JP5862802B2 - Carburizing steel - Google Patents

Description

The present invention relates to a carburizing steel that can improve impact resistance characteristics of various carburized steel parts without adjusting carburizing conditions for each carburized steel part.
This application claims priority based on Japanese Patent Application No. 2012-288131 for which it applied in Japan on December 28, 2012, and uses the content here.

  Mechanical structural parts may be damaged by suddenly receiving large stresses. In particular, in vehicle gears such as differential gears, transmission gears, and carburized shafts with gears, the tooth roots may be damaged due to impact destruction due to loads at the time of sudden start and stop of the vehicle. In order to prevent such a phenomenon, especially in the differential gear and the transmission gear, it is further desired to improve the impact value (impact resistance). By sufficiently improving the impact value of these mechanical structural parts, the amount of material used for the mechanical structural parts can be reduced, and the weight of the mechanical structural parts can be reduced.

  Conventionally, in the above-described parts, the toughness of the core is generally secured by using case-hardened steel having a C content of around 0.2%, such as JIS SCr420 and JIS SCM420, for example. Furthermore, the above-described parts are subjected to carburizing and quenching treatment and low-temperature tempering at around 150 ° C. to make the metal structure on the part surface a tempered martensite structure with a C content of around 0.8%. This increases the high cycle bending fatigue strength and wear resistance of the part.

  A conventional technique for improving the impact value will be described below. Patent Document 1 proposes a steel for gears in which the contents of Al, B, and N are defined and the impact fatigue resistance and surface fatigue strength are enhanced by solute B, and a gear using the steel. However, in the gear described in Patent Document 1, the de-B phenomenon occurs during carburizing, and the solute B in the gear surface layer disappears, so the impact value is not significantly improved.

  Patent Document 2 proposes a gear excellent in impact resistance obtained by regulating the contents of Mo, Si, P, Mn, and Cr, and in particular by increasing the content of Mo. However, since it is necessary to reduce the contents of Si, Mn, and Cr in increasing the Mo content, the gear described in Patent Document 2 has a decrease in strength due to a decrease in hardenability.

  Patent Document 3 proposes a case-hardened steel having high strength and high toughness obtained by containing an appropriate amount of Cu. However, at high temperatures, Cu in the steel becomes a liquid layer and promotes embrittlement of the steel. Therefore, there are restrictions on the manufacturing conditions of the case hardening steel described in Patent Document 3.

The inventors diligently investigated the relationship between carburization characteristics and impact resistance characteristics. As a result, as will be described later, it is said that reducing the amount of C entering the steel during carburizing and reducing the surface C concentration of the carburized material is effective for improving the impact value. The inventors have obtained knowledge. However, when the surface C concentration of the carburized material is too low, improvement in characteristics such as fatigue strength and wear resistance, which are the original purposes of the carburizing process, cannot be achieved. Therefore, in order to make impact resistance characteristics and characteristics such as fatigue strength and wear resistance compatible in the carburized steel parts, it is necessary to control the surface C concentration of the carburized steel parts to an appropriate level.
Reduction of the surface C concentration can be realized by lowering the carbon potential during the carburizing process. However, it is difficult to implement this in an actual production process using a carburizing furnace. This is because in the actual production process, the carburizing furnace needs to process a large amount of various parts having different applications simultaneously and continuously. The characteristics required for the parts to be carburized are not limited to the impact resistance characteristics as described above. For example, characteristics such as wear resistance and fatigue strength are also required for carburized steel parts. Therefore, reducing the carbon potential during carburizing treatment is effective for parts that mainly require impact resistance, but it has an adverse effect on parts that mainly require fatigue strength, resulting in a decrease in fatigue strength. Cause problems. If it is attempted to control the surface C concentration of the carburized steel part by carburizing under the condition that the carburizing ability is not exhibited so much, it is necessary to adjust the carburizing condition for each part. However, this leads to a decrease in productivity, which is not preferable for industrial use.
Accordingly, there is a need for a carburizing steel that can control the amount of C intrusion to an appropriate level even when carburizing is performed under strong carburizing conditions that can cope with carburizing of various parts.

  As a technique for controlling the surface C concentration, Patent Document 4 proposes a carburized steel part that suppresses excessive carburization by defining the relationship among the contents of Si, Ni, Cu, and Cr. However, the carburizing atmosphere used in this document is a carburizing atmosphere in which the steel surface C concentration is about 1.0%. When the steel surface C concentration is set to such a value, carbides are generated on the steel surface. In this case, it is impossible to realize a reduction in the surface C concentration effective for improving the impact value.

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

As described above, in carburized steel parts for machine structures, it is necessary to achieve both impact resistance and wear resistance. By sufficiently improving the impact resistance characteristics of the carburized material used for the carburized steel parts, the design of the parts can be changed so that the impact resistance characteristics of the parts are ensured while suppressing the amount of material used. Furthermore, the carburizing process in the actual production process of carburized steel parts for machine structures needs to be carried out under a single carburizing condition or as few types of carburizing conditions as possible for various parts with different applications. There is.
In the disclosed technologies of Patent Documents 1 to 4, there is a need to achieve both improvement in impact value and avoidance of productivity reduction, specifically, obtaining a carburized steel part excellent in impact resistance without adjusting carburizing conditions for each part. It was not possible to meet the needs.
INDUSTRIAL APPLICABILITY The present invention can provide a carburized steel part that is excellent in both impact value (impact resistance) and wear resistance when used as a material for a carburized steel part, and is carburized at the time of manufacturing the carburized steel part. It provides steel that does not require changing conditions.

  In order to realize a steel that can avoid the reduction in productivity and obtain a carburized steel part having an excellent impact value, the inventors have extensively and systematically changed the chemical composition and carburized material properties. Carburization and impact tests were performed on the steel. As a result, the following knowledge was obtained.

  FIG. 1 is a graph showing the relationship between the surface C concentration and impact value of a carburized material obtained by carburizing steel. The surface C density | concentration of steel rises by a carburizing process. In order to improve the impact value of the carburized material, it was found that it is effective to control the surface C concentration after carburizing to an appropriate level as shown in FIG.

Furthermore, the present inventors have found that controlling the surface C concentration of the carburized material as described above can be realized by adjusting the content of the alloy element dissolved in the steel. Specifically, the content of each alloy element is set within a predetermined range, and among the alloy elements in steel, the contents of Si, Ni, Al, and Sn in steel (unit: mass%) are set to [Si. %], [Ni%], [Al%], and [Sn%], the following formula (A) is satisfied, so that the surface C concentration of the carburized material becomes an appropriate value and the impact value is improved. The present inventors have clarified this, thereby completing the present invention.
42 ≧ 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] ≧ 8.5 (A)

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

(1) The carburizing steel according to one embodiment of the present invention has a chemical composition of mass%, C: 0.16 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.35. -1.45%, Cr: 0.05-3.0%, Al: 0.001-0.2%, Ni: 0.04-5.0%, Sn: 0.015-1.0%, 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 -1.0%, B: 0-0.005%, Nb: 0-0.3%, Ti: 0-0.3%, V: 0-1.0%, Ca: 0-0.01% Mg: 0 to 0.01%, Zr: 0 to 0.05%, Te: 0 to 0.1%, rare earth elements: 0 to 0.005%, and the balance: Fe and impurities, Si, Ni , Al and Sn content in mass% [Si ], [Ni%], [Al%], when expressed as [Sn%], satisfying the following formula (A).
42 ≧ 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] ≧ 8.5 (A)

(2) The carburizing steel according to the above (1) is such that the chemical composition is mass%, Mo: 0.05 to 1.0%, Cu: 0.01 to 1.0%, and B: 0. One or more of 0002 to 0.005% may be contained.

(3) In the carburizing steel according to (1) or (2), the chemical composition is mass%, Nb: 0.005 to 0.3%, Ti: 0.005 to 0.3%, And V: You may contain 1 type, or 2 or more types among 0.01-1.0%.

(4) In the carburizing steel according to any one of (1) to (3), the chemical composition is mass%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.00. 01%, Zr: 0.0005 to 0.05%, Te: 0.0005 to 0.1%, and rare earth elements: 0.0001 to 0.005%, or one or more of them may be contained. Good.

  If carburized steel parts are manufactured using the steel of the present invention, it is not necessary to adjust the carburizing conditions for each carburized steel part in order to improve the impact value of the carburized steel part. Accordingly, it is possible to improve the production efficiency by unifying the carburizing method and obtain carburized steel parts having excellent impact values, and the industrial effect of the present invention is extremely large.

It is a graph which shows the relationship between an impact value and surface C density | concentration. It is a schematic diagram which shows the cross section perpendicular | vertical to the extending direction of a notch of the Charpy impact test piece used in this invention. It is a schematic diagram which shows the measurement area | region of surface C density | concentration. It is a semilogarithmic graph which shows the relationship between surface C density | concentration and an impact value ratio. It is a graph which shows the relationship between 21 * [Si%] + 5 * [Ni%] + 40 * [Sn%] + 32 * [Al%] and surface C density | concentration and an impact value ratio.

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

  First, the reasons for limiting the chemical components of the steel according to this embodiment will be described. Hereinafter, “mass%”, which is a unit related to the content of alloy elements, is simply referred to as “%”. In the following description, the description related to steel (carburizing steel) applies also to carburized steel parts (carburized material) unless otherwise specified.

C: 0.16-0.30%
C content determines the intensity | strength of the core part of a carburized steel component, and also affects the effective hardened layer depth. In order to ensure the required core strength, the lower limit of the C content is 0.16%. On the other hand, since productivity will fall when there is too much C content, the upper limit of C content shall be 0.30%. The C content is preferably 0.18 to 0.25%.

Si: 0.01 to 2.0%
Si is an element effective for deoxidation of steel, and is an element effective for imparting strength and hardenability necessary for machine structural parts to carburized steel parts. Furthermore, the increase in the Si content reduces the carburizing properties during carburizing and improves the impact value of the carburized steel part. If the Si content is less than 0.01%, the effect is insufficient. Moreover, when Si content exceeds 2.0%, the decarburization at the time of manufacture will become remarkable and the intensity | strength and effective hardened layer depth of carburized steel components will be insufficient. For the above reasons, the Si content needs to be in 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 element effective for deoxidation of steel and an element effective for imparting necessary strength and hardenability to steel. If the Mn content is less than 0.35%, the martensitic transformation start temperature becomes high, causing self-tempering and decreasing the hardness. Further, if the Mn content exceeds 1.45%, the retained austenite is stably present in the steel even after the sub-zero treatment, and the strength of the steel is reduced. For the above reasons, the Mn content needs to be in the range of 0.35 to 1.45%. The Mn content is preferably 0.50 to 1.10%.

Cr: 0.05-3.0%
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 above reasons, the Cr content needs to be in 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 that precipitates in the steel as a nitride and has an effect of refining crystal grains. Furthermore, when the Al content is increased, the carburizing property 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) becomes coarse, which causes embrittlement of steel and carburized steel parts. For the above reasons, the Al content needs to be in the range of 0.001 to 0.2%. A 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. Furthermore, the increase in Ni content reduces the carburizing properties during carburizing, thereby improving the impact value of carburized steel parts. If the Ni content is less than 0.04%, the effect is insufficient. If the Ni content exceeds 5.0%, even if the sub-zero treatment is applied to the steel, the retained austenite is stably present in the steel and the strength of the steel is reduced. For the above reasons, the Ni content needs to be in the range of 0.04 to 5.0%. Preferably, the Ni content is 1.0 to 2.0%.

Sn: 0.015-1.0%
By increasing the Sn content, the carburizing property at the time of carburizing is lowered, 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, when Sn content exceeds 1.0%, the hot ductility of steel will fall. For the above reasons, the Sn content needs to be in the range of 0.015 to 1.0%. The preferred range for 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, when the S content exceeds 0.05%, the effect is saturated, and rather, grain boundary segregation occurs and grain boundary embrittlement occurs. For these reasons, the S content needs to be in the range of 0.004 to 0.05%. A preferable range of the S content is 0.01 to 0.04%.

N: 0.003 to 0.03%
N combines with Al, Ti, Nb, V, and the like in steel to form nitrides or carbonitrides. These nitrides and carbonitrides have an effect of suppressing coarsening of crystal grains. If the N content is less than 0.003%, the effect is insufficient. If the N content exceeds 0.03%, the effect is saturated. For the above reasons, the N content needs to be in the range of 0.003 to 0.03%. A preferable range of the N content is 0.005 to 0.008%.

O: 0.005% or less O forms an oxide in steel. This oxide may segregate at the grain boundaries to cause grain boundary embrittlement. O is an element that easily forms brittle fracture by forming hard oxide inclusions in steel. The O content needs to be limited to 0.005% or less. A preferable range of the O content is 0.0025% or less. Since it is preferable that the O content is small, the lower limit of the O content is 0%.

P: 0.025% or less P segregates at austenite grain boundaries during carburizing, thereby causing grain boundary fracture. In other words, P decreases the impact value of the carburized steel part. Therefore, it is necessary to limit the P content to 0.025% or less. A 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 substantial lower limit of the P content is usually about 0.004%.

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

Mo: 0 to 1.0%
Mo suppresses the segregation of P at grain boundaries, and is an effective element for improving the impact value of steel. If the Mo content exceeds 1.0%, the effect is saturated, so the upper limit of the Mo content needs to be 1.0%. The lower limit of the Mo content is 0%, but when Mo is contained and the above-described effects are obtained, the Mo content is preferably 0.05% or more. A further 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 improving the hardenability. If the Cu content exceeds 1.0%, the hot ductility decreases, so the upper limit of the Cu content needs to be 1.0%. The lower limit of the Cu content is 0%, but when Cu is contained to obtain the above-described effects, the Cu content is preferably 0.01% or more. A further preferable range of the Cu content is 0.01 to 0.2%.

B: 0 to 0.005%
B has a function of suppressing grain boundary segregation of P. B also has the effect of improving the grain boundary strength and the intragranular strength, and the effect of improving the hardenability, and these effects improve the impact value of the steel. If the B content exceeds 0.005%, the effect is saturated, so the upper limit of the B content needs to be 0.005%. The lower limit 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 further preferred range for the B content is 0.0005 to 0.003%.

  The steel according to the present embodiment further contains one or more of Nb, Ti, and V within the range shown below in order to prevent a decrease in impact value even when carburized for a long time. May be. However, it is not essential to contain these elements.

Nb: 0 to 0.3%
Nb produces Nb carbonitride in the steel. Even when so-called high-temperature carburizing with a carburizing temperature of 980 ° C. or higher is applied and when so-called long-term carburizing with a carburizing time of 10 hours or longer is applied, a suitable amount of Nb carbonitride is present in the steel. As a result, the austenite grains can be refined and the impact value can be prevented from decreasing. If the Nb content exceeds 0.3%, the machinability deteriorates, 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 effect is obtained by containing Nb, the Nb content is preferably 0.005% or more. A further preferred range for 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 so-called high-temperature carburizing with a carburizing temperature of 980 ° C. or higher is applied, and when so-called long-term carburizing with a carburizing time of 10 hours or longer is applied, suitable amounts of TiC and TiCS are present in the steel. As a result, the austenite grains can be made finer and the impact value of the steel can be prevented from being lowered. If the Ti content exceeds 0.3%, TiN-based precipitates increase and the fatigue properties of the steel deteriorate. For the above reasons, the upper limit of Ti content needs to be 0.3%. The lower limit of the Ti content is 0%, but when Ti is contained to obtain the above-described effects, the Ti content is preferably set to 0.005% or more. A further preferred range for the Ti content is 0.02 to 0.2%.

V: 0 to 1.0%
V produces V carbonitrides in the steel. Even when so-called high-temperature carburization with a carburizing temperature of 980 ° C. or higher is applied and when so-called long-time carburizing with a carburizing time of 10 hours or longer is applied, a suitable amount of V carbonitride is present in the steel. As a result, the austenite grains can be made finer and the impact value of the steel can be prevented from being lowered. If the V content exceeds 1.0%, the machinability of the steel is deteriorated. For the above reason, the upper limit of the V content needs to be 1.0%. The lower limit of the V content is 0%, but when V is contained and the above-described effects are obtained, the V content is preferably set to 0.01% or more. A further preferred range for 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 following ranges. . However, it is not essential to contain these elements.

Ca: 0 to 0.01%
Ca lowers the melting point of the oxide and softens due to temperature rise during cutting, thus improving machinability. However, if the Ca content exceeds 0.01%, a large amount of CaS is generated, and the machinability deteriorates. For the above reasons, it is desirable to set the upper limit of Ca content to 0.01%. The lower limit of the Ca content is 0%, but when Ca is contained to obtain the above-described effects, the Ca content is preferably 0.0005% or more. A further preferable range of the Ca content is 0.0005 to 0.0015%.

Mg: 0 to 0.01%
Mg is a deoxidizing element and generates an oxide in steel. Further, the Mg-based oxide formed by Mg tends to be a nucleus of MnS crystallization and / or precipitation. Further, the Mg sulfide becomes a composite sulfide of Mn and Mg, thereby spheroidizing MnS. Thus, Mg is an effective element for controlling the dispersion of MnS and improving machinability. However, if the Mg content exceeds 0.01%, a large amount of MgS is generated and the machinability of the steel is lowered. Therefore, it is desirable to set the upper limit of the Mg content to 0.01%. The lower limit of the Mg content is 0%, but when Mg is contained to obtain the above-described effects, the Mg content is preferably set to 0.0005% or more. A further preferred range for the Mg content is 0.0005 to 0.0015%.

Zr: 0 to 0.05%
Zr is a deoxidizing element and generates an oxide. Furthermore, the Zr-based oxide formed by Zr tends to be a nucleus of MnS crystallization and / or precipitation. 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 is saturated, so the upper limit of the Zr content is preferably set to 0.05%. The lower limit of the Zr content is 0%. However, when the above effect is obtained by containing Zr, the Zr content is preferably set to 0.0005% or more. In order to promote the spheroidization of MnS, the lower limit value of the Zr content is particularly preferably more than 0.003%.

Te: 0 to 0.1%
Te promotes the spheroidization of MnS and improves the machinability of the steel. Since the effect is saturated when the Te content exceeds 0.1%, the upper limit of the Te content is preferably set to 0.1%. The lower limit of the Te content is 0%, but when Te is contained to obtain the above-described effects, the Te content is preferably set to 0.0005% or more. A further preferred range for the Te content is 0.0005 to 0.0015%.

Rare earth elements: 0 to 0.005%
The rare earth element is an element that promotes the production of MnS by producing sulfides in the steel and these sulfides become MnS precipitation nuclei, and improves the machinability of the steel. However, if the total content of rare earth elements exceeds 0.005%, the sulfide becomes coarse and the fatigue strength of the steel is reduced, so the upper limit of the total content of rare earth elements needs to be 0.005%. There is. The lower limit of the total content of rare earth elements is 0%, but when the above effects are obtained by adding rare earth elements, the total content of rare earth elements is preferably 0.0001% or more. A further preferable range of the total content of rare earth elements is 0.001 to 0.003%.

  The steel according to the present embodiment contains the above-described alloy components, and the balance contains Fe and impurities. It is permissible for elements other than the above-mentioned alloy components to be mixed into the steel as impurities from the raw materials and production equipment as long as the mixed amount is at a level that does not affect the properties of the steel.

  It mentioned above regarding content of each alloy ingredient which steel concerning this embodiment contains. However, by controlling the content of each alloy component individually, a steel for obtaining a carburized steel part having a sufficient impact value under a single carburizing condition regardless of the shape of the carburized steel part is realized. It is not possible. The present inventors have further found that it is necessary to control the content of the alloy component based on the formula (1).

42 ≧ 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] ≧ 8.5 (1)
In the formula (1), [Si%], [Ni%], [Sn%], and [Al%] indicate the contents of Si, Ni, Sn, and Al in mass%. Below, the basis for derivation of Equation (1) will be described.

First, the present inventors will explain the contents of the study conducted to evaluate the impact resistance of the carburized material.
First, C: 0.20 mass%, Si: 0.24 mass%, Mn: 0.79 mass%, P: 0.014 mass%, S: 0.015 mass%, Cr: 1.21 mass%, Carburization containing Al: 0.031% by mass, Ni: 0.05% by mass, Sn: 0% by mass, N: 0.005% by mass, and O: 0.001% by mass with the balance being Fe and impurities. Steel was defined as the reference steel. Next, a Charpy impact test piece having an arc-shaped notch (notch) having an outer dimension of 10 mm × 10 mm × 55 mm and a radius of curvature of 10 mm and a depth of 2 mm shown in FIG. The impact test piece 3 was defined. For the Charpy impact test piece 3 formed using the reference steel as a material, first, the carburizing condition (hereinafter referred to as the reference carburizing) in which the processing temperature is 930 ° C., the processing time is 5 hours, and the carbon potential is 0.8. The Charpy absorbed energy at 25 ° C. of the carburized material obtained by performing gas carburizing in the case of gas carburizing and then tempering with a tempering temperature of 150 ° C. and a tempering time of 90 minutes is defined as a reference impact value. Defined.
Furthermore, the impact value ratio is determined by the Charpy at 25 ° C. of the carburized material obtained by carburizing and tempering the Charpy impact test piece 3 in accordance with the carburizing conditions (that is, the reference carburizing conditions) applied when obtaining the reference impact value. Absorbed energy was defined as the value divided by the reference impact value.
The reference steel described above is a steel having a chemical composition corresponding to SCr420, which is generally used as a gear steel, and is the same as that of Comparative Example 26 described later. The gas carburizing performed under the above-mentioned standard carburizing conditions is a general carburizing process performed for the manufacture of machine structural parts.
FIG. 2 shows the side shape of the Charpy impact test piece 3 described above (the shape of the cross section perpendicular to the extending direction of the notch). The radius of curvature of the notch 2 is 10 mm. The shape of the Charpy impact test piece 3 is different from the shape of a general Charpy impact test piece (for example, the shape defined in JIS-Z2242 “Charpy impact test method for metal material”). The shape of the notch 2 of the Charpy impact test piece 3 is determined with the intention of simulating the shape of the tooth root portion of the gear. By performing the Charpy impact test on the test piece having such a notch, it is possible to estimate the impact resistance characteristics at the tooth root portion of the gear. A test piece having such a notch is widely used as a test piece shape for measuring the impact resistance characteristics of carburized steel as described in, for example, Japanese Patent Application Laid-Open No. 2013-40376. The Charpy absorbed energy was measured according to JIS-Z2242 “Charpy impact test method for metal materials” except for the shape of the Charpy impact test piece 3. The implementation temperature of the Charpy impact test was 25 ° C. The Charpy impact test piece 3 was produced by machining.

Next, the present inventors forged, machined, and carburized various steels containing alloy elements within the above-described component ranges (however, the provisions relating to Formula 1 were not considered), and carburized materials obtained from these steels. The impact value ratio was determined. Furthermore, the present inventors measured the surface C density | concentration of each carburizing material.
A method for measuring the surface C concentration will be described below. First, the Charpy impact test piece 3 is formed along the notch surface (surface on which the notch is formed) of the Charpy impact test piece 3 subjected to gas carburizing treatment under the standard carburizing condition and the direction perpendicular to the notch 2. The cut surface was cut and polished. FIG. 3 shows a schematic view of the cut surface. Next, the C concentration was measured at intervals of 5 μm from the bottom surface of the notch 2 in the region of 5 to 50 μm (surface C concentration measurement region 1) in the height direction of the Charpy impact test piece 3. The C concentration was measured by EPMA. The size of the measurement point (EPMA electron beam diameter) was set to 5 μm. A value obtained by averaging the ten measurement data obtained as described above was defined as the surface C concentration. The unit of the surface C concentration is mass%.

  Thus, as a result of measuring the surface C concentration after carburizing when the chemical composition and carburizing material characteristics of the steel were changed extensively and systematically, the surface C concentration changed according to the amount of alloy element added. The present inventors have found out. This phenomenon is considered to be caused by a chemical interaction between the alloy element and C that enters the steel surface by carburization. Si, Ni, Al, and Sn have a particularly strong influence on the surface C concentration, and the surface C concentration decreased as the content of these elements increased. According to the above-described knowledge, in the chemical composition of the steel according to the present embodiment, the carburizability of the steel can be controlled by defining the relationship among the contents of Si, Ni, Al, and Sn by Equation 1. it can.

The impact value ratio needs to be 1.2 or more for the following reason. As described above, by sufficiently improving the impact resistance characteristics of carburized materials used in carburized steel parts, the design of the parts is changed so that the impact fracture resistance is ensured while the amount of material used is suppressed. be able to. In the technical field of mechanical structural parts, in order to carry out such design changes, the impact value is improved by 20% with respect to the above-mentioned reference impact value (impact value of SCr420 carburized under general carburizing conditions). It is said that it is necessary.
As described above, there is a correlation between the impact value ratio and the surface C concentration. FIG. 4 is a semilogarithmic graph showing the correlation between the surface C concentration of the carburized material and the impact value ratio. 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 carburized material having an impact value ratio of 1.2 or more, the surface C concentration of the carburized material subjected to gas carburizing under the standard carburizing condition is controlled to 0.75% or less. I found it necessary to do.
Here, the surface C concentration was subjected to multiple regression analysis with the contents of Si, Ni, Al, and Sn as factors. As a result, the following formulas (1 ′) and (2 ′) are used as critical conditions for obtaining a carburized material having a surface C concentration of 0.75 mass% when gas carburization is performed under the standard carburizing conditions. Got.
21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] = α (1 ′)
α = 8.5 (2 ′)

  FIG. 5 is a graph showing the relationship between the α value and the surface C concentration and the relationship between the α value and the impact value ratio. In FIG. 5, the α value of the data point to the left of the left broken line is less than 8.5, and the α value of the data point to the right of the right broken line is more than 42. 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 carburizing conditions. As the α value increases, the surface C concentration of the carburized material decreases, and accordingly, the impact value of the carburized material increases. Preferably, the α value is 12 or more.

  On the other hand, when the carburizing property is remarkably lowered, the wear resistance is remarkably lowered by reducing the surface hardness, and the strength as a carburized steel part is not sufficient. When gas carburizing is performed under the standard carburizing conditions, the surface hardness of the carburized material is desirably higher than HV550. In order to achieve this, the surface C concentration of the carburized material that has been gas carburized under the standard carburizing condition needs to be 0.4 mass% or more. Furthermore, in order to make the surface C concentration of the carburized material subjected to gas carburizing under the standard carburizing conditions 0.4 mass% or more, it has been found that the α value needs to be 42 or less. The surface C concentration of the carburized material that has been gas carburized under the standard carburizing conditions is more preferably 0.55% by mass or more, and in order to achieve this, the α value is preferably 25 or less. .

The carburizing method for obtaining carburized steel parts from the steel according to the present embodiment is preferably gas carburizing (which may be either a modified furnace method or a dropping method). In addition to carburizing, nitriding may be performed.
Although the effective hardened layer depth may be considered as an evaluation standard for carburized steel parts, the characteristics required for parts such as vehicle gears such as differential gears are higher in surface hardness than effective hardened layer depth. Strongly related to. Therefore, if the steel according to the present embodiment, which can control the surface C concentration of the carburized steel part to an appropriate level and thereby optimize the surface hardness, an advantageous effect on industrial use can be obtained.

  The steel according to the present embodiment is, for example, first formed into a round bar steel by hot rolling, then subjected to forging or cutting to form a gear or the like, and further carburized and quenched to be a carburized steel part. Good.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Various steel ingots having chemical components shown in Table 1-1 and Table 1-2 are forged into a square bar shape having a longitudinal cross-sectional dimension of 50 mm in length and 50 mm in width (50 mm × 50 mm), followed by soaking. After performing normalization and further normalization, it was further divided into four in a square bar shape having a longitudinal cross-sectional dimension of 25 mm in length and 25 mm in width. Each of the obtained bars has an arc-shaped notch (notch) having an outer dimension of 10 mm × 10 mm × 55 mm and a radius of curvature of 10 mm and a depth of 2 mm shown in FIG. 2 along its central axis. Charpy impact test specimens were collected. This test piece shape is the same as the Charpy impact test piece 3 described above. Next, the Charpy impact test piece was carburized. In Examples and Comparative Examples other than Comparative Example 29, gas carburizing was performed under carburizing conditions in which the processing temperature was 930 ° C., the processing time was 5 hours, and the carbon potential was 0.8. This processing condition is the same as the reference carburizing condition described above. In Comparative Example 29, gas carburizing was performed under carburizing conditions in which the processing temperature was 930 ° C., the processing time was 5 hours, and the carbon potential was 0.6. Tempering was carried out under conditions where the tempering temperature was 150 ° C. and the tempering time was 90 minutes.
After tempering, the surface C concentration of each sample was measured. The method for measuring the surface C concentration is as follows. First, the Charpy impact test piece was cut along a notch surface (surface on which a notch was formed) and a direction perpendicular to the notch of the Charpy impact test piece, and the cut surface was polished. Next, the C concentration was measured at intervals of 5 μm from the bottom surface of the notch 2 in the 5 to 50 μm region (surface C concentration measurement region 1) in the height direction of the Charpy impact test piece. The C concentration was measured by EPMA. The size of the measurement point (EPMA electron beam diameter) was set to 5 μm. A value obtained by averaging the ten measurement data obtained as described above was defined as the surface C concentration. The unit of the surface C concentration is mass%.
Moreover, the Charpy impact test was implemented after tempering and Charpy absorbed energy (impact value) was measured. The Charpy impact test was performed at a test temperature of 25 ° C. in accordance with the method defined in JIS-Z2242, except for the shape of the notch of the Charpy impact test piece.
Further, the impact value ratio of each sample was calculated by dividing the impact value of each sample by the impact value of Comparative Example 26. Note that the steel of Comparative Example 26 is the reference steel described above.
In addition, in order to evaluate the wear resistance of each sample, a wear test was performed on each sample to measure the wear depth. From the normalized 50 mm × 50 mm square bar produced by the above-described method, a cylindrical portion having a diameter of 26 mm and a length of 28 mm along the central axis, and a diameter of 24 mm and a length having the same central axis as the cylindrical portion. A wear test piece having a shape having a cylindrical grip portion having a thickness of 51 mm was collected. The grip portions are arranged at both ends of the cylindrical portion in the length direction. Further, this wear test piece was subjected to carburizing treatment under the same conditions as the Charpy impact test piece described above. The wear depth is the depth of wear generated in the wear test piece after pressing the roller against the cylindrical portion of the wear test piece and rotating the roller 1 million times. The wear test conditions were as follows. Samples with a wear depth of less than 30 μm were judged to have sufficient wear resistance.
Roller material: Bearing steel (SUJ2)
Roller hardness: HV700-800
Roller diameter: 130mm
Roller width: 18mm
Roller shape: R = 150mm crowning is formed on the outer periphery Roller contact force: Hertz stress 1500 MPa (surface pressure)
Slip rate: -100%

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

On the other hand, Comparative Examples 26 to 35 did not have preferable characteristics.
Since Comparative Examples 26 and 28 did not contain Sn, they were excessively carburized and had only low impact values compared to the Examples. Further, in Comparative Example 31, since the Sn content was below the specified range of the present invention, similarly to Comparative Examples 26 and 28, carburization was excessively performed, and only a low impact value was obtained as compared with the Examples. There wasn't.
Regarding Comparative Example 27, the content of each alloy element is within the specified range of the present invention, but 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] is the present invention. The specified range was exceeded. Thereby, the abrasion resistance of Comparative Example 27 was low.
Regarding Comparative Example 30, the content of each alloy element is within the specified range of the present invention, but 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] is the present invention. The carburization was excessive because it was below the specified range. Thereby, the comparative example 30 had only a low impact value compared with the Example.
Regarding Comparative Example 32, since the Sn content exceeded the specified range of the present invention, the hot ductility was lowered. Thereby, in Comparative Example 32, cracks frequently occurred on the surface of the obtained carburized material.
Regarding Comparative Example 33, since the Ni content exceeded the specified range of the present invention, the strength decreased. Thereby, the abrasion resistance of Comparative Example 33 was low.
Regarding Comparative Example 34, since the Al content exceeded the specified range of the present invention, embrittlement occurred. Thereby, the impact value ratio of the comparative example 34 was low.

  Reference Example 29 is the same steel as Comparative Example 26, but the carburizing conditions are different and the carbon potential (carburizing treatment of 0.6) is set low, so the surface C concentration is low and a good impact value is obtained. You can see that However, setting a low carbon potential in actual production is not suitable because it leads to a decrease in productivity.

1 Surface C concentration measurement area 2 Notch
3 Charpy impact test piece (carburized material)

Claims (4)

Chemical composition is mass%,
C: 0.16-0.30%,
Si: 0.01 to 2.0%,
Mn: 0.35 to 1.45%,
Cr: 0.05-3.0%,
Al: 0.001 to 0.2%,
Ni: 0.04 to 5.0%,
Sn: 0.015-1.0%,
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 elements: 0 to 0.005%, and the balance: Fe and impurities,
When the content of Si, Ni, Al and Sn is expressed as [Si%], [Ni%], [Al%], and [Sn%] in mass%, the following expression (1) is satisfied. Carburizing steel.
42 ≧ 21 × [Si%] + 5 × [Ni%] + 40 × [Sn%] + 32 × [Al%] ≧ 8.5 (1) The chemical composition is mass%,
Mo: 0.05-1.0%,
Cu: 0.01 to 1.0%, and B: 0.0002 to 0.005%
The carburizing steel according to claim 1, wherein one or more of them are contained. The chemical composition is mass%,
Nb: 0.005-0.3%
Ti: 0.005 to 0.3%, and V: 0.01 to 1.0%
The carburizing steel according to claim 1 or 2, wherein one or more of them are contained. The chemical composition is 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%
The carburizing steel according to any one of claims 1 to 3, wherein one or more of them are contained.

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