JP4807949B2 - Rolled steel bar for case hardening with excellent high-temperature carburizing characteristics - Google Patents

Description

  The present invention relates to a case-hardening rolled steel bar used as a material for machine parts used by carburizing in transportation equipment such as automobiles, construction machinery, and other industrial machines, and particularly bearings that require high-temperature carburizing characteristics. Further, the present invention relates to a rod-shaped rolled steel material for case hardening that is useful as a material for pulleys for CVT, shafts, gears, gears with shafts, and the like.

  Of machine parts used for automobiles, construction machinery, and other various industrial machines, parts that require particularly high strength are conventionally subjected to surface hardening heat treatment (skin hardening) such as carburizing, nitriding and carbonitriding. ing. For these applications, case-hardened steel defined by JIS standards such as SCr, SCM, SNCM, etc. is usually used. After forming into the desired part shape by machining such as forging and cutting, carburizing and carbonitriding It is manufactured through a finishing process such as polishing.

  In recent years, it has been desired to reduce the manufacturing cost and the lead time for the mechanical parts as described above, and the heat treatment time has been shortened by increasing the temperature of carburizing and carbonitriding. However, when the carburizing and carbonitriding temperature is increased, there arises a problem that the crystal grains of the steel material become coarse and the amount of heat treatment strain increases.

  Under such circumstances, the precipitation of carbides and nitrides containing elements such as Al, Nb, Ti, etc. in the steel material is finely precipitated to suppress the coarsening of crystal grains, and the carbonitriding temperature. In order to cope with the increase in the steel, development of a steel material capable of exhibiting the effect of preventing the coarsening of the crystal grains even in a higher temperature range is being developed. For example, in Patent Document 1, a proper amount of Nb is contained in case-hardened steel, and the precipitates made of Nb carbonitride are generated finely and in large quantities by optimizing the conditions for rolling the steel material. The crystal grain coarsening temperature is increased.

Patent Document 2 discloses a method of suppressing the coarsening of crystal grains under high temperature conditions by optimizing the contents of Nb, Al, Ti, and N in steel. Furthermore, in Patent Document 3, the content of N, sol-Al, Ti, and Nb in steel is specified, and a large number (5/10 μm ² or more) of Nb carbonitride and Nb-Al composite carbonitride are precipitated. Therefore, the coarsening of crystal grains is prevented.

However, in the method disclosed in Patent Document 1, an effect of preventing grain coarsening up to a temperature range of about 1030 ° C. is obtained, but a satisfactory effect of preventing grain coarsening is obtained at a high temperature range exceeding 1030 ° C. I can't. Further, in Patent Document 2, it is possible to obtain an appropriate effect of preventing coarsening of crystal grains by specifying the contents of Nb, Ti, Al, and N, but the effect is up to 1050 ° C. and exceeds 1050 ° C. In the high temperature range, a satisfactory crystal grain coarsening prevention effect cannot be obtained. This is considered to be due to the fact that only the content of Nb, Ti, and Al serving as a carbonitride source is determined, and no consideration has been given to the precipitation state of carbonitrides containing these elements. Further, even in Patent Document 3, the effect of preventing the coarsening of the crystal grains obtained is up to 1025 ° C., and a sufficient effect cannot be obtained in a high temperature range exceeding this. This is considered to be due to the fact that no consideration has been given to the size distribution by controlling only the number of carbonitride-based precipitates.
JP-A-4-371522 Japanese Patent No. 3510506 JP-A-9-78184

  As described above, various additions of Al, Nb, and Ti to steel materials have been studied as countermeasures for increasing the temperature of the grain coarsening prevention action to cope with an increase in carbonitriding temperature, but all are satisfactory. It cannot be said that it can be done, and further improvement is required.

  The present invention has been made by paying attention to such circumstances, and its purpose is to shorten the carburizing and carbonitriding treatment as a material for rod-like machine parts such as a CVT pulley where a carburizing depth is required. The object is to provide a rolled steel bar for case hardening that can exhibit an excellent effect of preventing grain coarsening even when carburizing at a higher temperature than the conventional example so that it can be performed in a timely manner.

The composition of the present invention that has solved the above-mentioned problems is in mass%,
C: 0.05 to 0.25%
Si: 0.05-2.0%,
Mn: 0.01 to 1.5%,
S: 0.005 to 0.2%,
Cr: 0.4 to 1.5%,
N: 0.0085 to 0.0219%,
Al: 0.058 to 0.062%,
Nb: 0.038 to 0.100%,
Ti: 0.008 to 0.012%,
And the balance is made of steel consisting of Fe and inevitable impurities, and all carbides, nitrides and carbonitrides having an equivalent circle diameter of 100 nm or more containing at least one element selected from Al, Nb, and Ti in the steel, It is a rolled steel bar for case-hardening having excellent crystal grain coarsening prevention characteristics during high-temperature carburization, in which ^two or more of them are attached or the number of combined precipitates is 0.5 × 10 ¹² pieces / m ² or less.

In addition to the above components, the steel material of the present invention is also effective to contain one or more elements selected from the groups shown in the following 1) to 6) according to the required properties.
1) Cu: 1.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%),
2) Mo: 1.0% or less (excluding 0%),
3) B: 0.0005 to 0.0030%,
4) Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%),
5) Mg: 0.0001 to 0.02%, Ca: 0.0001 to 0.02%, Te: 0. 0005 to 0.02%, REM: at least one selected from the group consisting of 0.0005 to 0.02%,
6) Zr: 0.2% or less (not including 0%) and / or V: 0.5% or less (not including 0%).

According to the present invention, the content of Al, Nb, Ti and the like in the steel is specified, and in particular, all of the equivalent circle diameters of 100 nm or more containing at least one element selected from Al, Nb, Ti in the steel. Carbide, nitride, carbonitride, two or more of them are attached, or the number of combined precipitates is suppressed to 0.5 × 10 ¹² pieces / m ² or less to prevent grain coarsening at higher temperatures. The effect can be enhanced, and for example, it is possible to provide a rolled steel bar for case hardening that does not cause crystal grain coarsening even in a temperature range exceeding 1050 ° C., or even in a high temperature range exceeding 1100 ° C. Therefore, it is possible to increase the processing temperature in preparation for shortening the carburizing or carbonitriding process, or it is possible to advance the carbonitriding to the deep part in a shorter time, and it has excellent dimensional accuracy as well as surface layer hardening characteristics and deep part impact characteristics Case-hardened parts can be provided.

As described above, in the present invention, as a rolled steel bar for case hardening excellent in high-temperature carburizing characteristics, the chemical components of the steel are specified, and Al, which becomes a fine precipitate source that exhibits an effect of preventing grain coarsening particularly during heating, By controlling each content of Nb, Ti and N, C to a specific range, and suppressing the coarsening of the precipitates, and generating a large number of fine particles as much as possible, the effect of preventing the coarsening of grains is more effective. As a means for exhibiting, all carbides, nitrides and carbonitrides having an equivalent circle diameter of 100 nm or more containing at least one element selected from Al, Nb, and Ti in steel are adhered. Or the number of composite precipitates (hereinafter sometimes referred to as “Al / Nb / Ti-containing precipitates”) is suppressed to 0.5 × 10 ¹² / m ² or less. Yes.

Hereinafter, the reason why the chemical composition of the steel is determined in the present invention will be clarified, and subsequently, the reason why the number of the Al / Nb / Ti-containing precipitates in the steel is suppressed to 0.5 × 10 ¹² / m ² or less will be clarified. I will make it.

  First, the reason for determining the chemical composition of steel will be described.

C: 0.05-0.30%;
C is an important element for securing the core hardness necessary for a machine part. If it is less than 0.05%, the static strength of the part is insufficient due to insufficient hardness. However, if the amount of C is too large, it becomes too hard and the forgeability and machinability deteriorate, so it is necessary to keep it to 0.30% or less. From such a viewpoint, the C content is more preferably 0.15% or more, further preferably 0.17% or more, and 0.25% or less, more preferably 0.23% or less.

Si: 0.01-2.0%;
Si acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving the internal quality, and is effective in ensuring the surface hardness of carburized parts by suppressing the decrease in hardness during tempering treatment. It is an element and requires addition of 0.01% or more. However, if the amount of Si is too large, the steel becomes too hard and the machinability and forgeability deteriorate, so 2.0% was set as the upper limit. The Si content is more preferably 0.02% or more, further preferably 0.05% or more, 0.8% or less, and further preferably 0.6% or less.

Mn: 0.01 to 2.0%;
Mn acts as a deoxidizer, has the effect of increasing the internal quality of steel by reducing oxide inclusions, and has the effect of significantly increasing the hardenability during carburizing and quenching. It is necessary to make it contain 0.01% or more in order to make it exhibit. However, if the amount is too large, not only the center segregation becomes prominent and the internal quality is deteriorated, but also the striped structure becomes prominent and the variation of the internal characteristics increases and the impact characteristics deteriorate. And The more preferable content of Mn is 0.2% or more, more preferably 0.3% or more, 1.5% or less, and further preferably 1.0% or less.

S: 0.005 to 0.2%;
S combines with Mn, Ti, etc. to form MnS inclusions, TiS inclusions, etc., and adversely affects the impact strength of the parts. Therefore, it is preferable to suppress as much as possible, and in the present invention where impact characteristics are required, the upper limit is set. Set to 0.2%. However, S, on the other hand, has the effect of improving the machinability, so when the machinability is strongly required, it should be contained in an appropriate amount, and preferably about 0.005% or more. In normal steel for machine structural use, it is preferably about 0.01% or more and 0.07% or less.

Cr: 0.01-2.0%;
Cr has the effect of increasing its hardness by dissolving in carbides such as Ti and Nb, and contributes to the improvement of wear resistance. Therefore, it is an alloy element often used in sliding parts such as gears and bearings, and it is desirable to contain 0.01% or more. Incidentally, in the case of JIS standard case-hardened steel (SCr420), the Cr content is set to 0.9 to 1.2%. However, if the Cr content exceeds 2.0%, the steel material becomes too hard and the machinability and forgeability deteriorate, so 2.0% was set as the upper limit. More preferably, it is 0.4% or more, more preferably 0.9% or more, 1.5% or less, and further preferably 1.2% or less.

N: 0.003-0.030%;
N combines with Al, Ti and Nb to form nitrides and carbonitrides, and has the effect of suppressing austenite grain growth during carburizing heating. It must be contained in an amount of 0.003% or more, preferably 0.005% or more. However, if the amount of N is too large, it will adversely affect hot workability and impact properties, so at most 0.030% or less, more preferably 0.025% or less, and even more preferably 0.020% or less. It is good to suppress.

Al: 0.01 to 0.12%;
Al is an element effective for adjusting the crystal grains of the steel structure. That is, Al combines with N in the steel to form a nitride, which exhibits the action of suppressing the growth of crystal grains during heat treatment. In addition, when Al is added in combination with Nb or Ti, which will be described later, a composite precipitate of Al nitride and Ti carbonitride that is more stable than an Al-based single precipitate, or a composite of Al nitride and Nb carbonitride A precipitate or a composite precipitate of Al nitride and Nb—Ti composite carbonitride is formed to enhance the grain coarsening action during high-temperature carburization. In order to exhibit these effects effectively, it is necessary to contain 0.01% or more. However, if the Al content is too high, hard and coarse non-metallic inclusions (Al ₂ O ₃ ) are generated and the impact strength is deteriorated, so the upper limit was set to 0.12%. A more preferable content of Al is 0.015% or more, more preferably 0.02% or more, and 0.10% or less, and further preferably 0.07% or less.

Nb: 0.01-0.20%;
Nb is an element that plays an especially important role in the present invention, and combines with N and C in steel to form nitrides, carbides or carbonitrides, and suppresses grain coarsening in the heating process during carburizing. If it is less than 0.01%, nitrides, carbides, or carbonitrides that are stable at high temperatures are not generated, so that the effect of preventing grain coarsening cannot be obtained. Moreover, Nb is added in combination with Al or Ti, so that it is more stable than a single precipitate containing Nb and is a more stable composite of Al nitride and Nb carbonitride, Nb-Ti composite carbonitride, or Al nitride. A composite precipitate of Nb—Ti composite carbonitride is formed and contributes to an improvement in the effect of preventing grain coarsening during high temperature carburization.

  However, if the Nb content is too large, coarse precipitates containing Nb are formed, and the Ostwald grain growth described later is accelerated to reversely deteriorate the crystal grain coarsening prevention characteristic. Therefore, it should be suppressed to 0.20% or less. is there. A more preferable content of Nb is 0.02% or more, more preferably 0.03% or more, 0.15% or less, and further preferably 0.10% or less.

Ti: 0.005 to 0.12%;
Ti is also an element that plays an important role in the present invention. That is, Ti in steel is combined with N and C to form carbides, nitrides, carbonitrides, and suppresses grain coarsening during high-temperature carburizing. Also, by adding Al and Nb in combination, a more stable Al nitride and Ti carbonitride composite precipitate, Nb-Ti composite carbonitride, or Al nitride and Nb than a single precipitate containing Ti. Forms composite precipitates of -Ti composite carbonitrides and contributes to improvement of crystal grain coarsening prevention characteristics. When the Ti content is less than 0.005%, the number of precipitated Ti carbonitrides and composite carbonitrides with other elements becomes insufficient, and satisfactory crystal grain coarsening prevention characteristics cannot be obtained. On the other hand, if the Ti content exceeds 0.12% and becomes excessively large, coarse Nb—Ti carbonitrides are formed to promote Ostwald growth and deteriorate the crystal grain coarsening prevention characteristics. The more preferable content of Ti is 0.008 or more and 0.10% or less, more preferably 0.05% or less.

  The essential constituent elements of the steel material used in the present invention are as described above, and the balance is Fe and inevitable impurities. Elements that are inevitably mixed include, for example, P (phosphorus) and O (oxygen), and the amount thereof is not particularly limited as long as it is an unavoidable impurity amount. Should be suppressed to 0.03 or less and O to 0.002% or less.

  Incidentally, P segregates at the grain boundaries and lowers the impact characteristics of the part, so it should be suppressed as little as possible, at most 0.03% or less, more preferably 0.015% or less, still more preferably 0.010. It is good to keep it below%. Further, O decreases the strength characteristics of the steel material, so 0.002% or less, more preferably 0.001% or less is preferable.

  In addition to the above essential elements, the steel used in the present invention is effective to contain the following selective elements in order to give additional desired characteristics as desired. What added the element is also contained in the technical scope of this invention.

Ni: 3.0% or less and / or Cu: 1.0% or less;
Ni and Cu are both elements that improve the corrosion resistance of steel, and can be added alone or in combination of two as required. Moreover, since Ni contributes to the improvement of impact resistance of steel, addition of an appropriate amount is effective. However, excessive addition of Ni and Cu leads to an increase in steel cost, and excessive addition of Cu also causes a decrease in hot workability, so Ni is 3.0% or less and Cu is 1.0% or less. Should be suppressed. A more preferable addition amount of Ni is 0.1 to 2.0%, more preferably 0.3 to 1.5%, and a more preferable addition amount of Cu is 0.1 to 0.8%, and more preferably 0.8. 2 to 0.6%.

Mo: 1.0% or less;
Mo is an element effective in suppressing the hardness reduction during the tempering process and ensuring the surface hardness of the carburized parts. In addition, it significantly enhances the hardenability during carburizing and quenching and suppresses hydrogen embrittlement resistance. But it is known to work effectively. However, even if added excessively, the effect is saturated, resulting in an increase in the cost of the steel material. Further, since the steel material is hardened and deteriorates the machinability, it should be suppressed to 1.0% or less even if added. is there. A more preferable addition amount of Mo is 0.1 to 0.8%, and more preferably 0.15 to 0.45%.

B: 0.0005 to 0.0030%;
B has the effect of greatly increasing the hardenability of the steel material in a small amount, and also has the effect of enhancing the impact strength by strengthening the grain boundaries. Such an effect is effectively exhibited by adding 0.0005% or more. However, these effects are saturated at about 0.0030%, and if the amount of B is too large, B nitride is easily formed and adversely affects cold and hot workability. In this case, it is preferable to adjust within the range of 0.0005 to 0.0030%, more preferably 0.0008 to 0.0025%, and still more preferably 0.0010 to 0.0020%.

Pb: 0.1% or less and / or Bi: 0.1% or less;
Pb and Bi are elements that contribute to improving the machinability of steel, and it is effective to add one or two of these when machinability is particularly required. However, if the added amount is too large, the strength of the steel material is lowered, so each should be suppressed to 0.1% or less, more preferably Pb + Bi to 0.1% or less. A more preferable addition amount as Pb + Bi is 0.02 to 0.08%, and more preferably 0.03 to 0.06%.

One or more of Ca: 0.0001-0.02%, Mg: 0.0001-0.02%, Te: 0.0005-0.02%, REM: 0.0005-0.02%;
Ca, Mg, Te, and REM have the effect | action which suppresses the expansion | swelling of the sulfide which exists in steel by adding 1 type, or 2 or more types, and improves an impact characteristic. In the case of Mg and Ca, such an effect is not exhibited effectively when the content is less than 0.0001%, and when it exceeds 0.02%, the steel strength may be decreased due to the formation of coarse oxides. . Therefore, Mg and Ca are each preferably 0.0001 to 0.02%, more preferably 0.001 to 0.010%.

  Similarly, if Te and REM are less than 0.0005%, the effect is not exhibited effectively, and if it exceeds 0.02%, hot ductility is significantly adversely affected, making it difficult to manufacture steel and process parts. Therefore, when adding Te and REM, each is selected from the range of 0.0005 to 0.02%, more preferably 0.001 to 0.01%, and still more preferably 0.002 to 0.005%. It is good.

Zr: 0.2% or less and / or V: 0.5% or less;
Zr and V form carbides and nitrides in the same manner as Nb and Ti, and exhibit a function of increasing the high-temperature stability of these carbonitrides by complex precipitation with carbonitrides of Al, Nb, and Ti. . However, if the amount is too large, coarse precipitates containing Zr and V are generated and exhibit the crystal grain coarsening preventing property. Therefore, Zr should be suppressed to 0.2% or less and V should be suppressed to 0.5% or less. The more preferable contents in consideration of the advantages and disadvantages thereof are 0.001 to 0.1% for Zr and 0.005 to 0.2% for V.

The number of Al, Nb, Ti-containing precipitates containing Al, Nb, Ti in steel and having an equivalent circle diameter of 100 nm or more is 1.0 × 10 ¹² pieces / m ² or less;
Coarse Al, Nb, Ti-containing precipitates (specifically, carbides, nitrides, carbonitrides containing them, or those in which they are attached or combined) in the hot rolled material When present, the Ostwald growth of the Al, Nb, Ti-containing precipitate during high-temperature carburization is promoted. Ostwald growth is a phenomenon in which coarse precipitates take in the fine precipitates existing in the system and grow. Since it disappears and the number thereof decreases, the crystal grain coarsening prevention characteristic deteriorates.

  Therefore, in the present invention, coarse Al / Nb / Ti-containing precipitates are reduced as much as possible, and fine precipitates are kept stable even during high temperature treatment for carburizing, thereby suppressing grain coarsening during high temperature carburizing. It is intended.

  When hot forging a steel material, it is said that Al-Nb-Ti-containing precipitates are usually dissolved at a high temperature of 1100 ° C. or higher at the time of heating for forging, but coarse precipitates are It does not dissolve sufficiently in the heating time that is performed before hot forging. Therefore, the grain coarsening prevention characteristic can be improved by reducing in advance the coarse Al, Nb, Ti-containing precipitates contained in the rolled steel bar. That is, since the total amount of Al, Nb, Ti-containing precipitates contained in the steel material containing the same amount of Al, Nb, Ti, N is substantially the same, the amount of coarse Al, Nb, Ti-containing precipitates As a result, the amount of fine Al / Nb / Ti-containing precipitates is greatly increased, and the presence of a large amount of the fine Al / Nb / Ti-containing precipitates reduces the effect of suppressing grain coarsening. It will be dramatically improved.

A more preferable number of Al.Nb.Ti-containing precipitates having an equivalent circle diameter of 100 nm or more is 0.8 × 10 ¹² / m ² or less, and more preferably 0.5 × 10 ¹² / m ² or less.

  In recent years, in order to reduce heat treatment costs and improve forging accuracy, it has been required to lower the heating temperature and forging temperature during forging to a temperature range in which solid solution of Al, Nb, Ti cannot be expected. Even under such circumstances, reducing the coarse Al, Nb, Ti-containing precipitates in the rolled steel bar is extremely useful for improving the crystal grain coarsening prevention characteristics.

  In cold forging, reducing the number of coarse Al, Nb, Ti-containing precipitates in the rolled steel bar is also effective for preventing grain coarsening.

In the present invention, the quantitative reference for the Al, Nb, Ti-containing precipitate is that an extracted replica is prepared from the D / 4 (D is the diameter) position of the rolled steel bar, and this is used as a transmission electron microscope (product of Hitachi, Ltd.) The name “H-800”) was used to photograph 5 fields of view (approximately 300 μm ² ) at 20,000 times, and the area of the precipitate was measured by image analysis using Image Pro made by Micromedia. Thus, it was converted into an equivalent circle diameter and used as a quantitative standard for size.

  On the other hand, the determination of whether or not the precipitate contains Al, Nb, Ti as the quantitative standard of the component is performed by quantitative analysis of the component with EDX (energy dispersive X-ray analyzer), and with the precipitate having an Al + Nb + Ti amount of 50% or more. Al / Nb / Ti-containing precipitates were used.

  In addition, in this invention, when it shows as an example of the precipitate contained in a steel bar, it is as the following FIGS. That is, FIG. 1 is an electron micrograph of an Al nitride-Nb carbonitride composite precipitate observed in the test steel bar, and FIG. 2 is an arrow 1, 2, 3, 4 attached to the photograph of FIG. 3 is a chart showing the characteristic X-ray intensity of the position, FIG. 3 is an electron micrograph of Al nitride similarly observed in the test steel bar, and FIG. 4 shows the characteristic X-ray intensity of the position of the arrow 5 in FIG. FIG. 5 is an electron micrograph of an Nb—Ti composite carbonitride that is also observed in the test steel bar, FIG. 6 is a chart showing the characteristic X-ray intensity at the position of the arrow 6 in FIG. FIG. 8 is an electron micrograph of Nb carbonitride observed in the test bar steel, and FIG. 8 is a chart showing the characteristic X-ray intensity at the position of arrow 7 in FIG.

In any case, in the present invention, in the steel material whose component composition is specified as described above, the number of precipitates containing Al, Nb, Ti having an equivalent circle diameter of 100 nm or more in the steel material is 1.0 × 10 ¹² pieces / m ² or less. Suppressed to a high temperature stability that hardly causes coarsening of crystal grains even when exposed to a high temperature range of, for example, 1050 ° C. or higher, or 1100 ° C. or higher at the time of carburizing as steel for case hardening. A steel for baking can be obtained. Therefore, if this case-hardening steel is used, the carburizing process can be performed at a higher temperature and in a shorter time, the thermal economy for the carburizing process can be improved and the processing efficiency can be greatly increased. It is possible to provide a case hardening steel that is very excellent in accuracy and core impact characteristics.

  The production conditions for obtaining the case-hardening steel having the above characteristics are not particularly limited, and steel that satisfies the above-mentioned requirements for the component composition is melted and cast, soaked, hot-rolled, or necessary according to a conventional method. The steel sheet may be re-heated and then cold-rolled to obtain a steel bar having a predetermined size. At this time, it is preferable to perform soaking before hot rolling in a temperature range of 1250 to 1350 ° C. for 2 hours or more.

  That is, when steel grades satisfying the above component composition are soaked at 1250 to 1350 ° C. for 2 hours or more, coarse Al, Nb, Ti-containing precipitates in the steel can be dissolved as much as possible in this soaking step. it can. In other words, if the soaking temperature is less than 1250 ° C., the coarse Al / Nb / Ti-containing precipitates cannot be sufficiently dissolved, and the coarse Al / Nb / Ti-containing precipitates remain after the subsequent hot rolling. Therefore, it becomes difficult to obtain a satisfactory crystal grain coarsening suppression effect.

  This soaking temperature is preferably high from the viewpoint of dissolution of coarse Al, Nb, Ti-containing precipitates, but the effect is saturated at about 1350 ° C. If the soaking temperature is increased further, the equipment Thermal deterioration becomes significant, and the cost load becomes excessive compared to the effect, so it is preferable to keep the temperature below 1350 ° C. A more preferable temperature for soaking is 1270 ° C. or higher and 1325 ° C. or lower.

  In addition, the soaking time varies depending on the soaking temperature, but if it is less than 2 hours, the time required for dissolution of coarse Al, Nb, Ti-containing precipitates cannot be secured, and the remaining is unavoidable and satisfactory. The effect of suppressing grain coarsening cannot be obtained. In order to proceed the dissolution of the precipitate by soaking more reliably, the soaking time should be 2.5 hours or longer, more preferably 3 hours or longer. The upper limit of the soaking time is not particularly specified, but when soaking is performed in the above temperature range, the effect is saturated in about 15 hours, more certainly in about 24 hours. Within hours, more preferably about 15 hours or less.

  Other production conditions are not particularly limited, but in order to reduce the size of the Al, Nb, Ti-containing precipitates as much as possible and finish the dissolution in a short soaking time, it is desirable to increase the cooling rate after melting as much as possible. .

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

Example 1
Steel materials having chemical compositions shown in Tables 1 and 2 are melted in a small melting furnace and cast to obtain a 150 kg ingot. The ingot is soaked under the conditions shown in Tables 1 and 2 and then cooled to 500 ° C. or lower by air cooling. Then, after reheating to 1000 ° C., hot rolling was performed at a finishing temperature of 900 ° C. to obtain a steel bar having a diameter of 50 mm.

For each of the obtained steel bars, the number density of the Al / Nb / Ti-containing precipitates having an equivalent circle diameter of 100 nm or more containing Al, Nb, and Ti in the steel material was measured by the following method, and extracted from the D / 4 position of the rolled steel bar. This was taken using a transmission electron microscope (trade name “H-800”) manufactured by Hitachi, Ltd., and photographic images of 5 fields of view (about 300 μm ² ) were taken at a magnification of 20,000, and loss photo images were made by Micromedia. The area of the precipitate was measured by image analysis using Image Pro and converted to an equivalent circle diameter, and the number density of precipitates having an equivalent circle diameter of 100 μm or more was measured.

  On the other hand, the determination of whether or not the precipitate contains Al, Nb, Ti as the quantitative standard of the component is performed by quantitative analysis of the component with EDX (energy dispersive X-ray analyzer), and with the precipitate having an Al + Nb + Ti amount of 50% or more. Al / Nb / Ti-containing precipitates were used.

  Moreover, about each test bar steel, the test piece for a crystal grain coarsening inhibitory effect confirmation effect was produced by cutting. As shown in FIG. 9, the test piece was in the form of a cylinder having a diameter of 8 mm and a height of 12 mm. Next, each test piece was hot compressed using a hot working simulator (trade name “THERMECMASTER-Z” manufactured by Fuji Electric Koki Co., Ltd.). The heating rate was 10 ° C./min, the hot heating temperature was 1250 ° C., the compression rate was 70%, and the cooling rate was 1 ° C./min. This test piece was annealed in a vacuum annealing furnace at 1100 ° C. for 3 hours and then cooled with water, and then the state of crystal grain coarsening in the cross section of the test piece was examined.

  As for the grain coarsening situation, 10 fields of view were observed at a magnification of 100 times using an optical microscope, and the grain size was larger than the grain size corresponding to the grain size number 5 defined in the JIS standard. Area (coarse grain area), the part that is smaller than the crystal grain size corresponding to the crystal grain size number 5 (grain size control part), the ratio of the coarse grain area to the visual field area (coarse grain ratio) and the average The crystal grain size number [= {the crystal grain size number of the sized part × the area ratio (%) of the sized part + the crystal grain size of the coarse part × coarse ratio (%)} / 100)] was determined.

  The coarse grain ratio is evaluated in three stages: 0% is very good (）), more than 0 is less than 5% is good (◯), and 5% or more is coarsened (×). Regarding the diameter, No. 9 or more was considered very good (（), Nos. 7 to 9 were good (（), and less than No. 7 were bad (x). From these results, it was judged that a crystal grain coarsening preventing property at a high temperature was good when both the coarse grain ratio and the average crystal grain size were good or very good at a heating temperature of 1100 ° C.

  In addition, for each test bar steel, a test piece having a diameter of 32 mm × 48 mm was prepared by processing the test piece, 70% forging was added at 1250 ° C., and then held at 1100 ° C. for 3 hours to simulate carburizing treatment, and then quenched. Temperature: Oil-cooled at 930 ° C, and then tempered at 170 ° C. The impact test piece prescribed | regulated to JISZ2242 was cut out from the center of this test piece, the impact test was done, and the core part hardness was investigated based on JISZ2244. The results are shown in Tables 3 and 4.

  From Tables 1 to 4, the following can be considered.

  No. No. 1 has a low core hardness due to insufficient C content in the steel. No. 5 has too low C content in the steel, so the core impact value is low and does not meet the object of the present invention.

  No. Nos. 6 and 9 have no Si. Nos. 10 and 13 show that the Mn content in the steel is outside the specified range. No. 15 has too much S content in the steel. Since No. 18 has too much Cr content in steel, the core impact value is low.

  No. Nos. 26 to 33 have a content of Al, Nb, Ti, N in the steel that is outside the specified range, so that the effect of preventing grain coarsening is poor. In 27, 29, 31, and 33, the core impact value is poor.

  No. No. 44 has an appropriate steel component composition, but the soaking temperature at the time of production is low and the soaking time is short. 45 steel components are also appropriate, but because the soaking time is extremely short, the coarse precipitates are not sufficiently dissolved, and the number density of Al, Nb, Ti-containing precipitates with an equivalent circle diameter of 100 nm or more is outside the specified range. As a result, a satisfactory crystal grain coarsening suppression effect cannot be obtained.

  On the other hand, those other than the above satisfy the specified requirements of the present invention, so they exhibit excellent crystal grain coarsening prevention action and have excellent performance in all core hardness and core impact characteristics. ing.

  In the above examples, the test results of the effect of preventing grain coarsening and impact properties were shown by a method simulating hot forging. However, the present invention is not limited to the application to hot forging as well as the present invention. The same effect can be obtained even when applied to inter-forging.

It is an electron micrograph of the Al nitride-Nb carbonitride composite precipitate observed in the test steel bar. It is a chart which shows the characteristic X intensity | strength of the position of the arrows 1, 2, 3, 4 attached | subjected to the photograph of FIG. It is an electron micrograph of Al nitride observed in a test steel bar. It is a chart which shows the characteristic X intensity | strength of the position of the arrow 5 attached | subjected to the photograph of FIG. It is an electron micrograph of the Nb-Ti composite carbonitride observed in a test bar steel. It is a chart which shows the characteristic X intensity | strength of the position of the arrow 6 attached | subjected to the photograph of FIG. It is an electron micrograph of Nb carbonitride observed in a test bar steel. It is a chart which shows the characteristic X intensity | strength of the position of the arrow 7 attached | subjected to the photograph of FIG. It is explanatory drawing which shows the test piece for the crystal grain coarsening prevention effect confirmation used in experiment.

Claims (7)

% By mass
C: 0.05 to 0.25%,
Si: 0.05-2.0%,
Mn: 0.01 to 1.5%,
S: 0.005 to 0.2%,
Cr: 0.4 to 1.5%,
N: 0.0085 to 0.0219%,
Al: 0.058 to 0.062%,
Nb: 0.038 to 0.100%,
Ti: 0.008 to 0.012%,
And the balance is made of steel consisting of Fe and inevitable impurities, and all carbides, nitrides and carbonitrides having an equivalent circle diameter of 100 nm or more containing at least one element selected from Al, Nb, and Ti in the steel, Two or more of them are attached or the number of combined precipitates is 0.5 × 10 ¹² pieces / m ² or less, and it is excellent for preventing grain coarsening during high-temperature carburizing and rolling for skin baking. Steel bar.   The skin according to claim 1, wherein the steel further contains Cu: 1.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%) as other elements. Rolled steel bar for baking.   The steel bar according to claim 1 or 2, wherein the steel further contains Mo: 1.0% or less (not including 0%) as another element.   The steel bar according to any one of claims 1 to 3, wherein the steel further contains B: 0.0005 to 0.0030% as another element.   The steel further contains, as another element, Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%). Rolled steel bar for skin hardening according to crab.   Still other elements of steel are Mg: 0.0001-0.02%, Ca: 0.0001-0.02%, Te: 0.0005-0.02%, REM: 0.0005-0. The rolled steel bar for case hardening according to any one of claims 1 to 5, comprising at least one selected from the group consisting of 02%.   The steel further contains, as other elements, Zr: 0.2% or less (not including 0%) and / or V: 0.5% or less (not including 0%). Rolled steel bar for skin hardening according to crab.