JPH06340924A - Method of manufacturing low yield ratio high strength steel - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for stably producing a steel material having a low yield ratio of 80% or less and a high strength of 580 MPa or more. [Constitution] Nb: 0.015 to 0.10% is included, and T
After hot rolling a steel material containing at least one of i and REM at a finishing temperature of 950 ° C or higher, it is immediately cooled to 750 to 600 ° C and then air-cooled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a tensile strength of 580 MPa or more and a yield ratio of 80% used for steel structures such as buildings.
The present invention relates to a method for producing a high-strength steel having a low yield ratio, and particularly to a method for producing a thick steel plate having a plate thickness of 20 mm or more.

[0002]

2. Description of the Related Art With the increase in size of steel structures such as construction, the steel materials used are required to have high strength. on the other hand,
From the viewpoint of safety of structures, that is, prevention of brittle fracture, low yield ratio is required. Low yield ratio steel has a large stress that can be tolerated even if stress above the yield point is applied and a large uniform elongation, so even if a large earthquake that exceeds the yield stress occurs in buildings, for example, it will not be destroyed. It is extremely advantageous.

Generally, the yield ratio tends to increase with increasing strength, and it is not easy to obtain a low yield ratio of 80% or less in a high-strength steel having a tensile strength of 580 MPa or more. Conventionally, as a method for producing a high-strength steel having a low yield ratio, there is a technique disclosed in, for example, Japanese Patent Publication No. 58-10442.
This is due to the direct quenching method after rolling and gradual cooling, after which the cooling start after rolling finish is delayed by gradual cooling to precipitate about 5 to 6% of ferrite, and then cooling is performed. This is a method of obtaining a low yield ratio by forming a two-phase mixed structure of ferrite and hardened phase. However, this method has a drawback in that the amount of ferrite differs depending on the cooling start temperature and the ratio of ferrite and the second phase changes, so that the mechanical properties such as the yield ratio greatly change. Especially 20m thick
For thick steel plates of about m or more, the air cooling waiting time from the rolling finishing temperature to the start of cooling is long, so the temperature difference in the plate thickness section is large, the yield ratio and mechanical properties vary widely, and the productivity of the rolling mill is remarkable. descend.

[0004]

SUMMARY OF THE INVENTION The present invention solves the problem that the air cooling waiting time from the rolling finishing temperature to the cooling start temperature is long in the conventional direct rolling quenching method as described above, and it is as low as 80% or less. It is an object of the present invention to provide a method for stably producing a steel material having a yield ratio and a high strength of 580 MPa or more.

[0005]

As a result of repeated studies to solve the problems of the conventional method, a steel having a proper composition containing Nb was rolled at a temperature of 950 ° C. or higher and immediately
It has been found that a high yield steel with a low yield ratio can be manufactured by quenching to a temperature range of 50 to 600 ° C. and then air cooling.

That is, in Nb-containing steel, the rolling temperature is 95
When the temperature is lowered to 0 ° C. or lower, Nb carbide precipitates even in the austenite region, so that the hardenability improvement and precipitation strengthening of Nb at the time of alpha transformation are reduced, but the Nb carbide in the austenite region is rapidly cooled from 950 ° C. By preventing the precipitation of carbides, the strengthening of Nb can be sufficiently exerted.

By adjusting the cooling stop temperature to 750 to 600 ° C., about 20 to 80% of ferrite is included (ferrite + pearlite) or (ferrite +
A bainite) structure can be obtained, and a low yield ratio and high strength can be combined. That is, the basic idea of the present invention is to increase the amount of ferrite phase in the product structure to obtain a low yield ratio more stably than in the prior art, and to supplement the strength that is reduced by precipitation of Nb. And

That is, in the present invention, C: 0.06 to 0.1
5%, Si: 0.01-0.60%, Mn: 1.00-
1.80%, P ≦ 0.015%, S ≦ 0.010%, N
b: 0.015 to 0.10%, Al ≦ 0.050%,
N: 0.0010 to 0.0060% included, and Ti:
0.005-0.030%, REM: 0.001-0.
A steel material containing 020% of one kind or two kinds and the balance Fe and unavoidable impurities is heated to a temperature of 1050 to 1300 ° C., and hot rolling is finished at a temperature of 950 ° C. or higher, and then 750 to 600 ° C. immediately. The present invention is a method for producing a high yielding steel having a low yield ratio of 580 MPa for construction, which is characterized in that it is water-cooled at a cooling rate of 1 ° C / s or more, and then air-cooled. 0.50%, N
i ≦ 0.50%, Cr ≦ 0.50%, Mo ≦ 0.20
%, V ≦ 0.10%, B ≦ 0.0030%, and a low yield ratio for construction 580 MPa class high tensile steel manufacturing method.

[0009]

The reason why the composition is limited in the present invention will be described below. C: 0.06 to 0.15% C is added to secure high strength, but if it is less than 0.06%, it is difficult to obtain a predetermined strength, while 0.
If it exceeds 15%, the weldability will decrease.
It was set to 0.15%.

Si: 0.01 to 0.60% Si is an element effective in improving the strength, but if it is less than 0.01%, its effect is small, and if it exceeds 0.60%, the heat affected zone of welding is affected. 0.01 to 0.6 because it lowers the toughness of
The range was 0%. Mn: 1.00 to 1.80% Mn is added in an amount of 1.00% or more to secure strength,
If added in excess of 1.80%, the susceptibility to weld cracking will increase and the toughness of the heat-affected zone will also decrease, so 1.00 to 1.80.
The range is%.

P ≦ 0.015%, S ≦ 0.010% P and S both reduce the toughness and ductility, so they were made 0.15% or less and 0.010% or less, respectively. Nb: 0.005 to 0.100% Nb forms a solid solution in γ iron to improve the hardenability in the alpha transformation and also precipitation strengthens the alpha matrix. In order to exert these reinforcing effects, it is necessary to add 0.005% or more of Nb, while addition of Nb is 0.10.
If it exceeds 0%, the weldability and the toughness of the heat-affected zone will be deteriorated, so the range was made 0.005 to 0.100%.

Al: 0.050% Al is used as a deoxidizing agent and fixes N in the steel, so it is effective in improving the toughness, but if it exceeds 0.050%, the toughness decreases, so it is 0.050%. Limited to less than%. N: 0.0010 to 0.0060% N is contained in steel as an unavoidable impurity, but since it combines with Ti to form TiN and suppresses grain coarsening in the weld heat affected zone. It is effective in improving the toughness of the heat-affected zone. Therefore, the minimum amount of N is 0.0010%
It is necessary, but if it is contained in excess of 0.0060%, the toughness of the heat-affected zone will be significantly reduced.
The range was limited to 0.0060%.

Ti: 0.005-0.030%, RE
M: 0.0010 to 0.0200% Ti and REM effectively contribute to improving the toughness of the heat-affected zone of large heat input welding, but T
If i is less than 0.005% and REM is less than 0.001%, the effect is small. On the other hand, when the content of Ti exceeds 0.030%, the excess N increases with respect to TiN, so that the toughness of the heat affected zone deteriorates. If REM exceeds 0.002%, the cleanliness is remarkably reduced. Therefore, Ti is 0.005
0.030% and REM were limited to the range of 0.001-0.020%.

Cu ≦ 0.50%, Ni ≦ 0.50% Both Cu and Ni are elements effective for improving hardenability and strength. Further, Ni has an effect of improving toughness.
However, if Cu is added and added in excess of 0.50%, the hot workability during rolling deteriorates. Further, excessive addition of Ni in excess of 0.50% has a problem in economy, so the upper limit was made 0.5%.

Cr ≦ 0.50%, Mo ≦ 0.20%, V
≤0.10%, B≤0.0030% Both elements are effective elements for enhancing hardenability and tempering softening resistance and for improving strength, but excessive addition causes an increase in yield ratio. , The weldability and the toughness are deteriorated, so the respective upper limits are set as described above.

Heating temperature: 1050 to 1300 ° C. A heating temperature of 1050 ° C. or higher is necessary for sufficiently solutionizing 0.0050% Nb necessary for precipitation strengthening. On the other hand, if the temperature exceeds 1300 ° C., the crystal grains become coarse and the toughness deteriorates, so the heating temperature was set in the range of 1050 to 1300 ° C. Finishing temperature: 950 ° C or higher Even when rolling in the austenite region, the rolling temperature is 95
When the temperature is lower than 0 ° C., Nb carbide precipitates and the amount of solid solution Nb decreases, so that precipitation strengthening by Nb decreases. Therefore, it is necessary to prevent the precipitation of Nb carbide by setting the finishing temperature to 950 ° C. or higher and quenching immediately after the completion of rolling.

Cooling after rolling: Quenching at 1 ° C./s or more After cooling at 950 ° C. or more, cooling rate is 1 ° C./s
By quenching as described above, precipitation of Nb carbide in the austenite region can be prevented. Further, by quenching at a cooling rate of 1 ° C./s or more in the alpha transformation region of 800 ° C. or lower, a fine structure required for high toughness can be obtained. Therefore, the cooling rate is set to 1 ° C./s or more.

Cooling stop temperature: 750 to 600 ° C. If the cooling stop temperature is higher than 750 ° C., sufficient strength cannot be obtained. On the other hand, when the cooling stop temperature is lower than 600 ° C,
Although high strength can be obtained, soft ferrite required for a low yield ratio is no longer produced, and a low yield ratio cannot be obtained. Therefore, the cooling stop temperature is limited to the range of 750 to 600 ° C. The steel material obtained as described above has a temperature of 600 ° C.
Even if tempered at the following temperatures, it has a low yield ratio of 80% or less and 5
A high tensile strength of 80 MPa or more can be obtained.

[0019]

EXAMPLES Steel materials having various compositions shown in Table 1 (symbols A to J)
Was heated to 1000 to 1250 ° C., hot-rolled under various conditions shown in Table 2, water-cooled to a cooling stop temperature, and then air-cooled to room temperature. When the plate thickness is 50 mm, the cooling rate is 3 ° C / s, 60 m
It was 2 ° C./s when m. Note that Comparative Example E2 was air-cooled to room temperature after hot rolling.

[0020]

[Table 1]

[0021]

[Table 2]

The tensile properties of the obtained steel materials are shown in Table 2. Steels A to G in Table 1 are inventive steels, and steels H to K are comparative steels. H steel is a normal 580 MPa grade steel for heat treatment, but since the Nb content is lower than the proper range, the manufacturing process of the present invention does not provide the predetermined yield strength and tensile strength. Similarly, for I steel, the C content is lower than the appropriate range, so the yield strength and tensile strength are too low. J steel has Mn and Mo
Since the amount is too high, high strength can be obtained, but the yield ratio becomes too high and exceeds the target yield ratio (≦ 80%). Further, the K steel does not have a predetermined high strength because the Mn content is too low.

Further, although A5, A6, A7, and E2 have component compositions within appropriate ranges, the heating temperature, finishing temperature, and cooling conditions are out of the appropriate ranges, so that the yield ratio is too high or the yield strength is high. And tensile strength are insufficient. On the other hand, A1-A4, B-by the method of the present invention
Each of the G steels has a low yield ratio of 75 to 78% and a high tensile strength of 600 to 660 MPa.

[0024]

According to the present invention, even in a thick steel plate, the waiting time from the rolling finishing temperature to the start of cooling is short, the productivity of the rolling mill is not lowered, and the yield ratio is 80% or less and the tensile strength is 580 MPa or more. It is possible to stably manufacture a high-strength steel having a low yield ratio and a high strength. Further, the method of the present invention does not require heat treatment for refining and is therefore excellent in economic efficiency and mass productivity.

Claims (2)

[Claims] 1. C: 0.06-0.15%, Si: 0.
01 to 0.60%, Mn: 1.00 to 1.80%, P ≦
0.015%, S ≦ 0.010%, Nb: 0.005
0.100%, Al ≦ 0.050%, N: 0.0010
To 0.0060%, and Ti: 0.005 to 0.
030%, REM: 0.001 to 0.020% of 1 type or 2 types, and the steel material consisting of the balance Fe and unavoidable impurities is heated to a temperature of 1050 to 1300 ° C, and hot rolling is performed at 950 ° C or more. Immediately after finishing at 750 ~ 750
A low yield ratio for construction of 580 MPa, which is characterized by performing water cooling up to 600 ° C. at a cooling rate of 1 ° C./s or more, and then air cooling.
Of high grade high strength steel. 2. Further, Cu ≦ 0.50% and Ni ≦ 0.5.
0%, Cr ≦ 0.50%, Mo ≦ 0.20%, V ≦ 0.
The method for producing a high yield steel having a low yield ratio for construction of 580 MPa class according to claim 1, characterized in that it contains at least one of 10% and B ≦ 0.0030%.