JP7168101B2 - High-strength steel member - Google Patents

Description

The present disclosure relates to a high-strength steel member having a tensile strength of 1300 MPa or more and 1600 MPa or less.

High-strength steel members, which are widely used in machines, automobiles, bridges, buildings, etc., are specified in, for example, JIS G 4104: 1979 and JIS G 4105: 1979, and have a C content of 0.20 to 0.35%. It is manufactured by quenching and tempering using medium carbon steel (chromium steel (SCr), chromium molybdenum steel (SCM), etc.). However, it is well known that the risk of hydrogen embrittlement (delayed fracture) increases when the tensile strength is 1300 MPa or more for any steel type. For this reason, the upper limit of the tensile strength of currently used high-strength steel members for construction is currently 1150 MPa class.

As conventional knowledge for improving the delayed fracture resistance of high-strength steel members, for example, Patent Document 1 proposes that it is effective to refine prior austenite grains and convert the structure to bainite. . It is true that the bainite structure is effective against delayed fracture, but the bainitization treatment increases the manufacturing cost. Patent Document 2 proposes that the delayed fracture resistance is improved by refining the prior austenite grains.

Further, Patent Document 3 proposes steel for bolts in which the delayed fracture resistance is improved by utilizing the hydrogen trapping effect of fine compounds.

In addition, Patent Document 4 proposes a high-strength steel material having high strength and improved delayed fracture resistance by using prior austenite grains having a predetermined grain size and an aspect ratio of 1.5 or more. .

In addition, Patent Document 5 provides a high-strength steel material having a high tensile strength of 1800 MPa or more and improved delayed fracture resistance by controlling the degree of Mn microsegregation and the amount of non-diffusible hydrogen. ing.

Patent Document 1: Japanese Patent Laid-Open Publication No. 3-243744 Patent Document 2: Japanese Patent Laid-Open Publication No. 3-243745 Patent Document 3: Japanese Patent Laid-Open Publication No. 10-17985 Patent Document 4: Japanese Patent Laid-Open Publication No. 2014-43612 Patent Document 5: Japanese Patent Application Laid-Open No. 2012-172247

However, although the steel members of Patent Documents 1 and 2 are improved in delayed fracture resistance, further improvement is desired in order to use them more stably.
In addition, the steel for bolts of Patent Document 3 also has restrictions on the structure, size, and shape of precipitates that exhibit hydrogen trapping ability in tests by the present inventors. No ability is obtained.

The technique disclosed in Patent Document 4 requires that the steel material be subjected to quenching and tempering treatment following hot rolling. As a result, the metallographic structure of the steel material disclosed in Patent Document 4 has elongated prior austenite grain boundaries, thereby achieving a hydrogen trapping effect. The manufacturing method disclosed in Patent Document 4 cannot be applied to steel members (for example, high-strength bolts, etc.) manufactured by performing quenching and tempering after molding at room temperature.
On the other hand, the technique of Patent Literature 5 assumes a spring steel material having a tensile strength of 1800 MPa or more. In general, tensile strength and delayed fracture resistance are in a trade-off relationship, and the characteristic level required in Patent Document 5 (tensile strength of 1800 MPa or more) and the steel material with a tensile strength of 1300 MPa or more and 1600 MPa or less are required. Characteristic levels are different. Even if the technique of Patent Document 5 is applied as it is, sufficient hydrogen trapping ability cannot be expressed in the strength range assumed in the present disclosure, and a different idea is required.

As described above, according to the conventional technology, there is a limit to manufacturing a high-strength steel member having a tensile strength of 1300 MPa or more and 1600 MPa or less and having drastically improved delayed fracture resistance.

Therefore, an object of the present disclosure is to provide a high-strength steel member having good delayed fracture resistance and a tensile strength of 1300 MPa or more and 1600 MPa or less.

Means for solving the above problems include the following aspects.

<1>
in % by mass,
C: 0.25 to 0.50%,
Si: 2.00 to 4.00%,
Mn: 0.20-2.00%,
V: 0.30 to 1.00%,
Mo: 0.05-1.00%,
N: 0.001 to 0.020%,
P: 0.015% or less, and S: 0.015% or less,
with the remainder consisting of Fe and impurities, having a chemical composition in which the value of the following formula (1) is greater than 200 and the value of the following formula (2) is 3.00 or less,
The steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member is a tempered martensite structure with an average aspect ratio of prior austenite grains of 1.3 or less,
A high-strength steel member having a tensile strength of 1300 MPa or more and 1600 MPa or less.
100×%C+50×%Si+200×%V+100×%Mo Expression (1)
%Mo/%V Formula (2)
Here, in formulas (1) and (2), %C, %Si, %V and %Mo represent the contents (% by mass) of C, Si, V and Mo, respectively.
<2>
in % by mass,
Cr: 0 to 2.00%,
Al: 0 to 0.100%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
W: 0 to 1.000%,
B: 0 to 0.0100%,
Ni: 0 to 3.00%,
Cu: 0 to 2.00%, and
REM: 0-0.0200%
The high-strength steel member according to <1>, which has a chemical composition containing one or more of
<3>
2. The steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member contains precipitates with a grain size of 50 nm or less in an area ratio of 0.50% or more, or The high-strength steel member according to claim 2.

According to the present disclosure, it is possible to provide a high-strength steel member having good delayed fracture resistance and a tensile strength of 1300 MPa or more and 1600 MPa or less.

An embodiment that is an example of the present disclosure will be described in detail below.
In this specification, a numerical range represented by "-" means a range including the numerical values before and after "-" as lower and upper limits.
In the numerical ranges described stepwise, the upper limit value or lower limit value described in a certain numerical range may be replaced with the upper limit value or lower limit value of another numerical range described stepwise.
In numerical ranges, the upper limit or lower limit described in a certain numerical range may be replaced with the values shown in the examples.
The term "process" is included in this term not only as an independent process, but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
The content of the element in the chemical composition is expressed as the amount of element (for example, the amount of C, the amount of Si, etc.).
"%" means "% by mass" for the content of elements in the chemical composition.
A "combination of preferred aspects" is a more preferred aspect.

The high-strength steel member according to the present embodiment (hereinafter also simply referred to as "steel member") contains predetermined elements within the range of the amount of each element described later, the value of the following formula (1) is more than 200, and It has a chemical composition in which the value of the following formula (2) is 3.00 or less.
100×%C+50×%Si+200×%V+100×%Mo Expression (1)
%Mo/%V Formula (2)
Here, in formulas (1) and (2), %C, %Si, %V and %Mo represent the contents (% by mass) of C, Si, V and Mo, respectively.

In the high-strength steel member according to the present embodiment, the steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member has an average aspect ratio of prior austenite grains of 1.3. It has the following tempered martensite structure, and the tensile strength of the steel member is 1300 MPa or more and 1600 MPa or less.
In addition, the tensile strength of a steel member is a value measured according to JIS Z 2241:2011.

The high-strength steel member according to the present embodiment is a high-strength steel member having good delayed fracture resistance and a tensile strength of 1300 MPa or more and 1600 MPa or less due to the above configuration. A high-strength steel member according to the present embodiment was found based on the following findings.

First, the present inventors analyzed the delayed fracture behavior in detail using steel members with various strength levels manufactured by quenching and tempering. As a result, the present inventors obtained the following findings. When steel members having the same hardness after quenching and tempering are compared, high temperature tempering after quenching improves the delayed fracture resistance. In addition, by precipitating fine precipitates (in particular, precipitating MC-based carbides exhibiting a secondary hardening phenomenon), it is possible to improve delayed fracture resistance while maintaining high strength.
However, during tempering, cementite, which requires only diffusion of carbon, usually precipitates, and using this cementite as a carbon supply source, MC-based carbide precipitates with dissolution of cementite. Therefore, there is a drawback that it takes a long time to precipitate enough MC-based carbides.

On the other hand, the present inventors also obtained the following findings. 2.00% or more (preferably 3.00% or more) and 4.00% or less of Si, 0.30% or more and 1.00% or less of V, and 0.05% or more and 1.00% or less of Mo By quenching and tempering at a high temperature of 550°C or higher, steel members containing You get a steel member.

The reason for this is considered as follows. When a steel member satisfying the above chemical composition is quenched and tempered at a high temperature of 550° C. or higher, fine precipitates are formed in the quenched martensite structure. In particular, Si suppresses the precipitation of cementite, making it possible to efficiently precipitate "MC-based carbides mainly composed of VC" exhibiting the temper secondary hardening phenomenon in a fine state with a grain size of 50 nm or less. Precipitation of these precipitates (especially MC-based carbides) occurs at least in the steel structure of the steel member in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member. It is believed that this improves the delayed fracture resistance while maintaining high strength.

The steel structure of the steel member located at a depth of 1.0 mm from the surface of the steel member and in a plane parallel to the surface of the steel member was focused on. This is because the stress triaxiality is generated starting from a portion with a depth of several hundred μm or more and a high degree of stress triaxiality. However, the precipitation of MC-based carbides is not limited to the position of 1.0 mm deep from the surface of the steel member, but is considered to progress at any location on the steel member.

Based on these findings, the present inventors have found that if the chemical composition of the steel member is optimally selected, it has excellent delayed fracture resistance even with a high tensile strength of 1300 MPa or more and 1600 MPa or less. It was concluded that a steel member applicable to concrete (PC) steel bars and the like could be realized.

Therefore, based on the above findings, it was found that the steel member according to the present embodiment is a high-strength steel member having good delayed fracture resistance and a tensile strength of 1300 MPa or more and 1600 MPa or less.

(chemical composition)
Hereinafter, reasons for limiting the chemical composition of the steel member according to the present embodiment will be described.

-C: 0.25 to 0.50%-
C is an essential element for ensuring the strength of steel members. If the amount of C is less than 0.25%, the required strength cannot be obtained. On the other hand, when the amount of C exceeds 0.50%, the toughness is deteriorated, and the delayed fracture resistance is also deteriorated. Therefore, the amount of C is set to 0.25 to 0.50%.
The lower limit of the amount of C is preferably 0.30% or more.
The upper limit of the amount of C is preferably 0.45% or less, more preferably 0.39% or less.

-Si: 2.00 to 4.00%-
Si has the effect of suppressing precipitation of cementite during tempering, and is an important element in the present disclosure. When a steel member containing 2.00% or more of Si is quenched and tempered, precipitation of coarse cementite is suppressed by the effect of Si during tempering, and fine ε-carbides are formed instead. As a result, fine precipitates are easily precipitated.
Since Si can suppress the precipitation of coarse cementite, it has the effect of promoting the precipitation of MC-based carbides. When the MC-based carbides are precipitated without going through cementite, the precipitated MC-based carbides are fine and in large numbers. Furthermore, Si has a solid-solution strengthening effect and has the effect of increasing the strength of steel. From this point of view, the amount of Si is set to 2.00% or more. On the other hand, if the amount of Si is excessively large, exceeding 4.00%, the above-mentioned effects are saturated, and at the same time, grain boundaries become embrittled due to grain boundary segregation of Si, which causes deterioration of delayed fracture resistance. Therefore, the amount of Si is set to 2.00 to 4.00%.
The lower limit of the Si content is preferably 2.50% or more, more preferably 3.00% or more.
The upper limit of the amount of Si is preferably 3.50% or less.
Incidentally, Si has been conventionally believed to impair the delayed fracture resistance because it lowers the ductility by solid-solution strengthening of ferrite. However, the inventors of the present invention have found that the delayed fracture resistance can rather be improved in the region of high Si content (Si: 2.00% to 4.00%).

-Mn: 0.20 to 2.00%-
Mn is not only necessary for deoxidation and desulfurization, but is also an effective element for improving hardenability for obtaining a martensite structure. If the Mn content is less than 0.20%, the above effect cannot be obtained. On the other hand, if the amount of Mn exceeds 2.00%, it segregates at the grain boundaries during heating in the austenitic region, embrittles the grain boundaries, and deteriorates the delayed fracture resistance. Therefore, the Mn content is set to 0.20 to 2.00%.
The lower limit of the Mn amount is preferably 0.50% or less.
The upper limit of the amount of Mn is preferably 1.50% or less.

-V: 0.30 to 1.00%-
V is an element effective for precipitating MC-based carbides that cause secondary hardening after tempering. However, if the amount of V is 0.30% or more, the above effect is small. When the amount of V exceeds 1.00%, the above effect saturates. Also, if the V content exceeds 1.00%, workability is impaired due to an increase in deformation resistance. Therefore, the V amount is set to 0.30 to 1.00%.
The lower limit of the V content is preferably 0.35% or more.
The upper limit of the V content is preferably 0.60% or less.

-Mo: 0.05 to 1.00%-
When Mo is contained in a steel member together with V, it precipitates as MC-based carbide due to a combined effect with V, and has the effect of further refining the MC-based carbide. However, the above effect is small unless the Mo content is 0.05% or more.
When the Mo content exceeds 1.00%, not only does the above effect saturate, but when the Mo content is excessive, M ₂ C-based carbide precipitates alone, suppressing the precipitation amount of MC-based carbide, so tempering is required. Reduce the amount of secondary curing. Therefore, the Mo content is set to 0.05 to 1.0%.
The lower limit of Mo content is preferably 0.20% or more.
The upper limit of Mo content is preferably 0.80% or less.

-N: 0.001 to 0.020%-
N has the effect of refining prior austenite grains and increasing yield strength by forming nitrides of V, Al, Nb, Ti, and the like. If the amount of N is less than 0.001%, the effect is small. When the amount of N exceeds 0.020%, the above effect saturates. Therefore, the amount of N is set to 0.001 to 0.020%.

-P: 0.015% or less-
P is contained in steel as an impurity. When the amount of P exceeds 0.015%, the delayed fracture resistance deteriorates. Therefore, the amount of P is set to 0.015% or less.
The P content should be as low as possible, and the lower limit of the P content is ideally 0%. However, removing P more than necessary increases the manufacturing cost. Therefore, the substantial lower limit of the P content is, for example, 0.002%.

-S: 0.015% or less-
S is contained in steel as an impurity. When the amount of S exceeds 0.015%, the delayed fracture resistance deteriorates. Therefore, the amount of S is set to 0.015% or less.
The S content is preferably as low as possible, and the lower limit of the S content is ideally 0%. However, removing S more than necessary increases the manufacturing cost. Therefore, the substantial lower limit of the amount of S is, for example, 0.002%.

-Formula (1)-
In the steel member according to the present embodiment, the value of the following formula (1) is set to more than 200 while satisfying the chemical composition of the above elements. As a result, it is possible to secure the tensile strength of the steel member of 1300 MPa or more by tempering at 550° C. or more, which will be described later, while improving the delayed fracture resistance of the steel member.
The value of formula (1) is preferably 220 or more, more preferably 250 or more, from the viewpoint of ensuring the tensile strength of the steel member and improving the delayed fracture resistance. However, from the viewpoint of ensuring toughness, the upper limit of the value of formula (1) is preferably 500 or less.
100×%C+50×%Si+200×%V+100×%Mo (1)
Here, %C, %Si, %V and %Mo represent the content (% by mass) of C, Si, V and Mo, respectively.

-Formula (2)-
The steel member according to the present embodiment has V and Mo as essential elements, and contains both elements in a predetermined ratio. In order to obtain a combined effect of V and Mo in steel, the value of the following formula (2) is set to 3.00 or less after satisfying the chemical composition of the above elements. As a result, a large amount of fine MC-based carbides are precipitated, and the delayed fracture resistance of the steel member can be improved while ensuring the steel member's tensile strength of 1300 MPa or more by tempering at 550° C. or higher, which will be described later. On the other hand, if the value of formula (2) exceeds 3.00, the precipitation of M ₂ C-based carbides is suppressed, and sufficient properties are not realized.
From the viewpoint of ensuring the tensile strength of the steel member and improving the delayed fracture resistance, the value of formula (2) is preferably 2.50 or less, more preferably 2.00 or less. However, from the viewpoint of hydrogen trapping amount, the lower limit of the value of formula (2) is preferably 0.30 or more.
%Mo/%V Formula (2)
Here, %V and %Mo represent V and Mo contents (% by mass), respectively.

- Selected elements -
The steel member according to the present embodiment may further contain one or more of the following elements in addition to the above elements. However, the object of the present disclosure can be achieved even in steels that do not contain any of the following elements.
Cr: 0-2.00% (preferably 0.05-2.00%)
Al: 0 to 0.100% (preferably 0.005 to 0.100%),
Ti: 0 to 0.300% (preferably 0.005 to 0.300%),
Nb: 0 to 0.300% (preferably 0.005 to 0.300%),
W: 0 to 1.000% (preferably 0.005 to 1.000%),
B: 0 to 0.0100% (preferably 0.0003 to 0.0100%),
Ni: 0 to 3.00% (preferably 0.05 to 3.00%),
Cu: 0-2.00% (preferably 0.05-2.00%), and REM: 0-0.0200% (preferably 0.0020-0.0200%).

-Cr: 0.05 to 2.00%-
Cr is an effective element for improving hardenability and increasing softening resistance during tempering. If the Cr content is less than 0.05%, the above effect cannot be sufficiently exhibited. On the other hand, when the Cr content exceeds 2.00%, deterioration in toughness, deterioration in cold workability, and deterioration in delayed fracture resistance are caused. Therefore, the Cr content is preferably 0.05 to 2.00%.
The lower limit of Cr content is more preferably 0.20% or more.
The upper limit of the Cr content is more preferably 1.50% or less.

-Al: 0.005 to 0.100%-
Al has the effect of preventing coarsening of austenite grains and fixing N by forming AlN during deoxidation and heat treatment. If the Al content is less than 0.005%, the above effects are difficult to achieve. When the amount of Al exceeds 0.100%, the above effects are saturated. Therefore, the Al content is preferably 0.005 to 0.100%.

-Ti: 0.005 to 0.300%-
Ti has the effect of preventing the austenite grains from coarsening by forming TiN during deoxidation and heat treatment, as with Al, and has the effect of fixing N as well. If the Ti content is less than 0.005%, the above effects are difficult to achieve. When the Ti amount exceeds 0.300%, the above effects are saturated. Therefore, the Ti content is preferably 0.005 to 0.300%. The upper limit of Ti may be 0.100% or 0.050%.

-Nb: 0.005 to 0.300%-
Nb is an element effective for refining austenite grains by forming carbonitrides. If the Nb content is less than 0.005%, the above effect may be insufficient. On the other hand, when the amount of Nb exceeds 0.300%, the above effect is saturated. Therefore, the Nb content is preferably 0.005 to 0.300%. The upper limit of Nb may be 0.100% or 0.050%.

-W: 0.005 to 1.000%-
W, like Nb, is an element effective in refining austenite grains by forming carbonitrides. If the W content is less than 0.005%, the above effect may be insufficient. On the other hand, when the amount of W exceeds 1.000%, the above effect is saturated. Therefore, the W content is preferably 0.005 to 1.000%.

-B: 0.0003 to 0.0100%-
B has the effect of suppressing intergranular fracture and improving delayed fracture resistance. Furthermore, B remarkably enhances the hardenability by segregating at the austenite grain boundaries. If the amount of B is less than 0.0003%, the above effects are difficult to achieve. When the amount of B exceeds 0.0100%, the above effect saturates. Therefore, the B content is preferably 0.0003 to 0.0100%. The upper limit of B may be 0.0050% or less.

-Ni: 0.05 to 3.00%-
Ni has the effect of improving the ductility, which deteriorates as the strength increases, and also improving the hardenability during heat treatment to increase the tensile strength. If the Ni content is less than 0.05%, the above effect is small. On the other hand, if the amount of Ni exceeds 3.00%, the effect corresponding to the content cannot be exhibited. Therefore, the Ni content is preferably 0.05 to 3.00%. The upper limit of Ni may be 1.00% or less, or 0.50% or less.

-Cu: 0.05 to 2.00%-
Cu is an effective element for increasing temper softening resistance. If the amount of Cu is less than 0.05%, the effect cannot be exhibited. If the amount of Cu exceeds 2.00%, the hot workability deteriorates. Therefore, the Cu content is preferably 0.05 to 2.00%. The upper limit of Cu may be 1.00% or less, or 0.50% or less.

-REM: 0.0020 to 0.0200%-
REM suppresses elongation of MnS particles during rolling and hot forging, and has the effect of suppressing cracking during cold forging. If the amount of REM is less than 0.0020%, it is difficult to exhibit the effect. When the amount of REM exceeds 0.0200%, the effect saturates. Therefore, the REM amount is preferably 0.0020 to 0.0200%. The upper limit of REM may be 0.0100%, or 0.0050%.

"REM" is a generic term for a total of 17 elements including Sc, Y, and lanthanoids, and the content of REM refers to the total content of one or more elements in REM. REM is generally contained in misch metal. Therefore, for example, REM may be added in the form of misch metal so that the amount of REM is within the above range.

-Remainder-
In the chemical composition of the steel member according to this embodiment, the balance is Fe and impurities.
Here, the term "impurity" refers to a component contained in raw materials or a component mixed in during the manufacturing process and not intentionally included. Furthermore, the impurities also include components that are included in an amount within a range that does not affect the performance of the steel member, even if they are components that are intentionally included.
The steel member according to the present embodiment contains, as impurities, other than P and S, for example, O, Pb, Ca, Mg, Sb, Bi, Co, As, Ta, Sn, In, Zr, Te, Se, and Zn. Possibility of containing elements such as In any element, if the content is within the following levels, it does not affect the delayed fracture resistance, and no particular problem occurs.
O: 0.0040% or less Pb: 0.01% or less Ca: 0.0010% or less Mg: 0.0010% or less Sb: 0.10% or less Bi: 0.10% or less Co: 0.10% or less As : 0.05% or less Ta: 0.10% or less Sn: 0.05% or less In: 0.01% or less Zr: 0.05% or less Te: 0.005% or less Se: 0.005% or less Zn: 0.10% or less

(Organizational form)
In the steel member according to the present embodiment, the steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member is tempered martensite with an aspect ratio of prior austenite grains of 1.3 or less. Organization. This is because the steel member according to the present embodiment is manufactured without hot working (such as hot rolling or hot forging) when reheating from room temperature to perform quenching and tempering. .

Here, "the steel structure is a tempered martensite structure" means that the metal structure is formed by quenching and tempering, and the main component is tempered martensite. A metal structure formed by quenching and tempering may contain a very small amount of untempered martensite. For example, a metal structure formed by quenching and tempering may contain a small amount of ferrite or cementite structure formed by tempering decomposition of retained austenite generated by quenching. These are difficult to distinguish separately from tempered martensite. If it can be determined, the object of the present disclosure can be achieved even if a small amount of structure (pearlite, ferrite, cementite) other than tempered martensite is included (specifically, 10% or less in terms of area ratio). That is, in the present disclosure, "tempered martensite structure" means a metal structure in which the main part of the metal structure is tempered martensite and the total area ratio of other structures (pearlite, ferrite, cementite) is 10% or less. Point.

(Average aspect ratio of prior austenite grain boundaries)
It is assumed that the steel member according to the present embodiment is quenched and tempered by being reheated from room temperature after being formed into the shape to be used at room temperature (or without undergoing a forming process). That is, in the present embodiment, the steel member is not subjected to elongation of prior austenite grains by hot rolling. Therefore, the austenite grains that are temporarily formed during the quenching process are isotropic. Therefore, even in the finished steel member, the prior austenite grain boundaries are isotropic, and the average aspect ratio of the prior austenite grains is 1.3 or less.

Here, the average aspect ratio of prior austenite grains is measured as follows.
A surface parallel to the surface parallel to the longitudinal direction of the steel member located at a depth of 1.0 mm from the surface of the steel member (hereinafter also referred to as "measurement surface") from an arbitrary part of the steel member to be measured. A site was taken, and the measurement surface of the sample was ground and etched to prepare a test specimen. Etching is performed by immersing the viewing surface in a 3-5% picral solution (mixed solution of picric acid and ethyl alcohol) at about 40-60° C. for 30 sec-2 min. After corrosion, immediately wash the observation surface of the sample thoroughly with water, and then quickly dry it with cool or warm air.
Next, an optical microscope image (magnification: 500 times) with a field of view of 200 μm×200 μm was taken from the etched measurement surface. The aspect ratio is measured for each prior austenite grain included in the captured image, and the average value of the measured aspect ratios (average aspect ratio) is calculated. The aspect ratio of the prior austenite grain boundary is the major axis (the distance between the two points with the longest distance in the prior austenite grain) and the minor axis (the maximum width of the prior austenite grain boundary in the direction perpendicular to the major axis). It was obtained by the ratio of

(precipitate)
In the steel member according to the present embodiment, it is preferable to precipitate fine MC-based carbides mainly composed of VC in the vicinity of the surface of the steel member. According to the findings of the inventors, in the steel member manufactured by the above-described composition system and the manufacturing method described later, precipitates with grain sizes of 50 nm or less in particular are mostly MC-based carbides mainly composed of VC. many. That is, by setting the precipitation amount of precipitates having a grain size of 50 nm or less to a predetermined amount or more, fine MC-based carbides mainly composed of VC are secured, and it becomes easier to achieve both tensile strength and delayed fracture resistance. Conceivable.

It is presumed that when minute MC-based carbides mainly composed of VC are present, the carbides function as hydrogen traps and the delayed fracture resistance is further improved. Furthermore, MC-based carbides mainly composed of VC have tempering secondary hardening ability and increase tensile strength. Such an effect tends to be reduced when the carbide is a non-MC carbide. A typical example of non-MC carbide is M ₂ C carbide.

In the steel member according to the present embodiment, the steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member has a grain size (specifically, equivalent circle diameter) of 50 nm or less. It is preferable that the precipitates contain 0.50% or more in terms of area ratio. By containing fine precipitates in an area ratio of 0.50% or more, the delayed fracture resistance is further improved while maintaining high strength. The reason for this is presumed to be that the above-mentioned MC-based carbides mainly composed of VC are sufficiently ensured, and the hydrogen trapping function and the secondary temper hardening phenomenon are realized.
The area ratio of precipitates is preferably 0.60% or more, more preferably 0.70%, from the viewpoint of securing tensile strength and improving delayed fracture resistance. However, from the viewpoint of ensuring toughness, the upper limit of the area ratio of precipitates is preferably 1.50% or less, more preferably 1.00% or less.

Specifically, the steel structure in a plane located at a depth of 1.0 mm from the surface of the steel member and parallel to the surface of the steel member has MC-based carbides mainly composed of VC (V-carbides) with an area ratio of 0.05 mm. It is particularly preferable to contain 50% or more.

Here, the MC-based carbides mainly composed of VC are not limited to carbides of V carbide alone, but are composite carbides of V carbides and carbides of elements other than V, and the mass ratio of V carbides to the total carbides. Also includes composite carbides with the highest amount of

The area ratio of precipitates is determined by using a transmission microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDS) after preparing a test piece by the extraction replica method. Specifically, it is as follows.

A portion having a surface (hereinafter also referred to as "measurement surface") parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member is sampled from an arbitrary portion of the steel member to be measured, A test piece is prepared by the extraction replica method.

Here, preparation of the test piece by the extraction replica method is as follows. First, the measurement surface of the sample taken from the steel member is electrolytically polished. After electropolishing, the measurement surface of the specimen is subjected to constant potential electrolysis at a potential of −200 mV using a 10% acetylacetone-1% tetramethylammonium chloride (TMAC)-methanol solution. The current density was 20 mA/cm ² . In addition, the sample of the present disclosure was electropolished under the condition that the current-carrying area was 0.8 cm ² . As a result, precipitates and inclusions are exposed from the measurement surface of the specimen. The energization time is 60 sec.
After attaching an acetylcellulose film to the measurement surface of the sample after electrolysis, the film is peeled off, and the deposits and inclusions are transferred onto the film. Carbon vapor deposition is performed on the transferred film to create a carbon vapor deposition film. The carbon-evaporated film is immersed in a methyl acetate solution to dissolve the acetylcellulose film, and an extracted replica film is obtained as a test piece. The prepared extraction replica membrane is disc-shaped with a diameter of 3 mm.

Next, five arbitrary fields of view (fields of size 5 μm×5 μm) on the surface (measurement surface) of the extraction replica film as the test piece are observed at a magnification of 50,000 times.

Next, among the precipitates present in the field of view to be observed, the area ratio of precipitates having an equivalent circle diameter of 50 nm or less is measured. Then, the average value of the area ratio of precipitates having an equivalent circle diameter of 50 nm or less observed in five fields of view is obtained as the area ratio of precipitates.

The components of the precipitate present in the field of view to be observed were analyzed by analysis of the electron beam diffraction pattern of TEM and analysis by EDS. As a result, among the examples to be described later, in the examples satisfying the requirements of the present disclosure, most of the precipitates having an equivalent circle diameter of 50 nm or less were MC carbides mainly composed of VC.

(tensile strength)
The tensile strength of the steel member according to this embodiment measured according to JIS Z 2241:2015 is 1300-1600 MPa. In general, steel members have a trade-off relationship between tensile strength and delayed fracture resistance. The steel member according to the present embodiment can achieve both high strength with a tensile strength of 1300 MPa or more and delayed fracture resistance. On the other hand, when the tensile strength exceeded 1600 MPa, sufficient delayed fracture resistance could not be ensured.

(hydrogen storage capacity)
The hydrogen storage capacity of the steel member according to this embodiment is preferably 4.0 ppm or more. When the hydrogen storage amount is 4.0 ppm or more, the hydrogen trapping ability of the steel becomes sufficient, and the delayed fracture resistance is further improved. In addition, ppm is a mass standard.
From the viewpoint of improving the delayed fracture resistance, the hydrogen storage amount is preferably 5.0 ppm or more, more preferably 6.0 ppm or more. According to the method of the present disclosure, the maximum hydrogen storage amount can be expected to be approximately 25.0 ppm.

The hydrogen storage capacity is measured as follows.
A round bar with a diameter of 7 mm and a length of 70 mm is taken from the steel member to be measured, and used as a test piece for investigating the amount of trapped hydrogen.
Next, a round bar test piece having a diameter of 7 mm and a length of 70 mm prepared by the above procedure is immersed in a solution of 20 mass % ammonium thiocyanate aqueous solution (20° C.) for 100 hours to charge with hydrogen. After removing the hydrogen-charged round bar test piece from the solution and leaving it at 20° C. for 72 hours in an air atmosphere, it was heated from room temperature to 400° C. at a heating rate of 100° C./hour using a gas chromatograph. , to measure the amount of hydrogen released from the sample piece.

(Manufacturing method of steel member)
An example of a method for manufacturing a steel member according to this embodiment will be described below.
In the method for manufacturing a steel member according to the present embodiment, the steel member having the above chemical composition is tempered at 550° C. or higher (preferably 580° C. or higher) after quenching.

The lower limit of the tempering temperature is set to 550° C. or higher in order to obtain a tensile strength of 1300 to 1600 MPa. If the tempering temperature is less than 550°C, the tensile strength may become high (more than 1600 MPa), leading to deterioration of delayed fracture resistance. If the tempering temperature is lower than 550° C., precipitation of MC-based carbides that function as hydrogen traps may be insufficient, and from this point, there is also a risk of impairing the delayed fracture resistance.

The upper limit of the tempering temperature does not have to be set, but if the tempering temperature is 700° C. or higher, the MC-based carbides mainly composed of VC may coarsen and the required strength may not be maintained. In addition, coarsened MC-based carbides have a low hydrogen trapping ability. Therefore, the upper limit of the tempering treatment temperature is preferably less than 700°C, more preferably less than 650°C.
The tempering holding time is preferably 20 to 120 minutes, more preferably 30 to 60 minutes, from the viewpoint of ensuring the tensile strength of the steel member and improving the delayed fracture resistance.
Cooling after tempering is not particularly limited, and examples thereof include air cooling.

As the quenching conditions, for example, the temperature is kept at 850 to 1000° C. for 30 to 60 minutes and then rapidly cooled.

Hereinafter, the present disclosure will be described more specifically with reference to examples. However, each of these examples does not limit the present disclosure.

The test materials having the chemical compositions shown in Table 1 were quenched and tempered under various conditions, and the "mechanical properties, structural morphology, and delayed fracture resistance" of the quenched and tempered test materials were evaluated. Details are as follows. In addition, in any of the examples and comparative examples shown in Table 1, the O content was 0.0040% or less.

(Test conditions)
The test material was a steel bar with a diameter of 7 mm and a length of 800 mm.
The condition of the quenching treatment was the condition of holding at 950° C. for 30 minutes and cooling with oil.
The tempering treatment was carried out at the temperature shown in Table 2 for 30 minutes followed by air cooling. The holding time at the tempering temperature was set at 30 minutes in consideration of industrial productivity.

(Evaluation of mechanical properties)
The tensile strength (denoted as "TS" in the table) of the test material was measured according to JIS Z 2241:2015.

(Evaluation of organizational form)
The following structural morphology of the test material was measured according to the described method.
・Area ratio of precipitate ・Average aspect ratio of prior austenite grain The metal structure was observed with an optical microscope, and confirmed to be a tempered martensitic structure.

(Evaluation of delayed fracture resistance)
The delayed fracture resistance was measured by applying a load of 70% of the tensile strength in an ammonium thiocyanate aqueous solution having a liquid temperature of 50° C. and a concentration of 20% by mass, and measuring the FIP fracture time. However, the test was stopped when the FIP rupture time reached 100 hours (indicated as "no rupture" in the table).
In Table 2, the notation of "-" indicates that the tensile strength of the test material was excessively low and the delayed fracture resistance was not evaluated.

(Evaluation of hydrogen storage capacity)
A round bar with a diameter of 7 mm and a length of 70 mm was taken from a bar-shaped steel material with a diameter of 7 mm and a length of 800 mm as a test material, and used as a test piece for investigating the amount of trapped hydrogen. Then, the hydrogen storage capacity of the test piece was measured according to the method described above.

Tables 1 and 2 show the results of these tests and evaluations.
In addition, in Tables 1 and 2, test material No. 1 to 21 are examples, and the others are comparative examples.

Test material No. which is an example. 1 to 21, when the predetermined chemical composition is satisfied and the steel structure is a tempered martensite structure with an average aspect ratio of prior austenite grains of 1.3 or less, the tensile strength is 1300 MPa or more and 1600 MPa or less while maintaining high strength. , excellent in delayed fracture resistance (FIP fracture time).

On the other hand, test material No. 1, which is a comparative example. In No. 22, since the predetermined amount of Mo is not added, the combined effect of V and Mo cannot be obtained, and although the strength is obtained, the hydrogen trap amount is low. As a result, the delayed fracture resistance deteriorated.
Test material no. In Nos. 23 to 28, since a predetermined amount of Si was not added, the formation of MC-based carbides mainly composed of VC was insufficient, and a sufficient area ratio of precipitates with a grain size of 50 nm or less could not be obtained, resulting in poor lag resistance. This is an example in which the breaking property is lowered.
Test material no. No. 29 is an example in which a predetermined amount of V was not added, so that a sufficient area ratio of precipitates with a grain size of 50 nm or less was not obtained, and the delayed fracture resistance was lowered.
Test material no. No. 30 is an example in which sufficient tensile strength was not obtained because the predetermined amount of C was not added.
Test material no. No. 31 is an example in which the amount of Si is excessive and segregation of Si at the grain boundaries makes the grain boundaries embrittled and deteriorates the delayed fracture resistance.
Test material no. No. 32 is an example in which the amount of Mn is excessive and segregation of Mn at the grain boundaries makes the grain boundaries embrittled and deteriorates the delayed fracture resistance.
Test material no. No. 33 is an example in which the delayed fracture resistance deteriorated as a result of containing an excessive amount of Cr.
Test material no. No. 34 is an example in which the value of formula (1) did not satisfy the predetermined range, so that a sufficient area ratio of precipitates with a grain size of 50 nm or less could not be obtained, and the delayed fracture resistance was lowered.
Test material no. In No. 35, the tempering temperature was high, and as a result of coarsening of precipitated carbides, the area ratio of precipitates with a grain size of 50 nm or less was insufficient, and the delayed fracture resistance was lowered. Furthermore, test material No. In No. 35, sufficient tensile strength could not be obtained due to the high tempering temperature.
Test material no. No. 36 is an example in which the tempering temperature was low, the precipitates with a grain size of 50 nm or less were not sufficiently precipitated, and the area ratio was insufficient, so the delayed fracture resistance was deteriorated. Furthermore, test material No. In No. 36, the tensile strength was excessive due to the low tempering temperature.
Test material no. 37, the value of formula (2) does not satisfy the predetermined range. The area ratio of precipitates with a grain size of 50 nm or less reached 0.50%, but as a result of V and Mo precipitating as M ₂ C-based carbides instead of MC-based carbides, MC-based carbides mainly composed of VC-based carbides were formed. is insufficient and the delayed fracture resistance is lowered.
In addition, actually test material No. The chemical composition of the precipitates with a grain size of 50 nm or less in No. 37 was analyzed by analysis of the electron beam diffraction pattern of TEM and analysis by EDS. As a result, test material No. In No. 37, M ₂ C-based carbides were generated in precipitates with an equivalent circle diameter of 50 nm or less, and it was confirmed that the ratio of MC-based carbides was reduced.

Claims (3)

in % by mass,
C: 0.25 to 0.50%,
Si: 2.00 to 4.00%,
Mn: 0.20-2.00%,
V: 0.30 to 1.00%,
Mo: 0.05-1.00%,
N: 0.001 to 0.020%,
P: 0.015% or less, and S: 0.015% or less,
with the remainder consisting of Fe and impurities, having a chemical composition in which the value of the following formula (1) is greater than 200 and the value of the following formula (2) is 3.00 or less,
The steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member is a tempered martensite structure with an average aspect ratio of prior austenite grains of 1.3 or less,
A high-strength steel member having a tensile strength of 1300 MPa or more and 1600 MPa or less.
100×%C+50×%Si+200×%V+100×%Mo Expression (1)
%Mo/%V Formula (2)
Here, in formulas (1) and (2), %C, %Si, %V and %Mo represent the contents (% by mass) of C, Si, V and Mo, respectively. in % by mass,
Cr: 0 to 2.00%,
Al: 0 to 0.100%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
W: 0 to 1.000%,
B: 0 to 0.0100%,
Ni: 0 to 3.00%,
Cu: 0 to 2.00%, and
REM: 0-0.0200%
The high-strength steel member according to claim 1, having a chemical composition containing one or more of 2. The steel structure in a plane parallel to the surface of the steel member located at a depth of 1.0 mm from the surface of the steel member contains precipitates with a grain size of 50 nm or less in an area ratio of 0.50% or more, or The high-strength steel member according to claim 2.