JP2004183065A - High strength steel for induction hardening, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel for induction hardening which has excellent induction hardenability, and further has excellent high strength properties, and to provide a production method therefor. <P>SOLUTION: The high strength steel for induction hardening has a composition comprising, by mass, 0.35 to 0.6% C, 0.01 to 1% Si, 0.2 to 2% Mn, 0.005 to 0.15% S, ≤0.35% (inclusive of 0%) Cr, 0.0005 to 0.005% B, 0.015 to 0.05% Al and 0.05 to 0.2% Ti, and in which the contents of N, P and O are limited, and the balance iron with inevitable impurities is included. In the steel, precipitates of Ti with a diameter of ≤0.2 μm are comprised in ≥5 pieces/μm<SP>2</SP>in the matrix of the structure after hot rolling, the ferrite crystal grain size is ≤20 μm, the ferrite fraction is ≤30%, and decarburization depth prescribed in JIS G 0558 is controlled to ≤0.2 mm DM-T. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００６８】
【表２】

【００６９】
【発明の効果】
本発明の高強度高周波焼入れ用鋼材とその製造方法を用いれば、冷間鍛造や切削加工時には冷間加工性に優れ、高周波焼入れ後には靭性に富んだ微細な組織が得られ、脆性破壊が抑制され、硬化層の硬さに見合った優れた強度特性を得ることが可能となり、産業上の効果は極めて顕著なるものがある。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength steel for induction hardening and a method for producing the same.
[0002]
[Prior art]
Various gears and shafts manufactured in the induction hardening process have a strong tendency to increase in strength in response to the recent increase in output of automobile engines or compliance with environmental regulations. Further, there is a strong demand for reduction of heat treatment distortion generated at the time of induction hardening in order to omit the polishing step and the correction step.
[0003]
Conventional induction hardened steels include C: 0.35 to 0.6%, Si: 0.01 to 0.15%, Mn: 1.0 to 1.8%, Mo: 0.05 to 0.8. %, S: 0.01 to 0.15%, Al: 0.015 to 0.05%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0. 002-0.01%, the amount of P, Cu, O is limited to a specific amount or less, or further contains one or two types of Nb and V, or one type of Cr and Ni Or a microstructure comprising ferrite and lamellar perlite, wherein the ferrite has a microstructure fraction of 35% or less and a ferrite crystal grain size of 20 μm or less. Excellent in workability, suitable for induction hardened shaft parts with excellent torsional fatigue strength after induction hardening Steel material (for example, Patent Document 1), C: more than 0.38 to 0.58%, Si: 0.01 to 0.15%, Mn: 0.85 to 1.7%, S: 0.005 to 0.15%, Cr: 0.35% or less, B: 0.0005 to 0.005%, Al: 0.015 to 0.05%, N: less than 0.007%, N containing Ti Depending on the amount, it is contained in the range of 0.015 to 3.4N + 0.02%, or one or two of Mo: 0.02 to 0.3% and Ni: 0.02 to 1.0%. And the microstructure is substantially a ferrite-pearlite structure, the ferrite crystal grain size is 25 μm or less, and the score of the ferrite band of the structure having a cross section parallel to the hot rolling direction is 1 to 5. Steel. In addition, the steel of the above component is heated at a heating temperature of 1050 ° C. or higher, a hot rolling finish temperature of 750 to 1000 ° C., and subsequently to a temperature range of 750 to 500 ° C. at a cooling rate of 1 ° C./sec or less following the hot rolling. There is a steel material for induction hardening which is excellent in cold workability due to a cooling method and has excellent high strength characteristics and low heat treatment distortion characteristics after induction hardening, and a method for producing the same (for example, Patent Document 2).
[0004]
Although these steel materials are designed to have high strength characteristics by specifying the steel composition and microstructure, the high strength characteristics are not yet sufficiently satisfactory.
[0005]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 8-283910 [Patent Document 2]
JP-A-11-236644
[Problems to be solved by the invention]
An object of the present invention is to provide a steel material for high-strength induction hardening, which is excellent in high-frequency hardenability and has high strength properties, and a method for producing the same.
[0007]
[Means to solve the problem]
The inventor of the present invention has intensively studied to solve the above problems. If the hardness of the induction hardened hardened layer is increased or the depth of the hardened layer is increased in order to increase the strength of the induction hardened material, the toughness will deteriorate and brittle fracture will occur. Deteriorates conversely. In order to suppress the brittle fracture and obtain the strength according to the hardness, the point is to reduce the prior austenite grain size of the induction hardened hardened layer and to improve the toughness of the induction hardened hardened layer. In order to reduce the prior austenite grain size of the induction hardened hardened layer, it is necessary to finely disperse a large amount of TiC-based precipitates before the induction hardening. As a result of intensive studies on a steel material manufacturing method for realizing this, the steel composition was selected so that TiC-based fine precipitates were formed, and after casting, slab rolling was performed without cooling to a temperature of A3 or less. The present invention was completed by finding that a high-strength induction hardened steel material in which a large amount of TiC-based precipitates were finely dispersed was obtained by performing low-temperature heating rolling using the obtained steel slab.
[0008]
The gist of the present invention is as follows.
[0009]
(1) In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05-0.2%
Containing
N: less than 0.007% (including 0%),
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
And the balance consists of iron and unavoidable impurities. The matrix of the structure after hot rolling has at least 5 precipitates of Ti having a diameter of 0.2 μm or less per 5 μm ² or more, and a ferrite crystal grain size. Is not more than 20 μm, the ferrite fraction is not more than 30%, and the decarburization depth specified by JIS G 0558: DM-T is 0.2 mm or less.
[0010]
(2) In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05-0.15%,
Nb: 0.002 to 0.05%
Containing
N: less than 0.007% (including 0%)
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
And the balance consists of iron and unavoidable impurities, and in the matrix of the structure after hot rolling, a precipitate of Nb with a diameter of 0.2 μm or less, a precipitate of Ti, or a composite composition of Nb and Ti The total number of the precipitates is 5 / μm ² or more, the ferrite crystal grain size is 20 μm or less, the ferrite fraction is 30% or less, and the decarburization depth specified by JISG 0558: DM-T0. A high-strength induction hardening steel material having a thickness of 2 mm or less.
[0011]
(3) In mass%,
Mo: 1% or less,
Ni: 2.5% or less V: 0.4% or less The steel material for high-strength induction hardening according to the above (1) or (2), which contains one or more of V and 0.4% or less.
[0012]
(4) In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05 to 0.15%
Containing
N: less than 0.007% (including 0%)
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
The steel is made by a process of performing slab rolling without cooling the steel consisting of iron and unavoidable impurities to a temperature below the A3 point after casting. The heating temperature is 900 to 1070 ° C. Hot rolling to 800 or 970 ° C., followed by hot rolling, followed by hot rolling to 800 or 500 ° C. in a temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./sec or less, to a wire or a bar; In the matrix of the structure after hot rolling, precipitates of Ti having a diameter of 0.2 μm or less are set to 5 / μm ² or more, the ferrite crystal grain size is 20 μm or less, the ferrite fraction is 30% or less, and JIS G 0558. Decarburization depth specified by: DM-T 0.2 mm or less, a method for producing a high-strength induction hardened steel material.
[0013]
(5) In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05-0.15%,
Nb: 0.002 to 0.05%
Containing
N: less than 0.007% (including 0%),
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
The steel is made by a process of performing slab rolling without cooling the steel consisting of iron and unavoidable impurities to a temperature below the A3 point after casting. The heating temperature is 900 to 1070 ° C. Hot rolling to a wire rod or a steel bar under the condition that the finishing temperature of hot rolling is 800 to 970 ° C., and the temperature range of 800 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./sec or less following the hot rolling. A total of 5 precipitates of Nb, Ti precipitates, or precipitates composed of a composite composition of Nb and Ti having a diameter of 0.2 μm or less in the matrix of the structure after hot rolling, and a total of 5 precipitates / μm ² or more; A ferrite crystal grain size of 20 μm or less, a ferrite fraction of 30% or less, and a decarburization depth specified by JIS G 0558: DM-T of 0.2 mm or less. Manufacture Law.
[0014]
(6) In mass%,
Mo: 1% or less,
Ni: 2.5% or less V: 0.4% or less The method for producing a steel material for high-strength induction hardening as described in (4) or (5) above, wherein one or two of V: 0.4% or less are contained. .
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0016]
In the present invention, when a steel slab manufactured by casting a steel by HCR (Hot Charge Rolling) and then performing slab rolling without cooling to a temperature below A3 point is used, and low-temperature heating and rolling is performed at the time of bar rod wire rolling, fine precipitation of TiC is obtained. The material is kept in a state where it is deposited as it is. As a result, it is effective for making the former austenite grain size of the induction hardened material thereafter fine. Further, when the low-temperature heating rolling is performed, the amount of precipitation strengthening is reduced as compared with the normal heating rolling and the high-temperature heating rolling that involve solution of TiC. Also, the total decarburization amount can be reduced, and the decarburization depth: DM-T 0.2 mm or less specified in JIS G 0558 can be achieved.
[0017]
As a result, the former austenite grain size of the induction hardened material can be reduced, and the high strength characteristics of the induction hardened material can be exhibited by suppressing brittle fracture. First, the reasons for limiting the components will be described.
[0018]
C is an effective element for giving the necessary strength to steel, but if it is less than 0.35%, the required strength cannot be secured, and if it exceeds 0.6%, it becomes hard and cold workable. , The toughness after induction hardening is remarkably deteriorated, and the strength is rather deteriorated. For the above reasons, the C content needs to be in the range of 0.35 to 0.6%. The preferred range is 0.4-0.56%.
[0019]
Si is an element effective for deoxidizing steel, and is also an element effective for imparting necessary strength and hardenability to steel and improving tempering softening resistance. In particular, it has a remarkable effect of increasing the grain boundary strength of the induction hardened material, suppressing grain boundary fracture, and increasing the strength through improvement of toughness. However, if the content is less than 0.01%, the effect is insufficient. On the other hand, if it exceeds 1%, the hardness is increased and the cold workability is deteriorated. For the above reasons, the content needs to be within the range of 0.01 to 1%. In addition, a preferable range when the cold workability is particularly important is 0.01 to 0.15%. In addition, a preferable range when the high strength property of the induction hardened material is emphasized is 0.2 to 1%, and a preferable range when the high strength property of the induction hardened material is particularly emphasized is 0.4 to 1%. 1%.
[0020]
Mn is an element effective for improving induction hardenability. In order to ensure sufficient hardenability with emphasis on high strength characteristics and low heat treatment distortion characteristics, the effect is insufficient if less than 0.2%. On the other hand, if it exceeds 2%, the hardness will be remarkably increased and the cold workability will be degraded, so it is necessary to be within the range of 0.2% to 2%.
[0021]
S forms MnS in steel and is added for the purpose of improving machinability. However, if less than 0.005%, its effect is insufficient. On the other hand, when the content exceeds 0.15%, the effect is saturated, and rather, grain boundary segregation is caused to cause grain boundary embrittlement. For the above reasons, the content of S needs to be in the range of 0.005 to 0.15%. The preferred range is 0.005 to 0.04%.
[0022]
Cr is an element effective for improving hardenability. However, Cr stabilizes the cementite by forming a solid solution in the cementite. For this reason, poor penetration of cementite is likely to occur during short-time heating in induction hardening, which causes uneven hardness. This behavior becomes remarkable especially when it exceeds 0.35%. For the above reasons, it is necessary to limit the content to 0.35% or less (including 0%). A preferred range is 0.15% or less. In order to strictly suppress unevenness in hardness, it is desirable to limit Cr to 0.15% or less.
[0023]
B is added aiming at the following two points. (1) At the time of induction hardening, the steel is given hardenability. {Circle around (2)} By improving the grain boundary strength of the induction hardened material, the strength is dramatically improved through improvement of toughness. If the addition is less than 0.0005%, the above effect is insufficient, and if it exceeds 0.005%, the effect is saturated, so the content needs to be within the range of 0.0005 to 0.005%. There is. The preferred range is 0.001 to 0.003%.
[0024]
Al is added as a deoxidizing agent. If it is less than 0.015%, the effect is insufficient. On the other hand, when the content exceeds 0.05%, AlN remains without being solutionized at the time of rolling and heating, and becomes a precipitation site of a precipitate of Ti, thereby deteriorating cold workability. For the above reasons, the content needs to be in the range of 0.015 to 0.05%. The preferred range is 0.02 to 0.04%.
[0025]
It is desirable to limit N as much as possible for the following two reasons. {Circle around (1)} B is added for the purpose of improving hardenability and strengthening the grain boundary as described above. However, the effect of these B is exhibited only in the state of solid solution B in steel, so the N content is reduced. Therefore, it is essential to suppress the generation of BN. {Circle around (2)} When N combines with Ti in steel, it forms coarse TiN which hardly contributes to grain control, and this is a precipitation site of NbC, Nb (CN) mainly composed of NbC and TiC, Ti (CN) mainly composed of TiC. This hinders the fine precipitation of these carbonitrides of Ti and Nb and adversely affects the refinement of the prior austenite grains of the induction hardened material. The above adverse effects are particularly remarkable when the N content is 0.007% or more. For the above reasons, it is necessary to limit the content to less than 0.007% (including 0%). A preferred range is 0.005% or less.
[0026]
Ti is the most important component in the present invention, and has an effect of binding with N to form TiN, fix solid solution N, and suppress the formation of BN. For this reason, the effect of increasing the induction hardening property by B added to the steel can be exhibited. In addition, when the steel is cast and slab-rolled without being cooled to the A3 point or less, Ti combines with C and N to form TiC and Ti (CN), and these precipitates are finely dispersed in the steel slab. I do. By performing bar rod wire rolling by low-temperature heating rolling using the steel slab, a large amount of Ti-based precipitates can be finely dispersed in the steel of the bar steel rod. This makes it possible to reduce the prior austenite grain size after induction hardening. In order to increase the strength of the induction hardened material, it is effective to increase the hardness of the induction hardened hardened layer.However, according to conventional studies, the toughness of the hardened layer deteriorates, causing brittle fracture. Instead, the strength often deteriorated. On the other hand, when a large amount of Ti-based precipitates are finely dispersed in the steel as described above to reduce the prior austenite grain size after induction hardening, toughness is improved and brittle fracture is suppressed, and as a result, It is possible to realize an unprecedented level of high strength.
[0027]
If the Ti content is less than 0.05%, it is insufficient to generate a large amount and fineness of TiC and Ti (CN), and if it exceeds 0.2%, precipitation hardening due to TiC becomes remarkable, and cold forging cracking occurs. This is not preferred because it causes
[0028]
Therefore, the content of Ti is set to 0.05 to 0.2%. The preferred range is 0.06-0.15%. Further, a preferable range for further reducing the size of the induction hardened material to improve toughness and increasing the strength is 0.07 to 0.15%.
[0029]
Since P is an element that increases the deformation resistance during cold forging and deteriorates toughness, cold workability deteriorates. In addition, the fatigue strength of the final product is degraded by embrittlement of the crystal grain boundaries of the component after induction hardening and tempering, so that it is desirable to reduce the fatigue strength as much as possible. Therefore, it is necessary to limit the content to 0.025% (including 0%). A preferred range is 0.015% or less.
[0030]
O forms oxide inclusions such as Al ₂ O _{3 in} steel. When a large amount of oxide-based inclusions is present in steel, the cold workability deteriorates. When the O content exceeds 0.0025%, the tendency becomes particularly remarkable. For the above reasons, it is necessary to limit the content to 0.0025% or less (including 0%). The preferred range is 0.002% or less.
[0031]
Nb has an effect similar to that of Ti, forms Nb (CN) by combining with C and N in steel, and is an element effective in increasing the strength through fine crystal grains of the induction hardened hardened layer. If less than 0.002%, the effect is insufficient. On the other hand, when the content exceeds 0.05%, the hardness of the material becomes hard and the cold workability deteriorates. For the above reasons, the content needs to be within the range of 0.002 to 0.05%. The preferred range is 0.005 to 0.04%. Further, a preferable range in which the grain size of the induction hardened material is further reduced to improve the toughness and increase the strength is 0.02 to 0.04%.
[0032]
Next, in the present invention, one, two or more of Mo, Ni, and V can be contained.
[0033]
Mo is an element that imparts strength and hardenability to steel, increases grain boundary strength of the induction hardened material, suppresses grain boundary fracture, and has a remarkable effect on increasing strength through improvement of toughness. However, if it is added in excess of 1%, the hardness is increased and the cold workability is deteriorated.
[0034]
Ni is also an element effective in imparting strength and hardenability to steel, but if added in excess of 2.5%, the hardness is increased and the cold workability is degraded. V is also an effective element for imparting strength and hardenability to steel, but if added in excess of 0.4%, hardness is increased and cold workability is degraded.
[0035]
Next, in the present invention, the matrix after hot rolling has 5 precipitates / μm ² or more of Ti having a diameter of 0.2 μm or less, and the reason for such limitation is described below.
[0036]
In order to reduce the former austenite grain size of the induction hardened material, it is effective to disperse a large amount and finely of the particles that pin the crystal grain boundaries. This is preferable because the number of pinning particles increases. In the present invention, the precipitate of Ti refers to TiC, TiN, Ti (CN). When fine precipitates of Ti having a diameter of 0.2 μm or less after hot rolling are dispersed at a total of 5 / μm ² or more, crystal grains are kept fine in a high-frequency heating temperature range, and after high-frequency quenching, A structure having excellent strength toughness having a fine prior austenite grain size is obtained. If the total number of fine precipitates is less than 5 / μm ² , such an effect is small. For the above reasons, it is necessary that 5 precipitates / μm ² or more of Ti having a diameter of 0.2 μm or less are dispersed in the matrix. A preferable range is 10 pieces / μm ² or more.
[0037]
Further, the reason why the microstructure is such that the ferrite crystal grain size is 20 μm or less and the ferrite fraction is 30% or less is that the grain size and the ferrite fraction are specified from the viewpoint of induction hardening. Because induction quenching is rapid heating, if the ferrite in the structure before induction quenching is coarse, the part of the ferrite, after austenitizing, the diffusion of carbon is insufficient, and the carbon concentration is lower than the added carbon concentration. After quenching, the hardness at that position decreases.
[0038]
When the ferrite crystal grain size exceeds 20 μm and the ferrite fraction exceeds 30%, the hardness unevenness after quenching becomes particularly remarkable, and it is easy to break down with low strength.
[0039]
The ferrite crystal grain size referred to in the present invention corresponds to a circle-equivalent diameter when the form of the ferrite is granular, and corresponds to a width when the form of the ferrite is plate-like. In the present invention, the structure other than ferrite is desirably pearlite or bainite, but there is no particular problem even if martensite is partially mixed.
[0040]
In addition, the decarburization depth is closely related to the strength. If the decarburization depth is large, the strength is reduced, and sufficient hardness of the surface layer cannot be obtained. In the present invention, by performing low-temperature heating rolling described below, the decarburization depth specified by JIG G0558: DM-T can be 0.2 mm or less, and can be made shallower than the conventional decarburization depth. High strength induction hardened steel is obtained. For this reason, in the present invention, the decarburization depth: DM-T defined by JIS G 0558 is limited to 0.2 mm or less.
[0041]
Next, the hot rolling conditions will be described.
[0042]
The steel comprising the component of the present invention is melted by a usual method such as a converter, an electric furnace, etc., the components are adjusted, and after casting, a slab rolling process is performed without cooling to a point A3 or lower to obtain a wire or a bar. The following low-temperature heat rolling is performed.
[0043]
That is, the hot rolling is performed at a finishing temperature of 800 to 970 ° C. at a heating temperature of just above the Ar ₃ point of 900 to 1070 ° C. Subsequent to the hot rolling, hot working is performed on the wire or the bar under the condition that the temperature is slowly cooled in a temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./sec or less.
[0044]
The heating temperature is set to a temperature just above the Ar ₃ point of 900 to 1070 ° C. so that fine TiC precipitates generated by slab rolling without cooling to a temperature below the A3 point after casting do not form a solid solution in the matrix. If the temperature is lower than 900 ° C., the rolling temperature is in a ferrite region, which is not preferable. If the temperature exceeds 1070 ° C., the precipitates are not preferable because they form a solid solution in the matrix. The heating temperature was set to 900 to 1070 ° C. in order to maintain a fine TiC precipitate as it is and to obtain a fine structure during induction hardening. When the heating temperature exceeds 1070 ° C., the total decarburization becomes remarkable. Therefore, from the viewpoint of suppressing the total decarburization, the heating temperature is limited to this temperature range.
[0045]
Next, the finishing temperature of the hot rolling is set to 800 to 970 ° C. for the following reason. If the finishing temperature is lower than 800 ° C., the decarburization of the rolled material proceeds, so that the total decarburization becomes conspicuous, and the surface hardness of the induction hardened material does not appear. On the other hand, when the finishing temperature exceeds 970 ° C., the ferrite grain size of the rolled material becomes coarse, and the induction hardening property is deteriorated. For the above reasons, the finishing temperature of the hot rolling is set to 800 to 970 ° C. The preferred temperature is 850-960 ° C.
[0046]
Next, after the hot rolling, the temperature range of 800 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./sec or less for the following reason. When the cooling rate exceeds 1 ° C./sec, the hardness of the rolled material increases, and the cold forgeability and cold workability deteriorate. Therefore, the cooling rate is limited to 1 ° C./sec or less. A preferred range is 0.7 ° C./sec or less. In addition, as a method of reducing the cooling rate, there is a method of installing a heat insulating cover or a heat insulating cover with a heat source behind the rolling line, thereby performing slow cooling.
[0047]
In the present invention, the size of the slab and the cooling rate during solidification are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied. In addition, when the steel of the present invention is not only a step of forming an as-rolled steel bar into a part by cold forging or cutting, but also undergoes an annealing step or warm / hot forging before cold forging or cutting, The present invention can also be applied to a case where a component is formed in a warm / hot forging process and a case where a component is entirely formed in a cutting process.
[0048]
【Example】
Hereinafter, the effects of the present invention will be more specifically described with reference to examples.
[0049]
Continuously molten converter steel having the composition shown in Table 1 was cast, and after casting, the steel was subjected to slab rolling without cooling to a temperature below the A3 point to obtain a 162 mm square steel slab (rolled material) Rolling conditions I). For the comparative steels c and d, after continuous casting, the steel was once cooled to room temperature, and then heated again to the point A3 or higher and subjected to slab rolling to obtain a 162 mm square steel slab (rolled material). Condition II).
[0050]
Subsequently, a steel bar having a diameter of 37 to 44 mm was manufactured by hot working. Table 2 shows the hot working conditions. The cooling rate after hot working was adjusted using a slow cooling cover installed on a cooling floor. Comparative steels a and b are JIS S45C and S53C.
[0051]
In order to examine the dispersion state of the precipitates of Ti and Nb in the steel bars after the hot working, the precipitates present in the matrix of the steel bars were sampled by the extraction replica method and observed with a transmission electron microscope. Observation was performed at about 30,000 magnifications in about 20 visual fields, and the number of precipitates of Ti, Nb, and composites of Ti and Nb having a diameter of 0.2 μm or less in one visual field were counted. It was converted to a number.
[0052]
The structure of the bar after rolling was observed, and the ferrite grain size and the ferrite fraction were measured. Further, the Vickers hardness of the bar steel after rolling was measured. Hardness was used as an index of cold forgeability and cold rollability. The total decarburization depth was also investigated.
[0053]
Next, the machinability of the as-rolled steel bars was evaluated using the life speed of a high-speed drill. The drill used was a high-speed drill having a diameter of 3 mm according to JIS-SKH51. The drilling conditions were 0.25 mm / rev for feed, 9 mm hole depth, and 2 liters / minute using spindle oil as cutting oil. In the evaluation test, a cutting-drill life curve was obtained from a drill life until cutting became impossible at various cutting speeds at various cutting speeds, and this was defined as a life speed.
[0054]
Next, the as-rolled steel bars were induction hardened at a frequency of 8.5 kHz, and then tempered at 170 ° C. for 1 hour. The hardness of the material was measured from the outermost surface, and this was defined as induction hardening hardness.
[0055]
In addition, a static torsion test piece having a parallel portion of 20 mm was collected from the as-rolled steel bar. Notches having a depth of 2 mm and a curvature of 1.0 R at the tip were provided above and below the center of the parallel portion of the test piece. This test piece was subjected to induction hardening at a frequency of 8.5 kHz, and then tempered at 170 ° C. for 1 hour. For the induction hardened material, the depth of the hardened layer and the prior austenite grain size of the hardened layer were measured. In addition, a static torsional test was performed to evaluate the static torsional strength.
[0056]
Table 2 shows the results of these investigations together with the hot working conditions. The high-speed drill life was indicated by a relative value when the drill life of Comparative Example 28 (S53C) was set to 100. Further, the fracture mode of the test piece in the static torsional strength test was described as ductility when fractured by shear ductility, and brittle when fractured brittlely due to main stress.
[0057]
Comparative Examples 27 and 28 are the characteristics of S45C and S53C of JIS, but the cutability of the present invention example is substantially equal to or better than that of Comparative Examples 27 and 28 when compared with the same amount of carbon. Shows sex. Next, when compared with the same amount of carbon, the present invention example is higher in induction hardening hardness than Comparative Examples 27 and 28. Further, the γ grain size is remarkably fine, and the static torsional fractures are all broken in the ductile mode, and excellent characteristics of torsional strength are obtained.
[0058]
Next, in Table 2, Comparative Example 20 is a case where the content of C is lower than the range specified in the present application, and the torsional strength characteristics of the induction hardened material are inferior to those of the present invention. Comparative Example 21 was a case where the content of C was more than the range specified in the present application, the cutability of the rolled material was poor, and the brittle fracture was exhibited in the torsional test of the induction hardened material. And the torsional strength characteristics are poor.
[0059]
Comparative Example 22 is a case where the content of Cr exceeded the range specified in the present application, and the penetration defect of carbide was remarkable during induction hardening, so that the induction hardening hardness was lower and the depth of the hardened layer was shallower. The torsional strength characteristics are inferior to the invention examples.
Comparative Example 23 is a case where the content of B was less than the range specified in the present application, the hardened layer depth of the induction hardened material was shallow, and in the torsion test of the induction hardened material, brittle fracture was exhibited. The torsional strength characteristics are inferior in comparison.
[0060]
Comparative Example 24 is a case where the content of Ti is lower than the range specified in the present application, and Comparative Example 26 is a case where the content of N is higher than the range specified in the present application. Is insufficient, the austenite grains of the hardened layer of the induction hardened material are not sufficiently refined, exhibit brittle fracture in the torsional test of the induction hardened material, and have a torsional strength characteristic compared to the present invention example. Inferior.
[0061]
Comparative Example 25 is a case where the content of Ti exceeds the range specified in the present application, and the hardness as-rolled becomes hard and the machinability is inferior. In the torsion test of the induction hardened material, the brittle fracture due to embrittlement due to TiN is exhibited, and the torsional strength characteristics are inferior to those of the examples of the present invention.
[0062]
Comparative Examples 29 and 30 are different from the present invention in a method of manufacturing a billet, and are a case where after casting, the steel is once cooled to a temperature not higher than the A3 point and then subjected to slab rolling. In each case, the number of precipitates as rolled was insufficient, the austenite grains in the hardened layer of the induction hardened material were not sufficiently refined, and brittle fracture was exhibited in the torsional test of the induction hardened material. The torsional strength characteristics are inferior to the invention examples.
[0063]
Comparative Example 31 is a case where the heating temperature of the hot working exceeded the range specified in the present application, the number of precipitates was lower than the range of the present invention, and the total decarburization depth was higher than the range of the present invention. And, the induction hardening hardness is low, the austenite grains of the hardened layer of the induction hardened material are not sufficiently refined, and the brittle fracture is exhibited in the torsional test of the induction hardened material, and the torsion is compared with the example of the present invention. Poor strength properties.
[0064]
Comparative Example 32 is the case where the finishing temperature of hot working exceeded the range specified in the present application, the ferrite grain size also exceeded the range specified in the present application, the induction hardening hardness was low, the induction hardening depth was shallow, and the torsional strength was high. The properties are poor.
[0065]
Comparative Example 33 is a case where the finishing temperature of hot working was lower than the range specified in the present application, the ferrite fraction also exceeded the range specified in the present application, the total decarburization depth also exceeded the range in the present invention, and Compared with the invention examples, the induction hardening hardness is lower, the induction hardening depth is shallower, and the torsional strength characteristics are inferior.
[0066]
Comparative Example 34 is a case where the cooling rate after hot working exceeded the range specified in the present application, and the hardness as rolled was high and the machinability was poor.
[0067]
[Table 1]

[0068]
[Table 2]

[0069]
【The invention's effect】
By using the high-strength induction hardening steel material of the present invention and a method for producing the same, excellent cold workability is obtained during cold forging or cutting, and a fine structure rich in toughness is obtained after induction hardening, and brittle fracture is suppressed. As a result, it is possible to obtain excellent strength characteristics commensurate with the hardness of the cured layer, and the industrial effect is extremely remarkable.

Claims (6)

In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05-0.2%
Containing
N: less than 0.007% (including 0%),
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
And the balance consists of iron and unavoidable impurities. The matrix of the structure after hot rolling has at least 5 precipitates of Ti having a diameter of 0.2 μm or less per 5 μm ² or more, and a ferrite crystal grain size. Is not more than 20 μm, the ferrite fraction is not more than 30%, and the decarburization depth specified by JIS G 0558: DM-T is 0.2 mm or less. In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05-0.15%,
Nb: 0.002 to 0.05%
Containing
N: less than 0.007% (including 0%)
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
And the balance consists of iron and unavoidable impurities, and in the matrix of the structure after hot rolling, a precipitate of Nb with a diameter of 0.2 μm or less, a precipitate of Ti, or a composite composition of Nb and Ti The total number of the precipitates is 5 / μm ² or more, the ferrite crystal grain size is 20 μm or less, the ferrite fraction is 30% or less, and the decarburization depth specified by JISG 0558: DM-T0. A high-strength induction hardening steel material having a thickness of 2 mm or less. In further mass%,
Mo: 1% or less,
The steel material for high-strength induction hardening according to claim 1 or 2, wherein the steel material contains one or more of Ni: 2.5% or less and V: 0.4% or less. In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05 to 0.15%
Containing
N: less than 0.007% (including 0%)
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
The steel is made by a process of performing slab rolling without cooling the steel consisting of iron and unavoidable impurities to a temperature below the A3 point after casting. The heating temperature is 900 to 1070 ° C. Hot rolling to 800 or 970 ° C., followed by hot rolling, followed by hot rolling to 800 or 500 ° C. in a temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./sec or less, to a wire or a bar; In the matrix of the structure after hot rolling, precipitates of Ti having a diameter of 0.2 μm or less are set to 5 / μm ² or more, the ferrite crystal grain size is 20 μm or less, the ferrite fraction is 30% or less, and JIS G 0558. Decarburization depth specified by: DM-T 0.2 mm or less, a method for producing a high-strength induction hardened steel material. In mass%,
C: 0.35 to 0.6%,
Si: 0.01-1%,
Mn: 0.2-2%,
S: 0.005 to 0.15%,
Cr: 0.35% or less (including 0%),
B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%,
Ti: 0.05-0.15%,
Nb: 0.002 to 0.05%,
Containing
N: less than 0.007% (including 0%),
P: 0.025% (including 0%),
O: 0.0025% or less (including 0%)
The steel is made by a process of performing slab rolling without cooling the steel consisting of iron and unavoidable impurities to a temperature below the A3 point after casting. The heating temperature is 900 to 1070 ° C. Hot rolling to a wire rod or a steel bar under the condition that the finishing temperature of hot rolling is 800 to 970 ° C., and the temperature range of 800 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./sec or less following the hot rolling. A total of 5 precipitates of Nb, Ti precipitates, or precipitates composed of a composite composition of Nb and Ti having a diameter of 0.2 μm or less in the matrix of the structure after hot rolling, and a total of 5 precipitates / μm ² or more; A ferrite crystal grain size of 20 μm or less, a ferrite fraction of 30% or less, and a decarburization depth specified by JIS G 0558: DM-T of 0.2 mm or less. Manufacture Law. In further mass%,
Mo: 1% or less,
6. The method for producing a high-strength induction hardening steel material according to claim 4, wherein one or more of Ni: 2.5% or less and V: 0.4% or less are contained.