JP2776174B2 - Manufacturing method of high tensile strength and high toughness fine bainite steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a building, bridges, production of 60~100kgf / mm ² grade thick steel plate having an excellent fine bainite weldability and low temperature toughness to be used for steel structures such as tanks It is about the law.

[0002]

2. Description of the Related Art As steel structures such as buildings, bridges, tanks, etc., become larger, steel materials used are required to have higher strength and thicker wall. On the other hand, a high-strength steel sheet excellent in weldability and low-temperature toughness is required from the viewpoint of structural safety, that is, prevention of brittle fracture. However, in general, the weldability and low-temperature toughness tend to decrease with increasing strength, and it is not easy to combine good weldability and low-temperature toughness with a high-tensile steel having a tensile strength of 60 kgf / mm ² or more. .

Conventional methods for producing high-strength steel plates include:
As disclosed in JP-A-2-27407, it is known to provide high strength and high toughness by applying a heat treatment method by quenching and tempering from an austenite temperature range. On the other hand, as another conventional method for producing a high-strength steel plate, a non-heat-treated controlled rolling / accelerated cooling method is known as disclosed in Japanese Patent Application Laid-Open No. 2-48606.

[0004]

In the former method among the conventional methods, there has been a problem that the production efficiency is reduced and the cost is high due to the reheating processing which is an off-line processing. On the other hand, the latter method has a problem in that coarse bainitic ferrite is generated at a relatively high temperature, and fine carbide cannot be precipitated inside the bainitic ferrite, so that sufficient toughness cannot be obtained. Was.

Therefore, it is necessary to provide a method of increasing the thickness of the wall without heat treatment (as it is rolled), and it is necessary to generate a fine bainite structure at a low temperature, thereby increasing the toughness while utilizing the high strength. is necessary. An object of the present invention is to provide a method for producing a 60 to 100 kgf / mm class ² high-strength steel which is non-heat-treated and has excellent weldability and low-temperature toughness by refining the bainite structure. .

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the method for producing a high-strength and high-toughness thick steel plate, and has been made by the present inventors by appropriately adjusting the chemical composition and further improving the chemical composition. After applying a cumulative pressure of 50% or more in the crystalline austenite region, the bainite transformation is performed during cooling, and then the bainite transformation is performed in which the carbide is finely dispersed in the bainite structure by maintaining the isothermal temperature or changing the cooling rate. by Rukoto avoids generating island martensite to inhibit the toughness, which has been completed to obtain a finding that it is possible to manufacture a high strength and high toughness steel plate, the gist is as follows .

(1) C: 0.01 to 0.20 by weight ratio
%, Si: 0.05 to 0.6%, Mn: 0.6 to 2.0%,
Nb: 0.005 to 0.08%, Ti: 0.005 to 0.0
3%, B: 0.0005 to 0.0020%, sol Al:
Material steel containing 0.05% or less and N: 0.008% or less, with the balance being Fe and unavoidable impurities,
After heating to a temperature of 0 to 1250 ° C., a reduction of a cumulative rolling reduction of 50% or more is applied at a temperature of 900 ° C. or less, and rolling is completed at a temperature of ₃ points or more and 800 ° C. or less.
Cool at a cooling rate of 10 ° C./s or more to a temperature of 80 ° C. or less and 450 ° C. or more, and after cooling, maintain an isothermal time of 580 ° C. or less and 450 ° C. or more for 10 s to 100 s, or At a temperature of 450 ° C. or more, a time of 10 s or more and 100 s or less is 0.5 ° C./s.
A method for producing high-tensile / high-toughness fine bainite steel, characterized by cooling at the following cooling rate.

(2) The raw steel further contains Cu: 1.5% or less, Cr: 0.5% or less, and Mo: 0.5% or less.
V: The method for producing a high-tensile / high-toughness fine bainite steel according to (1), characterized by containing at least one of 0.1% or less.

(3) The high tensile strength steel according to (1) or (2), wherein the material steel further contains Ni: 1% or less.
Manufacturing method of high toughness fine bainite steel.

[0010]

Hereinafter, the operation of the present invention will be described in detail based on experimental results of the inventors. First, the reasons for limiting the composition of steel will be described. C: 0.01 to 0.20% C is an element effective for increasing the strength.
1% or more is required. However, the upper limit needs to be 0.20% from the viewpoint of securing toughness and preventing a decrease in weld cracking resistance.

Si: 0.05 to 0.6% Si is a deoxidizing element, and therefore, it is necessary to add 0.05% or more of Si. However, if it exceeds 0.6%, the low temperature toughness of the heat affected zone is reduced, so that the upper limit is 0.6%.
It is necessary to

Mn: 0.6 to 2.0% Mn is an element effective for increasing the strength.
It is necessary to add 0.6% or more. However, if added in excess of 2.0%, the toughness deteriorates, so the upper limit needs to be 2.0%.

Nb: 0.005 to 0.08%, Ti: 0.00
005-0.03% Nb and Ti are effective elements for refining crystal grains.
For that purpose, it is necessary to add 0.005% or more of each. However, if Nb exceeds 0.08% and Ti exceeds 0.03%, the toughness deteriorates. Therefore, the upper limits are set to 0.08% for Nb and Ti, respectively.
Needs to be 0.03%.

B: 0.0005% to 0.0020% B is an element effective for improving hardenability and accompanying strength, and therefore, it is necessary to add 0.0005% or more of B. However, if added in excess of 0.0020%, the toughness deteriorates, so the upper limit must be 0.0020%.

Sol Al: 0.05% or less Al is an element effective as a deoxidizing agent and is also effective in refining crystal grains, but an excessive amount of Al generates harmful inclusions in the material. Therefore, the upper limit needs to be 0.05%.

N: 0.008% or less N forms nitrides together with Al and is effective for refining crystal grains. However, an excessive amount of N impairs the toughness of the welded portion, so the upper limit is 0.008. %.

Cu: 1.5% or less Cu is an element effective for increasing the strength. However, the addition of an excessive amount of Cu lowers the toughness, so the upper limit must be 1.5%.

Ni: 1% or less Ni is an element effective for improving low-temperature toughness.
The upper limit is set to 1% from the viewpoint of economy.

Cr: 0.5% or less, Mo: 0.5% or less,
V: 0.1% or less Cr, Mo, and V are effective elements for increasing the strength. However, since excessive addition lowers the toughness, the upper limit of the addition amount is set to 0.5% for Cr and Mo, respectively. , V need to be 0.1%.

Next, the reasons for limiting the rolling conditions will be described. Regarding the billet heating temperature, the upper limit temperature is set to 1250 ° C. to prevent coarsening of austenite crystal grains during heating.
The lower limit temperature is set to 1000 ° C. in order to form a solid solution of Nb effective for refinement of crystal grains during rolling and precipitation strengthening after rolling. In order to reduce the metal structure by rolling, 9
It is necessary to perform rolling at a temperature of 00 ° C. or less and a cumulative draft of 50% or more.

With regard to the rolling end temperature, when the rolling is completed at a temperature exceeding 800 ° C., a fine grain structure cannot be obtained.
If the rolling completion temperature Ar is less than _three points, the toughness deteriorates. For this reason, the rolling completion temperature is limited to a temperature range from 800 ° C. to _three Ar points. As for the cooling rate after the rolling, as a condition under which the structure is refined and the strength is increased by the formation of the bainite structure during the cooling, the temperature is accelerated to 580 to 450 ° C. at a cooling rate of 10 ° C./s or more. Requires cooling.

Further, in order to sufficiently obtain a fine bainite structure, 10 seconds at a temperature range of 580 to 450 ° C.
The temperature is kept isothermal for at least 100 s or less, or
In the temperature range of 0 to 450 ° C., it is necessary to gradually cool for 10 to 100 s at a cooling rate of 0.5 ° C./s or less. By holding for more than 100 s, carbides precipitated inside the bainitic ferrite become coarse and deteriorate toughness. Therefore, the holding time needs to be limited to 100 s or less.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of the present invention is as described above, but the embodiments will be described below. The test steel sheet was prepared by melting and casting steel having the chemical components shown in Table 1 by a conventional method, and subjecting the resulting steel slab to thermomechanical treatment in accordance with the manufacturing conditions shown in Table 2. The plate thickness is 20 mm to 50 mm. Test specimens were taken from these steel slabs and subjected to a tensile test and 2 mm
A V-notch Charpy impact test was performed.

[0024]

[Table 1]

[0025]

[Table 2]

As can be seen from the results of the above embodiment,
In the example of the present invention, when the tensile strength is 60 kgf / mm ² or more,
High toughness with a Charpy absorbed energy at 13 ° C. of 13 kgf · m or more is obtained. On the other hand, in the comparative example, either the tensile strength or the Charpy absorbed energy has not reached a satisfactory level.

[0027]

As described above, the manufacturing method according to the present invention has a tensile strength of 60 to 100 depending on its structure.
It has an excellent effect that a steel having a kgf / mm ² and a fine bainite structure having excellent low-temperature toughness can be produced.

Claims (3)

(57) [Claims] (1) C: 0.01 to 0.20% by weight,
Si: 0.05 to 0.6%, Mn: 0.6 to 2.0%, Nb
: 0.005 to 0.08%, Ti: 0.005 to 0.03
%, B: 0.0005 to 0.0020%, sol Al: 0.005%
Material steel containing 0.05% or less and N: 0.008% or less, with the balance being Fe and unavoidable impurities,
After heating to a temperature of １２1250 ° C., a reduction of a cumulative reduction rate of 50% or more is applied at a temperature of 900 ° C. or less, and Ar ₃ points or more and 8
Rolling is completed at a temperature of 00 ° C. or less, and
After cooling at a cooling rate of 10 ° C./s or more to a temperature of 0 ° C. or less and 450 ° C. or more to transform bainite, after cooling, 580
At a temperature of 450 ° C. or lower for 10 s to 100 ° C.
s or less at 580 ° C. or less and 450 ° C. or more at a cooling rate of 0.5 ° C./s or less for a time of 10 s or more and 100 s or less .
A method for producing a high-tensile, high-toughness fine bainite steel, characterized by forming fine bainite. 2. The steel material further comprises Cu: 1.5% or less;
: 0.5% or less, Mo: 0.5% or less and V: 0.1%
The method for producing a high-tensile / high-toughness fine bainite steel according to claim 1, comprising at least one of the following. 3. The method for producing a high tensile strength and high toughness fine bainite steel according to claim 1, wherein the base steel further contains Ni: 1% or less.