JPS6148531A - Manufacture of hot-rolled low-carbon steel sheet having superior deep drawability - Google Patents

Abstract

PURPOSE:To obtain a hot-rolled low-C steel sheet having superior deep drawability at a low cost by rolling a low-C Mn steel slab at a specified total draft in a specified rolling time in the final stage and carrying out finish rolling at the Ar3 point or below and coiling at a regulated temp. CONSTITUTION:A steel slab contg. <0.04wt% C and <0.5wt% Mn as principal components is hot rolled at >=40% total draft. At this time, rolling in the final stage or the preceding stage is carried out in <=0.3sec. Finish rolling is finished at the Ar3 point - the Ar3 point -50 deg.C, and the resulting steel sheet is quenched and coiled at 550-700 deg.C. By this method, the slab can be taken out at a low temp., and a hot rolled steel sheet having high deep drawability can be manufactured as a substitute for a cold rolled steel sheet without requiring any special element.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a thin hot-rolled steel sheet for processing with excellent deep drawability.

(Conventional technology) Conventionally, cold-rolled steel sheets have been generally used as thin hot-rolled steel sheets for processing, but since cold-rolled steel sheets are expensive, recently thin hot-rolled steel sheets have been used to replace cold-rolled steel sheets. However, there has been an increasing desire to reduce costs.

However, in order to maintain workability, hot-rolled steel sheets for processing are generally rolled at temperatures above the Ar3 transformation point to avoid the formation of a processed ferrite structure. is randomized, and the deep drawing properties of hot-rolled steel sheets are significantly inferior to those of cold-rolled steel sheets.

According to Japanese Patent Application Laid-Open No. 51-59718, in order to improve the r value (Lankford value, which indicates deep drawing properties) of a hot rolled sheet, the final rolling reduction is increased above the Ar5 transformation point, and 2
It states that 0% or more is necessary.

Furthermore, in order to increase the r value of hot-rolled sheets, it is necessary to
There is also a warm rolling method using precipitates such as AI-N as described in Japanese Patent No. 93836.

(Problems to be Solved by the Invention) In the method described in JP-A-51-59718, the texture is randomized due to the Ar3 transformation that occurs during cooling after rolling, so the r value of the hot rolled sheet is is 1.1 or less. Furthermore, since low carbon materials have a high A r s transformation point, high temperature extraction is required, resulting in high costs. In addition, the method described in JP-A No. 59-93836 has problems such as a large motor load on the rolling mill due to the very low temperature of rolling, and high costs due to At, etc. required for precipitates. There are drawbacks. The present invention has been made to solve these problems.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention implements high-temperature extraction at a finishing temperature of Ar5 50°C to Ars transformation point without using any special additive elements. without having to.

The purpose is to manufacture thin hot-rolled steel sheets with good deep drawing properties as a cold-rolling alternative material.

That is, in terms of weight ratio, C: less than 0.04 cm, Mn: 0
．． When hot rolling a steel slab whose main component is less than 5 mm, the final stage of finish rolling or the final stage of 91
Rolling from the front stage to the final stage rolling within 0.3 seconds,
A method of deep drawing strength characterized by rolling at a total rolling reduction of 40 inches or more, finishing finish rolling at 50°C to 50°C, and then winding at 550 to 700°C. This is a method for manufacturing thin hot-rolled steel sheets of good quality.

(Function) The above-mentioned constituent elements of the present invention are limited based on the respective functions that are lacking.

C: less than 0.04% is because if the carbon content exceeds this value, workability will deteriorate and elongation required for a thin steel plate for processing will not be obtained. At the same time, if more carbon is contained, the amount of austenite remaining in the structure immediately after rolling will increase, and if the subsequent cooling rate is fast, hard phases such as bainite will be formed, which will also affect workability. make things worse.

The reason why Mn is less than 0.5% is the same as the reason for limiting the amount of carbon described above, in order to prevent solid solution strengthening and generation of a hard phase.

Note that the lower the lower limits of both C and Mn, the better. The reason is,
This is because the steel sheet for processing is required to be soft, and the less C and Mn there are, the less solid solution strengthening and the generation of hard phases occur, and the steel sheet becomes softer. Also, deep drawing! It is known that the higher the (111) plane strength and the lower the (100) plane strength, the more effective the texture for improving the ll property is in increasing the r value. When recrystallization of ferrite occurs in high-purity steel, (iii) planes preferentially develop from grain boundaries. From this point of view, the lower the amount of CrMn, the better, preferably C: 0.01w
t% or less and Mn: 0.2wt% or less is manufactured under the manufacturing conditions of the present invention, a value of 1.1 or more can be obtained.

Therefore, Ti, Nb, etc. can replace solid solution C, N in steel materials.
(C,N), Nb Since it is captured by (C,N), it is effective for high purification, and may be added as appropriate.

We used low carbon copper ia (C<0.04%,
Mn < 0.5%), the final stage rolling or the rolling up to 91 stages before the final stage is within 0.3 seconds,
As a result of experiments by changing the total rolling reduction and final finishing rolling temperature, as shown in Fig. 1, the finishing rolling temperature was Ar3-
50℃~Ar3, 7 value ≧ 1.0 when the total rolling reduction rate up to the final stage of finishing or 91 stages before the final stage is 40 inches or more
It was discovered that a hot-rolled steel sheet having a diameter of 5 can be manufactured. In addition,
'Ar3 transformation point was determined by the following formula.

Immediately after the rolling was completed, the material was rapidly cooled and the microstructure was observed. As shown in Figure 2, the microstructure of the product rolled under the production conditions of the present invention was fine grains with a grain size number of 9.0 or higher on the entire surface. Ta. (1,...Figure 2, numbers shown in the figure are shown.

Same below. ) As shown in FIG. 3, this entire surface fine ferrite grain structure has an i value of 1.05 or more after winding, and has good deep drawing properties. However, when rolling is performed above the 1Ar5 transformation point in FIG. 2, recrystallization of austenite grains occurs, but a random texture is formed due to the r→α transformation. (2,5) Furthermore, when rolled below the Ar5 transformation point, the temperature range in the α+r two-phase region is narrow because it is a low carbon steel, and the rolling is mostly in the ferrite region.

This is why the reduction rate was low! D (4) When the rolling temperature is low (3,4), the ferrite is not completely recrystallized and remains as processed ferrite with high (100) plane strength. For this reason, an f value of 1.0 or more cannot be obtained.

The manufacturing conditions of the present invention, that is, the finish rolling temperature is Ar3
50℃~Ar3, finishing final stage rolling, or rolling from one stage before the final stage to the final stage within 0.3 seconds, with a total reduction rate of 40 inches or more, due to the high reduction rate and high finishing temperature. Recrystallization of ferrite was caused, and fine ferrite as shown in FIG. 2 (1) was obtained immediately after rolling, and the high r value shown in FIG. 1 was obtained by subsequent rolling treatment.

In other words, for the above reasons, the final stage or final stage 9
The total rolling reduction from the previous stage to the final stage was 40% or more, and the finishing temperature was Ar5 50°C to Ar3.

However, in actual rolling, the rolling reduction rate in the final two stages is 60 in total.
% is the limit.

It is preferable that the rolling reduction ratio in the final stage is 40% or more in one stage, but when continuous rolling is performed at high speed, strain may accumulate. Therefore, the present inventors investigated the relationship between final stage rolling amount time and microstructure for low carbon steel (C<0.0
4 wt%, Mn < 0.5 wt%), the total rolling reduction is 40% or more, and the finishing temperature is Ar3.
As a result of rolling at 50°C to Ar5, rapid cooling, and microstructural observation, as shown in Figure 4, the rolling amount time was
It has been found that fine ferrite grains with a grain size number of 9.0 or more can be obtained within 0.3 seconds. For this,
If the rolling amount time is within 0.3 seconds, the effect on the high r value is not impaired by the cumulative effect of strain.

In addition, the reason why the winding temperature range is 550 to 700°C is that as shown in Figure 3, at a winding temperature lower than 550°C, the fine ferrite grains obtained immediately after rolling are deep drawn. This is because it is not possible to grow ferrite grains with particle size numbers 6 to 8, which have good properties and processability.

The reason why the upper limit is set at 700° C. is that if the winding temperature is higher than this, grain growth will proceed, producing giant grains and causing rough skin.

In addition, if the amount of C is high during cooling from the finishing rolling temperature to the winding temperature, hard phases such as bainite and martensite may be generated, but the components of the present invention are
It is made of low carbon material of 0.04% or less, and the finishing temperature is Ar
Since the transformation point is 3 or lower, most of the particles are ferrite grains, so no hard phase is generated regardless of the cooling rate.

(Example) Next, the present invention will be explained in detail based on examples.

Rolling and winding were performed using steel having the chemical composition shown in Table 1, and the results obtained are shown in Table 2. The grain size immediately after rolling of the steel sheets indicated by ■■■■■ in Table 2, which were manufactured under the conditions within the scope of the present invention, is all grain size number 9.0.
If the grain size is 6.0 to 8.0 after winding, the hot-rolled sheet will have a high value of 1.05 or more. Also, the growth is maintained at over 42 inches. Compared to this,
If manufactured under conditions outside the scope of the present invention, as shown in ■■, the grain size after rolling will increase or become processed ferrite, the r value will be less than 1.0, and the f value ≧1. .05 could not be satisfied.

(Effects of the Invention) If Miko hot-rolled steel sheets are manufactured according to the present invention described above, a material quality equivalent to that of cold-rolled steel sheets can be obtained without using a cold rolling process, and moreover, it can be manufactured by finish rolling below the Ar3 transformation point. for,
The slab extraction temperature may be low. Therefore, a large cost reduction can be expected, and the economic effect is extremely large.

[Brief explanation of drawings]

Figure 1 shows the influence of the finishing temperature and the total rolling reduction in the final stage, which is less than 1 value, and Figure 2 shows the influence of the finishing temperature and the total rolling reduction in the final stage on the ferrite grain size number immediately after rolling. Figure 3 is a diagram showing the relationship between the ferrite grain size number and Y value after the winding process is completed, and Figure 4 is a diagram showing the influence of the final rolling amount time on the ferrite grain size number immediately after rolling. It is a diagram showing the influence.

Claims (1)

[Claims] When hot rolling a steel slab whose main components are C: less than 0.04% and Mn: less than 0.5% in terms of weight ratio, the final stage of finish rolling, or one stage before the final stage to the final stage. Rolling to rolling within 0.3 seconds, total reduction rate of 40%
Roll it so that it is above, Ar_3-50℃~Ar_3
A method for producing a thin, low-carbon hot-rolled steel sheet with excellent deep drawability, characterized in that finish rolling is completed at 550 to 700°C, and then rolling is performed at 550 to 700°C.