JP6520598B2 - High strength low alloy steel - Google Patents

Description

  The present invention is a high-strength low-alloy steel material, in particular, a high-strength low-strength steel suitable for use in automobiles, industrial machines, building structures, etc., having a high tensile strength of 1300 MPa or more, high elongation and excellent resistance to hydrogen embrittlement. It relates to an alloy steel material.

  In recent years, high strength steels having a tensile strength exceeding 1000 MPa tend to be used from the viewpoint of weight reduction, functions, and the like. However, when the tensile strength of steel materials exceeds 1000 MPa, hydrogen embrittlement becomes a serious problem. Hydrogen embrittlement is a phenomenon in which mechanical properties deteriorate more than the original value when hydrogen penetrates into steel materials. In addition, since the atomic radius of hydrogen is the smallest among all elements, it is inevitable to penetrate into the steel material.

  Then, various techniques are disclosed regarding the steel material which improved the hydrogen embrittlement resistance characteristic, and its manufacturing method.

  Patent Document 1 describes a hydrogen embrittlement resistant property having a bainite-based metal structure which contains expensive elements, Mo and Ni as essential elements and V as optional elements, and is isothermally transformed in a low temperature range near the Ms point. Techniques for superior high strength steels are disclosed.

  In Patent Document 2, Ca is contained to reduce alumina-based inclusions and, if necessary, expensive elements such as Mo, Ni and V are contained to cold-draw a tempered lower bainite structure, delayed fracture. Technology relating to steel wire that is less likely to cause

  Patent Document 3 heats and hardens a steel containing V as an essential element, Mo, Ni and the like as an optional element by heating to 900 to 1000 ° C., and then tempered at 550 to 650 ° C. to precipitate fine precipitates. There is disclosed a technology relating to a high strength steel excellent in hydrogen embrittlement resistance, which is generated to impart a hydrogen trapping effect.

  In Patent Document 4, 0.05 to 0.3% of V is contained as an essential element, Mo, Ni, etc. as an optional element, and a bainite-based structure excellent in ductility by extending old austenite grains is used as a substrate. A technique related to a high strength PC steel bar excellent in delayed fracture resistance is disclosed.

Japanese Patent Application Laid-Open No. 7-188840 Unexamined-Japanese-Patent No. 7-292442 JP 2002-194481 A Japanese Patent Application Laid-Open No. 9-078192

  The high strength steel disclosed in Patent Document 1 needs to contain expensive elements Mo and Ni at 0.3% or more and 0.1% or more by mass%, respectively. Moreover, the contents of Mo and Ni in “invention steels” 1 to 9 specifically shown in the example of this patent document 1 are 0.56 to 0.96% and 0.29 to 0.29% by mass, respectively. It is 0.55% and all are extremely expensive steels.

  The steel wire disclosed in Patent Document 2 certainly does not easily cause delayed fracture. However, in the technique of Patent Document 2, addition of Ca is essential at the time of melting to secure desired high strength and resistance to hydrogen embrittlement.

  The high strength steel disclosed in Patent Document 3 is indeed excellent in hydrogen embrittlement resistance. However, the high strength steel of Patent Document 3 has room for improvement in securing elongation among the three important properties required for high strength materials such as good strength, elongation and resistance to hydrogen embrittlement. There is.

  The high strength PC steel bar disclosed in Patent Document 4 is indeed excellent in hydrogen embrittlement resistance. However, the technique of Patent Document 4 needs to perform hot rolling at a low temperature of 700 to 900 ° C. in order to make the substrate a structure mainly composed of bainite which is excellent in ductility and in which old austenite grains are elongated. Equipment is required.

  The present invention is suitable for use in automobiles, industrial machines, building structures, etc. having a low content of expensive alloying elements, a tensile strength of 1300 MPa or more, a large elongation, and excellent hydrogen embrittlement resistance. , It aims at providing high strength low alloy steel material.

  MEANS TO SOLVE THE PROBLEM The present inventors acquired the following important knowledge, as a result of repeating earnest research about the method of improving the hydrogen embrittlement-proof characteristic of steel materials by low alloy design, in order to solve said subject.

  The chemical composition is, by mass%, the content of C exceeds 0.55% and the content of V exceeds 0.30%, the metal structure is bainite, and the cementite deposited is the lath interface Steels with a coverage of 10% or less and old austenite grains having a grain size number 6.0 or more as defined in JIS G 0551 (2013) are also low in the content of expensive elements such as Mo and Ni. Since it has sufficient resistance to hydrogen embrittlement and good elongation, it can be suitably used as a high strength part having a tensile strength of 1300 MPa or more.

  FIG. 1 is a view for schematically explaining the situation in which precipitated cementite covers the lath interface in various bainites. In (a), (b) and (c) in the figure, 80%, 5% and 0% of the percentage of cementite deposited on the lath interface (cementite does not precipitate on the lath interface but only inside) The case of precipitation is illustrated.

  The present invention has been made based on the above-described findings, and the gist thereof resides in the high strength low alloy steel material described below.

(1) Chemical composition is in mass%,
C: more than 0.55% and less than 0.70%,
Si: 0.05 to 0.50%,
Mn: 0.30 to 1.0%,
Cr: 0.5 to 1.5%,
V: more than 0.30% and less than 1.0%,
Al: 0.005 to 0.10%,
N: 0.0030 to 0.030%,
Mo: 0 to less than 0.30%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
The balance is Fe and impurities,
P, S and O as impurities are P: 0.030% or less, S: 0.030% or less and O: 0.010% or less,
The metal structure is bainite, and the bainite has a percentage of covered cementite covering the lath interface of 10% or less, and the prior austenite grain has a JIS grain size number of 6.0 or more.
Tensile strength is 1300MPa or more,
High strength low alloy steel.

  (2) The high strength and low alloy steel material according to the above (1), containing V: 0.50 to 1.0% by mass.

  (3) The high-strength low-alloy steel according to the above (1) or (2), containing, by mass%, Mo: 0.05% or more and less than 0.30%.

  (4) Any one of the above (1) to (3) containing one or more selected from, by mass%, Ti: 0.005 to 0.10% and Nb: 0.005 to 0.10% High strength low alloy steel material described in.

  The high-strength low-alloy steel material of the present invention has a low content of expensive alloy elements, a high elongation at a tensile strength of 1300 MPa or more, and an excellent resistance to hydrogen embrittlement, so it is suitable for automobiles, industrial machines, building structures, etc. It can be used suitably.

In various bainite, it is a figure which illustrates typically the condition where the deposited cementite coats the lath interface. (A), (b) and (c) in a figure show the case where the ratio which the precipitated cementite covers a lath interface is 80%, 5%, and 0%, respectively. It is a figure which shows the shape of the notch test piece used for evaluation of the hydrogen embrittlement-proof characteristic in the Example. Numerical values in the figure indicate dimensions (unit: mm).

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" of content of each element means "mass%."

(A) Chemical composition:
C: more than 0.55% and less than 0.70% C is an important element in the present invention, and is an essential element to improve the strength and to make the metal structure into bainite. In addition, it is an element necessary for generating fine precipitates, and is an element necessary for imparting a hydrogen trapping effect to a steel material. In addition, C also has the function of improving the hardenability and lowering the Ms point, which makes it easier to obtain a desired metal structure, and is an effective element also in terms of production. Furthermore, since C has the effect of reducing the occluded hydrogen concentration even with the same strength, the hydrogen embrittlement resistance can be improved even if the content of expensive elements such as Mo and Ni is reduced. In order to stably obtain the three effects of securing high strength of 1300 MPa or more by tensile strength, securing of Ms point which is easy to control to the desired metallographic structure, and securing of excellent hydrogen embrittlement resistance, C is 0 It must be contained in excess of 55%. On the other hand, when the content of C increases to 0.70% or more, the formation of grain boundary cementite is promoted, and a specific bainite having a ratio of covered cementite covering the lath interface of bainite is 10% or less (hereinafter, Since bainite other than “specific bainite” is generated, it is difficult to secure a desired metal structure, and sufficient hydrogen embrittlement resistance can not be obtained, and furthermore, the toughness is significantly deteriorated. Therefore, the content of C is more than 0.55% and less than 0.70%. The desirable lower limit of the C content is 0.60%, and the desirable upper limit is 0.65%.

Si: 0.05 to 0.50%
Si has a deoxidizing action, and also has an action of improving strength and hardenability. The improvement of the strength is effective for securing the tensile strength of 1300 MPa or more, and the improvement of the hardenability is advantageous from the viewpoint of production because a desired metal structure is easily obtained. In order to obtain these effects, the content of Si needs to be 0.05% or more. On the other hand, even if Si is contained in excess of 0.50%, the effect is not only saturation but also deterioration of toughness occurs. Therefore, the content of Si is set to 0.05 to 0.50%. A desirable lower limit of Si content is 0.10%, and a desirable upper limit is 0.30%.

Mn: 0.30 to 1.0%
Mn has an effect of improving hardenability and strength. The improvement of the strength is effective for securing the tensile strength of 1300 MPa or more, and the improvement of the hardenability is advantageous from the viewpoint of production because a desired metal structure is easily obtained. Mn also has an effect of forming sulfides by bonding with S, suppressing grain boundary segregation of S and improving hydrogen embrittlement resistance. In order to obtain these effects, the content of Mn needs to be 0.30% or more. On the other hand, when Mn is contained excessively, it segregates in grain boundaries and promotes intergranular crack-type hydrogen embrittlement fracture. Therefore, the content of Mn is set to 0.30 to 1.0%. The desirable lower limit of the Mn content is 0.60%, and the desirable upper limit is 0.90%.

Cr: 0.5 to 1.5%
Cr is an element effective to improve the strength. In addition, Cr inhibits the diffusion of C and suppresses the growth of carbides to promote the formation of specific bainite. In order to acquire these effects, it is necessary to contain Cr 0.5% or more. On the other hand, when Cr is excessively contained, toughness is degraded. Therefore, the content of Cr is set to 0.5 to 1.5%. The desirable lower limit of the Cr content is 0.8%, and the desirable upper limit is 1.2%.

V: more than 0.30% and not more than 1.0% V is the most important element in the present invention, and it is a fine precipitate formed by combining with C and / or N (carbonitride, carbide and A nitride, which will hereinafter be collectively referred to as “carbonitride”, is an element that significantly improves the resistance to hydrogen embrittlement by exhibiting a hydrogen trapping effect. In a non-tempered bainite base, in order to sufficiently ensure this effect, V needs to be contained in excess of 0.30%. However, when V is contained in an amount of more than 1.0%, the amount and size of precipitates increase, the toughness is degraded, and the hydrogen embrittlement resistance is degraded. Therefore, the content of V is more than 0.30% and not more than 1.0%. V also has the effect of improving the resistance to hydrogen embrittlement by refining the prior austenite grains of the carbonitride of V which has already been precipitated during the austenitizing heat treatment. The above-mentioned action is obtained at a high temperature during the austenitizing heat treatment, with the carbonitrides of V remaining in the austenite without being completely dissolved. Therefore, a desirable lower limit of V content is 0.50%, and a more desirable lower limit is 0.80%.

Al: 0.005 to 0.10%
Al is an element having a deoxidizing action. In order to sufficiently ensure this effect, it is necessary to contain Al by 0.005% or more. On the other hand, even if Al is contained in excess of 0.10%, the effect is saturated. Therefore, the content of Al is made 0.005 to 0.10%. The Al content in the present invention refers to the content in acid-soluble Al (so-called "sol. Al").

N: 0.0030 to 0.030%
N is an element necessary to form the above-described fine carbonitride of V, and is an element necessary to provide a steel material with a hydrogen trapping effect. In order to obtain this effect, the content of N needs to be 0.0030% or more. On the other hand, even if N is contained in more than 0.030%, its effect is not only saturated but also toughness is deteriorated. Therefore, the content of N is set to 0.0030% to 0.030%. The desirable lower limit of N content is 0.010%, and the desirable upper limit is 0.025%.

Mo: 0 to less than 0.30% Mo is an element that improves the stability of Fe carbides and improves the resistance to hydrogen embrittlement. Therefore, Mo may be contained as needed. However, in the present invention, good hydrogen embrittlement resistance can be secured by optimizing the content of other elements such as C and the metal structure, and Mo is a very expensive element. The large content of Mo greatly impairs the economy. Therefore, the Mo content in the case of being contained is made less than 0.30%. The upper limit of the Mo content is preferably 0.20%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of Mo content is 0.05%, and it is still more preferable that it is 0.10%.

Ti: 0 to 0.10%
Ti is an element that combines with C and / or N to form fine precipitates, and refines prior austenite grains to improve hydrogen embrittlement resistance. For this reason, Ti may be contained as needed. However, when Ti is contained in an amount of more than 0.10%, the amount of precipitates increases and the toughness is degraded. Therefore, the upper limit of Ti content in the case of making it contain is made into 0.10%. The upper limit of the Ti content is preferably 0.06%. In addition, in order to acquire the said effect stably, it is desirable that the minimum of Ti content is 0.005%, and it is still more desirable that it is 0.03%.

Nb: 0 to 0.10%
Nb is an element that combines with C and / or N to form fine precipitates, and refines prior austenite grains to improve hydrogen embrittlement resistance. For this reason, Nb may be contained as needed. However, when Nb is contained in an amount of more than 0.10%, the amount of precipitates increases and the toughness is degraded. Therefore, the upper limit of Nb content in the case of making it contain is made into 0.10%. The upper limit of the Nb content is desirably 0.06%. The lower limit of the Nb content is preferably 0.005%, and more preferably 0.03%, in order to stably obtain the above effects.

  It is desirable that the total amount in the case of containing Ti and Nb in combination is 0.08% or less.

  The high-strength low-alloy steel material of the present invention comprises the above-described elements, the balance being Fe and impurities, and P, S and O as impurities are P: 0.030% or less, S: 0.030% or less And O: have a chemical composition that is 0.010% or less.

  Here, "impurity" is a component that is mixed due to various factors such as ore, scrap, etc. and various factors of the manufacturing process when industrially manufacturing steel materials, and is acceptable within a range that does not adversely affect the present invention Means what is done.

P: 0.030% or less P is contained as an impurity and segregates at grain boundaries to lower the toughness and / or the hydrogen embrittlement resistance. If the content of P exceeds 0.030%, the above-mentioned adverse effect becomes remarkable. Therefore, the content of P is set to 0.030% or less. It is desirable that the content of P be as low as possible.

S: 0.030% or less S is contained as an impurity and, like P, segregates in grain boundaries to reduce the resistance to hydrogen embrittlement. When the content of S exceeds 0.030%, the above-mentioned adverse effect becomes remarkable. For this reason, the content of S is made 0.030% or less. It is desirable that the content of S be as low as possible.

O: 0.010% or less O (oxygen) is contained as an impurity and combines with Al to form an oxide. If the content is increased to more than 0.010%, problems such as excessive formation of oxides and a decrease in toughness occur. Therefore, the content of O is made 0.010% or less. It is desirable that the content of O be as low as possible.

(B) Metallographic structure:
The high-strength low-alloy steel material having the chemical composition described in the above (A) has a metal structure of bainite, and the bainite is the specific bainite described above. The prior austenite grains have a JIS grain size number of 6.0 or more.

  Even if the metallographic structure is bainite, the grain boundary strength remarkably decreases if the JIS grain size number of the prior austenite is less than 6.0, and the hydrogen embrittlement resistance is sufficient when the tensile strength is high strength of 1300 MPa or more No property is obtained.

  The larger the prior austenite grain size number is (ie, the smaller the prior austenite grain size is), the better, but in industrial production, the upper limit is JIS grain size number 12.5 or so.

  Even if the metallographic structure is bainite, if the bainite is not a specific bainite, sufficient hydrogen embrittlement resistance can not be obtained when the tensile strength is a high strength of 1300 MPa or more.

  In the case of high strength with a tensile strength of 1300 MPa or more, in order to obtain better hydrogen embrittlement resistance, it is desirable that the specific bainite has a small proportion of cementite covering the lath interface. , 0% is the most desirable.

(C) Strength of steel:
The high-strength low-alloy steel material according to the present invention has the metal structure described in the above (B) and is used for a part that requires a tensile strength of 1300 MPa or more. The upper limit of the tensile strength of the steel material is preferably 2000 MPa, and more preferably 1800 MPa.

  The above-described steel is manufactured by processing isothermally in a specific temperature range as described below and isothermally transformed into a steel processed into a predetermined shape using a steel having the chemical composition described in the paragraph (A). can do.

  That is, it austenitizes under the conditions according to the chemical composition of the steel material, prevents precipitation of ferrite and pearlite, and rapidly cools it into a salt bath or lead bath maintained at 400 ° C. or less and 350 ° C. or more. The above-described metallographic structure and tensile strength can be imparted by keeping the above-mentioned bath as it is, terminating the isothermal transformation at this temperature, and then cooling to room temperature. The steel manufactured as described above also has a large elongation of 10% or more. The upper limit of the elongation of the steel material is about 20%.

  When the austenitizing temperature exceeds 950 ° C., it may be difficult to obtain prior austenite grains having a JIS grain size of 6.0 or more, so the austenitizing temperature is desirably 950 ° C. or less. However, when the V content is 0.5% or more, if the austenitizing temperature exceeds 950 ° C. but is 1050 ° C. or less, a predetermined prior austenite grain having a JIS grain size number of 6.0 or more can be obtained. It is possible. On the other hand, in order to austenitize completely at the time of austenitizing process, it is desirable to make the austenitizing temperature into 850 degreeC or more.

  If the above bath temperature exceeds 400 ° C., even if bainite transformation, the amount of cementite deposited at the lath interface increases, and the ratio of the deposited cementite covering the lath interface exceeds 10% and the strength of 1300 MPa or more It will be difficult to get

  In addition, most of austenite is transformed to specific bainite in a few minutes by holding in the bath in the above temperature range, but bainite transformation of a part of the structure may be delayed due to the influence of the size of steel and / or the contained elements. . Therefore, the holding time in the bath is preferably 30 minutes or more, and more preferably 60 minutes or more. The upper limit is preferably about 80 minutes. There is no particular limitation on the cooling rate when cooling to room temperature after holding in the bath to complete the isothermal transformation.

  Continuous cooling treatment which can prevent the precipitation of ferrite and pearlite and can complete the transformation from austenite in the temperature range of 350 ° C. or more at 400 ° C. or less, the above-mentioned metal structure, tensile strength and elongation Of course, it is possible to produce a steel material having

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples, but the present invention is not limited to these examples.

  After steels A to O having the chemical compositions shown in Table 1 were melted and cast into molds, the ingots obtained were heated to 1250 ° C., and then hot forged to form round bars 25 mm in diameter.

  Steels A to I in Table 1 are steels having a chemical composition within the range defined by the present invention, while steels J to O are steels whose chemical compositions deviate from the conditions defined by the present invention.

  The round bar having a diameter of 25 mm obtained as described above was austenitized by holding it for 45 minutes at the temperature shown in Table 2, and then held in a lead bath under the conditions shown in Table 2 to perform isothermal transformation heat treatment. After the isothermal transformation heat treatment, it was allowed to cool in the air. With respect to steel A, as Test No. 11, a round bar of 25 mm in diameter was austenitized by holding it at 950 ° C. for 45 minutes, then oil quenching was performed, and tempering was performed at 460 ° C. for 60 minutes.

  Various investigations shown below were conducted using the above-described round bars of each of the isothermal transformation heat-treated steels, and the round bars with oil-quenched and the oil-quenched-tempered round bars of the steel A.

<1> Metallographic structure:
A round bar of 25 mm in diameter subjected to isothermal transformation heat treatment is cut through its center line in the longitudinal direction (hereinafter referred to as “longitudinal section”), a test piece is collected, and austenite grain size number number according to JIS G 0551 (2013) investigated. Specifically, the test piece is embedded in a resin and mirror-polished so that the longitudinal cross-section of the test piece becomes the test surface, and then etched with a picric acid saturated aqueous solution to which a surfactant is added as described in Annex JA of the JIS The former austenite grain boundaries were revealed, and observation was carried out with a 10 field optical microscope at a magnification of 200 times, and the former austenite grain size number was measured by the cutting method described in Appendix C of the above-mentioned JIS. In addition, using the test pieces obtained by vertically cutting the oil-quenched round bar of steel A, the prior austenite grain size number was investigated by the above-mentioned method.

  Furthermore, test bars were collected by traversing the 25 mm diameter round bar subjected to the isothermal transformation heat treatment and the oil quenching and tempering treatment. Then, the test piece was embedded in a resin such that the longitudinal cross-section of the test piece was the test surface, and after mirror-polished, the sample was etched with a 2% nital to observe the structure.

  Specifically, each test piece subjected to the isothermal transformation heat treatment was first observed with a 10-field optical microscope at 200 × magnification to determine the metal structure. Subsequently, the area ratio of specified bainite (the bainite in which the ratio of covered cementite to cover the lath interface is 10% or less) in bainite was observed for 10 fields of view at a magnification of 10000 × with a scanning electron microscope and determined by optical microscope observation I asked for. On the other hand, the oil-quenched-tempered test pieces were observed under a 10-field optical microscope at a magnification of 200 times, and it was confirmed that the metal structure consisted of tempered martensite and that bainite was not contained therein.

<2> Mechanical properties:
From the R / 2 part ("R" represents the radius of the round bar) of the 25 mm diameter round bar subjected to isothermal transformation heat treatment and oil quenching-tempering treatment, the diameter of the parallel part in the longitudinal direction is 6 mm and the gauge distance is 40 mm The round bar tensile test pieces were cut out and subjected to tensile test at room temperature to determine tensile strength and elongation. In addition, when the elongation was 10% or more, this was targeted as having a large elongation.

<3> Hydrogen embrittlement resistance:
Notched tensile test of the shape shown in FIG. 2 in the longitudinal direction from the R / 2 portion of each 25 mm diameter round bar at which a tensile strength of 1300 MPa or more and an elongation of 10% or more were obtained in the survey of <2> above. A piece was cut out, and the hydrogen embrittlement resistance was examined based on the presence or absence of breakage when conducting a constant load test under a cathode charge loaded with 70% stress of tensile strength for 200 hours. At that time, the current density of the cathode hydrogen charge was adjusted so that hydrogen was absorbed at a concentration of 1 ppm inside the test piece. The amount of hydrogen occluded inside the test piece was determined by using a 3% NaCl solution and using a temperature rising and desorbing device when the cathode hydrogen charge was performed for 72 hours at a current density of 0.8 to 1.5 mA / cm ^2. The amount of hydrogen released up to 350 ° C. when the temperature was raised at 10 ° C./min was used.

  Table 2 summarizes the results of the above surveys. In the column of hydrogen embrittlement resistance, the above test was conducted for 200 hours, and those without breakage were marked with "o", and those with breakage were marked with "x".

  From Table 2, Test Nos. 1 to 9 of the invention examples satisfying the conditions of the chemical composition and metal structure defined by the present invention have elongation of 14% in spite of having high tensile strength of 1300 MPa or more. It is clear that no breakage occurs in constant load tests under excess, even under cathodic charge, with good elongation and resistance to hydrogen embrittlement.

  On the other hand, in the case of the test No. 10 to 18 of the comparative example, it is not possible to simultaneously secure three important properties: tensile strength of 1300 MPa or more, elongation of 10% or more and good hydrogen embrittlement resistance .

  Although test No. 10 shows that the chemical composition of steel A used is within the range specified in the present invention, the area ratio of specified bainite in bainite in the metal structure is 77%, which deviates from the conditions specified in the present invention Therefore, the tensile strength was less than 1300 MPa.

  The test No. 11 has the chemical composition of the used steel A within the range specified in the present invention, and the JIS grain size number of the prior austenite is 10.6, but the tempered martensite structure obtained by the oil quenching and tempering treatment So the growth was less than 10%.

  The test No. 12 has the chemical composition of the used steel F within the range specified in the present invention, and although the metallographic structure is bainite, the prior austenite grains have a JIS grain size number as small as 5.4. For this reason, although 100% of the said bainite is specific bainite by area ratio, it is inferior to the hydrogen-proof embrittlement characteristic.

  Test No. 13 has a metal structure that satisfies the conditions specified in the present invention, but the C content of the steel J used is as low as 0.49%, which deviates from the conditions specified in the present invention. It did not meet.

  Test No. 14 has a metal structure that satisfies the conditions specified in the present invention, but the Mn content of the steel K used is as high as 1.20%, which deviates from the conditions specified in the present invention. It is inferior.

  Test No. 15 has a Cr content of 0.32% as low as steel L used and deviates from the conditions defined in the present invention, so it is inferior in hardenability and the metal structure becomes a mixed structure of ferrite and bainite. It deviates from the conditions specified by the invention. For this reason, the test No. 15 had a tensile strength of less than 1300 MPa.

  Although Test No. 16 satisfies the conditions specified in the present invention, the V content of the steel M used is as high as 1.32%, which deviates from the conditions specified in the present invention. It is inferior.

  Although Test No. 17 has a metallurgical structure satisfying the conditions specified in the present invention, the contents of P and S in the impurities of the used steel N are as high as 0.040% and 0.032%, respectively. Since it deviates from the specified conditions, it is inferior to the hydrogen embrittlement resistance.

  Test No. 18 shows that the metal structure satisfies the conditions specified in the present invention, but the contents of V and N of steel O used are as low as 0.18% and 0.0014%, respectively, and the conditions specified in the present invention It is inferior to hydrogen embrittlement resistance because

The high-strength low-alloy steel material of the present invention has a low content of expensive alloy elements, a high elongation at a tensile strength of 1300 MPa or more, and an excellent resistance to hydrogen embrittlement, so it is suitable for automobiles, industrial machines, building structures, etc. It can be used suitably.

Claims (4)

The chemical composition is in mass%,
C: more than 0.55% and less than 0.70%,
Si: 0.05 to 0.50%,
Mn: 0.30 to 1.0%,
Cr: 0.5 to 1.5%,
V: more than 0.30% and less than 1.0%,
Al: 0.005 to 0.10%,
N: 0.0030 to 0.030%,
Mo: 0 to less than 0.30%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
The balance is Fe and impurities,
P, S and O as impurities are P: 0.030% or less, S: 0.030% or less and O: 0.010% or less,
The metal structure is bainite, and the bainite has a percentage of covered cementite covering the lath interface of 10% or less, and the prior austenite grain has a JIS grain size number of 6.0 or more.
Tensile strength is 1300MPa or more,
High strength low alloy steel.   The high-strength low-alloy steel material according to claim 1, containing V: 0.50 to 1.0% by mass.   The high-strength low-alloy steel material according to claim 1 or 2, wherein the content of Mo is 0.05% or more and less than 0.30%. The high strength according to any one of claims 1 to 3, containing one or more selected from, by mass%, Ti: 0.005 to 0.10% and Nb: 0.005 to 0.10%. Low alloy steel.

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