JP6303866B2 - High-strength steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel material and a method for producing the same, and more particularly to a high-strength steel material having a delayed fracture resistance and a high-strength steel material having a strength of 1000 MPa to 1400 MPa and a method for producing the same.

  In recent years, steel materials are becoming stronger and have a longer service life from the viewpoint of reducing the weight and safety of transportation equipment such as automobiles, and improving the safety of elevated structures such as buildings, roads, and railways. It is requested. In general, when the strength of a steel material is increased, ductility and toughness tend to decrease. Therefore, material design and structure control are performed with an emphasis on improving the balance.

  Since the properties of steel materials vary greatly depending on the state of the microstructure, optimal structure design can be performed by appropriately selecting the chemical composition and process conditions. When the microstructure of steel materials is classified by the tensile strength level, the ferrite structure is 0.3 to 0.8 GPa, the pearlite structure is 0.8 to 1.2 GPa, the bainite structure is 0.5 to 1.6 GPa, and martensite. It is 0.6 to 4.4 GPa in the tissue.

  In order to improve the ductility of steel materials, various multiphase steels in which these structures are combined have been proposed. Typical examples are DP steel having a two-phase structure of ferrite and martensite, and TRIP steel made of ferrite, bainite and retained austenite, and exhibits good ductility. However, there are few DP steels and TRIP steels both having a strength level with a tensile strength exceeding 1.2 GPa.

  In recent years, structure control that combines strength and ductility has been proposed, such as the Q & P (Quench & Partitioning) process or the QPT (Q & P + tempering) process, and 2GPa grade steel has also been proposed along with the optimization of the composition. (See Non-Patent Documents 1 and 2.)

  Patent Document 1 discloses a steel sheet for a quenched member having a TS of 1.8 GPa or more and excellent toughness. Furthermore, Patent Document 2 contains 1.8 to 2.3% of V, and has a 1.8 GPa class TS by dispersing VC as a precipitate, and has a high strength heat excellent in bending workability. A rolled steel sheet is disclosed.

Japanese Patent Laid-Open No. 2007-302937 JP 2012-172237 A

J. G. Speer, D. V. Edmonds, F. C. Riggo, D. K. Matlock, Curr. Opin. Solid State Mater. Sci. 8, p. 219 (2004) H. Y. Li, X. W. Lu, W. J. Li, X. J. Jin, Metall. Mater. Trans. 41A, p. 1284 (2010)

  The steel manufactured by the Q & P process or the QPT process described in Non-Patent Documents 1 and 2 has a problem that the elongation remains at 10% or less. Further, in the cooling after the austenitizing heat treatment, a heat treatment is performed such that quenching is performed between the start temperature (Ms point) and the end temperature (Mf point) of the martensite transformation, and the temperature is again increased and maintained at a predetermined temperature. The process is complicated.

  Furthermore, a problem to be overcome in increasing the strength of steel materials is the problem of delayed fracture. The higher the strength of steel materials, the higher the risk of delayed fracture. In particular, for materials containing a large amount of carbon, improvement in delayed fracture resistance is strongly demanded. However, Patent Documents 1 and 2 do not consider the problem of delayed fracture at all.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a high-strength steel material that is excellent in delayed fracture resistance while being a high-carbon steel.

  As a result of intensive studies on a method for achieving both high strength and delayed fracture resistance of the steel, the present inventors have obtained the following knowledge.

  (A) In order to increase the strength, it is necessary to increase the carbon content.

  (B) In order to achieve both high strength and delayed fracture resistance, it is effective to increase the yield strength and the yield ratio (yield strength / maximum tensile strength).

  (C) In order to improve the yield strength, it is effective to make the steel metal structure a tempered structure of lower bainite or martensite.

  (D) In order to improve delayed fracture resistance, it is effective to subject the initial structure composed of the lower bainite structure or the martensite structure to a predetermined tempering treatment to spheroidize and refine the cementite.

  (E) In order to realize a structure in which spheroidized cementite is finely dispersed, it is desirable that the prior austenite grain size in the initial structure be refined.

  (F) In order to refine the prior austenite grain size of the initial structure while maintaining high strength, it is effective to contain Mo and B in combination.

  (G) Mo and B suppress the movement of dislocations and grain boundaries in the austenite region, and contribute to refinement of the austenite grain size. This is an effect resulting from the boride of Mo and B or the interaction of Mo-B.

  (H) Mo and B are also effective in improving hardenability. When the austenite grains are refined, the hardenability is lowered, so that ferrite is generated during cooling, and it becomes difficult to obtain a martensite structure or a bainite structure. However, the hardenability is improved by incorporating Mo and B in the steel in a composite manner, and an initial structure composed of a martensite structure or a bainite structure can be obtained while the prior austenite grain size is refined.

  (I) By performing a tempering treatment on the steel having the above initial structure, a metal structure that is a tempered structure of lower bainite or martensite in which the prior austenite grain size is fine and spheroidized cementite is finely dispersed is formed. A high strength steel material can be obtained.

  The present invention has been made on the basis of the above-mentioned knowledge, and the gist thereof is the following high-strength steel material and a manufacturing method thereof.

(1) The chemical composition is mass%,
C: more than 0.2% and 0.55% or less,
Si: 0.1 to 0.3%,
Mn: 0.1 to 0.3%,
Mo: 1-4%
Ti: 0.002 to 0.008%,
Al: 0.01-0.2%
B: 0.001 to 0.006%,
N: 0.001 to 0.015%,
V: 0 to 0.3%
Nb: 0 to 0.05%,
Balance: Fe and impurities,
Having a metal structure consisting of a tempered structure of either martensite or lower bainite with a prior austenite particle size of 3 μm or less,
The spheroidization rate of cementite contained in the metal structure is 0.8 or more,
A high-strength steel material having a tensile strength of 1000 to 1450 MPa.

(2) The chemical composition is mass%,
V: 0.05-0.3%
Nb: 0.01-0.05%
The high-strength steel material according to (1) above, which contains one or more selected from the above.

(3) having the chemical composition described in (1) or (2) above,
A steel having a single-phase structure selected from ferrite, pearlite, cementite and tempered martensite or a multi-phase structure in which two or more types are mixed, and having a metal structure having a ferrite or prior austenite grain size of 10 μm or less. ,
After heating to a temperature range of 800 to 900 ° C., cooling to a temperature range of 250 to 450 ° C. at a cooling rate of 10 ° C./s or more, holding at that temperature for 10 to 100 s, and then cooling,
A method for producing a high-strength steel material, comprising a tempering treatment step of holding for 10 min to 1 h in a temperature range of 400 to 680 ° C. after the heat treatment step.

(4) having the chemical composition described in (1) or (2) above,
A steel having a single-phase structure selected from ferrite, pearlite, cementite and tempered martensite or a multi-phase structure in which two or more types are mixed, and having a metal structure having a ferrite or prior austenite grain size of 10 μm or less. ,
After adding warm working so that the cross-sectional reduction rate is 20% or more in a temperature range of 500 to 750 ° C., heating to a temperature range of 800 to 900 ° C. and then cooling at a cooling rate of 10 ° C./s or more. A heat treatment step of cooling to a temperature range of 450 ° C., holding at that temperature for 10 to 100 s, and then cooling;
A method for producing a high-strength steel material, comprising a tempering treatment step of holding for 10 min to 1 h in a temperature range of 400 to 680 ° C. after the heat treatment step.

(5) having the chemical composition described in (1) or (2) above,
A steel having a single-phase structure selected from ferrite, pearlite, cementite and tempered martensite or a multi-phase structure in which two or more types are mixed, and having a metal structure having a ferrite or prior austenite grain size of 10 μm or less. ,
After cold working so that the cross-sectional reduction rate is 20% or more, the steel is heated to a temperature range of 800 to 900 ° C, and then cooled to a temperature range of 250 to 450 ° C at a cooling rate of 10 ° C / s or more. , A heat treatment step of holding at that temperature for 10 to 100 s and then cooling,
A method for producing a high-strength steel material, comprising a tempering treatment step of holding for 10 min to 1 h in a temperature range of 400 to 680 ° C after the heat treatment step.

  According to the present invention, according to the present invention, it is possible to obtain a high-strength steel material having good ductility and excellent delayed fracture resistance while having a high tensile strength of 1000 to 1450 MPa. It is possible to dramatically improve the properties and safety of the material. Therefore, the high-strength steel material according to the present invention is suitable for use as a structural material that supports transportation equipment such as automobiles, building members, roads, and railways.

  Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: More than 0.2% and 0.55% or less C is a basic element that improves the strength of the steel material. C finely disperses carbides in tempered martensite or tempered bainite, and contributes to high strength. In addition, it contributes to the improvement of elongation and local ductility. However, when the C content is 0.2% or less, it is difficult to obtain the effect by the above action. Therefore, C needs to be contained exceeding 0.2%. On the other hand, when the C content exceeds 0.55%, cementite is coarsened, and ductility and toughness are lowered. Therefore, the C content is 0.55% or less. The C content is desirably 0.25% or more, and more desirably 0.35% or more. Further, the C content is desirably 0.5% or less, and more desirably 0.45% or less.

Si: 0.1 to 0.3%
Si is an element that has the effect of suppressing inclusions in the steel by the deoxidation effect and preventing delayed fracture. However, if the Si content is less than 0.1%, it is difficult to obtain the above effects. Therefore, the Si content is 0.1% or more. On the other hand, if the Si content exceeds 0.3%, the prior austenite grain boundaries in the tempered martensite or tempered bainite structure weaken, and on the contrary, delayed fracture tends to occur. Therefore, the Si content is 0.1 to 0.3%. The Si content is desirably 0.15% or more, and desirably 0.25% or less.

Mn: 0.1 to 0.3%
Mn improves hardenability and facilitates the formation of martensite phase and bainite phase. However, if the Mn content is less than 0.1%, it is difficult to obtain the above effects. Therefore, the Mn content is 0.1% or more. On the other hand, if the Mn content exceeds 0.3%, the spheroidization of the carbide is suppressed and the delayed fracture resistance is deteriorated. Therefore, the Mn content is 0.1 to 0.3%. The Mn content is desirably 0.15% or more, and desirably 0.25% or less.

Mo: 1-4%
Mo is an element that contributes to the refinement of austenite grains by containing B in a composite manner. However, if the Mo content is less than 1%, it is difficult to obtain the above effects. Therefore, the Mo content is 1% or more. On the other hand, if the Mo content exceeds 4%, coarse carbides are generated at the prior austenite grain boundaries, and the delayed fracture resistance is lowered. Therefore, the Mo content is 1 to 4%. The Mo content is desirably 1.5% or more, and desirably 3.5% or less.

Ti: 0.002 to 0.008%
Ti is an element that produces carbon / nitrides such as TiC and TiN, and has an effect of suppressing the coarsening of crystal grains by a pinning effect on the growth of crystal grains. Further, Ti has an effect of reducing the solute N. In the steel containing B as will be described later, if solid solution N is present, B and N are combined to form BN, so that the effect of containing B is not exhibited. Therefore, Ti is an effective element for indirectly exerting the effect of B. However, if the Ti content is less than 0.002%, it is difficult to obtain the above effects. Therefore, the Ti content is set to 0.002% or more. On the other hand, if the Ti content exceeds 0.008%, coarse TiN is generated and delayed fracture resistance is lowered. Therefore, the Ti content is set to 0.002 to 0.008%. The Ti content is desirably 0.003% or more, and desirably 0.006% or less.

Al: 0.01 to 0.2%
Al has the effect of suppressing inclusions in the steel due to the deoxidation effect and preventing delayed fracture. However, if the Al content is less than 0.01%, it is difficult to obtain the above effects. On the other hand, if the Al content exceeds 0.2%, the oxides and nitrides are coarsened, and on the contrary, delayed fracture is promoted. Therefore, the Al content is set to 0.01 to 0.2%. The Al content is desirably 0.02% or more, and desirably 0.1% or less.

B: 0.001 to 0.006%
B is an element that improves the hardenability and facilitates the formation of a martensite phase and a bainite phase by being compounded with Mo. However, if the B content is less than 0.001%, it is difficult to obtain the above effects. Therefore, the B content is 0.001% or more. On the other hand, if the B content exceeds 0.006%, coarse carbides are generated at the prior austenite grain boundaries, and the local ductility and delayed fracture resistance deteriorate. Therefore, the B content is 0.001 to 0.006%. The B content is desirably 0.003% or more and desirably 0.005% or less.

N: 0.001 to 0.015%
N is an element that has the effect of suppressing the growth of austenite and ferrite and suppressing delayed fracture by forming nitrides. However, when the N content is less than 0.001%, it is difficult to obtain the above effects. On the other hand, when the N content exceeds 0.015%, the nitride becomes coarse, and on the contrary, delayed fracture is promoted. Therefore, the N content is 0.001 to 0.015%. The N content is desirably 0.002% or more, and desirably 0.010% or less.

V: 0 to 0.3%
V, like Mo, is an element that contributes to the refinement of austenite grains by being combined with B. Therefore, you may contain V as needed. However, if the V content exceeds 0.3%, coarse VC and VN are produced, and ductility and delayed fracture resistance are lowered. Therefore, the V content is 0 to 0.3%. The V content is desirably 0.25% or less. Moreover, when obtaining said effect, it is desirable to make V content 0.05% or more.

Nb: 0 to 0.05%
Nb, like Mo, is an element that contributes to the refinement of austenite grains by being combined with B. Therefore, you may contain Nb as needed. However, if the Nb content exceeds 0.05%, coarse NbC and NbN are generated, and ductility and delayed fracture resistance are lowered. Therefore, the Nb content is 0 to 0.05%. The Nb content is preferably 0.04% or less. In order to obtain the above effect, the Nb content is preferably 0.01% or more, and more preferably 0.015% or more.

P: 0.02% or less P is contained as an impurity, weakens the grain boundary, and causes delayed fracture. When the P content exceeds 0.02%, the deterioration of delayed fracture resistance becomes significant. Therefore, the P content is 0.02% or less. The P content is desirably 0.015% or less. A lower P content is desirable, but excessive reduction leads to a significant cost increase. From the viewpoint of a realistic manufacturing process and manufacturing cost, the P content is preferably 0.001% or more.

S: 0.005% or less S is contained as an impurity, weakens the grain boundary, and causes delayed fracture. When the S content exceeds 0.005%, the delayed fracture resistance deteriorates significantly. Therefore, the S content is 0.005% or less. The S content is desirably 0.004% or less. The smaller the S content, the better. However, excessive reduction leads to a significant cost increase. From the viewpoint of a realistic manufacturing process and manufacturing cost, the S content is preferably 0.0005% or more.

  The high-strength steel material of the present invention has a chemical composition comprising the above-described elements from C to S, the balance Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel sheets, and are permitted within a range that does not adversely affect the present invention. Means something.

2. Metallographic structure: Tempered structure of either martensite or lower bainite The high-strength steel material according to the present invention has a tensile strength of 1000 to 1450 MPa while improving ductility and delayed fracture resistance. It shall have a metal structure consisting of any tempered structure of the lower bainite. The tempered structure of martensite or lower bainite can be easily distinguished from other structures (ferrite, pearlite, upper bainite, etc.) from the orientation relationship. Usually, it can analyze by observation using a scanning electron microscope or a transmission electron microscope.

Cementite spheroidization ratio: 0.8 or more The tempered structure of martensite or lower bainite can be obtained by subjecting the initial structure composed of martensite or lower bainite to the tempering treatment described later. And the structure | tissue in which spherical cementite was disperse | distributed is obtained by performing said tempering process. If the spheroidization rate of cementite is less than 0.8, stress concentration tends to occur at the long-diameter tip of flattened cementite, and hydrogen tends to accumulate on that portion, which tends to be the starting point of fracture, so it is resistant to delayed fracture. Deteriorates. Therefore, the spheroidization rate of cementite contained in the metal structure is 0.8 or more.

  When the initial structure is a martensite structure single phase or a lower bainite structure single phase, a uniform carbide spheroidized structure is easily obtained after tempering. On the other hand, hard martensite may be mixed in bainite. That is, the lower bainite structure is cooled to the bainite transformation temperature range after the austenitizing heat treatment, and is generated after a predetermined holding time. At that time, with the bainite transformation, carbon redistribution occurs in the structure, and a hard martensite structure may be formed in a high carbon region. When the above structure is tempered, the cementite to be produced precipitates in the form of a plate at the grain boundary, and the spheroidization rate tends to be less than 0.8, which may adversely affect delayed fracture resistance. Therefore, the initial structure before the tempering treatment is preferably a martensite structure single phase or a lower bainite structure single phase.

  In the present invention, “a metal structure composed of a tempered structure of either martensite or lower bainite” allows the structure of cementite, ferrite, austenite, and the like to be included within a total range of less than 5%. To do.

Prior austenite grain size: 3 μm or less By setting the prior austenite grain size in the initial structure to 3 μm or less, carbides in the structure of martensite or lower bainite after tempering are easily spheroidized. The prior austenite grain size of martensite or lower bainite in the initial structure does not change even after tempering. Therefore, by setting the prior austenite grain size in the initial structure to 3 μm or less, the prior austenite crystal grain size in the tempered structure of martensite or lower bainite also becomes 3 μm or less.

3. Manufacturing method Although there is no restriction | limiting in particular about the manufacturing method of the high strength steel materials which concern on this invention, For example, it can manufacture by giving the heat processing and tempering process shown below with respect to steel which has said chemical composition.

3-1 Metal structure of steel before heat treatment The metal structure of steel to be subjected to heat treatment (hereinafter also referred to as “heat-treating steel”) is a single-phase structure selected from ferrite, pearlite, cementite, and tempered martensite. Or it is desirable to set it as the multiphase structure in which 2 or more types are mixed. The structure is preferably fine-grained. In the case of a composite structure of ferrite and pearlite or a composite structure of ferrite and cementite, the ferrite particle diameter is preferably controlled to 10 μm or less. In the case of tempered martensite, it is desirable that the prior austenite grain size be 10 μm or less. The crystal grain size is more preferably 5 μm or less, and further preferably 3 μm or less. The steel for heat treatment that satisfies the above structural requirements may be produced by any process of hot rolling, hot forging, and heat treatment.

3-2 Heat Treatment Step As described above, in the present invention, the heat treatment steel is first subjected to heat treatment including the following steps. By subjecting the heat-treating steel having the above metal structure to a heat treatment under appropriate conditions, it becomes possible to obtain a steel having an initial structure composed of a single martensite structure phase or a lower bainite structure single phase. Each step in the heat treatment process will be described in detail below.

a) Heating step In the heat treatment step of the present invention, heating is performed to a temperature range of 800 to 900 ° C. In order to austenize the structure, the heating temperature is desirably 800 ° C. or higher. On the other hand, when the heating temperature exceeds 900 ° C., the crystal grains become coarse and adversely affect ductility and toughness. Therefore, the heating temperature is desirably in the range of 800 to 900 ° C.

b) Cooling step After heating, cooling is performed to a temperature range of 250 to 450 ° C at a cooling rate of 10 ° C / s or more, and then cooling is stopped. If the cooling rate is less than 10 ° C./s, ferrite precipitates, and there is a possibility that the martensite structure single phase or the lower bainite structure single phase may not be obtained. On the other hand, when the cooling stop temperature exceeds 450 ° C., the transformation temperature range of bainite or martensite is not achieved, and ferrite may precipitate. On the other hand, when the cooling stop temperature is less than 250 ° C., spheroidization of cementite may be suppressed.

c) Holding step After cooling to the cooling stop temperature, the temperature is held for 10 to 100 s. Within this holding time, bainite or martensite transformation is performed, but if the holding time is less than 10 s, bainite transformation is not completed, and ductility and toughness may be deteriorated. On the other hand, if the holding time exceeds 100 s, the carbides may become coarse and adversely affect toughness.

3-3 Tempering step By performing a tempering treatment subsequent to the above heat treatment, a steel having a tempered structure of martensite or lower bainite is obtained from the steel having the initial structure. In the tempering process, the temperature is maintained at 400 to 680 ° C. for 10 min to 1 h. When the tempering temperature is less than 400 ° C., cementite does not spheroidize, which may adversely affect the delayed fracture resistance. On the other hand, when it exceeds 680 ° C., it may reversely transform into an austenite phase, and the lower bainite structure or martensite structure may collapse, which may adversely affect delayed fracture resistance.

  Further, when the tempering time is less than 10 minutes, it becomes impossible to sufficiently promote spheroidization of cementite. On the other hand, if the tempering time exceeds 1 h, tempering softening proceeds and a high strength of 1000 MPa or more may not be obtained.

3-4 Warm-working step If the above-mentioned steel is warm-worked before it is heat-treated, the subsequent heat-treatment tends to make the austenite crystal grains finer, and the lower austenite grain size fine lower bainite. It becomes easy to obtain a steel material having a metal structure composed of a structure. In the case where the warm working is performed before the heat treatment, it is desirable that the cross-section reduction rate is 20% or more in a temperature range of 500 to 750 ° C. When the warm working temperature exceeds 750 ° C. or when the cross-section reduction rate is less than 20%, the effect of warm working may not be exhibited. On the other hand, it is desirable that the warm working temperature is low, but in practice it is preferably 500 ° C. or higher, and more preferably 600 ° C. or higher. In addition, the higher the cross-sectional reduction rate, the higher is desirable, but the upper limit is desirably 50% in view of the performance of the equipment. The warm working can be performed by a general process such as plate rolling, hole rolling, warm pressing or the like.

3-5 Cold working step When cold working is performed on the above steel before the heat treatment, the austenite crystal grains are likely to be refined by the subsequent heat treatment, and the lower austenite grain size of the lower austenite is fine. It becomes easy to obtain a steel material having a metal structure composed of a structure. In the case where the cold working is performed before the heat treatment, it is desirable to carry out under the condition that the cross-sectional reduction rate is 20% or more. The higher the cross-sectional reduction rate, the higher is desirable, but the upper limit is preferably 50% in view of the performance of the equipment. The cold working can be performed by a general process such as rolling, pressing, or bending.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  After casting steel having the chemical composition shown in Table 1 by vacuum melting 150 kg of molten steel, the steel is heated at a furnace temperature of 1250 ° C., hot forged at a temperature of 950 ° C. or higher, and further shown below. The steel for heat treatment was produced by processing.

  About test numbers 1-8, after hot forging, it was air-cooled and it was set as the steel for heat processing. Test numbers 9 to 22 were air-cooled after hot forging, then reheated to 1050 ° C., hot-rolled by 6-pass multi-pass rolling, and finished rolling with a total reduction of 43% at 920 ° C. Thereafter, it was air-cooled to obtain a steel for heat treatment. Test Nos. 23 to 26 were subjected to water cooling after hot forging, and then subjected to tempering treatment at 600 ° C. for 10 minutes to obtain heat-treating steel.

  Test numbers 27 to 29 were water-cooled after hot forging, and then tempered at 600 ° C. for 10 minutes. Subsequently, after performing warm rolling with a total rolling reduction of 40% by multi-pass rolling at 600 ° C., the steel was air-cooled to obtain a heat-treating steel. Test Nos. 30 to 33 were air-cooled after hot forging, then reheated to 1250 ° C., hot-rolled by multi-pass rolling of 6 passes, and finished rolling with a total reduction ratio of 33% at 920 ° C. Then, after air cooling and cooling to room temperature, cold rolling with a total reduction of 40% was added to obtain a steel for heat treatment.

  For test number 34, air-cooled after hot forging, then reheated to 1050 ° C., hot-rolled by multi-pass rolling of 6 passes, and after finishing rolling with a total reduction of 43% at 920 ° C. After air cooling and cooling to room temperature, cold rolling with a total reduction of 40% was applied to obtain a heat-treating steel. Test numbers 35 and 36 were air-cooled after hot forging, then re-heated to 1050 ° C., hot-rolled by 6-pass multi-pass rolling, and finish-rolled at 920 ° C. with a total rolling reduction of 43%. Thereafter, it was air-cooled, and after that, it was warm-rolled at 600 ° C. by multi-pass rolling with a total rolling reduction of 30 or 40%, and then air-cooled to obtain a heat-treating steel.

  Test No. 37 was water-cooled after hot forging, then tempered at 600 ° C. for 10 min, subsequently subjected to warm rolling with multi-pass rolling at 600 ° C. with a total reduction of 40%, and then air-cooled. Steel for heat treatment was used. For test number 38, air-cooled after hot forging, then reheated to 1050 ° C., hot-rolled by multi-pass rolling of 6 passes, and after finishing rolling with a total reduction of 43% at 920 ° C. Air-cooled, cooled to room temperature, and used as it was for the next step, austenitizing heat treatment.

  Further, for test numbers 27-29, 35, and 36, 30-40% warm processing was applied to the heat treatment steel, and for test numbers 30-34, 40% cold processing was applied to the heat treatment steel. added.

  Specimens were cut out from the above steel for heat treatment and then mirror polished. Thereafter, the steel was corroded with nital or picric acid, and was observed with respect to 5 visual fields of each sample using a scanning electron microscope (magnification 3000 times), and the metal structure was identified and the average particle diameter was measured. Here, the average particle size means the ASTM nominal particle size obtained in accordance with ASTM-E112. The ASTM nominal particle size can be calculated by multiplying the average particle section by 1.13.

  The plate | board thickness of the steel for heat processing obtained at said process is all 2 mm. Each steel for heat treatment was subjected to heat treatment and tempering treatment to produce a steel material. The heat treatment includes a heating step, a cooling step, and a holding step. Each processing condition is as shown in Table 2. Thereafter, the characteristics of each steel material were evaluated. Evaluation items of characteristics are tensile characteristics, toughness (Charpy impact characteristics) and bending characteristics. The characteristic evaluation method will be described below.

<Tensile properties>
By taking a JIS No. 13 tensile test piece and conducting a tensile test, the values of 0.2% yield strength (MPa), maximum tensile stress (MPa), yield ratio, elongation (%), and width drawing (%) were measured. .

<Delayed fracture resistance>
Evaluation was performed in a low load test under hydrogen charge. The hydrogen charge condition was -1.2 V polarization with respect to the Ag / AgCl electrode in 3% saline. Under a hydrogen charge, the load was changed in increments of 100 MPa, the occurrence of cracks after a test time of 200 hours was investigated, and the average of the minimum additional stress that broke and the maximum additional stress that did not break was defined as the minimum lower limit stress. The value obtained by dividing the minimum lower limit stress by the tensile strength in the atmosphere was used as an index of delayed fracture resistance as the delayed fracture strength ratio. The size of the test piece is a plate-like tensile test piece having a plate thickness of 2 mm, a gauge length of 25 mm, and a gauge width of 6 mm.

  In the evaluation of local ductility and delayed fracture resistance, it was judged that the width drawing value was 10% or more and the delayed fracture strength ratio was 0.8 or more, respectively.

  As can be seen from Table 2, the test numbers 1, 5, 9, 13, 16, 19 and 23, which are comparative examples, are not tempered, so that the metal structure of the steel is martensite or the tempered structure of the lower bainite. As a result, the delayed fracture resistance was inferior. In Test Nos. 2 to 4, 6 to 8, and 24 to 26, since the prior austenite grain size was coarse, the cementite spheroidization ratio was less than 0.8, and as a result, the delayed fracture resistance was poor. Further, in Test Nos. 17 and 20, the cementite spheroidization ratio was less than 0.8 due to an inappropriate manufacturing method, and as a result, delayed fracture resistance was poor.

  In Test No. 35, the contents of Si and Mo are not within the prescribed ranges, and the prior austenite grain size and cementite spheroidization ratio do not satisfy the provisions of the present invention. It became. In Test No. 36, the contents of Mn and Mo were not within the prescribed ranges, and the prior austenite grain size did not satisfy the provisions of the present invention, so that the delayed fracture resistance was remarkably inferior. Furthermore, in the test numbers 37 and 38, the B content was out of the specified range, and the metal structure of the steel material was not a tempered structure of martensite or lower bainite.

  On the other hand, test numbers 10 to 12, 14, 15, 18, 21, 22, and 27 to 34, which are examples of the present invention, have a high tensile strength of 1000 to 1450 MPa or more. The width drawing value was 10% or more and the local ductility was excellent, and the delayed fracture strength ratio was 0.8 or more, which was excellent in delayed fracture resistance.

  According to the present invention, according to the present invention, it is possible to obtain a high-strength steel material having good ductility and excellent delayed fracture resistance while having a high tensile strength of 1000 to 1450 MPa. It is possible to dramatically improve the properties and safety of the material. Therefore, the high-strength steel material according to the present invention is suitable for use as a structural material that supports transportation equipment such as automobiles, building members, roads, and railways.

Claims (5)

Chemical composition is mass%,
C: more than 0.2% and 0.55% or less,
Si: 0.1 to 0.3%,
Mn: 0.1 to 0.3%,
Mo: 1-4%
Ti: 0.002 to 0.008%,
Al: 0.01-0.2%
B: 0.001 to 0.006%,
N: 0.001 to 0.015%,
V: 0 to 0.3%
Nb: 0 to 0.05%,
Balance: Fe and impurities,
Having a metal structure consisting of a tempered structure of either martensite or lower bainite with a prior austenite particle size of 3 μm or less,
The spheroidization rate of cementite contained in the metal structure is 0.8 or more,
A high-strength steel material having a tensile strength of 1000 to 1450 MPa. The chemical composition is mass%,
V: 0.05-0.3%
Nb: 0.01-0.05%
The high-strength steel material of Claim 1 containing 1 or more types selected from these. A method for producing the high-strength steel material according to claim 1 or 2,
Having the chemical composition of claim 1 or claim 2,
A steel having a single-phase structure selected from ferrite, pearlite, cementite and tempered martensite or a multi-phase structure in which two or more types are mixed, and having a metal structure having a ferrite or prior austenite grain size of 10 μm or less. ,
After heating to a temperature range of 800 to 900 ° C., cooling to a temperature range of 250 to 450 ° C. at a cooling rate of 10 ° C./s or more, holding at that temperature for 10 to 100 s, and then cooling,
A method for producing a high-strength steel material, comprising a tempering treatment step of holding for 10 min to 1 h in a temperature range of 400 to 680 ° C after the heat treatment step. A method for producing the high-strength steel material according to claim 1 or 2,
Having the chemical composition of claim 1 or claim 2,
A steel having a single-phase structure selected from ferrite, pearlite, cementite and tempered martensite or a multi-phase structure in which two or more types are mixed, and having a metal structure having a ferrite or prior austenite grain size of 10 μm or less. ,
After adding warm working so that the cross-sectional reduction rate is 20% or more in a temperature range of 500 to 750 ° C., heating to a temperature range of 800 to 900 ° C. and then cooling at a cooling rate of 10 ° C./s or more. A heat treatment step of cooling to a temperature range of 450 ° C., holding at that temperature for 10 to 100 s, and then cooling;
A method for producing a high-strength steel material, comprising a tempering treatment step of holding for 10 min to 1 h in a temperature range of 400 to 680 ° C. after the heat treatment step. A method for producing the high-strength steel material according to claim 1 or 2,
Having the chemical composition of claim 1 or claim 2,
A steel having a single-phase structure selected from ferrite, pearlite, cementite and tempered martensite or a multi-phase structure in which two or more types are mixed, and having a metal structure having a ferrite or prior austenite grain size of 10 μm or less. ,
After cold working so that the cross-sectional reduction rate is 20% or more, the steel is heated to a temperature range of 800 to 900 ° C, and then cooled to a temperature range of 250 to 450 ° C at a cooling rate of 10 ° C / s or more. , A heat treatment step of holding at that temperature for 10 to 100 s and then cooling,
A method for producing a high-strength steel material, comprising a tempering treatment step of holding for 10 min to 1 h in a temperature range of 400 to 680 ° C after the heat treatment step.