JP2011241465A - Rolled steel material for induction hardening and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled steel material for induction hardening, which is capable of achieving a high strength and a high base-material toughness even without being refined and forms a hardened layer excellent in toughness by induction hardening.SOLUTION: The rolled steel material for induction hardening comprises, by mass, 0.38 to 0.55% C, ≤1.0% Si, 0.20 to 2.0% Mn, ≤0.020% P, ≤0.10% S, 0.10 to 2.0% Cr, ≤0.10% Al, 0.0005 to 0.0030% B, ≤0.008% N, ≤0.047% Ti (wherein 3.4N≤Ti≤3.4N+0.02%) and the balance being Fe with impurities. The rolled steel material has a microstructure composed of ferrite, lamellar pearlite and spheroidal cementite, wherein the ferrite has an average crystal grain size of 10 μm or less, lamellar pearlite having a lamellar spacing of ≤200 nm accounts for 20 to 50% of the area of the microstructure of the lamellar pearlite, and the number of the spheroidal cementite is 4×10pieces/mmor more.

Description

  TECHNICAL FIELD The present invention relates to a rolled steel for induction hardening, that is, a rolled steel used by induction hardening, and a method for producing the same, and more specifically, it does not necessarily contain an expensive element and further includes a so-called “control” of quenching and tempering. The present invention relates to an induction-quenched rolled steel material that can ensure high strength and base material toughness without performing quality treatment, and is excellent in toughness of a hardened layer produced by induction hardening, and a method for producing the same.

  Among automotive parts, the rack bar used in the steering device is an important part that shows the role of the framework that steers the direction of travel of the automobile and connects both the left and right wheels. turn into. For this reason, high reliability is requested | required of the steel materials used for a rack bar.

  In addition, the rack bar conventionally uses a rolled steel material of medium carbon steel material, and after performing tempering treatment for quenching and tempering, a tooth mold part is formed by cutting and induction hardening is performed on the tooth mold part, that is, a high frequency It has been manufactured by rapid and short-time heating (hereinafter referred to as “high-frequency heating”) by induction heating with electric current, and then immediately or immediately after holding at the heated temperature, followed by quenching.

  And since it is necessary to prevent damage as mentioned above, the rack bar used by induction hardening is required to have excellent bending strength and impact characteristics. That is, although it is a necessary condition that the induction-hardened layer is less likely to crack, even if a crack occurs in the induction-hardened layer, the crack does not propagate to the base material and lead to breakage. Is required.

Therefore, the steel used as the rack bar material is
・ High strength,
・ High base material toughness,
・ Toughness of hardened layer generated by induction hardening,
It is required to be superior to all of the above.

  As steel materials used for such rack bars, for example, the following steel materials have been proposed.

  That is, in Patent Document 1, in mass%, C: 0.40 to 0.60%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.50%, and S: 0.004. Further, as other elements, Cr: 1.5% or less (excluding 0%), Al: 0.0005-0.10%, and N: 0.002-0. Containing at least one element selected from the group consisting of 020%, and further, if necessary, B: 0.0005-0.0020% alone or with Ti: 0.005-0.050% The balance is a steel bar composed of Fe and inevitable impurities, and by quenching and short-time tempering, the quenching and tempering structure of a portion of the depth D / 4 (D indicates the diameter of the steel bar) from the surface of the steel bar is obtained. "The tempered bainite structure and the tempered martensite structure In 20-100% (area percentage) "and" Play pearlite structure 0-50% (area percentage) "excellent bending properties are adjusted to the steel steering rack is proposed. This steel for steering racks is a furnace in which a steel material having the above-mentioned chemical composition is rolled, the obtained bar steel is heated to a temperature of 820 ° C. or higher, controlled and cooled to room temperature by water cooling, and then heated to an ambient temperature of 680 ° C. or higher. And tempering for 20 minutes or less for a short time and air cooling to room temperature.

  However, the tempering process increases costs. For this reason, there is a movement to reduce energy consumption and manufacturing man-hours by omitting the tempering treatment from the past. Steel with the same strength and toughness as steel tempered with hot rolling. Various manufacturing methods have been proposed.

For example, in Patent Document 2, in mass%, C: 0.30 to 0.50%, Si: 0.15 to 0.50%, Mn: 1.0 to 1.65%, S: 0.04 -0.1%, V: 0.08-0.2%, Al: 0.015-0.05%, the steel material which the remainder substantially consists of iron and an unavoidable impurity is heated to the temperature of 850-1000 degreeC And after hot rolling at a finishing temperature of 800 to 950 ° C., the rolled steel bar is reheated to a temperature of 850 to 1000 ° C., hot forged at a finishing temperature of 800 to 950 ° C., and then high strength method for producing a non-heat treated steel bar, characterized by cooling at a cooling rate of 0.3 to 10 ° C. / sec between 550 ° C. from a ₃ transformation point are proposed.

  In Patent Document 3, in mass%, C: 0.35 to 0.70%, Si: 0.1 to 1.5%, Mn: 0.5 to 2.0%, Cr: 1.5% or less , V: 0.2 to 1.0%, Al: 0.005 to 0.05%, and Nb: 0.002 to 0.05%, Ni: 0.2 to 1.% if necessary. 1% or more of 0%, Cu: 0.2-1.0%, Mo: 0.1-0.5%, consisting of the balance Fe and inevitable impurities, with ferrite-pearlite structure, and more than 10 nm A high-strength and high-toughness non-tempered steel having A / B of 1/20 or more, where A is the number of precipitates of 10 nm or less and B is the number of precipitates, has been proposed. As described in paragraph [0019] of Patent Document 3, this high-strength and high-toughness non-heat treated steel has a steel slab heating temperature of 800 to 930 ° C., rough rolling, and a finish rolling start temperature of 780 to 930. It is obtained by cooling at an average cooling rate at 700 to 400 ° C. at 0.3 to 5.0 ° C./s after rolling.

In Patent Document 4, the amount of carbon (C) in which the volume fraction of carbide including cementite is 20% or less, and mass%, Si: 0.80% or less, Mn: 0.05 to 3.0%, Al: a steel material including 0.10% or less, a diameter or a short side length of 5 mm or more, and a ferrite grain structure having an average grain size of 2 μm or less together with carbides in the entire T section Strength and high toughness bars have been proposed. This bar can further include one or more of Cu, Ni, Ti, Nb, V, Cr, Mo, W, Ca, REM, and B, and is 400 ° C. or higher Ac. _In a temperature range of ₃ or less, the steel material is obtained by multi-pass hole rolling.

In Patent Document 5, the component composition is, by mass%, 0.4 <C <2.0%, 0.1 <Si <1.0%, 0.1 <Mn <2.0%, P ≦ 0. .1%, S ≦ 0.5%, and the balance of Fe and impurity elements are kept at 900 ° C. to 1200 ° C. for 30 seconds or more, then cooled to 500 ° C. to 700 ° C. and 30% at this temperature. There has been proposed a method for producing a non-tempered steel material characterized by performing warm working after holding for at least 2 seconds. According to this production method, the average particle size of ferrite is 2.0 μm or less, the average particle size of cementite is 0.5 μm or less, the yield ratio is 0.75 or more, the Charpy impact value is 150 J / cm ² or more, and A non-tempered steel material composed mainly of ferrite and cementite can be obtained.

In Patent Document 6, a steel containing 2% or less of C is heated to Ac ₁ point or higher, and then in hot working in which deformation is applied, during the rolling, to a temperature range of Ar ₁ point or less and Ar ₁ -200 ° C. or higher. cooling, followed by plastic deformation of 15% or more was added at rolling Thereafter, whereby at least two or more times spheroidized structure controlled rolling pattern to reach a temperature range of less than ₁ point or more Ac ₃ point Ac by deformation heat generated A method of manufacturing a steel bar and a wire rod is disclosed.

In Patent Document 7, a steel containing 2% or less of C is heated to an Ac ₁ point or higher, and then in a hot working in which deformation is performed, a temperature range of Ae ₁ point or less and exceeding an Ar ₁ point during rolling. Then, plastic deformation of 15% or more is subsequently applied by finish rolling, thereby accelerating pearlite or bainite transformation, thereby generating these structures, and at the same time, ac ₁ point or more again by deformation heat, Ac ₃ A method for producing a steel bar and a wire having a spheroidized structure, characterized by repeating a controlled rolling pattern that reaches a temperature range of a point or an Ac _cm point at least twice.

  In Patent Document 8, in mass%, C: 0.35 to 0.50%, Si: 0.1 to 0.6%, Mn: 0.5 to 1.5%, Al: 0.005 to 0 0.02%, V: 0.05 to 0.50%, and if necessary, Cr: 0.60% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less , Ti: 0.2% or less, Nb: 0.10% or less, consisting of balance iron and inevitable impurities, ferrite fraction in steel structure is 20-40%, average pearlite lamellar A ferrite-pearlite multiphase structure in which the average grain size of ferrite surrounded by a large-angle grain boundary with an interval of 0.05 to 0.20 μm and a crystal orientation difference of 15 ° or more is 2 to 10 μm. High strength, high toughness non-heat treated steel bars have been proposed. This non-tempered steel bar is obtained by finish rolling a steel containing the above-mentioned amounts of C to V at a starting temperature of 750 to 900 ° C., and an average cooling rate of 10 ° C./second or more within 10 seconds after finishing rolling. It is obtained by starting quenching and cooling to a temperature of 500 to 700 ° C., and then cooling to 200 ° C. at an average cooling rate of 0.1 to 5 ° C./second.

Japanese Patent Laid-Open No. 2003-166036 JP 61-170513 A JP 2005-281837 A JP 2000-309850 A JP 2006-225735 A JP 59-136423 A JP 60-149723 A JP 2005-133155 A

  The technique proposed in the above-mentioned Patent Document 1 performs a tempering process, and is not desirable in terms of cost from the viewpoint of energy and manufacturing man-hours as described above. Furthermore, it is not desirable to perform a heat treatment such as a tempering treatment from the viewpoint of carbon dioxide reduction, which is an important issue in recent global warming countermeasures.

  Since the non-tempered steel bar of Patent Document 2 must contain 0.08% or more of V, and the non-tempered steel of Patent Document 3 must contain 0.2% or more of V as an essential element, In the current situation where raw material prices are soaring, the use of V, which is an expensive element, is not a desirable steel material in terms of cost. Furthermore, both the non-tempered steel bar of Patent Document 2 and the non-tempered steel of Patent Document 3 tend to cause coarsening of crystal grains in the hardened layer when the high-frequency heating is performed, and the toughness of the hardened layer is sufficient. I can't say that.

  In Examples of Patent Document 4, C: 0.42%, Si: 0.18%, Mn: 0.68%, P: 0.013%, S: 0.006%, etc. , Si, Mn, P, and S are rolled with repeated heating at 640 ° C., and are excellent in strength and toughness. However, in steel materials having such chemical components, even when high-frequency heating is performed for quenching, carbides in the steel materials easily dissolve in the matrix, so the crystal grains of the hardened layer are coarse. Therefore, it cannot be said that sufficient toughness is obtained in the cured layer.

  Moreover, in the Example of patent document 5, by mass%, C: 0.40%, Si: 0.25%, Mn: 0.76%, P: 0.02%, S: 0.03%, etc. After heating the steel material consisting of C, Si, Mn, P, and S to a temperature of 900 to 1200 ° C. and austenitizing the structure, cooling to 500 to 700 ° C. to make a “ferrite pearlite structure”, warm It is described that what has been processed has a high yield ratio and toughness. However, as in Patent Document 4, in the steel material having such a chemical component, cementite in the steel material easily dissolves in the matrix even when high-frequency heating is performed for quenching. For this reason, the crystal grains of the hardened layer are likely to be coarsened. Therefore, it cannot be said that sufficient toughness is obtained in the hardened layer.

  The technology proposed in Patent Document 6 provides a method for producing a soft steel bar or wire material, which significantly shortens the spheroidizing annealing time used to reduce the deformation resistance of steel for cold forging. It is intended to do. Therefore, neither the steel bar nor the wire manufactured by the method of Patent Document 6 is used by induction hardening while maintaining the shape of the hot-rolled steel material almost like a rack bar. In the first place, it is not intended to obtain a rolled steel material that is excellent in high strength and base material toughness, and also in the toughness of a hardened layer produced by induction hardening.

  Similar to the invention of Patent Document 6, the technique proposed in Patent Document 7 also significantly reduces the processing time of spheroidizing annealing performed to reduce the deformation resistance of the steel for cold forging, It aims at providing the manufacturing method of a steel bar and a wire. Therefore, the steel bar and wire manufactured by the method of Patent Document 7 are not used by induction hardening while maintaining the shape of the hot-rolled steel material almost like a rack bar, and the invention itself of Patent Document 7 is originally used. It is not intended to obtain a rolled steel material that is excellent in high strength and base material toughness, and also in the toughness of a hardened layer produced by induction hardening.

  It is described that the technique proposed in Patent Document 8 has high strength and high toughness by setting the pearlite lamellar spacing to 0.05 to 0.20 μm. However, since it is necessary to contain 0.05% or more of V as an essential element, it is not a desirable steel material in terms of cost since V, which is an expensive element, is used in the current situation of rising raw material prices. Further, in steel materials having such chemical components, when high-frequency heating is performed for quenching, carbides in the steel materials are easily dissolved in the matrix, so that the crystal grains of the hardened layer are likely to be coarsened. Therefore, it cannot be said that sufficient toughness is obtained in the cured layer.

  The present invention has been made in view of the above-described situation, and provides a rolled steel material that is used by induction hardening like a material of a rack bar and a manufacturing method thereof, and more specifically, a particularly expensive element is necessarily required. In addition, the present invention provides a rolled steel material for induction hardening and a method for producing the same, which can obtain high strength and base material toughness without performing tempering treatment, and is excellent in toughness of a hardened layer generated by induction hardening. With the goal.

The high strength and the base material toughness which are the objects of the present invention are a rolled steel material having a tensile strength of 760 MPa or more and a notch bottom radius of 1 mm and a notch width of 2 mm as defined in JIS Z 2242 (2005), respectively. Among the U-notch test pieces, a test piece having a notch depth of 2 mm (that is, a height under the notch of 8 mm) (hereinafter referred to as “2 mm U-notch Charpy impact test piece”) at a test temperature of 25 ° C. in a Charpy impact test. It means that the impact value is 200 J / cm ² or more, and the excellent toughness of the hardened layer means that the crack initiation strength of the hardened layer by a test method described later is a load of 20 kN or more in a three-point bending test. Means.

  In order to solve the above-described problems, the present inventors have conducted various laboratory studies on means for obtaining high strength and toughness without performing tempering treatment in a medium carbon steel material. Obtained knowledge.

  (A) Generally, strength and toughness show a trade-off relationship, and if the strength is set too high, the toughness is significantly lowered and desired characteristics cannot be obtained.

  (B) In a mixed structure of ferrite and pearlite, it is known that refinement of ferrite is effective as a means for improving the so-called “strength-toughness balance”. As a method for obtaining a fine ferrite structure, hot rolling is performed in a two-phase temperature range of austenite and ferrite, and the precipitation of fine ferrite from austenite is promoted by the work strain caused by rolling, and the ferrite is moved by the work strain. The ferrite can be refined by subjecting it to recrystallization. However, simply by hot rolling in the two-phase temperature range of austenite and ferrite, the austenite remaining at the end of hot rolling undergoes pearlite transformation during cooling, forming lamellar cementite with low lamellar spacing and low toughness. End up. For this reason, the target toughness level cannot be obtained.

  (C) On the other hand, even if it is layered cementite, the target toughness level can be obtained if the lamellar spacing is sufficiently small, specifically 200 nm or less. For this purpose, if the steel surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s after the end of hot rolling, that is, if the austenite is supercooled, the diffusion distance of C can be reduced and the lamellar spacing can be sufficiently reduced. can do.

  (D) Furthermore, if some cementite in pearlite can be precipitated in a spherical shape, toughness can be further enhanced. In addition, after introducing a large number of processing strains into austenite by hot rolling in the two-phase temperature range of austenite and ferrite, a large amount of spherical cementite precipitates by allowing them to act as precipitation sites for spherical cementite by supercooling. Can be made.

  (E) However, if bainite and / or martensite is generated by overcooling, the toughness is reduced, so the above-described overcooling is performed at a steel surface temperature of 600 ° C. or less within 5 s after the end of hot rolling. It is necessary to set the temperature to exceed the Ms point and then perform reheating so that the steel surface temperature is in the range of 500 to 700 ° C.

  (F) When the structure of the steel material is mainly composed of fine ferrite, lamellar pearlite having a lamellar spacing of 200 nm or less, and spherical cementite, both high strength and excellent toughness can be achieved.

  (G) Since Cr has the effect of uniformly refining spherical cementite, the steel material whose Cr content is adjusted is adjusted to a temperature exceeding the Ms point at a steel material surface temperature of 600 ° C. or less within 5 s after the hot rolling is completed, Thereafter, when cooling is performed so that the surface temperature of the steel material is in a range of 500 to 700 ° C. by recuperation, a large amount of spherical cementite containing Cr is precipitated. In the case of heating for a short time such as high-frequency heating, the spherical cementite containing Cr is hardly dissolved in austenite. For this reason, the above-mentioned spherical cementite, as TiN and pinning particles, suppresses the growth of austenite grains during induction heating, and as a result, the toughness of the induction hardening layer increases and also suppresses the occurrence of cracks in the induction hardening layer. it can.

  (H) By containing B in the steel material, it is possible to improve hardenability during induction hardening and to suppress segregation of P and S at the grain boundaries of austenite during induction heating. As a result, the toughness of the induction hardened layer is further increased, and the generation of cracks in the induction hardened layer can be further suppressed.

(I) In a steel material having the above-described structure, a test temperature in a Charpy impact test using a 2 mm U-notch Charpy impact test piece with a tensile strength of 760 MPa or more by achieving appropriate strengthening by solid solution strengthening and precipitation strengthening. The target characteristic that the impact value at 25 ° C. is 200 J / cm ² or more and the crack generation strength of the hardened layer by the test method described later is a load of 20 kN or more in a three-point bending test can be achieved.

  The present invention has been completed based on the above findings, and the gist of the present invention is a rolled steel for induction hardening shown in the following [1] to [3] and a method for producing a rolled steel for induction hardening shown in [4]. It is in.

[1] By mass%, C: 0.38 to 0.55%, Si: 1.0% or less, Mn: 0.20 to 2.0%, P: 0.020% or less, S: 0.10 %: Cr: 0.10 to 2.0%, Al: 0.10% or less, B: 0.0005 to 0.0030%, N: 0.008% or less, and Ti: 0.047% or less (provided that 3.4N ≦ Ti ≦ 3.4N + 0.02%), the balance is composed of Fe and impurities, and has a chemical component in which the value of fn1 represented by the following formula (1) is 1.20 or less. The area ratio of the microstructure of the lamellar pearlite is 20 to 50%, which is composed of ferrite, lamellar pearlite and spherical cementite, the ferrite has an average crystal grain size of 10 μm or less, and the lamellar pearlite has a lamellar spacing of 200 nm or less. And spherical cementer Induction hardening rolled steel for, wherein the number of bets is 4 × 10 ⁵ cells / mm ² or more.
fn1 = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V− (5/7) S (1)
However, C, Si, Mn, Cr, V, and S in the above formula (1) represent the content of each element in mass%.

  [2] One or more elements selected from Cu: 1.0% or less, Ni: 3.0% or less, and Mo: 0.50% or less in mass% instead of a part of Fe The rolled steel material for induction hardening as described in [1] above, comprising:

  [3] The chemical component may contain one or more elements of Nb: 0.10% or less and V: 0.30% or less in mass% instead of a part of Fe. Rolled steel for induction hardening according to the above [1] or [2].

[4] Fully continuous heat comprising two or more rolling steps after heating the rolled material having the chemical component according to any one of [1] to [3] to a temperature range of 670 to 850 ° C. After rolling in the intermediate rolling method, and after finishing the rolling in the final rolling step, the steel surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s, and then the steel surface temperature is 500 to 700 ° C. by reheating. A method for producing a rolled steel material for induction hardening that is cooled so as to satisfy the following range, wherein the all-continuous hot rolling method satisfies the following [1] and [2]: A method for producing rolled steel.
[1] The surface temperature of the material to be rolled during each rolling step is within a temperature range of 650 to 820 ° C.
[2] The total area reduction rate is 30% or more.

  Note that “3.4N” indicates 3.4 times the N content in mass%.

  The “impurities” in the remaining “Fe and impurities” refers to those that are mixed due to various factors in the manufacturing process, including raw materials such as ore or scrap, when industrially producing steel materials. .

  “Spherical cementite” refers to cementite having a ratio of the major axis L to the minor axis W (L / W) of 2.0 or less.

  “Fully continuous hot rolling method” means, for example, “rough rolling mill train—finish rolling mill train” or “rough rolling mill train—intermediate rolling mill train—finish rolling mill train”. In a rolling line using a tandem mill consisting of machine rows, it refers to a method in which a material to be rolled cannot be left between rolling machine rows. In addition, in the above, each rolling mill row | line includes not only the case where it is comprised from several rolling mills but what is comprised by one rolling mill.

The “total area reduction ratio” means that when the cross-sectional area before rolling of the material to be rolled in the all continuous hot rolling method is A ₀ and the area after leaving the final rolling mill is A _f , {( A ₀ -A _f ) / A ₀ } × 100 (%).

  Hereinafter, a lamellar pearlite having a lamellar spacing of 200 nm or less may be referred to as “fine lamellar pearlite”, and a lamellar pearlite having a lamellar spacing exceeding 200 nm may be referred to as “coarse lamellar pearlite”.

The rolled steel material for induction hardening according to the present invention does not necessarily contain expensive V, and the tensile strength is not less than 760 MPa and a 2 mm U notch Charpy impact test piece in the state of the rolled steel material without performing tempering treatment. In the Charpy impact test used, the impact value at a test temperature of 25 ° C. is 200 J / cm ² or more. Further, since the grain growth hardly occurs during induction hardening, the hardened layer has excellent toughness. It is suitable for use as a material for parts such as rack bars that require bending strength and impact characteristics. This induction-quenched rolled steel can be produced by the method of the present invention.

It is a figure explaining the 3 point | piece bending test done in the Example for the toughness investigation of the hardened layer produced | generated by induction hardening.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” of the content of each element means “mass%”.

1. Chemical composition:
C: 0.38 to 0.55%
C has the effect of improving the strength of steel, induction hardenability, and the strength of a hardened layer formed by induction hardening. However, if the content is less than 0.38%, the desired effect cannot be obtained. On the other hand, if the C content exceeds 0.55%, the toughness of the base material decreases and the hardened layer formed by induction hardening becomes brittle. Therefore, the content of C is set to 0.38 to 0.55%. In addition, in order to acquire the said effect stably, it is preferable that content of C shall be 0.40% or more and 0.50% or less.

Si: 1.0% or less Si is a deoxidizing element and is an element that improves the strength of ferrite by solid solution strengthening. Meanwhile, Si raises the A ₃ transformation point with increasing content is also an element to lower the strength of the induction hardening and curing layer formed by induction hardening. Then, with increasing content for A ₃ transformation point rises, the temperature region where the decarburization easily occurs austenite and ferrite in the cooling process after heating or hot rolling is the primary constituent phase spreads, the Si A steel material with a high content tends to cause decarburization. In particular, when the Si content exceeds 1.0%, a deoxidation effect and solid solution strengthening can be expected, but decarburization after hot rolling is likely to occur, and a hardened layer generated by induction quenching is generated. Toughness decreases. Therefore, the Si content is set to 1.0% or less. In addition, it is preferable that Si content shall be 0.8% or less. On the other hand, the content of Si is preferably 0.03% or more and more preferably 0.10% or more in order to ensure the strength by utilizing the above-described solid solution strengthening effect of Si. .

Mn: 0.20 to 2.0%
Mn is an element effective for improving the induction hardenability and the toughness of the hardened layer formed by induction hardening, and is an element for improving the strength of the ferrite by solid solution strengthening. However, when the Mn content is less than 0.20%, the desired effect cannot be obtained. On the other hand, when Mn is contained exceeding 2.0%, it becomes easy to produce bainite at the time of cooling after final rolling, resulting in deterioration of toughness. Therefore, the content of Mn is set to 0.20 to 2.0%. In order to stably obtain the above effect while keeping the alloy cost low, the Mn content is preferably 0.40% or more and 1.50% or less.

P: 0.020% or less P is contained as an impurity and causes grain boundary segregation and center segregation, leading to a decrease in base material toughness and toughness of a hardened layer generated by induction quenching. If it exceeds 020%, the base material toughness and the toughness of the hardened layer produced by induction quenching will be significantly reduced. Therefore, the content of P is set to 0.020% or less. The P content is preferably 0.010% or less.

S: 0.10% or less S is contained as an impurity. In addition, when S is positively contained, it combines with Mn to form MnS and has an effect of improving machinability, in particular, chip disposal, but if too much MnS is formed, machinability is improved. Even if it can, the toughness of the base material toughness and the toughness of the hardened layer generated by induction quenching will be reduced. Especially when the S content exceeds 0.10%, the toughness of the base material toughness and the hardened layer generated by induction hardening is tough. The decline is significant. Therefore, the content of S is set to 0.10% or less. The S content is preferably 0.08% or less. On the other hand, from the viewpoint of improving machinability, S is preferably contained in an amount of 0.010% or more, and more preferably 0.015% or more.

Cr: 0.10 to 2.0%
Cr is an indispensable element for uniform refinement of spherical cementite in hot rolled steel. Furthermore, Cr also has an effect of improving induction hardenability. These effects are exhibited when the Cr content is 0.10% or more. However, if the Cr content exceeds 2.0%, the effect of improving the uniform refinement of the spherical cementite and the induction hardenability is saturated, and the toughness of the base material is lowered. Therefore, the content of Cr is set to 0.10 to 2.0%. The Cr content is preferably 0.20% or more and 1.8% or less.

Al: 0.10% or less Al is an element mixed as an impurity. Al may be added for deoxidation. However, Al raises the A ₃ transformation point, lowering the induction hardening properties. In particular, when the Al content exceeds 0.10%, the induction hardenability is significantly lowered, and further, the base material toughness is deteriorated. Therefore, the upper limit of the Al content is set to 0.10%. The Al content is preferably 0.08% or less. On the other hand, in order to obtain the deoxidizing effect of AlN, the Al content is preferably 0.005% or more.

B: 0.0005 to 0.0030%
B is a temperature region in which austenite and ferrite are the main constituent phases. The upper limit is shifted to a high temperature side, and the effect of improving induction hardenability. Has the effect of suppressing segregation. The above effect is remarkable when the B content is 0.0005% or more. However, even if it contains B exceeding 0.0030%, the above-mentioned effect is saturated and the cost is increased. Therefore, the content of B is set to 0.0005 to 0.0030%. The B content is preferably 0.0010% or more and 0.0020% or less.

  Even in the case of containing B in an amount in the above range, when B is combined with N in steel to form BN and does not exist as so-called “solid solution B”, the above-described effects are obtained. I can't expect it. Therefore, in order to exhibit the effect of improving the induction hardenability of B and the effect of suppressing the segregation of P and S at the austenite grain boundaries during induction hardening, it is necessary to reduce the amount of N in the steel.

N: 0.008% or less N has a large affinity with B, and when it binds to B in steel to form BN, austenite and ferrite are the main constituent phases due to the inclusion of B. The effect of shifting the upper limit of the temperature region to the high temperature side, the effect of improving the induction hardenability, and the effect of suppressing the segregation of P and S at the austenite grain boundary during induction hardening cannot be expected. In particular, when the N content is increased and exceeds 0.008%, the above effect cannot be obtained. Therefore, the N content is set to 0.008% or less. In addition, it is preferable to reduce N content in steel as much as possible.

Ti: 0.047% or less Ti suppresses formation of BN by preferentially bonding with N in the steel, and B is present as a solid solution B, so that austenite and ferrite of B are the main constituent phases. It is an element effective for ensuring the effect of shifting the upper limit of the temperature region to the high temperature side, the effect of improving the induction hardenability of B, and the effect of suppressing the segregation of P and S at the austenite grain boundaries during induction hardening. However, if the Ti content is too high, Ti combines with C in the steel to form carbides, reducing the amount of solute C in the steel and increasing the proportion of ferrite. In addition, the toughness of the induction-hardened hardened layer is also reduced. In particular, when the Ti content increases and exceeds 0.047%, the induction hardenability is lowered and the toughness of the hardened layer is significantly lowered. Therefore, the Ti content is set to 0.047% or less.

3.4N ≦ Ti ≦ 3.4N + 0.02%
Even if the content of N and Ti is in the above-mentioned range, if the content of Ti is less than [3.4N], the fixation of N in the steel is insufficient and N binds to B. Therefore, the above-mentioned effect of B cannot be fully expressed. On the other hand, if the Ti content exceeds [3.4N + 0.02%], a reduction in induction hardenability and a reduction in toughness of the induction hardened layer are inevitable. Therefore, the Ti and N contents are set to 3.4N ≦ Ti ≦ 3.4N + 0.02%.

fn1 value: 1.20 or less In the present invention, the value of fn1, that is,
fn1 = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V− (5/7) S (1)
If the value represented by the formula is too large, it is strengthened excessively, leading to a decrease in the base material toughness. As will be described later, the rolled steel for induction hardening according to the present invention includes “fine ferrite, fine lamellar pearlite having an area ratio of 20 to 50% in the microstructure (that is, lamellar pearlite having a lamellar spacing of 200 nm or less). An excellent “strength-toughness balance” is ensured by forming a microstructure composed of “pearlite and spherical cementite”. Even when such a microstructure can be obtained, the value of fn1 is 1.20. Exceeding the target, the target base material toughness (impact value at a test temperature of 25 ° C. in a Charpy impact test using a 2 mmU notch Charpy impact test piece is 200 J / cm ² or more) cannot be obtained. Therefore, the value of fn1 represented by the above (1) is set to 1.20 or less.

  The value of fn1 is preferably 1.10 or less. The value of fn1 is preferably 0.65 or more from the viewpoint of securing strength.

  One of the rolled steel materials for induction hardening according to the present invention has a chemical component whose balance is Fe and impurities in addition to the above elements. As already described, “impurities” in “Fe and impurities” refers to those mixed from ore or scrap as a raw material or the environment when steel materials are industrially produced.

  Another chemical component of the induction-hardened rolled steel of the present invention is one or more elements selected from the following first group and second group instead of a part of Fe, that is, It may be a chemical component containing one or more elements of the first group and the second group as optional elements.

Group 1: Cu: 1.0% or less, Ni: 3.0% or less and Mo: 0.50% or less The elements of the first group, Cu, Ni and Mo, improve the induction hardenability and increase the strength. Since it has the effect | action which raises, in order to acquire this effect, you may make it contain in said range, respectively. Hereinafter, the elements of the first group will be described in detail.

Cu: 1.0% or less Cu, like C and Mn, has the effect of improving the induction hardenability and increasing the strength, and thus may contain Cu for increasing the strength. However, when the Cu content exceeds 1.0%, the hot workability is deteriorated. Therefore, the amount of Cu in the case of inclusion is set to 1.0% or less. In addition, it is preferable that the quantity of Cu in the case of making it contain is 0.80% or less.

  On the other hand, in order to surely obtain the above-described Cu strength improvement effect, the Cu content is preferably 0.05% or more, and more preferably 0.10% or more.

Ni: 3.0% or less Ni, like C and Mn, has the effect of improving the induction hardenability and increasing the strength, so Ni may be included for increasing the strength. However, when the Ni content exceeds 3.0%, the effect is saturated, and the cost is increased. Therefore, the amount of Ni in the case of inclusion is set to 3.0% or less. In addition, when Ni is contained, the amount of Ni is preferably set to 2.0% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving Ni strength, the Ni content is preferably 0.05% or more, and more preferably 0.10% or more.

Mo: 0.50% or less Mo, like C and Mn, has the effect of improving the induction hardenability and increasing the strength, so Mo may be contained for increasing the strength. However, when the Mo content exceeds 0.50%, the above effect is saturated and the cost is increased. Therefore, the amount of Mo in the case of inclusion is set to 0.50% or less. In addition, it is preferable that the quantity of Mo in the case of making it contain is 0.40% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the strength of Mo, the Mo content is preferably 0.05% or more, and more preferably 0.10% or more.

  Said Cu, Ni, and Mo can be contained only in any one of them, or 2 or more types of composites. The total amount of these elements in the case of inclusion may be 4.50% or less, but is preferably 3.20% or less.

Group 2: Nb: 0.10% or less and V: 0.30% or less Nb and V, which are elements of Group 2, have a crystal grain refining action. You may make it contain in the range. Hereinafter, the second group of elements will be described in detail.

Nb: 0.10% or less Nb has a function of forming precipitates in steel and refining crystal grains. Nb also has the effect of improving the strength of steel. However, when the content of Nb exceeds 0.10%, the effect is saturated, the cost increases, and the toughness is reduced. For this reason, the amount of Nb in the case of inclusion is set to 0.10% or less. In addition, it is preferable that the quantity of Nb in the case of making it contain is 0.08% or less.

  On the other hand, in order to stably obtain the Nb crystal grain refining effect, the Nb content is preferably 0.01% or more, and more preferably 0.015% or more.

V: 0.30% or less V has an action of forming precipitates in the steel and refining crystal grains. V also has the effect of improving the strength of the steel. However, if the content of V exceeds 0.30%, the effect is saturated, the cost increases, and the toughness is reduced. For this reason, the V amount in the case of inclusion is set to 0.30% or less. In addition, it is preferable to make the quantity of V in the case of making it contain into 0.25% or less.

  On the other hand, in order to stably obtain the V crystal grain refining effect, the V content is preferably 0.01% or more, and more preferably 0.02% or more.

  Said Nb and V can be contained only in any 1 type or 2 types of composites. The total amount of these elements in the case of inclusion may be 0.40% or less, but is preferably 0.33% or less.

  Also in the steel material of the chemical component containing the said arbitrary elements from Cu to V, the value of fn1 represented by said (1) formula needs to be 1.20 or less.

2. Microstructure:
The microstructure of the induction-hardened rolled steel material of the present invention having the chemical components described in the previous section is composed of ferrite, lamellar pearlite and spherical cementite, and the ferrite has an average crystal grain size of 10 μm or less, and the lamellar spacing of lamellar pearlite is 200 nm. The area ratio of the following lamellar pearlite (that is, fine lamellar pearlite) in the microstructure should be 20 to 50%, and the number of spherical cementite should be 4 × 10 ⁵ / mm ² or more.

  In addition to the chemical components, the steel has the above-mentioned microstructure of the steel material, so that it has high strength and base material toughness without any tempering treatment, and the toughness of the hardened layer generated by induction hardening is reduced. It is because it can also improve.

  In the rolled steel material for induction hardening of the present invention, when the area ratio of the fine lamellar pearlite in the microstructure of the lamellar pearlite is less than 20%, the target strength cannot be obtained. On the other hand, when the area ratio of the fine lamellar pearlite in the microstructure exceeds 50%, the toughness of the base material is lowered. Accordingly, the area ratio of the fine lamellar pearlite in the microstructure of the lamellar pearlite is defined as 20 to 50%.

  Of the lamellar pearlite, lamellar pearlite having a lamellar spacing exceeding 200 nm, that is, coarse lamellar pearlite may cause deterioration in strength and base material toughness. For this reason, it is desirable that the area ratio of the lamellar pearlite in the lamellar pearlite with the lamellar spacing exceeding 200 nm in the microstructure is 20% or less.

  In addition, regardless of the lamellar spacing, the area ratio of lamellar pearlite in the microstructure is desirably 50% or less. The smaller lamellar spacing is better, but the limit lamellar spacing obtained by pearlite transformation is about 50 nm.

  Furthermore, when the average crystal grain size of ferrite exceeds 10 μm, it is difficult to obtain target strength and base material toughness. Therefore, the average crystal grain size of ferrite is set to 10 μm or less. The average grain size of ferrite is preferably as small as possible for strengthening by grain refinement. However, in order to form submicron-order crystal grains, special processing conditions or equipment are required, which is industrial. It is difficult to realize. Therefore, as an industrially realizable size, the average crystal grain size of ferrite is about 1 μm.

  On the other hand, when the area ratio of the lamellar pearlite of the lamellar pearlite in the microstructure of the lamellar pearlite occupying the microstructure and the average crystal grain size of the ferrite satisfy the above conditions, the spherical cementite per unit area is a hardened layer during induction hardening. It affects the toughness of the part.

That is, the rolled steel material for induction hardening according to the present invention contains Cr, and about 0.5% of Cr is solid-solved in the cementite, and the cementite that dissolves such Cr at the time of induction hardening. When heated for a short time, it hardly dissolves in the matrix and has a pinning effect of crystal grain growth. However, when the number of spherical cementite is less than 4 × 10 ⁵ pieces / mm ² , the effect of suppressing crystal grain growth in the hardened layer during induction hardening cannot be sufficiently exerted, and the target toughness of the hardened layer can be obtained. Absent. Therefore, the number of spherical cementite was specified to be 4 × 10 ⁵ pieces / mm ² or more. The larger the number of spherical cementite, the better. However, it is substantially about 5 × 10 ⁷ pieces / mm ² .

  As described above, “spherical cementite” refers to cementite having a ratio of the major axis L to the minor axis W (L / W) of 2.0 or less. The number of spherical cementites per unit area is as follows. It can be calculated by a method.

  First, a cross section cut through the central axis of the rolled steel material and parallel to the rolling direction (hereinafter referred to as a “longitudinal section”) is embedded in a resin so that it becomes a test surface, mirror-polished, and then picric alcohol (picral) The microstructure is corroded with a liquid, and a microstructure image is taken for 10 fields of view using a scanning electron microscope (SEM) at a magnification of 5000 times. At this time, the area of each visual field is 25 μm × 20 μm.

Then, using the above-mentioned photographed image, the major axis L and the minor axis W of each cementite are individually measured by image processing software, and those having L / W exceeding 2.0 form lamellar. , Count the number of cementite having L / W of 2.0 or less, that is, spherical cementite, and finally calculate the number of spherical cementite per 1 mm ² (pieces / mm ² ).

The amount of Cr dissolved in the cementite can be calculated from the electrolytic extraction residue. For example, electrolysis is performed using a 10% AA-based electrolytic solution (10% acetylacetone, 1% tetraammonium chloride-methanol), a residue is collected with a 0.2 μm filter, and then the mass of the collected residue is measured. At the same time, after the acid decomposition treatment, ICP-AES (High Frequency Inductively Coupled Plasma Atomic Spectroscopy) is performed to measure the mass of Fe, Cr, and Mn in the residue. Then, if it is assumed that all the residues are M ₃ C-type carbides, that is, cementite, and the mass in the cementite is calculated, the amount of Cr finally dissolved in the cementite can be calculated. By this method, it was found that about 0.5% of the above-described Cr was dissolved in the cementite of the induction hardening rolled steel of the present invention.

3. Production method of induction hardening rolled steel:
The microstructure of the rolled steel for induction hardening according to the present invention described in the previous section can be easily obtained by, for example, hot rolling the material having the chemical components already described by the following rolling method and cooling. Can do.

  In addition, as a hot rolling method, the all-continuous hot rolling method provided with two or more rolling processes is suitable for manufacturing the rolled steel materials for induction hardening of this invention industrially. For this reason, the following description will be based on rolling by the above-described all-continuous hot rolling method (hereinafter sometimes referred to as “all-continuous hot rolling”).

3.1. Heating conditions:
The material to be rolled having the chemical components already described is heated to a temperature range of 670 to 850 ° C. in which austenite and ferrite are the main constituent phases, and then all continuous hot rolling is started.

  By the above heating, the Cr concentration in the cementite constituting the pearlite existing in the material to be rolled, that is, in the steel material before being processed into a predetermined shape by all-continuous hot rolling, can be increased. A part of stable cementite can remain in the matrix. Furthermore, by using such an initial structure, precipitation of fine ferrite from austenite is promoted by processing strain due to rolling, and ferrite can be dynamically recrystallized by processing strain to refine the ferrite. .

  The cementite remaining in the austenite becomes a precipitation site for work-induced cementite during the all-continuous hot rolling process, so that the precipitation of spherical cementite can be promoted.

  In addition, in the heating in the temperature range of 670 to 850 ° C. performed before hot rolling, not only the temperature of the material to be rolled (raw material) is raised to a predetermined area but also the temperature in the cross section of the raw material is uniform. In order to achieve this, heat treatment for a long time may be performed, and in this case, ferrite decarburization may occur on the surface of the material. Therefore, in order to suppress the ferrite decarburization, the heating time in the temperature range is preferably 3 hours or less.

3.2. Full continuous hot rolling conditions after heating:
In order to obtain the desired microstructure of the induction-strengthened rolled steel, the material to be rolled having the chemical components described above is heated under the conditions described in the above section “3.1.”, And then two or more When performing rolling by a fully continuous hot rolling method including a rolling process, the fully continuous hot rolling method preferably satisfies the following conditions [1] and [2].

[1] The surface temperature of the material to be rolled during each rolling step is within a temperature range of 650 to 820 ° C.
[2] The total area reduction rate is 30% or more.

  This is because the temperature in the all-continuous hot rolling is controlled, and in addition, the total area reduction by the rolling is set to a specific value or more, thereby promoting the processing-induced precipitation of ferrite from austenite. This is because a large number of processing strains are introduced into the ferrite to promote recrystallization, and precipitation of processing-induced spherical cementite is promoted into austenite.

  The above-described effects can be manifested in a state in which austenite and ferrite are retained as the main constituent phases. For this purpose, first, the surface temperature of the material to be rolled during each rolling step in all-continuous hot rolling is 820 ° C. or less. It is good to do.

  When the surface temperature of the material to be rolled during each rolling process exceeds 820 ° C., the dislocations introduced in the hot rolling easily disappear with the recovery and recrystallization of austenite. Processing-induced precipitation cannot be performed, and the above effect is difficult to obtain.

  On the other hand, when the surface temperature of the material to be rolled during each rolling step is lower than 650 ° C., many dislocations can be introduced, but austenite starts pearlite transformation by being held at that temperature. Therefore, not only fine lamellar pearlite cannot be generated by cooling after the end of all-continuous hot rolling, which will be described later, but also coarse lamellar pearlite, which is a phase with extremely large deformation resistance, is processed, so the mill load is extremely increased. Resulting in. Therefore, the surface temperature of the material to be rolled during each rolling step is preferably 650 ° C. or higher.

  In the case of the all-continuous hot rolling method, the temperature at the center of the material to be rolled rises due to processing heat generated by rolling. By providing one or more intermediate cooling steps before the rolling step, and performing intermediate cooling in the middle of continuous rolling, the central temperature of the material to be rolled is controlled to a desired temperature, and the main constituent phase is austenite. And the state which becomes a ferrite can be maintained.

  On the other hand, when performing intermediate cooling in the middle stage of all-continuous hot rolling, if the surface temperature of the material to be rolled is too low, in the austenite and ferrite that are the main constituent phases during the cooling or after the cooling ends Austenite starts pearlite transformation, and the pearlite is processed by subsequent rolling. In this case, although a part of the layered cementite constituting the pearlite structure is slightly divided, the aspect ratio of the cementite is not so small and the deformation resistance of the pearlite is extremely large, so the mill load is extremely increased. End up. Further, when the temperature of the material to be rolled is further lowered, the austenite is transformed into a hard phase such as bainite and martensite. When the transformation into the hard phase occurs, the hard phase is rolled, so that the surface of the material to be rolled is cracked during rolling.

  However, even if the surface temperature of the material to be rolled temporarily falls below 650 ° C. during the intermediate cooling step such as water cooling, austenite does not immediately start the pearlite transformation. In the intermediate cooling step, the austenite does not start the transformation unless the time Δt until the surface temperature of the material to be rolled is reheated to a temperature of 650 ° C. or higher after the end of cooling does not substantially exceed 10 s. And the surface of the material to be rolled at the start of the subsequent rolling process even when the surface of the material to be rolled is "supercooled", that is, when the temperature is lowered by intermediate cooling in the middle stage of all-continuous hot rolling If the temperature is reheated to 650 ° C. or more, the surface structure of the material to be rolled can be kept in a state in which austenite and ferrite are the main constituent phases.

  Therefore, the surface temperature of the material to be rolled during each rolling step in the full continuous hot rolling should satisfy the above condition [1], that is, “in the range of 650 to 820 ° C.”. Good.

  Even when the surface temperature of the material to be rolled during each rolling step is in the temperature range of 650 to 820 ° C., when deformation due to rolling progresses, the main constituent phases are stably maintained in the state of austenite and ferrite. May be difficult. For this reason, in two or more rolling mill rows, in particular, in the case of “rough rolling mill row—finish rolling mill row” or “rough rolling mill row” or “rough rolling mill row— In the case of “intermediate rolling mill train—finish rolling mill train”, in the “intermediate rolling mill train” and “finish rolling mill train”, in order to stably and reliably maintain austenite and ferrite as main constituent phases, The surface temperature of the material to be rolled is preferably 650 to 800 ° C.

  In order to more stably carry out the refinement of ferrite crystal grains and further the suppression of pearlite transformation in the rolling process and the dispersion of spherical cementite, in the latter rolling mill row in the above two or more rolling mill rows The surface temperature of the material to be rolled during the rolling process is more preferably 650 to 780 ° C.

  Even if the above condition [1] is satisfied, when the total area reduction in all-continuous hot rolling is less than 30%, the introduction of dislocations accompanying the processing is insufficient, so that processing from austenite Induced ferrite cannot be precipitated, and processing-induced spherical cementite may not be sufficiently precipitated.

  For the above reason, it is preferable that the total area reduction rate satisfies the above condition [2], that is, “30% or more”.

  Note that the total area reduction ratio in all-continuous hot rolling is 60% or more because the fine ferrite is stably precipitated from austenite by work-induced precipitation and sufficient work strain is introduced into the austenite. Is preferred. The upper limit of the total area reduction rate in all-continuous hot rolling is that if the total area reduction rate is extremely increased, as the finish rolling mill is approached, the rolling speed increases, processing heat is generated, and processing heat generation is suppressed. It is about 99.5% because the cooling equipment or rolling layout needs to be extended or expanded significantly.

3.3. Final cooling conditions after completion of all continuous hot rolling:
In order to obtain the desired microstructure of the induction-strengthened rolled steel, the rolled material having the chemical components described above is heated under the conditions described in the above section “3.1.”, And then the “3. .2. "After performing all-continuous hot rolling under the conditions described in the section" 2 ", the steel surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s. Cooling is preferably performed so that the surface temperature is in the range of 500 to 700 ° C.

After finishing all continuous hot rolling, that is, after finishing rolling in the final rolling process, the austenite after the final rolling process is exceeded by setting the steel surface temperature to 600 ° C. or less and exceeding the Ms point within 5 s. Since it becomes cold austenite and the steel surface temperature is in the range of 500 to 700 ° C. by subsequent recuperation, the so-called “supercooling degree” which is the difference between the A ₁ point (about 720 ° C.) in the equilibrium state and the above recuperation temperature. In addition to producing fine lamellar pearlite having a lamellar spacing of 200 nm or less, spherical cementite can be precipitated from austenite into which strain has been introduced by processing. Further, cooling the steel material so that the steel surface temperature is in the range of 500 to 700 ° C. by recuperation suppresses the formation of bainite, and Cr diffuses into the spherical cementite to stabilize the spherical cementite. It will also do.

  After the final rolling step is completed, when the time for which the steel surface temperature is 600 ° C. or less and exceeds the Ms point exceeds 5 s, that is, the steel surface temperature is 600 ° C. within 5 s after the final rolling step is completed. When it does not become below, even if the austenite after the final rolling process does not become supercooled austenite, and the steel material surface temperature is in the range of 500 to 700 ° C. by reheating thereafter, fine lamellar pearlite cannot be generated. On the other hand, when the steel material surface temperature is set to the Ms point or less within 5 s after the rolling in the final rolling process is finished, martensite is generated and fine lamellar pearlite cannot be obtained.

  Even if the steel surface temperature is set to 600 ° C. or less and exceeds the Ms point within 5 s, bainite is generated when the steel surface temperature is below 500 ° C. due to recuperation. It will cause a decline. Moreover, even if the steel material surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s, when the steel material surface temperature exceeds 700 ° C. due to double heat, the generated fine lamellar pearlite is reversely transformed. Therefore, a desired microstructure cannot be obtained. Therefore, after finishing the rolling in the final rolling step, the steel surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s, and then cooled so that the steel surface temperature is in the range of 500 to 700 ° C. by reheating. It is good to do.

  In addition, the steel material surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s described above, and then cooled to room temperature after cooling so that the steel material surface temperature is in the range of 500 to 700 ° C. by reheating. Is not particularly stipulated. For this reason, it may be determined as appropriate from, for example, air cooling (cooling), forced air cooling, mist cooling, etc. in consideration of manufacturing equipment, productivity and the like.

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

  Square billets (160 mm square and 10 m long) made of steels A to V having chemical components shown in Table 1 were prepared.

  Among the above steels, steels A to K are steels whose chemical components are within the range defined in the present invention. On the other hand, steels L to V are steels of comparative examples whose chemical components deviate from the range defined in the present invention.

  In Table 1, the Ms point is also shown for each steel.

  The above-mentioned square billet was set to 95 under the conditions shown as test numbers 1 to 28 in Table 2 by an all-continuous hot rolling line equipped with cooling equipment between the rolling mill rows below. 1% hot rolled and processed into a steel bar with a diameter of 40 mm.

・ Rough rolling mill row: composed of 6 rolling mills,
-First intermediate rolling mill row: composed of two rolling mills,
-Second intermediate rolling mill row: composed of four rolling mills,
-Finish rolling mill row: Consists of two rolling mills.

  In addition, the surface temperature of the to-be-rolled material at the time of rolling and the surface temperature of the to-be-rolled material in the cooling process after completion | finish of continuous rolling were measured using the radiation thermometer. And about the temperature history of the surface part and the center part of the material to be rolled at each stage of rolling and cooling, the surface temperature measurement value measured by the radiation thermometer, the cooling condition in each intermediate cooling equipment, each intermediate cooling equipment Considering the cooling conditions and rolling conditions in the air after leaving, numerical analysis was performed by the difference method.

  After the end of continuous rolling, that is, after finishing rolling by the second rolling mill in the finish rolling mill row, the cooling rate is controlled by the amount of water using a water cooling facility, and the steel surface within 5 s for test numbers 1 to 27 Cooling was performed so that the temperature was 600 ° C. or less and exceeded the Ms point. In addition, about the test number 27, the amount of cooling water was increased rather than the test numbers 1-26, and it cooled strongly. The material to be rolled after leaving the water-cooling facility increased the steel surface temperature due to recuperation. Test No. 28 was left as it was in the air without being cooled with water after the final rolling.

  In Table 2, the rough rolling mill row, the first intermediate rolling mill row, the second intermediate rolling mill row and the finish rolling mill row are respectively represented as “rough row”, “first intermediate row”, “second intermediate row” and The cooling equipment between the rough rolling mill row and the first intermediate rolling mill row is denoted as “finishing row”, and the cooling facility between the first rolling mill row and the first intermediate rolling mill row is referred to as “cooling equipment 1”. The equipment was indicated as “cooling equipment 2”, and the cooling equipment between the second intermediate rolling mill row and the finishing rolling mill row was designated as “cooling equipment 3”. In addition, the “cooling end time” after the final rolling in test numbers 1 to 27 is the water cooling time by the water cooling equipment, “temperature after cooling” is the temperature immediately after the end of water cooling, and “temperature after reheating” is the highest temperature reached by reheating. It is. On the other hand, the “temperature after cooling” in Test No. 28 in which the product was allowed to cool in the air as it was without water cooling after the final rolling was the temperature after 5 seconds had passed after cooling.

  It should be noted that the rolling start temperature, entry side temperature, exit side temperature, rolling end temperature, and post-cooling temperature after re-rolling and post-recuperation temperature shown in Table 2 were measured using a radiation thermometer. It is the surface temperature, and in the intermediate cooling step, the time Δt from the start of cooling until the surface temperature of the rolled material is reheated to 650 ° C. or higher after the end of cooling is the measured surface temperature measured with a radiation thermometer, each intermediate cooling Described by calculating from the temperature history of the surface of the material to be rolled obtained by numerical analysis by the differential method, considering the cooling conditions in the equipment, the cooling conditions in the air after leaving each intermediate cooling equipment, and the rolling conditions It is.

  Furthermore, for each steel bar obtained as described above, the following methods were used to investigate the microstructure, the amount of Cr dissolved in cementite, tensile properties, impact properties, and toughness of the hardened layer generated by induction hardening. did.

  The microstructure investigation was conducted as follows.

  That is, first, a test piece having a length of 20 mm was cut out from each steel bar having a diameter of 40 mm, embedded in a resin so that the longitudinal section of these test pieces became a test surface, mirror-polished, and then 3% nitrate alcohol (Nital The microstructure is formed by corroding with a liquid) and observing the R / 2 portion of each steel bar ("R" represents the radius of the steel bar) with an optical microscope or SEM. Phase identification was performed.

  When the phase constituting the microstructure is composed of ferrite, lamellar pearlite, and cementite, it is mirror-polished again and then corroded with picric acid alcohol (picral liquid), and the magnification is set to 5000 times using an SEM. Microstructure images were taken for the field of view. The area of each visual field is 25 μm × 20 μm.

Next, using the above-mentioned photographed image, the average crystal grain diameter of ferrite is obtained by image processing software, and the major axis L and minor axis W of each cementite are individually measured, and the L / W is 2.0 or less. The number of certain cementite, that is, the number of spherical cementite, was counted, and finally the number of spherical cementite per 1 mm ² area (pieces / mm ² ) was calculated. For lamellar pearlite, the particle size and lamellar number are measured for each pearlite, the average value λ of lamellar spacing is obtained by calculation, and fine lamellar pearlite with λ of 200 nm or less and coarse lamellar pearlite with λ greater than 200 nm. The area ratio of each lamellar perlite in the microstructure was determined by image processing software.

  The amount of Cr dissolved in cementite was calculated using the 10% AA electrolyte solution and the above-described method from the extracted residue.

  For the tensile properties, the 14A test piece (however, the diameter of the parallel part: 7 mm) defined in JIS Z 2201 (1998) is set so that the R / 2 part of each steel bar having a diameter of 40 mm is the central axis of the test piece. The sample was collected and a tensile test was performed at room temperature with a gauge distance of 35 mm to obtain a tensile strength (MPa).

As with the tensile test piece, a 2 mm U-notch Charpy impact test piece was sampled so that the R / 2 part of each steel bar with a diameter of 40 mm was the central axis of the test piece, and a Charpy impact test was conducted at 25 ° C. The impact value (J / cm ² ) was obtained by carrying out.

  The toughness investigation of the hardened layer produced by induction hardening was carried out as follows.

  That is, first, a 2 mm U-notch Charpy impact test piece was collected as described above, and induction hardening was performed so that the hardened layer depth at the U-notch portion (depth from the surface where the Vickers hardness is 450) was 1 mm. Various conditions were adjusted and induction hardening was performed. Thereafter, as in the case of actual parts, tempering treatment was performed at 180 ° C. for 2 hours for the purpose of preventing cracking after induction hardening.

  Next, using the test piece tempered after induction hardening, a three-point bending test was performed at a distance between supporting points of 50 mm and an indentation speed of 0.5 mm / min as shown in FIG. "Distance) curve" is collected, and the load when the load fluctuates due to pop-in, that is, a small crack is defined as "crack generation load", and the toughness of the hardened layer generated by induction hardening is evaluated by this load. did.

As already described, the tensile properties, impact properties, and toughness of the hardened layer produced by induction hardening are respectively set to a tensile strength of 760 MPa or more, an impact value of 200 J / cm ² or more, and a crack generation load of 20 kN. That's it.

  Table 3 shows the results of the above investigations. In Table 3, “F” in the “Microstructure” column indicates ferrite, “LP” indicates lamellar pearlite, “SC” indicates spherical cementite, and “B” indicates bainite. The “○” mark in the “Evaluation” column indicates that the tensile properties, impact properties, and toughness targets of the hardened layer generated by induction hardening are all satisfied, while the “×” mark indicates the above target. It means that even one of them is not satisfied. The “−” in test numbers 19, 23, 24 and 27 indicates that the phase is not “F + LP + SC” and is not investigated.

From Table 3, in the case of the steel bars of test numbers 1 to 11 that satisfy the conditions of chemical components and microstructure defined in the present invention, the evaluation is “◯”, and the desired properties without performing the tempering treatment, That is, the tensile strength was not less than 760 MPa, and the excellent mechanical property that the impact value at a test temperature of 25 ° C. in a Charpy impact test using a 2 mm U-notch Charpy impact test piece was not less than 200 J / cm ² , and further generated by induction hardening. It is clear that a rolled steel material excellent in the toughness of the hardened layer with a crack generation load of the hardened layer of 20 kN or more can be stably obtained at low cost.

  On the other hand, from Table 3, in the case of steel bars with test numbers 12 to 28 that deviate from at least one of the chemical components and microstructure conditions defined in the present invention, the evaluation is “x”, and the desired It is clear that the characteristics are not obtained and the tempering process cannot be omitted.

  That is, in the case of test number 12, the C content of the steel L used is as low as 0.32%, which is lower than the value specified in the present invention. For this reason, the strength of the rolled steel bar is lowered and the desired strength cannot be obtained, the strength of the hardened layer produced by induction hardening is low, and the crack generation load is as low as 9 kN.

In the case of test number 13, the C content of the steel M used is as high as 0.60%, which exceeds the value specified in the present invention. For this reason, the Charpy impact value of the base material is as low as 130 J / cm ^2, and further, the toughness of the hardened layer generated by induction hardening is lowered, and the crack generation load is as low as 17 kN.

In the case of test number 14, the Si content of the steel N used is as high as 1.20%, which exceeds the value specified in the present invention. For this reason, since the A ₃ transformation point rises and ferrite remains in the hardened layer generated by induction hardening, the strength is lowered and the crack initiation load is as low as 11 kN.

  In the case of test number 15, the Mn content of the steel O used is as low as 0.10%, which is lower than the value specified in the present invention. For this reason, the strength of the rolled steel bar is reduced, the desired strength cannot be obtained, the induction hardenability is low, the fine pearlite structure exists in the hardened layer generated by the induction hardening, and the toughness of the hardened layer is reduced. The crack generation load is as low as 9 kN.

In the case of test number 16, the Cr content of the steel P used is as low as 0.05%, which is lower than the value specified in the present invention. For this reason, since cementite cannot remain in the heating stage or in the middle of rolling, the number of spherical cementite enriched with Cr is as small as 2.0 × 10 ⁵ pieces / mm ^2, and the Charpy impact value of the base material is 100 J. / Cm ² and low. Furthermore, the toughness of the hardened layer produced by induction hardening is also reduced, and the crack initiation load is as low as 12 kN.

In the case of test number 17, the Cr content of the steel Q used is as high as 2.20%, which exceeds the value specified in the present invention. For this reason, although spherical cementite can be finely dispersed and high base material strength can be obtained, the base material has a low Charpy impact value of 120 J / cm ² .

  In the case of test number 18, the N content of the steel R used is as high as 0.015%, which exceeds the value specified in the present invention, and the Ti content is 0.030%, the value specified in the present invention. It is below. For this reason, the effect of improving the induction hardenability of B is reduced by forming BN, and the effect of suppressing segregation of P and S to the austenite crystal grain boundary during induction hardening by B is poor, and is generated by induction hardening. The toughness of the hardened layer is lowered, and the crack initiation load is as low as 9 kN.

In the case of test number 19, the steel S used does not contain B and Ti, and is lower than the value specified in the present invention. For this reason, the effect of shifting the upper limit of the temperature range where the austenite and ferrite of B are the main constituent phases to the high temperature side cannot be obtained, and the constituent phases of the microstructure after rolling are ferrite and coarse lamellar pearlite, The organization does not include. Therefore, although the target strength can be obtained due to the presence of the hard coarse lamellar pearlite phase, the Charpy impact value of the base material is as low as 120 J / cm ² due to the absence of the spherical cementite phase. Furthermore, the effect of improving the induction hardenability and the effect of suppressing the segregation of P and S to the austenite grain boundaries during induction hardening cannot be obtained, the toughness of the hardened layer generated by induction hardening is reduced, and the crack initiation load is 17 kN. Low.

  In the case of test number 20, the Ti content of the steel T used is as low as 0.008%, which is lower than the value specified in the present invention. For this reason, N which does not couple | bond with Ti but remains in steel couple | bonds with B, and forms BN. Therefore, since B does not exist as “solid solution B”, the effect of improving the induction hardenability of B and the effect of suppressing the segregation of P and S to the austenite grain boundaries during induction hardening cannot be obtained. The toughness of the layer is reduced and the crack initiation load is as low as 8 kN.

  In the case of test number 21, the Ti content of the steel U used is as high as 0.060%, which exceeds the value specified in the present invention. For this reason, excess residual Ti combines with C in the steel to form carbide (TiC), so the amount of C in the steel decreases, the proportion of ferrite increases, and induction hardenability decreases. Further, the toughness of the induction-hardened hardened layer also decreases, and the crack initiation load is as low as 9 kN.

In the case of test number 22, the value of fn1 of the steel V used is as high as 1.24, which exceeds the value specified in the present invention. For this reason, the Charpy impact value of the base material is as low as 140 J / cm ² .

In the case of Test No. 23 and Test No. 24, the chemical composition of Steel A used satisfies the conditions specified in the present invention, but both have a microstructure composed of ferrite and lamellar pearlite and do not contain spherical cementite. . Therefore, although the target intensity is e by the presence of the hard coarse Lamellar pearlite phase, by not contain spheroidal cementite phase, Charpy impact value of the base material, respectively, 110J / cm ² and 95 J / cm ² and lower, Furthermore, the toughness of the hardened layer produced by induction hardening is reduced, and the crack initiation load is as low as 16 kN and 15 kN, respectively.

Also in the case of test number 25, the chemical composition of the steel A used satisfies the conditions specified in the present invention, but the area ratio of the fine lamellar pearlite in the microstructure exceeds the conditions specified in the present invention. The number is below the conditions defined in the present invention. For this reason, the Charpy impact value of the base material is as low as 100 J / cm ² , the toughness of the hardened layer generated by induction hardening is lowered, and the crack generation load is as low as 19 kN.

Also in the case of test number 26, the chemical composition of the steel B used satisfies the conditions specified in the present invention, but the area ratio of the fine lamellar pearlite in the microstructure exceeds the conditions specified in the present invention. The number is below the conditions defined in the present invention. For this reason, the Charpy impact value of the base material is as low as 90 J / cm ² , the toughness of the hardened layer generated by induction hardening is lowered, and the crack generation load is as low as 17 kN.

Also in the case of test number 27, the chemical composition of the steel C used satisfied the conditions specified in the present invention, but did not undergo pearlite transformation. Therefore, the microstructure was composed of bainite having a hard phase, ferrite, and spherical cementite. It deviates from the conditions defined in the present invention. For this reason, the Charpy impact value of the base material is as low as 120 kJ / cm ² , the toughness of the hardened layer generated by induction hardening is reduced, and the crack generation load is as low as 10 kN.

  Even in the case of test number 28, the chemical composition of the steel C used satisfies the conditions specified in the present invention, but the microstructure of fine lamellar pearlite (that is, lamellar pearlite having a lamellar spacing of 200 nm or less) among lamellar pearlite. The area ratio occupied is 5%, which is lower than the conditions defined in the present invention. For this reason, the tensile strength is as low as 695 MPa.

The rolled steel material for induction hardening according to the present invention does not necessarily contain expensive V, and the tensile strength is not less than 760 MPa and a 2 mm U notch Charpy impact test piece in the state of the rolled steel material without performing tempering treatment. In the Charpy impact test used, the impact value at a test temperature of 25 ° C. is 200 J / cm ² or more. Further, since the grain growth hardly occurs during induction hardening, the hardened layer has excellent toughness. It is suitable for use as a material for parts such as rack bars that require bending strength and impact characteristics. This rolled steel material for induction hardening can be stably manufactured at low cost by the method of the present invention.

Claims (4)

In mass%, C: 0.38 to 0.55%, Si: 1.0% or less, Mn: 0.20 to 2.0%, P: 0.020% or less, S: 0.10% or less, Cr: 0.10 to 2.0%, Al: 0.10% or less, B: 0.0005 to 0.0030%, N: 0.008% or less, and Ti: 0.047% or less (however, 3. 4N ≦ Ti ≦ 3.4N + 0.02%), with the balance being Fe and impurities, having a chemical component in which the value of fn1 represented by the following formula (1) is 1.20 or less, The structure is composed of ferrite, lamellar pearlite and spherical cementite, the average grain size of ferrite is 10 μm or less, the lamellar pearlite whose lamellar spacing is 200 nm or less is 20% to 50% of the area ratio in the microstructure and spherical cementite of Induction hardening rolled steel for, wherein the number is 4 × 10 ⁵ cells / mm ² or more.
fn1 = C + (1/10) Si + (1/5) Mn + (5/22) Cr + 1.65V− (5/7) S (1)
However, C, Si, Mn, Cr, V, and S in the above formula (1) represent the content of each element in mass%.   The chemical component contains one or more elements of Cu: 1.0% or less, Ni: 3.0% or less, and Mo: 0.50% or less in mass% instead of a part of Fe. The rolled steel material for induction hardening according to claim 1.   The chemical component contains at least one element selected from Nb: 0.10% or less and V: 0.30% or less in mass%, instead of a part of Fe. Or the rolled steel materials for induction hardening of 2. The material to be rolled having the chemical component according to any one of claims 1 to 3 is heated to a temperature range of 670 to 850 ° C, and then rolled by an all-continuous hot rolling method including two or more rolling steps. Furthermore, after finishing the rolling in the final rolling step, the steel material surface temperature is set to a temperature exceeding 600 ° C. and exceeding the Ms point within 5 s, and then the steel material surface temperature is in the range of 500 to 700 ° C. by reheating. A method of manufacturing a rolled steel material for induction hardening to be cooled, wherein the all-continuous hot rolling method satisfies the following [1] and [2].
[1] The surface temperature of the material to be rolled during each rolling step is within a temperature range of 650 to 820 ° C. [2] The total area reduction is 30% or more.

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