JP6182489B2 - Case-hardened steel that has excellent cold forgeability and can suppress abnormal grain generation during carburizing. - Google Patents

Description

  The present invention relates to carburizing treatment and carbonitriding treatment after cold forging (hereinafter, these may be collectively referred to as “carburizing treatment”) in transportation equipment such as automobiles, construction machinery, and other industrial machines. The present invention relates to a case-hardened steel which is a material for machine structural parts manufactured by surface hardening heat treatment. More specifically, the present invention relates to a case hardening steel that has excellent cold forgeability and can suppress the occurrence of abnormal grains during carburizing.

  The material of machine structural parts that require high strength in transportation equipment, construction machinery, other industrial machines, etc., is an alloy steel material for mechanical structures such as SCr, SCM, SNCM, etc. defined by JIS standards, so-called skin Generally, hardened steel is used. After this case-hardened steel is formed into a desired part shape by machining such as cold forging or cutting, it is subjected to surface hardening heat treatment such as carburizing treatment or carbonitriding treatment, followed by polishing, etc. Manufactured.

  In the surface hardening heat treatment as described above, in order to shorten the lead time at the time of manufacture, the heat treatment time is shortened by increasing the temperature. However, when the temperature of the surface hardening heat treatment is increased, there is a problem that the crystal grains of the mechanical structural component are coarsened and the mechanical characteristics are deteriorated.

  For example, Patent Documents 1 and 2 have been proposed as techniques for preventing such coarsening of crystal grains. In these techniques, precipitates such as AlN, Nb (CN), TiC and the like are dispersed in steel to exert a pinning effect and prevent coarsening of crystal grains.

JP 2005-146303 A JP 2012-207244 A

  Like the technologies proposed so far, it is considered that the crystal grain coarsening prevention technology using the pinning effect by the precipitates uses fine precipitates of 10 nm or more. However, as a result of investigations by the present inventors, in the technology using the pinning effect that has been proposed so far, the density of precipitates is insufficient and the crystal grains are partially coarsened under the recent carburizing conditions at high temperatures. It was found that abnormal grains were generated. In the prior art, the density of precipitates is insufficient under recent carburizing conditions where the temperature is increased, and coarsening of prior austenite (hereinafter sometimes referred to as “old γ”) occurs, or under carburizing conditions where the heating rate is slow. Since pearlite undergoes austenite transformation (hereinafter sometimes referred to as “γ”) prior to ferrite, abnormal grain growth, which is partial old γ grain growth, is likely to occur at sites where pearlite aggregates excessively. found.

  The present invention has been made in view of the circumstances as described above, and the object thereof is excellent in cold forgeability, and suppresses grain coarsening in surface hardening heat treatment such as carburizing and carbonitriding. It is to provide a case-hardened steel that can prevent the occurrence of abnormal grains.

The case-hardened steel according to the present invention, which has been able to solve the above problems, is C: 0.10 to 0.30% (meaning mass%, hereinafter the same), Si: 0.01 to 0.50%, Mn: 0.30 to 0.80%, P: more than 0% to 0.030%, S: more than 0% to 0.020%, Cr: 0.80 to 2.00%, Al: 0.01 to 0.10%, N: more than 0% to 0.005%, Ti: 0.040 to 0.200%, B: 0.0005 to 0.0050%, the balance being iron, and inevitable impurities The density of the equivalent circle diameter Ti containing 10 nm or more and less than 200 nm and the density of carbonitride is 10 pieces / μm ² or more, and the density of precipitates containing the equivalent circle diameter 200 nm or more containing Ti and S is 0. .2 pieces / [mu] m ² or less, the metal structure is a pearlite ferrite mixed structure, the surface of the mixed structure The area ratio of pearlite is 25% or less with respect to the entire metal structure, and the area ratio of pearlite with an equivalent circle diameter of 100 μm or more is 10% or less with respect to the entire metal structure. In particular.

  The case-hardened steel of the present invention may further contain, as necessary, other elements: (I) Mo: more than 0% to 2.0%, (II) Cu: more than 0% to 0.10% and Ni: 0 It is also useful to contain at least one of more than% to 3.0, etc., and the characteristics of the case-hardened steel are further improved according to the type of element contained.

  According to the present invention, the chemical composition is appropriately adjusted, and among the carbides and carbonitrides containing Ti, a density equivalent to a circle equivalent diameter of 10 nm or more and less than 200 nm is ensured in a predetermined amount or more, and Ti and S are contained. Excellent cold forging because it suppresses the density of coarse precipitates, and further controls the pearlite to a predetermined area ratio and suppresses the area ratio of coarse pearlite while the metal structure is mainly composed of pearlite ferrite mixed structure. And can prevent the generation of abnormal particles during carburization.

It is the schematic diagram which showed the concept of the behavior of the heat processing pattern at the time of a carburizing process, and the precipitate and structure | tissue before and behind a carburizing process. It is the schematic diagram which showed the heat processing pattern at the time of a carburizing process.

  As disclosed in Patent Documents 1 and 2 above, fine precipitates containing Ti are effective in preventing crystal grain coarsening, but when the density is insufficient, the crystal grains are coarsened in the insufficient portion. In this state, abnormal particles are generated. In particular, due to the recent increase in carburizing temperature, the pinning effect by fine precipitates that has been proposed so far has not been able to sufficiently suppress the occurrence of abnormal grains.

  Therefore, the present inventors have studied the influence of fine precipitates and metal structures on the generation of abnormal grains, and have made extensive studies on the precipitation state of fine precipitates and metal structures that can suppress the generation of abnormal grains. As a result, among the fine precipitates, it is effective to prevent the coarsening of the crystal grains and suppress the generation of abnormal grains, among the carbides containing Ti and the carbonitrides, In order to secure a predetermined amount of the equivalent diameter of 10 nm or more and less than 200 nm (hereinafter sometimes referred to as “10 nm or more of Ti carbide and the like”) and (i) 10 nm or more of Ti carbide and the like, (Ii) It is important to suppress coarse precipitates having an equivalent circle diameter of 200 nm or more containing Ti and S (hereinafter sometimes referred to as “coarse Ti—S precipitates”), (iii) Furthermore, the present inventors have found that by controlling the metal structure appropriately to reduce the solid solution amount of Ti carbide of 10 nm or more in (i) above, the occurrence of abnormal particles during carburizing treatment can be suppressed, and the present invention has been completed.

  The present inventors considered the cause of occurrence of abnormal grains during carburization as shown in FIG. FIG. 1 is a schematic diagram showing the concept of the heat treatment pattern during carburizing treatment and the behavior of precipitates and structures before and after carburizing treatment. In the state before carburization, that is, in the heat treatment pattern STAGE 1, when there is massive pearlite in the metal structure, that is, pearlite having an equivalent circle diameter of 100 μm or more (hereinafter sometimes referred to as “pearlite agglomerated part”), 2 in the carburizing heating period. In the phase region, that is, in the heat treatment pattern STAGE 2, the pearlite undergoes austenite transformation and becomes a state of ferrite + austenite. During this time, Ti carbide of less than 10 nm or Ti carbonitride (hereinafter sometimes referred to as “Ti carbide of less than 10 nm”) precipitated in STAGE 1 gradually dissolves in the matrix as the solid solubility limit increases. At this time, if the heating rate is insufficient and the residence time in the two-phase region is increased, the pearlite aggregate part transforms into austenite having a higher solid solubility limit than ferrite, so that Ti carbide of 10 nm or more effective in suppressing abnormal grain generation. Etc. are dissolved in the matrix, the pinning effect is lost and the local crystal grains become coarse and abnormal grains are likely to be generated.In addition, when the temperature is raised to STAGE 3 in the heat treatment pattern, the temperature is maintained. As the solid solubility limit of the matrix is further increased, more Ti carbide of 10 nm or more is dissolved in the matrix and abnormal grains are generated. It becomes easier.

  As described above, the growth of crystal grains is deeply related to the aggregation of pearlite and the solid solution of fine Ti carbide, etc. In order to prevent grain coarsening, the reduction of pearlite aggregation and the pinning effect in the carburizing process It is considered effective to reduce solid solution in a matrix of 10 nm or more Ti carbide or the like that exhibits the above. In particular, it is important to make more effective use of the added Ti in order to reduce solid solution in a matrix of Ti carbide of 10 nm or more that exhibits a pinning effect. Therefore, the density of coarse Ti—S precipitates that are not effective for the pinning effect must be reduced.

As a result of further studies by the present inventors, it has been found that, by satisfying the following three requirements, generation of abnormal grains during carburizing treatment can be suppressed while ensuring excellent cold forgeability.
(A) Carbide with a circle equivalent diameter of 10 nm or more and less than 200 nm containing Ti and carbonitride, that is, a density of Ti carbonitride of 10 nm or more is 10 pieces / μm ² or more. (B) Contains Ti and S Precipitates having an equivalent circle diameter of 200 nm or more, that is, the density of coarse Ti—S precipitates is 0.2 pieces / μm ² or less. (C) (c-1) The metal structure is a pearlite ferrite mixed structure, The area ratio of the mixed structure is 80% or more, (c-2) the area ratio of pearlite is 25% or less with respect to the entire metal structure, and (c-3) the pearlite having an equivalent circle diameter of 100 μm or more, The area ratio of pearlite aggregates is 10% or less with respect to the entire metal structure.

  Hereinafter, each requirement will be described.

(A) Ti carbide having a density of Ti carbide of 10 nm or more is 10 pieces / μm ² or more Ti carbide having a density of 10 nm or more works effectively for preventing grain coarsening during carburizing treatment, and can suppress generation of abnormal grains. In order to effectively exhibit such an effect, the density is 10 pieces / μm ² or more, preferably 15 pieces / μm ² or more, and more preferably 20 pieces / μm ² or more. The upper limit of the density of Ti carbide of 10 nm or more is not particularly limited, but is usually 150 pieces / μm ² or less, preferably 120 pieces / μm ² or less, more preferably 100 pieces / μm ² or less.

In addition, a predetermined amount of Ti carbide or the like of less than 10 nm is used in order to more effectively exhibit the pinning effect and suppress the occurrence of abnormal particles without causing solid dissolution of the Ti carbide or the like of 10 nm or more in the matrix during the carburizing process. It is also effective to secure. Therefore, the density of Ti carbide or the like of less than 10 nm is preferably 10 pieces / μm ² or more, more preferably 15 pieces / μm ² or more. The upper limit of the density of Ti carbide or the like less than 10 nm is not particularly limited, but is usually about 300 pieces / μm ² . In addition, although the minimum of the size of Ti carbide | carbonized_material less than 10 nm is not specifically limited, Since there exists a measurement limit of measuring apparatuses, such as an electron microscope, it is about 2 nm normally.

  Ti carbides and the like in the present invention detect a peak indicating C or N and a Ti peak by elemental analysis using energy dispersive X-ray spectroscopy (EDX) or the like. Means precipitates.

(B) Density of coarse Ti—S precipitates is 0.2 pieces / μm ² or less When the density of coarse Ti—S precipitates is too large, the above (a) 10 nm or more effective in suppressing abnormal grain generation The number of Ti carbides and the like cannot be ensured. Therefore, the density of coarse Ti—S precipitates is 0.2 pieces / μm ² or less, preferably 0.15 pieces / μm ² or less, more preferably 0.10 pieces / μm ² or less. The smaller the amount of coarse Ti—S precipitates, the better. However, it is usually a value exceeding 0 / μm ² . The coarse Ti—S precipitate in the present invention means a precipitate from which Ti and S peaks are detected by elemental analysis using EDX or the like.

(C) (c-1) The metal structure is a pearlite ferrite mixed structure, and the area ratio of the mixed structure is 80% or more. The structure of the case hardening steel subjected to cold forging contains a lot of bainite and martensite. If it is, deformation resistance at the time of cold forging increases and coarsening of crystal grains is likely to occur at the time of subsequent carburizing, so it is necessary to mainly use a pearlite ferrite mixed structure. Specifically, the area ratio of the pearlite ferrite mixed structure is 80% or more of the total metal structure, more preferably 90% or more, and still more preferably 100%. The structure other than the pearlite ferrite mixed structure is not particularly limited, and is, for example, a single phase of bainite or martensite or a composite structure thereof.
(C-2) The area ratio of pearlite is 25% or less with respect to the entire metal structure. In order to reduce deformation resistance during cold forging, the area ratio of pearlite is 25% or less with respect to the entire metal structure, preferably 23 % Or less, more preferably 20% or less.
(C-3) The area ratio of the pearlite aggregated portion is 10% or less with respect to the entire metal structure. When there are too many pearlite aggregated portions, the above (a) a matrix of Ti carbides of 10 nm or more which is effective in suppressing abnormal grain generation As a result, the pinning effect is lost. Therefore, the area ratio of the pearlite aggregate part is 10% or less, preferably 8% or less, more preferably 5% or less with respect to the entire metal structure.

  In the present invention, as described above, in order to ensure Ti carbide of 10 nm or more in the steel, in addition to suppressing coarse Ti-S precipitates and controlling the metal structure, In order to exhibit basic characteristics, it is necessary to adjust the chemical composition appropriately. This will be described below.

C: 0.10 to 0.30%
C is an element necessary for ensuring the core hardness necessary for carburized parts. If the C content is less than 0.10%, the static strength as a carburized part is insufficient due to insufficient hardness. In order to effectively exert such effects, the C content is 0.10% or more, preferably 0.12% or more, more preferably 0.15% or more. However, when C is contained excessively, the amount of pearlite increases and the cold forgeability deteriorates. Therefore, the C content is 0.30% or less, preferably 0.28% or less, more preferably 0.25% or less.

Si: 0.01 to 0.50%
Si is an element that acts to improve the surface fatigue characteristics of mechanical structural parts by suppressing the decrease in tempering hardness. In order to effectively exhibit such effects, the Si content is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. However, if Si is contained excessively, it adversely affects the part formability such as machinability and forgeability. Therefore, the Si content is 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less.

Mn: 0.30 to 0.80%
Mn is an element effective for enhancing the hardenability during the carburizing process. Mn also acts as a deoxidizer, and is an element that has the effect of increasing the internal quality by reducing the amount of oxide inclusions in the steel. Furthermore, Mn also has an effect of preventing red heat embrittlement. In order to exhibit such an action effectively, the Mn content is 0.30% or more, preferably 0.35% or more, more preferably 0.400% or more. However, when Mn is contained excessively, the cold forgeability tends to be deteriorated and the variation of the material becomes large. Therefore, the Mn content is 0.80%, preferably 0.70% or less, more preferably 0.60% or less.

P: Over 0% to 0.030%
P is an element contained in the steel as an inevitable impurity, and segregates at the grain boundary to deteriorate the impact fatigue characteristics of the mechanical structural component. Therefore, the P content is 0.030% or less, preferably 0.025% or less, more preferably 0.020% or less. The smaller the P content, the better. However, it is difficult to make it 0% due to the limitation of the manufacturing process. Therefore, the P content is more than 0%, and usually at least about 0.0001% is included.

S: Over 0% to 0.020%
S is an element that combines with Mn to form MnS and improves machinability when cutting. In order to effectively exert such effects, the S content is more than 0%, preferably 0.0001% or more, more preferably 0.0003% or more. However, when S is contained excessively, crystal grains may be coarsened due to an increase in the density of coarse Ti-S precipitates or a decrease in the density of Ti carbides of 10 nm or more. Therefore, the S content is 0.020% or less, preferably 0.015% or less, more preferably 0.013% or less, and still more preferably 0.010% or less.

Cr: 0.80 to 2.00%
Cr is an element necessary for promoting carburization and forming a hardened layer on the steel surface. In order to effectively exert such effects, the Cr content is 0.80% or more, preferably 0.90% or more, and more preferably 1.00% or more. However, when Cr is excessively contained, excessive carburization is caused and the strength of the mechanical structural component is lowered. Therefore, the Cr content is 2.00% or less, preferably 1.95% or less, more preferably 1.90% or less, and still more preferably 1.80% or less.

Al: 0.01-0.10%
Al is an element that acts as a deoxidizer, and in order to effectively exert such an effect, the Al content is 0.01% or more, preferably 0.015% or more, more preferably 0.020% or more. is there. However, if Al is contained excessively, the deformation resistance of the steel increases and the cold forgeability deteriorates. Therefore, the Al content is 0.10% or less, preferably 0.080% or less, more preferably 0.060% or less.

N: Over 0% to 0.005%
N is an element necessary for forming Ti carbide or the like of 10 nm or more that acts to appropriately adjust the crystal grain size of the mechanical structural component. In order to exert such effects, the N content is more than 0%, preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more. However, when N is contained excessively, a large amount of nitrides such as AlN and TiN are formed in the steel, and the machinability and cold forgeability are deteriorated. Therefore, the N content is 0.005% or less, preferably 0.0045% or less, more preferably 0.0040% or less.

Ti: 0.040 to 0.200%
Ti is an element necessary for forming Ti carbide or the like of 10 nm or more that acts to appropriately adjust the crystal grain size of the mechanical structural component. In order to exert such an effect, the Ti content is 0.040% or more, preferably 0.045% or more, more preferably 0.050% or more. However, when Ti is excessively contained, TiN is excessively formed in the steel, and the machinability and cold forgeability are deteriorated. Therefore, the Ti content is 0.200% or less, preferably 0.180% or less, more preferably 0.150% or less.

B: 0.0005 to 0.0050%
B is an element that works particularly effectively to improve the hardenability in carburizing treatment even in a small amount, and is an element that strengthens the grain boundary and effectively works to improve impact strength. In order to effectively exert such effects, the B content is 0.0005% or more, preferably 0.0007% or more, more preferably 0.0010% or more. However, even if B is contained excessively, the above effect is saturated, and B nitride is generated to deteriorate the cold forgeability. Therefore, the B content is 0.0050% or less, preferably 0.0040% or less, more preferably 0.0030% or less.

  The basic components of the case-hardened steel according to the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that steel contains inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like.

  Furthermore, in the present invention, if necessary, (a) Mo: more than 0% to 2.0%, (b) Cu: more than 0% to 0.10%, and Ni: more than 0% to 3.0% It is also useful to contain at least one of the above, and the characteristics of the case-hardened steel are further improved according to the type of element contained.

Mo: more than 0% to 2.0%
Mo is an element that effectively acts to improve the hardenability in the carburizing process. In order to effectively exert such effects, the Mo content is preferably 0.05% or more, more preferably 0.08% or more, and further preferably 0.10% or more. However, when Mo is contained excessively, machinability and forgeability are deteriorated. Therefore, the Mo content is preferably 2.0% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

Cu: More than 0% to 0.10%, and Ni: More than 0% to 3.0% Cu and Ni, as in the case of Mo, are effective in improving the hardenability in the carburizing process. It is an element. Further, Cu and Ni are elements that are less likely to be oxidized than Fe, and thus are effective elements for improving the corrosion resistance of mechanical structural parts. In order to effectively exhibit these actions, the Cu content is more than 0%, preferably 0.03% or more, more preferably 0.04% or more, and further preferably 0.05% or more. The Ni content is more than 0%, preferably 0.03% or more, more preferably 0.05% or more, and further preferably 0.08% or more. However, when Cu is excessively contained, hot forgeability is lowered, and problems such as cracking are likely to occur. Therefore, the Cu content is preferably 0.10% or less, more preferably 0.08% or less. Moreover, since it will become expensive if Ni is contained excessively, Ni content becomes like this. Preferably it is 3.0% or less, More preferably, it is 2.5% or less, More preferably, it is 2.0% or less. Cu and Ni may contain either one or both.

  In order to produce the case-hardened steel of the present invention, a steel having a predetermined chemical composition is melted in accordance with a normal melting method, cast, split-rolled, and then subjected to bar rolling, in particular, in the series of rolling. It is preferable to appropriately adjust the heating temperature and heating holding time during the rolling of the steel bar and the cooling rate after the steel bar rolling. Specifically, after preheating at 600 to less than 750 ° C., heating at the time of lump is 1150 to 1250 ° C. for 0.5 to 1.0 hour, and heating at the time of rolling the steel bar is 0 to 800 to 1100 ° C. .5 to 1.5 hours. The average cooling rate after rolling the steel bar is 40 ° C./hour or less.

  In the present invention, the bulk rolling suppresses the generation of coarse Ti-S precipitates, and at the same time, suppresses the generation of abnormal grains by not dissolving Ti carbides generated in the casting stage as much as possible in the matrix. Precipitates serving as nuclei such as Ti carbides are secured.

  Also, in steel bar rolling, Ti carbide remaining in the split rolling is Ostwald-grown, and Ti carbide of 10 nm or more effective for abnormal grain growth reaches the above density, and the abnormal grain growth is suppressed during the cooling process. In addition, pearlite control is performed to ensure cold forgeability.

  In the conventional partial rolling, after the steel material is inserted into the main heating furnace, it is heated from room temperature to the partial rolling temperature, but because the residence time at 750 ° C. or higher where the amount of solid solution of Ti carbide or the like increases is long. Ti carbides of 10 nm or more were dissolved in the matrix. However, by preheating at a predetermined temperature to reach the partial rolling temperature to make the temperature of the steel material uniform, heating to the partial rolling temperature can suppress solid solution of Ti carbide or the like of 10 nm or more. In order to exhibit such an effect, the preheating temperature is preferably 600 ° C. or higher, more preferably 630 ° C. or higher. However, if the preheating temperature is too high, the amount of solid solution such as Ti carbide of 10 nm or more increases. Accordingly, the preheating temperature is preferably less than 750 ° C, more preferably 730 ° C or less. The preheating time is not particularly limited, and may be adjusted so that the temperature of the steel material is uniform. However, if the preheating time is too short, the temperature of the steel material varies, so the preheating time is preferably 0.5 hour. As mentioned above, More preferably, it is 1.0 hour or more. On the other hand, if the preheating time is too long, Ti carbide or the like of less than 10 nm is dissolved in the matrix, so that it is preferably 5.0 hours or less, more preferably 3.0 hours or less. When preheating is performed, after heating and holding in advance in a preheating furnace at a predetermined temperature, heating to a predetermined block rolling temperature in the main heating furnace may be performed.

  When the heating temperature at the time of the block rolling is below 1150 ° C., the load on the rolling mill at the time of the block rolling becomes large, and rolling to a desired shape becomes difficult. For this reason, heating temperature becomes like this. Preferably it is 1150 degreeC or more, More preferably, it is 1160 degreeC or more, More preferably, it is 1170 degreeC or more. However, if the heating temperature becomes too high, Ti carbides of 10 nm or more generated in the casting stage are dissolved in the matrix, and the density of coarse Ti—S precipitates increases. Therefore, the heating temperature at the time of the block rolling is preferably 1250 ° C. or less, more preferably 1230 ° C. or less, and further preferably 1200 ° C. or less. If the heating and holding time in the temperature range is too long, Ti carbide of 10 nm or more generated in the casting stage is dissolved in the matrix. Therefore, the heating and holding time is preferably 1 hour or less, more preferably 50 minutes or less. On the other hand, if the heating and holding time is too short, the temperature of the steel material is uneven, leading to variations in material quality. Therefore, the heating and holding time is preferably 30 minutes or more, more preferably 35 minutes or more.

  When the heating temperature at the time of rolling the steel bar is less than 800 ° C., the load on the steel bar rolling machine becomes large, and rolling to a desired shape becomes difficult. For this reason, the heating temperature at the time of steel bar rolling is preferably 800 ° C. or higher, more preferably 820 ° C. or higher, and further preferably 850 ° C. or higher. However, when the heating temperature at the time of steel bar rolling exceeds 1100 ° C., the density of Ti carbide or the like of 10 nm or more decreases. For this reason, heating temperature becomes like this. Preferably it is 1100 degrees C or less, More preferably, it is 1050 degrees C or less, More preferably, it is 1000 degrees C or less. On the other hand, if the heating and holding time in the above temperature range is too long, the density of Ti carbide or the like of less than 10 nm decreases. For this reason, the heat holding time is preferably 1.5 hours or less, more preferably 1.25 hours or less. On the other hand, if the heating and holding time is too short, there will be temperature irregularities in the steel material, leading to variations in material quality. Therefore, the heating and holding time is preferably 0.5 hours or more, more preferably 0.75 hours or more.

  After steel bar rolling, it is cooled to room temperature. However, if the cooling rate after steel bar rolling is too fast, excessive pearlite and pearlite agglomerates will be generated, causing deterioration of cold forgeability and abnormal grain growth, respectively. It becomes. For this reason, an average cooling rate becomes like this. Preferably it is 40 degrees C / hour or less, More preferably, it is 30 degrees C / hour or less, More preferably, it is 25 degrees C / hour or less.

  As described above, the case-hardened steel of the present invention can be obtained by satisfying the conditions of the heating temperature and heating holding time at the time of the block rolling and the steel bar rolling and the cooling rate after the steel bar rolling.

  Although the shape of the case hardening steel of this invention is not specifically limited, For example, it is a bar steel of (phi) 10-150mm. Machine structural parts obtained by carburizing the case-hardened steel of the present invention that satisfies these requirements, that is, the surface of which is carburized, can suppress the occurrence of abnormal grains and have excellent cold forgeability. It will be.

  Specific examples of mechanical structural parts using the case-hardened steel of the present invention include gears, shafts, continuously variable transmission (CVT) pulleys, constant velocity joints (CVJ), bearings, and the like. Is mentioned.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel satisfying the chemical composition shown in Table 1 below was melted in a melting furnace in accordance with a normal melting method to produce a steel piece.

  Using the obtained various steel pieces, after heating at the partial rolling temperature shown in Table 2 below and the heating and holding time, the partial rolling was performed and cooled to room temperature. In condition A, preheating was performed at 700 ° C. for 1 hour, and then heated to the block rolling temperature. Subsequently, the steel bar was rolled by heating at a steel bar rolling temperature and a heating and holding time shown in Table 2 below, thereby manufacturing a steel bar having a diameter of 23 mm.

  Observations of Ti carbides of less than 10 nm, Ti carbides of 10 nm or more, and coarse Ti—S precipitates were made by the following procedure.

(1) Measurement of the density of each precipitate The cross section of the obtained steel bar, that is, the cross section perpendicular to the axis of the steel bar, is mechanically polished and then electropolished to form a nital solution, ie, ethanol and 3% nitric acid. After etching with the mixed solution, a replica film was prepared by an extraction replica method in which carbon deposition was performed. The precipitate was observed by observing the steel bar diameter D / 4 position in the transverse section at 75,000 times using a transmission electron microscope H-800 manufactured by Hitachi, Ltd. The constituent elements of the observed deposits were measured by point analysis using an EDX analyzer EMAX-7000 manufactured by Horiba. A peak indicating C or N is detected, and a precipitate in which a peak of Ti is detected is determined as “a carbide and carbonitride containing Ti”, and a precipitate in which a peak of Ti and S is detected is “ It was judged as “precipitate containing Ti and S”. In addition, the precipitate which showed the same aspect in a transmission image determined the precipitate by determining with the same structural element. In addition, the density of each precipitate was measured by Sumitomo Metal Technology's Particle Analysis Ver. Measured by 3.0. The measurement visual field was 1.35 μm × 1.60 μm, and five visual fields were observed. The arithmetic average value was defined as the density of each precipitate. The results are shown in Table 3 below. In the table, the number density of carbides and carbonitrides containing Ti is each column of “Ti charcoal (nitride) density (pieces / μm ² )” according to the equivalent circle diameter, circles containing Ti and S. The number density of precipitates having an equivalent diameter of 200 nm or more is described in “Ti, S precipitate density (pieces / μm ² )”.

(Measurement of pearlite area ratio (%), ferrite area ratio (%), and pearlite area ratio (%) with an equivalent circle diameter of 100 μm or more)
Next, at the D / 4 position of the obtained steel bar, the cross section of the steel bar was mechanically polished, etched with a picral solution, and observed with an optical microscope at 100 times. The metal structure fraction was determined by Sumitomo Metal Technology's Particle Analysis Ver. 3.0. Furthermore, the area ratio in the visual field of pearlite having an equivalent circle diameter of 100 μm or more was measured and entered in the “perlite aggregate part area ratio (%)” column.

  Next, centering on the D / 4 position of the obtained steel bar, a cylindrical specimen having a diameter of 20 mm × 30 mm is prepared so that the longitudinal direction of the specimen is parallel to the rolling direction, and 50% in the longitudinal direction of the cylindrical specimen. Was subjected to cold compression, that is, cold forging. At this time, for the test piece in which cracking occurred, “cracking during cold forging” was entered in the table, and the following grain size number was not determined. On the other hand, as shown in FIG. 2, the test piece in which cracking did not occur during cold forging was carburized for 6 hours at CP (carbon potential) 0.8%, temperatures 930 ° C., 950 ° C., and 980 ° C. for 100 hours. After immersing in an oil bath at 0 ° C., a tempering treatment was performed at 170 ° C. for 120 minutes to obtain a test piece for measuring crystal grain size. The procedure for measuring the grain size is as follows.

(2) Judgment of grain size number of prior austenite grains After cutting out the plane parallel to the compression direction of the above test piece for grain size measurement and etching it with a nital solution, it was observed at a magnification of 100 using an optical microscope. The particle size number of the prior austenite grains was measured according to JIS G0551 (2005). The particle size number was measured at the surface layer portion at the compression end, that is, at a depth of 2 mm from the surface, and the particle size number of the portion where the crystal grains were the largest was measured to obtain the maximum γ particle size. Those having a maximum γ particle size of 6.0 or more were evaluated as “no occurrence” of abnormal particles. The results are shown in Table 3.

  Test No. in Table 3 Nos. 1 to 43 are steels that satisfy the chemical composition defined in the present invention under appropriate manufacturing conditions. Therefore, the density of coarse Ti-S precipitates such as Ti carbide of 10 nm or more, and the metal structure of the present invention are determined. It can be adjusted to meet the requirements, and abnormal grain generation during carburizing can be suppressed.

  On the other hand, test No. 44-60, the chemical composition or manufacturing conditions of the steel were inappropriate, so the density and metal structure of Ti carbides and coarse Ti-S precipitates of 10 nm or more and coarse Ti-S precipitates can be adjusted to the ranges specified in the present invention. However, cracks occurred during cold forging, or abnormal grains occurred during carburizing.

  No. No. 44 is an example using steel Z1 with a large amount of C, and the pearlite area ratio increased, and the cold forgeability deteriorated.

  No. 45 is an example using steel Z2 with a large amount of Si, and cracking occurred during cold forging.

  No. No. 46 is an example using steel Z3 with a large amount of Mn, and cracking occurred during cold forging.

  No. No. 47 is an example using steel Z4 with a large amount of S. Coarse Ti—S precipitates increased, the density of Ti carbides of 10 nm or more could not be secured, and abnormal grains were generated.

  No. No. 48 is an example using steel Z5 with a large amount of Al. The deformation resistance of the steel increased, and cracking occurred during cold forging.

  No. No. 49 is an example using steel Z6 with a large amount of N, and cracking occurred during cold forging. This is probably because nitrides such as AlN and TiN were excessively formed in the steel.

  No. 50 is an example using steel Z7 with a small amount of Ti, and the density of Ti carbide of 10 nm or more could not be secured, and abnormal particles were generated.

  No. 51 is an example using steel Z8 with a large amount of Ti, and cracking occurred during cold forging. This is presumably because excess TiN precipitated in the steel.

  No. 52 to 54 are examples in which the production condition B in which preheating was not performed at the time of the block rolling was adopted, and the density of Ti carbide or the like of 10 nm or more could not be secured, and abnormal particles were generated.

  No. 55 to 57 is an example in which the preheating is not performed, the heating and holding time at the time of the ingot rolling is long, and the production condition C in which the steel bar rolling is not performed at a predetermined temperature is adopted. The density could not be secured and abnormal particles were generated.

  No. Nos. 58 to 60 are examples in which the production condition D is used, in which preheating is not performed, the heating temperature at the time of the ingot rolling is high, the heating and holding time is long, and the cooling rate after the steel bar rolling is fast. The density of Ti carbide or the like could not be ensured, and the area ratio of the pearlite aggregation portion was high, and abnormal particles were generated.

Claims (3)

C: 0.10 to 0.30% (meaning mass%, hereinafter the same),
Si: 0.01 to 0.50%,
Mn: 0.30 to 0.80%,
P: more than 0% to 0.030%,
S: more than 0% to 0.020%,
Cr: 0.80 to 2.00%,
Al: 0.01 to 0.10%,
N: more than 0% to 0.005%,
Ti: 0.040 to 0.200%,
B: 0.0005 to 0.0050%
The balance is iron, and inevitable impurities,
Carbide having a circle-equivalent diameter of 10 nm or more and less than 200 nm containing Ti, and a density of carbonitride of 10 / μm ² or more
The density of precipitates having an equivalent circle diameter of 200 nm or more containing Ti and S is 0.2 pieces / μm ² or less,
The metal structure is a pearlite ferrite mixed structure, and the area ratio of the mixed structure is 80% or more,
Excellent cold, characterized in that the area ratio of pearlite is 25% or less with respect to the entire metal structure, and the area ratio of pearlite with an equivalent circle diameter of 100 μm or more is 10% or less with respect to the entire metal structure. Case-hardened steel that has forgeability and can suppress the occurrence of abnormal grains during carburizing.   Furthermore, Mo: More than 0%-2.0% of case hardening steel of Claim 1.   Furthermore, the case hardening steel of Claim 1 or 2 containing at least 1 sort (s) of Cu: more than 0%-0.10% and Ni: more than 0%-3.0%.