JP2007291497A - Steel material for high temperature carburizing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for high temperature carburizing in which the generation of grain coarsening can be suppressed even when carburizing is performed at a high temperature of >1,000°C with normalizing treatment before the carburizing treatment obviated. <P>SOLUTION: The steel material for high temperature carburizing contains, by mass, 0.10 to 0.30% C, 0.030 to 0.060% Nb, 0.0010 to 0.0030% Ti, 0.005 to 0.015% V, ≤0.060% Al and 0.0185 to 0.0300% N, wherein for carbonitrides and nitrides before carburizing treatment, the total amount of compound carbonitrides [NbTi(CN)], [NbV(CN)] and [NbTi(CN)] to be precipitated satisfies ≥0.010%, also, the amount of AlN to be precipitated satisfies ≤0.010%, and further, the number of the compound carbonitrides with a diameter of >20 to 80 nm is ≥300 pieces/1,000 μm<SP>2</SP>in total. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel material for high-temperature carburizing. Specifically, it is used as a material for machine structural parts, and even if the normalizing process before the carburizing process is omitted, when the parts are carburized at a high temperature exceeding 1000 ° C., no abnormal particles are produced, The present invention relates to a steel material for high-temperature carburization that can suppress the occurrence of crystallization.

  Hereinafter, in the present specification, when observed with an optical microscope at a magnification of 100 times, a state in which crystal grains having the largest frequency in the visual field occupy an area of 20% or more from the crystal grains having the largest frequency in the visual field occupy an area of 20% or more. As “mixed grains”, each crystal grain having a grain size number 3 or larger is referred to as “abnormal grain”, and a portion where no “abnormal grain” is present is referred to as “sized particle part”.

  In this specification, carburization or the occurrence of mixed grains in a heating and holding experiment simulating carburization is simply referred to as “coarse graining” or “coarse graining”, and the lowest level of coarse graining occurs. The temperature is referred to as “roughening temperature”.

  Conventionally, case hardening steel has been carburized in a carburizing gas atmosphere at around 930 ° C. Since AlN is relatively stable at the carburizing temperature of about 930 ° C., conventional case-hardened steel contains appropriate amounts of Al and N, and this Al and N are precipitated as AlN before the carburizing treatment, and austenite. By using it as a pinning particle, it has been attempted to suppress the occurrence of coarsening.

  On the other hand, in order to shorten the treatment time, recently, carburizing treatment at a high temperature exceeding 1000 ° C. has been studied. This is because, for example, if carburizing at 1050 ° C., the processing time can be shortened to 1/3 to 1/4 compared with the case of carburizing at 930 ° C.

  However, when carburizing at such a high temperature, a part of AlN is dissolved in the matrix (substrate) and the pinning effect is reduced, so that it is difficult to avoid coarsening. If coarsening occurs, the hardenability of the part increases, so that the heat treatment strain of the part increases, and further, the mechanical properties deteriorate.

  For this reason, even when carburizing at a high temperature exceeding 1000 ° C., there is a great demand for high-temperature carburizing steel that can suppress the occurrence of coarsening.

  Further, when carburizing at high temperature, a normalization process is performed off-line after hot forging, and countermeasures for preventing coarsening are taken by uniformly dispersing a large amount of pinning particles such as AlN. However, in recent years, from the viewpoint of cost reduction, there is an increasing demand for shortening the carburizing time and omitting the normalizing process before the carburizing process.

  Therefore, in order to meet the above-described demands, Patent Documents 1 to 3 propose various high-temperature carburizing steels using the pinning action of fine AlN and Nb (CN).

  Specifically, Patent Document 1 includes, in mass%, Nb: 0.001 to 0.10%, Al: 0.01 to 0.15%, and N: 0.01 to 0.03%, Optionally containing one or more selected from the group consisting of V, Ti, Ta, Zr, Te and rare earth elements and / or one or more selected from the group consisting of Pb, Bi and Se, N Has disclosed a “high temperature carburizing steel” that satisfies a specific formula and has a solid solution Al content of 0.01 to 0.10% after hot working represented by the specific formula .

  In Patent Document 2, in mass%, C: 0.1 to 0.4%, Si: 0.02 to 1.3%, Mn: 0.3 to 1.8%, S: 0.001 to 0. 15%, Al: 0.015 to 0.045%, Nb: 0.005 to 0.05%, N: 0.01 to 0.02%, and a specific amount of Cr, Mo, Ni and Containing one or more of V, limiting the content of P, Ti and O to a specific amount or less, and the amount of precipitation of NbC, NbN and Nb (CN) after hot forging 0.005% As described above, the precipitation amount of AlN is limited to 0.01% or less, and in addition, the bainite fraction, the pearlite fraction, and the ferrite grain size number in the structure after hot forging are also specified in a specific range. A "high-temperature carburized component shaped material excellent in crystal grain coarsening prevention characteristics that does not require a normalizing treatment after forging" has been disclosed.

  In Patent Document 3, in mass%, C: 0.1-0.40%, Si: 0.02-1.3%, Mn: 0.3-1.8%, S: 0.001-0. 15%, Al: 0.015 to 0.04%, Nb: 0.005 to 0.04%, N: 0.006 to 0.020%, and a specific amount of Cr, Mo, Ni and 1 type or 2 types or more of V are contained, and the content of P, Ti and O is limited to a specific amount or less, and the precipitation amount of Nb (CN) after hot rolling is 0.005% or more, of AlN The amount of precipitation is limited to 0.005% or less, or in addition to the above content, the number of Nb (CN) having a diameter of 0.1 μm or less in the steel matrix after hot rolling, after hot rolling Of bainite and the ferrite grain size number after hot rolling in a specific range. Preventing coarse grains properties superior hardening steel and its manufacturing method, and carburized component formed and fabricated material when "is disclosed.

Japanese Patent Laid-Open No. 2001-20038 JP 2001-303174 A WO99 / 05333

  The technique disclosed in Patent Document 1 described above contains specific amounts of Nb, Al, and N for the purpose of suppressing the occurrence of coarsening as much as possible during carburizing and quenching after warm forging and cold forging. Furthermore, after hot working, Al is not precipitated as AlN, but is contained in an appropriate amount in the form of solid solution Al, and fine AlN, NbC, NbN, and Nb (CN) are precipitated during heat treatment such as spheroidizing annealing. It is something to be made. In other words, a proper amount of Nb, Al and N is mixed and fine AlN, NbC, NbN and Nb (CN) are preliminarily precipitated by heat treatment before carburizing, and the pinning action of the precipitate is used. To do. However, a sufficient effect for suppressing the occurrence of coarsening during high-temperature carburization is not always obtained.

  The technology disclosed in Patent Document 2 defines the contents of Nb, Al, N, Ti and O for the purpose of preventing the occurrence of coarsening during high-temperature carburizing of parts that omit the normalizing process after hot forging. Then, after hot forging, Nb (CN) is finely precipitated more than a certain amount while Al is precipitated as AlN as much as possible, and AlN and Nb (CN) are newly finely precipitated during carburizing heating. Fine AlN and Nb (CN) are used as the pinning particles. However, as in the case of Patent Document 1, a sufficient effect is not necessarily obtained for suppressing the occurrence of coarsening during high-temperature carburization.

  The technique disclosed in Patent Document 3 specifies the contents of Nb, Al, N, Ti and O for the purpose of preventing the occurrence of coarsening during high-temperature carburization, and after hot rolling or hot forging, Nb (CN) is finely precipitated more than a certain amount while Al is precipitated as AlN as much as possible, and AlN is newly finely precipitated in the subsequent annealing process, normalizing process, or carburizing temperature rising process, Fine AlN and Nb (CN) are used as pinning particles. However, as in the case of Patent Document 1 and Patent Document 2, a sufficient effect is not necessarily obtained for suppressing the occurrence of coarsening during high-temperature carburization.

  Therefore, an object of the present invention is a material for machine structural parts, and even if the normalizing process before carburizing treatment is omitted, the occurrence of coarsening when the parts are carburized at a high temperature exceeding 1000 ° C. It is to provide a steel material for high-temperature carburizing that can be reliably suppressed.

  In order to solve the above-described problems, the present inventors made various studies, and as a result, obtained the following findings (a) to (i).

  (A) The coarsening caused by heating and holding at a high temperature has a driving force for crystal grain growth larger than the pinning force at the grain boundary due to precipitated particles such as AlN and Nb (CN) dispersed in the steel. This happens when The pinning force due to the precipitated particles depends on the number of the precipitated particles, and increases as the number of the precipitated particles increases. On the other hand, the driving force for crystal grain growth depends on the crystal grain size, and increases as the crystal grain size decreases.

  (B) When performing high-temperature carburization that is heated and held at a high temperature for a long time, as in Patent Document 1, if precipitating particles such as AlN and Nb (CN) are dispersed in the steel in advance, AlN is used at the initial stage of carburization. Although the crystal grains are finely maintained by the pinning force of Nb (CN) or the Nb (CN), when the holding time becomes long, the solid solution and aggregation of the precipitates progress and the pinning force decreases. As a result, the crystal grains The driving force for growth becomes relatively large with respect to the pinning force, and coarsening proceeds. In other words, coarsening cannot be prevented only by AlN or Nb (CN).

  (C) As in Patent Document 2 and Patent Document 3, when precipitates are finely precipitated during the carburizing heating, crystal grains in the initial stage of carburizing become fine, and the driving force for crystal grain growth increases. For this reason, sufficient effect is not acquired with respect to suppression of the coarsening generation | occurrence | production at the time of high-temperature carburizing.

  (D) In order to prevent coarsening during high-temperature carburizing, in order to secure a pinning force that exceeds the driving force for crystal grain growth during carburizing, it is more solid solution in the matrix than AlN or Nb (CN). It is necessary to deposit precipitates that are difficult to do.

  (E) Precipitates that are harder to dissolve in the matrix than the above-mentioned AlN and Nb (CN) are one or more of Ti and V and a composite carbonitride of Nb [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)] should be used, and such a composite carbonitride of Ti, V and Nb is composed of a very small amount of Ti and V together with a specific amount of Nb, C and N. It can be made to contain.

  (F) In order to prevent coarsening during high-temperature carburizing, it is important to deposit a specific amount or more in the stage before carburizing the Nb, Ti and V composite carbonitride described in (e) above. It is.

  (G) AlN tends to grow relatively coarsely. For this reason, as described in the above (f), even when a specific amount or more of Ti or V and Nb composite carbonitride is precipitated in the stage before carburizing treatment, the above Ti or V is used with coarse AlN as a nucleus. If the Nb composite carbonitride is agglomerated and coarsened, prevention of coarsening cannot be achieved. Therefore, in order to prevent coarsening during high-temperature carburizing, it is necessary to limit the amount of AlN deposited at the stage before carburizing.

  (H) [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are the composite carbonitrides of Ti, V, and Nb described above, [1] 20 nm or less [2] Over 20 nm and below 80 nm and [3] Over 80 nm. And among the above three categories, the diameter of [2] exceeds 20 nm and 80 nm or less contributes to prevention of coarsening during high-temperature carburizing. That is, when the diameter of [3] exceeds 80 nm, the pinning action is small because it is coarsened. The composite carbonitride of Nb and Ti or V is less soluble in the matrix than AlN or Nb (CN), but the [1] diameter is 20 nm or less during the carburizing process. There is a tendency to grow and coarsen those in which the diameter of [3] exceeds 80 nm after being substantially dissolved in the matrix. Therefore, neither contributes much to preventing coarsening during high temperature carburization.

  (I) Even when Ti or V and Nb composite carbonitride having a diameter of more than 20 nm and not more than 80 nm as described in (h) is precipitated, The effect of preventing crystallization cannot be obtained. Therefore, in order to prevent coarsening during high-temperature carburizing, in addition to precipitating more than a specific amount of the composite carbonitride of Ti and V and Nb in (f) above before carburizing treatment, It is necessary to deposit a large number of composite carbonitrides of Ti, V and Nb having a diameter exceeding 20 nm and not more than 80 nm.

  This invention is completed based on said knowledge, The summary exists in the steel material for high temperature carburizing shown to following (1) and (2).

(1) By mass%, C: 0.10 to 0.30%, Nb: 0.030 to 0.060%, Ti: 0.0010 to 0.0030%, V: 0.005 to 0.015% , Al: 0.060% or less and N: 0.0185-0.0300%, carbonitride and nitride in the steel before carburizing treatment, one or more of Ti and V and Nb The total amount of precipitation of the composite carbonitride satisfies 0.010% or more in terms of mass%, and the amount of precipitation of AlN satisfies 0.010% or less in terms of mass%, and among Ti and V exceeding 20 nm in diameter and 80 nm or less A steel material for high-temperature carburizing, characterized in that the total number of composite carbonitrides of one or more of Nb and Nb is 300 pieces / 1000 μm ² or more.
However, one or more of Ti and V and Nb composite carbonitride refers to [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)].

(2) By mass%, C: 0.10 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.10 to 2.00%, P: 0.025% or less, S: 0.10% or less, Cr: 0.5-2.5%, Nb: 0.030-0.060%, Ti: 0.0010-0.0030%, V: 0.005-0.015%, Al: 0.060% or less and N: 0.0185-0.0300%, Ni: 0.1-3.0% and Mo: 0.02-1.5%, one or two Containing seeds, the balance consisting of Fe and impurities, and for the carbonitrides and nitrides in the steel before carburizing treatment, the total precipitation amount of one or more of Ti and V and the composite carbonitride of Nb In addition to satisfying 0.010% or more by mass%, and the precipitation amount of AlN satisfying 0.010% or less by mass%, the diameter exceeds 20 nm. The number of composite carbonitride of one or more and Nb of Te 80nm or less of Ti and V is a total of 300/1000 .mu.m ² or more at a high temperature carburizing steel material, characterized in that.
However, one or more of Ti and V and Nb composite carbonitride refers to [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)].

  The diameter of the precipitate refers to the average value of the major axis and the minor axis, that is, “(major axis + minor axis) / 2”.

  Hereinafter, the inventions related to the steel materials for high-temperature carburization of the above (1) and (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  If the steel material for high-temperature carburizing of the present invention is used as a material for machine structural parts, the occurrence of coarsening is reliably suppressed when carburizing at a high temperature exceeding 1000 ° C even if the normalizing treatment before carburizing is omitted. As a result, the manufacturing cost can be rationalized by shortening the carburizing time.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition C: 0.10 to 0.30%
C forms, together with N, [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are composite carbonitrides of Ti, V and Nb, and is coarsened during high-temperature carburizing. Has the effect of suppressing C also has the effect of increasing the strength of the base material (dough) after carburizing and quenching. In order to obtain such an effect, a C content of 0.10% or more is necessary. However, if C is contained excessively, the machinability is lowered. In particular, when the C content exceeds 0.30%, the machinability is significantly deteriorated. Therefore, the content of C is set to 0.10 to 0.30%. A desirable range for the C content is 0.15 to 0.25%.

Nb: 0.030 to 0.060%
Nb forms [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)] together with C and N, and also Ti and V, and suppresses coarsening during high-temperature carburizing. Have However, when the content of Nb is less than 0.030%, the amount and number of composite carbonitrides are reduced. In particular, in carburizing at a high temperature exceeding 1000 ° C., the pinning force is reduced and the coarse particles It is not possible to obtain the effect of suppressing the crystallization. On the other hand, even if the content exceeds 0.060%, the effect of suppressing the coarsening is saturated and the machinability is deteriorated, so that the cost is increased and the economic efficiency is only impaired. Therefore, the Nb content is set to 0.030 to 0.060%. Note that the Nb content is preferably 0.035 to 0.050%.

Ti: 0.0010 to 0.0030%
Ti forms [NbTi (CN)] and [NbTiV (CN)], which are very stable precipitates even at high temperatures, and has the effect of suppressing coarsening during high-temperature carburization. However, the effect is poor when the Ti content is less than 0.0010%. On the other hand, when the Ti content exceeds 0.0030%, coarse Ti nitride is produced, and this is used as a nucleus to produce composite carbonitrides of Ti, V and Nb [NbTi (CN)] and [NbTiV (CN )] Is coarsely precipitated, so that a sufficient coarsening suppression effect cannot be ensured. Therefore, the Ti content is set to 0.0010 to 0.0030%.

V: 0.005 to 0.015%
V forms precipitates [NbV (CN)] and [NbTiV (CN)] that are very stable even at high temperatures, and has the effect of suppressing coarsening during high-temperature carburization. However, when the V content is less than 0.005%, the effect is poor, and a sufficient coarsening suppression effect cannot be ensured. On the other hand, when the content of V exceeds 0.015%, coarse V nitride is produced, and this is used as a nucleus, and is a composite carbonitride of Ti, V and Nb [NbV (CN)] and [NbTiV ( CN)] precipitates coarsely, so that a sufficient coarsening suppression effect cannot be ensured. Therefore, the content of V is set to 0.005 to 0.015%.

Al: 0.060% or less Al has a deoxidizing action of steel. However, when the content exceeds 0.060%, coarse AlN is generated, and the composite carbonitride of Ti, V, and Nb aggregates and coarsens with this coarse AlN as the nucleus, preventing coarsening. The effect is reduced. Therefore, the Al content is set to 0.060% or less. From the viewpoint of preventing the formation of coarse AlN, the Al content is preferably 0.050% or less. In order to surely obtain the deoxidizing action of steel, the Al content is preferably 0.010% or more.

N: 0.0185-0.0300%
N forms, together with C, [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are composite carbonitrides of Ti, V and Nb, and is coarsened during high-temperature carburizing. Has the effect of suppressing However, if the content is less than 0.0185%, in the case of carburizing at a high temperature exceeding 1000 ° C., the precipitation amount of the composite carbonitrides of Ti, V, and Nb becomes insufficient, thereby suppressing coarsening. The effect is not obtained. On the other hand, when the N content exceeds 0.0300%, coarse AlN is generated, and the composite carbonitride of Ti, V, and Nb aggregates and coarsens with this coarse AlN as the core, so that coarsening The suppression effect is reduced. Therefore, the N content is set to 0.0185 to 0.0300%. The N content is preferably 0.0185 to 0.0250%.

  For the above reason, the chemical composition of the steel for high temperature carburization according to the present invention (1) includes C, Nb, Ti, V, Al, and N in the above-described range.

  The steel material for high-temperature carburizing according to the present invention only needs to contain C, Nb, Ti, V, Al, and N in the above-described range, but C, Nb, Ti, V, Al, and In addition to N, together with Si, Mn, P, S and Cr in the following ranges, one or two of Ni and Mo are included, and the balance is preferably a steel material having a chemical composition composed of Fe and impurities. .

Si: 0.01 to 2.00%
Si is an element effective for deoxidizing steel, and is an element effective for imparting strength and hardenability. In order to acquire such an effect, it is preferable to contain Si 0.01% or more. However, if the Si content exceeds 2.00%, the machinability may deteriorate due to an increase in hardness. Therefore, the Si content is preferably 0.01 to 2.00%.

  Si, which has the effect of improving the rolling fatigue life after carburizing and quenching, is an element that promotes internal oxidation during carburizing. For this reason, the site that is internally oxidized may become the starting point of fatigue cracks. is there. Therefore, when using it in the heat-treated skin as carburized, it is more preferable that the Si content is 0.01 to 0.50%. On the other hand, when the surface layer is removed by machining after carburizing, when it is desired to increase the strength, the Si content is more preferably set to 0.50 to 2.00%.

  In addition, even if it contains Si, the effect of suppressing coarsening by [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are composite carbonitrides of Ti, V and Nb There is no effect.

Mn: 0.10 to 2.00%
Mn is an element that improves the hardenability of steel and has the effect of increasing the hardness of the parts after carburizing and quenching. In order to exert this effect, it is preferable to contain at least 0.10% of Mn. However, if it is excessively contained, the hardness before carburizing is increased and machinability is lowered. In particular, when the Mn content exceeds 2.00%, the increase in hardness before carburizing is increased. Since the machinability may be significantly lowered, the upper limit of the Mn content is preferably 2.00%. A more desirable range of the Mn content is 0.30 to 1.20%.

P: 0.025% or less P causes the steel to become brittle, and in particular, if its content exceeds 0.025%, the carburized hardened layer may become brittle, leading to a decrease in fatigue strength and the like. The content is preferably 0.025% or less. The P content is more preferably 0.015% or less.

S: 0.10% or less S, if its content exceeds 0.10%, the carburized hardened layer may become brittle and decrease fatigue strength, etc., so the S content is 0.10% or less. It is preferable that Note that S has an effect of improving machinability, and this effect is exhibited by containing 0.02% or more of S. Therefore, 0.02% or more of S is contained under the upper limit. May be.

Cr: 0.5 to 2.5%
Cr, like Mn, is an element that improves the hardenability of steel and has the effect of increasing the hardness of parts after carburizing and quenching. In order to exert this effect, it is preferable to contain at least 0.5% Cr. However, if the content exceeds 2.5%, the machinability may deteriorate, so the upper limit of the Cr content is preferably 2.5%. In addition, the more desirable range of Cr content is 0.7 to 2.0%.

One or two of Ni: 0.1 to 3.0% and Mo: 0.02 to 1.5% Ni and Mo are both effective elements for improving the hardenability. For this reason, it is preferable to contain 1 type or 2 types in 0.1% or more of Ni and 0.02% or more of Mo according to the magnitude | size of a component, a shape, and the hardening method after carburizing. However, if these elements are contained excessively, the cost will increase significantly. Therefore, when the above elements are contained, the upper limit is 3.0% for Ni and 1.5% for Mo. It is preferable that

  For the above reason, the chemical composition of the steel for high temperature carburizing according to the present invention (2) includes C, Si, Mn, P, S, Cr, Nb, Ti, V, Al, and N in the above-described ranges, One or two of Ni and Mo were contained, and the balance was made of Fe and impurities.

(B) Carbonitride and nitride in steel before carburizing treatment (B-1) One or more of Ti and V and Nb composite carbonitride and AlN
In order to prevent coarsening during high-temperature carburizing, it is necessary to finely disperse precipitates in order to ensure a pinning force that exceeds the driving force for crystal grain growth during carburizing.

  However, with AlN and Nb (CN), which have been conventionally used to suppress crystal grain growth during carburizing, solid solution and aggregation progress when the holding time at a high temperature exceeding 1000 ° C. is increased. As a result, the pinning force decreases, and as a result, the driving force for crystal grain growth becomes relatively large with respect to the pinning force, and the coarsening proceeds.

  Therefore, it is a composite carbonitride of Nb and one or more of Ti and V and NbTi (CN)], [NbV (CN)], and [NbV (CN)] that are less soluble in the matrix than AlN and Nb (CN). NbTiV (CN)] needs to be precipitated in the steel material at a stage before carburizing treatment so that the total precipitation amount is 0.010% or more, particularly in mass%.

  In addition, if AlN is precipitated before carburizing treatment, AlN tends to grow relatively coarsely during high-temperature carburizing, and a large amount of AlN is deposited, particularly, exceeding 0.010% by mass%. Then, with the coarse AlN as a nucleus, the Nb, Ti and V composite carbonitrides tend to aggregate and coarsen. And when the said composite carbonitride of Nb, Ti, and V coarsens, it does not contribute so much to the coarsening prevention at the time of high temperature carburization so that it may mention later. Therefore, the precipitation amount of AlN in the steel material before the carburizing treatment needs to be 0.010% or less in mass%.

  For the above reasons, in the present invention, the total amount of precipitation of one or more of Ti and V and the composite carbonitride of Nb with respect to carbonitrides and nitrides in the steel material before carburizing treatment is mass%. Therefore, it was decided that 0.010% or more and the precipitation amount of AlN should satisfy 0.010% or less in terms of mass%.

(B-2) Size and number of one or more of Ti and V and Nb composite carbonitride [NbTi (CN)], which is one or more of Ti and V and Nb composite carbonitride. [NbV (CN)] and [NbTiV (CN)] are cases where the total amount of precipitation as described in the above section (B-1) is mass% and is 0.010% or more. However, when the size and number of the particles are out of a specific range, it does not contribute much to prevention of coarsening during high-temperature carburization.

That is, [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are the composite carbonitrides of Nb, Ti and V, are more solid in the matrix than AlN and Nb (CN). Although it is difficult to melt, paying attention to its diameter,
[1] 20 nm or less,
[2] Over 20 nm and 80 nm or less,
[3] Over 80 nm,
If the diameter of [3] exceeds 80 nm, the pinning action is small because it is coarsened. In addition, those having a diameter of [1] of 20 nm or less tend to dissolve substantially in the matrix at the time of carburizing, and then grow and coarsen those having a diameter of [3] exceeding 80 nm.

  Therefore, both [1] and [3] do not contribute much to the prevention of coarsening during high-temperature carburization.

  For this reason, [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are complex carbonitrides of Nb, Ti and V, are dissolved in the matrix even at high temperatures exceeding 1000 ° C. In addition, it is necessary to deposit one having a diameter of more than 20 nm and not more than 80 nm so as to exhibit a pinning effect.

In addition, even when Nb and Ti or V composite carbonitrides having a diameter of more than 20 nm and not more than 80 nm are precipitated, the number thereof is small, in particular, the total number is 300 / When it is less than 1000 μm ² , the effect of preventing coarsening cannot be obtained.

For the above reason, in the present invention, the total number of one or more of Ti and V exceeding 20 nm in diameter and 80 nm or less and the composite carbonitride of Nb is 300/1000 μm ² or more. did.

  As described above, the diameter of the precipitate refers to the average value of the major axis and the minor axis, that is, “(major axis + minor axis) / 2”.

  The steel material for high temperature carburizing according to the present invention is preferably manufactured by the following method, for example.

First, steel ingots and slabs
-Heating temperature: 1270 ° C or higher,
・ Heating time: 30 minutes or more
After heating under the above conditions, it is rolled into pieces and finished into steel pieces.

Next, the steel piece
-Temperature rising rate: 15 ° C / min or more,
-Heating temperature: 950 ° C or higher,
-Forging finishing temperature: 950 ° C or higher,
Hot forging into the part shape under the conditions of
-Average cooling rate from forging finish temperature to 700 ° C: 60-500 ° C / min,
-Average cooling rate of 700-550 ° C: 20-60 ° C / min,
Cool under the conditions of

In addition, after carrying out the partial rolling as described above,
-Heating temperature: 1200 ° C or higher,
・ Heating time: 30 minutes or more
After heating under the above conditions, it is hot-rolled and finished into a steel bar or wire, and then this steel bar or wire is manufactured by hot forging and cooling in the same manner as described above. Also good.

  For example, a desired carburized part can be manufactured by subjecting the steel material for high-temperature carburizing obtained as described above to cutting into a part shape and then performing a high-temperature carburizing treatment.

  In the case of producing as described above, the most important thing for obtaining the carbonitride and nitride of the steel material for high-temperature carburizing of the present invention is the partial rolling conditions and the heating rate during hot forging. And the cooling rate.

When performing the ingot rolling, it is preferable that the heating temperature is 1270 ° C. or higher and the heating time is 30 minutes or more. If the temperature or time is lower, the large carbonitriding that exists at the stage of steel ingot or slab Of the composite carbonitride of Nb and one or more of Ti and V having a diameter of more than 20 nm and not more than 80 nm. This is because the total number decreases to less than 300/1000 μm ² and coarsening occurs.

  The steel material for high-temperature carburizing of the present invention is a state before one or more of Ti and V and a composite carbonitride of Nb is precipitated in a small amount after cooling by hot rolling or in the state before hot forging. Then, this minute amount of complex carbonitride is present in the steel material. Moreover, when the temperature rising rate at the time of hot forging is slow, one or more of Ti and V and Nb composite carbonitride precipitate in a slight amount during the temperature rising process.

  If the heating temperature or forging finish temperature is less than 950 ° C., a non-uniform structure is generated during hot forging. Therefore, the steel for high-temperature carburizing of the present invention is heated to 950 ° C. or more and hot forged, and the forging finish temperature is also high. It is desirable to manufacture at 950 ° C. or higher.

And if the temperature rising rate at the time of hot forging is slow, the composite carbonitride that existed before hot forging although it is a small amount, and the composite carbonitride that precipitates in a small amount during the temperature rising process grows, and is about 950 ° C. Even if kept in the forging temperature range, it cannot be sufficiently dissolved in the matrix. Therefore, the number of composite carbonitrides that are newly finely precipitated in the cooling process after hot forging decreases, and as a result, in the steel material before carburizing treatment, one of Ti and V having a diameter of more than 20 nm and not more than 80 nm. This is because it becomes difficult to precipitate the total number of the composite carbonitrides of seeds or more and Nb of 300 pieces / 1000 μm ² or more, resulting in coarsening. Therefore, the temperature increase rate during hot forging is preferably 15 ° C./min or more.

The average cooling rate of 700 ° C. from the finishing temperature of hot forging is preferably 60 to 500 ° C./min. If it is less than 60 ° C./min, decarburization tends to proceed, and subsequent cooling at 700 to 550 ° C. The total amount of precipitation of [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which is a composite carbonitride of Nb and one or more of Ti and V, in the surface layer part in the process This is because the mass% is less than 0.010%. On the other hand, in the case of exceeding 500 ° C./min, the overcooled austenite is present in the subsequent cooling process together with ferrite and pearlite transformed from austenite, and thus the composite coal that precipitates in the cooling process at 700 to 550 ° C. In the steel material, the dispersion state of the nitride becomes uneven, and the total number of composite carbonitrides of one or more of Ti and V exceeding 20 nm in diameter and 80 nm or less and Nb in the steel material is 300. This is because it is difficult to set the number of particles / 1000 μm ² or more, and the total amount of the composite carbonitride deposited is less than 0.010% by mass.

  Furthermore, the average cooling rate at 700 to 550 ° C. following the above cooling is preferably 20 to 60 ° C./min. If it is less than 20 ° C./min, the precipitation amount of AlN in the steel before carburizing is On the other hand, in the case of exceeding 60 ° C./min, one or more of Ti and V in the steel material before carburizing treatment and Nb composite carbonitride. This is because the total precipitation amount of certain [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)] is less than 0.010% in mass%.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
After the converter was melted, continuous casting was performed to produce steel slabs having the chemical composition shown in Table 1. This steel i is a steel whose chemical composition satisfies the conditions specified in the present invention.

  The above steel slabs were heated under various conditions shown in Table 2 and subjected to ingot rolling to obtain 180 mm × 180 mm steel slabs.

  Next, each steel slab was heated at a heating temperature of 1280 ° C. and a heating time of 40 minutes and then hot-rolled to produce a steel bar having a diameter of 35 mm.

  The steel bar having a diameter of 35 mm thus obtained was cut to a length of 52.5 mm and then heated to 1200 ° C., and under various conditions shown in Table 2, hot forging by upsetting and subsequent cooling were performed. An approximately 70 mm carburizing material was prepared.

  About each carburizing material having a diameter of about 70 mm prepared by placing it hot, 10 mm in diameter and 10 mm in length from its R / 2 part (where “R” represents the radius of the carburizing material). [NbTi (CN)], [NbV (CN)] and [NbTiV (CN), which are composite carbonitrides of Nb and one or more of Ti and V and Nb by electrolytic extraction separation analysis. )] And the precipitation amount of AlN.

  Here, the electrolytic extraction separation analysis method was performed under the following conditions <1> to <4>.

<1> “10% Acetylacetone-1% Tetramethylammonium Chloride-Methanol Solution”, which is a so-called “10% AA-methanol solution”, is used as the electrolytic solution, and constant potential (−100 mV vs SCE (saturated calomel electrode)) Alternatively, about 0.5 g of steel is electrolyzed with a constant current (20 mA / cm ² ).

  <2> After electrolysis, the electrolytic residue (the composite carbonitride and AlN aggregated in the electrolytic solution) with a pore size of 0.2 μm is suction filtered and collected.

  <3> The electrolytic residue recovered by the above <2> is decomposed with a mixed acid (sulfuric acid 1, phosphoric acid 1, water 1) and fixed with pure water, and then subjected to ICP (high frequency inductively coupled plasma) emission spectroscopy or Quantitative analysis of Nb, Ti, V and Al is performed by atomic absorption spectrometry.

  <4> For each element, the amount of electrolysis obtained in <3> above is converted into the content (mass%) in the steel material, and the total precipitation amount of the composite carbonitride is converted into the above Nb, Ti, and V conversions. The total content and the precipitation amount of AlN are represented by the above-mentioned converted content of Al.

  Moreover, about the R / 2 part of each carburizing raw material produced by setting up the said hot, it used for observation with a transmission electron microscope (TEM) by the extraction replica method.

  A TEM equipped with an energy dispersive X-ray detector (EDS) was used, and the composite form and shape of Nb, Ti and V contained in the composite carbonitride were confirmed from elemental analysis by EDS.

  The number of complex carbonitrides [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)] was determined as follows.

That is, with respect to those having a diameter of 80 nm or less, they were counted in 10 fields with a magnification of 100,000, and converted into the number per 1000 μm ² . At this time, those having a diameter of 20 nm or less were excluded from the objects of counting.

  Furthermore, from the R / 2 part of the carburizing material produced by setting up in the hot state, a test piece for cutting a cylindrical test piece having a diameter of 20 mm and investigating the graining temperature (hereinafter referred to as “roughening test”). It was used for. The coarsening test was performed by simulating a heat pattern of high-temperature carburization and holding the test piece at 960 to 1100 ° C. for 2 hours in an Ar atmosphere. In addition, the water cooling process was carried out after heat-holding.

  The vertical section of each test piece thus obtained was mirror-polished and corroded with a saturated aqueous picric acid solution to which a surfactant was added, and then observed with an optical microscope at a magnification of 100 times to measure the austenite grain size, The occurrence of mixed grains in which abnormal grains existed, the sized part size and the coarsening temperature were investigated.

  As already described, “mixed grain”, “abnormal grain”, “sized part” and “roughening temperature” in the present specification are defined as follows when observed with an optical microscope at a magnification of 100: It is based on. In addition, as described above, the occurrence of mixed grains is that “coarse grains occur” or “coarse grains”.

“Mixed grain”: a state in which a crystal grain having a size of 3 or more from the grain having the largest frequency in the field of view occupies an area of 20% or more,
“Abnormal Grain”: Each crystal grain having a grain size number larger than 3
“Sized part”: the part where no abnormal grain exists,
“Roughening temperature”: the lowest temperature at which mixed grains occur.

  Table 3 summarizes the above test results. In Table 3, the temperature corresponding to the austenite grain number indicated in italics is “coarsening temperature”, and the austenite grain size at the heating temperature exceeding the temperature was not measured. Indicated. The austenite particle size number expressed in italics indicates the particle size number in the sized portion when abnormal particles are formed and become mixed particles.

  From Table 3, in the case of Test No. 1, Test No. 2 and Test No. 4 that satisfy the conditions specified in the present invention, the coarsening temperature is all as high as 1060 ° C. or higher, and thus a high temperature exceeding 1000 ° C. It is clear that the occurrence of coarse graining is also suppressed when carburizing with.

  On the other hand, even when using steel A, which is a steel whose chemical composition satisfies the conditions specified in the present invention, carbonitrides and nitrides in steel before carburizing treatment, specifically, [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are complex carbonitrides of Nb and one or more of Ti and V, and AlN deviate from the conditions specified in the invention. In the case of Test No. 3 and Test Nos. 5 to 8, the coarsening temperature is 1000 ° C. at most and does not reach the target of the present invention.

That is, in the case of test number 3, the total number of one or more of Ti and V in the steel material before carburizing treatment exceeding 20 nm and 80 nm or less and the composite carbonitride of Nb is less than 300/1000 μm ^2. Therefore, the coarsening temperature was as low as 980 ° C.

  In the case of Test No. 5 and Test No. 8, the total precipitation amount of one or more of Ti and V and Nb composite carbonitride in the steel material before carburizing treatment is less than 0.010% by mass%. is there. For this reason, the coarsening temperature of Test No. 5 was 1000 ° C., and the coarsening temperature of Test No. 8 was 980 ° C.

In the case of test number 6, one or more of Ti and V in the steel material before carburizing treatment and Nb composite carbonitride [NbTi (CN)], [NbV (CN)] and [NbTiV ( CN)] is the total of the number of composite carbonitrides of Nb and one or more of Ti and V of less than 0.010% and more than 20 nm in diameter and 80 nm or less in mass%. because in less than 300/1000 .mu.m ^2, grain coarsening temperature was extremely low temperature of 960 ° C..

  Furthermore, in the case of the test number 7, since the precipitation amount of AlN in the steel material before carburizing treatment is mass% and exceeds 0.010%, the coarsening temperature was 1000 ° C.

(Example 2)
Continuous casting was performed after converter melting, and steel A1 to A12 and steel B1 to B8 slabs having chemical compositions shown in Table 4 were produced. Steels A1 to A12 in Table 4 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel B1-B8 is steel of the comparative example from which the chemical composition remove | deviated from the conditions prescribed | regulated by this invention.

  Steel A1 is basic steel. With respect to this steel A1, Nb content in steel A2 and steel A3, Ti content in steel A4 and steel A5, V content in steels A6 and A7, Al content in steel A8, and In Steel A9 and Steel A10, the N content was changed independently. In steel A11, Mo was not added to steel A1, and Ni was added alone. In steel A12, Ni and Mo were added in combination to steel A1.

  On the other hand, with respect to steel A1, steel B1 has a lower Nb content, steel B2 has a lower Ti content, steel B3 has a higher Ti content, steel B4 has a lower V content, steel B5 In order to increase the V content, steel B6 was changed to increase the Al content, steel B7 was changed to lower the N content, and steel B8 was changed to increase the N content.

  Each of the above slabs was heated at 1300 ° C. for 50 minutes (solution treatment) and then rolled into pieces to produce 180 mm × 180 mm steel pieces.

  Next, each steel slab was heated at a heating temperature of 1270 ° C. and a heating time of 35 minutes, and then hot-rolled to produce a steel bar having a diameter of 35 mm.

  The steel bar having a diameter of 35 mm thus obtained was cut into a length of 52.5 mm, and then hot forging by upsetting and subsequent cooling were performed under the following conditions to produce a carburizing material having a diameter of about 70 mm.

-Temperature rising rate: 20 ° C / min,
-Heating temperature: 1280 ° C
-Forging finishing temperature: 1020 ° C
-Average cooling rate from forging finish temperature to 700 ° C: 65 ° C / min,
-Average cooling rate of 700 to 550 ° C: 20 ° C / min.

  About each carburizing material having a diameter of about 70 mm produced by placing the above hot, a cylindrical sample having a diameter of 10 mm and a length of 10 mm is cut out from the R / 2 part, and Ti and the electrolytic extraction separation analysis method are used. The total amount of precipitation of [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are complex carbonitrides of one or more of V and Nb, and the amount of precipitation of AlN were determined.

  The electrolytic extraction separation analysis method was performed under the following conditions <1> to <4> that are the same as those in the case of Example 1.

That is,
<1> A so-called “10% AA-methanol solution” is used as an electrolyte, and about 0.5 g of steel is obtained by constant potential (−100 mV vs SCE (saturated calomel electrode)) or constant current (20 mA / cm ² ). Electrolyze.

  <2> After electrolysis, the electrolytic residue (the composite carbonitride and AlN aggregated in the electrolytic solution) with a pore size of 0.2 μm is suction filtered and collected.

  <3> The electrolytic residue recovered by the above <2> is decomposed with a mixed acid (sulfuric acid 1, phosphoric acid 1, water 1), and the volume is adjusted with pure water, and then Nb by ICP emission spectrometry or atomic absorption spectrometry. Quantitative analysis of Ti, V and Al is performed.

  <4> For each element, the amount of electrolysis obtained in <3> above is converted into the content (mass%) in the steel material, and the total precipitation amount of the composite carbonitride is converted into the above Nb, Ti, and V conversions. The total content and the precipitation amount of AlN are represented by the above-mentioned converted content of Al.

  Moreover, about the R / 2 part of each carburizing material produced by setting up in the said hot, the deposit was used for observation by TEM by the extraction replica method.

  A TEM equipped with EDS was used, and the composite form and shape of Nb, Ti and V contained in the composite carbonitride were confirmed from elemental analysis by EDS.

  The number of [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)], which are composite carbonitrides, was determined as follows in the same manner as in Example 1.

That is, with respect to those having a diameter of 80 nm or less, they were counted in 10 fields with a magnification of 100,000, and converted into the number per 1000 μm ² . At this time, those having a diameter of 20 nm or less were excluded from the objects of counting.

  Furthermore, a cylindrical test piece having a diameter of 20 mm was cut out from the R / 2 part of the carburizing material produced by setting up in the hot state and subjected to a coarsening test. This coarsening test was also performed by simulating a heat pattern of high-temperature carburization and holding the test piece at 960 to 1100 ° C. for 2 hours in an Ar atmosphere. In addition, the water cooling process was carried out after heat-holding.

  The vertical section of each test piece thus obtained was mirror-polished and corroded with a saturated aqueous picric acid solution to which a surfactant was added, and then observed with an optical microscope at a magnification of 100 times to measure the austenite grain size, The occurrence of mixed grains in which abnormal grains existed, the sized part size and the coarsening temperature were investigated.

  Table 5 summarizes the above test results. In Table 5, the temperature corresponding to the austenite grain number indicated in italics is “coarsening temperature”, and the austenite grain size at the heating temperature exceeding that temperature was not measured. Indicated. The austenite particle size number expressed in italics indicates the particle size number in the sized portion when abnormal particles are formed and become mixed particles.

  From Table 5, in the case of test numbers 9 to 20 that satisfy the conditions specified in the present invention, the coarsening temperature is as high as 1020 ° C. or higher, and therefore, even when carburizing at a high temperature exceeding 1000 ° C. It is clear that the occurrence of granulation is suppressed.

  On the other hand, in the case of test numbers 21 to 28 of comparative examples that deviate from the conditions defined in the present invention, the coarsening temperature is at most 1000 ° C. and does not reach the target of the present invention.

Test No. 21, Test No. 22, Test No. 24 and Test No. 27 are more than the values specified in the present invention for the contents of Nb, Ti, V and N in Steel B1, Steel B2, Steel B4 and Steel B7, respectively. Since both are low, both are one or more of Ti and V in the steel material before carburizing treatment and Nb composite carbonitride [NbTi (CN)], [NbV (CN)] and [NbTiV (CN) The total number of precipitated carbonic nitrides is less than 0.010% in mass% and more than one type of Ti and V exceeding 20 nm in diameter and not more than 80 nm and Nb combined carbonitride. Since the number of particles is less than 1000 μm ² , the coarsening temperature is low.

Test number 23 is because the Ti content in steel B3 is higher than the value specified in the present invention, and test number 25 is because the V content in steel B5 is higher than the value specified in the present invention. Respectively, coarse Ti nitride (TiN) and V nitride (VN) are produced, and this is used as a nucleus and is a composite carbonitride of one or more of Ti and V and Nb [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)] are precipitated. For this reason, the total number of the composite carbonitrides having a diameter exceeding 20 nm and not more than 80 nm in the steel material before carburizing is less than 300/1000 μm ² , and the coarsening temperature is low.

Test number 26 is because the content of Al in steel B6 is higher than the value specified in the present invention, and test number 28 is because the content of N in steel B8 is higher than the value specified in the present invention. In any case, the precipitation amount of AlN in the steel material before carburizing treatment exceeds 0.010%, and with the generation of coarse AlN, one or more of Ti and V having a diameter exceeding 20 nm and not exceeding 80 nm and Since the total number of Nb composite carbonitrides [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)] is less than 300/1000 μm ² , the coarsening temperature is low.

  Since the coarsening temperature of the steel material for high-temperature carburizing of the present invention is a high temperature exceeding 1000 ° C., if the steel material for high-temperature carburizing of the present invention is used as the material for machine structural parts, carburizing at a high temperature exceeding 1000 ° C. In addition, the occurrence of coarsening can be reliably suppressed, and the manufacturing cost can be rationalized by shortening the carburizing time. Furthermore, when using the steel material for high-temperature carburizing of the present invention, there is an effect that the normalizing treatment before carburizing can be omitted.

Claims (2)

In mass%, C: 0.10 to 0.30%, Nb: 0.030 to 0.060%, Ti: 0.0010 to 0.0030%, V: 0.005 to 0.015%, Al: About 0.060% or less and N: 0.0185 to 0.0300%, and carbonitride and nitride in steel before carburizing treatment, one or more of Ti and V and Nb combined carbonitriding One of Ti and V having a diameter exceeding 20 nm and not more than 80 nm, with the total amount of precipitation satisfying 0.010% or more by mass%, and the amount of precipitation of AlN satisfying 0.010% or less by mass% A steel material for high-temperature carburizing, characterized in that the total number of composite carbonitrides of Nb and Nb is 300/1000 μm ² or more.
However, one or more of Ti and V and Nb composite carbonitride refers to [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)]. In mass%, C: 0.10 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.10 to 2.00%, P: 0.025% or less, S: 0.10 % Or less, Cr: 0.5 to 2.5%, Nb: 0.030 to 0.060%, Ti: 0.0010 to 0.0030%, V: 0.005 to 0.015%, Al: 0 0.060% or less and N: 0.0185 to 0.0300% and Ni: 0.1 to 3.0% and Mo: 0.02 to 1.5% or one or two of them The balance consists of Fe and impurities, and for the carbonitrides and nitrides in the steel before carburization, the total precipitation amount of one or more of Ti and V and the composite carbonitride of Nb is mass%. 0.010% or more, and the precipitation amount of AlN satisfies 0.010% or less in terms of mass%, and the diameter exceeds 8 nm and is 8 by the number of one or more composite carbonitride of Nb of nm or less Ti and V in total, 300 / high temperature carburizing steel, characterized in that at 1000 .mu.m ² or more.
However, one or more of Ti and V and Nb composite carbonitride refers to [NbTi (CN)], [NbV (CN)] and [NbTiV (CN)].