JPH07242940A - Production of low yield ratio high tensile strength steel by rapid tempering - Google Patents

Abstract

PURPOSE:To produce a low yield ratio high tensile strength steel having high toughness by short time tempering treatment by subjecting a steel having a specified compsn. contg. no precipitation hardening elements such as Mb and V and added with equivalent amounts of Mn, Mo, B or the like to rapid heating and executing tempering without holding it. CONSTITUTION:A steel having a compsn. contg., by weight, 0.06 to 0.20% C, <=1.0% Si, 1.0 to 1.8 Mn, <=0.020% P, <=0.010% S, <=0.05% Al, <=0.5% Mo, <=0.03% B, <=0.02% Ti and <=0.006% N and furthermore contg. one or more kinds among <=1% Ni and each <=0.5% Cu and Cr, and the balance Fe is used. This steel is rolled, is thereafter air-cooled and is water-cooled from a two phase region of austenite and ferrite of 720 to 780 deg.C to an ordinary temp. to forme its microstructure into a mixed one of ferrite+(martensite-bainite). In this steel plate, scales are removed away. After that, it is heated to 800 to 1000 deg.C, is charged to a heat treating furnace and is heated at >=0.3 deg.C/sec temp. rising rate. Immediately after the surface temp. reaches a prescribed temp. of the A1 point -200 deg.C or above to the A1 point +50 deg.C or below, it is discharged to the outside of the furnace and is cooled to an ordinary temp. by air cooling.

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 low yield ratio, which is industrially feasible and has good toughness by a highly efficient tempering treatment.

[0002]

^2. Description of the Related Art Conventionally, most high-strength steels having a tensile strength of 580 N / mm ² or more have been manufactured by quenching and tempering. Although high strength can be obtained by such a manufacturing method,
At the same time, the yield strength was high, and the yield ratio was high. Further, since the manufacturing cost is high and the time required for manufacturing is long, there are problems such as poor productivity. For this reason, the so-called DQ method in which water cooling is performed immediately after rolling has begun to be used as a method for supplementing the conventional heat treatment method. Among the DQ methods, as a manufacturing method aiming at a high-strength steel having a low yield ratio, a technique is described in, for example, JP-A-3-264993.

However, with such a technique, the yield ratio is improved to a low level, but the toughness is not always good. In addition, since the tempering process is a conventional method, the productivity is poor and an increase in manufacturing cost is unavoidable (the conventional tempering process involves charging a steel sheet into a heat treatment furnace maintained at about 550 to 680 ° C). After the temperature of the steel sheet reaches a predetermined temperature, it is held for about 20 to 40 minutes and then air-cooled, which takes a long time to combine heating and holding).

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a technique for producing a high-strength steel sheet having a low yield ratio and good toughness by a highly efficient tempering treatment which is industrially possible. The steel produced according to the present invention has good toughness and high tensile strength with a low yield ratio, and can be applied to large buildings and the like.

[0005]

The gist of the present invention is as follows. (1) C: 0.06 to 0.20% by weight ratio, Si:
1.0% or less, Mn: 1.0 to 1.8%, P: 0.02
0% or less, S: 0.010% or less, Al: 0.05% or less, Mo: 0.5% or less, B: 0.003% or less, T
i: 0.02% or less, N: 0.006% or less, and N
i: 1% or less, Cu: 0.5% or less, Cr: 0.5% or less, one or two or more, and the balance consisting of Fe and inevitable impurities is rolled and air-cooled, and the surface temperature of the steel sheet is 720.
After removing the scale of the steel sheet from the two-phase region of austenite and ferrite of ~ 780 ° C to room temperature by water cooling and making the microstructure a mixed structure of ferrite + (martensite, bainite), the steel sheet is 800 to 1000 ° C. It was charged into a heat treatment furnace heated to 0 ° C. and heated at a temperature rising rate of 0.3 ° C./second or more, and the surface temperature reached a predetermined temperature of A ₁ point −200 ° C. or more and A ₁ point + 50 ° C. or less. After that, it is immediately taken out of the furnace and cooled to normal temperature by air cooling, which is a method for producing high tensile strength steel with low yield ratio by rapid tempering.

(2) C: 0.06 to 0.20 in weight ratio
%, Si: 1.0% or less, Mn: 1.0 to 1.8%,
P: 0.020% or less, S: 0.010% or less, Al:
0.05% or less, Mo: 0.5% or less, B: 0.003
% Or less, Ti: 0.02% or less, N: 0.006% or less, and Ni: 1% or less, Cu: 0.5% or less, Cr:
Two phases of austenite and steel with a steel plate surface temperature of 720 to 780 ° C. by recooling after cooling air-cooled or water-cooled to room temperature after rolling steel containing 0.5% or less of one or more and the balance of Fe and inevitable impurities. Heat to the area and then
A steel sheet having a microstructure of a mixed structure of ferrite + (martensite and bainite) was cooled to room temperature, the scale of the steel sheet was removed, and then the steel sheet was charged into a heat treatment furnace heated to 800 to 1000 ° C. After heating at a temperature rising rate of 3 ° C / sec or more, the surface temperature reaches a predetermined temperature of A ₁ point -200 ° C or more and A ₁ point + 50 ° C or less, and then immediately takes it out of the furnace and cools it by air to room temperature. A method for producing high-strength steel with low yield ratio by rapid tempering, which is characterized by cooling.

The basic idea of the present invention will be described below. It is said that it is effective for a high-strength steel with a low yield ratio to have a microstructure in which two phases, a soft phase and a hardened phase, coexist. As a method for producing such a steel, there is a method of reheating, heating to an austenite / ferrite two-phase coexistence region, followed by water cooling, or a method of water cooling from the two-phase coexistence region after rolling.
In the two-phase coexistence region, carbon (C) that can form a solid solution with the ferrite phase
Since the amount is about 0.02%, the diffusion of C into the austenite phase occurs, and the C-enriched austenite phase and C
The ferrite phase is separated into an extremely small amount. By water cooling from this state, a two-phase mixed structure of ferrite and martensite (including bainite) can be obtained. In these microstructures, dislocations introduced by transformation also exist, and the strength increases.

It is generally known that the yield strength depends on the strength of the soft ferrite phase, and the tensile strength depends on the strength of the hardened martensite phase. For this reason, the steel sheet manufactured by the above-described manufacturing method has low yield strength and high tensile strength, that is, high tensile strength with a so-called low yield ratio. In addition, the main metallurgical phenomena that occur in the tempering process are
From the short side of tempering, solid solution carbon atoms in martensite are discharged as cementite, and coarsening of cementite begins. Many dislocations in the microstructure generated during transformation are reduced and disappear. Cementite in bainite becomes coarse. A phenomenon in which solid solution carbon atoms are precipitated as a metal element other than Fe as a carbide proceeds. These phenomena are faster as the tempering temperature is higher and the tempering time is longer, and progress is delayed as the tempering temperature is lower and the tempering time is shorter.

In the conventional tempering method, a steel sheet is placed in a heat treatment furnace kept at a predetermined temperature and held for 20 to 40 minutes after the temperature of the steel sheet reaches a predetermined temperature. Is long, and all of the metallurgical phenomena mentioned above occur. For this reason, the coarsening of cementite and dislocations that contribute to the strength described in the metallurgy phenomenon are also reduced, and the strength is reduced, and at the same time, the toughness is also deteriorated.

That is, even if the strength is increased by quenching from the two-phase region, tempering causes a rapid decrease in strength and the toughness tends to deteriorate. Therefore, the conventional tempering method results in excessive tempering conditions, resulting in impaired strength and toughness. According to the research conducted by the present inventors, a method of eliminating a solid solution state of excess carbon and suppressing coarsening of cementite has good toughness that cannot be achieved by the conventional method, and a low yield ratio and a high tensile strength are obtained. I found that I could get it. In other words, it is a method in which the temperature is raised to a predetermined temperature by rapid heating and tempered without holding. One of these methods is already used as an induction heating method for large diameter pipes, but in the case of thick steel plates,
Since the thickness and width of the thick steel plates that are actually manufactured are extremely large, industrial commercialization was considered to be an unreasonable problem due to cost and temperature accuracy problems.

The present inventors have examined the optimum tempering method after quenching from the two-phase region and found that the following method is most suitable. That is, the two-phase region after rolling, and after rolling, after cooling to room temperature, it is heated to the two-phase region by reheating and quenched, and a steel plate having a microstructure of a mixed structure of ferrite + (martensite, bainite) After removing the scale on the surface, the steel sheet was placed in a heat treatment furnace heated to 800 to 1000 ° C., heated at a temperature rising rate of 0.3 ° C./sec or more, and the surface temperature A ₁ point −200 ° C. or more, A tempering method is characterized in that after reaching a predetermined temperature of A ₁ point + 50 ° C. or lower, it is immediately taken out of the furnace and air-cooled to room temperature.

First, when a steel sheet is rapidly heated in a heat treatment furnace at a temperature rising rate of 0.3 ° C./sec, the temperature of the steel sheet is greatly influenced by the scales on the front and back surfaces of the steel sheet, and particularly at a predetermined temperature as in the present invention. In the method not held by, the ultimate temperature of the steel plate largely depends on the adhered state of the scale in the plate. For this reason,
Material variation in the same heat-treated steel sheet is large, which is not suitable for practical use.

Further, the temperature of the heat treatment furnace is set to the tempering temperature (A ₁
The reason why the temperature is maintained at 800 to 1000 ° C, which is significantly higher than the point -200 ° C or higher and the A ₁ point + 50 ° C or lower), is that the heating rate is slower than 800 ° C (the heating rate increases as the holding temperature of the heat treatment furnace increases). As a result, the time of exposure to high temperature becomes long, coarsening of cementite and loss of dislocations occur, so that the strength and toughness deteriorate, and the initial purpose is not achieved. Also, if it exceeds 1000 ° C, the difference between the center temperature of the plate thickness and the surface temperature or the temperature difference between the end of the steel plate exceeds 100 ° C, and the strength and toughness change greatly at the steel plate portion.

Further, the reason for setting the temperature of the steel sheet surface (achieved temperature: tempering temperature) within the range of A ₁ point −200 ° C. to A ₁ point + 50 ° C. or less is that excessive carbon is present at a temperature lower than A ₁ point −200 ° C. This is because the solid solution state of does not disappear and the toughness does not recover. At a temperature of A ₁ point + 50 ° C. or higher, cementite coarsening becomes remarkable and not only the strength decreases, but also
This is because the toughness also deteriorates. Next, after reaching the predetermined temperature, the reason why the air is immediately taken out of the furnace without holding it and air-cooling is the same as the above-mentioned reason. Because of the coarsening.

According to the method of the present invention, there is a great merit in terms of productivity. The time required for tempering in the method of the present invention is significantly shorter than that in the conventional method. That is, although the processing time varies depending on the plate thickness, the processing time is about 1/3 to 1/5 of that of the conventional method, which is a great economical advantage. The steel of the present invention is capable of producing a steel sheet of an extremely good material by a tempering treatment for a short time which has not been conventionally,
The steel composition is important together with the manufacturing method, which will be described below.

Before describing the individual component control ranges of steel, the concept of the component system suitable for the rapid tempering treatment of the present invention described above will be described. On the premise of rapid tempering, a component system suitable for this is that the precipitation hardening element is not added to the (A) steel component. (B) The component system is such that the microstructure easily martensites during quenching. It is important to satisfy the following two conditions. Regarding (A), of the metallurgical phenomena occurring during tempering as described above, the phenomenon of (1) hardly occurs in the heat treatment method of the present invention. For this reason, the precipitation hardening becomes insufficient, and the effect cannot be expected even if Nb, V, and elements such as Cu in excess of 0.5% are added.

With regard to (B), the martensite formed during quenching is sufficiently decomposed into a fine cementite by the tempering treatment of rapid heating, and the toughness is improved. However, in the component system in which it is difficult to form martensite, bainite containing island-shaped martensite is formed during quenching. This island-shaped martensite is difficult to decompose during rapid tempering, and the improvement in toughness is insufficient. That is, the feature of the present invention is to suppress the formation of coarse bainite (including island-shaped martensite) during quenching, decompose martensite by rapid tempering, and reduce the coarsening of cementite and the reduction of dislocations that are effective in strength and toughness. Then, the strength and toughness are improved.

Next, the limit range of each alloy element will be described. C is an important element for securing strength,
If less than 0.06%, sufficient strength cannot be obtained, so the lower limit was made 0.06%. Further, if over 0.2%, the weldability deteriorates, so the upper limit was made 0.2%. Si is deoxidized,
Although it is an element contained in steel, the weldability deteriorates if a large amount is added, so the upper limit was made 1%.

Mn is an essential element for securing strength and toughness, and addition of 1.0% or more is effective. However, if 2.0% or more, the weldability is impaired, so the upper limit was made 2.0%.

In the steel of the present invention, impurities P and S are set to 0.020% and 0.010% or less, respectively. The reason is that the base material toughness and weldability are not deteriorated. Al is generally an element contained in steel for deoxidation, but if it exceeds 0.05%, surface cracking tends to occur during slab casting, so the upper limit was made 0.05%. Mo, together with Mn, is an important element in the steel of the present invention, but if added in excess of 0.5%, the effect of improving strength is small and the weldability is impaired.
Was set as the upper limit.

B has the effect of suppressing the transformation of austenite to ferrite during quenching and improving the hardenability. However, excessive addition rather reduces the hardenability, so the upper limit was made 0.003%. Ti is T
There are effects such that iN is generated to suppress coarsening of austenite grains during slab heating, and N is fixed to exert the effect of B. However, excessive addition forms TiC and significantly impairs toughness, so the upper limit is 0.02%.
And If N is too much, the hardenability improving effect of B is lost,
The upper limit was made 0.006% because surface cracks are likely to occur during slab casting.

Next, the reason for adding Ni and Cr will be described. Addition of these elements is an important element for improving the hardenability of steel and ensuring strength and toughness.
Since excessive addition impairs the toughness and weldability of steel and the toughness of the welded part, the upper limits of each are Ni: 1% or less, Cr: 0.5
% Or less. Moreover, addition of 0.5% or less of Cu is Ni
Like Cr and Cr, it is important for improving hardenability and ensuring strength and toughness, but if it exceeds 0.5%, Cu is usually used.
Although clusters are formed to improve the strength, precipitation hardening does not occur in the production method of the present invention, and further addition is not effective. Therefore, the upper limit is set to 0.5%.

[0023]

EXAMPLE Table 1 shows the chemical composition of the invention steel and the comparative steel, the manufacturing conditions of the steel sheet, and the mechanical properties of the base material. Further, FIG. 1 shows the relationship between the strength (TS), the toughness (vTrs), and the yield ratio (YR) of the invention steel and the comparative steel.

[0024]

[Table 1]

[0025]

[Table 2]

As shown in Table 1 and FIG. 1, the invention steels have low yield ratio (YR ≦ 85%) and high strength (TS ≧ 650 N / mm ² ).
Shows good toughness (vTrs ≦ −50 ° C.) and has a good balance between strength characteristics and toughness. On the other hand, in Comparative Steel 11, the quenching and tempering method is the same as that of the invention steel, but the strength is good because the Mn of the steel component is too high, but the yield ratio is high and the toughness is not so good. In Comparative Steel 12, the quenching and tempering method is the same as in the invention steel, but the yield ratio and strength are good because Mn is too low, but the toughness is insufficient. In Comparative Steel 13, the quenching and tempering method is the same as that of the invention steel, but since C is too low, the yield ratio and toughness are good, but the strength is insufficient.

In Comparative Steel 14, the quenching and tempering method is the same as that of the invention steel, but the toughness is insufficient because the C content is too high. In Comparative Steel 15, the quenching and tempering method is the same as that of the invention steel, but Nb and V are added in a considerable amount to improve the strength, but the effect is small and the strength is insufficient. In Comparative Steels 16 to 18, the steel composition is the same as that of the invention steels, but the production method is different from that of the invention steels, and thus all of them have a high yield ratio and are insufficient.

[0028]

INDUSTRIAL APPLICABILITY The present invention provides a technique for producing a high-strength steel sheet having a low yield ratio by a highly efficient tempering treatment which is industrially possible. The steel produced by the present invention has both a low yield ratio, high tensile strength and high toughness, and can be applied to large welded structures and the like.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between strength, yield ratio, and toughness of the present invention steel and comparative steel.

Claims (2)

[Claims] 1. By weight ratio, C: 0.06-0.20%, Si: 1.0% or less, Mn: 1.0-1.8%, P: 0.020% or less, S: 0. 0.010% or less, Al: 0.05% or less, Mo: 0.5% or less, B: 0.03% or less, Ti: 0.02% or less, N: 0.006% or less, and further, Ni: 1 % Or less, Cu: 0.5% or less, Cr:
0.5% or less of one or two or more, the balance consisting of Fe and inevitable impurities is rolled and air-cooled, and then water-cooled from the two-phase region of austenite and ferrite whose surface temperature is 720 to 780 ° C to room temperature. , A steel sheet having a microstructure of a mixed structure of ferrite + (martensite, bainite), after removing the scale of the steel sheet, was placed in a heat treatment furnace in which the steel sheet was heated to 800 to 1000 ° C,
The surface temperature is A when heated at a heating rate of 0.3 ° C / sec or more.
After reaching a predetermined temperature of ₁ point -200 ° C or higher and A ₁ point + 50 ° C or lower, it is immediately taken out of the furnace and cooled to normal temperature by air cooling. Manufacturing method. 2. The steel according to claim 1, after being rolled, air-cooled or water-cooled to room temperature and reheated so that the steel sheet surface temperature is 720 to 780.
Austenite at ℃, heated to the two-phase region of ferrite,
Then, water-cooling to room temperature, a steel sheet having a microstructure of a mixed structure of ferrite + (martensite, bainite),
After removing the scale of the steel plate, the steel plate is 800-1000
It is charged in a heat treatment furnace heated to ℃, heated at a temperature rising rate of 0.3 ℃ / sec or more, surface temperature A ₁ point-200 ℃ or more,
A method for producing a high-strength steel sheet with a low yield ratio by rapid tempering, which comprises: immediately after reaching a predetermined temperature of ₁ point + 50 ° C. or lower, immediately taking out of the furnace and cooling to normal temperature by air cooling.