JP6057626B2 - Machine structural steel with low heat treatment deformation - Google Patents

Description

  The present invention relates to a machine structural steel used as a power transmission component such as a gear and a shaft used in automobiles and industrial machines, for example, and more particularly to a machine structural steel having a small heat treatment deformation.

  It is known that deformation of a steel material (hereinafter referred to as “heat treatment deformation”) occurs due to heat treatment such as quenching. If there is such heat treatment deformation, the number of manufacturing processes will increase to correct the deformation, the defective part rate will increase if it cannot be corrected, or it will be caused by deformation when incorporated as a drive system part. There are adverse effects such as generating noise and vibration. Therefore, it is a very important problem in practice to keep the heat treatment deformation as small as possible.

  Conventionally, this heat treatment deformation is considered to be influenced by many factors other than steel, such as part shape, influence of pre-heat treatment process, physical properties of refrigerants such as quenching oil, and non-uniform cooling. It has been. Therefore, attempts have been made to reduce heat treatment deformation by optimizing them in various ways. For example, as a material countermeasure, a method has been proposed in which a soft ferrite phase is precipitated in the core of a hardened steel material to reduce heat treatment strain (see, for example, Patent Document 1).

  As an approach from the cooling method, a method using pressurized gas cooling instead of conventional oil quenching has been proposed (see, for example, Patent Document 2). In addition, a method for achieving uniform cooling of an object to be cooled using means for promoting or reducing the heat transfer coefficient has been proposed (for example, see Patent Document 3).

  In Patent Document 3, the means for promoting the heat transfer coefficient is due to the convection of the coating material that promotes cooling provided in the portion where cooling is delayed or the coolant formed around the portion where cooling is delayed, The means for reducing the heat transfer rate is said to be glass wool or a heat insulating coating material covering a portion where cooling is likely to proceed.

  However, the conventional method proposed above cannot always be a general-purpose means. This is because, for example, in the technique of Patent Document 1, a soft phase having a low strength may be introduced into a part, and in the technique of Patent Document 2, the heat treatment furnace itself has to be changed. This is because the technique of Patent Document 3 is not necessarily a general-purpose means, for example, it requires processing for individual heat-treated parts.

  On the other hand, the inventors of the present application ensure a sufficient steel strength without relying on the formation of a ferrite structure that is soft and may reduce the strength of the parts, and under a general quenching technique such as oil quenching, We have conducted intensive research on steel that can keep heat treatment deformation small even when the parts are not evenly cooled. As a result, by controlling the chemical composition of steel, the martensitic transformation start temperature (Ms point), and the hardenability measured by Jominy's one-end quenching method to an appropriate range, the heat treatment deformation can be kept small. This is the headline and the present invention.

JP 1997-111408 A JP 2008-121064 A JP 2010-174289 A

  The problem to be solved by the present invention is to provide a steel material with less heat treatment deformation made of mechanical structural steel used as power transmission parts such as gears and shafts used in automobiles and industrial machines.

The means of the present invention for solving the above-mentioned problems is that in the means of claim 1, in mass%, C: 0.20 to 0.30%, Si: 0.10 to 1.50%, Mn: 0 20 to 0.55%, P: 0.030% or less, S: 0.014% or less, Cr: 1.30 to 2.50%, Cu: 0.30% or less, Al: 0.008 to 0 300%, O: 0.0030% or less, N: 0.0020 to 0.0300%, steel for mechanical structure comprising the balance Fe and inevitable impurities, martensitic transformation start temperature of the steel material comprising the steel The (Ms point) is 460 ° C. or less, and the steel material is measured by the Jominy-type one-end quenching method, the hardness J1.5 at a distance 1.5 mm from the quenching end of the steel material and the hardness J9 at a distance 9 mm. (J9 / J1.5) calculated by the following equation (1) using The value is in the range of 0.70 to 0.85, and further calculated by the following equation (2) using J1.5 of hardness at a distance of 1.5 mm and J11 of hardness at a distance of 11 mm (J11 / J1.5) is a steel for machine structural use with small heat treatment deformation, characterized in that it is in the range of 0.67 to 0.78.
However,
(J9 / J1.5) = (hardness at a distance of 9 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (distance from the quenching end measured by the Jominy type one-end quenching method. 5mm hardness) ... Formula (1)
(J11 / J1.5) = (hardness at a distance of 11 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (distance from the quenching end measured by the Jominy type one-end quenching method. (Hardness at 5 mm) (2)

In the means of claim 2, in mass%, C: 0.20 to 0.30%, Si: 0.10 to 1.50%, Mn: 0.20 to 0.55%, P: 0.030% Hereinafter, S: 0.014% or less, Cr: 1.30 to 2.50%, Cu: 0.30% or less, Al: 0.008 to 0.300%, O: 0.0030% or less, N: It contains 0.0020 to 0.0300%, further contains one or two of Ti: 0.020 to 0.200%, Nb: 0.02 to 0.20%, and the balance Fe and inevitable impurities The steel material is made of mechanical structural steel, and the martensite transformation start temperature (Ms point) of the steel material is 460 ° C. or less, and the steel material is measured by Jominy one-end quenching method. J1.5 of hardness and distance 9mm at distance 1.5mm from the input end The value of (J9 / J1.5) calculated by the above equation (1) using J9 of hardness in the range is 0.70 to 0.85, and further, the hardness of J1 at a distance of 1.5 mm Heat treatment characterized in that the value of (J11 / J1.5) calculated by the above equation (2) is in the range of 0.67 to 0.78 using J11 of hardness at 0.5 and a distance of 11 mm It is a steel material for machine structures with little deformation.
However,
(J9 / J1.5) = (hardness at a distance of 9 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (distance from the quenching end measured by the Jominy type one-end quenching method. 5mm hardness) ... Formula (1)
(J11 / J1.5) = (hardness at a distance of 11 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (distance from the quenching end measured by the Jominy type one-end quenching method. (Hardness at 5 mm) (2)

  The reasons for limiting the components in the means of the above claims will be described below. In addition,% of each component element shows the mass%.

C: 0.20 to 0.30%
C is an element necessary for ensuring the strength after quenching and tempering of steel or the strength of the core after carburizing, quenching and tempering as machine structural parts, and is adjusted to a predetermined range in order to reduce heat treatment deformation. There is a need to. If the range is less than 0.20%, the strength cannot be secured, and if it exceeds 0.30%, deformation due to heat treatment becomes too large. Therefore, C is 0.20 to 0.30%, preferably 0.22 to 0.27%.

Si: 0.10 to 1.50%
Si is an element necessary for deoxidation and is an effective element for imparting necessary strength and hardenability to steel. However, if Si is less than 0.10%, the effect cannot be obtained, and if it exceeds 1.50%, the machinability is lowered. Therefore, Si is 0.10 to 1.50%, preferably 0.20 to 1.00%.

Mn: 0.20 to 0.55%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.10%, a sufficient effect on hardenability cannot be obtained, and if it exceeds 1.20%, the machinability is lowered. Therefore, Mn is 0.10 to 1.20%, preferably 0.20 to 0.80%, and more preferably 0.20 to 0.55%. By the way, more preferably, 0.20 to 0.55%, Mn: 0.10 to 1.20% of the limited range of Mn at the time of filing, Mn after correction: 0.20 to 0 .55%.

P: 0.030% or less P is an unavoidable element contained in scrap, but segregates at the grain boundary and lowers properties such as impact strength and bending strength. Therefore, P is set to 0.030% or less.

S: 0.014% or less S is an element that improves machinability, but generates MnS, which is a non-metallic inclusion, and lowers the toughness and fatigue strength in the lateral direction. Therefore, S is set to 0.030% or less. By the way, No. of the invention example which is the maximum value of the S component amount in Table 1 after correction. 8, S: 0.030% or less of the limited range of S at the time of filing is made S: 0.014% or less after correction.

Cr: 1.30 to 2.50%
Cr is an element necessary for ensuring hardenability. However, if Cr is less than 1.30%, a sufficient effect on hardenability cannot be obtained, and if it exceeds 2.50%, carburization is inhibited, and machinability is also deteriorated. Therefore, Cr is 1.30 to 2.50%, preferably 1.50 to 2.25%.

Cu: 0.30% or less Cu is an inevitable element contained from scrap, but has aging properties and an effect of increasing strength. However, when Cu exceeds 0.30%, the hot workability decreases. Therefore, Cu is made 0.30% or less.

Al: 0.008 to 0.300%
Al is an element used as a deoxidizing material, and also binds to N and precipitates as AlN as will be described later, thereby bringing about an effect of suppressing grain coarsening. In order to obtain this effect, Al needs to be added in an amount of 0.008% or more. On the other hand, if Al is added in excess of 0.300%, large alumina inclusions are formed, and fatigue characteristics and workability are deteriorated. Therefore, Al is made 0.008 to 0.300%, preferably 0.014 to 0.200%.

O: 0.0030% or less O is an element inevitably contained in steel. However, if O exceeds 0.0030%, workability and fatigue strength are reduced due to an increase in oxide. Therefore, O is set to 0.0030% or less, preferably 0.0020% or less.

N: 0.0020 to 0.0300%
N precipitates finely as AlN or Nb nitride in the steel and brings about an effect of preventing coarsening of crystal grains, and 0.0020% or more needs to be added to obtain the effect. However, if it exceeds 0.0300%, nitrides increase, and fatigue strength and workability decrease. Therefore, N is 0.0020 to 0.0300%, preferably 0.0020 to 0.0200%. However, especially in steel containing 0.020% or more of Ti, N is made 0.0020 to 0.0100% in order to avoid a decrease in fatigue strength due to excessive TiN formation.

Ti: 0.020 to 0.200%
Ti combines with C in steel to form carbides finely and brings about the effect of preventing crystal grain coarsening. To obtain this effect, it is necessary to add 0.020% or more of Ti. . On the other hand, the addition of Ti exceeding 0.200% impairs machinability. Therefore, Ti is set to 0.020 to 0.200%.

Nb: 0.02 to 0.20%
Nb forms carbides or nitrides, and has an effect of preventing crystal grain coarsening. In particular, nano-order sized NbC or Nb (C, N) finely dispersed in steel suppresses the growth of crystal grains. If Nb is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.20%, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, Nb is 0.02 to 0.20%, preferably 0.02 to 0.12%.

  Further, the reasons for limiting the Ms point and the hardenability measured by the Jominy one-side quenching method other than the reasons for limiting the above components will be described.

Ms point: 460 ° C. or lower In order to reduce the heat treatment deformation of the steel material in the means of the present invention, it is necessary to regulate the martensitic transformation start temperature (Ms point) to 460 ° C. or lower. The reason why heat treatment deformation can be reduced by restricting the Ms point to 460 ° C. or less is that, even when the parts are not cooled when quenched, the martensitic transformation occurs in the temperature range where the cooling performance of the refrigerant is high. This is because it can be avoided, and as a result, it can be suppressed that the martensitic transformation time largely deviates depending on the parts. Therefore, the Ms point is restricted to 460 ° C. or lower, but preferably the Ms point is restricted to 450 ° C. or lower. In this case, the heat treatment deformation means bending of the shaft-shaped part, falling of the gear teeth, or twisting.

(J9 / J1.5) value: 0.70 to 0.85, (J11 / J1.5) value: 0.67 to 0.78
From the J1.5 hardness at a distance of 1.5 mm from the quenching end of the steel material measured by the Jominy-type one-end quenching method of the steel material, the J9 hardness at a distance of 9 mm, and the J11 hardness at a distance of 11 mm The reason why the calculated (J9 / J1.5) value is regulated to 0.70 to 0.85 and the (J11 / J1.5) value is regulated to 0.67 to 0.78 is based on this range. This is because the heat treatment deformation of the steel material can be suppressed small. The heat treatment deformation referred to here is the bending of the shaft-shaped part, the gear teeth falling or twisting after quenching, and the change in the dimension (length, diameter, thickness, etc.) of the part before and after quenching. Although the mechanism by which heat treatment deformation is suppressed by controlling the Jominy hardenability within an appropriate range has not yet been fully elucidated, heat treatment deformation can be achieved even if Jominy hardenability is too low or too high. It has been experimentally confirmed by the present inventors that this increases. Presumably, in this range of steel with Jominy hardenability, bainite transformation occurs moderately prior to martensite transformation in the quenching cooling process, the steel material strength is increased, and martensite is not easily deformed to some extent. It is believed that heat treatment deformation can be suppressed because transformation starts. On the other hand, if the hardenability is too low, the bainite transformation occurs excessively, so that the heat treatment deformation increases due to the influence of the bainite transformation itself, and if the hardenability is too high, the bainite structure relaxes the heat treatment deformation. It is thought that the deformation due to heat treatment increases because of the small amount of heat.

  Quenching and carburizing and quenching to harden the parts after processing the steel into parts by limiting the steel component and Ms point of the means of the present invention and by limiting the hardenability as measured by the Jomini-type one-end quenching method It is possible to reduce the heat treatment deformation when performing the above. As a result, the present invention has beneficial effects such as improvement of component yield, simplification and abolition of component correction processes, and omission of gear tooth grinding for noise and vibration countermeasures. Can do.

EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated with reference to a table | surface below.
In order to obtain mechanical structural steels used as power transmission parts such as gears and shafts used in automobiles and industrial machines, No. 1 of the invention examples shown in Table 1 were obtained . 2, No. 5, no. 6, no. 14, no. 16, no. Steel consisting of 22 component compositions, the remainder Fe and inevitable impurities was melted in a vacuum induction melting furnace to obtain a 100 kg steel ingot.

As in the above-described examples of the present invention, as a steel for machine structural use used as a power transmission component such as a gear or a shaft used in automobiles, industrial machines, etc., the comparative example No. 1-4 and No.1. Steel consisting of 10 to 12 components, the balance Fe and inevitable impurities was melted in a vacuum induction melting furnace to obtain 100 kg of steel ingot.

  First, these ingots of the present invention and comparative examples were heated at 1250 ° C. for 5 hours and then forged into a steel bar having a diameter of 32 mm. Next, normalizing was performed by heating and holding at 900 ° C. for 1.5 hours and then air cooling. Subsequently, a test piece having a diameter of 20 mm and a length of 80 mm was produced from a steel bar having a diameter of 32 mm, and a groove having a depth of 5 mm, a width of 8 mm, and a length of 80 mm was applied to the side surface of the test piece. When quenching was performed by this groove processing, the cooling rate was varied greatly depending on the portion in the test piece. Moreover, the length of the test piece was measured after the groove processing. Further, the radius and groove width were measured at each position of 2 mm, 20 mm from the end of the test piece, and 40 mm which is the center of the test piece length. Then, after carburizing these test pieces at 930 ° C., the temperature was lowered to 850 ° C. in the furnace, and further maintained for 1 hour, and then quenched into a quenching oil at 60 ° C. After quenching, the specimens that were sufficiently cooled were measured for the bend and length of the specimen and the radius and groove width at each position of 2 mm, 20 mm from the specimen edge, and 40 mm, which is the center of the specimen length.

  For bending after heat treatment, hold both ends of the test piece with V block, and measure the maximum and minimum displacement on the circumference of the center of the test piece with a dial gauge when the test piece is rotated once. The difference between the maximum displacement and the minimum displacement was obtained by dividing by 2. In this measurement, the displacement of the bottom portion of the groove existing on the circumference of the test piece was ignored. In addition, as an index of heat treatment deformation, a difference in length of the test piece before and after the heat treatment was obtained, and the absolute value thereof was evaluated. Furthermore, the radius before and after the heat treatment at each position of the total three locations of 2 mm, 20 mm from the end of the test piece, and 40 mm which is the center of the length of the test piece, and the groove width dimension measurement result, After obtaining the dimensional change amount of the groove width, the values obtained by subtracting the minimum value from the maximum value among the dimensional change amounts at the three locations are defined as the radius change amount and the groove width change amount, respectively. It was evaluated as an index of deformation.

  In addition, a test piece having a diameter of 3 mm and a length of 10 mm is determined from the steel bar having a diameter of 32 mm after the above normalization, and the Ms point, which is the martensitic transformation start temperature of the steel material, is measured using a fully automatic transformation recording measuring device. did. The Ms point in the present embodiment is measured and measured under the condition that the cooling process of the component is assumed. In the present embodiment, the above-described grooved test piece having a diameter of 20 mm has an oil temperature of 60 ° C. Assuming oil quenching, the cooling rate during quenching was measured at 30 ° C./s. For the measurement of the hardenability of the steel material by the Jominy one-side quenching method, a test piece was prepared from the forged steel bar having a diameter of 32 mm and the “steel hardenability test method (one end) specified in JIS G 0561”. The test was conducted under the conditions according to the quenching method) and evaluated.

Table 3 shows the Ms point measured for the steel of the present invention, J1.5 of the hardness at a distance of 1.5 mm, J9 of the hardness at a distance of 9 mm from the quenching end measured by the Jomini type one-end quenching method, and Each value of the hardness J11 at a distance of 11 mm, and the obtained (J9 / J1.5) value and (J11 / J1.5) value are shown. Further, the bending (unit: mm) evaluated after quenching the above test piece, the absolute value of the difference in length of the test piece before and after the heat treatment (unit: mm), and the radius change of the test piece before and after the heat treatment determined by the above method An amount (unit: mm) and a groove width change amount (unit: mm) are shown. Invention Example No. As shown in Table 3, the steel material consisting of 2, 5-6, 14, 16, and 22 has a martensite transformation start temperature, that is, an Ms point in the range of 421 to 444 ° C., and (J9 / J1.5 of this steel material). The value of the formula (1) shown below in the range of 0.74 to 0.84 , and the value of the formula (2) shown below in (J11 / J1.5) is 0.69 to 0.77 . In the range, the bending after the heat treatment is 0.005 to 0.025 mm, the absolute value of the difference in the length of the test piece before and after the heat treatment is 0.003 to 0.018 mm, and the radius change before and after the heat treatment The amount was 0.004 to 0.008 mm, and the change in groove width before and after the heat treatment was 0.011 to 0.022 mm.
However,
(J9 / J1.5) = (hardness at a distance of 9 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (1.5 mm from the quenching end measured by the Jominy type one-end quenching method) (Hardness) …… (1)
(J11 / J1.5) = (hardness at a distance of 11 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (a distance of 1.5 mm from the quenching end measured by the Jominy type one-end quenching method) (Hardness) …… (2)

  Similarly, Table 4 shows the measured Ms point of the steel of the comparative example, J1.5 of the hardness (HRC) at a distance of 1.5 mm from the quenching end measured by the Jomini type one-end quenching method, and the hardness at a distance of 9 mm. Jominy hardenability which is J9 of thickness (HRC) and J11 of hardness (HRC) at a distance of 11 mm, and the obtained values of (J9 / J1.5) and (J11 / J1.5) are shown. . Further, the bending of the test piece after quenching (unit: mm), the absolute value of the difference in length of the test piece before and after the heat treatment (unit: mm), and the radius change of the test piece before and after the heat treatment determined by the above method The amount (unit: mm) and the groove width change amount (unit: mm) are shown.

No. of the above invention examples 2, No. 5, no. 6, no. 14, no. 16, no. No. 22 , the composition range excluding inevitable impurities other than Fe, Ni, and Mo of the steel material is shown in Table 1, and the Ms point is set to 388 to 444 ° C. of 460 ° C. or less, and is measured by the Jominy one-side quenching method. The hardenability is appropriately controlled, and the value of (J9 / J1.5) calculated from the formula (1) is within the range of 0.70 to 0.85, calculated from the formula (2) (J11 / By setting the value of J1.5) in the range of 0.67 to 0.78 , the bending of the test piece after heat treatment is reduced to a small range of 0.005 to 0.025 mm, and the length of the test piece before and after the heat treatment is further increased. The absolute value of the difference in thickness is set to a small range of 0.003 to 0.018 mm, the change in radius before and after the heat treatment is set to a small range of 0.004 to 0.008 mm, and the change in groove width before and after the heat treatment is set to 0. depression in a small range of .011~ 0.022 mm Rukoto could be.

On the other hand, No. of the above comparative example . 1-4 and No.1. In Table 2, the composition range excluding inevitable impurities other than Fe, Ni, and Mo of steel materials 10 to 12 is shown in Table 2. The remaining 6 cases , excluding 1 example , were out of the composition range of the present invention. These No. 1-4 and No.1. The steels of Comparative Examples 10 to 12 have a value (J9 / J1.5) of 0.70 to 0. 0 determined by the formula (1) and the formula (2) based on the hardness measured by the Jomini type one-end quenching method. It is out of the range of 85, and the value of (J11 / J1.5) is out of the range of 0.67 to 0.78 . In addition, these No. 1-4 and No.1. 10-12 is a bend 0.050~ 0.080 mm test piece after the heat treatment, both larger than in the steel of the invention examples. In addition, these No. 1-4 and No.1. In the steels of Comparative Examples 10 to 12 , one or more values of the absolute value of the difference in length of the test pieces before and after the heat treatment, or the radius change amount and the groove width change amount before and after the heat treatment are the values of the present invention. Bigger than steel. Therefore, among the comparative examples, the bending of the test piece after the heat treatment, the absolute value of the difference in length of the test piece before and after the heat treatment, and the radius change amount and the groove width change amount before and after the heat treatment are all the same as the steel of the present invention example. There was no one equivalent.

The Ms point satisfies the claims of the present invention, and the values of (J9 / J1.5) and (J11 / J1.5) satisfy the claims of the present invention . 2, No. 5, no. 6, no. 14, no. 16, no. 22 shows that the bending of the test piece after the heat treatment, the absolute value of the difference in length of the test piece before and after the heat treatment, and the radius change amount and the groove width change amount before and after the heat treatment are substantially smaller than the comparative example, and the heat treatment deformation is It is suppressed. The steel material of the present invention is used after being tempered after being subjected to heat treatment with hardening for hardening parts such as carburizing and quenching.

  From the above, after the steel material was processed into parts by the limitation of the steel components and the Ms point which is the martensitic transformation start temperature and the limitation of the hardenability measured by the Jominy one-side quenching method, carburizing and quenching was performed. For example, the heat treatment deformation when the heat treatment with hardening for hardening the parts is performed can be reduced. As a result, the steel material according to the present invention is a steel material applicable to power transmission parts such as gears and shafts used in automobiles and industrial machines.

Claims (2)

By mass%, C: 0.20 to 0.30%, Si: 0.10 to 1.50%, Mn: 0.20 to 0.55%, P: 0.030% or less, S: 0.014 %: Cr: 1.30-2.50%, Cu: 0.30% or less, Al: 0.008-0.300%, O: 0.0030% or less, N: 0.0020-0.0300 %, The balance is Fe and inevitable impurities, and the steel material is martensitic transformation start temperature (Ms point) of 460 ° C. or less. Using the following formula (1), J1.5 of the hardness at a distance of 1.5 mm from the quenching end of the steel material measured by the method and J9 of the hardness at a distance of 9 mm was calculated (J9 / J1.5 ) In the range of 0.70 to 0.85, and the distance 1. The value of (J11 / J1.5) calculated by the following equation (2) using J1.5 of hardness at mm and J11 of hardness at distance 11 mm is in the range of 0.67 to 0.78. A machine structural steel material having a small heat treatment deformation.
However,
(J9 / J1.5) = (hardness at a distance of 9 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (distance from the quenching end measured by the Jominy type one-end quenching method. (Hardness of 5mm) …… Formula (1)
(J11 / J1.5) = (hardness of a distance of 11 mm from the quenching end measured by the Jomini-type one-end quenching method) ÷ (distance from the quenching end measured by the Jomini-type one-end quenching method. (Hardness of 5mm) …… Formula (2) By mass%, C: 0.20 to 0.30%, Si: 0.10 to 1.50%, Mn: 0.20 to 0.55%, P: 0.030% or less, S: 0.014 %: Cr: 1.30-2.50%, Cu: 0.30% or less, Al: 0.008-0.300%, O: 0.0030% or less, N: 0.0020-0.0300 Is a steel for machine structural use that contains 1 or 2 types of Ti: 0.020-0.200%, Nb: 0.02-0.20% , and the balance Fe and inevitable impurities. The steel material made of the steel has a martensite transformation start temperature (Ms point) of 460 ° C. or less, and the steel material has a hardness at a distance of 1.5 mm from the quenching end of the steel material measured by the Jominy one-end quenching method. J1.5 of the thickness and J9 of the hardness at a distance of 9 mm, the following formula ( ), The value of (J9 / J1.5) calculated in the range of 0.70 to 0.85, and further using J1.5 of hardness at a distance of 1.5 mm and J11 of hardness at a distance of 11 mm, A steel material for machine structure having a small heat treatment deformation, characterized in that the value of (J11 / J1.5) calculated by the following formula (2) is in the range of 0.67 to 0.78.
However,
(J9 / J1.5) = (hardness at a distance of 9 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (distance from the quenching end measured by the Jominy type one-end quenching method. (Hardness of 5mm) …… (1)
(J11 / J1.5) = (hardness at a distance of 11 mm from the quenching end measured by the Jomini type one-end quenching method) ÷ (a distance of 1.5 mm from the quenching end measured by the Jominy type one-end quenching method) (Hardness) …… (2)