JP2006328473A - Method for producing non-heat treated high strength bolt - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a non-heat treated high strength bolt with which the relaxation of this bolt is improved and the suitable low temperature toughness as this bolt is obtained. <P>SOLUTION: At first, a steel material having the prescribed components for producing the non-heat treated high strength bolt, is melted (S10), and successively, this steel material is rolled into a wire rod, and a patenting treatment is applied. Further, the cross section is drawn to make the prescribed steel wire (S12). Thereafter, a press-forming is applied into the bolt-shape by deciding a reduction of area from the diameter of the steel wire to the cross section having effective diameter (S14). Then, the tensile stress is loaded in the axial direction of the formed bolt. This loading is applied with the tensile stress having not more than the elastic limit of the bolt material (S16). Then, under this loading or after separating from this loading, the prescribed heat treatment is applied (S18-S24). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a non-tempered high-strength bolt, and more particularly to a method for manufacturing a non-tempered high-strength bolt excellent in relaxation characteristics.

  In recent years, demand for high-strength bolts having a tensile strength of 1300 MPa or more, for example, in response to demands for weight reduction of parts intended to increase the scale of buildings, improve the earthquake resistance of structures, or improve the fuel efficiency of vehicles such as automobiles. Is growing. Common high-strength bolt steel uses medium carbon alloy steel, such as SCM435, SCM440, SCr440, etc., and the required strength is ensured by tempering by quenching and tempering. Called temper bolt. However, in general high-strength bolts, there is a known risk of so-called delayed fracture when the tensile strength exceeds about 1200 MPa.

  The cause of delayed fracture is intricately intertwined, and it is difficult to generally identify the cause. Factors that influence delayed fracture are said to be related to hydrogen, and tempering temperature, structure, material hardness, grain size, and various alloying elements have been recognized for some time. Effective methods have not been established, and various methods have been proposed by trial and error.

  For example, in patent document 1, in tempered bolt steel, in order to improve a delayed fracture phenomenon, the alloy composition is defined precisely. There, in addition to the single composition ratio of C, Si, Mn, P, S, Cr, Mo, Al, Ti, and N, limiting conditions among a plurality of elements are shown, and besides these elements, Ni It is stated that addition of elements such as, V is also effective. Patent Documents 2 and 3 disclose that high strength is obtained by performing a strong wire drawing process on a steel wire having a pearlite-based structure.

  In addition to taking into account the suppression of the delayed fracture phenomenon as described above, a so-called relaxation phenomenon (stress relaxation) may occur during use that causes a reduction in tightening force, that is, loosening of the bolt. For example, when left standing at 150 ° C., the load may be reduced by 30%.

  In Patent Document 4, a steel material having a pearlite structure of 80% or more is drawn by a patenting process (fine pearlite process) and formed into a bolt shape by cold heading. It is disclosed that the delayed fracture characteristics and relaxation characteristics are improved by applying the treatment.

  Patent Document 5 points out that as a cause of the relaxation phenomenon, a steel bolt is subjected to plastic working such as drawing in the manufacturing process, so that a large number of movable dislocations are introduced into the crystal lattice. That is, under tensile stress such as tightening of bolts, it is stated that these dislocations move in the threaded portion to cause elongation, and stress relaxation occurs due to the elongation. And it touches on the blueing process of the steel volt | bolt of patent document 4, and this process method still says that the improvement of a relaxation characteristic is not enough. Then, the relaxation characteristics can be improved by screwing the bolt into the bolt holder, applying a tensile stress in the axial direction exceeding the elastic limit, and performing a heat treatment between 200 ° C. and 300 ° C. It is disclosed.

Japanese Patent No. 2614659 JP 11-315349 A JP 2001-294881 A JP 2001-348618 A JP 2004-116624 A

  As described above, in the environment where high-strength bolts are used, problems with delayed fracture and relaxation phenomena are known. However, the contents of Patent Documents 1 to 3 do not improve the relaxation characteristics. Further, Patent Document 5 states that the improvement of the relaxation characteristics is not yet sufficient only by the simple bluing heat treatment disclosed in Patent Document 4. Although the method of Patent Document 5 is promising, the inventors of the present application conducted experiments and found that this method reduces toughness at low temperatures. For example, when a Charpy impact test is performed, the experimental results at low temperatures are not good. Therefore, even if the high-strength bolt according to the method of Patent Document 5 has a predetermined performance at room temperature, there is a concern that the impact characteristics may be lowered at a low temperature such as in a cold region.

  An object of the present invention is to provide a method for producing a high-strength bolt that improves relaxation characteristics and has appropriate low-temperature toughness in a non-tempered high-strength bolt.

  The manufacturing method of the non-tempered high strength bolt according to the present invention is based on the facts found in the experiment to improve the relaxation characteristics and low temperature toughness of the non-tempered high strength bolt. is there. The experiment is to apply a tensile stress below the elastic limit to the bolt after bolt forming, and then heat-treat in that condition, but it is appropriate for the area reduction rate, which is the reduction rate of the wire cross-sectional area in bolt forming. The present inventors have found that when heat treatment is performed with an appropriate tensile stress, the relaxation properties are improved and suitable low temperature toughness is obtained. In the following, these experimental results are used as a method for producing a non-tempered high strength bolt.

  The method for producing a non-tempered high-strength bolt according to the present invention comprises a component ratio in mass% of 0.7% to 1.0% C, 0.1% to 1.0% Si, The steel material containing 0.15% or more and 1.2% or less Mn, P limited to 0.03% or less, and S limited to 0.03% or less is rolled, patented, and drawn. A steel wire, a forming step of forming the steel wire into a bolt shape with a predetermined reduction in area, and a step of applying tensile stress in the axial direction to the formed bolt, the elastic limit of tensile stress In the relationship between the load ratio against stress and the area reduction ratio, the load ratio is 0, the area reduction ratio is 5%, the load ratio is 0, the area reduction ratio is 60%, and the load ratio is 1. Surrounded by load conditions with an area ratio of 60%, a load ratio of 1 and a reduction in area of 3%, a load ratio of 0.5 and an area reduction of 3% That is a load loading step for loading the tensile stress in the range of load conditions, the heat treatment step of heat-treating the bolts after loading the load at 50 ° C. or higher 450 ° C. or less, comprising a.

  Moreover, it is preferable that a shaping | molding process sets an area reduction rate and heat processing so that the volt | bolt after a heat treatment process may have the tensile strength of 1300 Mpa or more.

  According to the method for producing a non-tempered high-strength bolt according to the present invention, it is possible to improve relaxation characteristics and to have appropriate low-temperature toughness.

  Embodiments according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a flowchart showing a procedure of a method for manufacturing a non-tempered high strength bolt. In order to manufacture a non-tempered high-strength bolt, first, steel of a predetermined component is melted (S10). Next, this steel material is rolled into a wire material, subjected to a patenting process (fine pearlite process), and then further reduced in cross-sectional area to obtain a predetermined steel wire (S12). After that, a reduction rate, which is a reduction rate of the area from the cross-sectional area of the diameter of the steel wire to the cross-sectional area of the effective diameter of the bolt, is determined and formed into a bolt shape (S14). Then, the formed bolt is subjected to a tensile stress in the axial direction. This load is applied with a tensile stress below the elastic limit of the bolt material. That is, (load load / elastic limit load) = load load ratio, and the load load ratio is set to 1 or less (S16). Then, a predetermined heat treatment is performed on the bolt after loading. That is, a predetermined heat treatment is performed under this load (S18), and when these steps are completed, the product is appropriately cooled and unloaded (S20). Alternatively, unloading is performed after load application (S22), and then heat treatment is performed (S24). Thereby, a non-tempered high strength bolt is obtained.

  In the following, the processing conditions in each of these steps will be described in order, and then the test results such as relaxation characteristics, low temperature toughness, etc. will be described for various examples within the range of processing conditions determined as such and comparative examples of the surrounding conditions. .

  Predetermined components of each constituent element of steel in S10 are determined as follows from the viewpoint of balance of properties required for steel materials. In this steel material, the elements that determine the range of the component ratio are C, Si, and Mn, and the elements that suppress the component ratio to a certain level are P and S. In addition to these, there are impurities such as deoxidizing elements such as Al, N, O, Cu, and Cr as contained in the steel material, and their components are determined using, for example, general standards for steel material production. be able to. In addition, a component ratio is shown by the mass%.

  C: 0.7% or more and 1.0% or less In order to obtain a high-strength bolt excellent in delayed fracture resistance, it is necessary to create a pearlite wire drawing structure. If C is less than 0.7%, the fraction of pearlite is small, and if it exceeds 1.0%, the fraction of pro-eutectoid cementite increases and brittleness increases. Therefore, the component of C is 0.7% or more and 1.0% or less.

  Si: 0.1% or more and 1% or less Si is an element excellent in solid solution strengthening ability, and 0.1% or more is necessary to obtain a high-strength bolt. On the other hand, since Si increases the activity of cementite, it has an action of suppressing precipitation of cementite. That is, if the amount of Si is too large, it is difficult to increase the strength in a short time because precipitation of solute C is suppressed in the heat treatment of S18 described later. In particular, if Si is added in excess of 1%, precipitation of C is remarkably delayed, so the Si component is 0.1% or more and 1% or less.

  Mn: 0.15% or more and 1.2% or less First, Mn has an effect of suppressing hot brittleness due to S contained as an impurity, and is an indispensable element for steel products that are mass-produced by rolling. In order to exhibit the effect, 0.15% or more is necessary. On the other hand, the addition exceeding 1.2% is not preferable for excessively improving the hardenability of the steel material and stably generating a pearlite-based structure. Therefore, the Mn component is set to 0.15% or more and 1.2% or less.

  P: 0.03% or less P is an undesirable element that segregates at grain boundaries, lowers the bond strength of the grain boundaries, and induces delayed fracture characteristics. If P exceeds 0.03%, the delayed fracture resistance is significantly deteriorated, so the component of P is set to 0.03% or less.

  S: 0.03% or less S, like P, is an element that segregates at grain boundaries and lowers the bond strength at the grain boundaries, and is a harmful element that causes hot brittleness during rolling. If S exceeds 0.03%, the delayed fracture resistance is significantly deteriorated, so the S component is 0.03% or less.

  Next, the temperature range of the heat treatment in S18 will be described. This heat treatment is performed in order to prevent the dislocations introduced by the tensile stress load in S16 from moving after tightening in actual use of the bolt, that is, to improve relaxation resistance. To pin the dislocation. Therefore, the dislocation density and the C diffusion rate are related. If the treatment temperature exceeds 450 ° C., the diffusion of C is too fast, and it becomes difficult to stably form a C precipitation form having a pinning force. Further, if the treatment temperature is less than 50 ° C., the diffusion of C is too slow, which is inconvenient for industrial use. From the above viewpoint, the temperature range of the heat treatment of S18 is set to 50 ° C. or more and 450 ° C. or less.

  Next, the relationship between the area reduction rate in the bolt forming of S14 and the load condition of the load ratio corresponding to this in S16 will be described. The processing conditions related to each other are the processing conditions for improving the relaxation characteristics and the delayed fracture characteristics based on the steel wire obtained from the steel material of S10 through S12, and thus were determined by experiments. The experiment is representative of the component ratio of the steel material described in connection with S10, and the steel material having the component ratio shown in FIG. 2, that is, mass%, 0.83% C, 0.23% Si, 0.76 A steel material containing a component ratio of% Mn, 0.008% P, and 0.003% S was used. And corresponding to the process of S12, this was rolled into a wire with an arbitrary wire diameter, and a patenting process was performed to obtain a steel wire with a predetermined wire diameter with an appropriate reduction in area.

  The steel wire was formed into a bolt shape by forging. At that time, a plurality of bolts were created under various surface reduction rates. This corresponds to the process of S14. A plurality of bolts of each area reduction rate are subjected to a tensile test as they are, and correspond to the process of S16, and load in the axial direction is applied at various load ratios, and then 200 ° C for 30 minutes as a representative condition of S18 To those subjected to the heat treatment, those subjected to the tensile test and relaxation test after those treatments, and those subjected to low-temperature Charpy impact by cutting out the shape of the bolt.

  The method of applying an axial load corresponding to the process of S16 may use a tension method in which a bolt is applied to a tension device such as a tensile tester using an appropriate jig, or an appropriate screw jig. A rotary fastening method in which a bolt is screwed by rotational fastening may be used. The load / load ratio is calculated based on the load corresponding to the elastic limit obtained in advance using bolts having the same characteristics.

  FIG. 3 is a diagram illustrating the elastic limit. When a load is applied by the tensile method, a test bolt is applied to a tensile tester to obtain a stress-strain diagram or a load-elongation diagram as shown in FIG. The load is set as the limit, and the load ratio = (load corresponding to load / elastic limit) is calculated based on the corresponding load. When a load is applied by the rotation tightening method, for example, using a nut runner, the rotation angle of the bolt and the torque required for the rotation are obtained, the torque-rotation angle diagram shown in FIG. The point B out of the region is set as the elastic limit, and the load / load ratio = (load torque / torque corresponding to the elastic limit) is calculated based on the corresponding torque.

  Among the evaluation methods, the tensile test determined the tensile strength. In the relaxation test, a load corresponding to the elastic limit of the bolt is applied as a load at room temperature, and the holding interval length at both ends of the bolt is kept as it is for 24 hours, and the rate of decrease in load before and after that is measured. I went there. Further, the low temperature Charpy test was performed at −30 ° C. assuming a cold region. For the judgment criteria in these test results, a commercially available high-strength bolt having a tensile strength of 1100 MPa was prepared, and a tensile test, a room temperature relaxation test, and a low-temperature Charpy test were performed under the same conditions.

  Figure 4 shows the results of a room temperature relaxation test performed on bolts formed with various area reduction ratios, loaded with various load ratios, and subjected to heat treatment at 200 ° C for 30 minutes after unloading. Shown in Fig. 4 shows the reduction ratio during bolt forging on the horizontal axis, the tensile stress against the elastic limit in the axial direction, that is, the load ratio on the vertical axis, and the test at the position of the corresponding condition of each bolt. The evaluation of the result is shown by ○ or ×. The evaluation of the test results was made by comparing the load reduction rate under the above conditions of the room temperature relaxation test with the load reduction rate of the commercial 1100 MPa bolt, which is the criterion, and ◯ for the load reduction rate superior to the criterion, and inferior load reduction The rate was marked with an x.

  Similarly, the result of the low-temperature Charpy test is shown in FIG. The meanings of the horizontal and vertical axes are the same as in FIG. The evaluation of the test result is based on 40% of the absorbed energy of 1100 MPa bolt, which is the standard for judgment, as the judgment level, ○ for the result of higher absorption energy, and × for the result of lower absorption energy. I went there.

  In addition, as a result of the tensile test about the bolt of various conditions shown in FIG. 4, FIG. 5, all the bolts had a tensile strength of 1700 MPa or more. In addition, although it is possible to set the area reduction ratio during bolt forging to exceed 60%, the deformation resistance during processing increases and the burden on the mold increases, making it unsuitable for mass production.

  From the results of FIGS. 4 and 5, it can be seen that the range in which the room temperature relaxation characteristics are good and the low temperature Charpy characteristics representing low temperature toughness are also suitable is the processing condition range shown as “invention range” 10 in FIG. 6. That is, in the relationship between the load ratio and the area reduction ratio, the load ratio is 0, the area reduction ratio is 5%, the load ratio is 0, the area reduction ratio is 60%, and the load ratio is 1. High strength if the processing condition range is 60%, the load ratio is 1, the area reduction is 3%, and the load ratio is 0.5 and the area reduction is 3%. The relaxation characteristics of the bolt are improved, and appropriate low temperature toughness can be obtained.

  As described above, even if the tensile load applied in the axial direction is less than the elastic limit, the reason why the anti-relaxation characteristic equivalent to or better than that of a commercial bolt of 1100 MPa class can be obtained has not been grasped at present. . However, as shown in FIG. 4 to FIG. 6, since it is related to the area reduction ratio at the time of bolt forming, the dislocation group propagated at the time of bolt forming, the dislocation generated and moved by the load stress below the elastic limit, and It can be presumed that the physical metallurgy phenomenon that occurs between the iron carbides that pin the dislocations contributes to the improvement of the relaxation resistance.

  In the following, for the eight examples and the five comparative examples, the component ratio of the steel material in S10 is mainly changed. In addition, the area reduction rate during bolt forming in S14, the load ratio in S16, and the heat treatment conditions in S18 are as follows. The test evaluation results for each bolt obtained by changing the combination will be described.

  FIG. 7 is a diagram showing component ratios of five types of steel materials used in the experiments of the example and the comparative example. These steel numbers A to E are all within the range of the component ratio of each constituent element described in S10. Steels with these various chemical compositions were melted (S10), rolled into various wire rods, patented, and drawn with various surface reduction rates to create steel wires (S12). When forming bolts from this steel wire, bolts were created in which the surface area reduction rate, which is the rate of reduction of the area from the steel wire diameter to the effective diameter of the bolt, was made in various sizes (corresponding to S14). . Tensile tests were performed on these bolts to determine the elastic limit. These bolts were subjected to various tensile stresses in the axial direction (corresponding to S16), and heat-treated at various temperatures (S18). The tensile stress was applied by the above-described tension method or rotary fastening method.

  The bolts thus produced were subjected to a tensile test, a relaxation test, and a Charpy test. The contents of these tests are the same as those described with reference to FIGS. FIG. 8 summarizes these processing conditions and test results. The evaluation methods for the relaxation test and the Charpy test are the same as those described with reference to FIGS.

  In FIG. 8, trial numbers 1 to 8 are in this example, and non-tempered high-strength bolts having both excellent relaxation resistance and low-temperature toughness were obtained. Test numbers 9 to 13 are comparative examples. Test No. 9 is an example in which the low-temperature toughness deteriorates because the tensile stress in the axial direction is set to a stress exceeding the elastic limit. Test No. 10 is an example in which the relaxation resistance characteristics deteriorated because the area reduction rate during bolt forging was zero and no tensile stress was applied in the axial direction. In Test No. 11, since the area reduction ratio and the axial load stress ratio at the time of bolt forging are conditions outside the processing condition range shown as “invention range” 10 shown in FIG. 6, the relaxation resistance characteristics deteriorated. It is an example. Test No. 12 uses steel No. E that contains C in which the composition of the steel material is outside the range of the component ratio of each constituent element described in relation to S10, so that the amount of proeutectoid cementite increases and embrittlement causes low temperature. This is an example in which toughness has deteriorated. Test number 13 is an example in which the relaxation resistance characteristics deteriorated because the heat treatment conditions outside the heat treatment temperature range described in connection with S18 were used.

It is a flowchart which shows the procedure of the manufacturing method of the non-tempered high strength bolt in embodiment which concerns on this invention. In embodiment which concerns on this invention, it is a figure which shows the typical component ratio of steel materials. It is a figure explaining an elastic limit. In embodiment which concerns on this invention, it is a figure which shows the result of having performed the normal temperature relaxation test under various process conditions. In embodiment which concerns on this invention, it is a figure which shows the result of having performed the low-temperature Charpy test under various process conditions. In embodiment which concerns on this invention, it is a figure which shows an appropriate process condition range by the relationship between a surface-reduction rate and a load ratio. It is a figure which shows the component ratio of the various steel materials used for the Example and the comparative example. It is the figure which summarized the content of the Example and the comparative example.

Explanation of symbols

  10 Processing condition range.

Claims (2)

In a component ratio in mass%, 0.7% or more and 1.0% or less of C, 0.1% or more and 1.0% or less of Si, and 0.15% or more and 1.2% or less of Mn A step of rolling a steel material containing P, 0.03% or less and S limited to 0.03% or less, patenting and drawing to form a steel wire;
A forming step of forming a steel wire into a bolt shape at a predetermined area reduction rate;
This is a process of applying an axial tensile stress to the formed bolt. The relationship between the load ratio of the tensile stress to the elastic limit stress and the area reduction ratio is 0 and the area reduction ratio is 5%. , And the load ratio is 0 and the area reduction ratio is 60%, the load ratio is 1 and the area reduction ratio is 60%, the load ratio is 1, the area reduction ratio is 3%, and the load ratio is 0. A load loading step of applying a tensile stress in a load condition range surrounded by a load condition of 5% in area reduction rate of 5;
A heat treatment step of heat-treating the loaded bolt at 50 ° C. or higher and 450 ° C. or lower;
A method for producing a non-tempered high-strength bolt, comprising: In the manufacturing method of the non-tempered high strength bolt according to claim 1,
The forming step includes setting the area reduction rate and the heat treatment so as to have a tensile strength of 1300 Mpa or more.

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