KR101745224B1

KR101745224B1 - Steel for carburizing

Info

Publication number: KR101745224B1
Application number: KR1020157016420A
Authority: KR
Inventors: 다츠야 고야마; 마나부 구보타; 스구루 요시다
Original assignee: 신닛테츠스미킨 카부시키카이샤
Priority date: 2012-12-28
Filing date: 2013-12-25
Publication date: 2017-06-08
Also published as: KR20150086365A; WO2014104113A1; JP5862802B2; CN104884660B; JPWO2014104113A1; CN104884660A

Abstract

The steel according to the present invention has a chemical composition of 0.16 to 0.30% of C, 0.01 to 2.0% of Si, 0.35 to 1.45% of Mn, 0.05 to 3.0% of Cr, 0.001 to 0.2% of Al, 0.004 to 0.05% of N, 0.003 to 0.03% of N, 0.005% or less of O, 0.025% or less of P, 0 to 1.0% of Mo, 0 to 1.0% of Cu, 0 to 1.0% of B, 0 to 0.005% of B, 0 to 0.3% of Nb, 0 to 0.3% of Ti, 0 to 1.0% of V, 0 to 0.01% of Ca, 0 to 0.01% of Mg, 0 to 0.01% of Zr And the content of Si, Ni, Al, and Sn in terms of% by mass, [Si%], [Ni%], , [Al%] and [Sn%], the relationship of "42? 21 x [Si%] + 5 x [Ni%] +40 x [Sn%] +32 x [Al%]? 8.5" .

Description

Steel for carburizing {STEEL FOR CARBURIZING}

The present invention relates to a carburizing steel capable of improving the impact resistance characteristics of various carburized steel parts without adjusting the carburization conditions for each carburized steel part.

The present application claims priority based on Japanese Patent Application No. 2012-288131 filed in Japan on December 28, 2012, the contents of which are incorporated herein by reference.

Mechanical structural parts may be damaged by a sudden and large stress. Particularly, in the case of a vehicle gear such as a differential gear, a transmission gear, and a gear with attached gear, there is a case where the derandom is damaged by impact breakdown due to sudden acceleration of the vehicle and load at the time of stopping the vehicle. In order to prevent such a phenomenon, particularly in the case of differential gears and transmission gears, it is further desired to improve the impact value (impact resistance characteristics). By sufficiently improving the impact value of these mechanical structural parts, it is possible to reduce the amount of material used for the mechanical structural parts, thereby achieving weight reduction of mechanical structural parts.

Conventionally, in the above-described parts, the toughness of the core portion is secured by using, as a material, indium oxide having a C content of about 0.2%, such as JIS SCr 420 and JIS SCM 420, for example. In the above-mentioned parts, the carburizing quenching treatment and the low temperature tempering at about 150 캜 are carried out to make the metal structure of the surface of the component a tempering martensite structure having a C content of 0.8%. This improves the high cycle bending fatigue strength and wear resistance of the component.

The prior art for improving the impact value will be described below. Patent Document 1 proposes a gear steel for which the contents of Al, B, and N are specified, the impact resistance fatigue property and the strength of the surface are increased by the solid solution B, and a gear using the same. However, in the gear disclosed in Patent Document 1, the debinding occurs at the time of carburizing, and the solid solution B in the gear surface layer disappears, so that the improvement in the impact value is small.

Patent Document 2 proposes a gear having excellent impact resistance, which is obtained by specifying the content of Mo, Si, P, Mn, and Cr, and in particular by increasing the Mo content. However, in order to increase the Mo content, it is necessary to reduce the content of Si, Mn, and Cr, so that in the gear disclosed in Patent Document 2, the strength is lowered due to the deterioration of the hardenability.

Patent Document 3 proposes a progesterous steel having high strength and high toughness, which is obtained by containing an appropriate amount of Cu. However, at high temperatures, Cu in the steel becomes a liquid layer, which promotes embrittlement of the steel. Therefore, the manufacturing conditions of the proliferation steel described in Patent Document 3 are limited.

The inventors of the present invention have investigated the relationship between the carburization characteristics and the impact resistance characteristics. As a result, as described later, the inventors of the present invention have obtained the knowledge that it is effective to lower the penetration amount of C penetrating into the steel during carburization to lower the surface C concentration of the carburizing material to improve the impact value. However, when the surface C concentration of the carburizing material is too low, improvement in characteristics such as fatigue strength and abrasion resistance as the original purpose of the carburization treatment can not be achieved. Therefore, in order to make the properties such as impact resistance, fatigue strength, and wear resistance compatible with each other in the carburized steel parts, it is necessary to control the surface C concentration of the carburized steel parts to an appropriate level.

The reduction of the surface C concentration can be realized by lowering the carbon potential at the carburizing treatment. However, it is difficult to carry out this in an actual production process using a carburizing furnace. This is because, in an actual production process, it is necessary to simultaneously and successively carry out a large number of processes for various components having different uses for the carburization furnace. The characteristics required for a carburizing component are not limited to the impact resistance characteristics as described above. For example, properties such as abrasion resistance and fatigue strength are also required for carburized steel parts. Therefore, lowering of the carbon potential at the time of carburizing treatment is effective for parts for which impact resistance characteristics are mainly required, but has an adverse effect on parts for which fatigue strength is mainly required, thereby causing a problem caused by a decrease in fatigue strength. If it is attempted to control the surface C concentration of a carburized steel component by carburizing under conditions in which sticking property is not exerted much, it is necessary to adjust the carburizing conditions for each component. However, this leads to a decrease in productivity, which is undesirable for industrial use.

Therefore, there is a need for a carburizing steel capable of controlling the penetration amount of C to an appropriate level even if the carburizing treatment is carried out under somewhat strong carburizing conditions capable of coping with carburization of various parts.

As a control technique of the surface C concentration, a carburized steel component which suppresses excessive carburization by defining the relation of contents of Si, Ni, Cu, and Cr in Patent Document 4 has been proposed. However, in this document, the carburizing atmosphere is a carburizing atmosphere in which the surface C concentration of the steel is set to about 1.0%. When the surface C concentration of the steel is set to such a value, carbide is generated on the surface of the steel. In this case, reduction of the surface C concentration effective for improving the impact value can not be realized.

Japanese Patent Application Laid-Open No. 2008-179848 Japanese Patent Application Laid-Open No. 1-108347 Japanese Patent No. 3927355 Japanese Patent Application Laid-Open No. 2007-291486

As described above, in a carburized steel component for mechanical structure, it is necessary to combine an impact resistance property and an abrasion resistance. By sufficiently improving the impact resistance characteristics of the carburizing material used for the carburizing steel part, it is possible to change the design of the parts so as to secure the impact resistance characteristics of the parts while suppressing the amount of material used. The carburizing treatment in the actual production process of the carburized steel component for mechanical structure needs to be carried out under a single carburizing condition or under as few carburizing conditions as possible for various components having different uses.

In the techniques disclosed in Patent Documents 1 to 4, it is necessary to satisfy both the requirement to both improve the impact value and the avoidance of the productivity deterioration, specifically, to obtain a carburized steel component having excellent impact resistance characteristics without adjusting the carburizing condition for each part I could not.

INDUSTRIAL APPLICABILITY The present invention can provide a carburized steel part excellent in both of impact value (impact resistance property) and wear resistance when used as a material for a carburizing steel part, and it is also necessary to change the carburizing condition It is to provide a river that does not.

The present inventors carried out carburization and impact tests on steels whose chemical composition and carburizing material properties were varied in a wide range and systematically in order to realize a steel capable of obtaining a carburized steel component having a good impact value while avoiding a decrease in productivity Respectively. As a result, the following explanations were obtained.

1 is a graph showing the relationship between the surface C concentration and the impact value of the carburizing material obtained by carburizing the steel. The carburizing treatment increases the surface C concentration of the steel. In order to improve the impact value of the carburizing material, it has been found that it is effective to control the surface C concentration after carburization to an appropriate level as shown in Fig.

Further, the inventors of the present invention have found that controlling the surface C concentration of the carburizing material as described above can be realized by adjusting the content of the alloy element employed in the steel. Specifically, the content of each alloy element is set within a predetermined range, and the content (unit: mass%) in the steel of Si, Ni, Al and Sn among the alloying elements in the steel is defined as [Si%], [Ni The present inventors have found that the surface C concentration of the carburizing material becomes an appropriate value and the impact value is improved by satisfying the following formula (A) when the following formula (A) is satisfied: Thereby completing the present invention.

42? 21 x [Si%] + 5 x [Ni%] + 40 x [Sn%] + 32 x [Al%]? (A)

The present invention has been made based on the above-described novel findings, and the gist of the present invention is as follows.

(1) A steel according to one embodiment of the present invention has a chemical composition of 0.16 to 0.30% of C, 0.01 to 2.0% of Si, 0.35 to 1.45% of Mn, 0.05 to 3.0% of Cr, 0.05 to 3.0% of Cr, 0.001 to 0.2% of Ni, 0.04 to 5.0% of Ni, 0.015 to 1.0% of Sn, 0.004 to 0.05% of S, 0.003 to 0.03% of N, 0.005% or less of O, 0 to 0.3%, V: 0 to 1.0%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Cu: 0 to 1.0% % Of Si, Ni, Al, and Sn in mass%, Zr: 0 to 0.05%, Te: 0 to 0.1%, rare earth elements: 0 to 0.005% , [Ni%], [Al%] and [Sn%] satisfy the following expression (A).

42? 21 x [Si%] + 5 x [Ni%] + 40 x [Sn%] + 32 x [Al%]? (A)

(2) The steel according to the above (1) is characterized in that the above chemical composition contains at least one of Mo: 0.05 to 1.0%, Cu: 0.01 to 1.0%, and B: 0.0002 to 0.005% .

(3) The steel according to the above (1) or (2) is characterized in that the above chemical composition contains at least one of Nb in an amount of 0.005 to 0.3%, Ti in an amount of 0.005 to 0.3%, and V in an amount of 0.01 to 1.0% Two or more species may be contained.

(4) The steel according to any one of (1) to (3), wherein the chemical composition is 0.0005 to 0.01% of Ca, 0.0005 to 0.01% of Mg, 0.0005 to 0.05% of Zr, Te: 0.0005 to 0.1%, and rare earth elements: 0.0001 to 0.005%.

When the carburized steel component is manufactured using the steel of the present invention, it is not necessary to adjust the carburization conditions for each carburized steel component in order to improve the impact value of the carburized steel component. Therefore, the manufacturing efficiency can be improved by unifying the carburizing method, and a carburized steel component having an excellent impact value can be obtained, and the industrial effect according to the present invention is extremely large.

1 is a graph showing the relationship between the impact value and the surface C concentration.
Fig. 2 is a schematic view showing a cross section perpendicular to the extending direction of a cut-away portion of a Charpy impact test piece used in the present invention. Fig.
3 is a schematic diagram showing a measurement region of the surface C concentration.
4 is a graph of the number of flights showing the relationship between the surface C concentration and the impact value ratio.
FIG. 5 is a graph showing the relationship between 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] +32 × [Al%] and the surface C concentration and impact ratio ratio.

Hereinafter, embodiments for carrying out the present invention will be described in detail.

First, the reason for limiting the chemical composition of the steel according to the present embodiment will be described. Hereinafter, "% by mass ", which is a unit relating to the content of the alloy element, is simply described as "% ". In the following description, the description of steel (carburizing steel) is also applied to carburizing steel parts (carburizing material) unless otherwise specified.

C: 0.16 to 0.30%

The C content determines the strength of the core portion of the carburized steel component and also affects the effective hardened layer depth. In order to secure the required core strength, the lower limit value of the C content is set to 0.16%. On the other hand, if the C content is too large, the composition will decrease, so the upper limit of the C content is set to 0.30%. The C content is preferably 0.18 to 0.25%.

Si: 0.01 to 2.0%

Si is an effective element for deoxidizing steel and is an effective element for imparting strength and hardenability required for mechanical structural parts to carburized steel parts. Further, the increase in the Si content lowers the settling property upon carburization and improves the impact value of the carburized steel component. If the Si content is less than 0.01%, the effect is insufficient. When the Si content exceeds 2.0%, decarburization at the time of production becomes remarkable, and the strength of the carburized steel part and the depth of the effective hardened layer are insufficient. For the reasons described above, it is necessary to set the Si content within the range of 0.01 to 2.0%. The Si content is preferably 0.2 to 1.5%.

Mn: 0.35 to 1.45%

Mn is an effective element for deoxidation of steel and is an effective element for imparting necessary strength and hardenability to steel. If the Mn content is less than 0.35%, the martensitic transformation starting temperature is increased, self-tempering occurs, and the hardness is lowered. Further, if the Mn content exceeds 1.45%, the retained austenite is stably present in the steel even after the subzero treatment, and the strength of the steel is lowered. For the reasons described above, it is necessary to set the Mn content within the range of 0.35 to 1.45%. The Mn content is preferably 0.50 to 1.10%.

Cr: 0.05 to 3.0%

Cr is an effective element for imparting necessary strength and hardenability to steel. If the Cr content is less than 0.05%, the effect is insufficient. If the Cr content exceeds 3.0%, the effect is saturated. For the reasons described above, it is necessary to set the Cr content within the range of 0.05 to 3.0%. The Cr content is preferably 0.2 to 1.5%.

Al: 0.001 to 0.2%

Al is an element effective for deoxidation of steel and is an element which becomes a nitride and precipitates in the steel to exert the effect of grain refinement. Further, when the Al content is increased, the steepness of the steepness of the steel is lowered, thereby improving the impact value of the carburized steel part. If the Al content is less than 0.001%, the effect is insufficient. On the other hand, if the Al content exceeds 0.2%, the precipitate (Al nitride) coarsens and becomes a cause of brittleness of steel and carburized steel parts. For the reasons described above, it is necessary to set the Al content within the range of 0.001 to 0.2%. The preferable range of the Al content is 0.01 to 0.15%.

Ni: 0.04 to 5.0%

Ni is an effective element for imparting necessary strength and hardenability to steel. In addition, the Ni content increases, and the carburizing property at the time of carburization is lowered, whereby the impact value of the carburized steel component is improved. When the Ni content is less than 0.04%, the effect is insufficient. If the Ni content exceeds 5.0%, the residual austenite stably exists in the steel even when the steel is subjected to the subzero treatment, and the strength of the steel is lowered. For the above reason, it is necessary to set the Ni content within the range of 0.04 to 5.0%. Preferably, the Ni content is 1.0 to 2.0%.

Sn: 0.015 to 1.0%

The increase in the Sn content leads to a decrease in the tendency to sink during carburization, thereby improving the impact value of the carburized steel part. If the Sn content is less than 0.015%, the effect is insufficient. On the other hand, if the Sn content exceeds 1.0%, the hot ductility of the steel decreases. For the reasons described above, it is necessary to set the content of Sn within the range of 0.015 to 1.0%. The preferable range of the Sn content is 0.02 to 0.1%.

S: 0.004 to 0.05%

S forms MnS in the steel, thereby improving the machinability of the steel. If the S content is less than 0.004%, the effect is insufficient. On the other hand, if the S content exceeds 0.05%, the effect becomes saturated and causes grain boundary segregation rather than causing grain boundary embrittlement. For the above reason, it is necessary to set the content of S within the range of 0.004 to 0.05%. The suitable range of the S content is 0.01 to 0.04%.

N: 0.003 to 0.03%

N combines with Al, Ti, Nb, and V in the steel to produce nitrides or carbonitrides. These nitrides and carbonitrides have an effect of suppressing coarsening of crystal grains. When the N content is less than 0.003%, the effect is insufficient. When the N content exceeds 0.03%, the effect becomes saturated. For the reasons described above, it is necessary to set the N content within the range of 0.003 to 0.03%. The preferable range of the N content is 0.005 to 0.008%.

O: 0.005% or less

O forms oxides in the steel. This oxide may segregate in the grain boundaries to cause grain boundary brittleness. In addition, O is an element that forms a hard oxide inclusion in the steel and tends to cause brittle fracture. The O content needs to be limited to 0.005% or less. The preferable range of the O content is 0.0025% or less. Since it is preferable that the O content is small, the lower limit value of the O content is 0%.

P: not more than 0.025%

P is segregated at the austenite grain boundary at the time of carburization, thereby causing grain boundary fracture. That is, P lowers the impact value of the carburized steel part. Therefore, it is necessary to limit the P content to 0.025% or less. The preferable range of the P content is 0.01% or less. Since it is preferable that the P content is small, the lower limit value of the P content is 0%. However, if P is removed more than necessary, the manufacturing cost increases. Therefore, the lower limit of the P content is generally about 0.004%.

The steel according to the present embodiment may further contain one or more of Mo, Cu, and B to increase the impact value. However, it is not essential to contain these elements.

Mo: 0 to 1.0%

Mo is an effective element for improving the impact value of steel because it inhibits P from segregating in the grain boundary. If the Mo content exceeds 1.0%, the effect becomes saturated, so the upper limit of the Mo content needs to be 1.0%. The lower limit of the Mo content is 0%. However, when Mo is added to obtain the above-mentioned effect, the Mo content is preferably 0.05% or more. A more preferable range of the Mo content is 0.05 to 0.25%.

Cu: 0 to 1.0%

Cu is an element effective for improving the hardenability of steel and is an element for improving the impact value of steel by improvement of hardenability. If the Cu content exceeds 1.0%, the hot ductility is lowered, so the upper limit of the Cu content should be 1.0%. The lower limit value of the Cu content is 0%, but when the above effect is obtained by containing Cu, the Cu content is preferably 0.01% or more. A more preferable range of the Cu content is 0.01 to 0.2%.

B: 0 to 0.005%

B has an effect of suppressing grain boundary segregation of P. B also has an effect of improving the grain boundary strength and strength of the grain and an effect of improving the hardenability, and these effects improve the impact value of the steel. When the B content exceeds 0.005%, the effect becomes saturated, so the upper limit of the B content should be 0.005%. The lower limit value of the B content is 0%, but when the above effect is obtained by containing B, the B content is preferably 0.0002% or more. A more preferable range of the B content is 0.0005 to 0.003%.

The steel according to the present embodiment may contain at least one of Nb, Ti, and V within the range shown below in order to prevent the impact value from lowering even when carburization for a long time is carried out. However, it is not essential to contain these elements.

Nb: 0 to 0.3%

Nb produces Nb carbonitride in the steel. A so-called high-temperature carburization with a carburization temperature of 980 DEG C or higher and a so-called long-term carburization with a carburization time of 10 hours or more are applied, a suitable amount of Nb carbonitride is present in the steel, The lowering of the value can be prevented. When the Nb content exceeds 0.3%, the machinability is deteriorated, so the upper limit of the Nb content is set to 0.3%. The lower limit value of the Nb content is 0%, but when the above-mentioned effect is obtained by containing Nb, the Nb content is preferably 0.005% or more. A more suitable range of the Nb content is 0.02 to 0.05%.

Ti: 0 to 0.3%

Ti produces fine TiC and / or TiCS in the steel. When a so-called high-temperature carburization with a carburization temperature of 980 占 폚 or more is applied and a so-called long-term carburization with a carburization time of 10 hours or more is applied, the appropriate amounts of TiC and TiCS are present in the steel and the austenite- It is possible to prevent a decrease in the impact value of the steel. When the Ti content exceeds 0.3%, the precipitates of the TiN main body are increased, and the fatigue characteristics of the steel are lowered. For these reasons, it is necessary to set the upper limit of the Ti content to 0.3%. The lower limit of the Ti content is 0%, but when Ti is contained to obtain the above-mentioned effect, the Ti content is preferably 0.005% or more. A more suitable range of the Ti content is 0.02 to 0.2%.

V: 0 to 1.0%

V produces V carbonitride in the river. Even when a so-called high-temperature carburization with a carburization temperature of 980 DEG C or higher is applied and a so-called long-term carburization with a carburization time of 10 hours or longer is applied, a suitable amount of V carbonitride is present in the steel and the austenite ingress can be made fine, It is possible to prevent the impact value from lowering. When the V content exceeds 1.0%, the machinability of the steel is deteriorated. For the reasons described above, it is necessary to set the upper limit value of the V content to 1.0%. The lower limit of the V content is 0%, but when the above effect is obtained by adding V, the V content is preferably 0.01% or more. A more suitable range of the V content is 0.03 to 0.1%.

In order to improve machinability, the steel according to the present embodiment may further contain one or more of Ca, Mg, Zr, Te, and rare earth elements within the ranges shown below. However, it is not essential to contain these elements.

Ca: 0 to 0.01%

Ca lowers the melting point of oxides and softens due to temperature rise during cutting, thereby improving machinability. However, when the Ca content exceeds 0.01%, a large amount of CaS is produced and the machinability is lowered. For the reasons described above, it is preferable to set the upper limit value of the Ca content to 0.01%. The lower limit of the Ca content is 0%. However, when Ca is added to obtain the above-mentioned effect, the Ca content is preferably 0.0005% or more. A more suitable range of the Ca content is 0.0005 to 0.0015%.

Mg: 0 to 0.01%

Mg is a deoxidizing element and produces oxides in the steel. Further, the Mg-based oxide formed by Mg tends to become the nucleus of crystallization and / or precipitation of MnS. Further, the sulfide of Mg becomes a composite sulfide of Mn and Mg, thereby making the MnS spheroidized. Thus, Mg is an effective element for controlling the dispersion of MnS and improving the machinability. However, when the Mg content exceeds 0.01%, a large amount of MgS is produced and the machinability of the steel is lowered. Therefore, the upper limit of the Mg content is preferably 0.01%. The lower limit value of the Mg content is 0%. However, when the above effect is obtained by containing Mg, the Mg content is preferably 0.0005% or more. A more preferable range of the Mg content is 0.0005 to 0.0015%.

Zr: 0 to 0.05%

Zr is a deoxidizing element and produces oxides. Further, the Zr-based oxide formed by Zr tends to become the nucleus of crystallization and / or precipitation of MnS. Thus, Zr is an effective element for controlling the dispersion of MnS and improving the machinability. However, if the amount of Zr exceeds 0.05%, the effect becomes saturated, so that the upper limit of the Zr content is preferably 0.05%. The lower limit value of the Zr content is 0%, but when the above effect is obtained by containing Zr, the Zr content is preferably 0.0005% or more. Further, in order to promote spheroidization of MnS, it is particularly preferable that the lower limit value of the Zr content is set to more than 0.003%.

Te: 0 to 0.1%

Te promotes spheroidization of MnS, thereby improving the machinability of the steel. If the Te content exceeds 0.1%, the effect becomes saturated, and therefore it is preferable to set the upper limit of the Te content to 0.1%. The lower limit of the Te content is 0%, but when Te is added to obtain the above effect, the Te content is preferably 0.0005% or more. A more suitable range of the Te content is 0.0005 to 0.0015%.

Rare earth elements: 0 to 0.005%

The rare earth element generates a sulfide in the steel, and this sulfide becomes an precipitation nucleus of MnS, thereby accelerating the production of MnS, thereby improving the machinability of the steel. However, when the total content of the rare earth elements exceeds 0.005%, the sulfide becomes coarse and the fatigue strength of the steel is lowered. Therefore, it is necessary to set the upper limit value of the total content of rare earth elements to 0.005%. The lower limit value of the total content of the rare earth elements is 0%, but when the above effects are obtained by containing the rare earth elements, the total content of the rare earth elements is preferably 0.0001% or more. A more preferable range of the total content of the rare earth elements is 0.001 to 0.003%.

The steel according to the present embodiment contains the above-described alloy component, and the remaining portion contains Fe and impurities. It is permissible that the elements other than the above-described alloying elements are mixed as impurities in the steel from the raw material and the manufacturing apparatus so long as the mixing amounts do not affect the steel properties.

The content of each alloy component included in the steel according to the present embodiment has been described above. However, it is not possible to realize a steel for obtaining a carburized steel component having a sufficient impact value under a single carburizing condition, regardless of the shape of the carburized steel component, only by individually controlling the content of each alloy component. The present inventors have also found that it is necessary to control the content of the alloy component based on the formula (1).

42? 21 x [Si%] + 5 x [Ni%] + 40 x [Sn%] + 32 x [Al%]? (One)

In the formula (1), [Si%], [Ni%], [Sn%] and [Al%] represent the content of Si, Ni, Sn and Al in mass%. The basis for deriving Equation (1) will be described below.

First, the inventors of the present invention will examine the contents of the examination for evaluating the impact resistance of the carbur material.

First, an alloy comprising 0.20 mass% of C, 0.24 mass% of Si, 0.79 mass% of Mn, 0.014 mass% of P, 0.015 mass% of S, 1.21 mass% of Cr, 0.031 mass% of Al, 0.05 mass% 0% by weight of Sn, 0.005% by weight of N, and 0.001% by weight of O, with the remainder being Fe and impurities were defined as reference steels. Subsequently, a Charpy impact test piece having an arc-shaped notch (notch) having an external dimension of 10 mm x 10 mm x 55 mm and a curvature radius of 10 mm and a depth of 2 mm, shown in Fig. 2, . For the Charpy impact test piece 3 formed from the reference steel as a material, the carburized impact test specimen 3 was first subjected to a carburizing condition (hereinafter also referred to as a reference carburizing condition) at a treatment temperature of 930 캜, a treatment time of 5 hours, and a carbon potential of 0.8 Carburized carbon steel was subjected to tempering at a tempering temperature of 150 DEG C and a tempering time of 90 minutes, and the Charpy absorbed energy at 25 DEG C was defined as a reference impact value.

In addition, the impact value ratio is determined by carrying out carburizing and tempering on the Charpy impact test piece 3 according to the carburizing condition (i.e., the reference carburizing condition) applied when obtaining the reference impact value, and setting the Charpy absorbed energy at 25 캜 to the reference impact value Is defined as the divided value.

The reference steel described above is a steel having a chemical composition equivalent to SCr420 which is generally used as a gear steel, and is the same as the steel of Comparative Example 26 described later. Gas carburization carried out under the standard carburization conditions described above is a general carburization treatment performed for the production of mechanical structural parts.

Fig. 2 shows a side surface shape of the Charpy impact test piece 3 (the shape of a cross section perpendicular to the drawing direction of the cut). The radius of curvature of the cut 2 is 10 mm. The shape of the Charpy impact test piece 3 is different from the shape of a typical Charpy impact test piece (for example, the shape specified in JIS-Z2242 "Charpy impact test method of metallic material"). The shape of the notch 2 of the Charpy impact test piece 3 is determined by simulating the shape of the tooth portion of the gear. By performing the Charpy impact test on the test piece having such a notch, the impact resistance characteristic in the tooth portion of the gear can be estimated. The test piece having such a cut is widely used as a test piece shape for measuring the impact resistance characteristics of a carburized steel material as disclosed in, for example, JP-A-2013-40376. The Charpy absorbed energy was measured in accordance with JIS-Z2242 " Charpy impact test method of metal material " except for the shape of the Charpy impact test piece 3. The execution temperature of the Charpy impact test was 25 ° C. Charpy impact test specimen 3 was produced by machining.

Next, the present inventors have found that various steels containing alloying elements are forged, machined, and carburized in the above-mentioned composition range (but the provisions of the formula 1 are not taken into consideration), and the impact ratio Respectively. The present inventors also measured the surface C concentration of each carburizing material.

The method of measuring the surface C concentration will be described below. First, the Charpy impact test piece 3 was cut along the direction perpendicular to the notched surface (cut surface) of the Charpy impact test piece 3 subjected to gas carburization under the reference carburization condition and the notch 2, and the cut surface was polished. Fig. 3 shows a schematic view of a section. Subsequently, the C concentration was measured at intervals of 5 占 퐉 in a region (surface C concentration measuring region 1) of 5 to 50 占 퐉 toward the height direction of the Charpy impact test piece 3 from the bottom surface of the notch 2. C concentration was measured by EPMA. The size of the measurement point (electron beam diameter of EPMA) was set to? 5 μm. The surface C concentration was obtained by averaging the ten measurement data obtained thereby. The unit of surface C concentration is mass%.

As a result of measuring the surface C concentration after the carburization when the chemical composition and the carburization material properties of the steel were changed widely and systematically, the present inventors found that the surface C concentration varies depending on the amount of the alloy element added. It is considered that this phenomenon is caused by a chemical interaction between the alloy element and C that invades the steel surface by carburization. The effects of Si, Ni, Al, and Sn particularly on the surface C concentration were strong, and the surface C concentration decreased with the increase of the content of these elements. According to the above finding, the relationship between the contents of Si, Ni, Al, and Sn in the chemical composition of steel according to the present embodiment is defined by Equation 1, whereby the steepness of the steel can be controlled.

The impact value ratio should be 1.2 or more for the following reason. As described above, by sufficiently improving the impact resistance characteristics of the carburizing material used for the carburizing steel part, it is possible to change the design of the parts so as to secure the impact resistance and the fracture resistance while suppressing the amount of material used. In the technical field of mechanical structural parts, it is considered that the impact value needs to be improved by 20% with respect to the reference impact value (impact value of SCr420 carburized under the general carburizing condition) in order to carry out such a design change.

As described above, there is a correlation between the impact value ratio and the surface C concentration. Fig. 4 is a graph showing the relationship between the surface C concentration and the impact value ratio of the carburizing material. In Fig. 4, the impact value ratio of the data points below the broken line is less than 1.2. As shown in Fig. 4, in order to obtain a carburizing material having an impact ratio ratio of 1.2 or more, it was necessary to control the surface C concentration of the carburizing material subjected to gas carburization under the standard carburizing condition to 0.75% or less.

Here, the surface C concentration was subjected to multiple regression analysis with the content of each of Si, Ni, Al, and Sn as a factor. As a result, the following formulas (1 ') and (2') were obtained as critical conditions for obtaining a carburizing material having a surface C concentration of 0.75 mass% when gas carburization was performed under the standard carburization conditions.

21 x [Si%] + 5 x [Ni%] + 40 x [Sn%] + 32 x [Al%] = (One')

α = 8.5 ... (2')

5 is a graph showing the relationship between the alpha value and the surface C concentration and the relationship between the alpha value and the impact value ratio. 5, the alpha value of the data point to the left of the dashed line to the left is less than 8.5, and the alpha value of the data point to the right of the dashed line to the right is 42 seconds. As can be seen from this graph, when the α value is 8.5 or more, the surface C concentration becomes 0.75 mass% or less when gas carburization is performed under the standard carburization condition. As the value of alpha increases, the surface C concentration of the carburizing material decreases, and the impact value of the carburizing material increases accordingly. Preferably, the alpha value is 12 or more.

On the other hand, when the carburizing property is remarkably lowered, the surface hardness is lowered and the abrasion resistance is remarkably lowered, so that the strength as a carburized steel component becomes insufficient. When the gas carburization is performed under the standard carburization condition, it is preferable that the surface hardness of the carburizing material is higher than HV550. In order to accomplish this, the surface C concentration of the carburizing material subjected to the gas carburization under the standard carburizing condition needs to be 0.4 mass% or more. Further, it was found that it is necessary to set the alpha value to 42 or less in order to make the surface C concentration of the carburizing material subjected to the gas carburization under the standard carburization condition to 0.4 mass% or more. Further, it is more preferable that the surface C concentration of the carburizing material subjected to gas carburization under the standard carburizing condition is 0.55 mass% or more. To achieve this, it is preferable to set the value of alpha to 25 or less.

The carburizing method for obtaining the carburized steel component from the steel according to the present embodiment is preferably gas carburization (any of the denitrification method and the enemy stock). In addition to the carburizing, the carburizing may be performed.

In addition, although the depth of the effective hardened layer is sometimes considered as an evaluation standard for the carburized steel component, the characteristics required for components such as a vehicle gear such as a differential gear are strongly related to the surface hardness than the effective hardened layer depth. Therefore, by using the steel according to the present embodiment in which the surface C concentration of the carburized steel part can be controlled to an appropriate level and thereby the surface hardness can be optimized, a favorable effect in industrial use can be obtained.

The steel according to the present embodiment may be made, for example, a round bar steel by hot rolling first, followed by forging or cutting to form a gear or the like, and carburizing and quenching may be performed to form a carburized steel component .

Example

Next, an embodiment of the present invention will be described. The conditions in the embodiment are examples of conditions employed for confirming the feasibility and effect of the present invention, and the present invention is not limited to this example condition. The present invention can adopt various conditions as long as the objects of the present invention are achieved without departing from the gist of the present invention.

Various steel ingots having the chemical compositions shown in Table 1-1 and Table 1-2 were cut into a square bar shape having a longitudinal dimension of 50 mm and a width of 50 mm (50 mm x 50 mm), and then subjected to cracking and sizing , And the cross-sectional dimension in the longitudinal direction was divided into four pieces in the shape of a bar having a length of 25 mm and a width of 25 mm. Charpy impact test pieces having an arc-shaped cutout (notch) having an external dimension of 10 mm x 10 mm x 55 mm and a radius of curvature of 10 mm and a depth of 2 mm, as shown in Fig. 2, were taken along the central axis from the obtained bars. This test piece shape is the same as the Charpy impact test piece 3 described above. Subsequently, the Charpy impact test piece was carburized. In the Examples and Comparative Examples other than Comparative Example 29, the gas carburization was carried out under the carburization conditions in which the treatment temperature was 930 캜, the treatment time was 5 hours, and the carbon potential was 0.8. This treatment condition is the same as the above-mentioned reference carburization condition. In Comparative Example 29, the gas carburization was carried out under the carburization conditions in which the treatment temperature was 930 DEG C, the treatment time was 5 hours, and the carbon potential was 0.6. The tempering was carried out under the condition that the tempering temperature was 150 DEG C and the tempering time was 90 minutes.

After the tempering, the surface C concentration of each sample was measured. The method of measuring the surface C concentration is as follows. First, the Charpy impact test piece was cut along the notched surface (cut surface) of the Charpy impact test piece and the direction perpendicular to the cut, and the cut surface was polished. Subsequently, the C concentration was measured from the bottom surface of the cut 2 at a 5 占 퐉 interval in a region (surface C concentration measuring region 1) of 5 to 50 占 퐉 toward the height direction of the Charpy impact test piece. C concentration was measured by EPMA. The size of the measurement point (electron beam diameter of EPMA) was 5 [micro] m. The surface C concentration was obtained by averaging the ten measurement data obtained thereby. The unit of surface C concentration is mass%.

Charpy impact test was performed after tempering, and Charpy absorption energy (impact value) was measured. The Charpy impact test was carried out at a test temperature of 25 DEG C in accordance with the method specified in JIS-Z2242, except for the shape of the notch of the Charpy impact test piece.

The impact value of each sample was divided by the impact value of Comparative Example 26 to calculate the impact value ratio of each sample. The steel of Comparative Example 26 is the reference steel described above.

Further, in order to evaluate the abrasion resistance of each sample, a wear test was performed on each sample to measure the wear depth. A cylindrical portion having a diameter of 26 mm and a length of 28 mm and a cylindrical grip portion having the same central axis as the cylindrical portion and having a diameter of 24 mm and a length of 51 mm were formed along the central axis of a 50 mm x 50 mm square bar produced by the above- Shaped abrasion test specimens were taken. The grip portion is disposed at both ends in the longitudinal direction of the cylindrical portion. Carburized test pieces were carburized under the same conditions as the aforementioned Charpy impact test pieces. Wear depth refers to the depth of abrasion that occurs in a wear test piece after the roller is pressed against the cylindrical portion of the wear test specimen and the roller is rotated one million rotations. The abrasion test conditions were as follows. A sample having a wear depth of less than 30 mu m was judged to have sufficient abrasion resistance.

Roller material: Bearing steel (SUJ2)

Hardness of rollers: HV 700 ~ 800

Roller diameter: 130mm

Roller width: 18mm

Roller shape: Creates a crowning of R = 150mm on the outer circumference

Roller contact force: Hertz stress 1500 MPa (surface pressure)

Sliding ratio: -100%

Table 2 shows surface C concentration, impact value ratio, and wear depth of each sample. Comparative Example 26 has a chemical composition corresponding to SCr420 specified in JIS-G 4053 generally used as a gear steel, and has a composition of 21 x Si% + 5 x Ni% + 40 x Sn% + 32 × [Al%] was 6.3, and when gas carburization was carried out under the standard carburization condition, the impact value was 10 J / cm 2. The impact value ratios of Examples 1 to 25 were all 1.3 or more, and it was clear that they had excellent impact strength. For example, in Inventive Example 1, 21 x [Si%] + 5 x [Ni%] + 40 x [Sn%] + 32 x [Al%] is 41.1 and the surface C concentration is 0.46% .

In contrast, Comparative Examples 26 to 35 did not have desirable characteristics.

In Comparative Examples 26 and 28, Sn was not contained, so that carburization was carried out excessively, and only a low impact value was obtained as compared with the examples. In Comparative Example 31, since the Sn content was below the specified range of the present invention, the carburization was excessively performed in the same manner as in Comparative Examples 26 and 28, and only a low impact value was obtained as compared with the Examples.

In the case of Comparative Example 27, the content of each alloy element was within the range of the present invention, but 21 x Si% + 5 x Ni% + 40 x Sn% + 32 x Al% Of the present invention. As a result, the abrasion resistance of Comparative Example 27 was low.

In Comparative Example 30, the content of each alloy element was within the range specified in the present invention, but 21 x Si% + 5 x Ni% + 40 x Sn% + 32 x Al% , The carburizing was carried out excessively. As a result, Comparative Example 30 had only a low impact value as compared with the Examples.

In Comparative Example 32, since the Sn content exceeded the specified range of the present invention, the hot ductility deteriorated. As a result, in Comparative Example 32, the surface of the obtained carburizing material was frequently cracked.

In Comparative Example 33, since the Ni content exceeded the range specified in the present invention, the strength was lowered. As a result, the abrasion resistance of Comparative Example 33 was low.

In Comparative Example 34, embrittlement occurred because the Al content exceeded the specified range of the present invention. As a result, the impact value ratio of Comparative Example 34 was low.

Reference Example 29 is the same steel as Comparative Example 26, but since the carburizing conditions are different and the carbon potential (carburizing treatment of 0.6) is set low, the surface C concentration is low and a good impact value is obtained . However, setting the carbon potential to a low value in actual production leads to a decrease in productivity, which is unsuitable.

[Table 1-1]

[Table 1-2]

[Table 2]

1: surface C concentration measurement area
2: notch (notch)
3: Charpy impact test piece (carburizing material)

Claims

Chemical composition, in% by mass,
C: 0.16 to 0.30%
Si: 0.01 to 2.0%
Mn: 0.35 to 1.45%
Cr: 0.05 to 3.0%
Al: 0.001 to 0.2%
Ni: 0.04 to 5.0%,
Sn: 0.015 to 1.0%
S: 0.004 to 0.05%,
N: 0.003 to 0.03%
O: 0.005% or less,
P: 0.025% or less,
Mo: 0 to 1.0%,
Cu: 0 to 1.0%,
B: 0 to 0.005%,
Nb: 0 to 0.3%,
Ti: 0 to 0.3%,
V: 0 to 1.0%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
Zr: 0 to 0.05%,
Te: 0 to 0.1%,
Rare earth element: 0 to 0.005%, and
Balance parts: Fe and impurities,
(1) is satisfied when the content of Si, Ni, Al and Sn is represented by [Si%], [Ni%], [Al%] and [Sn% Dragon River.
42? 21 x [Si%] + 5 x [Ni%] + 40 x [Sn%] + 32 x [Al%]? (One)

The method according to claim 1, wherein the chemical composition comprises, by mass%
Mo: 0.05 to 1.0%
Cu: 0.01 to 1.0%, and
B: 0.0002 to 0.005%
, And at least one of them.

3. The method according to claim 1 or 2, wherein the chemical composition is expressed in mass%
0.005 to 0.3% of Nb,
Ti: 0.005 to 0.3%, and
V: 0.01 to 1.0%
, And at least one of them.

3. The method according to claim 1 or 2, wherein the chemical composition is expressed in mass%
Ca: 0.0005 to 0.01%
Mg: 0.0005 to 0.01%
Zr: 0.0005 to 0.05%
Te: 0.0005 to 0.1%, and
Rare earth element: 0.0001 to 0.005%
, And at least one of them.

4. The method according to claim 3, wherein the chemical composition comprises, by mass%
Ca: 0.0005 to 0.01%
Mg: 0.0005 to 0.01%
Zr: 0.0005 to 0.05%
Te: 0.0005 to 0.1%, and
Rare earth element: 0.0001 to 0.005%
, And at least one of them.

The method according to claim 1, wherein the chemical composition comprises, by mass%
Nb: 0 to 0.012%
By weight based on the total weight of the carburizing steel.

The method according to claim 1, wherein the chemical composition comprises, by mass%
Ti: 0 to 0.001%
By weight based on the total weight of the carburizing steel.