content necessary to obtain a high r value for a thin steel sheet has already been described.

The above description has been made based on experiments to improve the r value of cold-rolled thin steel sheets, but the effects of the present invention can be similarly expected for hot-rolled thin steel sheets, as will be described in Examples below.

(Example) Next, converter steel having the composition shown in Table 2 was continuously cast in a normal process and then hot-rolled into a hot-rolled thin plate (sample steel boiling 8.16), which was further cold-rolled. Cold-rolled thin plate (sample steel Al~7.
A9 to 15) were annealed under the soaking conditions and cooling conditions listed in Table 2, and then the r value of each specimen was measured and shown in the same manner.

In the same table, the test steels A 8 and 16 are simply annealed hot rolled steel sheets, and the r value of normal hot rolled steel sheets is A 16.
As shown in 1. ],, about 0, but by annealing according to the method of the present invention, it is possible to obtain an r value of about ρ1.2 as shown in A8, and the drawability can be improved.

Test copper metals 1 to 7. Boiled 9 to 11 are made from ultra-low carbon steel C: 0
．． Comparative copper metals 13 to 15 which deviate from the regulation according to the present invention have r values when cold-rolled steel sheets of up to 15% are cooled to 600'O at a cooling rate that conforms to the above-mentioned formula (1).
It is shown in comparison with 12. As shown in the same table, by selecting the cooling rate v2 according to the amount of C so as to satisfy the requirements for the heating temperature range and the ultimate cooling temperature and to conform to equation (1), the composition of components other than C can be reduced to hot rolled steel sheets. By applying this to continuous knot annealing or box annealing, the r value can be greatly improved and excellent drawing properties can be obtained.

[Brief explanation of drawings]

FIG. 1 is a graph showing the influence of the average cooling rate from the heating temperature of the steel sheet to the Ar transformation point temperature on the r value of cold rolled thin steel sheets having different C contents. Patent applicant: Kawasaki Steel Corporation

Claims (1)

[Scope of Claims] L: Heating or soaking to a temperature between AC process and Ac8 transformation points when completely annealing a thin steel plate with a carbon content of 0.15 weight or less, said heating or soaking. A method for improving drawability of a thin copper plate, which combines the following steps: performing cooling control that satisfies the conditions shown in the following formula while the temperature reaches 600°C during subsequent cooling. C-V ≦ 0.2 C: Carbon content of thin steel plate, (wt%) V = average cooling rate, (0/sec)