JP5867285B2 - Bolt steel - Google Patents

Description

  The present invention relates to a steel material for bolts, and more particularly, to a steel material for bolts having a tensile strength of 1200 MPa or more after quenching and tempering and a very small variation in tensile strength.

  In recent years, with the reduction in weight of automobiles and various industrial machines and the increase in size of building structures, high-strength bolts having high safety have been demanded. Therefore, in order to impart a predetermined axial force necessary for fastening to the bolt without variation, it is an urgent task to develop a steel material for bolts with a small variation in tensile strength.

  In particular, in bolts that are tightened in the plastic zone in order to obtain a high axial force, an axial force corresponding to the tensile strength of the bolt is applied, so the variation in the tensile strength of the bolt is the main factor in the variation in axial force. Become. Therefore, in order to improve the safety of the bolt, it is necessary to strictly suppress the variation in the tensile strength of the bolt.

  As a steel material for bolts, that is, steel used as a bolt material, for example, chromium molybdenum steel such as SCM435 defined in JIS G 4053 (2008) is used. In general, after hot rolling, spheroidizing annealing is performed for the purpose of improving cold workability, and then the wire is drawn or drawn and then finished into a bolt shape by cold forging. And finally, in order to ensure predetermined intensity | strength, a quenching and tempering process is performed and it becomes a bolt product. Hereinafter, wire drawing and drawing are simply referred to as “wire drawing”.

  Patent Documents 1 and 2 describe a method of shortening the processing time required for spheroidizing annealing by making the structure after hot rolling single by bainite by continuous cooling and isothermal transformation. Patent Document 3 discloses that by restricting the spheroidization rate of carbide, the workability of the steel for heat treatment before quenching is improved and the solid solution of carbide is promoted in the heating process during quenching. A steel material for heat treatment suitable for a heat treatment material has been disclosed, which has the capability of producing a high-strength molded product that can be sufficiently baked by low temperature and short time heating.

JP-A-48-26618 JP 60-9832 A JP 2011-195957 A

  The inventions described in Patent Documents 1 and 2 have not been studied at all in terms of suppressing variations in the tensile strength of steel materials after quenching and tempering, and it is necessary to strictly control the tensile strength. It is not necessarily suitable as a steel material for high-strength bolts. The invention described in Patent Document 3 relates to a heat-treating steel material capable of obtaining high strength even when quenched after being heated at a low temperature for a short time. However, there is still room for improvement in order to suppress the variation in the tensile strength of the steel material after quenching and tempering and to use it as a bolt material for tightening the plastic region.

  Among steels for bolts, because of the high strength as rolled, when using a steel material with low cold workability, spheroidizing annealing after hot rolling (hereinafter referred to as the following) so as not to reduce the life of the die for cold forging After performing wire drawing and wire drawing, a step of further spheroidizing annealing (hereinafter also referred to as “secondary annealing”) is provided to sufficiently reduce the strength, and then the final Drawing and cold forging are often performed.

  At this time, since annealing is performed twice, the carbides spheroidized by annealing are easily coarsened, and the coarsened carbides are difficult to be completely dissolved in the matrix during heating for subsequent quenching. Sometimes. Also, in a normal quenching furnace, the temperature in the furnace is not uniform, and the quenching temperature varies depending on the location, so there is also a difference in the degree of undissolved carbide, resulting in the tension of the bolt product after tempering. Variation in strength increases.

  Furthermore, generally used mass quenching furnaces for bolts are continuous furnaces with a soaking temperature of 860 to 900 ° C., and raising the holding temperature to 900 ° C. or more to promote solid solution of carbides due to equipment constraints. Often difficult. For this reason, there is a strong demand for a steel material for bolts that can sufficiently dissolve carbides even when quenched at 860 to 900 ° C.

  The present invention solves the problems of the prior art as described above, and even in quenching at 860 to 900 ° C., the carbides are uniformly and sufficiently dissolved, and the variation in tensile strength after tempering is extremely small, and An object of the present invention is to provide a steel material for high-strength bolts having a tensile strength of 1200 MPa or more.

  This invention is made | formed in order to solve said subject, The summary exists in the steel materials for bolts shown to following (1) and (2).

(1) The chemical composition of the steel material is mass%, C: 0.30 to 0.40%, Si: 0.01 to 0.40%, Mn: 0.10 to 1.0%, P: 0.00. 030% or less, S: 0.030% or less, Al: 0.005 to 0.10%, Cr: 0.90 to 1.8%, Mo: 0.10 to 2.0%, and N: 0.003 It has a structure that contains ~ 0.030%, the balance is Fe and impurities, and the ratio of the number of carbides with an equivalent circle diameter of 1.0 μm or more among the carbides with an equivalent circle diameter of 0.5 μm or more is 10% or less. (However, after primary annealing and secondary annealing, and before quenching and tempering , excluding the case where the structure contains a pearlite structure.) Steel material for bolts having a tensile strength of 1200 MPa or more after quenching and tempering .

  (2) The steel material for bolts according to (1) above, wherein the chemical composition of the steel material contains, in place of a part of Fe, mass% and further Nb: 0.10% or less.

  The steel material for high-strength bolts of the present invention has a tensile strength of 1200 MPa or more after quenching and tempering and has a very small variation in tensile strength.For example, it is used for fastening important mechanical structures such as automobile engine parts, It is suitable as a bolt material that is required to impart high strength and a predetermined axial force necessary for fastening to the bolt without variation.

1. Chemical composition of steel The reasons for limitation of each element are as follows. In the following description, “%” of the content of each element means “mass%”.

C: 0.30 to 0.40%
C has the effect | action which raises hardenability and improves intensity | strength. In order to obtain sufficient hardenability and stably and reliably obtain a tensile strength of 1200 MPa or more, it is necessary to contain 0.30% or more of C. However, even if C is contained in an amount exceeding 0.40%, the carbide tends to be coarsened, and it is difficult to dissolve the carbide during quenching. Therefore, the content of C is set to 0.30 to 0.40%. In addition, in order to sufficiently exhibit the effect of improving the strength of C, the lower limit of the C content is preferably set to 0.32%. In this case, a tensile strength of 1300 MPa or more should be secured stably and reliably. Can do. On the other hand, in order to reliably prevent the coarsening of the carbide, it is desirable that the upper limit of the C content is 0.38%.

Si: 0.01-0.40%
Si is an element effective for deoxidation, and in order to fully exhibit this effect, it is necessary to contain 0.01% or more. On the other hand, when it contains exceeding 0.40%, the moldability to the bolt by cold forging will fall remarkably. Therefore, the Si content is set to 0.01 to 0.40%. In order to fully exhibit the deoxidation action of Si, it is desirable to contain 0.05% or more. In order to facilitate bolt forming by the cold forging method, it is desirable that the upper limit of the Si content is 0.28%.

Mn: 0.10 to 1.0%
Mn has the effect of increasing the hardenability and improving the strength. In order to fully exhibit this effect, it is necessary to contain 0.10% or more. On the other hand, when it contains exceeding 1.0%, the concentration to a carbide | carbonized_material will be remarkable and the solid solution of the carbide | carbonized_material at the time of hardening will be overdue. Therefore, the content of Mn is set to 0.10 to 1.0%. In order to obtain stable hardenability, it is desirable to contain 0.25% or more. In order to sufficiently dissolve the carbide during quenching, the upper limit of the Mn content is preferably 0.59%, and more preferably 0.55%.

P: 0.030% or less P is contained as an impurity in the steel and segregates at grain boundaries to reduce toughness and delayed fracture resistance. In particular, when its content exceeds 0.030%, toughness and anti-resistance Decreased delayed fracture property becomes remarkable. Therefore, the content of P is set to 0.030% or less. The content of P is preferably as low as possible.

S: 0.030% or less S is contained as an impurity in steel and segregates at the grain boundaries to lower toughness and delayed fracture resistance. In particular, when its content exceeds 0.030%, toughness and resistance to resistance are reduced. Decreased delayed fracture property becomes remarkable. Therefore, the content of S is set to 0.030% or less. The content of S is preferably as low as possible.

Al: 0.005-0.10%
Al combines with N to form nitrides, which effectively work for fine graining by the pinning effect and improve delayed fracture resistance. In order to fully exhibit the effect, it is necessary to contain 0.005% or more. However, even if Al is contained in an amount exceeding 0.10%, the above effect is saturated and the toughness deteriorates. Therefore, the content of Al is set to 0.005 to 0.10%. Moreover, in order to ensure better toughness, it is desirable that the upper limit of the Al content be 0.06%. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

Cr: 0.90 to 1.8%
Cr has the effect of increasing the hardenability and improving the strength. In order to obtain a tensile strength of 1200 MPa or more, it is necessary to contain 0.90% or more of Cr. However, even if Cr is contained in excess of 1.8%, it is concentrated in carbides, and solid solution of carbides during quenching is delayed. Therefore, the Cr content is set to 0.90 to 1.8%. Further, in order to sufficiently dissolve the carbide during quenching, it is desirable that the upper limit of the Cr content is 1.5%.

Mo: 0.10 to 2.0%
Mo has the effect | action which raises hardenability and improves intensity | strength. Mo also contributes to precipitation strengthening by forming carbides, and has the effect of improving strength without lowering the tempering temperature. In order to obtain a tensile strength of 1200 MPa or more, it is necessary to contain 0.10% or more of Mo. However, even if Mo is contained in an amount exceeding 2.0%, the effect is saturated and the cost is increased, and it is concentrated in carbides, and solid solution of carbides during quenching is delayed. Therefore, the Mo content is set to 0.10 to 2.0%. In order to fully exhibit the effect of improving the strength of Mo, the lower limit of the Mo content is desirably 0.31%, and more desirably 0.33%. In this case, a tensile strength of 1300 MPa or more can be reliably ensured. Further, in order to sufficiently dissolve the carbide during quenching, the upper limit of the Mo content is desirably 1.8%, and more desirably 1.5%.

N: 0.003-0.030%
N combines with Nb and Al to form Nb nitrides and carbonitrides and Al nitrides, and effectively works for fine graining by the pinning effect and improves delayed fracture resistance. In order to fully exhibit the effect, it is necessary to contain 0.003% or more. However, if its content becomes excessive and exceeds 0.030%, nitrogen blowholes are generated during melting, which tends to cause wrinkles during processing. Therefore, the N content is set to 0.003 to 0.030%. In order to secure better delayed fracture resistance, it is desirable that the lower limit of the N content be 0.005%.

  The steel for bolts according to the present invention contains the above-described elements, and the balance has a chemical composition composed of Fe and impurities. Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something. The steel for bolts according to the present invention may further contain the following amount of Nb instead of part of Fe.

Nb: 0.10% or less Nb combines with C and N to form carbides, nitrides and carbonitrides, effectively works for fine graining by the pinning effect, and improves delayed fracture resistance as needed May be included. However, since these effects are saturated when the content exceeds 0.10%, the upper limit of the amount when Nb is included is set to 0.10%. In addition, in order to express this effect stably, it is desirable to contain Nb 0.005% or more.

2. Steel structure The steel material for bolts of the present invention is the number of carbides having an equivalent circle diameter of 1.0 μm or more out of carbides having an equivalent circle diameter of 0.5 μm or more after primary annealing, wire drawing and secondary annealing. It has a structure with a ratio of 10% or less.

  The equivalent circle diameter of carbide can be measured by the following method. The R / 2 position (“R” is the radius of the steel wire) of the cross-section of the Picral-etched steel wire was observed at a magnification of 5000 times using a scanning electron microscope, and 6 fields randomly selected Take an image. A binarization process was performed on the captured image, and the area of carbide was obtained from the image data, and converted to a circle-equivalent diameter. When using a binarized image, a carbide having an equivalent circle diameter of 0.5 μm or more can be discriminated properly and accurately even if there is a fine unevenness in the structure. Carbides of 5 μm or more were measured.

  Carbides having an equivalent circle diameter of 1.0 μm or more are particularly difficult to dissolve during quenching by heating at 860 to 900 ° C. and have a significant effect on variations in tensile strength. Carbides with a size of 0 μm or more are coarse carbides.

  Of the carbides having a circle-equivalent diameter of 0.5 μm or more, which is a measurement target, the number ratio of coarse carbides of 1.0 μm or more needs to be 10% or less. In order to sufficiently dissolve the carbide during quenching, the smaller the amount of coarse carbide, the better. The number ratio of coarse carbide is preferably 5% or less, and more preferably 3% or less.

3. Manufacturing Method of Bolt The steel material for bolts according to the present invention only needs to satisfy the above chemical composition and structure, and the manufacturing method is not particularly limited. For example, when the steel material is a steel wire, the number ratio of coarse carbides can be reduced by manufacturing by the following method.

  The steel having the above chemical components is subjected to hot finish rolling and then scraped off and cooled using a stealmore cooling device. In that case, it is desirable that the scraping temperature is 850 ° C. and the end point temperature of the steermore conveyor is 550 ° C., both of which can tolerate an error of ± 100 ° C. It is desirable that the cooling rate from the scraping temperature to the Stealmore end point temperature is 2 to 10 ° C./second.

  By cooling under the above conditions, the steel material after rolling can have a structure in which the area ratio of bainite is 60% or more and the balance is composed of ferrite and pearlite. By making the area ratio of bainite, in which fine carbides are uniformly precipitated, 60% or more, it becomes possible to suppress the coarsening of the carbides during spheroidizing annealing.

  In order to prevent the carbide from coarsening due to the spheroidizing annealing, the primary annealing is preferably performed under a temperature condition lower than that of the secondary annealing. Specifically, primary annealing is performed by holding at 650 ° C. or more and less than 720 ° C. for 1 to 10 hours, and then wire drawing is performed at a surface reduction rate of 5 to 40%, and further, 1 to 10 at 720 to 780 ° C. It is desirable to perform secondary annealing while maintaining the time.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  All seven types of steels A to G having chemical components shown in Table 1 were melted in a converter and slabs were produced with a continuous casting machine. These cast slabs were divided and rolled into 160 mm square steel pieces, heated at 1120 ° C., and hot-rolled to obtain 18 mm diameter wires. After that, it is scraped off at 850 ° C., air-cooled at a cooling rate shown in Table 2 to 550 ° C., which is the end point temperature of the Stealmore conveyor, by a Stealmore cooling device, and then allowed to cool to room temperature. Was made.

  Steels A to C having the chemical composition shown in Table 1 are steels whose chemical compositions are within the range specified in the present invention, and Steels D to G are comparative examples whose chemical compositions are outside the specified range of the present invention. Of steel.

  After collecting five steel wires from each coil end after the above hot rolling, R / 2 part of the cross section of each steel wire subjected to nital etching was obtained at a magnification of 500 times using an optical microscope. Six visual fields were randomly observed for each steel wire, and the average value of the bainite area ratios in a total of 30 visual fields was determined and used as the bainite area ratio in each test number. The results are also shown in Table 2.

  Further, the above rolling coils were each annealed at the temperatures shown in Table 2 for 4.5 hours, subjected to primary annealing, drawn to a diameter of 15.8 mm, and then held at 755 ° C. for 4.5 hours. Next annealing was performed. After collecting five steel wires from the coil end after the secondary annealing, the R / 2 part of the cross-section of the steel wire subjected to Picral etching was used for each steel at a magnification of 5000 times using a scanning electron microscope. Six lines were picked up randomly for each line, and the obtained image was binarized. Then, the average value of the number ratio of coarse carbides in a total of 30 fields was obtained and used as the coarse carbide number ratio in each test number. . The measurement results are shown in Table 2. The measurement object is a carbide having an equivalent circle diameter of 0.5 μm or more, and the coarse carbide is a carbide having an equivalent circle diameter of 1.0 μm or more.

  Thereafter, 30 steel wires having a diameter of 15.8 mm and a length of 150 mm were further collected from the coil end, and randomly placed in a heat treatment having a temperature distribution of ± 30 ° C. in the furnace with respect to the set temperature. After heating at the set temperature shown in 2 for 30 minutes, quenching was performed to quench in oil. And it tempered by heating for 30 minutes at the temperature shown in Table 2, and produced the steel wire of tensile strength 1200MPa or more.

  From the steel wire after the quenching and tempering treatment, a JIS No. 14A tensile test piece was sampled and measured for tensile strength at room temperature. The tensile test was performed on all 30 steel wires, and the average value, standard deviation, and relative standard deviation (%: (standard deviation / average value) × 100) of tensile strength were determined.

  From Table 2, test numbers 1 to 3, which are examples of the present invention, are within the range defined by the present invention in chemical composition, and are coarse carbides having an equivalent circle diameter of 1.0 μm or more among carbides having an equivalent circle diameter of 0.5 μm or more. It can be seen that the relative proportion of tensile strength is as low as 0.41 to 0.54% and the variation in tensile strength is extremely small.

  On the other hand, the test numbers 4 to 7 which are comparative examples have contents of C, Mn, Cr and Mo exceeding the upper limits specified in the present invention, respectively, and the number ratio of coarse carbides is the specified range of the present invention in all of them. Therefore, the relative standard deviation of tensile strength was as high as 0.93 to 1.13%, and sufficient strength stability could not be obtained.

  Moreover, test number 8 which is a comparative example is the number of coarse carbides because the chemical composition is within the range specified by the present invention, but the cooling rate after rolling is slow and the area ratio of bainite is low. The ratio does not satisfy the conditions specified in the present invention. Therefore, the relative standard deviation of tensile strength was as high as 1.27%. Furthermore, test number 9 also has the chemical composition within the range defined by the present invention, but the number ratio of coarse carbides satisfies the conditions defined by the present invention due to the high primary annealing temperature. Absent. Therefore, as in Test No. 8, the relative standard deviation of tensile strength was as high as 1.16%.

  From the above results, it has been clarified that the inventive example having a low number ratio of coarse carbides can strictly suppress the variation in tensile strength after quenching and tempering as compared with the comparative example.

  The steel material for high-strength bolts of the present invention has a tensile strength of 1200 MPa or more after quenching and tempering, and has a very small variation in tensile strength. Therefore, it is suitable for use in automobiles, various industrial machines and building structures. . In particular, it is used as a material for bolts that are used for fastening important mechanical structures such as automobile engine parts, and that is required to apply a predetermined axial force required for fastening with high strength to the bolts without variation.

Claims (2)

The chemical composition of the steel material is% by mass, C: 0.30 to 0.40%, Si: 0.01 to 0.40%, Mn: 0.10 to 1.0%, P: 0.030% or less , S: 0.030% or less, Al: 0.005-0.10%, Cr: 0.90-1.8%, Mo: 0.10-2.0% and N: 0.003-0. Containing 030%, the balance being Fe and impurities, and having a structure in which the number ratio of carbides having an equivalent circle diameter of 1.0 μm or more out of carbides having an equivalent circle diameter of 0.5 μm or more is 10% or less (provided that A steel material for bolts having a tensile strength of 1200 MPa or more after quenching and tempering, after the primary annealing and secondary annealing, and before quenching and tempering , excluding the case where the structure contains a pearlite structure.   The steel composition for bolts according to claim 1, wherein the chemical composition of the steel material contains, in place of a part of Fe, mass% and further Nb: 0.10% or less.