JPH0813031A - Method for producing steel sheet with excellent low temperature toughness and weld splitting resistance - Google Patents

Abstract

(57) [Summary] (Corrected) [Purpose] Suitable manufacturing method for stably obtaining tempered high-strength thick steel plate with a tensile strength of 590 MPa or more and excellent in weldability and low-temperature toughness and having a plate thickness of 40 mm or more. . [Composition] C: 0.01 to 0.15, Si:
0.005-0.5, Mn: 0.6-2.0, Nb:
0.01-0.07, Ti: 0.005-0.03 and sol. It contains Al: 0.005 to 0.08, Cu: 0 to 2.0, Ni: 0 to 1.0, Cr: 0 to
1.0, Mo: 0 to 1.50, V: 0 to 0.10 and B: 0 to 0.0020, and the balance is Fe and inevitable impurities, and N in them is 0.006. In the following, P _CM = C + 1/30 · Si + 1/20 · Mn + 1/2
0 / Cu + 1/60 / Ni + 1/20 / Cr + 1/15
· Mo + 1/10 · V + 5B ............... the (a) a type _{P CM} is 0.18 less than the steel raw material of 1000-12
When heated to 50 ° C, the cumulative rolling reduction in the unrecrystallized γ region is 50%
The above hot rolling, accelerated cooling to 300 ° C or less, Ac
A tempering treatment is carried out by holding at ₁ point or less for ₁₀₀ to 4000 seconds.

Description

Detailed Description of the Invention

[0001]

The present invention has a tensile strength of 590 MPa.
It relates to a steel plate having the above performance and excellent in weldability and low temperature toughness, and particularly relates to a manufacturing method suitable for stably obtaining a tempered high tensile thick steel plate having a plate thickness of 40 mm or more having the above characteristics. .

[0002]

2. Description of the Related Art Conventionally, many alloy elements have been added to thick steel plates used for welded structures such as ships, offshore structures, bridges and tanks in order to secure strength. Therefore, such a high strength steel sheet tends to have a high carbon equivalent, and its weldability is a problem. In particular, in order to prevent cold cracking after welding, it is necessary to preheat at an appropriate temperature before welding, which is an obstacle in promoting the cost reduction of welding work.

On the other hand, in the case of steel in which the amount of alloying elements added is reduced in order to improve weldability, a complete quenched structure cannot be obtained even by the quenching treatment, and sufficient toughness cannot be obtained.

Aiming at improving weldability and toughness
A method for producing 590 MPa grade high strength steel is disclosed in Japanese Patent Laid-Open No. 64-42526.
JP-A-1-96329, JP-A-1-149923, JP-A-1-156424, JP-A-1-219122 and JP-A-2-20.
It is disclosed in each publication of 5627.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The methods disclosed in Japanese Patent Laid-Open No. 6-156424 and Japanese Patent Application Laid-Open No. 1-156424 provide a steel sheet with excellent toughness, but have a problem in securing the toughness in the weld heat affected zone (HAZ) due to the high Si content. Therefore, it is necessary to reduce the Si content.

According to the method disclosed in Japanese Patent Laid-Open No. 1-96329, it is possible to obtain a bainite structure from fine recrystallized γ by performing hot rolling with a cumulative rolling reduction of 40% or more in the γ recrystallization region. However, in the transformation from recrystallized γ with few lattice defects, nucleation in γ grains cannot be expected, and a fine bainite-ferrite mixed structure necessary for improving toughness cannot be obtained.

Even in the method disclosed in Japanese Patent Application Laid-Open No. 1-149923, since Nb, which has a high effect of suppressing the recrystallization of γ, is not added, it is transformed from recrystallized γ, and fine bainite necessary for improving toughness is obtained. It is not possible to obtain a mixed structure of and ferrite.

In the method disclosed in Japanese Patent Laid-Open No. 1-219122, since the cooling rate after hot rolling is controlled to obtain a structure mainly composed of ferrite, in order to secure the strength, a mixed structure of bainite and ferrite is used. Higher carbon equivalents than in the case are required, and deterioration of weldability and low temperature toughness is unavoidable.

In the method disclosed in Japanese Patent Application Laid-Open No. 2-205627, since the strength maintenance depends on the addition of B, the hardenability is remarkably lowered in the strong reduction rolling, and the heat reduction of less than 30% is performed. It is necessary to limit the rolling to hot rolling, and improvement of toughness due to sufficient refinement of the structure cannot be expected.

[0010]

The summary of the present invention is as follows.
It is in the method of manufacturing a steel sheet of (1) to (3).

(1) C: 0.01 to 0.15% by weight, Si: 0.1
005 to 0.05%, Mn: 0.6 to 2.0%, Nb: 0.01 to 0.07%, T
i: 0.005-0.03% and sol.Al: 0.005-0.08%, and further Cu: 0-2.0%, Ni: 0-1.0%, Cr: 0
~ 1.0%, Mo: 0 to 1.50%, V: 0 to 0.10% and B:
0 to 0.0020% of one or more, the balance consisting of Fe and inevitable impurities, N in the impurities is 0.006% or less, and the weld crack susceptibility composition P _CM (%) represented by the following formula (a) is 0. .
A method for producing a steel sheet having excellent low-temperature toughness and weld crack resistance (hereinafter, referred to as a first method), which is characterized in that a material steel having a number of less than 18 is processed in the following steps.

A hot rolling step of heating to 1000 to 1250 ° C. to make the cumulative reduction in the unrecrystallized γ region 50% or more, followed by a step of accelerating cooling to 300 ° C. or less, and then 100 at a temperature of Ac ₁ point or less. Process for tempering that holds for ~ 4000s (sec) P _CM = C + 1/30 ・ Si + 1/20 ・ Mn + 1/20 ・ Cu + 1/60 ・ Ni + 1/20 ・ Cr + 1/15 ・ Mo + 1/10 ・ V + 5B ・ ・・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (A) However, the element symbol is the content of each element (% by weight)
Indicates.

(2) The material steel having the composition of (1) above is
The method for producing a steel sheet excellent in low temperature toughness and weld crack resistance (hereinafter, referred to as a second method), which is characterized in that it is treated in the step of.

Hot rolling step of heating to 1000 to 1250 ° C. to make the cumulative reduction in the unrecrystallized γ region 50% or more, followed by accelerated cooling to a temperature range of 580 to 450 ° C. At the cooling stop temperature Step of holding isothermal for 100 to 4000s (seconds) Step of cooling to warm state After that Step of tempering at a temperature of Ac ₁ point or less if necessary (3) Raw material steel of the above (1) component Is processed in the following steps (1) to (4) to produce a steel sheet having excellent low temperature toughness and weld crack resistance (hereinafter referred to as a third method).

Hot rolling step of heating to 1000 to 1250 ° C. to make the cumulative reduction in the non-recrystallized γ region 50% or more, and then accelerated cooling to a temperature range of 580 to 450 ° C. From the cooling stop temperature Within a time range of 100 to 4000 s (seconds), a step of slow cooling at a cooling rate of 1 ° C / s or less, a step of further cooling to a warm state, and then a tempering treatment at a temperature of Ac ₁ point or less as required. Performing the process In each of the above material steels, Cu, Ni, Cr, Mo, V and B
None of them may be added. The lower limit when positively contained is 0.05% for Cu, 0.05% for Ni, 0.05% for Cr, and 0 for Mo.
It is desirable to set it to 05%, 0.01% for V and 0.0005% for B.

The above "warm state" means a temperature state in the range of 100 to 300 ° C.

From the viewpoint of solving the above-mentioned problems, the inventors of the present invention have conducted research on a method for producing a 590 MPa grade steel sheet having excellent weldability and toughness, and as a result, have found that Nb addition with a limited alloying element content is added. 590MPa-class preheating with excellent weldability and toughness by optimally rolling low-Si steel in the non-recrystallization temperature range of γ to form a fine bainite-ferrite mixed structure with carbide and carbonitride dispersion. For the first time, we have found that stable production of free steel sheets is possible.

[0018]

The reason for limiting the method of the present invention as described above will be described in detail below based on experimental results and the like. First, the reasons for limiting the chemical composition of the raw steel that is the subject of the method of the present invention will be described. % Means% by weight.

C: 0.01 to 0.15% C is an element effective for increasing strength, and in order to obtain the effect, a C content of 0.01% or more is necessary. However, from the viewpoint of ensuring toughness and preventing deterioration of weld crack resistance, the upper limit must be 0.15%.

Si: 0.005 to 0.05% Si is an effective element for deoxidation. To obtain the effect,
A Si content of 0.005% or more is required. However, if the Si content exceeds 0.05%, the low temperature toughness of the HAZ decreases, so the upper limit was made 0.05%.

Mn: 0.6 to 2.0% Mn is an element effective for increasing strength, and in order to obtain the effect, the Mn content must be 0.6% or more. But,
If the Mn content exceeds 2.0%, the toughness deteriorates, so the upper limit was made 2.0%.

Nb: 0.01 to 0.07% Nb is an element effective for refining crystal grains, and in order to obtain the effect, a Nb content of 0.01% or more is necessary. However, if the Nb content exceeds 0.07%, the toughness deteriorates, so
The upper limit was 0.07%.

Ti: 0.005 to 0.03% Ti is an element effective for refining crystal grains, and in order to obtain the effect, the Ti content must be 0.005% or more.
However, if the Ti content exceeds 0.03%, the toughness deteriorates, so the upper limit was made 0.03%.

Sol.Al: 0.005 to 0.08% Al is an element effective for deoxidizing and refining crystal grains.
To obtain this effect, the sol.Al content must be 0.005% or more. However, if the sol.Al content exceeds 0.08% and becomes excessive, harmful inclusions are produced in the steel, so the upper limit must be 0.08%.

N: 0.006% or less N is an impurity element contained inevitably. N content is
If the amount exceeds 0.006% and the HAZ toughness is impaired, the upper limit was made 0.006%. Within this range, Al
It produces nitrides and effectively contributes to the refinement of crystal grains.

In addition to the above-mentioned constituent steels,
It can contain one or more of Cu, Ni, Cr, Mo, V and B.

Cu: Upper limit 2.0% Cu may be added without any addition, but is an element effective for increasing strength.
When positively obtaining this effect, the Cu content is preferably 0.05% or more. However, if the Cu content exceeds 2.0%, the toughness decreases, so the upper limit was made 2.0%.

Ni: upper limit 1.0% Ni may be added without any addition, but is an element effective for improving low temperature toughness. To positively obtain this effect, the Ni content is preferably 0.05% or more. However, the Ni content is
Even if it exceeds 1.0%, it is not possible to obtain the strength increase and toughness improvement commensurate with the cost increase, so the upper limit was made 1.0%.

Cr: upper limit 1.0% Cr may be added without any addition, but is an element effective for increasing strength.
To positively obtain this effect, the Cr content is preferably 0.05% or more. However, if the Cr content exceeds 1.0%, the toughness decreases, so the upper limit was made 1.0%.

Mo: Upper limit 1.50% Mo may be added without any addition, but is an element effective for increasing strength.
When positively obtaining this effect, the Mo content is preferably 0.05% or more. However, if the Mo content exceeds 1.50%, the toughness decreases, so the upper limit was made 1.50%.

V: upper limit 0.10% V may be added without any addition, but it is an element effective for increasing strength.
To positively obtain this effect, the V content is preferably 0.01% or more. However, if the V content exceeds 0.10%, the toughness decreases, so the upper limit was made 0.10%.

B: Upper limit 0.0020% B may be added without any addition, but it is an element effective for improving the hardenability and the accompanying increase in strength. In order to positively obtain this effect, the lower limit of B content is preferably 0.0005%.
However, if the B content exceeds 0.0020%, the toughness decreases, so the upper limit was made 0.0020%.

The above material steel must further have a weld crack susceptibility composition P _CM (%) represented by the following formula (a) of less than 0.18.

P _CM = C + 1/30 ・ Si + 1/20 ・ Mn + 1/20 ・ Cu + 1/60 ・ Ni + 1/20 ・ Cr + 1/15 ・ Mo + 1/10 ・ V + 5B ・ ・ ・ ・ ・ ・ ・ (a ) However, the element symbol is the content of each element (% by weight)
Indicates.

If this P _CM is 0.18 or more, it is impossible to prevent cracking after welding when welding thick steel plates exceeding 40 mm without preheating.

Next, the reasons for limiting the conditions of hot rolling and heat treatment will be described. What is most important in the method of the present invention is to generate a fine mixed structure of bainite and ferrite by nucleation at grain boundaries and in grains from γ which preserves the working strain introduced by hot rolling.

In any of the first to third methods of the present invention, the material steel is heated to 1000 to 1250 ° C. and hot rolled so that the cumulative rolling reduction in the unrecrystallized γ region is 50% or more. .

The upper limit of the heating temperature of the material steel is 1250 ° C. in order to prevent coarsening of austenite crystal grains during heating,
On the other hand, the lower limit was set to 1000 ° C. in order to form a solid solution of Nb which is effective in refining crystal grains during hot rolling and precipitation strengthening after hot rolling.

Regarding the rolling reduction conditions, if hot rolling is completed with the cumulative rolling reduction of γ in the unrecrystallized temperature range being less than 50%, a fine crystal structure cannot be obtained. Therefore, in order to refine the bainite structure by hot rolling,
It is necessary to perform hot rolling such that the cumulative rolling reduction of γ in the unrecrystallized region is 50% or more.

In the first method of the present invention, after the above hot rolling, immediately after accelerated cooling to 300 ° C. or lower, a tempering treatment is carried out at a temperature of Ac ₁ point or lower for ₁₀₀ to 4000 s (seconds). .

After hot rolling, in order to obtain a fine structure and a fine mixed structure of bainite and ferrite in a well-balanced state, it is immediately accelerated-cooled to 300 ° C. or lower. The desirable cooling rate at this time is 10 ° C./s or more. The desirable cooling stop temperature is 250 ° C. As a cooling method, a spray method, a roller quench method, or the like is preferably used.

When continuously cooled to a temperature of 300 ° C. or lower as described above, carbides important for securing strength are precipitated, and after cooling, in order to remove transformation strain introduced by accelerated cooling, It is necessary to perform tempering treatment at a temperature of Ac ₁ point or less. A desirable tempering temperature is 450 ° C or higher so that the balance between strength and toughness is not hindered.

If the holding time of the tempering treatment is less than 100 s, carbides are not sufficiently precipitated. On the other hand, when it exceeds 4000 s, the precipitated carbides become coarse. Therefore, the holding time for tempering is 100 s
More than 4000s and less.

After the above tempering, the cooling rate was 10 ° C. by water cooling.
It is desirable to cool down to about 100 ° C above s / s.

In the second method of the present invention, hot rolling is performed under the same conditions as in the first method, immediately followed by accelerated cooling to a temperature range of 580 ° C. to 450 ° C., and then 100 to 4000 seconds at the cooling stop temperature.
After being kept isothermal for a period of time, it is cooled to the temperature in the warm state, and if necessary, tempered at a temperature of Ac ₁ point or lower.

After hot rolling, in order to obtain a finer structure and a finer mixed structure of bainite and ferrite in a more balanced manner, it is immediately accelerated-cooled to a temperature range of 580 to 450 ° C. If the cooling stop temperature exceeds 580 ° C, fine carbides cannot be sufficiently precipitated in bainite. On the other hand, if the temperature is lower than 450 ° C, fine carbides are precipitated but the bainite structure itself cannot be made fine.

The desirable cooling rate at this time is 10 ° C./s or more. As a cooling method, a spray method, a roller quench method, or the like is preferably used.

If the holding time at the cooling stop temperature is less than 100 s, the carbide will not be sufficiently precipitated. On the other hand, when it exceeds 4000 s, the precipitated carbides become coarse. Therefore, the holding time after accelerated cooling is set to 100s or more and 4000s or less.

After the above isothermal holding, cooling is continued until the temperature reaches a warm state (about 100 to 300 ° C.). In this case, it is desirable to cool with water cooling to about 250 ° C at a cooling rate of 10 ° C / s or more. If the tempering process described below is not performed thereafter, the product is further cooled to room temperature.

After cooling to the warm temperature or room temperature as described above, if necessary, tempering is further performed at a temperature of Ac ₁ point or lower. This tempering treatment may be carried out when it is necessary to further improve the balance between strength and toughness by removing the strain introduced by cooling and finely precipitating carbides. The temperature when carrying out this tempering treatment is
In order to obtain the above-mentioned effects, it is necessary to limit the Ac to ₁ point or less, preferably 450 ° C. or more. The desired holding time for tempering is 2000s. After tempering, it is better to cool to about 100 ° C with water cooling at a cooling rate of 10 ° C / s or more.

In the third method of the present invention, the first and second
Hot rolling under the same conditions as in the above method, followed by accelerated cooling according to the second method above, and then from the cooling stop temperature
Within the cooling time range of 100 s to 4000 s, slow cooling is performed at a cooling rate of 1 ° C./s or less, followed by further cooling to a warm state temperature. If the subsequent tempering process is not performed, the product is further cooled to room temperature. Then, similarly to the second method, if necessary, tempering treatment is further performed at a temperature of Ac ₁ point or lower.

Under the mild cooling condition, if the cooling time is less than 100 s, the carbides are not sufficiently precipitated, while if it exceeds 4000 s, the precipitated carbides become coarse. If the cooling rate exceeds 1 ° C./s, the effect of slow cooling cannot be obtained.

After the above slow cooling, cooling is further performed until the temperature reaches a warm state (about 100 to 300 ° C.). By this cooling, it is possible to obtain a well-balanced mixed structure of fine bainite in which fine carbide is precipitated and ferrite. The conditions in this case and the subsequent process when the tempering process is not performed may be the same as those in the second method of the present invention described above.

Then, if necessary, tempering treatment is performed.
The effect, desirable conditions and method are the same as in the case of the second method of the present invention described above.

[0055]

EXAMPLES Steels having the chemical compositions shown in Table 1 were melted by a conventional method,
The obtained raw material billet was subjected to thermo-mechanical treatment according to the production conditions shown in Table 2 to produce a test steel sheet having a thickness of 40 to 65 mm. Specimens were taken from 1/4 of the thickness of these steel sheets, and the tensile test of the base metal and the 2mmV notch Charpy impact test and the HAZ 2mmV notch Charpy impact test without preheating and diagonal Y-shaped weld cracking were performed. The test was conducted.
Table 3 shows the results.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

As shown in Table 3, the base steel sheet manufactured according to the method of the present invention has a tensile strength of 590 MPa or more and a absorbed energy of 215 J or more in the Charpy impact test at -80 ° C. Moreover, in HAZ, it is -40
Absorbed energy of 200 J or more is obtained in Charpy impact test at ℃. No cracks occurred in the oblique Y-shaped weld crack test without preheating.

[0060]

According to the method of the present invention, the tensile strength is 590 M.
It is possible to manufacture a high-strength steel sheet having a Pa value of Pa or more and good low temperature toughness. In this steel sheet, cracking does not occur even if preheating before welding is omitted.

Claims (3)

[Claims] 1. C: 0.01 to 0.15% by weight, Si: 0.005
~ 0.05%, Mn: 0.6-2.0%, Nb: 0.01-0.07%, Ti:
0.005 to 0.03% and sol.Al: 0.005 to 0.08%, further containing Cu: 0 to 2.0%, Ni: 0 to 1.0%, Cr: 0
1.0%, Mo: 0 to 1.50%, V: 0 to 0.10% and B: 0
~ 0.0020% of one or more, the balance consisting of Fe and inevitable impurities, N in the impurities is 0.006% or less, and the weld crack susceptibility composition P _CM (%) represented by the following formula (a) is 0.18.
The material steel which is less than 1000 is heated to 1000 to 1250 ° C, hot-rolled so that the cumulative rolling reduction in the unrecrystallized γ region is 50% or more, and then accelerated cooling to 300 ° C or less, and then Ac ₁ A method for producing a steel sheet having excellent low temperature toughness and weld crack resistance, which is characterized by performing a tempering treatment in which the temperature is maintained at a temperature below the point for 100 s to 4000 s. _{P CM = C + 1/30} · Si + 1/20 · Mn + 1/20 · Cu + 1/60 · Ni + 1/20 · Cr + 1/15 · Mo +1/10 · V + 5B ············ (a) where Element symbol is the content (% by weight) of each element
Indicates. 2. C: 0.01 to 0.15% by weight, Si: 0.005% by weight
~ 0.05%, Mn: 0.6-2.0%, Nb: 0.01-0.07%, Ti:
0.005 to 0.03% and sol.Al: 0.005 to 0.08%, further containing Cu: 0 to 2.0%, Ni: 0 to 1.0%, Cr: 0
1.0%, Mo: 0 to 1.50%, V: 0 to 0.10% and B: 0
~ 0.0020% of one or more, the balance consisting of Fe and inevitable impurities, N in the impurities is 0.006% or less, and the weld crack susceptibility composition P _CM (%) represented by the following formula (a) is 0.18.
The material steel that is less than 1000 is heated to 1000 to 1250 ° C, hot rolling is performed to achieve a cumulative reduction of 50% or more in the unrecrystallized γ range, and then accelerated to a temperature range of 580 ° C or less and 450 ° C or more. After cooling, it is isothermally held at the cooling stop temperature for 100s or more and 4000s or less, then cooled to a warm state, and then tempered at a temperature of Ac ₁ point or less, if necessary. A method of manufacturing a steel sheet having excellent toughness and weld crack resistance. _{P CM = C + 1/30} · Si + 1/20 · Mn + 1/20 · Cu + 1/60 · Ni + 1/20 · Cr + 1/15 · Mo +1/10 · V + 5B ············ (a) where Element symbol is the content (% by weight) of each element
Indicates. 3. By weight%, C: 0.01 to 0.15%, Si: 0.005
~ 0.05%, Mn: 0.6-2.0%, Nb: 0.01-0.07%, Ti:
0.005 to 0.03% and sol.Al: 0.005 to 0.08%, further containing Cu: 0 to 2.0%, Ni: 0 to 1.0%, Cr: 0
1.0%, Mo: 0 to 1.50%, V: 0 to 0.10% and B: 0
~ 0.0020% of one or more, the balance consisting of Fe and inevitable impurities, N in the impurities is 0.006% or less, and the weld crack susceptibility composition P _CM (%) represented by the following formula (a) is 0.18.
The material steel that is less than 1000 is heated to 1000 to 1250 ° C, hot rolling is performed to achieve a cumulative reduction of 50% or more in the unrecrystallized γ range, and then accelerated to a temperature range of 580 ° C or less and 450 ° C or more. After cooling, after slowly cooling at a cooling rate of 1 ° C / s or less within a time period of 100s or more and 4000s or less from the cooling stop temperature,
A method for producing a steel sheet having excellent low-temperature toughness and weld crack resistance, which is characterized by further cooling to a warm state, and then performing a tempering treatment at a temperature of Ac ₁ or less as required. _{P CM = C + 1/30} · Si + 1/20 · Mn + 1/20 · Cu + 1/60 · Ni + 1/20 · Cr + 1/15 · Mo +1/10 · V + 5B ············ (a) where Element symbol is the content (% by weight) of each element
Indicates.