JP2009299180A - High strength steel having excellent delayed fracture resistance, high strength bolt, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel having excellent delayed fracture resistance even in an environment where corrosion is severe, and to provide a method for producing the high strength steel. <P>SOLUTION: The high strength steel having excellent delayed fracture resistance is characterized in that the depth of a nitrided layer in the surface is ≥200 μm. Particularly, in the case the compressive residual quantity of the surface is ≥200 MPa, its delayed fracture resistance improves. The steel comprises proper amounts of C, Si and Mn, comprises one or two kinds selected from V and Mo, also satisfies the relation of V+Mo/2>0.4%, and further comprises one or more kinds selected from Cr, Nb, Cu, Ni and B. By increasing the nitrogen concentration by ≥0.02% than the average nitrogen concentration to a depth of at least ≥200 μm from the surface of the steel, the intrusion of diffusible hydrogen is remarkably suppressed, and its delayed fracture resistance improves. Further, since the carbides, nitrides and carbonitrides of V and Mo are finely precipitated, intruded diffusible hydrogen is made harmless, so as to increase a limit diffusible hydrogen content. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel material and a bolt excellent in delayed fracture resistance, particularly a high strength steel material (wire rod, PC steel rod) having a tensile strength of 1300 MPa or more and excellent in delayed fracture resistance, a high strength bolt, and a method for producing the same. It is about.

  High strength steels used in many machines, automobiles, bridges, and buildings are medium carbon steels with a C content of 0.20 to 0.35%, such as JIS G 4104 and JIS G 4105, SCr. , SCM or the like is used to perform a tempering process. However, it is well known that the risk of delayed fracture increases when the tensile strength exceeds 1300 MPa for any steel type.

  As a technique for improving the delayed fracture resistance of high-strength steel, a method of making the structure bainite is effective, and steel in which the prior austenite grains are further refined is disclosed in Patent Document 1, and steel in which segregation of steel components is suppressed. It is disclosed in Patent Documents 2 and 3. However, while the bainite structure contributes to the improvement of delayed fracture resistance, there is a problem that the alloy cost and the heat treatment cost are increased in order to form the bainite structure. In addition, due to the refinement of prior austenite grains and the suppression of component segregation, the delayed fracture resistance has not been significantly improved.

  Further, Patent Documents 4 to 6 disclose improvement in delayed fracture resistance by strong wire drawing pearlite. However, it is possible to increase the cost by wire drawing or to manufacture a wire having a large diameter. Have difficulty.

  As described above, the conventional technology has a limit in producing high-strength steel with greatly improved delayed fracture resistance at low cost.

Japanese Patent Publication No. 64-4566 JP-A-3-243744 JP-A-3-243745 JP 2000-337332 A JP 2000-337333 A JP 2000-337334 A

  In addition, even if fine precipitates are dispersed and hydrogen trapped, it is difficult to suppress delayed fracture resistance in a corrosive environment where there is a large amount of hydrogen entering from the outside.

  This invention is made in view of said subject, Comprising: It aims at provision of the high strength steel materials (wire rod, PC steel bar), the high strength bolt which have the outstanding delayed fracture resistance, and its manufacturing method. It is.

The gist of the present invention is as follows.
(1) In mass%,
C: 0.10 to 0.55%,
Si: 0.01 to 3%,
Mn: 0.1 to 2%,
In addition,
V: 1.5% or less,
Mo: 3.0% or less,
1 type or 2 types, and the contents of V and Mo are
V + 1 / 2Mo> 0.4%
Meets the requirements of
Cr: 0.05 to 1.5%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%,
Ni: 0.01-4%,
B: 0.0001 to 0.005%,
In which the balance is composed of Fe and inevitable impurities, and the structure is mainly tempered martensite, and the nitrogen concentration from the surface to at least 200 μm depth is 0.02% or more higher than the average nitrogen concentration. High strength steel with excellent delayed fracture resistance.
(2) Furthermore,
Al: 0.003 to 0.1%,
Ti: 0.003 to 0.05%,
Mg: 0.0003 to 0.01%
Ca: 0.0003 to 0.01%,
Zr: 0.0003 to 0.01%
The high strength steel material having excellent delayed fracture resistance according to (1), characterized by containing one or more of the following.
(3) The surface has a nitride layer having a nitrogen concentration of 0.02% or more higher than the average nitrogen concentration, and the depth of the nitride layer is 200 μm or more and 1000 μm or less from the surface (1) or A high-strength steel material having excellent delayed fracture resistance as described in (2).
(4) The high strength steel material having excellent delayed fracture resistance according to any one of (1) to (3), wherein the area ratio of tempered martensite is 85% or more.
(5) The high-strength steel material having excellent delayed fracture resistance according to any one of (1) to (4), wherein the compressive residual stress on the steel material surface is 200 MPa or more.
(6) In mass%,
C: 0.10 to 0.55%,
Si: 0.01 to 3%,
Mn: 0.1 to 2%,
In addition,
V: 1.5% or less,
Mo: 3.0% or less,
1 type or 2 types, and the contents of V and Mo are
V + 1 / 2Mo> 0.4%
Meets the requirements of
Cr: 0.05 to 1.5%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%,
Ni: 0.01-4%,
B: 0.0001 to 0.005%,
In which the balance is composed of Fe and inevitable impurities, and the structure is mainly tempered martensite, and the nitrogen concentration from the surface to at least 200 μm depth is 0.02% or more higher than the average nitrogen concentration. High strength bolt with excellent delayed fracture resistance.
(7) Furthermore,
Al: 0.003 to 0.1%,
Ti: 0.003 to 0.05%,
Mg: 0.0003 to 0.01%
Ca: 0.0003 to 0.01%,
Zr: 0.0003 to 0.01%
The high-strength bolt excellent in delayed fracture resistance according to (6), characterized by containing one or more of the above.
(8) The surface has a nitride layer having a nitrogen concentration of 0.02% or more higher than the average nitrogen concentration, and the depth of the nitride layer is 200 μm or more and 1000 μm or less from the surface (6) or A high-strength bolt excellent in delayed fracture resistance as described in (7).
(9) The high-strength bolt excellent in delayed fracture resistance according to any one of (6) to (8), wherein the area ratio of tempered martensite is 85% or more.
(10) The high-strength bolt excellent in delayed fracture resistance according to any one of (6) to (9), wherein the compressive residual stress on the surface of the steel material is 200 MPa or more.
(11) The method for producing a high-strength steel material according to any one of (1) to (5), wherein the steel material having the component according to (1) or (2) is processed into a desired shape. A method for producing a high-strength steel material excellent in delayed fracture resistance, characterized by nitriding the steel material at a nitriding temperature exceeding 500 ° C. and not exceeding 650 ° C.
(12) The method for manufacturing a bolt according to any one of (6) to (10), wherein after the steel material having the component according to (5) or (6) is processed into a bolt, the bolt is A method for producing a high-strength bolt excellent in delayed fracture resistance, characterized by performing nitriding at a nitriding temperature exceeding 500 ° C. and not exceeding 650 ° C.

  The present invention makes it possible to provide high-strength steel materials (wire rods, PC steel bars), high-strength bolts and manufacturing methods thereof that maintain delayed fracture resistance even in severe corrosion environments, and the industrial contribution is extremely remarkable. It is.

It is a hydrogen release curve of the hydrogen analysis by a temperature rising method. It is a nitrogen concentration profile by EDX. It is a top view of the test piece used for the delayed fracture test of steel materials. It is explanatory drawing of a delayed fracture test apparatus. It is the temperature and humidity pattern of a corrosion acceleration test. It is explanatory drawing of the amount of limit diffusible hydrogen.

  It is known that delayed fracture of steel involves hydrogen in steel. In addition, it is known that the process of hydrogen intrusion into steel is accompanied by corrosion during actual environment use. The invading diffusible hydrogen diffuses into the tensile stress concentration part and causes delayed fracture.

  FIG. 1 schematically shows a temperature-hydrogen release rate curve obtained when a steel material is heated at a rate of temperature increase of 100 ° C./h. Diffusible hydrogen peaks in the vicinity of 100 ° C. in FIG. It has something. In the present invention, the amount of hydrogen measured from room temperature to 400 ° C. is defined as the amount of diffusible hydrogen when the sample is heated.

  When the amount of diffusible hydrogen is small, delayed fracture does not occur, and when the amount of diffusible hydrogen is large, delayed fracture occurs. The minimum amount of diffusible hydrogen at which delayed fracture occurs is referred to herein as the limit diffusible hydrogen amount. The amount of critical diffusible hydrogen varies depending on the steel. A higher limit diffusible hydrogen amount is preferable because delayed fracture is less likely to occur.

  The present inventor performed various nitriding treatments on various high-strength steel materials, and examined hydrogen penetration characteristics and delayed fracture resistance characteristics through corrosion acceleration tests and exposure tests. As a result, steel in which V or Mo or their carbides, nitrides and carbonitrides are finely precipitated increases the amount of critical diffusible hydrogen by detoxifying the invading diffusible hydrogen. However, when the strength is further increased, the amount of intrusion hydrogen from the environment increases, and the amount of intrusion hydrogen increases compared to the amount of limit diffusible hydrogen, which causes a problem of delayed fracture.

  On the other hand, the surface of the steel material is nitrided to form a nitrided layer, specifically, the nitrogen concentration is increased by 0.02% or more from the average nitrogen concentration from the surface of the steel material to a depth of at least 200 μm or more. Thus, it was found that invasion of diffusible hydrogen is greatly suppressed and the delayed fracture resistance is improved. Further, it has been found that rapid cooling after nitriding treatment generates compressive residual stress on the surface and improves delayed fracture resistance. In particular, in a high-strength bolt in which a steel material is processed and strain is introduced into the surface layer, the formation of a nitrided layer is promoted and the nitrogen concentration is increased, so that the delayed fracture resistance is significantly improved.

  Hereinafter, the present invention will be described in detail.

  The high-strength steel material and high-strength bolt of the present invention are made of steel having a predetermined component, and a nitride layer is formed on the surface thereof.

  First, the nitride layer on the surface will be described. As a result of various studies, it has been found that the amount of hydrogen intrusion is greatly reduced when a region where the nitrogen concentration is 0.02% or more higher than the average nitrogen concentration is 200 μm or more from the surface. Therefore, the region where nitrogen is high is determined as a region where the nitrogen concentration is 0.02% or more higher than the average nitrogen concentration. And the area | region with high nitrogen concentration was limited to 200 micrometers or more from the surface. The upper limit of the thickness of the nitride layer is not specified, but if it exceeds 1000 μm, there is a problem that the productivity is lowered and the cost is increased.

  A steel material (wire or PC steel rod) made of such steel and having a nitride layer on its surface, a cross section of a bolt is polished, and an energy dispersive X-ray fluorescence analyzer (referred to as EDX) or wavelength dispersive X-ray fluorescence X When line analysis is performed with a line analyzer (referred to as WDS), the nitrogen concentration decreases from the surface layer portion toward the center portion as shown in FIG. 2, and the nitrogen concentration from the surface to a certain depth is 0.02 from the average nitrogen concentration. Since the profile becomes higher by at least%, it is possible to discriminate a range where the surface nitrogen concentration is 0.02% higher than the average nitrogen concentration. The distance from the surface of the region where the nitrogen concentration in FIG. 2 is 0.02% or more higher than the average nitrogen concentration is defined as the nitrided layer depth.

  The measurement of the nitride layer depth can be obtained as a simple average of any five locations.

  The average nitrogen concentration is the amount of nitrogen contained in the steel material or bolt before nitriding. Therefore, the average nitrogen concentration of the steel material or bolt after nitriding can be obtained as the nitrogen concentration at a depth where the numerical value is constant by measuring the nitrogen concentration from the surface by EDX or WDS. The nitrogen concentration may be measured at a depth of 2000 μm or more from the surface. It is also possible to grind 2000 μm or more from the surface, collect an analytical sample, and measure the average nitrogen concentration by chemical analysis.

  Next, the reason which limited the component of steel materials is demonstrated.

  C: C is an essential element for securing the strength of the steel material, but if it is less than 0.10%, the required strength cannot be obtained, and if it exceeds 0.55%, ductility and toughness are reduced. Also, delayed fracture resistance is reduced. Therefore, the C content is limited to a range of 0.10 to 0.55%.

  Si: Si is an element that increases the strength by the solid solution hardening action, but if the Si content is less than 0.01%, the effect is insufficient, and if it exceeds 3%, the effect is saturated. Therefore, the Si content is limited to 0.01 to 3%.

  Mn: Mn is not only necessary for deoxidation and desulfurization, but also to increase the hardenability for obtaining a martensite structure, and to obtain high strength by lowering the transformation temperature of a pearlite structure and a bainite structure. It is an effective element. However, if the Mn content is less than 0.1%, the effect is insufficient, and if it exceeds 2%, segregation occurs at the grain boundary during austenite heating, embrittles the grain boundary, and degrades delayed fracture resistance. Let Therefore, the Mn content is limited to a range of 0.1 to 2%.

  The present invention contains one or two of V and Mo as follows.

  V: V is an effective element for increasing the hardenability for obtaining a martensite structure and increasing the softening resistance during tempering, and for obtaining high strength by lowering the transformation temperature of a pearlite structure and a bainite structure. is there. Further, V precipitates finely as carbide, nitride or carbonitride during tempering to increase the strength of the steel material, and traps diffusible hydrogen contributing to delayed fracture and renders it harmless. If the V content exceeds 1.5%, the effect becomes saturated. Therefore, the content of V is set to a range of 1.5% or less.

  Mo: To increase the hardenability for obtaining the martensite structure and increase the softening resistance during the tempering treatment as in the case of V, and to obtain high strength by lowering the transformation temperature of the pearlite structure and the bainite structure. It is an effective element. Moreover, Mo precipitates finely as carbide, nitride, or carbonitride during tempering to increase the strength of the steel material, and traps diffusible hydrogen that contributes to delayed fracture and renders it harmless. If the Mo content exceeds 3.0%, the effect becomes saturated. Therefore, the Mo content is set to a range of 3.0% or less.

Furthermore, it contains one or two of V and Mo, and the contents of V and Mo are
V + 1 / 2Mo> 0.4%
It is necessary to satisfy the following conditions. Both V and Mo precipitate as carbides, nitrides or carbonitrides, and trap diffusible hydrogen. In the case of V alone, the effect cannot be sufficiently obtained when the V content is 0.4% by mass or less. In the case of Mo alone, if the Mo content is less than 0.8% by mass, the effect cannot be sufficiently obtained. And when it contains both V and Mo, the effect cannot fully be acquired as content of V and Mo is V + 1 / 2Mo <= 0.4%. Therefore, the contents of V and Mo are set to V + 1 / 2Mo> 0.4%.

  Further, for the purpose of increasing the strength, one or more of Cr, Nb, Cu, Ni and B are contained.

  Cr: Cr is an effective element for increasing the hardenability for obtaining the martensite structure and increasing the softening resistance during the tempering process, and for obtaining high strength by lowering the transformation temperature of the pearlite structure and the bainite structure. is there. If the Cr content is less than 0.05%, it is difficult to obtain the effect sufficiently, and if it exceeds 1.5%, the toughness may be deteriorated. Therefore, the Cr content is in the range of 0.05 to 1.5%.

  Nb: Nb, like Cr, V, and Mo, increases the hardenability for obtaining a martensite structure and increases the softening resistance during tempering, and lowers the transformation temperature of the pearlite structure and bainite structure to increase the strength. It is an effective element to obtain. If the Nb content is less than 0.001%, it is difficult to obtain the effect sufficiently, and if it exceeds 0.05%, the effect is saturated. Therefore, the Nb content is set to 0.001 to 0.05%.

  By adding Cu: Cu, it is possible to improve the hardenability, increase the temper softening resistance, and increase the strength by the precipitation effect. However, if the Cu content is less than 0.01%, it is difficult to obtain the effect sufficiently, and if it exceeds 4%, grain boundary embrittlement may occur and the delayed fracture resistance may be deteriorated. Therefore, the Cu content is in the range of 0.01 to 4%.

  Ni: Ni has the effect of improving hardenability and improving ductility that decreases with increasing strength. However, if the Ni content is less than 0.01%, it is difficult to obtain the effect sufficiently, and even if the content exceeds 4%, the effect is saturated. Therefore, the Ni content is in the range of 0.01 to 4%.

  B: B has an effect of suppressing grain boundary fracture and improving delayed fracture resistance. Further, B segregates at the austenite grain boundaries and remarkably increases the hardenability. However, if the content of B is less than 0.0001%, it is difficult to obtain the effect sufficiently, and if it exceeds 0.005%, B carbide or Fe carbon boride is generated at the grain boundary to cause grain boundary embrittlement. As a result, delayed fracture resistance decreases. Therefore, the B content is in the range of 0.0001 to 0.005%.

  Furthermore, for the purpose of refining the structure, one or more of Al, Ti, Mg, Ca and Zr can be contained.

  Al: Al forms Al oxide and Al nitride by deoxidation and heat treatment, and prevents austenite grains from coarsening. As a result, the effect of suppressing the deterioration of the delayed fracture resistance is exhibited. This effect is slightly insufficient when the Al content is less than 0.003%, and is saturated when the content exceeds 0.1%. Therefore, it is preferable to make Al content into the range of 0.003-0.1%.

  Ti: Ti is an element that forms oxides and nitrides similarly to Al, prevents coarsening of austenite grains, and suppresses deterioration of delayed fracture resistance. This effect is somewhat insufficient if the Ti content is less than 0.003%, and if it exceeds 0.05%, coarse Ti carbonitrides become coarse during heating for rolling, processing or heat treatment, and toughness is increased. descend. Therefore, the Ti content is preferably in the range of 0.003 to 0.05%.

  Mg: Mg has deoxidation and desulfurization effects, and forms Mg oxide, Mg sulfide, Mg-Al, Mg-Ti, Mg-Al-Ti composite oxide and composite sulfide, etc. Prevent coarsening of austenite grains. Thereby, although there exists an effect which suppresses deterioration of a delayed fracture resistance, this effect is a little inadequate when content of Mg is less than 0.0003%, and will be saturated when it exceeds 0.01%. Therefore, the Mg content is preferably in the range of 0.0003 to 0.01%.

  Ca: Ca has a deoxidizing and desulfurizing effect, and also prevents the austenite grains from coarsening by forming Ca oxide, Ca sulfide, Al, Ti, Mg composite oxide or composite sulfide. Suppresses the degradation of delayed fracture resistance. This effect is slightly insufficient when the Ca content is less than 0.0003%, and is saturated when it exceeds 0.01%. Therefore, the Ca content is preferably in the range of 0.0003 to 0.01%.

  Zr: Zr forms Zr oxides, Zr sulfides, complex oxides and composite sulfides of Al, Ti, Mg, Zr, etc., prevents austenite grains from coarsening and suppresses deterioration of delayed fracture resistance To do. This effect is somewhat insufficient when the Zr content is less than 0.0003%. On the other hand, the effect is saturated even if Zr is contained in excess of 0.01%. Therefore, the Zr content is preferably in the range of 0.0003 to 0.01%.

  Next, the organization form of the present invention will be described.

The structure form of the high-strength steel material of the present invention is a structure mainly composed of tempered martensite. An tempered martensite structure having a balance of one or more of retained austenite, bainite, pearlite, and ferrite in the tempered martensite structure is defined as a tempered martensite-based structure. It is possible to obtain such a structure by reheating from Ac ₁ transformation temperature to Ac ₃ transformation temperature + 250 ° C., or by quenching after hot rolling and tempering at a temperature of more than 500 ° C. to 650 ° C. or by nitriding. it can. For the measurement of the martensite structure ratio, an average value obtained by performing nital etching on a polished C cross section and measuring five visual fields in a range of 0.04 mm ² with an optical microscope can be used. Since the structure of the present invention is mainly tempered martensite, the ductility and toughness are good at a tensile strength of 1300 MPa or more.

  The compressive residual stress on the steel surface is generated by rapid cooling after nitriding, and improves delayed fracture resistance. In particular, when a compressive residual stress of 200 MPa or more is generated, the delayed fracture resistance is remarkably improved. Therefore, the surface compressive residual stress is preferably 200 MPa or more. The residual stress can be measured using an X-ray residual stress measuring device. In the measurement, after measuring the residual stress on the surface, etching is performed by 25 μm at a time by electrolytic polishing, and the residual stress in the depth direction is measured. Residual stress measurement measures arbitrary three places and uses the average value.

  If the tensile strength of the steel material is 1300 MPa or more, the frequency of occurrence of delayed fracture increases remarkably, so that the effect of improving delayed fracture resistance by forming a nitrided layer on the steel material surface becomes significant. It is technically difficult for the upper limit of the tensile strength to exceed 2200 MPa. The tensile strength may be measured according to JIS Z 2241.

  The critical diffusible hydrogen content of delayed fracture was determined by injecting hydrogen into the delayed fracture specimen shown in FIG. 3 and then applying a load of 90% of the tensile strength with the tester shown in FIG. The upper limit is the amount of diffusible hydrogen when not. In the testing machine shown in FIG. 4, when applying a tensile load to the test piece 1, the balance weight 2 is installed at one end of the lever having the fulcrum 3 as a fulcrum, and the test piece 1 is installed at the other end. . Hydrogen charging for allowing hydrogen to enter the delayed fracture test piece is performed by an electrolytic charging method. In addition, after hydrogen charging, Cd plating was applied to the surface of the test piece in order to prevent escape of diffusible hydrogen, and the sample was left at room temperature for 3 hours in order to homogenize the hydrogen concentration inside the test piece. The amount of diffusible hydrogen is obtained by measuring the integrated value of the amount of hydrogen released from room temperature to 400 ° C. by gas chromatography using a sample heated at 100 ° C./h.

  Next, the manufacturing method of the high strength steel materials and high strength bolts of the present invention will be described.

  The manufacturing method of the high-strength steel material of the present invention is a method in which steel comprising a predetermined component is melted in accordance with a conventional method, and is cast into a steel piece by casting, and then heated to hot work or in addition to this, a steel material (wire, PC steel rod) and a nitride layer is formed on the surface. After hot working, cold working and heat treatment may be appropriately performed. Moreover, the manufacturing method of the high intensity | strength bolt of this invention uses the steel which consists of a predetermined component as a wire according to a conventional method, makes it a bolt by cold or warm processing, and forms a nitrided layer on the surface.

  The nitride layer on the surface of the steel can be formed using a general nitriding method such as “gas nitriding method”, “gas soft nitriding method”, “plasma nitriding method”, “salt bath nitriding method” and the like.

  The temperature during the nitriding treatment is also performed by finely depositing V, Mo carbide, nitride or carbonitride. If it is 500 ° C. or lower, V, Mo carbide, nitride or carbonitride is not sufficiently precipitated, and if it exceeds 650 ° C., the strength is insufficient. It limited to 650 degrees C or less.

  When the nitriding time is less than 1 hour, a region with a high nitrogen concentration cannot be obtained from the surface to a depth of 200 μm, so that it is 1 hour or longer. Moreover, although the upper limit of processing time is not prescribed | regulated, since a merit is not acquired if it exceeds 12 hours, it is preferable to set it as 12 hours or less.

Steel having the chemical composition shown in Table 1 was melted and hot worked according to a conventional method to obtain a wire rod. The steel material was quenched by heating at 850 ° C. to 1000 ° C., which is a temperature within the range of Ac ₁ transformation temperature to Ac ₃ transformation temperature + 250 ° C., and then a nitride layer was formed by gas soft nitriding. In gas soft nitriding, the treatment was performed under the conditions of a temperature range of 500 to 650 ° C., an ammonia volume ratio in the treatment gas atmosphere of 30 to 50%, and a treatment time of 1 to 12 hours. The nitriding temperature was the temperature shown in Table 2. Further, the hot-worked wire was processed into a bolt, quenched under the same conditions, and a nitrided layer was formed by gas soft nitriding.

The average tempered martensite ratio was obtained by performing nital etching on the polished C cross section and measuring five visual fields in the range of 0.04 mm ² with an optical microscope. The remaining structure of the tempered martensite was one or more of retained austenite, bainite, pearlite, and ferrite.

  The tensile strength was measured according to JIS Z 2241.

  Polish the cross section of the sample, analyze any five locations with EDX, measure the nitrogen concentration distribution from the surface layer, and determine the depth of the region where the nitrogen concentration is 0.02% or more higher than the average nitrogen concentration as “nitride layer depth” " In addition, the difference between the average nitrogen concentration and the nitrogen concentration at a depth of 200 μm from the surface layer was measured. In all cases, the average value at five locations was taken as the measured value.

  The residual stress was measured using an X-ray residual stress measuring device. In the measurement, after measuring the residual stress on the surface, etching was performed by 25 μm by electrolytic polishing, and the residual stress in the depth direction was measured. Residual stress measurement was performed at three arbitrary locations and the average value was used.

  The amount of intrusion hydrogen was determined by measuring 30 cycles of the corrosion test with the pattern shown in FIG. 5, removing the corrosive layer on the surface with sand blasting, performing hydrogen analysis with a temperature rising method, and measuring hydrogen from room temperature to 400 ° C. Amount.

  The critical diffusible hydrogen content of delayed fracture was such that the test piece of FIG. 3 was charged with hydrogen, the surface was Cd-plated, and allowed to stand at room temperature for 96 hours, and 90% of the tensile strength was measured with the tester shown in FIG. A constant load delayed fracture test was performed under load, and in the graph of rupture time-diffusible hydrogen amount schematically shown in FIG. 6, the maximum value of the diffusible hydrogen amount of the test piece that did not break after 100 hours or more did. In addition, hydrogen charge was performed using the electrolytic hydrogen charge method, and the hydrogen level was changed by the charge current.

  Comparing the amount of penetrating hydrogen and the amount of critical diffusible hydrogen for delayed fracture, if the amount of critical diffusible hydrogen is greater than the amount of penetrating hydrogen, delayed fracture does not occur, and conversely, the amount of critical diffusible hydrogen is smaller Will cause delayed fracture. Therefore, the delayed fracture resistance was evaluated based on the amount of invading hydrogen and the amount of critical diffusible hydrogen.

  Table 2 shows the test results of the steel materials. No. in Table 2 Nos. 1 to 15 have a surface nitride layer (layer whose nitrogen concentration is 0.02% or more higher than the average nitrogen concentration) thickness of 200 μm or more, and the difference between the average nitrogen concentration and the nitrogen concentration at a depth of 200 μm is 0.1. 02% or more, all have tensile strength of 1300 MPa or more, intrusion hydrogen amount in corrosion promotion test is 1.0 ppm or less, limit diffusible hydrogen amount is 2.0 ppm or more, and limit diffusible hydrogen is more than intrusion hydrogen amount. The amount is larger.

  On the other hand, No. which is a comparative example. No. 16 is an example in which the amount of C, Si, and Mn is small and the strength is low. No. 17 is the amount of C. 18 is the amount of Mn. No. 19 is an example in which the delayed fracture resistance is deteriorated due to the large amount of Cr. No. No. 20 is an example in which the content of V + Mo / 2 is less than 0.4% and hydrogen trapping cannot be sufficiently performed, so that the limit diffusible hydrogen amount is lower than the intrusion hydrogen amount and the delayed fracture resistance is deteriorated. No. 21 is the amount of Cu. 22 is the amount of Ni. No. 23 is an example in which the delayed fracture resistance is deteriorated because the amount of B is large. No. No. 24 is an example in which the nitrogen concentration in the vicinity of the surface is 0.02% or more higher than the average nitrogen concentration, but the thickness of the nitride layer is less than 200 μm, the amount of invading hydrogen is large, and the delayed fracture resistance is deteriorated. No. No. 25 is an example in which the thickness of the nitrogen enriched layer by the nitriding treatment was less than 200 μm, and the amount of invading hydrogen was large and the delayed fracture resistance was deteriorated.

  Furthermore, the test results of the bolts are shown in Table 3. The tempered martensite ratio, tensile strength, nitrided layer depth, residual stress, hydrogen penetration amount, and critical diffusion hydrogen amount of delayed fracture were measured in the same manner as steel materials. From Table 2 and Table 3, No. No. 1 to 15 were processed into bolts and then nitrided. 26 to 40 show that the amount of invading hydrogen is further suppressed as compared with the wire.

1 test piece 2 balance weight 3 fulcrum

Claims (12)

% By mass
C: 0.10 to 0.55%,
Si: 0.01 to 3%,
Mn: 0.1 to 2%,
In addition,
V: 1.5% or less,
Mo: 3.0% or less,
1 type or 2 types, and the contents of V and Mo are
V + 1 / 2Mo> 0.4%
Meets the requirements of
Cr: 0.05 to 1.5%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%,
Ni: 0.01-4%,
B: 0.0001 to 0.005%,
In which the balance is composed of Fe and inevitable impurities, and the structure is mainly tempered martensite, and the nitrogen concentration from the surface to at least 200 μm depth is 0.02% or more higher than the average nitrogen concentration. High strength steel with excellent delayed fracture resistance. further,
Al: 0.003 to 0.1%,
Ti: 0.003 to 0.05%,
Mg: 0.0003 to 0.01%
Ca: 0.0003 to 0.01%,
Zr: 0.0003 to 0.01%
The high-strength steel material having excellent delayed fracture resistance according to claim 1, comprising one or more of the following.   3. The surface according to claim 1, wherein the surface has a nitride layer having a nitrogen concentration of 0.02% or more higher than the average nitrogen concentration, and the depth of the nitride layer is 200 μm or more and 1000 μm or less from the surface. High strength steel with excellent delayed fracture resistance.   The high-strength steel material excellent in delayed fracture resistance according to any one of claims 1 to 3, wherein the area ratio of tempered martensite is 85% or more.   The high-strength steel material excellent in delayed fracture resistance according to any one of claims 1 to 4, wherein the compressive residual stress on the steel material surface is 200 MPa or more. % By mass
C: 0.10 to 0.55%,
Si: 0.01 to 3%,
Mn: 0.1 to 2%,
In addition,
V: 1.5% or less,
Mo: 3.0% or less,
1 type or 2 types, and the contents of V and Mo are
V + 1 / 2Mo> 0.4%
Meets the requirements of
Cr: 0.05 to 1.5%,
Nb: 0.001 to 0.05%,
Cu: 0.01 to 4%,
Ni: 0.01-4%,
B: 0.0001 to 0.005%,
In which the balance is composed of Fe and inevitable impurities, and the structure is mainly tempered martensite, and the nitrogen concentration from the surface to at least 200 μm depth is 0.02% or more higher than the average nitrogen concentration. High strength bolt with excellent delayed fracture resistance. further,
Al: 0.003 to 0.1%,
Ti: 0.003 to 0.05%,
Mg: 0.0003 to 0.01%
Ca: 0.0003 to 0.01%,
Zr: 0.0003 to 0.01%
The high-strength bolt excellent in delayed fracture resistance according to claim 6, comprising one or more of the following.   8. The surface according to claim 6, wherein the surface has a nitride layer having a nitrogen concentration of 0.02% or more higher than the average nitrogen concentration, and the depth of the nitride layer is 200 μm or more and 1000 μm or less from the surface. High strength bolt with excellent delayed fracture resistance.   The high strength bolt excellent in delayed fracture resistance according to any one of claims 6 to 8, wherein the area ratio of tempered martensite is 85% or more.   The high-strength bolt excellent in delayed fracture resistance according to any one of claims 6 to 9, wherein the compressive residual stress on the surface of the steel material is 200 MPa or more.   It is a manufacturing method of the high strength steel materials of any one of Claims 1-5, Comprising: After processing the steel materials which have the component of Claim 1 or 2 into a desired shape, this steel materials are nitriding temperature Is a method for producing a high-strength steel material excellent in delayed fracture resistance, characterized by nitriding at a temperature exceeding 500 ° C. and not exceeding 650 ° C.   It is a manufacturing method of the volt | bolt of any one of Claims 6-10, Comprising: After processing the steel material which has a component of Claim 5 or 6 into a volt | bolt, the nitriding temperature of this bolt is 500 degreeC. A method for producing a high-strength bolt excellent in delayed fracture resistance, characterized by nitriding at a temperature exceeding 650 ° C.

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