JPH0676619B2 - High-strength steel sheet manufacturing method and heat treatment method of processed products thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a steel sheet suitable for high-strength members for automobiles or industrial equipment, specifically, it has a relatively low strength until it is subjected to forming. The present invention relates to a method for manufacturing a high-strength steel sheet for working which is easy to process and has high toughness which is enhanced by appropriate heat treatment after working, and a heat treatment method for a worked product.

(Prior Art) The so-called hot rolled steel sheet produced by continuous hot rolling is
As a relatively inexpensive structural material, it is widely used in various industrial equipment such as the above-mentioned automobile. Many members are formed by press working for their applications, and therefore hot rolled steel sheets are often required to have excellent workability. On the other hand, the structural member is also required to have high strength, but to achieve both high strength and excellent workability,
Usually difficult.

Therefore, various materials have been developed which have low strength and good workability in the material stage before processing, and which have high strength by appropriate heat treatment after processing. For cold-rolled steel sheets, so-called bake-hardening high-strength steel sheets, which are soft before processing and easy to work, and which increase in yield strength by hardening during baking coating after processing, have already been put into practical use. Recently, bake hardening type hot rolled steel sheets have been studied, and patents have been filed.

For example, Japanese Examined Patent Publication No. 62-180021 discloses a method for producing a bake hardening type high strength hot rolled steel sheet, wherein a steel having a specific chemical composition containing a large amount of N is quenched after hot rolling. . This method obtains bake hardenability by utilizing the strain aging of solid solution N, but according to the experimental results of the inventors of the present application, the bake hardenable high strength hot rolled steel sheet obtained by this method is Although the yield strength after baking is greatly increased,
The increase in tensile strength was slight. The reason is solid solution N
It is considered that this is because the main strengthening mechanism is the fixation of dislocations due to, and the strengthening by precipitates has not been reached.

When the increase in tensile strength is small, the improvement in fatigue properties is small. It is reported that the fatigue property has a strong correlation with the tensile strength, and the fatigue property increases as the tensile strength increases (Hirakawa et al., Sumitomo Metals Vol.33 (1981) No.4).
P.121).

Therefore, if the increase in tensile strength is small, the effect of improving the fatigue properties required for high strength members for automobiles and industrial equipment, which are the main applications of these steel sheets, is small, and the practical value becomes poor.

(Problem to be Solved by the Invention) The problem to be solved by the present invention is that before processing, it is soft, and by reheating at a relatively low temperature after processing, the tensile strength effective in improving fatigue properties increases significantly. It is intended to provide a method for producing a high-strength steel sheet for processing having good toughness, and an optimal heat treatment method for a processed product using the steel sheet as a raw material.

(Means for Solving the Problems) The present inventors have conducted continuous hot rolling under the conditions of continuous hot rolling to form a steel sheet by continuously hot rolling a steel slab having a specific chemical composition containing one or both of Ti and V. It was found that the above problems can be achieved by adjusting That is, if the finishing temperature of hot rolling is high and the coiling temperature is low to suppress precipitation of Ti and V, solid solution Ti
And a steel plate with a large amount of solid solution V is obtained. This steel sheet is soft before being worked, and solid solution Ti and solution V are precipitated at a relatively low temperature after working to enhance the strength.

Furthermore, if the final pass of finish rolling is set to a large reduction and the steel sheet is rapidly cooled immediately after rolling, a fine ferrite structure is formed and the toughness is improved.

The present invention is based on such findings, and the gist thereof is (i) and (ii) below.

(I)% by weight, C: 0.005 to 0.060%, Si: 1.20% or less, M
n: 1.5% or less, one or both of Ti and V in total 0.02
~ 0.20%, P as an impurity is 0.035% or less, after heating the billet to a temperature of 1100 ℃ or more, the final pass reduction ratio of finish rolling is 30% or more, finish rolling temperature is 920 ℃ or more It is characterized in that continuous hot rolling is performed, cooling is started within 3 seconds after the completion of rolling, the material is rapidly cooled to a temperature of 700 ° C or less at a cooling rate of 20 ° C / sec or more, and then wound at a temperature of 500 ° C or less. And a method for manufacturing a high-strength steel sheet.

(Ii) A high-strength steel sheet manufactured by the method according to claim 1, after being formed into a predetermined shape, heated at a temperature of 300 ° C. or higher and 800 ° C. or lower for 1 minute or longer. Heat treatment method.

(Operation) Hereinafter, the constituent features of the present invention and the operation thereof will be described in detail.

First, the reason why the chemical composition of the steel slab and the manufacturing conditions in the method for manufacturing a high-strength steel sheet according to the first invention of the present application are limited as described above will be described together with the working effects.

C: C is an important element that improves the tensile strength by combining with Ti and V and precipitating as TiC and / or VC during reheating after working. If the C content is less than 0.005%, desired precipitation hardening cannot be expected. On the other hand, if it exceeds 0.060%, the structure before processing and reheating after processing is hardened, resulting in poor workability and the hardened portion is rather softened during reheating. Therefore, even if TiC or / and VC are precipitated, the increase in tensile strength is small as a whole. For this reason, the content of C is set to 0.005 to 0.060%. It is preferably 0.015 to 0.040%.

Si: Si is a preferred element that improves strength and ductility through solution hardening. However, if added more than necessary, the weldability will deteriorate, so the content should be 1.20% or less.

Mn: Mn is also a preferable element that improves strength and ductility by solution hardening like Si. However, if the content is more than necessary, it promotes the formation of a low-temperature transformation structure such as hard martensite or bainite mainly composed of non-diffusion transformation, and the same problem as when the content of C is too large occurs. The amount. It is preferably 1.0% or less.

Ti and V: Ti and V are the most important elements in the present invention,
The content is 0.02 to 0.20% in total of one kind or two kinds.

Ti and V combine with C at the time of reheating after processing, and TiC, V
It precipitates and hardens as (C, N), but the total content is
If it is less than 0.02%, the amount of Ti and V that form a solid solution is small, and the desired precipitation hardening cannot be obtained. On the other hand, the total amount is 0.20%
If the amount is larger, the steel sheet is hardened by the low temperature transformation strengthening, and the same problem occurs as when the contents of C and Mn are too large.

The steel slab of the raw material in the first invention of the present application may be any as long as it contains at least the element in the above range. If necessary, 0.10% or less Nb, 0.0100% or less Ca, 0.0030%
A steel slab containing one or more of the following B may be used.

Nb has an effect of accelerating precipitation hardening auxiliaryly, Ca has an effect of improving workability, and B has an effect of improving resistance to cracking by being endured.

It is preferable that the amount of P as an impurity is as small as possible. In particular, since the steel sheet produced by the method of the present invention is apt to have temper embrittlement upon reheating, it is preferable to suppress it to 0.035% or less. It is more preferably 0.010% or less.

The first invention of the present application provides a steel slab having such a chemical composition,
Hot rolling is performed under the conditions described in (a) to (c) below,
The feature is that the rolled steel sheet is rapidly cooled and wound.

(A) Continuous hot rolling: In the hot rolling of the method of the present invention, at the same time as obtaining a hot-rolled steel sheet having a predetermined thickness, Ti or V in the steel is dissolved as much as possible, and Ti or V is dissolved. The purpose is to suppress the precipitation of carbon as a carbonitride. Furthermore, it aims at preventing deterioration of toughness by high temperature finishing.

For that purpose, it is necessary to subject the steel slab having the above chemical composition heated to a temperature of 1100 ° C. or more to rolling. Once, 11
Billets cooled to temperatures below 00 ° C must be reheated to above this temperature. However, so-called direct rolling can be carried out as it is, as long as it is a steel slab that is directly sent from the continuous casting or decomposition rolling step and maintains a temperature of 1100 ° C. or higher. When the heating temperature is lower than 1100 ° C, the solid solution amount of Ti and V is small, and the amount of increase in tensile strength during reheating after working is small.

The hot rolling needs to be continuous hot rolling in which the final pass reduction rate of the finish rolling is 30% or more and the finish rolling temperature is 920 ° C or more.

When the finishing temperature is lower than 920 ° C., solid solution Ti or solid solution V precipitates as carbonitrides, and as a result, the amount of increase in tensile strength during reheating after working becomes small. The preferred finish rolling temperature is 950 ° C or higher. On the other hand, if the final pass reduction rate of finish rolling is less than 30%, the toughness deteriorates because the austenite grain refining effect is insufficient. A preferable final pass reduction ratio is 40% or more.

b) Cooling after rolling: Cooling conditions are set so as to suppress the growth of austenite grains after rolling to prevent coarsening, and to refine the ferrite grains generated after transformation to improve toughness.

To do so, at least 3 after rolling the steel sheet after rolling
Starts cooling within seconds, and 700 at a cooling rate of 20 ° C / sec or more
It is necessary to rapidly cool it to below ℃.

If the cooling start time exceeds 3 seconds after the end of rolling, it is not possible to suppress the growth of austenite grains, and
When the cooling rate is slower than 20 ° C./sec or when the cooling end temperature is higher than 700 ° C., the ferrite grains formed after transformation become coarse and the toughness decreases. The preferable cooling start time is within 2 seconds after the end of rolling.

The higher the cooling rate is, the more preferable it is. Therefore, the upper limit is not particularly limited. The upper limit rate of cooling that can be industrially performed is about 100 ° C / sec.

c) Take-up: The take-up condition is determined so as to sufficiently secure the amount of solid solution Ti and the amount of solid solution V required to bring about an increase in tensile strength upon reheating after forming.

For that purpose, it is necessary to wind the cooled steel plate at a temperature of 500 ° C. or lower. At a winding temperature of more than 500 ° C., solid solution Ti and solid solution V are precipitated, and the increase in tensile strength during reheating after working is small.

As described above, the steel sheets manufactured under the conditions (a) to (c) are soft and easy to work since they have a large amount of solid solution Ti and V in the state of being subjected to processing, and solid solution Ti is dissolved by reheating after processing. And the solid solution V precipitates as TiC and / or VC and is hardened, so that the tensile strength greatly increases.

To remarkably increase the tensile strength, the reheating is preferably performed under the heat treatment conditions of the second invention of the present application. That is, after processing the high-strength steel sheet manufactured by the above method into a predetermined shape,
1 minute or more at a temperature of 300 ° C to 800 ° C, preferably 5
Heat more than a minute.

If the heating temperature is lower than 300 ° C, solid solution Ti and solid solution V are not precipitated, and if the heating time is shorter than 1 minute, these precipitates are not sufficiently obtained, and as a result, the tensile strength does not increase at all. Little rise. Further, at a heating temperature exceeding 800 ° C, thermal distortion occurs in the processed product.

This reheating can be done locally or totally depending on the application of the processed product. Moreover, the heating means is not particularly limited. For example, high-frequency induction heating can be used for heating the whole, and a gas burner can be used for heating locally.

According to the knowledge of the present inventors, the processed product is preferably processed by 5% or more. A precipitation site is introduced by processing, and the tensile strength is more effectively increased in a short time.

Next, the present invention will be further described with reference to examples.

(Example) Steel having a chemical composition shown in Table 1 was melted in a laboratory 50 kg vacuum melting furnace and hot forged, or a slab having a thickness of 60 mm was manufactured by a mold, and heat-treated under the conditions shown in Table 2. It was rolled and cooled, and then wound to produce a hot rolled steel sheet having a thickness of 2 mm.

The slab manufactured by hot forging was reheated to the temperature shown in Table 2 after forging, while the slab cast from the mold was kept at 1200 ° C and directly subjected to hot rolling. .

A test piece was taken from each of the obtained hot-rolled steel sheets, and the tensile strength (Ti) and elongation (El) of the hot-rolled steel sheet and pre-strain of 8% were given to the test piece, and then reheating shown in the same table. The amount of increase in tensile strength (ΔTS) and the Charpy property (vTrs) after heat treatment were measured at temperature and reheating time. The results are shown in Table 2. As is clear from Table 3, No. 1 obtained by the method of the present invention
Steel sheets No. 9 have relatively low tensile strength and large elongation when hot rolled. That is, even though it is soft and has good workability, it is possible to obtain 7-
An increase in tensile strength (ΔTS) of 17 kgf / mm ² is obtained. Also, vTrs of -40 ° C or lower are obtained.

On the other hand, rolling, cooling, winding and reheating conditions obtained under the conditions out of the range specified in the present invention are:
Like No. 10, No. 11 and No. 16, sufficient solid solution Ti and / or solid solution V cannot be obtained so that the increase in tensile strength is small, or No. 12, No. 13 and No. As in No. 14, the ferrite grains are coarsened and the toughness is poor. In addition, as in No.15, sufficient solid solution Ti and / or V cannot be obtained, and the ferrite grains are coarsened so that the increase in tensile strength is small and the toughness is poor. As in .18, sufficient solid solution Ti and / or solid solution V is not precipitated, so that the increase in tensile strength is small or one of the characteristics is inferior.

Further, those using a steel slab having a chemical composition out of the range specified in the present invention, even if the manufacturing conditions satisfy the conditions of the present invention, solid solution Ti and / / like No. 19 and No. 24. Or, the increase in tensile strength is small without the precipitation of solid solution V occurring, or
As in No. 20, No. 22 and No. 25, since the tensile strength increases greatly due to transformation strengthening while hot rolling, the transformation strengthening decreases and solid solution Ti and / or solid The contributions of the precipitation strengthening of the molten V cancel each other out, resulting in a small increase in tensile strength. No. 21 has a large increase in ΔTS, but S
As the result of the carbon dioxide arc welding test,
Arc welding failure occurred, and No. 23 had poor toughness due to high P content.

(Effects of the Invention) As described above, according to the method of the present invention, a steel sheet which is soft before processing and has excellent workability, and which has excellent toughness in which tensile strength greatly increases by heating at a relatively low temperature after processing is manufactured. be able to.

Further, the heating after processing is sufficient at a low temperature as compared with the quenching temperature adopted in the conventional quenching-strengthened high-strength steel sheet, and since it is not necessary to cool it thereafter,
Almost no distortion occurs in the processed product. Therefore, the steel sheet obtained by the method of the present invention is most suitable for high strength members for automobiles and various industrial equipment.

Claims (2)

[Claims] 1. C: 0.005-0.060%, Si: 1.20% by weight
Hereafter, Mn: 1.5% or less, one or both of Ti and V in a total amount of 0.02 to 0.20%, and a steel slab having P as an impurity of 0.035% or less are heated to a temperature of 1100 ° C. or more, and then finished. Rolling final pass reduction is 30% or more, finish rolling temperature is 920
Perform continuous hot rolling at ℃ or more, start cooling within 3 seconds after the end of rolling, quench at a cooling rate of 20 ℃ / s or more to 700 ℃ or less, and then wind at 500 ℃ or less A method for producing a high-strength steel sheet, which is characterized in that 2. A high-strength steel sheet manufactured by the method according to claim 1, after being formed into a predetermined shape, at 300 ° C. or higher, 8
A method for heat treating a processed product, which comprises heating at a temperature of 00 ° C or lower for 1 minute or more.