JP4411253B2 - Hot forged parts with excellent delayed fracture resistance and method for producing the same - Google Patents

Description

  The present invention relates to a hot forged part excellent in delayed fracture resistance and a manufacturing method thereof, and more particularly to a hot forged part excellent in delayed fracture resistance having a tensile strength of 1600 MPa class or more and a manufacturing method thereof. is there.

  High-strength steel used in various industrial fields such as automobiles, machinery, bridges, civil engineering and construction is chrome steel (SCr) and chrome molybdenum steel (SCM) defined in JIS G 4104 and JIS G 4105, for example. The medium carbon steel has a C concentration of 0.30 to 0.45% by mass, and is manufactured by quenching and tempering the steel. However, it is well known that the above steel materials have an increased risk of hydrogen embrittlement when the tensile strength exceeds 1300 MPa, in particular, the risk of delayed fracture due to hydrogen entering from the environment during use. Therefore, for example, in the case of construction, the upper limit is a steel material having a tensile strength of 1150 MPa class.

  As conventional knowledge for improving delayed fracture resistance of high-strength steel, for example, in the invention described in Patent Document 1, it is proposed that it is effective to refine prior austenite grains and to bainite the structure. Yes.

  In addition to the invention described in Patent Document 1, Patent Documents 2 to 4 propose inventions related to the refinement technology of prior austenite grains. It has already been pointed out in Patent Document 8 that this has not been reached. Further, in Patent Documents 5 to 7, the austenite grains are elongated and the intergranular spacing is narrowed with respect to the direction of propagation of fracture cracks, thereby exhibiting substantially the same effect as the refinement mechanism, and delayed fracture resistance. An invention for improving the above has been proposed. However, these inventions do not mention high-strength steel having a tensile strength of 1600 MPa or more, and the effect of improving delayed fracture resistance in high-strength steel remains unknown.

  In addition, the method of bainiteizing the structure has an effect of improving the delayed fracture resistance, but raises the manufacturing cost required for the bainite treatment.

  Further, in Patent Document 8 described above, a critical hydrogen amount (limited diffusion hydrogen amount) in which delayed fracture occurs by dispersing and distributing oxides, carbides, nitrides alone or composite precipitates that trap hydrogen in steel. ) Has been proposed to improve delayed fracture resistance.

  According to the present invention, it can be said that one of the mechanisms for improving delayed fracture resistance is the technical idea of utilizing carbides produced by quenching and tempering treatment, but the component that effectively utilizes the effect to the maximum The method of limiting the design and manufacturing method was not clearly limited.

Japanese Patent Publication No. 03-243744 Japanese Examined Patent Publication No. 61-064815 Japanese Patent Publication No. 64-004566 Japanese Patent Publication No. 03-243745 Japanese Patent Application Laid-Open No. 09-078191 Japanese Patent Application Laid-Open No. 09-078192 Japanese Patent Application Laid-Open No. 09-078193 JP 2000-026934 A

  As shown in the above-described situation so far, there is a limit to the production of high-strength steel with drastically improved delayed fracture characteristics in the prior art.

  Therefore, the present invention is typified by delayed fracture, which appears as a problem especially as the strength is increased, by finding the essential components for solving this problem, the manufacturing method and the essential condition of the microstructure form, and the manufacturing process conditions for realizing it. The present invention provides a hot-forged part that can advantageously prevent hydrogen embrittlement and has excellent delayed fracture resistance with a tensile strength of 1600 MPa or more, and a method for producing the same.

  In order to solve the above-mentioned problems, the present invention has been made only after intensive studies on the chemical composition, manufacturing method, and microstructure of steel materials.

The gist of the present invention is as follows.
(1) By mass%, C: 0.2 to 0.6%, Si: 0.05 to 0.5%, Mn: 0.1 to 2%, Mo: 0.5 to 10%, Al: 0 0.005 to 0.5%, with the balance being composed of Fe and inevitable impurities, composed of a tempered martensite structure, and the longitudinal length of the prior austenite grains and the length perpendicular to the longitudinal direction Having a microstructure with a ratio of 1.5 (hereinafter referred to as aspect ratio) of 1.5 or more, a limit diffusible hydrogen of 1.5 ppm or more, and a tensile strength of 1600 MPa or more, delayed fracture resistance Hot forged parts with excellent characteristics.
(2) Furthermore, it contains any one of V: 0.05 to 1%, Ti: 0.01 to 1%, Nb: 0.01 to 1%, or two or more by mass%. A hot forged part having excellent delayed fracture resistance as described in (1) above.
(3) Further, any one of Cr: 0.1 to 2%, Ni: 0.05 to 1%, Cu: 0.05 to 0.5%, B: 0.0003 to 0.01% by mass% Or a hot forged part excellent in delayed fracture resistance according to (1) or (2) above, characterized by containing at least one kind.
(4) The steel having the component composition according to any one of claims 1 to 3 is heated at 900 to 1300 ° C, and hot working is performed at a reduction in area of 10% or more between 780 and 1000 ° C. After finishing, it is cooled at a cooling rate of 5 ° C./second or more within 20 seconds and then tempered at 550 to 700 ° C. .
(5) Delayed fracture resistance according to (4) above, characterized by finishing at room temperature to 700 ° C. before or after the tempering, or before and after the tempering. Manufacturing method for hot forged parts with excellent resistance.

  The present invention solves the various problems described above, and can provide a hot forged part that prevents hydrogen embrittlement represented by delayed fracture in high-strength steel, and a method for manufacturing the same.

  First, the reason why the chemical components of the steel material are limited in the present invention will be described. In addition,% means the mass%.

  C is added as an effective component for improving the strength of the steel, but if it is less than 0.2%, sufficient quenching does not occur during the quenching heat treatment and the strength is insufficient. On the other hand, excessive addition exceeding 0.6% causes deterioration of basic material properties such as excessive increase in strength and increase in crack sensitivity. Therefore, the limited range of the C concentration is set to 0.2 to 0.6%.

  In addition to functioning as a deoxidizing element, Si is a component necessary for securing the strength of the base material. But the effect is saturated. Therefore, the limited range of the Si concentration is set to 0.05 to 0.5%.

  Mn needs to be added in an amount of 0.1% or more in order to ensure the strength and toughness of the base material. However, the addition of more than 2% is resistant to hydrogen for reasons such as excessive increase in strength and increased microsegregation. Impairs embrittlement characteristics. Therefore, the limited range of the Mn concentration is set to 0.1 to 2%.

  Mo is an alloy element that generates carbides. In the present invention, it has been clarified that the precipitation of the carbides not only ensures normal temperature and high temperature strength, but also the precipitate interface functions as a hydrogen trap site. If it is less than 0.5%, the hydrogen trap function cannot be exhibited. On the other hand, if it exceeds 10%, the hardenability is excessively increased and the base material toughness is impaired. It was. However, in order to ensure the tensile strength, the above range is further limited to 1.5 to 10%.

  Al is a strong deoxidizing element, but if it is less than 0.005%, a sufficient deoxidizing effect cannot be obtained. On the other hand, even if it exceeds 0.5%, the effect is saturated. Therefore, the limited range of the Al concentration is set to 0.005 to 0.5%.

  Next, in addition to the above components, the reason for the regulation related to the concentration ranges of V, Ti and Nb which are alloy elements selectively added in the present invention will be described.

  V is an alloy element that alone or in combination with other carbides and carbonitride-constituting elements Mo, Ti, and Nb to form carbides and carbonitrides, contributing to precipitation strengthening and improving hydrogen trapping ability. is there. If the amount of V added is less than 0.05%, the amount of carbonitride deposited is insufficient and the above effect cannot be obtained. On the other hand, if it exceeds 1%, the amount of carbonitride deposited becomes excessive and the toughness is impaired. From the above, the limited range of the V concentration was set to 0.05 to 1%.

  Ti is an alloy element that forms carbonitrides alone or in combination with V or Nb, and contributes to precipitation strengthening, and the precipitate functions as a hydrogen trap, thereby preventing hydrogen embrittlement such as delayed fracture. Improve the conversion characteristics. If the Ti concentration is less than 0.01%, the amount of precipitation is insufficient, so the functions of precipitation strengthening and hydrogen trapping are insufficient, and the effect is saturated even if it exceeds 1% or more. Therefore, the limited range of the Ti concentration is set to 0.01 to 1%.

  Nb is an alloy element constituting carbonitride by itself or in combination with V or Ti, and contributes to precipitation strengthening, and the precipitate functions as a hydrogen trap, so that hydrogen resistance such as delayed fracture is obtained. Improve embrittlement characteristics. Since the precipitation amount is insufficient when the Nb concentration is less than 0.01%, the precipitation strengthening and the function as a hydrogen trap are insufficient, and even when it exceeds 1%, the solution temperature becomes high and industrially. Solutionization in the heating furnace used is insufficient, coarse carbonitrides are dispersed, and the contribution to precipitation strengthening and hydrogen trapping ability are insufficient. From the above, the limited range of the Nb concentration was set to 0.01 to 1%.

  Next, in addition to the above components, the reason for the regulation relating to the concentration range of Cr, Ni, Cu, B, which are alloy elements selectively added in the present invention, will be described.

  Cr is an element necessary for improving the hardenability and increasing the softening resistance during the tempering treatment. However, if it is less than 0.1%, the effect cannot be sufficiently exhibited. Lowering and cold workability will be caused. Therefore, the limited range of the Cr concentration is set to 0.1 to 2%.

  Ni is added to improve ductility, which deteriorates with increasing strength, and to improve the hardenability during heat treatment and improve the tensile strength. If the Ni concentration is less than 0.05%, the effect is small, and even if it exceeds 1%, an effect commensurate with the concentration cannot be exhibited. Therefore, the Ni concentration is limited to 0.05 to 1%.

  Cu is an effective element for increasing the tempering softening resistance. However, if it is less than 0.05%, the effect cannot be exerted, and if it exceeds 0.5%, the hot workability is lowered. Limited to 0.05 to 0.5%.

  B has an effect of suppressing grain boundary fracture and improving delayed fracture resistance. Further, B segregates at the prior austenite grain boundaries and remarkably improves the hardenability. However, if it is less than 0.0003%, the effect cannot be exhibited, and if it exceeds 0.01%, the effect is saturated. Therefore, the concentration range of B is limited to 0.0003 to 0.01%.

  Next, in the present invention, hot forged parts having excellent delayed fracture resistance can be manufactured by performing hot working under appropriate heat treatment conditions and then performing appropriate tempering and finishing. I found.

  First, the reason for heating at 900 to 1300 ° C. is to secure the temperature necessary for once sufficiently melting the alloy elements of the material, while simultaneously heating the material to the hot working temperature. If it is less than 900 ° C., it is difficult not only to secure a temperature necessary for the subsequent hot working, but also solution of the alloy elements is insufficient. On the other hand, when heating is performed at a temperature exceeding 1300 ° C., the material temperature necessary for hot working can be secured, but the upper limit is set to 1300 ° C. because it is economically inefficient.

  The reason why the hot working temperature is limited to the range of 780 to 1000 ° C. is to keep the austenite structure elongated by hot working without being recrystallized. In hot working at a temperature exceeding 1000 ° C., recrystallization occurs immediately after the working and the elongated austenite structure disappears. On the other hand, at temperatures below 780 ° C., ferrite transformation occurs in part, and when hot working is performed, processing strain accumulates and delayed fracture resistance is reduced.

  The reason why the area reduction ratio between 780 and 1000 ° C. was set to 10% or more is that when the area reduction ratio is less than 10%, the processing amount is insufficient and it is insufficient to obtain an elongated austenite structure of the surface layer portion. . Within the above range, the area reduction rate is preferably limited to 15% or more. This is because the tensile strength increases with an increase in the processing amount, and the cost can be suppressed by reducing the alloy elements for securing the tensile strength.

  The reason for cooling at a cooling rate of 5 ° C./second or more within 20 seconds after finishing the hot working is to keep the austenite structure without being recrystallized. In both the case where it exceeds 20 seconds until the start of cooling and the case where the cooling rate is less than 5 ° C./second, recrystallization occurs and the elongated austenite structure disappears, so that it does not contribute to the improvement of the delayed fracture resistance.

  The reason for tempering at 550 to 700 ° C. after cooling is to improve strength and delayed fracture resistance by tempering secondary precipitation of alloy carbide. Although the optimum tempering temperature range was limited to 550 to 700 ° C., the reason for this is that tempering secondary precipitation is insufficient when the temperature is lower than 550 ° C., and over aging when the temperature exceeds 700 ° C. This is because stabilization and coarsening reduce the contribution to improving strength and improving delayed fracture resistance.

  When finishing is required, it is desirable to heat and process from the viewpoint of preventing cracking and reducing deformation resistance. However, in order to prevent overheating of alloy carbides that are secondarily precipitated by tempering, the processing temperature Was set to 700 ° C. In finishing processing exceeding 700 ° C., the strength and delayed fracture resistance are reduced due to overaging of the alloy carbide, and it is impossible to produce a high-strength part excellent in delayed fracture resistance, which is the object of the present invention.

  Hereinafter, the effects of the present invention will be described more specifically with reference to examples. Examples in the production of high-strength bolts are shown as representatives of hot forged parts. In the manufacture of bolts, the header processing and the thread rolling process correspond to the finishing processing shown in the claims.

  A round bar material having the chemical composition shown in Table 1 was heated, then hot worked into a 12 mmφ bar steel at a predetermined temperature, and subjected to accelerated cooling after hot working. Thereafter, tempering, header processing, and thread rolling were performed under the conditions shown in the table, and the tensile properties and delayed fracture resistance of the bolts were confirmed.

  The tensile properties such as tensile strength were obtained from the results of the tensile test, and the delayed fracture properties such as the limit diffusible hydrogen content (iron and steel, Vol. 83 (1997), p454), In the state in which hydrogen is forcibly introduced into the steel by the electrolytic hydrogen charging method and the tensile stress is applied at a constant load 0.9 times the tensile strength (hereinafter referred to as a constant load test) It was determined by analyzing and measuring the hydrogen concentration.

  Here, the electrolytic hydrogen charging method is a method in which the steel material is immersed in an aqueous solution of ammonium thiocyanate, an anode potential is generated on the surface of the steel material, and hydrogen is taken into the steel material. In this method, the amount of hydrogen in the steel can be adjusted by variously adjusting the setting conditions such as the concentration of the ammonium thiocyanate aqueous solution, the charging time, and the current value.

  In the constant load test, Cd plating was applied to the bolt surface in advance in order to avoid escape of hydrogen from the steel material surface under test. In this test, the bolt held without breaking for about 100 hr in a constant load state was judged as “no break”.

  Bolts that broke in the constant load test and bolts that did not break were collected immediately after the test, and after removing the plating, the amount of released hydrogen was measured by temperature analysis using a gas chromatograph. In the present invention, the measurement was performed at a temperature rising rate of 100 ° C./hr.

  The diffusible hydrogen that affects the delayed fracture phenomenon is hydrogen that is trapped at a hydrogen trap site with a relatively weak binding force, and has a peak in which the amount of emission decays with time in temperature rising analysis. In the present invention, the hydrogen release peak detected up to 400 ° C. is the target, and the cumulative value of the released hydrogen amount detected up to 400 ° C. by the temperature rise analysis is defined as the diffusible hydrogen amount.

  The measurement was carried out according to the above procedure, and the rupture time and diffusible hydrogen content of the bolts used in the test were arranged, and the maximum diffusible hydrogen content was limited diffusion among the steel materials that did not break in the constant load test. The amount of reactive hydrogen was defined, and if this value was high, the delayed fracture resistance was evaluated as excellent.

  The distribution of the prior austenite grains was confirmed by microstructural observation, and the aspect ratio of the prior austenite grains was determined.

  As shown in Table 2, all of the inventive examples have high tensile strength of 1600 MPa or more, and the amount of limit diffusible hydrogen is 1.5 ppm or more, respectively, and is judged to have excellent delayed fracture resistance. The

  On the other hand, the comparative example shown in a table | surface is demonstrated. Since the comparative example 22 has Mo below the limited range of the present invention, the strength is insufficient. Since Comparative Examples 23 and 24 were not hot-worked during the solution treatment before tempering, the aspect ratio was less than 1.5, the critical diffusible hydrogen was low, and the delayed fracture resistance was insufficient. . In Comparative Examples 25, 26, 27, 28, and 30, the machining finishing temperature is out of the scope of the present invention. Further, in Comparative Example 29, the machining finishing temperature is out of the limited range of the present invention. Thus, since the time until finishing after processing is out of the limited range of the present invention, the aspect ratio is less than 1.5, the critical diffusible hydrogen is low, and the delayed fracture resistance is insufficient. In Comparative Examples 29 and 31, since the time until finishing after processing is outside the limited range of the present invention, the aspect ratio is less than 1.5, the critical diffusible hydrogen is low, and the delayed fracture resistance is not good. It will be enough. In Comparative Example 32, the cooling rate is out of the limited range of the present invention, so that the aspect ratio is less than 1.5, the critical diffusible hydrogen is low, and the delayed fracture resistance is insufficient. In Comparative Examples 33 and 34, the tempering temperature is out of the limited range of the present invention, so that the strength is insufficient or the delayed fracture resistance is insufficient.

  In addition, in the manufacture of bolts, finishing processes such as header processing and screw rolling are indispensable steps. Therefore, in the embodiment, the manufacturing method corresponding to the present invention described in claim 4, that is, manufacturing without performing the finishing process. Although the method is not described, the present invention according to claim 5, ie, finishing, is applied to a hot-forged part that is not subjected to finishing by applying the manufacturing method according to the present invention according to claim 4. Obviously, it has the same mechanical properties as when processing.

  As is clear from the above examples, the hydrogen embrittlement resistance typified by delayed fracture resistance of high strength members having a tensile strength of 1600 MPa or more can be greatly improved. As hot forged parts such as high-strength bolts, it can be applied to parts used in the fields of automobiles, machinery and civil engineering, and the industrial effects such as weight reduction, high efficiency, and safety improvement are extremely remarkable. It is.

Claims (5)

% By mass
C: 0.2 to 0.6%
Si: 0.05 to 0.5%,
Mn: 0.1 to 2%,
Mo: 0.5 to 10%,
Al: 0.005 to 0.5%
The balance of the length of the prior austenite grains in the longitudinal direction and the length perpendicular to the longitudinal direction (hereinafter referred to as the aspect ratio). Hot forging with excellent delayed fracture resistance, characterized in that it has a microstructure of 1.5 or more, limit diffusible hydrogen is 1.5 ppm or more, and tensile strength is 1600 MPa or more. parts. Furthermore, in mass%,
V: 0.05 to 1%
Ti: 0.01 to 1%,
Nb: 0.01 to 1%
The hot forged part having excellent delayed fracture resistance according to claim 1, wherein the hot forged part contains at least one of these. Furthermore, in mass%,
Cr: 0.1 to 2%,
Ni: 0.05 to 1%,
Cu: 0.05 to 0.5%,
B: 0.0003 to 0.01%
The hot forged part excellent in delayed fracture resistance according to claim 1 or 2, characterized by containing any one or more of the following.   After heating the steel having the component composition according to any one of claims 1 to 3 at 900 to 1300 ° C and performing hot working with a reduction in area of 10% or more between 780 and 1000 ° C, and finishing the steel. A method for producing a hot forged part excellent in delayed fracture resistance, characterized by cooling at a cooling rate of 5 ° C./second or more within 20 seconds and then tempering at 550 to 700 ° C.   The heat excellent in delayed fracture resistance according to claim 4, wherein the heat treatment is performed at room temperature to 700 ° C prior to the tempering, after the tempering, or before and after the tempering. Manufacturing method for inter-forged parts.