JP2001064724A - Production of 60 kilo class high tensile strength steel excellent in weldability and toughness after strain aging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing 60 kilo class reheated, quenched and tempered steel excellent in weldability and toughness after strain aging and having excellent low temp. toughness even after cold working such as bending. SOLUTION: Steel contg., by weight, 0.06 to 0.10% C, 0.1 to 0.5% Si, 1.2 to 1.8% Mn, 0.1 to 0.5% Cr, 0.01 to 0.05% Nb, 0.002 to 0.070% sol. Al and 0.001 to 0.004% N, moreover, added with Mo, Cu, Ni, V, Ti and Ca in accordance with desired performance capacities and also satisfying P cm<=0.20%, Ceq (WES)<=0.42% and 0.50<=Si+3Cr<=1.25% is reheated and quenched from >=Ar3 and is thereafter tempered at 400 to 630 deg.C, where Pcm=C+Mn/20+Si/30+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and Ceq (WES)=C+Mn/6+Si/24+ Ni/40+Cr/5+Mo/4+V/14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hydraulic iron pipe, a pressure vessel, a line pipe, and a marine structure.
The present invention relates to a method for manufacturing a kilo-class structural steel, particularly a steel having excellent low-temperature toughness even after cold working such as bending and excellent in toughness after strain aging.

[0002]

2. Description of the Related Art When a steel is plastically deformed in the cold, a phenomenon called strain aging embrittlement, which deteriorates toughness, occurs. Research on strain aging embrittlement has been mainly conducted on thin steel sheets for automobile bodies, but in recent years the demand for structural reliability has increased, and thick steel sheets are not only processed at the material stage but also processed. The toughness after plastic deformation due to accidents and accidents has come to be regarded as a problem.

As a test for evaluating strain aging embrittlement, a strain aging Charpy test in which a 5% tensile prestrain is applied and a Charpy test is performed after aging treatment at 250 ° C. for 1 hour is known. The number of cases required as one is increasing.

As techniques for suppressing strain aging embrittlement of thick steel plates, Japanese Patent Application Laid-Open Nos. 5-320820 and 59-18-18 are disclosed.
No. 2,915 and JP-A-56-127750, but none of these technologies are intended for general 600 MPa class thick steel plates.

Japanese Patent Application Laid-Open No. Hei 5-320820 discloses a low yield point hardened steel for a spherical bow having a tensile strength of 400 MPa. The steel material structure is sized to prevent toughness deterioration after strain aging, but the C content is 0.002 to 0.03.
%, Which is a component composition containing almost no other strengthening elements, and cannot be applied to 60 kg steel.

JP-A-59-182915 discloses a production method for suppressing strain aging embrittlement in a 500 MPa grade TMCP steel. Focusing on the fact that embrittlement after cold working is caused by concentration of strain in the ferrite phase of ferrite bainite structure when cold working of 50 kg of TMCP steel, the solid solution N and solid solution C in ferrite are cooled. This is a technique of reducing the temperature by controlling the stop temperature and suppressing the embrittlement of the ferrite phase. For this reason, it cannot be applied to 60 kg class steel which is cooled to around room temperature and becomes a quenched structure.

Japanese Patent Application Laid-Open No. 56-127750 discloses a 600M
A technique for suppressing strain aging embrittlement of Pa class steel is described. However, in this technique, in a VN precipitation type steel, strain aging embrittlement caused by containing 0.01% or more of N can be suppressed by adding Ca or Mg. Is shown. However, the present technology has a
The effect is exhibited only for VN steel manufactured with s roll or quota.
A steel having a high weldability of 2% or more and a Pcm of 0.25% or more is described, and does not meet the demands of current general consumers.

[0008]

As described above, as described above,
A technique for suppressing embrittlement after plastically deforming a 60-kilometer thick steel material excellent in welding workability has not yet been completed. The present invention provides a method for producing a 60-kg class high-strength steel having excellent weldability and excellent toughness even after strain aging. Specifically, a fracture transition temperature vTs ( The present invention provides a method for producing a 60 kg class high-strength steel having an aged value of -40 ° C or lower.

[0009]

Means for Solving the Problems Conventionally, the mechanism of toughness deterioration after strain aging is as follows. In the case of a thin steel sheet, the interaction between C and N dissolved in the steel and the dislocation due to strain application causes the dislocation of the dislocation. It is known that movement is hindered, yield points rise and embrittle. However, the present inventors conducted a snake peak measurement of the solute C and N of steels having different degrees of toughness degradation after strain aging by using an internal friction measurement method for thick steels. The amounts of dissolved C and N are less than 3 ppm, and there is no positive influence of the amounts of dissolved C and N on the toughness degradation after strain aging of thick steel. The fact that the steel had a bainite-major structure was presumed to be due to structural differences.

Therefore, the present inventors varied the composition of the steel material and the heat treatment conditions, and studied in detail the effect of the structure on the strain aging characteristics. The aging characteristics were grasped.

1. Decreasing the C content reduces toughness degradation after strain aging.

2. The balance between strength and toughness of the base material is 6 due to the tendency of the base phase to soften due to tempering and the precipitation of Nb carbonitride.
It changes around 30 ° C. FIG. 1 schematically shows the effect of the tempering temperature on the strength and toughness of the base material.
On the lower side than around ° C., as the tempering temperature rises, Nb carbonitride precipitates consistently, the strength increases, and the toughness slightly deteriorates.

The toughness after strain aging also deteriorates as the tempering temperature increases, but the difference in toughness before and after strain aging is small. on the other hand,
On the higher side than around 630 ° C., as the tempering temperature rises, the matrix further softens, and at the same time, Nb carbonitride grows, the degree of coordination with the matrix decreases, and the strength-raising effect is lost. . Although the base metal toughness recovers, the toughness after strain aging does not recover and deteriorates, so that the difference in toughness before and after strain aging increases.

3. The structure changes by tempering, and cementite is relatively finely precipitated as the tempering temperature is lower.
The precipitation sites were observed to be dispersed in the prior austenite grain boundaries, bainite packet boundaries and in the prior austenite grains. When the tempering temperature increases,
Cementite became coarse and coarse, and the precipitation sites were only the former austenite grain boundaries and the packet boundaries of bainite.

4. The smaller the austenite grain size during quenching and the smaller the packet size of bainite, the less the toughness degradation after strain aging.

5. The presence of film-like ferrite at the prior austenite grain boundaries reduces toughness degradation after strain aging. It is effective to add an appropriate amount of Si and Cr in combination to form a film-like ferrite.

6. The fracture unit of the brittle fracture surface in the Charpy impact test after strain aging corresponds to the packet size of bainite.

From these results, the toughness degradation of the Nb-containing thick steel is due to the precipitation of cementite, and the toughness after strain aging depends on the size and the amount of cementite that precipitates at the packet boundary between the prior austenite grain boundary and bainite. It was understood that they would be controlled. As a factor affecting the size and amount of precipitation of cementite, C
Amount and tempering temperature, indirectly increasing the number of precipitation sites for a certain amount of cementite, the former austenite crystal grains having the effect of reducing the precipitate size, the packet size of bainite, the film form which precipitates on the former austenite grain boundary Ferrite was observed.

That is, the main manufacturing conditions that affect the toughness after strain aging are the C content and S that affect ferrite formation.
The slab heating temperature affects the i content, the Cr content, the tempering temperature, the prior austenite crystal grain size and the packet size of bainite.

The present invention has been made based on the above findings and further studied.

1. By weight%, C: 0.06-0.10
%, Si: 0.1 to 0.5%, Mn: 1.2 to 1.8
%, Cr: 0.1 to 0.5%, Nb: 0.01 to 0.0
5%, sol. Al: 0.002 to 0.07%, N:
0.001 to 0.004%, and Pcm ≦ 0.2
0%, Ceq (WES) ≦ 0.42%, 0.50% ≦ S
Manufacture of 60 kg class high strength steel excellent in weldability and toughness after strain aging characterized by reheating and quenching steel satisfying i + 3Cr ≦ 1.25% from Ar 3 or more and then tempering at 400 to 630 ° C. Method.

However, Pcm = C + Mn / 20 + Si / 3
0 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5B, Ceq (WES) = C + Mn / 6 +
Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 1
4 is assumed.

2. As a steel composition, Mo:
2. The method for producing a 60 kg high strength steel excellent in weldability and toughness after strain aging according to 1 which contains one or two of 0.02 to 0.3% and Cu: 0.1 to 0.6%. .

3. As a steel composition, Ni:
3. The method for producing a 60 kg class high strength steel excellent in weldability and toughness after strain aging according to 1 or 2, which contains 0.1 to 0.5%.

4. As a steel composition, V:
4. The method for producing a 60 kg high strength steel excellent in weldability and toughness after strain aging according to any one of 1 to 3 containing 0.01 to 0.08%.

[5] As steel composition, further in weight% Ti:
0.005 to 0.02%, Ca: 0.001 to 0.00
5. The method for producing a 60 kg high strength steel excellent in weldability and toughness after strain aging according to any one of 1 to 4 containing 4% of one or two kinds.

6. Heating temperature during quenching is Tmin. (℃)
The method for producing a 60 kg high strength steel excellent in weldability and toughness after strain aging according to any one of 1 to 5, characterized in that the temperature is set to Tmax.

However, Tmin. (° C.) = (6770 / (2.
26-log (0.005C)))-373 + 2t Tmax. (° C.) = 910 + 2t t: plate thickness (mm)

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION The component composition of the present invention is described below
The manufacturing conditions will be described.

1. Component composition C: 0.06% or more and 0.10% or less C is added to secure a predetermined strength. If it is less than 0.06%, it is difficult to secure a tensile strength of 60 kg class in the case of a thick material, and if it exceeds 0.10%, the toughness after strain aging deteriorates. Add 10% or less.

Si: 0.1% or more and 0.5% or less Si is a strong ferrite-forming element, and is added to form a film-like ferrite at a prior austenite grain boundary. If less than 0.1%, the effect is not enough, and 0.5%
If the amount exceeds 0.1%, the effect is saturated and the toughness of the heat affected zone is significantly deteriorated.

Mn: 1.2% or more and 1.8% or less Mn is added to secure a predetermined strength. 1.2%
If the thickness is less than 60%, it is difficult to secure a tensile strength of 60 kg in the case of a thick material, and if it exceeds 1.8%, the toughness of the heat affected zone is significantly deteriorated, so that it is 1.2% or more and 1.8% or less. Added.

Cr: 0.1% or more and 0.5% or less Cr is a strong ferrite-forming element and is added to form a film-like ferrite at a prior austenite grain boundary.
If it is less than 0.1%, its effect is insufficient, and if it exceeds 0.5%, the hardenability is remarkably increased, and it becomes difficult to form a film-like ferrite.

Nb: 0.01% or more and 0.05% or less Nb suppresses austenite recrystallization during rolling, activates austenite grain boundaries during direct quenching, and facilitates formation of film ferrite. . In addition, it is added as Nb carbide during tempering and is effective in increasing the strength.
If the content is less than 0.01%, those effects are insufficient, and 0.05%
%, The toughness is deteriorated due to remarkable strengthening of precipitation of Nb carbide, so that 0.01% or more and 0.05% or less are added.

Sol. Al: 0.002% or more and 0.07
% Or less Al is added for deoxidation. sol. 0.00 in Al content
If it is less than 2%, its effect is not sufficient, and if it exceeds 0.07%, the surface flaws of the steel material are likely to occur, so it is added in an amount of 0.002% or more and 0.07% or less.

N: not less than 0.001% and not more than 0.004% N is combined with Al or Ti at the time of rolling and heating to form AlN,
Generates TiN and refines austenite. 0.
If it is less than 001%, the effect is not enough, and 0.004%
If the content exceeds 0.001%, since even after quenching and tempering, remarkable strain aging embrittlement is caused by solid solution N, 0.001% or more
4% or less.

Pcm ≦ 0.20, Ceq (WES) ≦
0.42 Pcm, Ceq (WES) is an index of the low-temperature cracking property of the weld and the toughness of the weld heat affected zone. When Pcm exceeds 0.20%, low-temperature cracking may occur in welding without preheating. , Ceq (WES) exceeds 0.42, the toughness of the heat-affected zone in large heat input welding is significantly deteriorated, so that Pcm ≦ 0.
20, CeqWES ≦ 0.42. Where Pcm =
C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60
+ Cr / 20 + Mo / 15 + V / 10 + 5B, Ceq
(WES) = C + Mn / 6 + Si / 24 + Ni / 40 +
Cr / 5 + Mo / 4 + V / 14.

In the present invention, the component ranges are further defined so as to satisfy the following parameters.

Si + 3Cr: 0.50% or more and 1.25%
The following parameter is a parameter that can form a film-like ferrite at the former austenite grain boundary of steel having the above-mentioned component range.
If i + 3Cr is less than 0.50%, the effect is not sufficient, and if i + 3Cr exceeds 1.25%, the formation of film-like ferrite due to excessive hardenability is suppressed, so that 0.50% or more and 1.25% or more. The following is assumed.

The above is the basic composition and parameters of the steel of the present invention. In order to improve desired properties, it is possible to add Mo, Cu, Ni, V, Ti, and Ca singly or in combination. .

Mo: 0.02% to 0.3%, C
u: One or two kinds of Mo of 0.1% or more and 0.6% or less Mo is added because it improves the strength and is particularly effective for thick-walled materials. If it is less than 0.02%, the effect is not sufficient, and 0.3%
%, The weldability and the toughness of the heat affected zone are significantly degraded. Cu is added to improve the strength.

If the amount is less than 0.1%, the effect is not sufficient.
If added in excess of 0.6%, the risk of Cu cracking increases, so the content is made 0.1% or more and 0.6% or less.

Ni: 0.1% or more and 0.5% or less Ni is added to improve toughness. If it is less than 0.1%, the effect is not sufficient, and if it exceeds 0.5%, the cost of the steel material rises remarkably.

V: 0.01% or more and 0.08% or less V precipitates as carbide during tempering and is added to improve the strength. If it is less than 0.01%, the effect is not sufficient, and if it exceeds 0.08%, the toughness deteriorates due to remarkable precipitation strengthening of V carbide, so the content is made 0.01% or more and 0.08% or less.

Ti: 0.005% or more and 0.02% or less,
Ca: one or two types of 0.001% or more and 0.004% or less Ti and Ca are added to improve the base material toughness and the toughness of the weld heat affected zone. Ti forms TiN at the time of rolling heating or welding, and refines the austenite grain size.

If the content is less than 0.005%, the effect is not sufficient. If the addition exceeds 0.02%, a composite carbide of TiNb precipitates during rolling, and the amount of precipitation of Nb carbide during tempering becomes insufficient. 0.005
% To 0.02% or less.

Ca is present in the steel as Ca sulfide and reduces the austenite grain size during rolling heating or welding. If less than 0.001%, the effect is not enough,
When added in excess of 0.004%, the cleanliness is significantly degraded by a large amount of Ca sulfate, so that the content is made 0.001% or more and 0.004% or less.

Further, in the present invention, it is desirable to regulate B, O, P and S within the following ranges.

B: 0.0002% or less, O: 0.001
% To 0.004% B is treated as an impurity element in the present invention. At the time of quenching, if it exists as solid solution B, the formation of film-like ferrite at the prior austenite grain boundaries is suppressed. O is an unavoidable impurity, but if it is less than 0.001%, the production cost becomes expensive, and if it exceeds 0.004%, a large amount of Ca sulphate aggregates and the cleanliness deteriorates.
Not less than 0.004%.

P ≦ 0.010%, S ≦ 0.002% P and S are impurity elements, P ≦ 0.010%, S ≦ 0.0
When it is 02%, the segregation at the center is reduced, and the toughness and weldability at the center of the plate thickness are improved.

2. Manufacturing conditions Hardening temperature: Ar3 or more In order to secure hardenability to obtain predetermined strength and toughness,
It is necessary that the structure during quenching be an austenitic single phase, and the quenching temperature is Ar3 or higher. Ar3 is, for example, Ar3 = 910-310C-80Mn-20Cu-15
It is determined as Cr-55Ni-80Mo.

In the present invention, as a more appropriate quenching condition according to the components and the plate thickness, the heating temperature during quenching is set to Tmin. (° C.).
~ Tmax. (° C).

However, Tmin. (° C.) = (6770 / (2.
26-log (0.005C)))-373 + 2t Tmax. (° C.) = 910 + 2t t: sheet thickness (mm) In the Nb-containing steel targeted by the present invention, in order to secure strength, the steel sheet is increased in thickness. Increasing the heating temperature during quenching, increasing the amount of Nb solid solution, increasing quenching properties and increasing Nb carbonitride during tempering, and when the Nb content is relatively low and the C content is high Since the amount of Nb solid solution decreases, it is also desirable to increase the heating temperature at the time of quenching, and it is preferable that the heating temperature be Tmin. on the other hand,
If the heating temperature is high, the tempering temperature for adjusting the strength during quenching becomes high, and the toughness after strain aging is deteriorated.

Tempering temperature: 400 ° C. or more and 630 ° C. or less Tempering is performed for the purpose of recovering toughness due to softening of the parent phase and improving the strength-toughness balance by strengthening the precipitation of Nb carbonitride. The effect is not clear below 400 ° C,
If it exceeds 400, the toughness after strain aging is remarkably deteriorated.
The temperature is set to 00 ° C or more and 630 ° C or less.

[0055]

EXAMPLES Table 1 shows the chemical components of the test steels used in the examples (the remainder not shown consists essentially of Fe and inevitable impurities). After heating the slab having these chemical components, 3
It was rolled to 2 to 100 mm. After rolling, reheating and quenching,
After that, tempering was performed. Table 2 shows the manufacturing conditions and the characteristics of the steel sheet.

As mechanical properties, strength, toughness and toughness after strain aging were determined. The tensile test was a test using a JIS No. 14A (14φ) test piece taken from 1/4 t.

The impact test was a test using a 2 mm V notch Charpy impact test piece (JIS No. 4 standard test piece) sampled so that the longitudinal direction was perpendicular to the rolling direction from 1/4 t. The toughness after strain aging was determined by applying a 5% tensile pre-strain to a plate-like test piece, aging at 250 ° C. for 1 hour, and extracting a 2 mm V notch Charpy impact test piece in the tensile direction.

Hereinafter, embodiments will be described in detail. Steel types A to G in Table 1 are steels having a component composition satisfying the invention described in any one of claims 1 to 5, and steel types H to J are each of the components, Pcm, Ceq (WES), or any of Si + 3Cr. Is out of the scope of the invention. Steel numbers 1 to 7 in Table 2 are production examples using steel types A to G, and are examples of the invention according to any one of claims 1 to 6.

The vTs after strain aging is -40 ° C. or less, the change in vTs before and after strain aging is small, and good strain aging embrittlement resistance is obtained. Steel Nos. 8 to 11 are inferior in strength or toughness after strain aging when quenching and tempering conditions are outside the scope of the invention described in any of claims 1 to 6. Steel Nos. 12 to 14 are production examples based on steel types H to J, and have a component composition outside the scope of the present invention, and are inferior in strain aging embrittlement resistance.

[0060]

[Table 1]

[0061]

[Table 2]

[0062]

According to the present invention, at the time of reheating and quenching, film-like ferrite is generated at the former austenite grain boundary, and the substantial grain boundary area is increased, so that the strain concentrated on the cementite precipitated during tempering. It is possible to provide a method for producing a 60 kg class reheat quenched and tempered steel which is excellent in toughness after strain aging and excellent in weldability, and is extremely industrially effective.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing the effect of tempering temperature on strength and vTs before and after strain aging.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA01 AA04 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 BA01 CC03 CF01 CF02 CF03

Claims (6)

[Claims] C .: 0.06 to 0.10% by weight,
Si: 0.1-0.5%, Mn: 1.2-1.8%, C
r: 0.1 to 0.5%, Nb: 0.01 to 0.05%,
sol. Al: 0.002 to 0.07%, N: 0.00
1 to 0.004%, and Pcm ≦ 0.20%, C
eq (WES) ≦ 0.42%, 0.50% ≦ Si + 3C
Manufacture of 60 kg class high strength steel excellent in weldability and toughness after strain aging characterized by reheating and quenching steel satisfying r ≦ 1.25% from Ar 3 or more and then tempering at 400 to 630 ° C. Method. However, Pcm = C + Mn / 20 + Si / 30
+ Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 +
V / 10 + 5B, Ceq (WES) = C + Mn / 6 + S
i / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
And 2. The steel composition further includes Mo: 0.1 in weight%.
The method for producing a 60 kg high-strength steel excellent in weldability and toughness after strain aging according to claim 1, which contains one or two types of 02 to 0.3% and Cu: 0.1 to 0.6%. . 3. The steel composition further includes Ni: 0.1% by weight.
3. The method for producing a 60 kg high strength steel having excellent weldability and toughness after strain aging according to claim 1 or 2 containing 1 to 0.5%. 4. The steel composition further has a V: 0.0% by weight.
The method for producing a 60 kg high strength steel having excellent weldability and toughness after strain aging according to any one of claims 1 to 3 containing 1 to 0.08%. 5. The steel composition further comprises Ti: 0.1% by weight.
005-0.02%, Ca: 0.001-0.004%
The method for producing a 60 kg high-strength steel excellent in weldability and toughness after strain aging according to any one of claims 1 to 4, which comprises one or two of the following. 6. The heating temperature during quenching is from Tmin.
The method for producing a 60 kg high strength steel having excellent weldability and toughness after strain aging according to any one of claims 1 to 5, wherein the maximum strength is set to max. However, Tmin. (° C) = (67
70 / (2.26-log (0.005C)))-37
3 + 2t Tmax. (° C.) = 910 + 2t t: plate thickness (mm)