JPH06145782A - Production of high tensile strength steel plate - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel plate free from dispersion in quality at a low cost by hot-rolling a high tensile strength steel slab into plate and then subjecting the high-temp. hot rolled plate directly after hot rolling to hardening and tempering under specific temp. conditions. CONSTITUTION:A slab of a high tensile strength steel which has a composition containing, by weight, 0.05-0.17% C, 0.10-0.50% Si, 0.90-1.60% Mn, 0.010-0.06% V, and 0.015-0.080% Al or further containing, if necessary, Ni and Mo by 0.10-0.60% and 0.03-0.20%, respectively, or 0.01-0.030% Nb and 5-15ppm B is hot-rolled at 1000-1300 deg.C, and the resulting high-temp. hot rolled plate is directly hardened. After hardening by means of water quenching is stopped at a temp. between the martensitic transformation initiating temp. (Ms point) and the temp. higher than this initiation temp. by 50-100 deg.C, the steel plate is successively cooled down to room temp. and subjected to ordinary tempering treatment. By this method, the heat treated high tensile strength steel plate of >=600Mpa grade can be produced at a low cost, in an as-hot rolled state while obviating the necessity of a cold rolling stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel sheet having a temper of 600 MPa or more. The direct quenching method, in which steel is hot-rolled immediately after hot rolling, is being put into practical use as an energy-saving and process-saving method.
Steel material obtained by performing tempering after this direct quenching, compared with the steel material obtained by a normal reheating quenching-tempering method, the additive component is sufficiently solid-soluted in austenite, and further hardenability is improved. It is known that high strength can be imparted.

[0002]

2. Description of the Related Art In this direct quenching process, water cooling is carried out promptly after the completion of rolling. At this time, a method is adopted in which the water cooling end temperature is below the martensitic transformation start temperature (hereinafter referred to as Ms point). (See JP-A-2-270913 and JP-A-4-173920).

[0003]

However, water cooling is not possible.
If it is applied up to the Ms point or lower, there is a problem in that the following disadvantage occurs. That is, since the strength of the as-quenched steel becomes too high, the appropriate tempering temperature becomes high, and for example, the strength of a 600 Mpa-class high-strength steel is often close to the upper limit, which deteriorates the workability of the steel sheet. Since the tempering temperature becomes more sensitive to fluctuations, it becomes more susceptible to variations in furnace temperature. Since the strength of the as-quenched steel is high, the straightening effect of the in-line hot leveler is small, and steel sheets with an unstable strain shape cannot be straightened until after tempering.

Therefore, the present invention solves the above problems,
It is an object of the present invention to propose a manufacturing technology that makes the material characteristics extremely stable and high-performance after being directly quenched and then tempered.

[0005]

[Means for Solving the Problems]

According to the present invention, a steel sheet obtained by hot rolling a slab for high-strength steel is directly quenched while keeping the temperature high, water cooling is stopped immediately above the Ms point, and then air cooling is performed. After that, it is a method for manufacturing a high-strength steel sheet, characterized by performing a tempering treatment.

The method of the present invention is characterized in that the water cooling stop temperature in direct quenching is directly above the Ms point,
The respective steps prior to the direct quenching may be carried out under the conditions based on the normal direct quenching method for high-strength steel, but the procedure of the present invention including the steps prior to the direct quenching will be specifically described below.

First, it is preferable to use a material having high hardenability for the high tension slab of the present invention. For example, C: 0.05 to 0.17 wt% (hereinafter simply referred to as%), Si: 0.
10 to 0.50%, Mn: 0.90 to 1.60%, V: 0.010 to 0.06%,
Al: 0.015 to 0.080% is an essential component, and Ni and Mo are 0.10 to 0.60% and 0.03 to 0.03% depending on the plate thickness and application.
0.20%, depending on the case, Nb content is 0.010 to 0.030, B
Is added in the range of 5 ppm to 15 ppm to control the hardenability.

Here, the reason for setting the above-mentioned component composition range is as follows. C: 0.05 to 0.17% If it is less than 0.05%, the hardenability is remarkably deteriorated and high tensile steel cannot be obtained, while if it exceeds 0.17%, the weldability is deteriorated. Si: 0.10 to 0.50% 0.10% is the lower limit for killed steel, and if it is less than this, it causes internal defects such as blowholes. While 0.50
%, The weldability and heat affected zone toughness are adversely affected. V: 0.010 to 0.06% Even if a trace amount is effective for improving hardenability, if it is less than 0.010%, the effect is small, while if it exceeds 0.06%, the effect does not increase, so the range is 0.010 to 0.06%. . Al: 0.015 to 0.080% There is a grain refining effect that controls the toughness, 0.015% is the lower limit for exerting the grain refining effect, and conversely, if the content exceeds 0.08%, the effect does not increase. For Ni and Mo, the effect starts from the lower limit value, and the effect increases as the content increases, but it is an expensive component and the upper limit is determined from the economical efficiency.
Nb and B are effective even when added in a trace amount, and are less effective below the lower limit, and do not increase even if the amount exceeds the upper limit.

The slab is then heated to a temperature range of 1000-1300 ° C. That is, in order to form a solid solution of the added components
A temperature of 1000 ° C or higher is required. On the other hand, if the temperature exceeds 1300 ° C, the austenite grains become coarse, and the subsequent rolling does not promote grain refinement and deteriorates toughness.
Heating to a temperature range of 1300 ° C. is preferred.

Further, in order to obtain a steel plate having high strength and high toughness, in the hot rolling, working strain is introduced and solid solution N is added.
Is substantially zero, and it is preferable that the solid solution B is sufficiently segregated in the austenite grain boundaries and the deformation zone by maintaining the temperature in a predetermined temperature range for a predetermined time from the end of rolling to the start of quenching.

Therefore, in order to effectively introduce working strain, the rolling reduction at 900 ° C. or less and at Ar ₃ points or more is 40% or more, and the upper limit of the rolling finishing temperature is 900 ° C. If the rolling finish temperature is less than Ar ₃ points, the toughness deteriorates and the material in the sheet thickness direction becomes non-uniform. Therefore, the lower limit of the rolling finish temperature is Ar ₃ points.

The steel sheet that has been subjected to the above steps is directly subjected to quenching treatment, but it is essential that water cooling is stopped immediately above the Ms point and then air cooling is performed. Here, it is preferable that the cooling stop temperature is just above the Ms point where the martensitic transformation does not start, from the viewpoint of strength assurance, but considering the plate temperature control technology in actual operation, the Ms point to the Ms point. It is desirable that the temperature is within the range of +50 to 100 ° C.

[0014]

According to the present invention, when directly quenching a steel sheet that has been kept at a high temperature after hot rolling, water quenching is stopped just above the Ms point, whereby the quenching process is completed in the bainite transformation structure. Therefore, the strength level in the quenched state can be suppressed within a range in which the in-line straightening effect can be sufficiently obtained. Further, even if the temperature in the tempering treatment performed after the quenching treatment is variously changed, the change in strength after tempering is small, so that the appropriate temperature range can be made extremely wide.

Note that the normal tempering-free material (TMCP
By utilizing the control technology applied to steel, that is, the technology of low temperature heating and rolling in the non-recrystallization zone + rolling in the two-phase zone,
It is possible to obtain excellent toughness equivalent to or higher than that of the conventional method.

[0016]

Example Steels having the respective compositional components shown in Table 1 were melted by a vacuum melting method. Then, according to each condition shown in Table 2, slab heating, hot rolling and direct quenching were performed. Test pieces were taken from each of the steel sheets thus obtained and subjected to a tensile test. The results are also shown in Table 2.

[0017]

[Table 1]

[0018]

[Table 2]

From Table 2, the tempering temperature sensitivity in the comparative example is 40 Mpa / 30 ° C., while the steel sheet according to the present invention has a very stable characteristic of 5 Mpa / 30 ° C.

In the above embodiments, the case of 600 Mpa steel has been described, but the present invention is not limited to this, and it is needless to say that the present invention can be applied to 700 Mpa steel and 800 Mpa steel.

[0021]

According to the present invention, it is possible to widen the appropriate temperature range in the tempering process after the direct quenching, and it is possible to obtain a product with extremely small variation in material.

Claims (1)

[Claims] 1. A steel plate obtained by hot-rolling a slab for high-strength steel is directly quenched while keeping the temperature high, and water cooling is stopped immediately above the martensite transformation start temperature.
A method for manufacturing a high-strength steel sheet, which is characterized by performing air-cooling and then tempering.