JP2019077911A - Steel member and manufacturing method of steel member - Google Patents

Abstract

To provide a steel member capable of achieving both of high strength and delayed fracture resistance, while avoiding specific treatments or excessive cost rise.SOLUTION: There is provided a steel ember containing, 0.20%≤C≤0.75%, 0.01%≤Si≤2.00%, 0.10%≤Mn≤5.00%, P≤0.05%, S≤0.05%, 0.01%≤Cu≤2.00%, 0.01%≤Ni≤3.00%, 0.01%≤Cr≤3.00%, 0.01%≤Mo≤2.00%, Al≤0.20% (excluding 0%), N≤0.05% (excluding 0%) and the balance Fe with inevitable impurities, and having a substrate consisting of a martensite phase or a mixed phase of the martensite phase and a bainite phase, and a surface later occupying a surface of the substrate, containing N of 1 mass% or more and consisting of a γ phase. The steel member is manufactured by molding a steel material and conducting a hardening treatment, then conduction a nitriding treatment using a nitriding gas to introduce an N atom to the surface of the steel material, and further removing the nitriding gas to conduct a heat treatment of the steel material.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel member and a method of manufacturing the steel member, and more particularly to a high strength steel member used as a fastening member including a bolt and the like, and a method of manufacturing such a steel member.

  High-strength steels are widely used as materials for fastening members such as bolts and as structural steels that constitute various members in automobiles and the like. For example, there is an increasing demand for weight reduction of various members for the purpose of reducing fuel consumption of automobiles, etc. In order to ensure sufficient strength as a whole as a member even if weight reduction, high strength steel is further enhanced Needs to be However, if the steel material is strengthened, delayed fracture is likely to occur. Generally, it is empirically known that delayed fracture is likely to occur when the tensile strength of a steel material is 1200 MPa or more.

  Therefore, attempts have been made to achieve both tensile strength and delayed fracture resistance of steel materials. For example, in Patent Document 1, a bolt made of a steel material having a predetermined component composition is heat-treated to have a tensile strength of 1400 MPa or more, and then the surface of the bolt in a liquid or in the presence of a liquid film. A high intensity bolt manufacturing method is disclosed in which a pulsed laser beam is focused and irradiated to generate compressive residual stress on the bolt surface. Further, in Patent Document 2, after heat treatment is performed on a bolt made of a steel material having a predetermined component composition, an ultrasonic vibration terminal is pressed against the lower part of the neck of the bolt to perform a striking process. A method of property improvement is disclosed.

  In addition, martensitic steel is usually used as a steel material having a tensile strength exceeding 1200 MPa, and the above-mentioned Patent Documents 1 and 2 also apply a predetermined treatment to the martensitic steel. On the other hand, austenitic steel is known to be superior in delayed fracture resistance as a strength comparison to martensitic steel. This is considered to be because, in the FCC structure of austenitic steel, the rate of hydrogen diffusion that causes delayed fracture is about one digit lower than that of BCC structure of martensitic steel.

JP, 2006-291295, A Unexamined-Japanese-Patent No. 2006-131990

  The methods disclosed in Patent Literatures 1 and 2 are all intended to enhance the delayed fracture resistance of the steel material by performing a predetermined treatment in a state where the tensile strength of the steel material is enhanced by heat treatment. . As a result, even if the delayed fracture resistance can be compatible with high strength, special treatment is required to improve the delayed fracture resistance, so the versatility is poor and the manufacturing cost of the steel increases. .

  Furthermore, as described above, although austenitic steels are superior in resistance to delayed fracture as a strength comparison as compared to martensitic steels, austenitic steels generally have lower tensile strength than martensitic steels. In the austenitic steel, in order to obtain a high tensile strength such as 1200 MPa or more, it is necessary to add a large amount of Ni and Co, which increases the material cost of the steel material.

  The problem to be solved by the present invention is to provide a steel member capable of achieving both high tensile strength and delayed fracture resistance while avoiding special treatment and excessive cost increase, and a method for producing such a steel member. It is to do.

  In order to solve the above-mentioned subject, the steel member concerning the present invention is 0.20% ≦ C ≦ 0.75%, 0.01% ≦ Si ≦ 2.00%, 0.10% ≦ Mn ≦% by mass. 5.00%, P ≦ 0.05%, S ≦ 0.05%, 0.01% ≦ Cu ≦ 2.00%, 0.01% ≦ Ni ≦ 3.00%, 0.01% ≦ Cr ≦ 3.00%, 0.01% ≦ Mo ≦ 2.00%, Al ≦ 0.20% (but not including 0%), N ≦ 0.05% (but not including 0%) A matrix consisting of Fe and unavoidable impurities, a matrix consisting of a martensitic phase or a mixed phase of a martensite and a bainite phase, and the surface of the matrix, containing at least 1 mass% of N, consisting of a gamma phase And a surface layer.

  Here, the steel member may have a tensile strength of 1200 MPa or more.

  Moreover, content of each said element sets mass% as a unit, and 4.4 [C] +1.2 [Si] -0.7 [Mn] +0.1 [Cu] +1.2 [Ni] -0. It is preferable to satisfy the relationship of 2 [Cr] +2.0 [Mo] ≧ 0.

  The content of each of the elements is, in terms of mass%, 0.4 [C]-26.3 [Si] + 67.0 [Mn] + 9.8 [Cu] + 53.7 [Ni]-16.2. It is good to satisfy the relationship of Cr] +13.1 [Mo] ≧ 42.

  The steel member may further contain, by mass%, at least one selected from V ≦ 1.0%, Ti ≦ 1.0%, Nb ≦ 1.0%.

  Content of each said element is -37.8 [C] +71.7 [Si] +0.7 [Mn] -14.5 [Cu] -4.5 [Ni] +0.8 in mass% as a unit. It is preferable to satisfy the relationship of [Cr] +80.5 [Mo] <100.

  It is good for content of N in the said surface layer to be 3 mass% or more.

  The steel member may be a fastening member.

  In the method of manufacturing a steel member according to the present invention, after forming a steel material and performing a hardening treatment, N atoms are introduced to the surface of the steel material by a nitriding treatment using a nitriding gas, and the nitriding gas is further removed Then, the steel member as described above is manufactured by heat-treating the steel material.

  Here, the heat treatment is preferably performed in a nitrogen atmosphere.

  The steel member according to the present invention has high tensile strength when the base is formed of the martensitic phase having the above-described predetermined component composition, or a mixed phase of the martensitic phase and the bainitic phase. On the other hand, the diffusion of hydrogen into the steel member is suppressed by having the surface layer made of γ phase on the surface of such a base. As a result, the delayed fracture resistance of the steel member is enhanced. Thus, the steel member according to the above-described invention achieves both high strength and delayed fracture resistance.

  The formation of γ phase on the surface of martensitic structure or bainitic structure can be conveniently achieved by introducing N at a concentration of 1% by mass or more. The introduction of N can be carried out by nitriding treatment widely used as a treatment process of steel materials. Therefore, the steel member which makes high strength and delayed fracture resistance compatible as mentioned above can be manufactured without passing through a special treatment process. Moreover, the said component composition does not contain expensive components, such as large amounts of Ni and Co, and can also avoid the excessive cost rise.

  Here, when the tensile strength of the steel member is 1200 MPa or more, sufficient tensile strength as a high strength steel can be secured.

  In addition, the content of each element is 4.4 [C] + 1.2 [Si]-0.7 [Mn] + 0.1 [Cu] + 1.2 [Ni]-0.2, based on mass%. In the case where the relationship of [Cr] +2.0 [Mo]] 0 is satisfied, it is easy to obtain particularly high tensile strength in the steel member.

  Content of each element is 0.4 [C]-26.3 [Si] + 67.0 [Mn] + 9.8 [Cu] + 53.7 [Ni]-16.2 [Cr] in a unit of mass%. When the relationship of +13.1 [Mo] Mo 42 is satisfied, it is easy to generate γ phase in the surface layer of the steel member.

  In the case where the steel member further contains, by mass%, at least one selected from V ≦ 1.0%, Ti ≦ 1.0%, Nb ≦ 1.0%, delayed fracture resistance due to toughness reduction. It becomes easy to improve especially the tensile strength of a steel member, avoiding the fall of a characteristic.

  Content of each element is -37.8 [C] + 71.7 [Si] + 0.7 [Mn]-14.5 [Cu]-4.5 [Ni] + 0.8 [unit of mass%]. When the relationship of Cr] +80.5 [Mo] <100 is satisfied, the formation of coarse carbides such as M6C carbides which lowers the delayed fracture resistance can be suppressed, and particularly high delayed fracture resistance can be obtained.

  When the content of N in the surface layer is 3% by mass or more, by forming the γ phase capable of highly suppressing the diffusion of hydrogen as the surface layer, the delayed fracture resistance of the steel member is effectively achieved. It can be enhanced.

  When the steel member is a fastening member, in the fastening member such as a bolt, high tensile strength is required in view of its use, and from its shape, it is required to suppress delayed fracture which tends to occur depending on the site. However, by forming the fastening member as a steel member having a surface layer on the surface of the base as described above, it is possible to achieve both high strength and delayed fracture resistance.

  In the method of manufacturing a steel member according to the above-described invention, a martensitic structure or a mixed phase structure of a martensitic phase and a bainitic phase can be obtained by performing a hardening process on a steel material having a predetermined component composition. Thereafter, nitriding treatment is performed to introduce N atoms into the surface of the steel material, and heat treatment is performed, whereby the surface layer portion of the martensitic structure or the bainitic structure can be transformed to the γ phase. As a result, the surface of the base comprising the martensitic phase or the mixed phase of the martensitic phase and the bainitic phase giving high tensile strength has a low diffusion rate of hydrogen and a surface layer comprising the γ phase which contributes to the improvement of the delayed fracture resistance. The steel member which it has can be manufactured. The component composition of the steel member does not contain a large amount of expensive component elements, and each processing step is not special either, so it is highly expensive while avoiding the need for special processing and excessive cost increase. A steel member having both strength and delayed fracture resistance can be manufactured.

  Here, when the heat treatment is performed in a nitrogen atmosphere, the N atoms introduced to the surface layer in the preceding nitriding treatment are less likely to escape in the heat treatment step. As a result, the surface layer portion composed of the γ phase is reliably formed on the surface of the steel member, and it becomes easy to contribute to the improvement of the delayed fracture resistance.

It is the optical microscope image which observed the cross section of the steel materials, (a) shows the observation image of the sample 1 and (b) the sample 2. It is a X-ray-diffraction pattern of steel materials, (a) shows the measurement result of the sample 1 and (b) the sample 2. FIG. It is a figure explaining the shape of the test piece used in the Example, (a) is a rough processing goods of a tensile test piece, (b) is the refined processing goods. (C) is a rough product of the delayed fracture test piece, and (d) is its finely processed product. In (c) and (d), the whole view (left) and the enlarged view (right) of the center notch part are shown. It is a schematic diagram explaining the method of the cantilever bending test in a delayed fracture test.

  Below, the steel member concerning one embodiment of the present invention, and the manufacturing method of a steel member are explained in detail.

[Structure of steel member]
The steel member according to one embodiment of the present invention has most of its volume constituted by a matrix which has the following component composition and is composed of a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase. The surface layer is formed to occupy the surface of the base. The surface layer has a component composition in which N is added to substantially the same component as the base, and is composed of a γ phase.

  The type and shape of the steel member are not particularly limited, but a suitable example may be a fastening member such as a bolt. In the steel member, the surface of the surface layer may be further treated by plating or the like.

(Configuration of base)
The base that occupies most of the steel members according to the present embodiment contains the following elements, with the balance being Fe and unavoidable impurities. The types of additive elements, component ratios, and reasons for limitation are as follows. The unit of component ratio is mass%.

0.20% ≦ C ≦ 0.75%
C dissolves in the matrix during quenching to form a martensitic structure, thereby improving the tensile strength and hardness of the steel member. By setting the content of C to 0.20% ≦ C, high tensile strength, for example, tensile strength of 1200 MPa or more can be secured. Preferably, 0.40% ≦ C.

  On the other hand, when the content of C is excessive, coarse carbides are likely to be increased, and the toughness of the steel member is lowered. If the toughness decreases, the delayed fracture resistance also decreases. From the viewpoint of securing sufficient delayed fracture resistance as the entire steel member, C ≦ 0.75%. Preferably, C ≦ 0.70%.

0.01% ≦ Si ≦ 2.00%
Si has an effect as a deoxidizer and an effect to improve the tensile strength of the steel member. From the viewpoint of obtaining those effects, the content of Si is 0.01% ≦ Si. Preferably, 0.20% ≦ Si.

  On the other hand, when the content of Si is excessive, the γ phase is less likely to be generated as the surface layer, and the delayed fracture resistance is degraded. From the viewpoint of securing sufficient delayed fracture resistance as the entire steel member, Si ≦ 2.00%. Preferably, Si ≦ 1.00%.

0.10% ≦ Mn ≦ 5.00%
Mn has the effect of enhancing the hardenability of steel members. In addition, it has the effect of facilitating the generation of the γ phase on the surface of the base. From the viewpoint of obtaining those effects, the content of Mn is 0.10% ≦ Mn. Preferably, 1.00% ≦ Mn.

  On the other hand, when the content of Mn is excessive, residual γ phase is easily generated. That is, at the time of quenching, the γ phase (austenite phase) tends to remain in the matrix. Then, the tensile strength of the steel member is reduced. From the viewpoint of avoiding such a situation, Mn ≦ 5.00%. Preferably, Mn ≦ 4.00%.

・ P ≦ 0.05%
P is an element which may be mixed in the steel member at the impurity level. The incorporation reduces the toughness of the steel member and, as a result, reduces the delayed fracture resistance of the steel member. Therefore, although it is preferable not to contain P, if P <= 0.05%, those effects do not become serious and you may contain. Preferably, P ≦ 0.02%.

・ S ≦ 0.05%
S is also an element that may be mixed in the steel member at the impurity level. The incorporation reduces the toughness of the steel member and, as a result, reduces the delayed fracture resistance of the steel member. Therefore, although it is more preferable not to contain S, if S <= 0.05%, those effects do not become serious and you may contain. Preferably, S ≦ 0.02%.

· 0.01% Cu Cu 2.00 2.00%
Cu has the effect of enhancing the hardenability of the steel member. It also has the effect of facilitating the generation of the γ phase in the surface layer. From the viewpoint of obtaining those effects, the content of Cu is 0.01% ≦ Cu.

  On the other hand, if the content of Cu is excessive, the material cost of the steel member becomes high. From the viewpoint of suppressing an excessive increase in material cost, Cu ≦ 2.00%. Preferably, Cu ≦ 1.00%.

0.01% ≦ Ni ≦ 3.00%
Ni also has the effect of enhancing the hardenability of the steel member. It also has the effect of facilitating the generation of the γ phase in the surface layer. From the viewpoint of obtaining those effects, the content of Ni is 0.01% ≦ Ni. Preferably, 0.10% ≦ Ni.

  On the other hand, if the content of Ni is excessive, the material cost of the steel member becomes high. From the viewpoint of suppressing an excessive increase in material cost, Ni ≦ 3.00%. Preferably, Ni ≦ 2.00%.

0.01% ≦ Cr ≦ 3.00%
Cr has the effect of enhancing the hardenability of the steel member. From the viewpoint of obtaining the effect of improving the hardenability, the content of Cr is set to 0.01% ≦ Cr. Preferably, 0.20% ≦ Cr.

  On the other hand, if the content of Cr is excessive, the material cost of the steel member becomes high. From the viewpoint of suppressing an excessive increase in material cost, Cr ≦ 3.00%. Preferably, Cr ≦ 2.00%.

· 0.01% Mo Mo 2.00 2.00%
Mo contributes to enhancing the tensile strength and hardness of the steel member by forming secondary precipitated carbides. It also has the effect of facilitating the generation of the γ phase in the surface layer. From the viewpoint of obtaining those effects, the content of Mo is 0.01% ≦ Mo. Preferably, 0.15% ≦ Mo.

  On the other hand, when the content of Mo is excessive, the material cost of the steel member becomes high. From the viewpoint of suppressing an excessive increase in material cost, Mo ≦ 2.00%.

· Al 0.20 0.20% (but excluding 0%)
Al functions as a deoxidizing agent, and combines with N to form AlN, and is added to the steel member because it suppresses the coarsening of old γ grains. By suppressing the coarsening of the old γ grains, the embrittlement of the steel member is suppressed. Since the addition of Al has a high effect even at a small amount, the lower limit of the content is not particularly provided. Preferably, it is preferable to contain 0.005% or more.

  On the other hand, when a large amount of Al is contained, the workability of the steel member is lowered. Therefore, the upper limit of the content is 0.20%.

・ N ≦ 0.05% (but excluding 0%)
N is added to the steel member because it combines with Al to form AlN and has the effect of suppressing the coarsening of old γ grains. Since the addition of N has a high effect even at a small amount, the lower limit of the content is not particularly provided. On the other hand, since the effect of the former γ grain coarsening suppression is saturated at a content of about 0.05%, the upper limit of the addition amount is set to 0.05%. Preferably, 0.002% or more may be contained.

  The base of the steel member according to the present embodiment contains the predetermined amount of C, Si, Mn, Cu, Ni, Cr, Mo, Al, N, and contains P and / or S below the upper limit amount. The rest is composed of Fe and unavoidable impurities. Here, As, Sn, Sb, O, etc. of 0.010 mass% or less are assumed as unavoidable impurities. As, Sn, and Sb may not lead to embrittlement of the steel member, and O may lead to a decrease in fatigue strength, and therefore, it is preferable not to contain each exceeding 0.010 mass%.

  The base of the steel member according to the present embodiment may optionally contain one or more elements selected from the following elements in addition to the above-described essential elements.

・ V ≦ 1.0%, Ti ≦ 1.0%, Nb ≦ 1.0%
V, Ti and Nb all have the effect of improving the tensile strength of the steel member. However, when the content of these elements is excessive, the toughness of the steel member is lowered. If the toughness decreases, the delayed fracture resistance also decreases. In order to secure sufficient delayed fracture resistance, the upper limit of the content is 1.0%. Although V, Ti and Nb exert the effect of improving the tensile strength even if added in small amounts, the lower limit of the content is not particularly specified, but from the viewpoint of sufficiently obtaining the effects, the content of each element Is preferably 0.04% or more.

  Furthermore, in the component composition of the matrix of the steel member according to the present embodiment, the content of each element preferably satisfies at least one of the following relational expressions. In each of the relational expressions, the notation [] indicates the content of each element in mass%.

First, it is preferable that the component composition of the base satisfies the following relationship (1).
4.4 [C] + 1.2 [Si]-0.7 [Mn] + 0.1 [Cu] + 1.2 [Ni]-0.2 [Cr] + 2.0 [Mo] 0 0 (1)

The formula value on the left side of the formula (1) is an amount serving as an index of the tensile strength of the steel member, and the larger the value, the higher the tensile strength. In particular, when the formula value is 00, it is easy to obtain a tensile strength of 1200 MPa or more in the steel member. It is particularly preferred that the formula value is 3.03.0.

Moreover, it is preferable that the component composition of a base satisfy | fills the relationship of the following (2).
0.4 [C] -26.3 [Si] +67.0 [Mn] +9.8 [Cu] +53.7 [Ni] -16.2 [Cr] +13.1 [Mo] 3.142 (2)

  The formula value on the left side of the formula (2) is an index that indicates the ease of formation of the γ phase in the surface layer of the steel member, and the larger the value is, the more it passes through nitriding treatment and heat treatment to It is easy to generate In particular, when the formula value is 4242, it is easy to form the γ phase excellent in the delayed fracture resistance improvement effect on the surface layer. It is particularly preferred that the formula value is 100100.

Furthermore, it is preferable that the component composition of the base satisfies the following relationship (3).
-37.8 [C] + 71.7 [Si] + 0.7 [Mn]-14.5 [Cu]-4.5 [Ni] + 0.8 [Cr] + 80.5 [Mo] <100 (3)
The formula value on the left side of the formula (3) is a quantity serving as an index indicating the ease of formation of coarse carbides such as M6C carbides. When coarse carbides represented by M6C carbides and the like precipitate, delayed fracture tends to occur in the steel member. As the value of the equation on the left side of the equation (3) is larger, such carbides are more likely to precipitate and the delayed fracture resistance of the steel member becomes lower. However, when the formula value is suppressed to less than 100, coarse carbides such as M6C carbides are less likely to be formed, and the delayed fracture resistance of the steel member is enhanced.

  The structure of the base of the steel member according to the present embodiment is composed of a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase. By subjecting a steel material having the above component composition to a heat treatment including a quenching treatment, it is possible to obtain a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase. It is preferable that the base is almost entirely composed of a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase, and the residual γ phase is 30% or less. Further, alloy carbides are contained in the martensitic phase and the bainitic phase.

(Structure of surface)
The surface layer is formed so as to occupy the surface of the base in the steel member according to the present embodiment, and is continuous with the base. The surface layer has almost the same composition as the above base except that it contains high concentration of N. It is preferable that the shift from the content in the base of the content of each component element other than N in the surface layer is suppressed to 0.3% or less based on the content in the base.

  The surface layer contains 1% by mass or more of N. The content of N is preferably 3% by mass or more. From the viewpoint of the characteristics of the surface layer, the upper limit of the content of N is not particularly provided, but the content of N that can be practically introduced into the surface layer by the manufacturing method described later is about 4.5 mass% or less .

  The structure of the surface layer is composed of the γ phase (austenite phase). That is, the surface layer consisting of the γ phase occupies a partial region in the depth direction from the surface of the base consisting of the martensite phase or the mixed phase of the martensite phase and the bainite phase. Since the surface layer is made of the γ phase, as described in detail later, the penetration of hydrogen from the outside air is suppressed, which contributes to the improvement of the delayed fracture resistance of the steel member. In the surface layer, the transformation from the martensite phase or the bainite phase to the γ phase takes place almost over the entire area, and the occupancy ratio of the γ phase in the surface layer is preferably 80% by volume or more. In the present specification, "consisting of a γ phase" refers to a state in which the γ phase is a mother phase, including the case of containing a small amount of martensitic structure, bainite structure or the like.

  The improvement of the delayed fracture resistance of the steel member by the surface layer is effectively achieved by the surface layer containing 1 mass% or more of N atoms as described above. In the case where the surface layer contains 3% by mass or more of N atoms, the effect is particularly high.

  The thickness of the surface layer is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of improving the delayed fracture resistance, the thickness is preferably 5 μm or more in average value. More preferably, it is 10 μm or more. The thicker the surface layer, the higher the effect of improving the delayed fracture resistance. Therefore, there is no upper limit to the thickness, but the thickness of the surface layer that can be practically formed by the manufacturing method described later is about 30 μm or less. is there.

  The surface layer may be formed on the entire surface of the base constituting the steel member or may be formed only on the surface of a partial region. However, in order to improve the delayed fracture resistance in the entire steel member, it is preferable to form the surface layer over the entire surface of the base. As described later, it is preferable to form the surface layer on the entire surface of the matrix also from the viewpoint of simplicity in forming the surface layer by nitriding treatment and heat treatment.

(Characteristics of steel members)
As mentioned above, in the steel member concerning this embodiment, the surface layer which contains N atom is formed in the surface of the base which constitutes the major part. The base is composed of a martensite phase or a mixed phase of a martensite phase and a bainite phase, while the surface layer is composed of a γ phase (austenite phase).

  In the steel member according to the present embodiment, high tensile strength is obtained because the base is composed of the martensitic phase and the bainitic phase. The tensile strength of the entire steel member is preferably 1200 MPa or more. If it has a tensile strength of 1200 MPa or more, it can be suitably used for applications as various high strength steels including a fastening member. More preferably, it is preferable to have a tensile strength of 1600 MPa or more. The tensile strength of the steel member can be measured according to JIS Z2241.

  The martensitic phase and bainitic phase constituting the matrix have high tensile strength, but have low delayed fracture resistance. However, in the steel member according to the present embodiment, the surface of the matrix has a surface layer composed of γ phase, and the surface layer plays a role of improving the delayed fracture resistance of the entire steel member.

  The delayed failure of steel materials is caused by the penetration of hydrogen from the outside air. In the martensitic phase and the bainitic phase having the BCC crystal structure, diffusion of hydrogen is likely to occur, so that the delayed fracture resistance is lowered. On the other hand, in the γ-phase FCC crystal structure, the diffusion rate of hydrogen is about one digit lower than that in the BCC crystal structure. Therefore, the surface of the matrix consisting of the martensite phase or the mixed phase of the martensite phase and the bainite phase is covered, and the γ phase suppresses the penetration of hydrogen from the outside air by the presence of the surface layer consisting of the γ phase. be able to. As a result, it is difficult for hydrogen from the outside air to reach the base that occupies most of the steel members, and the occurrence of delayed fracture of the steel members due to the penetration of hydrogen is suppressed. In this way, high delayed fracture resistance is achieved for the steel member as a whole.

The delayed fracture resistance of the steel member can be evaluated, for example, using a 30-hour delayed fracture strength ratio as an index. Specifically, hydrochloric acid having a concentration of 0.1 N is dropped onto a test piece obtained by processing a steel material into a predetermined shape, as shown in the subsequent examples. Before and after dropping, the test piece is subjected to a cantilever bending test, and the ratio of the breaking stress (30-hour dropping stress σ ₃₀ ) after the dropping to the breaking stress (static bending breaking stress σ _B ) before the dropping is 30 The time delay fracture strength ratio is taken as (σ ₃₀ / σ _B ). Here, the dropping of hydrochloric acid is performed as an accelerated test for evaluating the penetration of hydrogen into the steel member, and the 30-hour delayed fracture strength ratio is 30 hours of hydrochloric acid based on the bending rupture strength of the original steel member. It can be considered to be the ratio of the flexural breaking strength of the steel member after the dropping. As the value is larger, it means that the bending rupture strength of the steel member does not decrease significantly even after the dropping of hydrochloric acid, and it is shown that delayed fracture due to hydrogen penetration is less likely to occur. The flexural rupture strength of the steel member has a high correlation with the tensile strength, and the delayed fracture strength ratio evaluated as the extent of the reduction in flexural rupture strength by the above evaluation method is the extent of the tensile strength degradation It can also be used as an index that reflects the delayed fracture resistance characteristics.

In the steel member according to the present embodiment, the 30-hour delayed fracture strength ratio evaluated as described above is preferably 0.6 or more. More preferably, the 30-hour delayed breaking strength ratio is preferably 0.7 or more.

  In general, in steel materials having high tensile strength, delayed fracture is likely to occur, and in particular, when the tensile strength is 1200 MPa or more, the reduction in delayed fracture resistance becomes remarkable. However, in the steel member according to the present embodiment, the effect of the coexistence of the base composed of the martensite phase or the mixed phase of the martensite phase and the bainite phase and the surface layer composed of the γ phase covering the surface is high. Both tensile strength and high delayed fracture resistance are achieved. As described above, the component composition of the steel member does not contain a large amount of expensive component elements. In addition, as described later, the formation of the surface layer on the surface of the matrix can be easily performed in a versatile process such as nitrogen introduction and phase transition by nitriding treatment and heat treatment. In this way, improvements in tensile strength and delayed fracture resistance can be achieved without using special processes and while avoiding excessive increases in material and processing costs.

  The steel member preferably has high hardness in addition to high tensile strength from the viewpoint of mechanical strength. The steel member may have a hardness of 38 HRC or more, or even 47 HRC or more.

[Method of manufacturing steel member]
Next, a method of manufacturing a steel member according to an embodiment of the present invention will be described. The steel member as described above can be manufactured using the manufacturing method according to the present embodiment.

  In the manufacturing method according to the present embodiment, the steel member having the surface layer on the surface of the base is implemented by sequentially performing (1) processing to a predetermined shape, (2) hardening, and (3) surface layer formation in this order. Manufacture. Each step will be described below.

(1) Processing to a predetermined shape The steel having the component composition described above is melted and cast in a vacuum induction furnace or the like. Then, processing is performed to the shape of a predetermined steel member, that is, the shape of a bolt or the like.

  The processing can be performed, for example, by rolling, forging and machining. Forging and machining can be performed, for example, by the steps of roughing and roughing parts. As the member roughing process, hot forging and cold forging can be exemplified, and as the member precision machining, cold forging and cutting can be exemplified. In addition to these processes, heat treatment such as normalizing may be performed as appropriate.

(2) Quenching Quenching is performed on a steel member processed into a predetermined shape. For example, after heating a steel member to 800 to 1200 ° C., quenching is performed at 60 ° C./min. It may carry out by cooling at the above speed. Cooling is preferably performed by oil cooling. After quenching, substantially the entire steel member becomes a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase.

(3) Surface layer formation A surface layer forming treatment is performed on a steel member substantially consisting of a martensite phase or a mixed phase of a martensite phase and a bainite phase obtained as described above, and a martensite phase or martensite The surface of the matrix composed of the mixed phase of the phase and the bainite phase is in the state of being coated on the surface layer composed of the γ phase.

  The surface formation is performed by two steps of nitriding treatment and subsequent heat treatment. N atoms are introduced to the surface of the steel member by nitriding treatment. Then, by the heat treatment, the transition of the surface region into which N atoms have been introduced to the γ phase is completed to form the surface layer. The region present inside the surface layer is tempered by heating in the nitriding process and heat treatment process, and becomes a matrix consisting of a martensite phase or a mixed phase of a martensite phase and a bainite phase.

  The nitriding treatment can be performed by heating the steel member in a nitriding gas. The nitriding treatment may be carried out by the narrow definition nitriding in which only N atoms are introduced into the surface of the steel member or by soft nitriding in which N atoms and C atoms are introduced simultaneously.

  A known nitriding gas such as a gas containing ammonia may be appropriately used as the nitriding gas. In addition, it is preferable to set the heating temperature of the steel member during the nitriding treatment to a lower temperature than the heating in the preceding quenching step. In particular, the temperature is preferably 500 to 700 ° C. The time for performing the nitriding treatment may be set appropriately so that introduction of N atoms can be sufficiently performed, but 1 to 3 hours can be exemplified. If the temperature of nitriding treatment is lower than 500 ° C., and if the time of nitriding treatment is shorter than one hour, it becomes difficult to sufficiently introduce N atoms to the surface of the steel member, and the N atom concentration becomes high. Surface layer is difficult to form. on the other hand. If the temperature of nitriding treatment exceeds 700 ° C., and if the time of nitriding treatment exceeds 3 hours, the precipitation of cementite phase tends to lower the tensile strength as a whole of the steel member, and also tends to lower the hardness.

  By adjusting the type and concentration of the nitriding gas and the heating temperature, it is possible to change the concentration of N atoms in the surface layer obtained through the heat treatment. In addition, the thickness of the surface layer can be changed by adjusting those parameters and the time of nitriding treatment.

In the heat treatment subsequent to the nitriding treatment, the steel member is heated in a state where the nitriding gas used for the nitriding treatment is removed from the atmosphere. The heat treatment can be performed in the air or in an inert gas atmosphere such as nitrogen (N ₂ ). Preferably, the heat treatment is performed in a nitrogen atmosphere. As a result, N atoms introduced to the surface of the steel member in the previous nitriding process are prevented from escaping into the atmosphere during heat treatment, and finally, a surface layer containing a high concentration of N atoms is formed. It is easy to do.

  The heating temperature of the steel member in the heat treatment is preferably about the same as the heating temperature in the nitriding treatment. In particular, the temperature is preferably 500 to 700 ° C. The heat treatment time may be set appropriately so that the γ phase can be sufficiently formed, but 1 to 8 hours can be exemplified. If the temperature of the heat treatment is lower than 500 ° C., and if the heat treatment time is shorter than one hour, it is difficult to complete the transition to the γ phase sufficiently, and it becomes difficult to form the surface layer having the γ phase . On the other hand, if the heat treatment temperature exceeds 700 ° C., and if the heat treatment time exceeds 8 hours, the precipitation of the cementite phase tends to lower the tensile strength as a whole of the steel member, and also tends to lower the hardness.

  The nitriding treatment and the heat treatment may be performed separately or continuously. When the two processes are performed separately, the nitriding treatment is performed at a high temperature, and then the nitriding gas is removed from the atmosphere, and the steel member is allowed to cool or cool once, and then the steel member is heated again to the required temperature. Heat and heat treat. On the other hand, when performing both treatments continuously, after performing nitriding treatment at high temperature, removing the nitriding gas while keeping the steel member at high temperature, adjusting the temperature of the steel member and the atmosphere if necessary After the substitution of the above, heat treatment is performed as it is. In the case where the nitriding treatment and the heat treatment are performed at the same temperature, it is sufficient to remove the nitriding gas after the completion of the nitriding treatment and appropriately substitute the atmosphere for the heat treatment such as nitrogen gas as appropriate. From the viewpoint of shortening the time required to form the surface layer, it is preferable to perform both treatments continuously.

  As described above, by performing nitriding treatment and heat treatment, which are general-purpose steps, on steel members having a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase, N atoms can be formed on the surface of the matrix. It is possible to form a surface layer having a high concentration and taking the γ phase. The surface-covered base contains only a very small amount of N atoms, and takes a martensitic phase or a mixed phase of a martensitic phase and a bainitic phase.

  Hereinafter, the present invention will be more specifically described using examples.

[Confirmation of structure of steel members]
Regarding the steel member according to one embodiment of the present invention, the state of the structure was confirmed and compared with a conventional general high-strength steel.

<Preparation of sample>
The steel material of sample 1 was prepared as a steel member concerning one embodiment of the present invention. The composition of the matrix is, in terms of mass%, C: 0.58%, Si: 0.15%, Mn: 2.18%, P: 0.022%, S: 0.013%, Cu: 0 .14%, Ni: 0.27%, Cr: 0.13%, Mo: 1.00%, Al: 0.067%, N: 0.028%, the balance being from Fe and unavoidable impurities It was supposed to be The nitriding treatment was performed at 550 ° C. for 1 hour, and the subsequent heat treatment was performed at 600 ° C. for 2 hours in a nitrogen atmosphere. The composition of the components and the treatment conditions are the same as those of Example 12 in Tables 1 and 2 described later.

  On the other hand, a steel material of sample 2 was prepared as a conventional general steel member. Here, one type of high strength bolt steel was used. The component composition contains C: 0.60%, Si: 2.00%, Mn: 0.30%, Cr: 1.00%, Mo: 1.00% in mass%, and the balance is It consisted of Fe and unavoidable impurities. As processing to the steel materials after machining to a predetermined shape, hardening at 930 ° C. and tempering at 605 ° C. are performed.

<Confirmation of surface formation>
Optical microscopic observation of the cross section was performed on the steel materials of Sample 1 and Sample 2. Preparation and observation of the cross-sectional sample were performed in the state where the Ni plating layer was formed on the surface.

  An observation image of the sample 1 is shown in FIG. 1 (a) and the sample 2 is shown in FIG. 1 (b). In the observation image of the sample 2 in FIG. 1B, it can be seen that the entire cross section is composed of the same tissue.

  On the other hand, in the observation image of the sample 1 in FIG. 1 (a), as indicated by the dotted line in the drawing, there is a site observed brighter than the base on the surface of the base. This indicates that the base and the crystal structure are different, and sites giving different reflectances exist in the surface layer of the base.

<Confirmation of crystal structure>
Next, identification of the crystal structure of sample 1 and sample 2 was performed by X-ray diffraction. The measurement was performed using a Co tube as a radiation source.

FIG. 2 (a) shows the X-ray diffraction pattern obtained for Sample 1 and FIG. 2 (b) for Sample 2. FIG. In the result of sample 2 in FIG. 2 (b), only a diffraction peak of α-Fe indicating that the tissue has a BCC crystal structure is observed. From this, it is confirmed that the martensitic phase is formed in sample 2.

  On the other hand, in addition to the peak derived from α-Fe, the peak derived from γ-Fe (FCC structure) is observed in the measurement result of the surface layer portion of sample 1 in FIG. 2 (a). This indicates that in the sample 1, in addition to the martensitic phase, the γ phase is formed. This γ phase can be correlated with the tissue of the surface layer observed in the microscopic image of FIG. 1 (a). In addition, in X-ray diffraction, since it measures to the depth of about 30 micrometers from surface layer, it is thought that in the sample 1, the diffraction peak of alpha-Fe of a base is also detected.

  From the results of microscopic observation and X-ray diffraction of the cross section described above, in the conventional general high-strength steel, the entire structure is made of martensitic phase, while in the steel member according to the embodiment of the present invention, the surface of the matrix It was confirmed that there were different surface layers of base and crystalline structure. And it was confirmed that the surface layer consists of gamma phase.

[Relationship between alloy composition of steel members and manufacturing method and characteristics]
Next, the alloy composition of the steel member and the conditions in the manufacturing process were variously changed to evaluate the relationship with various characteristics.

<Preparation of sample>
A steel member containing the component elements (unit: mass%) shown in Tables 1 and 3 as samples according to Examples and Comparative Examples, the balance being Fe and unavoidable impurities, and having undergone the processing shown in Tables 2 and 4 Was produced. Specifically, 150 kg of steel having each component composition was melted in a vacuum induction furnace and forged into a round bar of φ 20 mm at 1250 ° C. After that, it was normalized. Normalization was performed by heating at 950 ° C. for 1 hour and air cooling.

  The processed material obtained above was machined to produce a rough test piece of a tensile test piece shown in FIG. 3 (a) and a rough test piece of a delayed fracture test piece shown in FIG. 3 (c). And the quenching process was performed with respect to these rough-worked goods. The quenching treatment was performed by oil cooling after heating at 900 ° C. for 1 hour.

  Each rough-processed product that has undergone quenching is further machined to produce a tensile test specimen refined product shown in FIG. 3 (b) and a delayed fractured test specimen refined product shown in FIG. 3 (d). did.

  The surface layer forming process was performed on each of the obtained finely processed products. The surface layer formation was performed by nitriding treatment and subsequent heat treatment. The nitriding treatment was performed by heating the sample in an ammonia atmosphere. The heating temperature and the heating time are shown in each table. The heat treatment was performed under the atmosphere, heating temperature and heating time shown in each table. The nitriding treatment and the heat treatment were performed as continuous steps without cooling the sample between them.

<Evaluation method>
Each evaluation shown below was performed with respect to the test piece obtained as mentioned above. Each evaluation is performed at room temperature unless otherwise stated.

(Confirmation of the presence or absence of M6C carbide)
SEM observation was performed on the cross section of the bottom of the notch of the delayed fracture test piece. In the observation image, it was determined whether M6C carbide was present.

(Measurement of N concentration in surface layer)
Elemental analysis using an electron probe microanalyzer (EPMA) was performed on the cross section of the bottom of the notched portion of the delayed fracture test piece to measure the concentration of N atoms in the surface layer.

(Measurement of surface thickness)
The thickness of the surface layer was measured in the SEM image used to confirm the presence or absence of the M6C carbide.

(Measurement of tensile strength)
About the tensile strength test piece (JIS 14A) shown in FIG. 3 (b), the tensile test was conducted by the method according to JIS Z2241, and the tensile strength was measured.

(Evaluation of delayed fracture resistance)
For the delayed fracture test pieces shown in FIG. 3 (d), a cantilever bending test was conducted before and after the 30-hour hydrochloric acid dropping to determine a 30-hour delayed fracture strength ratio.

  The cantilever bending test was conducted by the method as schematically shown in FIG. That is, one end of the elongated test piece 1 was fixed by the holding tool 2 and the other end was fixed to one end of the arm 3 having a length of 400 mm. Then, a weight 4 was attached to the other end of the arm 3, and a bending stress was applied to the test piece 1 by the gravity acting on the weight 4. The value of the bending stress at the time of occurrence of breakage due to bending was recorded at the site where the notch in the center of the test piece 1 was provided, and this was taken as the bending breakage stress.

  Here, hydrochloric acid (HCl) was continued to be dropped to the center notch of the test piece 1 for 30 hours. Then, before and after dropping, the above-described cantilever bending test was performed to measure the bending breaking stress. The concentration of hydrochloric acid is 0.1 N, and the dropping rate is 0.2 ml / min. And

And the ratio of the breaking stress at break for 30 hours (σ ₃₀ ) after bending for 30 hours to the static bending breaking stress (σ _B ), which is the bending break stress before dropping the hydrochloric acid From this, the 30-hour delayed fracture strength ratio (σ ₃₀ / σ _B ) was calculated.

<Result>
The component composition of the steel member (base) concerning each Example and a comparative example is shown to Table 1, 3. Moreover, in Table 2, 4, the conditions of each process performed with respect to the steel member of each Example and a comparative example, and the obtained evaluation result are shown. In Tables 1 and 2, the example group showing the same level of tensile strength is shown divided by thick lines. The column of “specimen type” according to each comparative example in Tables 3 and 4 shows the characteristic of the component composition and the treatment process in comparison with the example group. Further, in the column of N concentration in surface layer and surface layer thickness in Table 4, the description of “−” indicates that N at a concentration higher than that of the base is not contained, and that there is no surface layer.

  In Comparative Example 1, the γ phase is not formed on the surface of the base since the nitriding treatment using the nitriding gas is not performed. On the other hand, in Comparative Example 2, since the heat treatment was not performed after the nitriding treatment, the γ phase was not formed on the surface of the matrix. In these comparative examples, the delayed fracture resistance is lowered corresponding to the absence of the surface layer consisting of γ phase on the surface of the matrix, and the 30-hour delayed fracture strength ratio is a small value far below 0.6. I am taking

  In Comparative Examples 3 to 5, the content of C is below the lower limit of 0.20% by mass. Moreover, in connection with it, the formula value (value of the left side) of Formula (1) is less than zero. As a result, the tensile strength of the steel member has a small value of less than 1200 MPa.

  In Comparative Examples 6 and 7, the value of the equation (2) is less than 42. Corresponding to that, the γ phase is not formed on the surface of the matrix. As a result, as in Comparative Examples 1 and 2, the delayed fracture resistance is lowered, and the 30-hour delayed fracture strength ratio takes a small value of less than 0.6.

  In Comparative Example 8, the heating temperature in the nitriding treatment and the heat treatment is as low as 475.degree. As a result, no γ phase is formed on the surface of the matrix. Also in Comparative Examples 9 and 10, the time of the nitriding treatment or the subsequent heat treatment is as short as 0.2 hours. As a result, again, no γ phase is formed on the surface of the matrix. Also in these comparative examples, the delayed fracture resistance is lowered corresponding to the absence of the surface layer consisting of the γ phase on the surface of the base, as in the case of the above-mentioned Comparative Examples 1, 2, 6, 7. The 30-hour delayed fracture strength ratio takes a small value of less than 0.6.

  In Comparative Examples 11 to 17, the contents of C, Si, P, S, V, Ti, and Nb exceed the predetermined upper limit values. Corresponding to that, the delayed fracture resistance of the steel member is lowered, and in any case, the 30 hour delayed fracture strength ratio is less than 0.6.

  In Comparative Examples 18 and 19, Al and N are not contained, respectively. Corresponding to that, the delayed fracture resistance of the steel member is lowered, and in any case, the 30 hour delayed fracture strength ratio is less than 0.6. This is interpreted that the old γ grains are coarsened and the structure is embrittled because Al or N is not contained.

  As described above, in each of the comparative examples, N atoms are contained at a concentration of 1 mass% or more due to the component composition of the steel member or the condition of the treatment, and the surface layer that takes the γ phase is not provided. Poor in delayed fracture characteristics. Or it is inferior in either the tensile strength as a whole steel member, or a delayed fracture characteristic due to the component composition.

  On the other hand, in each of the examples, N atoms are contained at a concentration of 1 mass% or more, and the γ phase is contained through an appropriate surface layer forming step having a predetermined component composition and comprising nitriding treatment and heat treatment. Has a surface to take. As a result, it has high tensile strength of about 1200 MPa or more and high delayed fracture resistance of about 0.6 or more at a 30 hour delayed fracture strength ratio.

  In each of the examples, focusing on the tensile strength, in Examples 44 and 45, the heating temperature in the heat treatment after the nitriding treatment is 750 ° C., which is higher than that in the other examples, and the tensile strength is Is relatively low. In the other embodiments, tensile strengths significantly exceeding 1200 MPa are obtained.

  In Tables 1 and 2, there is a tendency that the content of C increases and the value of Formula (1) increases as the examples below the table in which tensile strength increases. This indicates that the effect of enhancing the tensile strength of the steel member is enhanced as the content of C increases and as the value of the equation (1) increases. In particular, in the groups of Examples 28 to 43 in which the formula value of the formula (1) is as large as 4.8 or more in almost all the examples, particularly high tensile strength of 1900 MPa or more is obtained.

  Next, attention is paid to the delayed fracture resistance. In each example, as the concentration of N atoms in the surface layer becomes higher, there is a tendency that the 30-hour delayed fracture strength ratio becomes larger. This indicates that the presence of the surface layer having a high concentration of N atoms improves the delayed fracture resistance. In particular, when the concentration of N atoms in the surface layer is 3.0 mass% or more as in Examples 12, 13, 19, 20, 25 to 27, 31 to 33, 39 to 43, the table is shown. The value of the 30-hour delayed breaking strength ratio is large in each of the example groups having the same degree of tensile strength partitioned therein.

  Further, when the atmosphere in the heat treatment after the nitriding treatment is nitrogen, the concentration of N atoms in the surface layer is higher than that in the case of the air, and the 30-hour delayed fracture strength ratio is larger. For example, in the set of Example 10 and Example 12, and the set of Example 24 and Example 25, the component composition is similar, and the processing conditions are the same except for the atmosphere at the time of heat treatment. In the heat treatment in the nitrogen atmosphere, the concentration of N atoms in the surface layer is higher in Examples 12 and 25 in which the heat treatment is performed in the atmosphere than in Examples 10 and 24 in which the heat treatment is performed in the air. The intensity ratio is large. In each of the examples in which the concentration of N atoms in the surface layer listed above is 3.0 mass% or more, the heat treatment is performed in a nitrogen atmosphere.

  Furthermore, in each Example in which the formula value of Formula (3) exceeds 100, the formation of M6C carbide is confirmed. And in those cases, the value of the 30-hour delayed breaking strength ratio is relatively small. For example, in the combination of Example 23 and Example 24, the content of C is very close in the component composition, and the treatment conditions are the same, but the formula value of Formula (3) is separated by 100 across There is. M6C carbides are precipitated only in the case of Example 23 in which the formula value of the formula (3) exceeds 100. In the case of Example 23, the 30-hour delayed breaking strength ratio is smaller.

  The embodiments and examples of the present invention have been described above. The present invention is not limited to these embodiments and examples, and various modifications can be made.

Claims (10)

In mass%,
0.20% ≦ C ≦ 0.75%,
0.01% ≦ Si ≦ 2.00%,
0.10% ≦ Mn ≦ 5.00%,
P ≦ 0.05%,
S ≦ 0.05%,
0.01% ≦ Cu ≦ 2.00%,
0.01% ≦ Ni ≦ 3.00%,
0.01% ≦ Cr ≦ 3.00%,
0.01% ≦ Mo ≦ 2.00%,
Al ≦ 0.20% (but excluding 0%),
N ≦ 0.05% (but excluding 0%)
And the balance consists of Fe and unavoidable impurities,
A base composed of a martensite phase or a mixed phase of a martensite phase and a bainite phase;
A steel member, which occupies the surface of the base, contains 1% by mass or more of N, and a surface layer made of a γ phase.   The steel member according to claim 1, having a tensile strength of 1200 MPa or more. The content of each element is, in terms of mass%,
4.4 [C] + 1.2 [Si]-0.7 [Mn] + 0.1 [Cu] + 1.2 [Ni]-0.2 [Cr] + 2.0 [Mo] 0 0
The steel member according to claim 1 or 2, characterized by satisfying the following relationship. The content of each element is, in terms of mass%,
0.4 [C] -26.3 [Si] +67.0 [Mn] +9.8 [Cu] +53.7 [Ni] -16.2 [Cr] +13.1 [Mo] ≧ 42
The steel member according to any one of claims 1 to 3, satisfying the following relationship: Furthermore, in mass%,
V ≦ 1.0%,
Ti ≦ 1.0%,
Nb ≦ 1.0%
The steel member according to any one of claims 1 to 4, containing at least one selected from the group consisting of The content of each element is, in terms of mass%,
-37.8 [C] + 71.7 [Si] + 0.7 [Mn]-14.5 [Cu]-4.5 [Ni] + 0.8 [Cr] + 80.5 [Mo] <100
The steel member according to any one of claims 1 to 5, satisfying the following relationship.   The steel member according to any one of claims 1 to 6, wherein the content of N in the surface layer is 3% by mass or more.   The steel member according to any one of claims 1 to 7, which is a fastening member. After forming steel material and hardening treatment,
N atoms are introduced to the surface of the steel by nitriding treatment using a nitriding gas,
Furthermore, by removing the nitriding gas and performing heat treatment on the steel material,
A method for producing a steel member, comprising producing the steel member according to any one of claims 1 to 8.   The method for manufacturing a steel member according to claim 9, wherein the heat treatment is performed in a nitrogen atmosphere.