JPH09157742A - Manufacture of high tensile strength steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high tensile strength steel plate for welded structure, used for purposes requiring high precision stress design, reduced in dispersion of yield strength, and having high yield ratio, by specifying the composition of a steel and regulating the ferrite ratio before the initiation of accelerated cooling to a prescribed value or above, in a manufacturing method where accelerated cooling is performed after controlled rolling. SOLUTION: A steel slab, which has a composition containing, by weight, 0.04-0.18% C, 0.05-0.8% Si, 0.2-1.7% Mn, and <=0.07% Al, further containing one or more kinds among <=0.03% Ti, <=0.05% Nb, <=0.1% V, <=0.7% Cu, <=0.7% Ni, <=1.3% Cr, and <=0.8% Mo, each including 0%, and having the balance Fe, is prepared. This steel slab is hot-rolled at a finishing temp. Tf( deg.C) not lower than the T1( deg.C) represented by equation (1), and accelerated cooling is started at a cooling starting temp. Ts( deg.C) not higher than the T2( deg.C) represented by equation (2). By this method, the dispersion of yield strength, caused by the dispersion of the proportion of ferrite phases comprising in the steel and the unevenness of cooling, can be remarkably reduced while obviating the necessity of reheating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low yield ratio, high tensile strength steel plate used for construction, civil engineering, etc., which requires a highly accurate stress design, and is particularly useful for individual steel plates. The present invention relates to a method for manufacturing a steel sheet with a small variation in yield strength within a single steel sheet or within a single steel sheet.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of ensuring safety in the event of a large earthquake, the ultimate yield strength design that prevents the collapse of the column by yielding the beam prior to the column yielding and absorbing the seismic energy. The law has begun to be applied in structures such as buildings.

In order to sufficiently absorb the seismic energy,
The steel material needs to be largely deformed, which requires a low yield ratio. Further, in order to yield the beam prior to the yielding of the column as intended by the designer, in addition to this, it is also required that the yield strength of the steel material be small.

As a technique for obtaining a low yield ratio in high-strength steel having a tensile strength of 490 MPa or more, for example, Japanese Patent Laid-Open No. 64-52023 discloses a ferrite and austenite two-phase region after controlled rolling and accelerated cooling. Reheat,
A method of forming a mixed structure of a soft ferrite phase and a hard bainite phase or martensite phase by quenching is disclosed. This method reheats, and it is expected that the yield strength will not fluctuate much, but the production efficiency will decrease and the manufacturing cost will increase due to the increase in heat treatment steps.

On the other hand, as a method for obtaining a mixed structure online, Japanese Patent Application Laid-Open No. 55-41927 discloses a manufacturing method in which controlled rolling is performed in the austenite region, followed by air cooling to the two-phase region and accelerated cooling. . This method, in the austenite region, performs rolling with a high area reduction rate and accelerates the precipitation of the ferrite phase to aim at securing the target characteristic value.However, since the area reduction rate is high, the cooling start temperature Even with the same steel sheet, even a small variation often causes a large variation in the transformation rate, which may cause a significant variation in the yield strength.

The technique disclosed in Japanese Unexamined Patent Publication No. Hei 6-271934 differs from the technique disclosed in Japanese Unexamined Patent Publication No. 64-52023 in that accelerated cooling after controlled rolling is 600 ° C.
This is a method in which the temperature is stopped to about a certain temperature, the two-phase region is heated again, and then accelerated cooling is performed.

In this method, the variation in the amount of the ferrite phase is reduced by reheating, and at the same time, the stop temperature of the first accelerated cooling is made relatively high, and the reheating treatment is performed online. However, as compared with the technique disclosed in Japanese Patent Laid-Open No. 64-52023, although it is easier to go online, it is a reheating treatment and a reduction in production efficiency cannot be avoided.

[0008]

As described above, the conventional techniques have problems that the characteristic values vary and the production efficiency is low. Therefore, without reheating, only high-productivity online processes such as controlled rolling and accelerated cooling have a low yield ratio, and there is little variation in properties, especially yield strength, and high tensile strength for inexpensive construction. Development of a method for manufacturing a steel sheet is required.

The target value of the characteristics of the steel sheet produced by the method of the present invention is as follows: yield ratio: 80% or less, and as an index of variation in yield strength, the variation range of yield strength 3σ (σ is standard deviation, hereinafter 3σYS ): 30 MPa or less, tensile strength: 490 MPa or more.

[0010]

The first aspect of the present invention, in% by weight,
C: 0.04 to 0.18%, Si: 0.05 to 0.8
%, Mn: 0.2 to 1.7%, Al: 0.07% or less, and Ti: 0.03% or less, Nb: 0.
05% or less, V: 0.1% or less, Cu: 0.7% or less,
Ni: 0.7% or less, Cr: 1.3% or less, Mo: 0.
A steel piece containing 8% or less (including 0) of one kind or two kinds or more, and the balance substantially consisting of Fe and unavoidable impurities is represented by the finishing temperature Tf (° C) by the formula (1). Done T
This is a method for producing a high-strength steel sheet in which hot rolling is performed at 1 (° C.) or higher, and accelerated cooling is started from a temperature at which a cooling start temperature Ts (° C.) is T2 (° C.) or lower represented by the formula (2). T1 (° C) = 932-305 × C (%) − 78 × Mn (%) − 17 × Cu (%) − 52 × Ni (%)-4 × Cr (%)-4 × Mo (%) (1) T2 (° C.) = 1001-350 × C (%)-70 × Mn (%)-20 × Cu (%)-60 × Ni (%)- 5 × Cr (%) − 5 × Mo (%) − 0.15 × Tf (° C.) (2) [Note that the finishing temperature Tf (° C.) is Tf and the cooling start temperature. Ts (° C) is Ts, T1 (° C) and T2 (° C) are T1 and T2, respectively, and Ar3 transformation point (° C) is A.
Abbreviated as r3. The second invention is a method for producing a high-strength steel sheet, in which the steel sheet produced according to the first invention is further reheated to 450 to 650 ° C and tempered.

In order to solve the above-mentioned problems, the present inventors first produced a method by accelerated cooling from a two-phase region after controlled rolling, as disclosed in Japanese Patent Laid-Open No. 55-41927. The cause of the variation in the yield strength of the high-strength steel sheet was investigated in detail.

As a result, the degree of variation in yield strength is
In addition to variations in the rate of ferrite phase in the steel sheet (ratio of ferrite phase in the steel. Hereinafter referred to as ferrite rate), it is greatly affected by uneven cooling (variation in cooling rate within the steel sheet) during accelerated cooling. I found a thing.

Further, through repeated studies, it was found that by setting the ferrite rate before the start of accelerated cooling to a certain value or more, the variations in yield strength due to variations in the ferrite rate and uneven cooling can be greatly reduced. .

That is, by using this method,
It is possible to significantly reduce the variation in yield strength not only within one steel plate but also between steel plates.

The reasons for limiting the above conditions will be described below. C is 0.04 in the steel in order to secure the strength of the steel.
%, It is necessary to contain at least 0.1%, but if the content exceeds 0.18%, the weldability deteriorates significantly. Therefore, the content of C is set to 0.04 to 0.18%.

Si must be contained as a deoxidizing agent in an amount of 0.05% or more, but if it exceeds 0.8%, the weldability and ductility are reduced. Therefore, the Si content is 0.05
~ 0.8%.

Mn is an element essential for improving strength and toughness, and its lower limit is 0.2%. On the other hand, if a large amount is added in excess of 1.7%, the hardenability is remarkably increased and the crack susceptibility during welding is increased. Therefore, the Mn content is set to 0.2 to 1.7%.

Al is an element having a deoxidizing action,
Since the deoxidizing action can be replaced by Si, the lower limit value is not specified. On the other hand, if its content exceeds 0.07%, the cleanliness of the steel is reduced and the toughness of the welded portion is reduced. Therefore, the upper limit value is set to 0.07%.

Ti, Nb, V, Cr and Mo are all alloying elements that contribute to the improvement of strength, and are added if necessary. However, in the case of any element, when the amount thereof is large, weldability and toughness are deteriorated. The upper limit of each is Ti: 0.03%, Nb: 0.05%, V: 0.
1%, Cr: 1.3%, Mo: 0.8%. In addition,
The lower limit values are all 0.

Cu is an element effective in securing strength, but a large amount of Cu exceeding 0.7% causes cracking during hot rolling. Therefore, the upper limit value is 0.7%. The lower limit is 0.

Ni is also effective in securing strength. It also has an effect of preventing the occurrence of cracking due to Cu during hot rolling. However, these effects tend to be saturated at about 0.7%. Therefore, the upper limit is 0.7%
And The lower limit of Ni is also 0.

Next, the reasons for limiting the manufacturing method will be described. When the finishing temperature of hot rolling is lower than Ar3 (excluding Ar3), the produced ferrite phase undergoes working, and the yield ratio increases. As a result of studying the relationship between Tf and the component exerted on Ar3 of the high-strength steel sheet subjected to controlled rolling, the present inventors have found that Ar3 can be expressed by equation (3).

That is, Ar3 is a function of the component and Tf, and since Tf needs to be Ar3 or more as described above, Tf is expressed by equation (1) from equations (3) and (4). The temperature is equal to or higher than T1. [In the formula (3),
Tf is obtained by setting Tf = Ar3 and set as T1. ] T
The upper limit temperature of f is 950 ° C. or less so that the crystal grains do not become extremely large.

Ar3 = 1072-350 × C (%)-90 × Mn (%)-20 × Cu (%)-60 × Ni (%)-5 × Cr (%)-5 × Mo (%)-0 .15 × Tf ・ ・ ・ ・ ・ ・ ・ ・ (3) Tf ≧ Ar3 ・ ・ ・ ・ ・ ・ (4) T1 = 932-305 × C (%)-78 × Mn (%)-17 XCu (%)-52xNi (%)-4xCr (%)-4xMo (%) (1) Acceleration after controlled rolling in order to lower the yield ratio. Cooling must be started in a two-phase region where a soft ferrite phase is formed, that is, a temperature region of Ar3 or lower. However, Ts is Ar3
Even if it is less than the above, when the ferrite ratio is not large, the yield strength varies with the change of the ferrite ratio. Further, the effect of reducing the yield ratio is not sufficient.

The present inventors have examined in detail the relationship between the accelerated cooling start temperature and the ferrite ratio that affect the variation in yield strength. As a result, in steels with various compositions, even if the ferrite ratio varies due to Ts variation by starting accelerated cooling from a temperature range where the ferrite rate is above a certain value, the variation in yield strength is significantly reduced. I found that I could do it.

Further, it was also found that the temperature at which the ferrite ratio becomes equal to or higher than the above constant value can be represented by T2 in the equation (2). Therefore, Ts after controlled rolling is determined in the temperature range of T2 or lower represented by the formula (2) in consideration of mechanical properties such as required strength. Lower limit temperature is Ac1 transformation point (℃)
It is.

T2 = 1001-350 × C (%)-70 × Mn (%)-20 × Cu (%)-60 × Ni (%)-5 × Cr (%)-5 × Mo (%)-0 .15 × Tf (2) Manufacturing conditions other than the above, that is, heating temperature during hot rolling, start temperature and reduction rate of controlled rolling, and cooling rate and stop temperature of accelerated cooling. Etc. may be determined in consideration of the characteristics such as the desired strength, and in the present invention, the above component range,
Conditions other than the temperature range are not specified.

Reduction of the low yield ratio and the variation of the yield strength is
Although it is sufficiently achievable by limiting the production conditions as described above, if the distortion of the steel sheet is severely limited in construction materials, etc., by reheating after rolling and cooling under the conditions as described above. It is effective to apply tempering treatment.
If the tempering treatment temperature for removing the strain is less than 450 ° C., the treatment time becomes long. Further, when the temperature exceeds 600 ° C., the material changes significantly. Therefore, the temperature range is 4
It is set to 50 to 600 ° C.

[0029]

EXAMPLES Specific examples of the present invention will be described below. 220 with ingredients within the scope of the invention shown in Table 1
The steel plates A and B having a thickness of mm were heated to 1150 ° C. in the factory, controlled rolling, and accelerated cooling to obtain steel plates having a thickness of 50 mm.

[0030]

[Table 1]

First, the effect of controlled rolling was investigated by changing Tf in the range of -40 to + 80 ° C with respect to T1 represented by the equation (1). In this case, Ts was fixed at a temperature 10 ° C. lower than T2 represented by the formula (2). next,
Tf is fixed at 850 ° C. and Ts is −40 to T2.
The steel plate which changed in the range of +40 degreeC was manufactured, and the influence was seen.

The characteristics are evaluated after the above-mentioned processing
These steel sheets were tempered at 550 ° C. In each case, the cooling rate during accelerated cooling was 6 ° C./sec, the cooling stop temperature was 500 ° C. or less for A steel,
Steel B was set to 100 ° C. or lower. Twenty test pieces were taken from various positions in the steel sheet that were separated from each other.

In steel A, Tf is -40 with respect to T1.
FIG. 3 shows the relationship between the average yield strength, the average tensile strength, the average yield ratio, the variation of the yield strength in the steel sheet, and Tf when the temperature is changed in the range of to + 80 ° C. Moreover, the result in B steel is shown in FIG.

As is apparent from FIGS. 3 and 4, under the condition of the present invention in which Ts is lower than T2 represented by the equation (2), 3σYS is 30 MPa or less regardless of Tf.
The average yield ratio can be set to 80% or less by setting Tf to T1 or more, which is the range of the present invention.
On the other hand, when Tf is set to a temperature lower than T1 outside the range of the present invention, it is found that the average yield ratio exceeds 80%.
(In the figure, the points at the positions where Tf-T1 is 0 or + indicate the examples. The points at the -position are comparative examples.)
The results are shown in the case where Tf was fixed at 850 ° C. and the Ts was changed in the range of −40 to + 40 ° C. with respect to T2 in the A steel. The average yield ratio is 80% or less even when Ts changes, but when Ts exceeds T2 represented by the equation (2), 3σYS exceeds 30 MPa. (In the figure, the point where Ts-T1 is 0 or-indicates an example. The point where + is a comparative example.) Further, FIG.
This is the result for steel, but the tendency is similar to that for A steel.

Steel pieces of 220 to 250 mm in thickness having the components within the scope of the present invention shown in Table 2 are manufactured in the factory at 1100 to 100 mm.
After heating to 1200 ° C., controlled rolling under the conditions shown in Table 3 and accelerated cooling were performed to manufacture three steel plates each having a thickness of 40 to 75 mm under each condition. After the accelerated cooling, there are steel sheets that have been tempered and steel sheets that have not been tempered. In both cases, the cooling rate during accelerated cooling was 6 ° C / sec, and the cooling stop temperature was 100.
The temperature was set to ℃ or below.

A total of 20 test pieces (6 to 7 pieces from various positions of one steel sheet separated from each other) were collected from three steel sheets manufactured under the same conditions.

C1 to H1 are examples of the present invention. Also, C2 to H2, C3, and G3 have too low Tf or Ts.
Is a comparative example of whether is too high.

[0038]

[Table 2]

[0039]

[Table 3]

Average yield strength, average tensile strength,
Table 4 shows the average yield ratio and 3σYS in and between the steel sheets. Each of the steel sheets manufactured by the method of the present invention has an average yield ratio of 80% or less and 3σYS of 30 MPa.
It is found that the yield ratio is low and the variation of the yield strength within and between the steel sheets is small.

[0041]

[Table 4]

On the other hand, the yield ratios of C2, F2 and G2 of the comparative examples having Tf lower than T1 exceeded 80%. Further, Cs, D2, E2, G of Comparative Examples in which Ts is higher than T2
3 and H2, 3σYS exceeds 30 MPa.

[0043]

As described above, according to the present invention, a high-strength steel sheet having a tensile strength of 490 MPa or more can be obtained by the online controlled rolling and the accelerated cooling process which are excellent in productivity.
It is possible to manufacture a steel sheet having excellent properties such as yield ratio: 80% or less, yield strength variation 3σYS (yield variation range 3σ): 30 MPa or less. With the completion of the manufacturing method of the present invention, it is possible to provide, at low cost, high-strength steel for welded structures that is used for construction, civil engineering, etc., and requires highly accurate stress design. large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between Ts and variations in average yield strength, tensile strength, yield ratio, and yield strength of A steel.

FIG. 2 is a diagram showing the relationship between the average yield strength, the tensile strength, the yield ratio, the variation of the yield strength, and Ts of B steel.

FIG. 3 is a diagram showing the relationship between the average yield strength, tensile strength, yield ratio, variation in yield strength, and Tf of steel A.

FIG. 4 is a diagram showing the relationship between Tf and variations in average yield strength, tensile strength, yield ratio, and yield strength of B steel.

Claims (2)

[Claims] 1. C: 0.04 to 0.18% by weight,
Si: 0.05 to 0.8%, Mn: 0.2 to 1.7%,
Al: 0.07% or less, and further, Ti: 0.
03% or less, Nb: 0.05% or less, V: 0.1% or less, Cu: 0.7% or less, Ni: 0.7% or less, Cr:
1.3% or less, Mo: 0.8% or less (all of which include 0), and one or more types are contained, and the balance is substantially F.
The steel piece composed of e and unavoidable impurities is finished at a finishing temperature Tf.
Hot rolling is performed at a temperature (° C) equal to or higher than T1 (° C) represented by the formula (1), and accelerated cooling is performed at a temperature Ts (° C) equal to or lower than T2 (° C) represented by the formula (2). A method for manufacturing a high-strength steel sheet, the method comprising: T1 (° C.) = 932-305 × C (%) − 78 × Mn (%) − 17 × Cu (%) − 52 × Ni (%)-4 × Cr (%)-4 × Mo (%). ........ (1) T2 (° C.) = 1001-350 × C (%)-70 × Mn (%)-20 × Cu (%)-60 × Ni (%)-5 × Cr (%) ) -5 x Mo (%) -0.15 x Tf (° C) ... (2) 2. C: 0.04 to 0.18% by weight,
Si: 0.05 to 0.8%, Mn: 0.2 to 1.7%,
Al: 0.07% or less, and further, Ti: 0.
03% or less, Nb: 0.05% or less, V: 0.1% or less, Cu: 0.7% or less, Ni: 0.7% or less, Cr:
1.3% or less, Mo: 0.8% or less (all of which include 0), and one or more types are contained, and the balance is substantially F.
The steel piece composed of e and unavoidable impurities is finished at a finishing temperature Tf.
Hot rolling is performed at a temperature (° C.) equal to or higher than T1 (° C.) represented by the formula (1), and accelerated cooling is performed at a temperature Ts (° C.) equal to or lower than T2 (° C.) represented by the formula (2). Starting from 4 more
A method for producing a high-tensile steel sheet, which comprises performing a tempering treatment by heating in a temperature range of 50 to 650 ° C. T1 (° C.) = 932-305 × C (%) − 78 × Mn (%) − 17 × Cu (%) − 52 × Ni (%)-4 × Cr (%)-4 × Mo (%) ... ........ (1) T2 (° C.) = 1001-350 × C (%)-70 × Mn (%)-20 × Cu (%)-60 × Ni (%)-5 × Cr (%) ) -5 x Mo (%) -0.15 x Tf (° C) ... (2)