0001]The present disclosure relates to high-strength steel member.
BACKGROUND
[0002]Machinery, automobiles, bridges, among the steel member used in the building, especially a steel member requiring high strength, for example, the JISG 4104, JIS G 4105 provisions of chromium steel or chromium molybdenum steel, quenching and tempering and We are using Te. Moreover, as the gears, perform quenching after performing carburization, there is also a steel member to obtain a high strength.
　The quenching, Yakiireru after heating to a high temperature comprising steel members and austenite phase. However, hydrogen from the atmosphere during heating when entering in the steel member, cause the quenching cracks are generated after quenching. For example, when the tempering temperature as high strength steel members 0.99 ~ 200 ° C. and lower, the hydrogen penetrates into the steel member is not sufficiently discharged by tempering, ductility or toughness after tempering is reduced during hardening there is.
[0003]
　Here, relates the hydrogen embrittlement resistance of high strength steel member (tensile strength 1000MPa or more steel member), for example, Patent Document 1, V, Nb, and the Ti added to steel prior austenite grain refinement it is described that be has been enabled to improve the delayed fracture resistance.
　Patent Documents 2 to 4, a fine deposits with hydrogen trapping ability by hot tempering after quenching is described a technique for improving the delayed fracture resistance are dispersed in the steel.
[0004]
Patent Document 1: Japanese Patent Laid-Open 3-243745 discloses
Patent Document 2: Japanese Patent 2000-26934 JP
Patent Document 3: Japanese Patent 2006-45670 JP
Patent Document 4: Japanese Patent 2001-288539 No.
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　However, in the conventional techniques described in Patent Documents 1 to 4 and the like, for example, after quenching, the delayed fracture resistance of high strength steel member such as applying cold tempering of 0.99 ~ 200 ° C. to be drastically improved there has been a limit.
[0006]
　Accordingly, one aspect of the object of the present disclosure is to provide a is one delayed high strength steel member having excellent fracture characteristics of hydrogen embrittlement resistance.
Means for Solving the Problems
[0007]
　Means for solving the problems of one embodiment of the present disclosure includes the following aspects.
[0008]
<1>
　by
　mass%,
　C:
　0.10 ~ 0.50%, Si: 0.02 ~ 2.00%,
　Mn: 0.05 ~ 2.00%, Cr: 0.10 ~ 2.00% ,
　Ti: 0.20 ~ 1.00%, and
　N:
　0.0020 ~
　0.0250% Al: 0 ~ 0.100%, V:
　0 ~ 0.50%, Nb: 0 ~ 0.50%,
　Mo:
　0 ~ 1.00%, B:
　0 ~ 0.0100%, Cu: 0 ~ 2.00%, and
　Ni: 0 ~ 3.00%,
　and contains the chemical composition comprising the balance Fe and impurities have,
　have a tensile strength of 1000MPa or more,
　at a position of depth of 1mm from the surface of the steel member, the average is 30 ~ 200 nm in an average circle equivalent diameter size, and Ti carbide, Ti nitrides and composites thereof at least one Ti precipitates in area% 0.10% or more containing selected from the group consisting of compounds And,
　high-strength steel member containing non-diffusible hydrogen released in a temperature range of 400 ~ 800 ° C. during the Atsushi Nobori leaving hydrogen Analysis least 0.5 mass ppm.
<2>
　in terms of% by mass,
　Al:
　0.005 ~ 0.100%,
　V: 0.01 ~ 0.50%, Nb: 0.01 ~ 0.50%, and
　Mo: 0.01 ~ 1.00%,
　1 kind or 2 high strength steel member according to <1> having a chemical composition containing the above species.
<3>
　mass%
　B: 0.0003 ~ 0.0100%
　high-strength steel member according to <1> or <2> having a chemical composition containing.
<4>
　in
　mass%, Cu: 0.05 ~ 2.00%, and Ni: having a chemical composition containing one or two 0.05 to 3.00% <1> to the <3> high strength steel member according to any one.
<5>
　The average aspect ratio of the Ti precipitates is 1.0-3.0 <1> to <4> high strength steel member according to any one of.
The invention's effect
[0009]
　According to one aspect of the present disclosure can provide is one delayed high strength steel member having excellent fracture characteristics of hydrogen embrittlement resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
When [1] is measured greater than "round steel bar size φ10mm × L50mm", when collecting a test piece for measuring the content of non-diffusible hydrogen from the measurement target, the sampling position of the test piece description is a schematic view for.
DESCRIPTION OF THE INVENTION
[0011]
　Will be described in detail below in embodiments wherein an example of the present disclosure.
　In this specification, numerical ranges expressed using "to" means a range including numerical values described before and after "to" as the lower and upper limits.
　The content of elements in chemical composition, element content (for example, C content, Si content, etc.) is denoted as.
　Further, the content of elements of the chemical composition, "%" means "% by mass".
[0012]
　High strength steel member according to the present embodiment (hereinafter, simply referred to as "steel member") has a predetermined chemical composition is a steel member than the tensile strength of 1000 MPa. Incidentally, the tensile strength of the steel member is a value measured according to JIS-Z2241 (2015 years).
　Then, the steel member according to the present embodiment, at the position of depth of 1mm from the surface of the steel member, the average is 30 ~ 200 nm in an average circle equivalent diameter size, and Ti carbide, Ti nitride and composite compounds thereof at least one of Ti precipitates contained in area% 0.10% or more, the non-diffusible hydrogen 0.5 mass ppm, which is released at 400 ~ 800 ° C. during the Atsushi Nobori leaving hydrogen analysis is selected from the group consisting of containing more.
[0013]
　Steel member according to the present embodiment, the above-described configuration, a high strength steel member having excellent delayed fracture resistance is one of hydrogen embrittlement resistance. Then, the steel member according to the present embodiment were found by the finding that following.
[0014]
　The present inventors, using various intensity levels of the steel member produced by quenching and tempering treatment, were analyzed in detail the delayed fracture behavior is one of the phenomena of hydrogen embrittlement.
　Delayed fracture, of the hydrogen entering from the outside environment in the steel member, it is already clear that in particular due to the diffusible hydrogen diffuse at room temperature in a steel member. The diffusible hydrogen in the "curve showing the relationship between the hydrogen release rate from the temperature and the steel member" obtained a steel member when heated at a rate of 100 ° C. / time, a peak temperature of about 100 ° C. It has can be measured from the curve.
　Therefore, if such does not diffuse by trapping hydrogen entering from the external environment to some portion of the steel member, it is possible to detoxify the hydrogen to absorbed hydrogen, delayed fracture can be prevented.
　The presence of the capture site hydrogen (hereinafter referred to as "hydrogen trap sites") may compare the peak temperature and the peak height of the curve of hydrogen release resulting hydrogen-charged before and after the steel member is heated at 100 ° C. / Time It can be judged by. Then, (also referred to as the "hydrogen trapping capacity") the amount of hydrogen trapped in a certain hydrogen trap sites can be determined by the area integral value of the peak.
[0015]
　Accordingly, the present inventors have found that after quenching, the delayed fracture resistance of the steel member subjected to low temperature tempering of 0.99 ~ 200 ° C., it was carried out following evaluation. Test pieces of round steel bar having a diameter of 10mm with an annular notch 30 and heated to 20 minutes in an atmosphere of 1 atm with 100% hydrogen, was quenched by water cooling, it returned 30 minutes baked at 0.99 ° C.. Thereafter, the test piece, the test piece in the air loaded with static load (90% of the tensile strength), measures the time until fracture. Thus, to evaluate the delayed fracture resistance. Incidentally, the delayed fracture resistance of the longer steel members rupture time means that it is good.
[0016]
　As a result, the present inventors found that, at the location of 1mm deep from the surface of the steel member, chosen from the average particle diameter is the 30 ~ 200 nm, and Ti carbide, Ti nitrides and the group consisting of complex compounds thereof at least one Ti precipitates steel member having a steel structure containing at area% 0.10% or more is found that excellent delayed fracture resistance.
　Then, having such steel structure, the steel member excellent in delayed fracture resistance, after heat treatment at the heat treatment conditions, when the heating leaving hydrogen analysis at a rate of 100 ° C. / time, 400 ~ 800 ° C. or less the hydrogen release peak indicating that the hydrogen that is stably trap is released into the hydrogen trapping sites consisting of Ti precipitates obtained temperature range. The amount of hydrogen released (hydrogen trapping capacity) was more than 0.5 mass ppm.
[0017]
　Here, the inventors have a patent document 2 "fine deposits with hydrogen trapping ability by hot tempering after quenching described in (JP 2000-26934 JP) was dispersed in the steel resistant It was compared with the technology "to improve the delayed fracture characteristics. As a result, we obtained the following findings. Fine Ti precipitates (Ti carbide, Ti nitride and at least one Ti precipitates are selected from the group consisting of complex compounds thereof), when a large amount containing 0.20% or more of Ti, more precipitation at high temperatures to. Therefore, without tempering, it is possible to deposit at the time of heating at the time of quenching, and trapped hydrogen is released at higher temperatures. As it can be seen from this that, in order to trap the stable hydrogen, the hydrogen which has entered from the heating atmosphere during quenching trap during quenching cooling allows harmless hydrogen in the subsequent low temperature tempering. Therefore, compared with the technique of Patent Document 2 it was found that excellent delayed fracture resistance.
[0018]
　Incidentally, the delayed fracture resistance is lowered when excessively containing C. Further, coarse particles are generated during quenching when not containing a predetermined amount of N, delayed fracture resistance is lowered. Therefore, as described later, the C content from 0.10 to 0.50 percent, the N amount is from 0.0020 to 0.0250 percent.
[0019]
　By the above findings, the steel member according to the present embodiment, the above configuration has been found to be a high strength steel member having excellent delayed fracture resistance is one of hydrogen embrittlement resistance.
　The established techniques for forming a hydrogen trap sites "Ti carbide, Ti nitride and at least one Ti precipitates are selected from the group consisting of complex compounds thereof" is finely precipitated steel structure steel member It has been.
　Incidentally, focusing on the steel structure of the steel member at the position of depth of 1mm from the surface of the steel member, delayed fracture inside a and stresses three Jikudo surface from a depth of several hundred μm or more of the steel member due to hydrogen embrittlement high sites is to generate become a starting point.
[0020]
　The following is a detailed description of a steel member according to the present embodiment.
[0021]
(Hydrogen trapping capacity)
　will be described first reasons for limiting the hydrogen trapping capacity is the most important point with respect to improvement of the delayed fracture characteristics of high strength steel members (i.e., the content of non-diffusible hydrogen).
　Diffusible hydrogen that cause delayed fracture of the steel member obtained by tempering at a low temperature after quenching is entering from the heating atmosphere in the steel member during quenching. For example, carburizing and quenching, or the time of quenching by heating with cooking RX gas (endothermic modified gas), hydrogen several mass ppm from entering during the heating to the austenite region. Since the diffusion coefficient of hydrogen martensite structure obtained by quenching small, the return cold baked after quenching, it is difficult to completely release hydrogen, it is possible that hydrogen embrittlement occurs.
[0022]
　Such heating in the atmosphere, when Ru quenching, when the captured stably hydrogen to some hydrogen trap sites, increasing the content of non-diffusible hydrogen after quenching, it is possible to suppress the hydrogen embrittlement become. That is, hydrogen released when it is reheated to a temperature range of 400 ~ 800 ° C. after quenching is a stably trapped into hydrogen trap sites, does not contribute to detoxified hydrogen embrittlement.
[0023]
　Therefore, the steel member according to the present embodiment, the steel member comprising a non-diffusible hydrogen released in a temperature range of 400 ~ 800 ° C. during the Atsushi Nobori leaving hydrogen Analysis least 0.5 mass ppm. In other words, the hydrogen trapping capacity (amount of non-diffusible hydrogen) and 0.5 mass ppm.
　Hydrogen trapping capacity, in view of the delayed fracture resistance improves, preferably at least 0.8 mass ppm, more preferably not less than 1.0 mass ppm. However, from the viewpoint of suppression of forgeability decreases due to the increase of the precipitates, the upper limit of the content of the non-diffusible hydrogen is preferably not more than 3.0 mass ppm.
[0024]
　Then, by the amount of non-diffusible hydrogen released in a temperature range of 400 ~ 800 ° C. during the Atsushi Nobori leaving hydrogen analysis to control the steel structure is not less than 0.5 mass ppm, to improve the delayed fracture properties possible to become.
[0025]
　Here, heating withdrawal hydrogen analysis is performed as follows. First, a steel member to be measured, collecting a test piece of round steel bar size φ10mm × L50mm. Next, the "gas chromatograph ShikiNoboru temperature hydrogen analyzer", the test piece was heated at 100 ° C. / time, analyzing the amount of hydrogen released in each temperature (mass).
　Then, obtain the hydrogen release curve showing the relationship between the amount of hydrogen released as the temperature. By the peak area integral values of hydrogen release curve, non-diffusible hydrogen amount emitted in a temperature range of 400 ~ 800 ° C., i.e. obtaining the hydrogen trapping capacity (amount of non-diffusible hydrogen).
　Here, when the measurement target is greater than the "round steel bar size φ10mm × L50mm", the test piece, the position is the outer peripheral surface of the 1mm deep from the surface of the measuring object "circle steel bar size φ10mm × L50mm" It is referred to as scraped specimen from the measurement target (see FIG. 1). In FIG. 1, OM steel member to be measured, SP denotes a test piece.
　On the other hand, if the measurement object is less than the "round steel bar size φ10mm × L50mm" specimens, the raw measurement target sample piece. Also test piece is smaller than the "round steel bar size φ10mm × L50mm", because there is no change in the value of the content of non-diffusible hydrogen is measured.
　Even if a-out annular notch provided in the test piece by an annular notch existence, no variation in the value of the content of non-diffusible hydrogen is measured.
[0026]
(Steel structure)
　steel member according to the present embodiment, at a position on the surface from a depth of 1mm of the steel member, the average is 30 ~ 200 nm in an average circle equivalent diameter size, and Ti carbide, Ti nitrides and their at least one Ti precipitates are selected from the group consisting of complex compounds containing at area% 0.10% or more. That is, the existence ratio of Ti precipitates in area% and 0.10% or more.
[0027]
　Ti precipitates have hydrogen trapping ability, the hydrogen trapping sites releases hydrogen at relatively high temperature of 400 ~ 800 ° C.. Then, during the quenching of the steel members by the presence of Ti precipitates having hydrogen trapping ability, a non-diffusible hydrogen that can be stably trapped. In other words, the hydrogen trapping capacity (amount of non-diffusible hydrogen) can be at least 0.5 mass ppm. Thereby, it is possible to improve the delayed fracture properties of the steel member.
　Although having also hydrogen trapping ability Ti oxides, for securing the forging, it is preferable to not include Ti oxides on the steel member.
[0028]
　In Ti precipitates, Ti carbide, Ti nitride and composite compounds thereof (i.e. Ti carbonitride) is, (Ti occupies more than 50 atomic% of metal sites) Ti as a metal component as a main, FCC (face it is a compound having a cardiac cubic) structure.
[0029]
　Abundance of Ti precipitates increases the hydrogen trapping capacity, in terms of delayed fracture properties improve, preferably not less than 0.10% by area%, more preferably at least 0.20%. However, from the viewpoint of toughness ensuring the presence ratio of Ti precipitates 1.00% or less are preferred in terms of area%, more preferably 0.50% or less.
　Incidentally, the existence ratio of Ti precipitates means the existence ratio of the total Ti precipitates contained in the steel member.
[0030]
　The average size of Ti precipitates while ensuring the tensile strength, enhanced hydrogen trapping capacity, in terms of delayed fracture properties improve, preferably 100nm or less in average circle equivalent diameter, and more preferably not more than 80 nm. Further, from the same viewpoint, the average size of Ti precipitates more preferably 60 nm.
[0031]
　The average aspect ratio of Ti precipitates while ensuring the tensile strength, enhanced hydrogen trapping capacity, in terms of delayed fracture properties improve, but 1.0-3.0 preferable. The upper limit of the average aspect ratio of Ti precipitates is more preferably 2.0, more preferably 1.5.
[0032]
　Here, the presence rate of Ti precipitates, the average size (mean circle equivalent diameter), each measurement of the average aspect ratio of Ti precipitates Ti precipitates Test pieces were produced by extraction replica method, energy dispersive X carried out using a linear analyzer (EDS) with a transmission electron microscope (TEM). More specifically, it is as follows.
　From any site of the measurement subject to the steel member, were collected portion having at a depth of 1mm from the surface of the steel member (hereinafter also referred to as a "measurement surface"), the extraction replica method to prepare a test piece.
　Next, observed by TEM-EDS any area of the measurement surface of the specimen (area size 5 [mu] m × 5 [mu] m) at a magnification of 30,000.
　Next, the components of the precipitates present in the visual field of observation, analysis by analysis and EDS electron diffraction pattern of the TEM, to identify Ti precipitates.
　Then, to calculate the area ratio of all the Ti precipitates present in the visual field to be observed.
　Then, the above operation was performed 5 times, the mean value of the area of the resulting Ti precipitates and abundance of Ti precipitates.
[0033]
　On the other hand, obtaining the equivalent circle diameter of all the Ti precipitates present in the visual field to be observed.
　Then, the above operation was performed 5 times, and the average size of the obtained average value of Ti precipitates "equivalent circle diameter" (average equivalent circle diameter) is.
[0034]
　Also, determine the long axis length and short axis length of all the Ti precipitates present in the visual field to be observed. Long axis length of Ti precipitates is the maximum diameter of Ti precipitates. Minor axis length of Ti precipitates is the maximum length of a length along the perpendicular direction to the long axis of Ti precipitates.
　Then, the above operation was performed five times, resulting in "aspect ratio (= ratio of the major axis and the minor axis length (major axis length / minor axis length))" mean the Ti precipitates the average aspect ratio.
[0035]
　Steel member according to the present embodiment, from the viewpoint of delayed fracture properties improved, it is preferable to have a prior austenite grains are refined.
　The particle size of the prior austenite grains (hereinafter referred to as "old γ grain size") is at a position at a depth of 1mm from the surface of the steel member, a circle equivalent diameter is preferably 5 ~ 50 [mu] m, more preferably 10 ~ 40 [mu] m, 15 even more preferably ~ 30μm.
[0036]
　Old γ grain size is determined by the following methods.
　After from any site of the measurement subject to the steel member, were collected portion having at a depth of 1mm from the surface of the steel member (hereinafter also referred to as a "measurement surface") were polished embedded measurement surface of the sample taken, etched with picral solution as etchant (HCl, mixed solution of picric acid and alcohol). The measuring surface of the sample optical microscope photograph taken (250 times), the γ grain boundaries were photographed binarized digitally measures the particle size of the old γ grains and calculate the average.
[0037]
(Chemical Composition)
　steel member according to the present embodiment, from the viewpoint of delayed fracture properties improve, by mass%, C: 0.10 ~ 0.50% , Si: 0.02 ~ 2.00%, Mn: 0 .05 ~ 2.0%, Cr: 0.10 ~ 2.00%, Ti: 0.20 ~ 1.00%, N: 0.0020 ~ 0.0250%, Al: 0 ~ 0.100%, V: 0 ~ 0.50%, Nb : 0 ~ 0.50%, Mo: 0 ~ 1.00%, B: 0 ~ 0.0100%, Cu: 0 ~ 2.00%, and Ni: 0 ~ containing 3.00%, it is preferred to have a chemical composition the balance being Fe and impurities.
　Here, in the chemical composition of the steel member according to the present embodiment, Al, V, Nb, Mo , B, Cu, and Ni, an optional component, i.e., a good component be free steel member. However, if the inclusion of these components, these components are preferably to be contained in more than the lower limit of the amount of each component which will be described later.
[0038]
C ·: 0.10 ~ 0.50%
　C is an essential element in securing a tensile strength of the steel member (hereinafter referred to as "strength"). C amount is not required strength can be obtained with less than 0.10%. On the other hand, the C content deteriorates the toughness exceeds 0.50%, the delayed fracture properties deteriorate. Therefore, C amount is set to 0.10 to 0.50%. C amount, in terms of strength and toughness, preferably 0.20 to 0.40%.
[0039]
Si ·: 0.02 ~
　2.00% Si is the solid solution hardening effect, an effect of increasing the strength of the steel members. Si content above action can not be exhibited less than 0.02%. On the other hand, when the Si content exceeds 2.00%, the effect is saturated, the effect commensurate with the amount can not be expected. Therefore, Si amount is set to 0.02 to 2.00%. The amount of Si, from the viewpoint of exhibiting a solid solution hardening effect, preferably 0.20 to 2.00%.
[0040]
Mn ·: 0.05 ~ 2.00% Mn
　is not only necessary for deoxidation and desulfurization, is an effective element to increase hardenability to obtain martensite. Mn amount is not the above effect obtained is less than 0.05%. On the other hand, when the Mn amount exceeds 2.00%, Mn precipitate was segregated at the grain boundaries during heating to the austenite region, it causes embrittle grain boundaries, thereby deteriorating the delayed fracture resistance. Therefore, Mn amount is set to 0.05 to 2.00%. Mn amount, from the viewpoint of improving the hardenability and delayed fracture resistance, is preferably from 0.50 to 1.50%.
[0041]
Cr ·: 0.10 ~
　2.00% Cr is an element effective for increasing the softening resistance at the time of increase and tempering treatment hardenability. Cr amount is above effect can not be sufficiently exhibited less than 0.10%. On the other hand, Cr content exceeds 2.00%, the toughness deteriorates, leading to cold workability deteriorates. Therefore, Cr amount is set to 0.10 to 2.00%. Cr amount, improving the hardenability, in view of the toughness and cold workability deterioration suppression, preferably from 0.50 to 1.50%.
[0042]
· Ti: 0.20 ~
　1.00% Ti is selected from a relatively high temperature fine Ti precipitates having hydrogen trapping ability (Ti carbide, Ti nitrides and the group consisting of complex compounds thereof that 400 ~ 800 ° C. at least one Ti precipitates) to form an element which contributes to the breakdown characteristics improved Re delayed is. Further, Ti is an effect of preventing coarsening of austenite grains by forming a TiN in deoxidation and the heat treatment also has the effect of fixing the N. Ti content is less than 0.20%, these effects can not be exhibited. On the other hand, if the Ti amount exceeds 1.00%, not dissolved even by heating at the time of rolling, and remaining coarse Ti precipitates, adversely affects the machinability or toughness. Therefore, Ti amount is set to 0.20 to 1.00%. Ti content, the formation of fine Ti precipitates, from the viewpoint of cutting or toughness, preferably 0.30 to 0.80%, more preferably 0.40 to 0.60%.
[0043]
N ·: 0.0020 ~ 0.0250%
　N forms a Ti nitride, an element which contributes to the breakdown characteristics improved Re delayed. Further, N represents, Al, V, by forming a nitride of Nb, the effect of the increase in miniaturization and yield strength of prior austenite grains. N content is less than 0.0020%, those effects are small. On the other hand, when the N amount exceeds 0.0250%, those effects are saturated. Therefore, N amount is set to 0.0020 to 0.0250%. N content, the delayed fracture resistance improvement, in view of increasing miniaturization and the yield strength of prior austenite grains, preferably 0.0030 to 0.0150 percent.
[0044]
　Here, the chemical composition of the steel member according to the present embodiment, by mass%, Al: 0 ~ 0.100%, V: 0 ~ 0.50%, Nb: 0 ~ 0.50%, and Mo: 0 it may contain one or more to 1.00%. Preferably, in mass%, Al: 0.005 ~ 0.100%, V: 0.01 ~ 0.50%, Nb: 0.01 ~ 0.50%, Mo: 0.01 ~ 1.00% is to contain one or more.
[0045]
Al ·: 0.005 ~
　0.100% Al, along with the effect of preventing the coarsening of austenite grains by forming an AlN during deoxidation and heat treatment, is an element effective to secure the N. The Al content is less than 0.005%, less likely these effects can be exerted. On the other hand, when the Al content exceeds 0.100% These effects are easily saturated. Therefore, Al is preferably 0.005 to 0.100%.
[0046]
V ·: 0.01 ~ 0.50%
　V is, TiC and complex precipitation, an element which contributes to fine dispersion of the precipitates. Also, V is, by generating a carbonitride, is an effective element in order to refine the austenite grains. However, the effect is less unless the amount V is 0.01% or more, the amount of V tends to saturate more than 0.50%. Further, the exceeded amount V is 0.50%, easily processability is deteriorated by the increase of deformation resistance. Therefore, V amount is preferably 0.01 to 0.50 percent.
[0047]
Nb ·: 0.01 ~
　0.50% Nb is, TiC and complex precipitated similarly to V, an element which contributes to fine dispersion of the precipitates. Further, Nb is by generating carbonitrides, is an effective element in order to refine the austenite grains. However, the effect, Nb amount is insufficient is less than 0.01% tends to saturate the Nb amount exceeds 0.50%. Therefore, Nb amount is preferably 0.01 to 0.50%.
[0048]
Mo ·: 0.01 ~
　1.00% Mo is, TiC and complex precipitated similarly to V, an element which contributes to fine dispersion of the precipitates. However, the effect, Mo amount is insufficient is less than 0.01% tends to saturate the Mo content exceeds 1.00%. Further, when the Mo content exceeds 1.00%, workability is easily impaired by the increase of deformation resistance. Therefore, Mo content is preferably 0.01 to 1.00%.
[0049]
　The chemical composition of the steel member according to the present embodiment, by mass%, B: 0 may contain ~ 0.0100%. Preferably, in mass%, B: is to contain 0.0003 to 0.0100 percent.
[0050]
B ·: 0.0003 ~ 0.0100%
　B suppresses grain boundary fracture, is an element improving the delayed fracture resistance. Also, B, by segregating the austenite grain boundaries, is also an element significantly increase the hardenability. However, the effect is hardly exhibited in the B amount is less than 0.0003%, the B content is likely to saturate more than 0.0100%. Therefore, B amount is preferably from 0.0003 to 0.0100%. B amount is hardenability, and from the viewpoint of improving the delayed fracture resistance, and more preferably 0.0003 to 0.0050%.
[0051]
　The chemical composition of the steel member according to the present embodiment, by mass%, Cu: 0 ~ 2.00%, and Ni: 0 ~ 3.00% of may contain one or two. Preferably, in mass%, Cu: 0.05 ~ 2.00%, and Ni: is to contain one or two 0.05 to 3.00%.
[0052]
Cu ·: 0.05 ~
　2.00% Cu is an element effective for increasing the softening resistance at the time of tempering. The Cu content is less than 0.05%, the effect is difficult to be exhibited. On the other hand, when the Cu content exceeds 2.00%, hot workability tends to deteriorate. Therefore, Cu amount is preferably 0.05 to 2.00%. Cu content, from the viewpoint of hot workability deterioration suppression, and more preferably 0.05 to 1.00%.
[0053]
Ni ·: 0.05 ~
　3.00% Ni is an element for improving the ductility deteriorates with an increase in strength. Ni is also an element to increase the tensile strength to improve the hardenability during the heat treatment. The Ni content is less than 0.05%, they less effective. On the other hand, when the Ni content exceeds 3.00%, then they effect saturated, the effect commensurate with the amount being exerted hardly. Therefore, Ni amount is preferably 0.05 to 3.00%.
[0054]
　In the chemical composition of the steel member according to the present embodiment, the remainder being Fe and impurities.
　Here, the impurity components contained in the raw material, or a component mixed in the manufacturing process, it refers to a do not have intentionally containing component. Further, impurities, even components intentionally contained, including components in an amount within a range not affecting the performance of the steel member.
　As the impurity, for example, P, S, and the like. P content, and S content for, for example, from the viewpoint of not affecting the delayed fracture resistance, the P content and S content are each preferably 0 to 0.015%. However, from the viewpoint of reduction of de P cost and de S costs, P amount, and the lower limit of the S amount may be greater than 0%.
[0055]
(Method of manufacturing a steel member)
　manufacturing method of a steel member according to the present embodiment, in order to respond to the diversity of the heat treatment conditions at the time the steel member prepared in the rolling process during production of the rolled steel member serving as the steel member material it is important to preliminarily deposit the Ti precipitates showing a trapping ability.
[0056]
　For example, when applying the rolling bar steel member as a material of the steel member, when the steel bar rolling, after solid solution Ti compound by heating steel slab having the above chemical composition a (billet) to a temperature above 1250 ° C., finish rolling temperature: 900 to hot rolling at ~ 1000 ° C., then, the average cooling rate: cooling at 40 ° C. / sec or less until 700 ~ 750 ° C.. Thereby, it is possible to deposit Ti precipitates aim. At this time, Ti precipitates isotropically deposited.
[0057]
　Here, the heating temperature of the billet (billet), refers to the surface temperature of the steel strip. Further, the finish rolling temperature, it refers to the surface temperature of the rolled steel bar member immediately after finish rolling. The average cooling rate after the finish rolling, refers to a surface cooling rate of rolling steel bar member after finish rolling.
[0058]
　After rolling steel bar members to precipitate a Ti precipitates aim is heated to the austenite region (e.g. 850 ~ 1050 ° C.), and quenched by cooling to 20 ~ 100 ° C. or less cooling rate 40 ° C. / s, temperature 0.99 ~ 200 ° C., by performing low-temperature tempering at time 15-60 minutes, the steel member according to the present embodiment is obtained.
[0059]
　Even when adopting a method for producing a steel member that is not based on rolling, by controlling the solid solution and precipitation of appropriate compounds, it is possible to form a Ti precipitates aim in the steel member.
Example
[0060]
　Hereinafter, the present disclosure will be described more specifically by way of Examples. However, these examples are not intended to limit the present disclosure.
[0061]
　The test materials having the chemical compositions shown in Table 1, it was heated to a temperature shown in Table 2, hot-rolled in the finishing rolling temperature shown in Table 2, cooled to 700 ° C. at an average cooling rate shown in Table 2 in rolling to 20 mm in diameter, were produced annular notch with a test piece made of round steel bar size φ10mm × L50mm (notch depth 2 mm, notch bottom radius 0.25 mm, a notch angle of 60 degrees).
　The test piece conditions simulating the carburizing heating atmosphere or RX gas heating (1 atm, a mixed atmosphere of 50% hydrogen and Ar, heating temperature 1000 ° C., heating time 30 minutes) was heated at the cooling rate 40 ° C. / s water cooled to 20 ° C. below, after quenching and tempered for 20 minutes at 0.99 ° C..
　However, a comparative steel No. 28, 30 minutes at 520 ° C., a comparative steel No. 29, was tempered for 40 minutes at 400 ℃.
[0062]
　The obtained test piece was measured rupture time by the constant load test 3000kgf which was made the upper limit 100 hours. In addition, it was also measured tensile strength.
　Separately, on specimens immediately after tempering in accordance with already described method, implemented heating withdrawal hydrogen analysis to measure the hydrogen trapping capacity is released in a temperature range of 400 ~ 800 ° C.. Further, according to the method described above, the old γ grain size, abundance of Ti precipitates was measured an average aspect ratio of Ti average size of the precipitates (the average equivalent circle diameter), and Ti precipitates.
[0063]
[Table 1]

[0064]
[Table 2]

[0065]
　In Table 1 to Table 2, No. 1-19 are examples steel, the others are comparative steels. As seen in the table, Example steels are all a hydrogen trapping ability of more than 0.5 mass ppm. Therefore, it can be seen that excellent delayed fracture resistance.
[0066]
　On the other hand, it is a comparative steel No. 20, 21, 22 in order Ti content is too low, small size of the Ti precipitates, or Ti precipitates absent, hydrogen trapping amount is low example.
　It is a comparative steel No. 23, since Ti content was excessive, TiC is not completely dissolved when heated rolling, becomes coarse carbides, hydrogen trapping amount is low example.
　It is a comparative steel No. 24, since the C content was excessive, an example of the delayed fracture resistance is lowered.
　It is a comparative steel No. 25 has a low Cr content, because there was not enough hardenability, tensile strength after quenching is low, an example in which not withstand the load of the constant load test.
　It is a comparative steel No. 26 has a low N content of the steel, coarse grains are generated during heating of the quenching, an example of the delayed fracture resistance is lowered.
　It is a comparative steel No. 27, the heating temperature during rolling is low, it can not be sufficiently dissolved the Ti compound, coarse Ti precipitates occurs and an example of the delayed fracture resistance is lowered.
　It is a comparative steel No. 28, high temperature tempering, since Ti precipitates most precipitated during tempering, reduced the size of Ti precipitates, an example of the delayed fracture resistance is lowered.
　It is a comparative steel No. 29, since the tempering temperature was high, tensile strength after tempering is low, an example in which not withstand the load of the constant load test.
　Therefore, comparison steels, it can be seen that the delayed fracture resistance is low.
[0067]
　Incidentally, disclosure of Japanese Patent Application No. 2017-123347 its entirety is incorporated herein by reference.
　All documents described herein, patent applications, and technical standards, each individual publication, patent application, and that the technical specification is incorporated by reference to the same extent as if marked specifically and individually, It incorporated by reference herein.

WE CLAIM

[Requested item 1]
　By
　mass%,
　C:
　0.10 ~ 0.50%, Si: 0.02 ~
　2.00%, Mn: 0.05 ~ 2.00%, Cr: 0.10 ~
　2.00%, Ti: 0.20 to 1.00%, and
　N:
　0.0020
　~ 0.0250% Al: 0 ~
　0.100% V: 0 ~ 0.50% Nb: 0 ~ 0.50%
　Mo: 0
　1.00% ~, B:
　0 ~ 0.0100%, Cu: 0 ~ 2.00%, and
　Ni: 0 ~ 3.00%,
　and contains, has a chemical composition the balance being Fe and impurities ,
　and a tensile strength of 1000MPa or more,
　at a position of depth of 1mm from the surface of the steel member, the average is 30 ~ 200 nm in an average circle equivalent diameter size, and made of Ti carbide, Ti nitride and composite compounds thereof at least one Ti precipitates are selected from the group containing at area% 0.10% or more,
　High strength steel member containing non-diffusible hydrogen released in a temperature range of 400 ~ 800 ° C. or higher 0.5 wt ppm in temperature leaving hydrogen analysis.
[Requested item 2]
　By
　mass%,
　Al: 0.005 ~ 0.100%,
　V: 0.01 ~ 0.50%, Nb: 0.01 ~ 0.50%, and
　Mo: 0.01 ~ 1.00%,
　the high strength steel member according to claim 1 having a chemical composition containing one or more kinds.
[Requested item 3]
　By mass%
　B: 0.0003 ~ 0.0100%
　high-strength steel member according to claim 1 or claim 2 having a chemical composition containing.
[Requested item 4]
　By
　mass%, Cu: 0.05 ~ 2.00%, and Ni: any of claims 1 to 3 having a chemical composition containing one or two 0.05 to 3.00% 1 high strength steel member according to claim.
[Requested item 5]
　The average aspect ratio of Ti precipitates, high-strength steel member according to any one of claims 1 to 4 is 1.0 to 3.0.