JP4485424B2 - Manufacturing method of high-strength bolts with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a method for producing a high-strength bolt excellent in delayed fracture resistance, and particularly relates to a method for producing a high-strength bolt excellent in delayed fracture resistance having a tensile strength of 1600 MPa or more.

  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, It is a medium carbon steel with 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 steel materials described above have an increased risk of hydrogen embrittlement, especially the risk of delayed fracture due to hydrogen entering from the environment during use, when the tensile strength exceeds 1300 MPa. Therefore, for example, in the case of construction, the upper limit is a steel material having a tensile strength of 1150 MPa class.

  As a conventional finding to improve delayed fracture resistance of high strength steel, delayed fracture resistance is improved by eliminating high temperature tempering of cementite that precipitates in film form at the prior austenite grain boundaries of martensitic steel produced by quenching. It is known. However, on the other hand, if the martensitic steel is tempered and kept at a high temperature for a long time, the strength is remarkably lowered and it is difficult to increase the strength. Therefore, in the invention described in Patent Document 1, delayed fracture resistance is improved by performing high-temperature and short-time tempering by high-frequency induction heating without greatly reducing the strength. However, the present invention is intended for steel bars having a tensile strength of about 1500 MPa at most, and cannot be applied as it is in a strength region requiring a tensile strength of 1600 MPa class or higher, which is the technical field of the present invention.

  On the other hand, a strengthening method by secondary precipitation of alloy carbide or alloy carbonitride by tempering is known as means for increasing the strength. This alloy carbide is distributed and distributed in steel and has the function of trapping hydrogen. Therefore, the delayed fracture resistance is improved by increasing the critical hydrogen amount (critical diffusion hydrogen amount) that causes delayed fracture. An invention that can be used is disclosed in Patent Document 2.

  However, when tempering at high temperature and short time is applied in the component system in the present invention, while film-like cementite distributed in the prior austenite grain boundaries disappears, secondary precipitated alloy carbides become over-aged, leading to an increase in strength. There is a concern that both the contribution and the contribution to the delayed fracture resistance due to the hydrogen trap are greatly reduced. Therefore, in order to apply high-temperature and short-time tempering to steel materials that function alloy carbides as hydrogen traps, it is necessary to find a combination of selection of appropriate chemical species and chemical composition ranges and appropriate high-temperature and short-time tempering conditions. It was.

JP 58-001016 A JP 2000-026934 A

  As shown in the above-mentioned situation, there has been a limit to the production of high-strength steel with drastically improved delayed fracture characteristics in the prior art.

  Therefore, the present invention finds components that solve this problem, process conditions that realize the essential conditions of the manufacturing method, and advantageously hydrogen embrittlement represented by delayed fracture phenomenon that appears as a problem with increasing strength. The present invention provides a method for producing a high-strength bolt that can be prevented and has excellent delayed fracture resistance with a tensile strength of 1600 MPa or more.

  In order to solve the above-mentioned problems, the present invention has been achieved only after intensive studies on the chemical composition of the steel material and the manufacturing method such as heat treatment.

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: 1.5 to 10%, Al: 0 The steel containing 0.005 to 0.5% and the balance of Fe and inevitable impurities is made into a martensite structure by quenching from 900 ° C. or higher, and then from 300 ° C. or higher to 5 ° C./second at the steel surface temperature. 1. Critical diffusible hydrogen, characterized in that it is rapidly heated to 600 ° C. or higher and 750 ° C. or lower at the above temperature rising rate, held for 0 to 20 seconds in the temperature range, and then allowed to cool or accelerate. 5ppm or more, tensile strength 1600MP a than on the delayed method for producing excellent high strength bolts fracture characteristics.
(2) Further, it is characterized by containing one or more of V: 0.05 to 1%, Ti: 0.01 to 1%, Nb: 0.01 to 1% by mass%. The manufacturing method of the high intensity | strength bolt excellent in the delayed fracture resistance as described in said (1).
(3) Further, by mass%, Cr: 0.1-2%, Ni: 0.05-1%, Cu: 0.05-0.5%, B: 0.0003-0.01% The method for producing a high-strength bolt excellent in delayed fracture resistance according to the above (1) or (2), comprising a seed or two or more kinds.

  The present invention solves the above-mentioned various problems, and can provide a bolt 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 0.5% or less, a sufficient 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. 10%. 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. Furthermore, B segregates at the prior austenite grain boundaries and significantly improves the hardenability. However, if less than 0.0003%, the effect cannot be exhibited, and if it exceeds 0.01%, the effect is saturated. The B concentration range was limited to 0.0003 to 0.01%.

  Next, tempering conditions at a high temperature and a short time required in the present invention will be described. First, it is assumed that the microstructure of the steel material before tempering is a martensite structure. In order to secure the martensite structure, rapid cooling from the austenite region is necessary. At the same time, however, the alloy carbide needs to be once solutionized in order to cause secondary precipitation of the alloy carbide. It was set to above ℃. However, even if the solution is formed at an excessively high temperature, the effect hardly changes, so that the quenching start temperature is desirably in the range of 900 to 1000 ° C.

  After quenching, the rate of temperature increase in tempering is at least 300 ° C. or more and 5 ° C./s or more at the steel surface temperature, if the temperature increase rate is less than 5 ° C./s in the temperature range of 300 ° C. or more, Sufficient holding time at high temperature will be secured, alloy carbide produced by secondary precipitation will be over-aged, resulting in a decrease in strength and delayed fracture resistance, and the desired strength and delayed fracture resistance This is because no longer can be obtained.

  Moreover, in this invention, it limited so that tempering temperature was hold | maintained in the high temperature state from 0 s to the maximum 20 s as needed in the range of 600 degreeC or more and 750 degrees C or less. The reason for this limitation is as follows. When tempering is performed at a temperature lower than 600 ° C., secondary precipitation becomes insufficient. On the other hand, when the tempering temperature exceeds 750 ° C. and when the holding time at the tempering temperature exceeds 20 s, the alloy formed by secondary precipitation This is because carbides cause overaging and decrease strength and delayed fracture resistance, so if the tempering temperature range and holding time depart from the limited range, the desired strength and delayed fracture resistance will not be obtained. is there.

  In the bolt manufacturing process, in addition to the quenching and tempering described above, a molding process such as header processing, straightening, and thread rolling is added. However, these processes may be performed either before or after quenching and tempering. Absent.

  Moreover, although the manufacturing method of the present invention is intended for bolts, it is obvious that it can also be applied to the manufacture of steel parts other than bolts and can manufacture parts with excellent delayed fracture resistance. It is.

  Hereinafter, the effects of the present invention will be described more specifically with reference to examples. After manufacturing a 12mmφ round bar with the chemical composition shown in Table 1, quenching and tempering, header processing, straightening and thread rolling are performed under the conditions shown in Table 2, and the tensile properties and delayed fracture resistance properties of the steel are confirmed. did. For tempering, in order to ensure a temperature rising rate of 5 ° C./second or more, high-frequency induction heating was employed to perform rapid heating.

  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, in order to avoid the escape of hydrogen from the steel material surface under test, Cd plating is applied to the steel material surface in advance, and a circumferential V-notch notch is formed at the center position for breaking at the center of the sample. Was given. In this test, the steel material that was held without breaking for about 100 hr under a constant load was determined as “no breakage”.

  Both the steel material that broke in the constant load test and the steel material 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 apparatus. 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 sample were arranged. The maximum diffusible hydrogen content was defined as the critical diffusible hydrogen content among steel materials that did not break in the constant load test. If this value is high, it was evaluated that the delayed fracture resistance is excellent.

  The invention examples shown in Tables 1 and 2 all have a high tensile strength of 1600 MPa or more, and the amount of limit diffusible hydrogen is 1.6 ppm or more, both of which are 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 17 has Mo less than the limited range of the present invention, the strength is insufficient. In Comparative Example 18, since the quenching temperature is lower than the limit range of the present invention, the alloy carbide is not sufficiently solutionized, and both the strength and the amount of critical diffusible hydrogen are low. In Comparative Example 19, the temperature increase rate during tempering is slower than the limited range of the present invention, and in Comparative Example 20, the holding time at the tempering temperature is longer than the limited range of the present invention. In both cases, overaging occurs and the strength is insufficient. In Comparative Example 21, since the tempering temperature is lower than the limited range of the present invention, secondary precipitation of alloy carbide becomes insufficient, and the amount of critical diffusible hydrogen becomes low. In Comparative Example 22, the tempering temperature is higher than the limited range of the present invention, and thus overaging occurs and the strength is insufficient. In Comparative Example 23, the quenching temperature was lower than the limited range of the present invention, and the holding time at the tempering temperature was longer than the limited range of the present invention. In Comparative Example 24, the rate of temperature increase during tempering was slower than the limited range of the present invention, and the holding time at the tempering temperature was longer than the limited range of the present invention. In Comparative Example 25, since the tempering temperature is lower than the limited range of the present invention, although the retention time at the tempering temperature is longer than the limited range of the present invention, secondary precipitation of the alloy carbide becomes insufficient, and the critical diffusion. The amount of reactive hydrogen is low.

  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. In addition to high-strength bolts, it can be applied to members used in the fields of automobiles, machinery and civil engineering, and industrial effects such as lighter members, higher efficiency, and improved safety are extremely remarkable. In addition to bolts, it can also be applied to parts that require high strength and delayed fracture resistance.

Claims (3)

% By mass
C: 0.2 to 0.6%
Si: 0.05 to 0.5%,
Mn: 0.1 to 2%,
Mo: 1.5 to 10%,
Al: 0.005 to 0.5%
Steel, the balance of Fe and unavoidable impurities is made into a martensite structure by quenching from 900 ° C. or higher, and then from 300 ° C. or higher to 600 ° C. at a rate of temperature increase of 5 ° C./second or higher. It is rapidly heated up to 750 ° C. or more and 750 ° C. or less, held for 0 to 20 seconds in the temperature range, and then allowed to cool or accelerated cooling. delayed production method of excellent high strength bolts fracture characteristics over a following.
Furthermore, in mass%,
V: 0.05 to 1%
Ti: 0.01 to 1%,
Nb: 0.01 to 1%
The manufacturing method of the high strength bolt excellent in the delayed fracture resistance of Claim 1 characterized by including 1 type, or 2 or more types 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 method for producing a high-strength bolt excellent in delayed fracture resistance according to claim 1 or 2, characterized by containing one or more of the following.