JP5316058B2 - Steel for heat treatment - Google Patents

Description

  The present invention is suitable as a material for steel parts such as chain parts, gear parts, clutch parts, seat belt parts, etc., which are manufactured by subjecting steel materials processed into a predetermined shape to heat treatment such as quenching and tempering or austempering. It relates to a steel material for heat treatment.

  Steel parts such as chain parts, gear parts, clutch parts, and seat belt parts are required to have high strength, high toughness, high fatigue characteristics, and wear resistance characteristics. For this reason, a steel material having a high C content of 0.2% by mass or more is used for the material of these steel parts so that the above-described characteristics can be obtained by performing various heat treatments such as quenching and tempering and austempering. . Therefore, the steel material used for the material of these steel parts (hereinafter also referred to as “steel material for heat treatment”) is essentially hard and inferior in workability.

  On the other hand, the steel material for heat treatment is processed into the shape of a steel part by press molding or punching before the heat treatment. Therefore, good workability is required for the steel for heat treatment. For example, when processing such as press molding or punching is performed, it is preferable that the steel for heat treatment is soft. Therefore, in order to make a steel material with high C content, essentially hard and inferior in workability, into a soft and highly workable steel material for heat treatment, the carbides in the steel material are spheroidized into steel materials such as hot-rolled steel sheets and cold-rolled steel sheets It is often performed to perform annealing for a long time (hereinafter referred to as “spheroidizing annealing”).

  In addition, the heat treatment steel material processed to the shape of the steel part is heat treated to increase the strength and obtain a high strength steel part. Therefore, the heat treatment steel material has high hardenability. Desired. If the steel for heat treatment has low hardenability, many phases and structures such as ferrite and pearlite are formed in the cooling process of heat treatment in addition to the originally intended low temperature transformation phase such as martensite and bainite. This is because there is a case that cannot be obtained. This tendency is particularly significant when the size of the steel part is large or when the cooling medium used for the heat treatment is high in temperature because the cooling rate in the heat treatment is low. Therefore, a steel material to which an alloy element having an effect of improving hardenability is added is often used. For example, chrome steel, chrome molybdenum steel, nickel chrome steel and the like specified in JIS G 3311.

  In addition, when quenching is performed in heat treatment, tempering is often further performed to improve toughness. Here, when tempering is performed at a tempering temperature of 250 to 550 ° C., temper embrittlement in which toughness is lowered despite a decrease in strength may occur. This temper embrittlement is said to be caused by segregation of alloying elements at the grain boundaries during tempering, and it is known that the addition of Mo is effective as a means for suppressing this. Therefore, when tempering is performed in a temperature range where temper embrittlement occurs, a steel material added with Mo is often used. For example, chromium molybdenum steel and nickel chromium molybdenum steel specified in JIS G 3311.

  Further, when austemper is applied in the heat treatment, the austemper stops the cooling in a higher temperature region than the quenching and makes the steel structure bainite by causing the isothermal transformation. Steel materials having a high amount and high hardenability, for example, high carbon steel materials having a C content of 0.7 to 0.85% are often used. However, as disclosed in Patent Document 1, it contains Cr and Mo. Thus, a steel material having a C content that is not so high may be used.

  As described above, in order to provide steel parts with high strength and high toughness, the heat-treating steel material used as the raw material is made of Cr and Cr so as to enhance hardenability or further suppress temper embrittlement. Steel materials containing Mo (hereinafter also referred to as “Cr, Mo-containing steel materials”) may be used.

By the way, heat treatment such as quenching and tempering and austempering is performed by heating a steel material for heat treatment that has been processed into the shape of a steel part to a temperature range of Ac ₃ points or more to re-dissolve carbides in the steel material, The intended steel structure is obtained by cooling.

  However, Cr and Mo-containing steel materials have a drawback that re-dissolution of carbides in the steel materials may be significantly delayed as compared with steel materials not containing Cr and Mo. That is, if the heating temperature at the time of re-dissolving the carbide is low or the heating time is short, the re-dissolution of the carbide may be insufficient and the intended steel structure and strength may not be obtained. In addition, carbides that remain without being re-dissolved (hereinafter, also referred to as “residual carbides”) serve as a starting point for fracture and cause a reduction in the toughness and fatigue strength of steel parts.

  Furthermore, as described above, the steel for heat treatment is often manufactured by spheroidizing annealing in order to improve workability. The carbides spheroidized by this spheroidizing annealing tend to delay the re-solidification in the heat treatment due to the shape factor, and this tendency is particularly remarkable in the steel sheet having a high alloy element content. Therefore, the Cr and Mo-containing steel materials subjected to spheroidizing annealing tend to have a particularly large amount of residual carbide after heat treatment.

Therefore, it is important to promote the re-dissolution of carbides of Cr and Mo-containing steel materials.
By the way, as a means for promoting the re-dissolution of carbides in the heat treatment, it is generally considered most effective to reduce the carbide particle size before the heat treatment. Technology has been proposed.

  For example, Patent Document 2 describes a manufacturing technique for obtaining a steel sheet in which carbide is easily dissolved during heating by crushing carbide by cold rolling. Further, Patent Document 3 describes that the average carbide particle size is 1.0 μm or less in order to re-dissolve carbide in short-time heating such as induction hardening.

Japanese Patent Publication No. 7-5970 JP 7-197193 A Japanese Patent Laid-Open No. 11-80884

  However, although the above technique is effective for promoting re-solution dissolution in the heat treatment of carbide of low alloy steel, it is insufficient as a means for promoting re-solution dissolution in the heat treatment of carbide of Cr and Mo-containing steel.

  For example, the method described in Patent Document 2 is not applicable to chromium molybdenum steel specified in JIS G 3311 because the Cr content is limited to 0.25% or less. Since the method described in Patent Document 3 allows a Cr content of up to 1.2%, it can be applied to chromium molybdenum steel defined in JIS G 3311. However, according to the study by the present inventors, it has been found that for Cr and Mo-containing steel materials, simply reducing the carbide particle size is not sufficient as a method for promoting the solid solution of carbide in heat treatment. .

  As described above, regarding Cr and Mo-containing steel materials that are excellent in hardenability and properties after tempering, an effective method for promoting the re-dissolution of carbides in heat treatment has not yet been established.

  However, in order to reliably re-dissolve the carbide in the heat treatment, heating at a high temperature for a long time may be performed, but this involves a decrease in productivity and an increase in manufacturing cost. For this reason, in recent years, with the progress of the heat treatment technology, the heating conditions in the heat treatment have been lowered and shortened. Therefore, there is a need for a method for promoting the re-dissolution of carbide with respect to Cr and Mo-containing steel materials in which re-solution of carbide tends to be delayed.

  Therefore, the present invention is to effectively reduce the residual carbide by easily re-dissolving the carbide during the heat treatment even when the carbide is spheroidized by spheroidizing annealing on the Cr and Mo-containing steel materials. It aims at providing the steel material which can do.

  The present inventors conducted detailed research on carbide re-solution behavior of Cr and Mo-containing steel materials by applying heat treatment equivalent to continuous heat treatment using Cr and Mo-containing steel materials subjected to spheroidizing annealing after hot rolling. Went. As a result, the following new findings were obtained.

(A) M ₂₃ C ₆ is observed in the carbides contained in the Cr and Mo-containing steel materials in addition to cementite (M ₃ C) observed in general carbon steel. Here, M consists of Cr, Mo, Mn, and Fe.

(B) Comparing the re-solution behavior during heat treatment between cementite and M ₂₃ C ₆ , the re-solution of M ₂₃ C ₆ is delayed. Therefore, the steel material containing a large amount of M ₂₃ C ₆ tends to have a large amount of residual carbide after heat treatment.

(C) In order to make the re-solution behavior of carbide to the same level as that of general carbon steel, the peak intensity (I _M ) of the (422) plane of M ₂₃ C ₆ and the (211) plane of cementite in the X-ray diffraction test The ratio (I _M / I _θ ) with _respect to the peak intensity (I _θ ) of the steel should be 0.3 or less, and the Cr and Mo-containing steel material obtained in this way can easily re-dissolve carbides in the heat treatment. Since it progresses and the residual carbides after heat treatment are effectively reduced, the heating conditions can be reduced in temperature and shortened.

The present invention has been completed based on the above-mentioned new findings, and the gist thereof is as follows.
(1) By mass%, C: 0.4% or more 56 % or less, Si: 0.5% or less, Mn: 0.2% or more and 1.2% or less, P: 0.05% or less, S: 0.02% or less, Cr: 0.6% or more 5% or less, Mo: 0.02% to 0.4% and sol. Al: containing 0.2% or less, the balance having a chemical composition consisting of Fe and impurities, and M23C6 (where M consists of Cr, Mo, Mn and Fe) in the X-ray diffraction test ( 422) A steel material for heat treatment, characterized in that the ratio (IM / Iθ) of the peak intensity (IM) of the plane to the peak intensity (Iθ) of the (211) plane of cementite is 0.3 or less.

  (2) The chemical composition contains one or two selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in mass% instead of a part of Fe. The steel for heat treatment as set forth in (1) above, wherein

  (3) The chemical composition is selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less in mass% instead of a part of Fe. The steel material for heat treatment as described in (1) or (2) above, which contains one kind or two or more kinds.

  According to the present invention, even when spheroidizing annealing is performed on a Cr or Mo-containing steel material, the carbide re-dissolves easily during the heat treatment and the residual carbide is effectively reduced. Can do. Therefore, the heat-treating steel material according to the present invention achieves lowering and shorter heating conditions, has high productivity, and is economical.

The chemical composition and structure of the heat-treating steel material of the present invention and the production method thereof will be described below.
1. Chemical composition of steel The chemical composition of the steel according to the present invention will be described. In the following description, “%” indicating the chemical composition of steel means “% by mass” unless otherwise specified.

(1) C: 0.4% or more and 0.7% or less C is an important element that determines the strength of the steel material after the heat treatment. When the C content is less than 0.4%, the strength of the steel material after the heat treatment is low, and the high strength required for the steel part targeted by the present invention cannot be obtained. Therefore, the C content is 0.4% or more. On the other hand, if the C content exceeds 0.7%, the strength after heat treatment becomes too high and the deterioration of toughness becomes remarkable, or the absolute amount of carbide increases, so that residual carbides are likely to occur, and toughness and fatigue Reducing the strength. Therefore, the C content is 0.7% or less.

(2) Si: 0.5% or less Although Si is an element generally contained as an impurity, it has an action of improving hardenability, and therefore may be positively contained. However, if the Si content exceeds 0.5%, the workability deterioration due to the strength increase of the steel material before the heat treatment becomes remarkable. Therefore, the Si content is 0.5% or less.

(3) Mn: 0.2% or more and 1.2% or less Mn is an element having an effect of improving hardenability, and is an element effective for ensuring the strength of the steel material after the heat treatment. If the Mn content is less than 0.2%, it is difficult to obtain an effect of improving hardenability. Therefore, the Mn content is 0.2% or more. On the other hand, if the Mn content exceeds 1.2%, the workability deterioration due to the strength increase of the steel material before the heat treatment becomes remarkable, or the toughness deterioration accompanying the strength increase of the steel material after the heat treatment becomes remarkable. In addition, residual carbide is easily generated by inhibiting re-dissolution of carbides in the heat treatment, which may reduce the toughness and fatigue strength of the steel material after the heat treatment. For this reason, Mn content shall be 1.2% or less. Preferably it is 0.8% or less.

(4) P: 0.05% or less P is an element contained as an impurity and has an action of reducing the toughness of the steel material after heat treatment. Therefore, the lower the P content, the better. When the P content exceeds 0.05%, the toughness deterioration of the steel material after heat treatment becomes significant. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less, More preferably, it is 0.015% or less.

(5) S: 0.02% or less S is an element contained as an impurity, and has the effect of reducing the toughness of the steel material after heat treatment. Therefore, the lower the S content, the better. If the S content exceeds 0.02%, the toughness deterioration of the steel material after heat treatment becomes significant. Therefore, the S content is 0.02% or less. Preferably it is 0.01% or less, More preferably, it is 0.005% or less.

(6) Cr: 0.6% or more and 1.5% or less Cr is an element having an effect of improving the hardenability, and is an important element for securing the strength of the steel material after the heat treatment. If the Cr content is less than 0.6%, the effect of improving the hardenability may not be sufficiently obtained. Therefore, the Cr content is 0.6% or more. On the other hand, if the Cr content exceeds 1.5%, the spheroidizing of the carbide is inhibited in the spheroidizing annealing performed to improve the workability of the steel material before the heat treatment, so the workability of the steel material before the heat treatment is spheroidized. It becomes difficult to improve by annealing. In addition, residual carbides are likely to be generated due to inhibition of carbide re-dissolution during heat treatment, which may reduce the toughness and fatigue strength of the steel material after heat treatment. Therefore, the Cr content is 1.5% or less. Preferably it is 1.2% or less.

(7) Mo: 0.02% or more and 0.4% or less Mo is an element having an effect of improving the hardenability, and is an important element for securing the strength of the steel material after the heat treatment. Furthermore, since it has the effect | action which suppresses tempering brittleness, it is an effective limit for improving the toughness of the steel materials after heat processing. If the Mo content is less than 0.02%, the effect of the above action may not be sufficiently obtained. Therefore, the Mo content is 0.02% or more. On the other hand, if the Mo content exceeds 0.4%, the effect due to the above action is saturated, leading to an increase in cost. Therefore, the Mo content is 0.4% or less. Preferably it is 0.3% or less.

(8) sol. Al: 0.2% or less Since Al is an element having an action of deoxidizing molten steel, it may be positively contained. However, sol. If the Al content exceeds 0.2%, the effect due to the above action is saturated, leading to an increase in cost. Further, Al is so has the effect of raising the _three points A, making it difficult to heat treatment inviting high temperature for a long time of heating conditions necessary for thermal processing. Therefore, sol. The Al content is 0.2% or less. Preferably it is 0.1% or less. Since Si can also be used for deoxidation of molten steel, sol. The lower limit of the Al content need not be specified. However, since the deoxidizing ability of Al is higher than that of Si, sol. The Al content is preferably 0.005% or more.

(9) Cu: 1.0% or less, Ni: 1.0% or less Cu and Ni are optional elements, have the effect of improving hardenability, and are effective in ensuring the strength of the steel material after heat treatment. Therefore, you may contain 1 type or 2 types. However, even if each content exceeds 1.0%, the effect due to the above effect is saturated, leading to an increase in cost. Therefore, each content is made 1.0% or less.

(10) Ti: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less Ti, Nb, and V are optional elements, have the effect of improving hardenability, and are further carbides. In addition, one or two or more of them may be contained because they have the action of forming a nitride and suppressing austenite grain growth during the heat treatment to improve the toughness of the steel material after the heat treatment. However, the excessive content promotes the precipitation of fine carbides, and the workability deterioration accompanying the increase in the strength of the steel before heat treatment becomes remarkable. Therefore, the content of each element is 0.1% or less.

(11) Others By setting the value of C / (Cr × Mo) (where each element symbol indicates the content of each element in steel (unit: mass%)) to 3.0 or more, It is possible to effectively suppress the formation of M ₂₃ C ₆ in which re-dissolution is delayed as compared with cementite, and to facilitate the re-dissolution of carbide during the heat treatment of the Cr and Mo-containing steel material, It becomes easy to reduce effectively. Therefore, C, Cr and Mo are preferably contained so as to satisfy the following formula (1).
C / (Cr × Mo) ≧ 3.0 (1)
Thus, it is possible to effectively suppress the formation of M ₂₃ C _6, Cr, it is easier to general carbon steel and comparable to redissolved behavior of carbides Mo-containing steels.

2. The peak intensity ratio between M ₂₃ C ₆ and cementite in the X-ray diffraction test In the present invention, it is a carbide unique to Cr and Mo-containing steel materials, and suppresses the formation of M ₂₃ C ₆ in which re-dissolution is delayed in heat treatment. As in the case of carbon steel, carbide is made substantially cementite, so even if the carbide is spheroidized by spheroidizing annealing, the carbide re-dissolves easily during the heat treatment. And making it possible to effectively reduce residual carbides.

Therefore, the ratio (I _M / I _θ ) between the peak intensity (I _M ) of the (422) plane of M ₂₃ C ₆ and the peak intensity (I _θ ) of the (211) plane of cementite in the X-ray diffraction test is 0. .3 or less. When the ratio (I _M / I _θ ) exceeds 0.3, the ratio of M ₂₃ C ₆ is large, so that re-solution dissolution in the heat treatment is delayed, and the re-solution of carbide during the heat treatment easily proceeds, It becomes difficult to effectively reduce the residual carbide.

3. Production method The steel material for heat treatment of the present invention may be produced by any production method as long as it satisfies the chemical composition and the peak intensity ratio of M ₂₃ C ₆ and cementite in the X-ray diffraction test.

For example, molten steel melted to the above chemical composition can be produced as a steel ingot by continuous casting, or as a steel slab by partial rolling after casting, and as a hot-rolled steel sheet by further hot rolling it can. The melting method and the like at this time may be conventional methods, but from the viewpoint of productivity, the casting process is preferably a continuous casting method. The hot-rolled steel sheet may be further annealed or pickled. Furthermore, annealing may be performed after cold rolling, or cold rolling may be performed after annealing. The annealing may be performed by continuous annealing or box annealing. However, if the annealing temperature is too low, M ₂₃ C ₆ tends to precipitate, and therefore the annealing temperature is preferably 600 ° C. or higher, more preferably 650 ° C. or higher. From the viewpoint of improving the workability of the steel sheet before the heat treatment, it is preferable that the final annealing is spheroidizing annealing by box annealing.

  Using a laboratory vacuum furnace, steel having the chemical composition shown in Table 1 was melted at the laboratory level. These cast ingots are forged into 20 mm thick hot rolling slabs, soaked at 1200 ° C. and hot rolled to finish 4 mm thick, simulating forced air cooling or water spray in a hot strip mill. The steel sheet was cooled to 600 ° C. under cooling conditions, immediately put into a furnace maintained at a temperature of 600 ° C., and cooled in the furnace to simulate the cooling after winding to obtain a hot rolled steel sheet. The hot-rolled steel sheet thus obtained was held at a temperature of 700 ° C. for 30 hours in an Ar atmosphere, and then allowed to cool, and both front and back surfaces were ground 1 mm at a time to produce a hot-rolled annealed sheet having a thickness of 2 mm. .

After chemically polishing the surface of the hot-rolled annealed plate, an X-ray diffraction test was performed, and the peak intensity (I _M ) of the (422) plane of M ₂₃ C ₆ and the peak intensity (I _θ ) of the (211) plane of cementite The ratio (I _M / I _θ ) was investigated.

  Next, the hot-rolled annealed plate was cut into 20 mm × 30 mm, held at a temperature of 900 ° C. in an Ar atmosphere for 10 to 80 minutes, and then water quenched. The steel plate after water quenching was cut, the cross-section was mirror-polished, and further Picral etching was performed, and then the SEM observation was performed at a 1/4 depth position of the plate thickness at 2000 times. SEM photographs taken using image analysis software were analyzed, and the area ratio of residual carbides was measured.

The ratio (I _M / I _θ ) between the peak intensity (I _M ) of the (422) plane of M ₂₃ C ₆ before heat treatment and the peak intensity (I _θ ) of the (211) plane of cementite, and residual carbides after the heat treatment The change in area ratio is shown in Table 2. Furthermore, in order to quantify the dissolution behavior of carbide during heating, the ratio of residual carbide after heating for 80 minutes to the residual carbide area ratio after heating for 10 minutes was determined, and the dissolution rate during this period was determined.

  Table 2 shows the results together.

The ratio (I _M / I _θ ) between the peak intensity (I _M ) of the (422) plane of M ₂₃ C ₆ and the peak intensity (I _θ ) of the (211) plane of cementite is 0.3 or less, that is, M ₂₃ C _{In the} example of the present invention in which the formation of ₆ was suppressed, the dissolution rate was all 90% or more, whereas the ratio (I _M / I _θ ) exceeded 0.3, that is, M ₂₃ C ₆ was large. In the produced comparative example, it was less than 10%. In this way, by suppressing the formation of M ₂₃ C _6, redissolved carbide in the heat treatment is promoted, residual carbides is reduced.

Claims (3)

% By mass, C: 0.4% or more 56 % or less, Si: 0.5% or less, Mn: 0.2% or more and 1.2% or less, P: 0.05% or less, S: 0.02% or less, Cr: 0.6% or more 5% or less, Mo: 0.02% to 0.4% and sol. Al: 0.2% or less, with the balance being a chemical composition consisting of Fe and impurities, M422C (where M consists of Cr, Mo, Mn and Fe) in the X-ray diffraction test (422) ) Plane peak intensity (IM) and cementite (211) plane peak intensity (Iθ) ratio (IM / Iθ) is 0.3 or less, a steel for heat treatment.   The chemical composition contains one or two selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in mass%, instead of a part of Fe. The steel material for heat treatment according to claim 1.   The chemical composition may be one selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less in mass%, instead of part of Fe. The steel for heat treatment according to claim 1 or 2, comprising two or more kinds.