JPH04354823A - Production of 60kgf/mm tensile strength secondary steel having <=80% yield ratio - Google Patents

Abstract

PURPOSE:To manufacture a low yield ratio high tensile strength steel sheet having low yield ratio with high safety, furthermore having yield point elongation and high in tensile strength. CONSTITUTION:A slab contg., by weight, 0.05 to 0.20% C, 0.05 to 0.60% Si, 0.60 to 2.00% Mn, <=0.04% P, <=0.02% S and 0.001 to 0.1% Al, furthermore contg. one or >= two kinds among <=1.00% Cu, <=1.50% Ni, <=1.00% Cr, <=1.00% Mo, <=0.10% V, <=0.10% Ti and <=0.005% B and the balance Fe with inevitable impurities is hot-rolled so as to regulate its finishing temp. to <=950 deg.C, is successively cooled at <=20 deg.C/s cooling rate, is thereafter reheated to the temp. range of (the Ac3-100 deg.C) to the Ac3 point, is successively subjected to primary cooling to 500 to 700 deg.C at <=6 deg.C/s cooling rate and is thereafter subjected to rapid cooling at >=10 deg.C/s cooling rate.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is mainly used in construction with a tensile strength (TS) of 80% or less and a yield ratio of 60 kgf/mm.
The present invention relates to a method for producing second grade steel plate.

0002

2. Description of the Related Art As steel structures such as buildings, bridges, and tanks become larger in size, steel materials used are required to have higher strength. From the viewpoint of structural safety, that is, prevention of brittle fracture, a low yield ratio is required. Low-yield ratio steels have a high allowable stress before breaking even if stress exceeding the yield point is applied, and they also have a large uniform elongation and excellent plastic deformability, so they can be used in construction, for example, to exceed the yield stress. Even if a large earthquake occurs, the structure will deform but not break, giving it an advantage in terms of earthquake resistance.

However, in general, the yield ratio tends to increase with increasing strength, and the yield ratio increases with tensile strength of 60 kgf/mm2
It is not easy to obtain a low yield ratio of 80% or less with the above high tensile strength steels. Conventionally, methods for obtaining high tensile strength steel with low yield ratio include, for example,
Japanese Patent Publication No. 55-50090 and Japanese Patent Publication No. 2-34721
Although it is disclosed in the publication, in both cases, the steel is made from Ac1 point
A low yield ratio is obtained by hardening from the two-phase region of three Ac points or one Ar point to three Ar points to create a two-phase mixed structure of ferrite + hard phase.

However, the stress-strain curve of the low yield ratio steel produced by the above-mentioned method has a drawback that it does not have a clear yield shelf and the initial work hardening results in a large round curve.

[0005]

[Problems to be Solved by the Invention] The present invention provides a high-strength steel plate with a low yield ratio of 80% or less, which has a yield shelf, and has a tensile strength of 60 kgf/mm2 or more, which has even higher safety. The purpose is to provide a manufacturing method.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention, as a result of various studies, proposes that in order to obtain a steel plate with a low yield ratio and a yield shelf, it is necessary to The present invention was constructed based on the new discovery that the structure of 2 after proper rolling, cooling, and further reheating in the two-phase region.
It consists of stage cooling, and the structure of the steel is made to include fine polygonal ferrite.

That is, the present invention provides C:0 in terms of weight ratio.
．． 05-0.20%, Si: 0.05-0.60%, M
n: 0.60-2.00%, P: 0.04% or less, S:
0.02% or less, Al: 0.001 to 0.10%, Cu: 1.00% or less, Ni: 1.50
% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.10% or less, Ti: 0.10% or less, B:
Contains 0.005% or less of one or more types, the balance being Fe
and unavoidable impurities at a finishing temperature of 9.
Hot rolling is carried out so that the temperature is below 50℃, and then 20℃
After cooling at a cooling rate of ℃/s or less, (Ac3-10
Reheat to a temperature range of 0 °C to 3 points of Ac, and then continue heating to 6
A tensile strength of 60 kg with a yield ratio of 80% or less, characterized by primary cooling from 500 to 700 °C at a cooling rate of 10 °C/s or higher, and then rapid cooling at a cooling rate of 10 °C/s or higher.
This is a method for manufacturing f/mm2 class steel.

[0008]

[Operation] The reason for limiting the range of components of the present invention is as follows. C: 0.05 to 0.20% (weight%, omitted below) C is required to be 0.05% or more to ensure strength, but
If it exceeds 0.20%, weldability and toughness will deteriorate.
The content was set at 0.05 to 0.20%.

Si: 0.05 to 0.60% Si is required as a deoxidizing agent in an amount of 0.05% or more, but addition of 0.60% or more deteriorates ductility and toughness. 60%
And so. Mn: 0.60-2.00% Mn is required at least 0.60% to ensure strength, but if it exceeds 2.00%, weldability deteriorates, so 0.60-2.00%
And so.

P: 0.04% or less, S: 0.02% or less Both P and S deteriorate toughness and ductility, so 0.02% or less, respectively.
It was limited to 0.04% and 0.02% or less. Al: 0.00
1 to 0.10% Al is required as a deoxidizing agent in an amount of 0.001% or more, but if it is 0.10% or more, the toughness deteriorates, so the content is set to 0.001 to 0.10%.

[0011] Cu: 1.00% or less, Ni: 1.50%
Below, Cr: 1.00% or less, Mo: 1.00% or less,
V: 0.10% or less, B: 0.005% or less, Ti:
0.10% or less Cu, Ni, Cr, Mo, V, B, Ti
Both are useful for improving strength, but excessive addition deteriorates weldability and ductility, so the upper limit for each is set to Cu: 1.0.
0%, Ni: 1.50%, Cr: 1.00%, Mo: 1
．． 00%, V: 0.10%, B: 0.005%, Ti
:0.10%.

[0012] After the steel having the above-mentioned composition system is made into a slab by the usual ingot-forming or continuous casting method, it is heated to a temperature equal to or higher than the austenitizing temperature and then hot-rolled. In hot rolling, when the finishing temperature of rolling exceeds 950℃, the heating strain recovers.
Furthermore, recrystallization occurs, and deformation bands that become ferrite precipitation sites are not formed. In addition, the cooling rate after rolling is 20℃/
At temperatures exceeding s, ferrite does not precipitate, only martensite,
Alternatively, the structure will consist of only bainite, or only martensite and bainite. In such a case, even if the next step of reheating and cooling is performed, there is no yield shelf in the stress-strain curve, and the work hardening is rapid, making it impossible to obtain the plastic deformability required as a structural steel material. Therefore, the rolling finishing temperature was set at 95
The cooling rate after rolling was 0° C. or lower and the cooling rate after rolling was 20° C./s or lower.

Next, reheating is performed, and the reheating temperature is (A
It is limited to a temperature range of c3-100 °C) to Ac3. If the heating temperature exceeds the Ac3 point, the effect of the previous step is lost, and if it is lower than (Ac3-100°C), fine polygonal ferrite cannot be obtained in the subsequent cooling. For this reason, the reheating temperature should be changed to (Ac3-100℃)
-Ac3.

[0014] If the primary cooling rate after reheating is faster than 6°C/s, polygonal ferrite will not precipitate and the stress of the steel plate will decrease.
The distortion curve becomes a round curve. For this reason, the primary cooling rate was set to 6° C./s or less. Also, successively 10℃/s
Although rapid cooling is performed at the above cooling rate, if the material is rapidly cooled from a temperature lower than 500° C., bainite transformation or martensitic transformation does not occur, and the target strength cannot be obtained. Rapid cooling start temperature is 70
If the temperature exceeds 0° C., polygonal ferrite will not be generated during cooling, and the primary cooling effect will be lost. For this reason, the water cooling start temperature was limited to 500 to 700°C. Furthermore, if the cooling rate is lower than 10° C./s, bainite transformation or martensitic transformation will not be completed sufficiently and the target strength will not be obtained, so 10° C./s was set as the lower limit.

Bainite or martensite produced under the above heat treatment conditions can be further tempered to improve its toughness. Therefore, it is more preferable to further perform a tempering treatment.

[0016]

[Example] Table 1 shows the chemical components and Ac3 transformation points of the test materials. Test materials A to D are steels within the composition range of the present invention, and E and F
is a comparison steel.

[0017]

[Table 1]

Hot rolling was carried out under the conditions shown in Table 2, and the plate thickness was 3.
After forming a 0 mm thick steel plate, it was subjected to the heat treatment shown in the same table. The resulting mechanical properties are shown in Table 3.

[0019]

[Table 2]

[0020]

[Table 3]

[0021] For all steel plates, the heating temperature during hot rolling was 1150°C, the rolling reduction at 950°C or lower was 70%, and the final plate thickness was 30 mm. The steel produced within the scope of the present invention exhibits the necessary strength and toughness, has a low yield ratio and a clear yield shelf, and has sufficient plastic deformability as a structural steel material.

[0022] Comparative steels outside the scope of the present invention either do not have target values for strength, toughness, or yield ratio, or have stress-strain curves that are round. In addition, Steel No. 11 has too much Cr in its steel components, Steel No. 14 has too high a rolling finishing temperature, and Steel No. 17 and Steel No. 18 have cooling conditions after reheating that are outside the scope of the present invention. Therefore, the reheating temperature of Steel No. 22 is too low, so the stress-strain curve does not have a yield shelf, and its plastic deformability is inferior to that of the steel of the present invention.

Steel No. 10 lacks C in the steel components, and Steel No. 16 and Steel No. 21 lack strength because the cooling conditions after reheating are outside the range of the present invention.

[0024]

Effects of the Invention The present invention provides a TS 60kgf/mm2 with a yield ratio of 80% or less and a clear yield shelf of 1% or more by appropriately controlling rolling and subsequent heat treatment.
It became possible to stably produce grade steel. This has improved the problem that conventional low yield ratio steels suffer from significant initial work hardening and are unable to fully demonstrate the effects of low yield ratios.

Claims (1)

[Claims] Claim 1: C: 0.05 to 0.20 in weight ratio
%, Si: 0.05-0.60%, Mn: 0.60-2
．． 00%, P: 0.04% or less, S: 0.02% or less,
Contains Al: 0.001 to 0.1%, and further contains Cu:
1.00% or less, Ni: 1.50% or less, Cr: 1
．． 00% or less, Mo: 1.00% or less, V: 0.10%
Hereinafter, a slab containing one or more of Ti: 0.10% or less, B: 0.005% or less, and the balance consisting of Fe and unavoidable impurities is heated at a finishing temperature of 950°C or less. Rolling is performed, followed by cooling at a cooling rate of 20°C/s or less, then reheating to a temperature range of (Ac3-100°C) to Ac3 points, and then cooling at a cooling rate of 6°C/s or less to 500°C to 700°C. First cool down to 1, then 1
A method for producing class 2 steel with a tensile strength of 60 kgf/mm and a yield ratio of 80% or less, characterized by rapidly cooling at a cooling rate of 0° C./s or more.