JPH06240353A - Production of 780mpa class high tensile strength steel excellent in weldability and low temperature toughness - Google Patents

Abstract

PURPOSE:To provide the method for producing 780MPa class high tensile strength steel excellent in weldability and low temp. toughness. CONSTITUTION:Steel contg. 0.01 to 0.20% C, <=0.04% Si, 0.6 to 2.0% Mn, 0.005 to 0.08% Nb, 0.005 to 0.03% Ti, 0.05 to 1.0% Cu, 0.5 to 4.0% Ni, 0.005 to 0.08% sol.Al and <=0.006% N is heated to 1000 to 1250 deg.C and is rolled down at <=900 deg.C at >=50% cumulative draft, and the rolling is completed at the Ar3 point to 900 deg.C. Immediately after that, it is subjected to accelerated cooling to 580 to 300 deg.C, is cooled and is thereafter isothemtally held for 10 to 100sec or is cooled for 10 to 100sec at <=0.5 deg.C/s cooling rate. Next, the operation of reheating it to the Ac3 point to (the Ac3 point+1000C) and thereafter subjecting it to hardening treatment is executed for one time or repeated for >=two times, and after that, tempering treatment is executed at the Ac1 point or below. The steel may moreover be incorporated with one or more kinds among 0.05 to 1.0% Cr, 0.05 to 1.50% Mo, 0.01 to 0.10% V and 0.0005 to 0.0020% B.

Description

Detailed Description of the Invention

[0001]

The present invention has a tensile strength of 780 MPa.
It is related to the manufacturing method of high-strength steel that has the above performance and is excellent in weldability and low-temperature toughness.
The present invention relates to a method for producing a heat-treated high-strength steel suitable for stably producing the thick steel.

[0002]

2. Description of the Related Art In recent years, the tendency of welded structures to increase in size has become more and more remarkable, and structural steel sheets used for these structures have been increasing in tension and thickness. For example, 100 mm thick 780 MPa class high-strength steel sheets have been used for the rack material of jack-up type oil drilling rigs. However, the weldability and low temperature toughness tend to decrease with increasing strength, and it is said that the 780MPa class high strength steel sheet that is currently generally used has both sufficient toughness and weldability. There is a demand for a 780 MPa grade thick steel plate that is difficult and has excellent performance from the viewpoint of structural safety, that is, prevention of brittle fracture.

As a conventional method for producing a 780 MPa class high strength thick steel plate, as disclosed in Japanese Patent Laid-Open No. 195242/1989, after rolling is completed, continuous reheating is carried out twice, quenching is followed by one tempering. It is known that the toughness is improved by a total of three heat treatment methods of performing.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
According to the method disclosed in Japanese Patent Publication No. 42, the austenite grain size before the second quenching cannot be made sufficiently fine, and desired toughness cannot be obtained. Therefore, it is necessary to optimize the amount of the added alloy element and further devise the first heat treatment method.

The object of the present invention is to obtain fine austenite grains during reheating by forming a bainite structure having fine carbide and carbonitride dispersion in the first stage heat treatment. Toughness and low temperature toughness
An object of the present invention is to provide a method for manufacturing 780 MPa grade high strength thick steel.

[0006]

The summary of the present invention is as follows.
It is in the manufacturing method of (1) and (2).

(1) In mass%, C: 0.01 to 0.20%, Si:
0.04% or less, Mn: 0.6 to 2.0%, Nb: 0.005 to 0.08%,
Ti: 0.005-0.03%, Cu: 0.05-1.0%, Ni: 0.5-4.
Steel containing 0%, sol.Al: 0.005 to 0.08% and N: 0.006% or less, with the balance Fe and unavoidable impurities,
After heating to a temperature of 1000 to 1250 ° C, a reduction of cumulative reduction of 50% or more is applied at a temperature of 900 ° C or less, rolling is completed at a temperature of Ar ₃ points or more and 900 ° C or less, and then as the first heat treatment. Immediately, accelerate cooling to 580 ℃ or less and 300 ℃ or more, and after cooling, at 580 ℃ or less and 300 ℃ or more, for 10 seconds or more.
Keep isothermal for 100 seconds or less, or 580 ℃ or less 300
0.5 ℃ / s in 10 seconds or more and 100 seconds or less at a temperature of ℃ or more
Cooling at the following cooling rate, then as the second heat treatment, reheating to a temperature range of Ac ₃ point or higher and (Ac ₃ point + 100 ° C) or lower, and then quenching treatment is repeated once or twice or more. The method for producing a 780 MPa class high strength steel excellent in weldability and low temperature toughness, which comprises performing tempering treatment at a temperature of Ac ₁ point or lower.

(2) The steel described in (1) above has a further mass%
At Cr: 0.05 to 1.0%, Mo: 0.05 to 1.50%, V: 0.01
To 0.10% and B: 0.0005 to 0.0020%, and a method for producing a 780 MPa class high strength steel excellent in weldability and low temperature toughness according to (1) above.

From the above viewpoints, the present inventors have earnestly studied to provide a means for producing a 780 MPa grade thick steel having excellent weldability and toughness, and as a result, the strength of the 780 MPa grade high strength steel is reduced. It was found that it is most effective to make the austenite grain size fine during reheating in order to improve the toughness without causing it.

However, in the conventional method in which the first treatment of the third heat treatment method is simply a quenching treatment, a coarse upper bainite or a mixed structure of this and martensite is formed, which is desirable as the structure before the second hardening. It is not suitable as a method for obtaining fine austenite grains.

In order to solve the above problems, a high toughness of 780M is obtained from both the aspects of thermomechanical treatment and optimization of the amount of added alloy elements.
As a result of examining the production method of the Pa class high strength steel, the following
I found the means of.

In the first-stage heat treatment, a unique processing of applying a cumulative reduction of 50% or more in the unrecrystallized austenite region, accelerating cooling under a constant condition, and then maintaining the same temperature or slowly cooling under a constant condition after cooling. By the heat treatment method, the structure before the second heat treatment can be a bainite structure in which carbides and carbonitrides are finely dispersed.

When the steel having the above bainite structure as a pre-structure is reheated to a temperature range of Ac ₃ point or higher and (Ac ₃ point + 100 ° C.) or lower, extremely fine austenite grains are formed as compared with the conventional method. can get.

In addition to these original processing heat treatment methods,
Manufacture of 780MPa class high-strength steel with excellent toughness and excellent weldability by controlling the Si content, which adversely affects the toughness, especially the toughness of the weld heat affected zone, to 0.04 mass% or less. can do.

[0015]

The operation of the present invention will be described in detail below based on the results of experiments conducted by the present inventors.

First, the reason why the chemical composition of the raw steel is limited as described above will be explained. In addition, all "%" mean the mass%.

C: 0.01 to 0.20% C is an element effective for increasing strength, and for this purpose, the C content must be 0.01% or more. However, from the viewpoint of ensuring toughness and preventing deterioration of weld crack resistance, the upper limit is 0.20%.
And

Si: 0.04% or less Si is an element necessary for deoxidation, but the Si content is 0.04% or less.
%, The low-temperature toughness of the heat-affected zone of the weld is reduced, so the content was made 0.04% or less.

Mn: 0.6 to 2.0% Mn is an element effective for increasing strength, and for this purpose, the Mn content must be 0.6% or more. However, if it exceeds 2.0%, the toughness deteriorates, so the upper limit was made 2.0%.

Nb: 0.005 to 0.08% Nb is an element effective for refining crystal grains, and for this purpose, the Nb content must be 0.005% or more. But 0.
If it exceeds 08%, the toughness deteriorates, so the upper limit was made 0.08%.

Ti: 0.005-0.03% Ti is an element effective for refining crystal grains, and for this purpose, the Ti content must be 0.005% or more. But 0.
If it exceeds 03%, the toughness deteriorates, so the upper limit was made 0.03%.

Sol.Al: 0.005 to 0.08% Since Al is an element effective as a deoxidizing agent and also for refining crystal grains, 0.005% or more of sol.Al is contained.
However, if the sol.Al content exceeds 0.08% and becomes excessive,
The upper limit is 0.08% because it creates inclusions that are harmful to the properties of steel.
And

N: 0.006% or less N forms a nitride together with Al and is effective for refining the crystal grains, but if the N content exceeds 0.006% and becomes excessive, the toughness of the welded portion is impaired. The upper limit was set to 0.006%.

Cu: 0.05 to 1.0% Cu is an element effective for increasing the strength, and for this purpose, the Cu content must be 0.05% or more. However, if the Cu content exceeds 1.0%, the toughness decreases, so the upper limit was made 1.0%.

Ni: 0.5 to 4.0% Ni is an element effective for improving the low temperature toughness, and for that purpose, it is necessary to contain 0.5% or more. But,
When the Ni content exceeds 4.0%, the strength increase and toughness improvement cannot be obtained to meet the cost increase due to the addition, so the upper limit was made 4.0%.

In the steel used as the raw material of the method of the present invention, in addition to the above-mentioned elements, any one of the following elements
One kind or two or more kinds can be contained.

Cr: 0.05 to 1.0% Cr is an element effective for increasing the strength. For that purpose, the Cr content must be 0.05% or more, but if it exceeds 1.0% in an excessive amount, the toughness decreases. Therefore, the upper limit was made 1.0%.

Mo: Mo: 0.05 to 1.50% Mo is an element effective for increasing the strength. For that purpose, the Mo content must be 0.05% or more, but if the Mo content exceeds 1.50%, it becomes too much. The upper limit is 1.5 to reduce toughness.
%.

V: 0.01 to 0.10% V is an element effective for increasing the strength, and for that purpose, the V content must be 0.01% or more, but if the V content exceeds 0.10%, the toughness becomes tough. The upper limit is 0.1 to lower
%.

B: B: 0.0005 to 0.0020% B is an element effective in improving hardenability and accompanying increase in strength, and for this purpose, the B content must be 0.0005% or more. However, if the B content exceeds 0.0020%, the toughness deteriorates, so the upper limit was made 0.0020%.

Next, each condition of rolling and heat treatment will be described.

The most important point in the production method of the present invention is the pretreatment conditions necessary for obtaining fine austenite grains when reheated to a temperature range of Ac ₃ point or higher and (Ac ₃ point + 100 ° C.) or lower, That is, it is the first-stage thermo-mechanical treatment condition, and it is essential to obtain a bainite structure having carbide and carbonitride finely dispersed therein under this condition.

The reasons for limiting the conditions of rolling and heat treatment as described above will be described in detail below.

1) Heating temperature of raw steel billet The upper limit temperature is set to 1250 ° C. in order to prevent coarsening of austenite crystal grains during heating, while it is effective for refining the crystal grains during rolling and strengthening precipitation after rolling. The minimum temperature is set to 1000 ° C to form a solid solution of Nb.

In order to refine the bainite structure by rolling, it is necessary to roll at a temperature of 900 ° C. or lower with a cumulative reduction of 50% or more.

2) When the rolling completion temperature is higher than 900 ° C. or the rolling reduction is insufficient (less than 50%), a fine austenite grain structure cannot be obtained, and the rolling completion temperature is Ar ₃ point. If it is less than 100%, the toughness deteriorates. For this reason, the rolling completion temperature is Ar ₃ points or more 9
Limited to a temperature range of 00 ° C or less.

3) First-stage heat treatment (cooling temperature range and cooling rate immediately after completion of rolling) As a condition for obtaining a sufficient structure refinement and strength increase due to bainite structure formation, a temperature range of 580 to 300 ° C. It needs to be cooled. The most desirable cooling rate condition at this time is accelerated cooling of 10 ° C./s or more.

In order to sufficiently obtain a fine bainite structure, after carrying out the above-mentioned accelerated cooling, it is kept isothermal in the temperature range of 580 to 300 ° C. for 10 seconds or more and 100 seconds or less, or Instead of isothermal holding, it is necessary to perform slow cooling at a cooling rate of 0.5 ° C / s or less in the temperature range of 580 to 300 ° C for a period of 10 seconds to 100 seconds.

If the isothermal holding time is less than 10 seconds, a fine bainite structure cannot be obtained sufficiently. On the other hand, if it exceeds 100 seconds, the bainitic ferrite (ferrite part that constitutes the bainite structure) will unite and the toughness will deteriorate. Therefore, the retention time must be 10 seconds or more and 100 seconds or less.

The same effect as described above can be obtained by performing the above-mentioned slow cooling for a predetermined time instead of holding the same temperature.

4) Second-stage heat treatment (reheating, quenching) Further, in the reheating before the quenching treatment, in order to prevent the austenite grains from coarsening, the heating temperature is Ac ₃ points or higher, ( Ac ₃ points +100 ℃) It is necessary to limit to below. If the heating temperature is less than Ac ₃ point, untransformed ferrite remains, and uniform and fine austenite grains cannot be obtained. On the other hand, if it exceeds (Ac ₃ point + 100 ° C), the austenite grains become coarse and the toughness deteriorates.

By repeating such quenching treatment two or more times and making the second quenching temperature lower than the first quenching temperature, the structure can be made finer and the toughness can be improved.

5) Tempering Treatment This is carried out in order to improve the strength / toughness balance by removing the strain caused by quenching and precipitating carbides finely. Then, in order not to hinder the balance between strength and toughness, the tempering temperature must be limited to Ac ₁ point or lower, and is preferably 450 ° C. or higher.

[0044]

[Example] From a raw steel slab (thickness: 160 mm) having the chemical composition shown in Table 1 manufactured and melted by a conventional method, a thickness of 40 to 40 is obtained under the manufacturing conditions shown in Tables 2 (1) and 2 (2). A 65 mm steel plate was manufactured.

The evaluation of the obtained steel sheet is 1 / thick of the thickness of the steel sheet.
Specimens were taken from 4 parts and a tensile test and a 2 mmV notch Charpy impact test were used. These results are shown in Table 2 (1)
And also shown in Table 2 (2).

[0046]

[Table 1]

[0047]

[Table 2 (1)]

[0048]

[Table 2 (2)]

From Table 2 (1) and Table 2 (2), the tensile strength of the steel sheet of the example of the present invention manufactured under the conditions defined in the present invention
It shows a strength of 780 MPa or more and an absorbed energy of 215 J or more in the Charpy impact test at -80 ° C.
It can be seen that a high strength steel with excellent low temperature toughness is obtained. The high-strength steel produced by the method of the present invention has a low C content and a low Si content, and therefore has excellent weldability.

[0050]

According to the method of the present invention, the tensile strength is 780
It is possible to manufacture a high-strength steel having a MPa grade and a good low-temperature toughness which is excellent in weldability.

Claims (2)

[Claims] 1. In mass%, C: 0.01 to 0.20%, Si: 0.04
% Or less, Mn: 0.6 to 2.0%, Nb: 0.005 to 0.08%, Ti:
0.005-0.03%, Cu: 0.05-1.0%, Ni: 0.5-4.0
%, Sol.Al: 0.005 to 0.08% and N: 0.006% or less, with the balance being Fe and inevitable impurities, 10
After heating to a temperature of 00 to 1250 ℃, a reduction of cumulative reduction of 50% or more is applied at a temperature of 900 ℃ or lower, rolling is completed at a temperature of Ar ₃ points or higher and 900 ℃ or lower, and then as the first heat treatment. ,
Immediately perform accelerated cooling to a temperature of 580 ° C or less and 300 ° C or more, and after cooling, hold isothermally at a temperature of 580 ° C or less and 300 ° C or more for 10 seconds or more and 100 seconds or less, or at a temperature of 580 ° C or less and 300 ° C or more. , 10 seconds or more and 100 seconds or less at a cooling rate of 0.5 ° C / s or less, and then as the second heat treatment, Ac ₃
It is recommended to reheat to a temperature range above the point above (Ac ₃ points + 100 ° C) and then to perform quenching once or more times, and then perform tempering at a temperature below the Ac ₁ point. A method for producing 780 MPa class high strength steel with excellent weldability and low temperature toughness. 2. In mass%, C: 0.01 to 0.20%, Si: 0.04
% Or less, Mn: 0.6 to 2.0%, Nb: 0.005 to 0.08%, Ti:
0.005-0.03%, Cu: 0.05-1.0%, Ni: 0.5-4.0
%, Sol.Al: 0.005 to 0.08% and N: 0.006% or less, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.50%,
Steel containing at least one of V: 0.01 to 0.10% and B: 0.0005 to 0.0020% and the balance being Fe and inevitable impurities is heated to a temperature of 1000 to 1250 ° C, and then accumulated at a temperature of 900 ° C or less. Reduction of 50% or more is applied, Ar ₃ points or more 900
Rolling is completed at a temperature of ℃ or less, then as the first heat treatment, it is immediately accelerated cooled to a temperature of 580 ℃ or less and 300 ℃ or more, and after cooling, it is 580 ℃ or less and 300 ℃ or more for 10 seconds or more.
Keep isothermal for 100 seconds or less, or 580 ℃ or less 300
0.5 ℃ / s in 10 seconds or more and 100 seconds or less at temperature above ℃
Cooling at the following cooling rate, then as the second heat treatment, reheating to a temperature range of Ac ₃ point or higher and (Ac ₃ point + 100 ° C) or lower, and then quenching treatment is repeated once or twice or more. The method for producing a 780 MPa class high strength steel excellent in weldability and low temperature toughness, which comprises performing tempering treatment at a temperature of Ac ₁ point or lower.