JPS6240319A - Manufacture of steel sheet having superior deep drawability by continuous annealing - Google Patents

Abstract

PURPOSE:To manufacture steel sheets having superior deep drawability in a high yield by subjecting an Al killed steel having a spacified composition contg. a very small amount of B and a reduced amount of Al to hot rolling, coiling at a low temp., cold rolling and continuous annealing. CONSTITUTION:A steel slab having a composition consisting of <=0.05% C, <=0.8% Si, <=1.0% Mn, <=0.10% P, <=0.05% S, <0.010% acid-soluble Al, <=40ppm N, 2-<30ppm B, one or more among 0.03-0.15% Cr, 0.01-0.15% V, 0.02-0.16% Mo and 0.02-0.30% W and the balance Fe with inevitable impurities is hot rolled, coiled at 400-<700 deg.C, descaled and cold rolled at >=40% draft. The resulting steel sheet is continuously annealed as usual. By this method, a steel sheet having superior deep drawability can be manufactured at a low coiling temp.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a thin steel sheet with excellent deep drawability by continuous annealing.

[Conventional technology]

Known techniques for producing cold-rolled steel sheets for deep drawing by continuous annealing using low carbon aluminum killed steel can be roughly divided into two types.

The first method is a so-called high-temperature winding method in which the winding temperature after hot rolling is set to a high temperature of 700° C. or higher.

This is due to cementite aggregation due to high temperature winding and A
This is aimed at the effect of precipitation and coarsening of RN. However, with the high temperature winding method, a thick oxide film is formed on the surface of the steel sheet after winding, resulting in a marked drop in pickling properties, and the material inside the coil cannot be improved because the quality of the inner and outer circumferential parts of the coil, where the cooling rate is high, cannot be increased. The uniformity is extremely poor, and the yield is greatly reduced. JP-A No. 58-37128 discloses a method of improving the material uniformity inside the coil by controlling the cooling rate by inserting the coil into a heat retention furnace or attaching a heat retention cover after high-temperature winding. However, in this case as well, problems arise, such as a decrease in pickling properties, a complicated process, and an increase in costs.

On the other hand, the second method is based on low-temperature winding. As a method corresponding to this,
As typified by Publication No. 6, a method of adding B to aluminum-killed steel, a method of coarsening grains after annealing by reducing the amount of Al and limiting the hot rolling reduction rate and cold rolling reduction rate (Japanese Patent Application Laid-open No. 58
-104124), a method using B-added aluminum killed steel and limiting the heating temperature and finishing temperature during hot rolling (Japanese Patent Application Laid-open No. 117834/1983), etc. are known. However, upon detailed investigation by the present inventors, it was found that with these methods, relatively large grains can be obtained, yield strength is relatively low, and elongation is quite high; The r value, which is an index of drawability, is only low compared to the case of high-temperature winding. Therefore, although it can be adopted as a simple manufacturing technique for soft steel sheets, it is not satisfactory as a manufacturing method for deep drawing steel sheets.

[Problem that the invention seeks to solve]

As mentioned above, in the conventional method, sufficient deep drawability (r
High-temperature winding is indispensable in order to obtain this value), but this method has several problems such as a decrease in yield and a decrease in pickling properties. Under these circumstances, when manufacturing a deep drawing steel plate by continuous annealing using aluminum killed steel, there is a strong desire to develop a method for lowering the winding temperature after hot rolling.

That is, the problem to be solved by the present invention is that in a method of manufacturing thin steel sheets with excellent deep drawability by continuous annealing using low carbon aluminum killed steel as a raw material, the coiling temperature after hot rolling must be raised. The disadvantage is that it does not.

[Means for solving problems]

The gist of the present invention is as follows.

(1) C: 0.05% or less, Si: 0.8% or less.

Mn: 1.0% or less, P: O, 10% or less -8ho.

0.05% or less, acid-soluble Al: less than 0.010%, N: 40
ppm or less, B: 2 ppm or more and less than 30 ppm, and C
r: 0.03% or more and 0.15% or less, V: 0.01-0
．． 15%, Mo: 0.02-0.16%.

W: A steel containing one or more of 0.02 to 0.30%, the rest consisting of Fe and unavoidable impurities, is hot rolled according to a conventional method at a temperature of 400°C or more and less than 700°C. 1. A method for producing a thin steel sheet having excellent deep drawability by continuous annealing, the method comprising: first winding, then cold rolling at a reduction rate of 40% or more, and continuous annealing.

(2. In the invention of claim (1), a method for manufacturing a thin steel sheet with excellent deep drawability by continuous annealing, characterized in that the C content range is less than 0.02%.

In order to obtain good material quality, especially deep drawability (r value) when producing aluminum killed steel by continuous annealing, it is necessary to make coarse cementite agglomerates during coiling after hot rolling, and to The important point is how to coarsely precipitate the N inside as large-sized precipitates and render them harmless.

As a result of detailed study on a method for manufacturing thin steel sheets with excellent deep drawability without using high-temperature coiling, the present inventors found that adding a small amount of B to ordinary low carbon steel, AΩ
A novel finding that high r-values can be obtained even at low temperatures only when three conditions are simultaneously met: significantly reducing the amount added and adding a small amount of carbide-forming elements that have a relatively weak tendency to form carbides. The present invention was completed based on this knowledge.

Specifically, first, by adding a small amount of B, N in steel can be precipitated as BN.

In low carbon aluminium-killed steel, N precipitates as AlN, but this AlN precipitates at a low ratio to the total N amount at low coiling temperatures, and precipitates extremely finely during annealing, suppressing grain growth.

This causes deterioration of deep drawability. Since BN has a greater tendency to form precipitates with N than AlN, the precipitation initiation temperature is higher, and coarse precipitates occur even during low-temperature winding, making it possible to eliminate the adverse effects of N.

The second necessary factor is to significantly reduce the amount of Al added at the same time as adding a small amount of B. As a result, even during low-temperature winding, the cementite in the hot-rolled sheet aggregates and precipitates into coarse lumps, resulting in a high r value after cold-rolling and annealing.

The third necessary factor is to simultaneously add a small amount of a carbide-forming element that has a relatively weak tendency to form carbides. This combined addition of carbide-forming elements promotes coarse aggregation of cementite during low-temperature winding.

Although the details of the mechanism by which this phenomenon occurs only when the addition of a small amount of B, a significant reduction in the amount of Al added, and the addition of a small amount of carbide-forming elements are simultaneously satisfied are unknown, the present inventors have proposed such a new method for low carbon steel. I found some insight. Based on this knowledge, the present inventors have completed a method for manufacturing a thin steel sheet with excellent deep drawability by low-temperature winding and continuous annealing using low carbon steel.

The above phenomenon of the present invention becomes even more remarkable when the C content in the steel is less than 0.02%. This is C
By setting the content to less than 0.02%, C completely dissolves in the α phase to pass through the α single phase region during cooling after hot rolling, and then coarsely precipitates at the ferrite grain boundaries. by.

A known technique for adding carbide-forming elements such as B and Cr is disclosed in Japanese Patent Publication No. 60-24173.

JP-A-59-123720, JP-A-58-486
There are publications such as Publication No. 34. The present invention differs from conventional known technology in the following points.

The publicly known technology of Japanese Patent Publication No. 60-24173 is 11B-C
The pickling properties of scale are improved by the combined addition of r,
This prevents uneven precipitation of flN. That is, the significance of adding B and Cr is fundamentally different from that of the present invention. In addition, the amount of Al1 in the steel in the known technique is only specified from the viewpoint of reducing ○ and ensuring the yield of B. Therefore, acid-soluble Al is one of the essential factors for coagulating cementite during low-temperature winding.
This is essentially different from the present invention in which a reduction in is defined.

The acid-soluble Al described in Table 2 and Examples in that patent is in a significantly higher range and different compared to the present invention. Furthermore, since the significance of the B-Cr composite addition in the patent is different from the significance of the addition of B and (carbide forming elements such as Cr) in the present invention, the amount of B added in the patent is in a higher range than in the present invention. .

In JP-A-59-123720, Nb.

Cr, Ti, and B are added for slab equiaxed crystallization, and are fundamentally different from the present invention. B,
Regarding the addition of Cr, there is no mention of the necessity of adding both in combination, nor does it exist in any of the Examples. Furthermore, since Al is effective in fixing N, a lower limit is set, which is completely different from the present invention.

The technology disclosed in Japanese Patent Application Laid-open No. 58-48634 is basically:
This shows that the properties of steel sheets are improved by lowering P and N.
This is different from the present invention, which is based on coarsening of carbides by adding carbide-forming elements such as B and Cr and reducing acid-soluble Al. Regarding Afl, it is said that 0.010 to 0.040% is preferable, and there is no component corresponding to the present invention in the examples.

None of the above-mentioned disclosed technologies satisfy all of the above three requirements of the present invention, and cementite is finely precipitated during low-temperature winding, resulting in a low r value.

[Effect]

The following five aspects of the present invention will be explained in more detail.

The reason for limiting the chemical components is as follows.

C is an element that determines the amount of cementite, and is 0.05%
If it exceeds 0.05%, the r value tends to decrease due to an increase in the amount of coarsely precipitated cementite itself, so the upper limit is set at 0.05%. The most desirable range is a C content of less than 0.02%. This is because C passes through the α single phase region during cooling after hot rolling, so that C is completely dissolved in the α phase, and then carbides are coarsely precipitated at the ferrite grain boundaries.

Si is an effective element for increasing strength.

If the content exceeds 0.8%, chemical conversion treatment properties and hot-dip galvanizing properties will deteriorate, so the upper limit is set at 0.8%.

Mn is also an effective element for increasing strength, but at 1.0
If the amount exceeds 1.0%, addition is economically undesirable, and the upper limit is 1.0%.

P can be used effectively to increase strength most efficiently, but if it exceeds 0.1%, the risk of secondary processing embrittlement increases, so 0.1% is the upper limit.

S is precipitated and fixed as MnS, but if the content increases, an unnecessary increase in the amount of Mn added will result, so 0.05%
The following shall apply.

Al is an important component of the present invention, and if it is 0.010% or more, only a low r value can be obtained in low-temperature winding, similar to conventional low carbon aluminum killed steel, so it is set to less than o, o io%. As shown in the Examples below, the most desirable range is less than 0.005%.

Since N tends to slightly deteriorate the material quality due to an increase in the amount of precipitated nitrides, it is set at 40 ppm or less.

B is an important component of the present invention, and if it is less than 2 ppm, there is no effect on eliminating the adverse effects of N or on coarse aggregation of cementite, so the lower limit is set at 2 ppm.

Moreover, at 3Qppm or more, the effect is similar to that of conventional B-added aluminum killed steel, and coarse agglomeration of cementite does not occur, and the content is less than 30ppm. As is clear from the examples, the most preferable range is 2 PPm or more and less than 20 PPm.

Cr, V, Mo, and W are each 0.03%.

If the content is less than 0.01%, 0.02%, and 0.02%, the effect of coarsening cementite, which is a feature of the present invention, will be lost, so 0.03% and 0.01%, respectively.

0.02%, 0.02% as the lower limit, 0.15 respectively
%, 0.15%, 0.16%, and 0.30%, the deep drawability deteriorates, so 0.015% and 0.015%, respectively.
The upper limits are 15%, 0.16%, and 0.30%. Cr,
It is necessary to contain at least one kind of V, Mo, and W,
Even if two or more types are contained, the effect remains the same.

The other components are Fe and unavoidable impurities.

The molten steel whose chemical composition has been adjusted to the above range is made into a slab by continuous casting or ingot forming. As the hot rolling method, either a method in which the slab is cooled to a temperature below the Ar3 transformation point or a method in which the slab is directly hot rolled as it is cast is possible. There is no need to particularly specify conditions such as the temperature history and rolling reduction of the hot rolling process. There is no upper limit for the winding temperature from the viewpoint of material properties, but pickling performance decreases at temperatures above 700°C, so the winding temperature should be lower than 700°C, and 400°C or higher.
At a lower temperature, cementite becomes difficult to coarsely aggregate and the deep drawability deteriorates, so the temperature is set at 400°C or higher. The hot rolled coil obtained by this method is descaled and then cold rolled.

Since a sufficient r value cannot be obtained at a pressure rate of less than 40%, the lower limit of the cold rolling rate is set at 40%.

Next, recrystallization annealing is performed, and the annealing method is continuous annealing. As continuous annealing, any process for manufacturing various types of cold rolled steel sheets, galvanized, tin plated, chrome plated, etc., is possible. As a condition for continuous annealing, there is no need to specify the annealing temperature as long as the steel plate temperature is equal to or higher than the recrystallization temperature, and over-aging treatment for the purpose of reducing the aging properties of the steel plate may be performed as necessary. . Skin pass rolling after annealing.

Antirust treatment, application of lubricant, etc. may be performed as necessary.

〔Example〕

Steel with the components shown in Table 1 is melted, made into a slab by continuous casting, heated to a temperature range of 1000 to 1250°C, then hot rolled at a temperature of 890°C or higher,
It was wound up at a temperature of 50°C to form a coil, and after pickling, it was cold rolled at a rolling reduction of 80%.

Next, continuous annealing was performed, including soaking at (600° C. to 775° C.) for -60 seconds and subsequent overaging treatment at 350° C. for 3 minutes, to obtain a cold rolled steel plate. At the stage of hot-rolled coils, the pickling properties were measured, and the r value was also measured. Acid washability was evaluated by immersing the sample in 8% hydrochloric acid (80°C) and measuring the time until the scale was completely peeled off. The manufacturing conditions and measured values are shown in Table 2, and Figure 1 shows the effects of Al addition amount, B addition amount, and C addition. The manufactured steel sheet has a high r-value despite being rolled at a low temperature of less than 700°C, and there is no deterioration in pickling properties associated with high-temperature winding.

〔Effect of the invention〕

According to the present invention, it is possible to produce thin steel sheets with excellent deep drawability at a lower coiling temperature than conventional ones, and problems such as lower yields and lower pickling properties associated with high-temperature coiled materials are resolved. It is obvious that it can be done, and it is extremely advantageous.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the effects of B, Cr, and soluble AIC. Patent Applicant Nippon Steel Corporation Representative Patent Attorney Yasuku Furushima 1■

Claims (2)

[Claims] (1) C: 0.05% or less, Si: 0.8% or less, Mn
: 1.0% or less, P: 0.10% or less, S: 0.05%
Below, acid-soluble Al: less than 0.010%, N: 40ppm
Below, B: 2 ppm or more and less than 30 ppm, and Cr: 0
．． 03% or more and 0.15% or less, V: 0.01-0.15
%, Mo: 0.02-0.16%, W: 0.02-0.
A steel containing one or more of 30% and the rest consisting of Fe and unavoidable impurities is hot rolled according to a conventional method, coiled at a temperature of 400°C or more and less than 700°C, and then rolled at a rolling reduction of 40°C. A method for producing a thin steel sheet with excellent deep drawability by continuous annealing, characterized by cold rolling at a rolling temperature of 10% or more and continuous annealing. (2) The method for manufacturing a thin steel sheet with excellent deep drawability by continuous annealing according to claim 1, characterized in that the C content range is less than 0.02%.