JP3579307B2 - 60kg-class direct quenched and tempered steel with excellent weldability and toughness after strain aging - Google Patents

Description

【００５２】
【表２】

【００５３】
【発明の効果】
本発明によれば、直接焼入れ時に、旧オーステナイト粒界に膜状のフェライトが生成され、実質的な粒界面積が増大されるため、焼戻しにおいて析出するセメンタイトに集中する歪が小さく、歪時効後の靭性に優れると共に、溶接性に優れる６０キロ級直接焼入れ焼戻し鋼の提供が可能で、産業上その効果は極めて大きい。
【図面の簡単な説明】
【図１】焼入れ後のミクロ組織を模式的に示す図
【図２】焼入れ焼戻し後のミクロ組織を模式的に示す図
【図３】焼入れ焼戻し後、歪を付加した場合のミクロ組織を模式的に示す図
【図４】旧オーステナイト粒界に、膜状のフェライトが析出している状態を模式的に示した図
【図５】旧オーステナイト粒界に膜状のフェライトが析出した組織の焼戻し後の状態を模式的に示す図
【図６】膜状フェライトの生成に及ぼすＳｉとＣｒの影響を示す図
【図７】溶接熱影響部シャルピー衝撃試験の試験片採取位置を示す図[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a 60-kg class structural steel used for penstocks, pressure vessels, line pipes, marine structures, and the like. In particular, the steel has excellent low-temperature toughness even after cold working such as bending. It is about excellent steel.
[0002]
[Prior art]
When steel is plastically deformed in the cold, a phenomenon called strain aging embrittlement, which deteriorates toughness, occurs. Research on strain aging embrittlement has been conducted mainly on thin steel sheets for automobile bodies. However, 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.
[0003]
As a test for evaluating strain aging embrittlement, a strain aging Charpy test in which a 5% tensile pre-strain is applied and a Charpy test is performed after aging treatment at 250 ° C. for 1 hour is known. Cases are increasing.
[0004]
Techniques for suppressing strain aging embrittlement of thick steel sheets include JP-A-5-320820, JP-A-59-182915, and JP-A-56-127760, all of which have a general thickness of 600 MPa class. It is not a technology for thin steel plates.
[0005]
JP-A-5-320820 discloses a low yield point hardened steel for a spherical bow having a tensile strength of 400 MPa. The steel composition is sized to prevent the deterioration of toughness after strain aging, but the content of C is 0.002 to 0.03%, and the composition of the component hardly contains other strengthening elements. It cannot be applied to 60 kg steel.
[0006]
JP-A-59-182915 discloses a manufacturing method for suppressing strain aging embrittlement in TMCP type 500 MPa class 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 for reducing the stop temperature by controlling the stop temperature and suppressing the embrittlement of the ferrite phase. For this reason, it cannot be applied to a 60-kilometer steel that is cooled to around room temperature and becomes a quenched structure.
[0007]
Japanese Patent Application Laid-Open No. 56-127750 discloses a technique for suppressing strain aging embrittlement of 600 MPa class steel. However, this technique applies to strain-age embrittlement caused by containing 0.01% or more of N in VN precipitation type steel. Can be suppressed by adding Ca or Mg. However, the present technology exerts its effect only on VN steel manufactured by as roll or quota, and the steel of Example also has a high C content of 0.12% or more and a Pcm of 0.25% or more. It describes steel with poor weldability and does not meet the demands of current general consumers.
[0008]
[Problems to be solved by the invention]
As described above, a technique for suppressing embrittlement after plastically deforming a 600 MPa class thick steel material excellent in welding workability has not yet been completed. The present invention is to provide a 60 kg class quenched and tempered steel having excellent weldability and excellent toughness even after strain aging. Specifically, the structure is coarser than that of a reheat treated material, and the toughness is improved. Provided is a direct quenched and tempered steel, which is inferior, and particularly deteriorates in toughness when subjected to plastic deformation, and has a fracture surface transition temperature vTs (aged) of −40 ° C. or lower in a strain-aged Charpy test, and is a 60 kg class direct quenched and tempered steel.
[0009]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on the cause of toughness degradation after undergoing plastic deformation of direct quenched and tempered steel, and techniques for preventing the toughness, and the toughness degradation is brought about by the mechanism shown in FIGS. It has been found that an increase in the grain boundary area is effective in preventing such a problem.
[0010]
FIG. 1 schematically shows the as-quenched microstructure. In the case of a direct hardened and tempered steel of 60 kg class having a low component design: low Pcm and low CeqW (abbreviation of Ceqwes), the microstructure of austenite is reduced. Due to the difficulty, the bainite structure becomes coarser than the microstructure of the reheat-quenched and tempered material.
[0011]
FIG. 2 shows the microstructure in the case of tempering after quenching, in which cementite precipitates at the grain boundaries. When the tempering temperature is the same, cementite is coarsened by a direct quenched material having a smaller grain boundary area than a reheated material.
[0012]
FIG. 3 shows a situation where strain is applied to the quenched and tempered steel, and the strain is concentrated around the cementite. Since the strain concentration increases as the precipitated cementite becomes coarser, the strain embrittlement of the direct quenched and tempered steel becomes larger than that of the reheated and quenched and tempered steel. To prevent this, it is effective to increase the grain boundary area to refine the cementite. Based on the above findings, the present inventors studied a method for increasing the grain boundary area, and as shown in FIG. 4, produced a film-shaped ferrite of several μm or less at the former austenite grain boundary during direct quenching. In this case, the grain boundary area is substantially increased, the cementite is refined (FIG. 5), and such a structure suppresses austenite recrystallization during rolling and activates austenite grain boundaries during transformation. The inventors have found that the present invention can be obtained by adding Nb and adding an appropriate amount of SI and Cr, which are ferrite-forming elements, and have completed the present invention.
[0013]
That is, the present invention provides: In mass%, C: 0.04 to 0.09%, Si: 0.1 to 0.5%, Mn: 1.2 to 1.8%, Cr: 0.1 to 0.5%, Nb: 0.01-0.05%, sol. Al: 0.002 to 0.07%, N: 0.001 to 0.004%, B ≦ 0.0002% , balance substantially consisting of Fe, Pcm ≦ 0.20%, Ceq (WES) Weldability satisfying ≦ 0.42%, 0.50% ≦ Si + 3Cr ≦ 1.25% and fracture transition by strain aging Charpy test after aging treatment at 250 ° C. for 1 hour after 5% tensile prestrain 60 kg class direct quenched and tempered steel with excellent toughness after strain aging at a temperature of -40 ° C or less.
Here, 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.
[0014]
However, 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.
[0015]
2. After the weldability and 5% tensile pre-strain according to 1, further containing one or two types of steel compositions, Mo: 0.02 to 0.3% and Cu: 0.1 to 0.6% by mass% . 60 kg class direct quenched and tempered steel with excellent toughness after strain aging , having a fracture surface transition temperature of −40 ° C. or less in a strain aging Charpy test after aging treatment at 250 ° C. for 1 hour .
[0016]
3. Strain-aging Charpy test after 1 hour of aging treatment at 250 ° C. after 1% weldability and 5% tensile pre-strain according to 1 or 2, further containing Ni: 0.1-0.5% by mass as steel composition A 60-kg class direct quenched and tempered steel with excellent toughness after strain aging in which the fracture surface transition temperature due to aging is -40 ° C or less .
[0017]
4. The steel composition further contains V: 0.01 to 0.08% by mass%, after 1% weldability and 5% tensile prestrain, and after aging treatment at 250 ° C. for 1 hour. 60 kg class direct quenched and tempered steel excellent in toughness after strain aging with a fracture surface transition temperature of −40 ° C. or less according to strain aging Charpy test .
[0018]
5. 5. The weldability according to any one of 1 to 4, which further contains one or two of steel composition: Ti: 0.005 to 0.02% and Ca: 0.001 to 0.004% by mass%. % 60% -class direct quenched and tempered steel with excellent fracture toughness after strain aging , having a fracture surface transition temperature of −40 ° C. or less by strain Charging test after aging treatment at 250 ° C. for 1 hour after% tensile prestrain .
[0020]
6. The weldability according to any one of 1 to 5 , wherein P ≦ 0.010% and S ≦ 0.002% in mass%, and aging at 250 ° C. for 1 hour after 5% tensile prestrain. A 60-kg class direct quenched and tempered steel having excellent toughness after strain aging, having a rupture transition temperature of −40 ° C. or less in a strain-aged Charpy test after treatment.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the component composition in the present invention will be described.
[0022]
C: 0.04% or more and 0.09% or less C is added to secure a predetermined strength. If it is less than 0.04%, it is difficult to secure a tensile strength of 60 kg class in the case of a thick material, and if it exceeds 0.09%, the toughness after strain aging deteriorates. 0.09% or less.
[0023]
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 during direct quenching. If it is less than 0.1%, the effect is not sufficient, and if it exceeds 0.5%, the effect is saturated and the toughness of the heat affected zone is significantly deteriorated.
[0024]
Mn: 1.2% or more and 1.8% or less Mn is added to secure a predetermined strength. If it is less than 1.2%, it is difficult to secure a tensile strength of 60 kg class 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 not less than 1.2%. 0.8% or less.
[0025]
Cr: 0.1% or more and 0.5% or less Cr is a strong ferrite-forming element, and is added because direct quenching forms 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.
[0026]
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. It is added as Nb carbide during tempering and is effective in increasing the strength. If it is less than 0.01%, these effects are insufficient, and if it exceeds 0.05%, the toughness deteriorates due to remarkable precipitation strengthening of Nb carbide, so that 0.01% or more and 0.05% or less are added.
[0027]
sol. Al: 0.002% or more and 0.07% or less Al is added for deoxidation. sol. If the Al content is less than 0.002%, the effect is not sufficient, and if added in excess of 0.07%, the surface flaws of the steel material are likely to occur. .
[0028]
N: 0.001% or more and 0.004% or less N is combined with Al or Ti at the time of rolling and heating to form AlN and TiN, and to refine austenite. If the content is less than 0.001%, the effect is not sufficient, and if the content exceeds 0.004%, remarkable strain aging embrittlement due to solid solution N occurs even after quenching and tempering, so that the content is 0.001% or more and 0.004% or less. I do.
[0029]
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, and Ceq ( If (WES) exceeds 0.42%, the heat-affected zone toughness of large heat input welding is significantly deteriorated, so that Pcm ≦ 0.20% and CeqWES ≦ 0.42%. Here, Pcm = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B, CeqWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14.
[0030]
In the present invention, the component range is further defined so as to satisfy the following parameters.
[0031]
Si + 3Cr: 0.50% or more and 1.25% or less This parameter is capable of forming a film-like ferrite at the former austenite grain boundary of steel in the above component range, and when Si + 3Cr is less than 0.50%, If the effect is not sufficient, and if it exceeds 1.25%, the formation of film-like ferrite is suppressed due to excessive hardenability, so the content is made 0.50% or more and 1.25% or less. FIG. 6 shows the effect, and formation of a film-like ferrite is recognized according to this rule.
[0032]
The above are the basic composition and parameters of the steel of the present invention. In order to improve desired properties, Mo, Cu, Ni, V, Ti, and Ca are added alone or in combination, and B, which are inevitable impurities, are added. It is possible to regulate P and S.
[0033]
One or two types of Mo having a Mo content of 0.02% or more and 0.3% or less and a Cu content of 0.1% or more and 0.6% or less improve the strength and are particularly effective for thick-walled materials. If it is less than 0.02%, the effect is not sufficient, and if it exceeds 0.3%, the weldability and the toughness of the heat affected zone are significantly deteriorated, so that the content is made 0.02% or more and 0.3% or less. Cu is added to improve the strength.
[0034]
If the content is less than 0.1%, the effect is not sufficient. If the content exceeds 0.6%, the possibility of Cu cracking increases, so the content is made 0.1% or more and 0.6% or less.
[0035]
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.
[0036]
V: 0.01% or more and 0.08% or less V is precipitated as carbide at the time of 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.
[0037]
One or two types of Ti, Ca of 0.005% or more and 0.02% or less and Ca: 0.001% or more and 0.004% or less are added to improve the base metal 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.
[0038]
If the content is less than 0.005%, the effect is not sufficient. If the content exceeds 0.02%, a composite carbide of TiNb precipitates during rolling, and the amount of precipitation of Nb carbide during tempering becomes insufficient, resulting in a decrease in strength. Therefore, the content is made 0.005% or more and 0.02% or less.
[0039]
Ca is present in the steel as Ca sulfide and reduces the austenite grain size during rolling heating or welding. If the content is less than 0.001%, the effect is not sufficient, and if the content exceeds 0.004%, the cleanliness is significantly deteriorated by a large amount of Ca sulfate, so the content is made 0.001% or more and 0.004% or less.
[0040]
B: 0.0002% or less B is treated as an impurity element in the present invention. At the time of direct quenching, if it is present as solid solution B, the formation of film ferrite at the prior austenite grain boundaries is suppressed.
[0041]
P ≦ 0.010%, S ≦ 0.002%
P and S are impurity elements. When P ≦ 0.010% and S ≦ 0.002%, the segregation at the center is reduced, and the regulation is performed to improve the toughness and weldability at the center of the plate thickness.
[0042]
The steel of the present invention limits its production method to direct quenching and tempering. It is difficult to obtain a 60 kg class tensile strength with a steel plate having a thickness of 30 mm or more having a composition satisfying Pcm ≦ 0.20% and CeqWES ≦ 0.42% by reheating, quenching and tempering. Manufactured by tempering.
[0043]
【Example】
Table 1 shows the chemical components of the test steels used in the examples. (The remainder not shown is substantially composed of Fe.) A 250 mm thick slab having these chemical components was heated to 980 to 1100 ° C and then rolled to 28 to 80 mm. Immediately after the rolling, the steel was directly quenched from a temperature of Ar 3 or more, and then tempered in the range of 400 to 630 ° C.
[0044]
The microstructure after the heat treatment was observed at a magnification of 250 to 600 times by SEM, and the presence or absence of film-like ferrite at the grain boundaries was examined. As mechanical properties, strength, toughness and toughness after strain aging were determined. The tensile test was a test using a JIS G14A (14φ) test piece taken from 1/4 t.
[0045]
The impact test was carried out using a 2 mm V notch Charpy impact test specimen sampled from 1/4 t at a test temperature of -100 to 0 ° C. The toughness after strain aging was determined by applying a 5% tensile prestrain to a plate-like specimen, aging at 250 ° C. for 1 hour, and collecting a 2 mm V notch Charpy impact specimen in the tensile direction at −100 to 20 ° C. Was tested.
[0046]
The weldability test was a maximum hardness test of the heat-welded heat-affected zone in accordance with JISZ3101. In addition, the toughness of the heat affected zone was measured using a 2 mm V notch Charpy impact test specimen (notch position: HAZ 1 mm, taken from a groove joint (welding condition: CO2 welding with a heat input of 30 kJ / cm). (Shown). Table 2 shows the test results. The results of the Charpy impact test of the heat affected zone show the minimum value of the results of three test pieces. Hereinafter, embodiments of the present invention will be described in detail.
[0047]
Among the steels 1 to 13, steels 4, 7, 9, and 10 are example steels having a component composition satisfying any of the inventions according to claims 1 to 6, and steels 1 to 3, 6, 11, and 11 are: B with the exception that exceed 0.0002% in reference steel component composition satisfying the present invention, the steel 14-21 each of claims 1 to 6 in more than Si + 3Cr is less than 0.50 or 1.25 Any of the described inventions have component compositions outside the scope of the invention.
[0048]
As shown in Table 2, when steels 1 to 13 are directly quenched and tempered (DQT), film-like ferrite is observed at the prior austenite grain boundary. The Charpy impact test results of these steels after strain aging are all good, with a fracture surface transition temperature (vTs) of -40 ° C or less, and are compared with the Charpy impact test results before strain aging (normally described as vTs in the table). The deterioration (degradation degree in the table) is as small as 0 to 15 ° C.
[0049]
Furthermore, steels 4, 5, 7, 8, 9, 10, 12, and 13 in which B ≦ 0.0002% have a small deterioration of 10 ° C. or less, and when B is regulated, prevention of toughness deterioration due to strain aging is even better. It will be. On the other hand, in steels 14 to 21, no film-like ferrite was observed at the prior austenite grain boundaries, and the Charpy impact test results after strain aging showed that the fracture surface transition temperature (vTs) was inferior to −35 ° C. or higher and the degree of deterioration was low. It is as large as 20 to 35 ° C.
[0050]
Since steels 1 to 13 are low-component systems with Pcm ≦ 0.19 and CeqW ≦ 0.42, the maximum hardness of the weld heat-affected zone is Hv300 or less, and the toughness of the weld heat-affected zone of HAZ 1 mm is 50 J at a test temperature of −40 ° C. As described above, excellent low-temperature cracking resistance and weld toughness are obtained. In addition, despite being a low-component system, it is manufactured by direct quenching and tempering, so that a strength of 60 kg class is obtained.
[0051]
[Table 1]

[0052]
[Table 2]

[0053]
【The invention's effect】
According to the present invention, at the time of direct quenching, film-like ferrite is generated at the prior austenite grain boundaries, and the substantial grain boundary area is increased, so that the strain concentrated on the cementite precipitated during tempering is small, and after strain aging. It is possible to provide a 60-kg class direct quenched and tempered steel having excellent toughness and excellent weldability, and its effect is extremely large in industry.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a microstructure after quenching. FIG. 2 is a diagram schematically showing a microstructure after quenching and tempering. FIG. 3 is a diagram schematically showing a microstructure when strain is added after quenching and tempering. FIG. 4 is a diagram schematically showing a state in which a film-like ferrite is precipitated at a prior austenite grain boundary. FIG. 5 is a diagram after tempering of a structure in which a film-like ferrite is precipitated at a prior austenite grain boundary. FIG. 6 is a diagram schematically showing the effect of Si and Cr on the formation of film ferrite. FIG. 7 is a diagram showing a test piece sampling position in a Charpy impact test of a heat affected zone of a weld.

Claims (6)

In mass%, C: 0.04 to 0.09%, Si: 0.1 to 0.5%, Mn: 1.2 to 1.8%, Cr: 0.1 to 0.5%, Nb: 0.01-0.05%, sol. Al: 0.002 to 0.07%, N: 0.001 to 0.004%, B ≦ 0.0002% , balance substantially consisting of Fe, Pcm ≦ 0.20%, Ceq (WES) ≤0.42%, 0.50% ≤Si + 3Cr≤1.25% Weldability and tensile transition after 5% pre-strain, aging treatment at 250 ° C for 1 hour, strain transition by fracture aging Charpy test 60 kg class direct quenched and tempered steel with excellent toughness after strain aging at a temperature of -40 ° C or less.
Here, 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. The weldability and 5% tensile prestrain according to claim 1, wherein the steel composition further contains one or two of Mo: 0.02 to 0.3% and Cu: 0.1 to 0.6% by mass%. Thereafter, a 60 kg class direct quenched and tempered steel excellent in toughness after strain aging, in which a fracture transition temperature in a strain aging Charpy test after aging treatment at 250 ° C. for 1 hour is -40 ° C. or less. 3. The strain aging according to claim 1 or 2, wherein the steel composition further contains Ni: 0.1 to 0.5% by mass% after the aging treatment at 250 ° C. for 1 hour after 5% tensile prestrain. A 60-kg class direct quenched and tempered steel excellent in toughness after strain aging, having a rupture transition temperature of −40 ° C. or lower in a Charpy test. 4. The aging treatment at 250 [deg.] C. for 1 hour after the weldability and 5% tensile prestrain according to claim 1, further containing V: 0.01 to 0.08% by mass% as a steel composition. 60 kg class direct quenched and tempered steel excellent in toughness after strain aging, having a rupture transition temperature of −40 ° C. or less in the subsequent strain aging Charpy test. The weldability according to any one of claims 1 to 4, wherein the steel composition further contains one or two of Ti: 0.005 to 0.02% and Ca: 0.001 to 0.004% by mass%. And 60 kg class direct quenched and tempered steel excellent in toughness after strain aging, having a transition transition temperature of −40 ° C. or less according to a strain aging Charpy test after aging treatment at 250 ° C. for 1 hour after 5% tensile prestrain. The weldability and 5% tensile prestrain according to any one of claims 1 to 6, wherein P ≦ 0.010% and S ≦ 0.002% in mass%, 1 hour at 250 ° C. A 60 kg class direct quenched and tempered steel excellent in toughness after strain aging, having a rupture transition temperature of −40 ° C. or less in a strain aging Charpy test after aging treatment.

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