JP2007107027A - Steel material and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material, even in the case a drive shaft or the like with small diameters are produced, which can secure excellent strength, toughness or the like, and also has satisfactory machinability, and to provide its production method. <P>SOLUTION: A steel material comprising prescribed amounts of C, Si, Mn, S, Ti, Cr and B, and the balance iron with inevitable impurities, and in which the occupancy area ratio of ferrite is ≥30% and H<SB>RB</SB>is 85 to 93 is subjected to induction hardening. The surface of the steel material after being subjected to the hardening shows 650 to 730 by H<SB>V</SB>. Further, provided that the distance from the surface to a part showing 392 by H<SB>V</SB>is defined as effective hardened layer depth (t), and the radius or thickness of the steel material is defined as (r), the hardened layer ratio t/r is ≥0.5. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel material and a manufacturing method thereof, and more particularly to a steel material suitable as a material for a drive shaft, for example, and a manufacturing method thereof.

  A drive shaft that constitutes a traveling engine of an automobile is generally made of steel. This type of steel material is required to be relatively easy to perform cutting and the like. On the other hand, in order to ensure the durability of the drive shaft, it is necessary that the shearing stress leading to fracture is high when torsional torque is applied, in other words, the torsional strength is high. In order to obtain such a drive shaft, for example, a JIS-S40C equivalent material is selected as the steel material.

By the way, with the recent increase in interest in environmental protection, various studies have been made to improve the fuel efficiency of automobiles in order to reduce the amount of exhaust gas such as CO ₂ and NOx. From this point of view, attempts have been made to reduce the weight by reducing the size of the components of the automobile.

  When a small-sized drive shaft is produced from the same steel material, the strength and the like are usually lowered. For this reason, even when a small-sized drive shaft is produced, a steel material capable of ensuring sufficient strength and the like is desired.

  In order to meet this demand, for example, Patent Document 1 proposes that a drive shaft is composed of a steel material in which the composition ratio, surface hardness, martensite ratio, and hardening depth ratio of constituent elements satisfy predetermined numerical values. Yes.

  Patent Document 2 discloses a steel for induction hardening in which the structure area ratio of ferrite with respect to contained C and the crystal grain size of ferrite are equal to or less than predetermined values. According to Patent Document 2, this induction hardening steel is suitable as a material for a drive shaft.

JP-A-10-36937 JP 2002-69566 A

  The present invention has been made in connection with the above-described technology, and an object of the present invention is to provide a steel material that exhibits excellent characteristics even when the member has a small size, and a method for manufacturing the same.

In order to achieve the above object, the steel material according to the present invention is, in mass%, C: 0.47 to 0.52%, Si: 0.45 to 0.55%, Mn: 0.65 to 0.00. 75%, S: 0.025-0.035%, Ti: 0.02-0.03%, Cr: 0.05-0.15%, B: 0.001-0.003%, The balance is iron and inevitable impurities,
The surface Vickers hardness is 650-730,
It is characterized in that the cured layer ratio t / r is 0.5 or more, where the distance from the surface up to the portion showing 392 in Vickers hardness is the effective cured layer depth t and the radius or thickness is r. To do.

  By setting the hardness and the hardened layer ratio in this way, hardness and strength are ensured. This is because a substance having a high hardness generally has a high strength. In addition, since the hardness is not excessively large, toughness is ensured and brittle fracture is difficult to occur.

  And it can also be set as the steel material excellent in machinability by setting it as an above-described component and composition ratio.

  N in this steel material is preferably 0.01% or less by mass%. In this case, BN and TiN are difficult to generate. Accordingly, it is possible to avoid the hardness from being excessively increased and the workability and the machinability from being lowered while the quenching proceeds without being inhibited.

  Furthermore, it is preferable that at least one of Mo for improving the grain boundary strength of the steel material or Al having a deoxidizing effect is included. The proportions of Mo and Al may be 0.04 to 0.09% and 0.002 to 0.004% (mass%), respectively. Of course, 0.04 to 0.09% Mo and 0.002 to 0.004% Al may be present simultaneously.

In the present invention, when the surface Vickers hardness is 650 to 730 and the distance from the surface to the portion showing 392 in Vickers hardness is the effective hardened layer depth t and the radius or thickness is r, t / r Is a method for producing a steel material of 0.5 or more,
In mass%, C: 0.47 to 0.52%, Si: 0.45 to 0.55%, Mn: 0.65 to 0.75%, S: 0.025 to 0.035%, Ti: 0.02 to 0.03%, Cr: 0.05 to 0.15%, B: 0.001 to 0.003%, the balance is iron and inevitable impurities, and the occupied area ratio of ferrite is It is characterized by subjecting a raw material steel having a B-scale Rockwell hardness of 85 to 93 to 30% or more and induction hardening.

  That is, by subjecting the raw material steel having the components and composition ratios described above to induction hardening, a steel material having excellent hardness and strength and good machinability can be obtained.

  By using a steel material exhibiting such characteristics, it is possible to produce a member having excellent strength even with a small size. This steel material can be suitably employed as a material for a drive shaft, for example.

  Moreover, as described above, it is preferable that at least one of Mo for improving the grain boundary strength of the steel material or Al having a deoxidizing effect is included. The Mo ratio may be 0.04 to 0.09% by mass, and the Al ratio may be 0.002 to 0.004%.

  According to the present invention, the steel material is configured by setting the components, composition ratio, hardness, and hardened layer ratio. As a result, a steel material having excellent strength, particularly torsional strength, and good machinability can be obtained.

  Since this steel material has good machinability and is rich in composition deformability, it is extremely easy to process. By using a steel material exhibiting such characteristics, it is possible to produce a member having excellent strength even with a small size. Moreover, since it is excellent in toughness, it is hard to produce a crack during a process.

  Moreover, since this steel material has high torsional strength, it is suitable as a material for a long shaft member, for example, a drive shaft.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of a steel material and a manufacturing method thereof according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is an overall schematic side view of the drive shaft 10 along the longitudinal direction. The drive shaft 10 is a long solid body, and the diameter of the measurement site denoted by reference numeral 12 in FIG. 1 is 24 mm.

  Here, the steel material which is the material of the drive shaft 10 contains C, Si, Mn, S, Ti, Cr, and B in addition to iron and inevitable impurities.

  C is a component for ensuring the strength and hardness of the drive shaft 10 after quenching and tempering, and the composition ratio is 0.47 to 0.52% (the numbers are mass%, the same applies hereinafter). Set to If it is less than 0.47%, it will be difficult to ensure the hardness. On the other hand, if the content is more than 0.52%, the hardness increases excessively, so that it is not easy to perform plastic deformation processing, cutting processing, or the like. In particular, the deformability in cold working is reduced.

  Si is an element useful for deoxidation of the drive shaft 10 and is contained in the drive shaft 10 at a ratio of 0.45 to 0.55%. If it is less than 0.45%, the deoxidation effect is poor. On the other hand, if it exceeds 0.55%, the hardness will increase excessively, so that it will not be easy to perform plastic deformation or cutting. In particular, the deformability in cold working is reduced.

  Mn improves the induction hardenability of the drive shaft 10. That is, due to the presence of Mn, when induction hardening is performed on the drive shaft 10, the hardness is significantly increased as compared with that before the quenching. In order to reliably obtain this effect, the composition ratio of Mn is set to 0.65 to 0.75%. If it exceeds 0.75%, the hardness will increase excessively, so it will not be easy to perform plastic deformation or cutting. In particular, the deformability in cold working is reduced.

  S is a component that forms MnS in the structure of the drive shaft 10 together with Mn, thereby improving the machinability of the drive shaft 10. The composition ratio of S is set to 0.025 to 0.035%. If it is less than 0.025%, it is difficult to improve the machinability, and if it exceeds 0.035%, the deformability particularly during cold working is reduced.

  Ti plays a role of capturing free N in the drive shaft 10. When free N is trapped in this way, the effect of adding B described later becomes more remarkable. The composition ratio of Ti is set to 0.02 to 0.03%. If it is less than 0.02%, the effect of capturing free N is poor. On the other hand, if it exceeds 0.03%, Ti is excessively present, so that machinability and deformability during cold working are reduced.

  Cr is a component that improves the hardenability of the drive shaft 10. In other words, the presence of Cr significantly increases the hardness of the drive shaft 10 after quenching. The composition ratio of Cr is set to 0.05 to 0.15%. If it is less than 0.05%, it is difficult to obtain this effect. On the other hand, if it exceeds 0.15%, Cr concentrates in cementite, which prevents carbon from dissolving in the steel during quenching.

  B is a component that improves the grain boundary strength. Moreover, the hardenability of the drive shaft 10 is also improved. This effect is reduced when N is present in excess. This is because BN is generated from B and surplus N. Therefore, as described above, this effect is secured by capturing N with a predetermined amount of Ti.

  The composition ratio of B is set to 0.001 to 0.003% (10 to 30 ppm). If it is less than 10 ppm, the effect of improving the grain boundary strength is poor. Moreover, when it exceeds 30 ppm, hardenability will be reduced.

  Here, if the free N is excessively present in the drive shaft 10, it binds to B as described above to generate BN, and as a result, hardenability and the like are reduced. Further, when TiN is produced by combining with Ti, the hardness is excessively increased and it becomes difficult to perform processing, and the toughness is lowered. In order to avoid such a situation, in the present embodiment, the composition ratio of N existing in the drive shaft 10 is set to 0.01% or less.

  The drive shaft 10 may further contain Mo and Al.

  Mo is an element useful for improving the grain boundary strength of the drive shaft 10 after quenching, and particularly for improving the torsional strength. The composition ratio of Mo is set to 0.04 to 0.09%. If it is less than 0.04%, it is difficult to improve the grain boundary strength. On the other hand, if it exceeds 0.09%, the hardness will increase excessively, so that it will not be easy to carry out plastic deformation or cutting. In particular, the deformability during cold working is reduced.

Al is a component that contributes to deoxidation in the same manner as Si. When the Al composition ratio is less than 0.002%, the deoxidation effect is poor. Further, when Al is present in excess, increased oxide impurities such as Al ₂ O _3, as a result, fatigue properties, deformability during plastic deformation decreases. For this reason, the upper limit of Al is set to 0.004%.

  Further, when the structure of the drive shaft 10 is observed, the occupied area ratio of ferrite existing in the structure is 30% or more. That is, when the area of the entire visual field is 100%, ferrite accounts for 30% or more. Due to the presence of ferrite in such a ratio, the drive shaft 10 after quenching exhibits excellent toughness.

Furthermore, the hardness of the surface of the drive shaft 10 is 85 to 93 when expressed in B scale Rockwell hardness (H _RB ). By setting the surface hardness within this range, the strength of the drive shaft 10 after quenching and tempering is ensured.

  The drive shaft 10 is manufactured by, for example, turning or rolling a cylindrical workpiece made of a steel material having the above-described components / composition ratio.

  Next, the drive shaft 10 is subjected to quenching / tempering treatment.

  In the present embodiment, an induction hardening method in which heating is performed with a high frequency is employed as the quenching. That is, first, the surface of the drive shaft 10 is rapidly heated by the high frequency induction current, and then the cooling liquid is jetted onto the surface to perform rapid cooling. For example, when the induction current frequency and output are approximately 1 to 40 kHz and 50 to 100 kW, the heating time may be 1 to 5 seconds.

  Next, a tempering process is performed on the drive shaft 10 within a temperature range from 100 ° C. to a point immediately below the A1 point. As a result, the residual stress is removed from the drive shaft 10, and it is possible to suppress the aging of the drive shaft 10 and the occurrence of cracks.

The Vickers hardness (H _V ) of the surface of the drive shaft 10 subjected to the quenching and tempering treatment as described above is 650 to 730. A steel material having a high hardness generally has a high strength, and if it has such a hardness, the toughness will not be insufficient.

In the drive shaft 10, the hardness decreases as shown in FIG. 2 in the radial direction, that is, from the surface to the inside. In the present embodiment, thus measuring the hardness of the drive shaft 10 from the surface side and the distance to the site showing the 392 H _V a (depth) and the effective case depth t. Since the diameter of the measurement site 12 is 24 mm as described above, 12 mm on the horizontal axis of the graph in FIG. 2 represents the center in the radial direction at the measurement site 12.

  When the radius of the drive shaft 10 is r, the cured layer ratio t / r, which is the ratio of the effective cured layer depth t to the radius r, is 0.5 or more. If it is less than 0.5, the thickness of the effective hardened layer is not sufficient, so that the torsional strength of the drive shaft 10 becomes small.

That is, the H _V surface 650-730, by the hardened layer ratio t / r of 0.5 or more, a drive shaft 10 which is excellent in strength and toughness is obtained.

  In the above-described embodiment, the drive shaft 10 is illustrated and described as a steel material. However, the final product is not particularly limited to this, and may be other things such as an outer member constituting a constant velocity joint. Good.

  0.48% for C, 0.50% for Si, 0.72% for Mn, 0.029% for S, 0.08% for Cr, 20 ppm for B, 0.01% for Ni, and P for P A steel ingot containing 0.006%, Cu 0.02%, O 11 ppm and N 35 ppm was prepared. Next, hot forging and induction hardening were performed on the ingot to produce 15 cylindrical workpieces having a diameter of 18.5 mm. The drive shaft 10 was subjected to induction hardening and tempering treatment under various conditions, and the effective hardened layer depth t and the hardened layer ratio t / r were variously changed. Thereafter, the shear stress of each cylindrical workpiece was measured. The results are also shown in FIG. A large value of the shear strength means that the static torsional strength is large.

  FIG. 4 is a graph showing the relationship (average value) between the cured layer ratio t / r and the shear stress. For comparison, the relationship between t / r and shear stress in S40C of the same size is also shown.

  From FIG. 3 and FIG. 4, it is clear that a cylindrical workpiece having a large shear stress can be formed by increasing the hardened layer ratio t / r and setting each component to a predetermined composition ratio.

  A workpiece exhibiting such characteristics is suitable as a raw material for the drive shaft 10.

It is the whole schematic side view along the longitudinal direction of the drive shaft which consists of steel materials concerning this Embodiment. It is a graph which shows the relationship between the distance from the surface in the drive shaft of FIG. 1, and hardness. It is a graph which shows the effective hardened layer depth t in each cylindrical body shape workpiece | work, hardened layer ratio t / r, and shear stress. It is a graph which shows the relationship between the hardened layer ratio r / t in a cylindrical body-shaped workpiece | work and S40C cylindrical body-shaped workpiece | work, and a shear stress.

Explanation of symbols

10 ... Drive shaft

Claims (5)

In mass%, C: 0.47 to 0.52%, Si: 0.45 to 0.55%, Mn: 0.65 to 0.75%, S: 0.025 to 0.035%, Ti: 0.02 to 0.03%, Cr: 0.05 to 0.15%, B: 0.001 to 0.003%, the balance is iron and inevitable impurities,
The surface Vickers hardness is 650-730,
A steel material characterized in that a hardened layer ratio t / r is 0.5 or more, where an effective hardened layer depth t is a distance from the surface up to a portion showing 392 in Vickers hardness and a radius is r.   The steel material according to claim 1, wherein N is 0.01% or less by mass%.   The steel material according to claim 1 or 2, further comprising at least one of Mo: 0.04 to 0.09% and Al: 0.002 to 0.004% by mass%. . When the surface Vickers hardness is 650 to 730, the distance from the surface to the portion showing 392 in Vickers hardness is t, the effective hardened layer depth t, and the radius or thickness is r, t / r is 0.5 or more A method of manufacturing a steel material,
In mass%, C: 0.47 to 0.52%, Si: 0.45 to 0.55%, Mn: 0.65 to 0.75%, S: 0.025 to 0.035%, Ti: 0.02 to 0.03%, Cr: 0.05 to 0.15%, B: 0.001 to 0.003%, the balance is iron and inevitable impurities, and the occupied area ratio of ferrite is A method for producing a steel material, characterized by subjecting a raw material steel having a B-scale Rockwell hardness of 85 to 93 to 30% or more and induction hardening.   5. The manufacturing method according to claim 4, wherein the raw material steel further contains Mo: 0.04 to 0.09% and Al: 0.002 to 0.004% by mass%. Steel material manufacturing method.

Images