JP2011241466A - 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 has chemical components comprising, 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, 0.010 to 0.10% Al, 0.004 to 0.03% N and the balance being Fe with impurities, wherein a value of [C+(1/10)Si+(1/5)Mn+(5/22)Cr+1.65V-(5/7)S] is ≤1.20, and the total content of Si, Mn and Cr satisfies 1.2 to 3.5%. 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, the lamellar pearlite accounts for ≤20% (including 0%) of the area of the microstructure, and the number of the spheroidal cementite is 6×10pieces/mmor more. The rolled steel material may further comprise specific amounts of one or more elements chosen from Cu, Ni, Mo, Ti, Nb and V, respectively.

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 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, for steel materials used as rack bar materials that require the above characteristics,
・ 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 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, by plastic deformation of 15% or more by finish rolling, and subsequently promoting pearlite or bainite transformation, these structures are formed, and at the same time, Ac ₁ point or more, Ac ₃ again by deformation heat. 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.

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

  The technique proposed in the above-mentioned Patent Document 1 performs a tempering process, and as described above, is not desirable in terms of cost rather than energy and manufacturing man-hours. 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.

  The non-heat treated steel bar of Patent Document 2 needs to contain 0.08% or more of V, and the non-heat treated steel of Patent Document 3 needs to contain 0.2% or more of V. In this situation, the use of V, which is an expensive element, was 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 of the hardened layer when the high-frequency heating is performed, and the toughness of the hardened layer is sufficient. I couldn'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 the above-mentioned 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 reduces the processing time of spheroidizing annealing performed 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 for induction hardening while maintaining the shape of the hot-rolled steel material like a rack bar. Patent Document 6 itself is originally used, It is not intended to obtain a rolled steel material having high strength and base material toughness, and further excellent toughness of a hardened layer produced by induction hardening.

  Similarly to 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 material for cold forging, that is, It aims at providing the manufacturing method of a wire. Therefore, the steel bars and wires manufactured by the method of Patent Document 7 are not used by induction hardening while maintaining the shape of the hot-rolled steel material like a rack bar, and Patent Document 7 itself is high in the first place. It is not intended to obtain a rolled steel material that is excellent in strength and base material toughness, and also in the toughness of a hardened layer produced by induction hardening.

  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 tensile strength of 600 MPa or more in the state of rolled steel, 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. The impact value means 150 J / cm ² or more, and the excellent toughness of the hardened layer means that the cracking strength of the hardened layer by the test method described later is a load of 10 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 region of austenite and ferrite, and precipitation of ferrite from austenite is promoted by processing strain due to rolling to increase the ferrite fraction and processing strain. Thus, the ferrite is dynamically recrystallized to refine the ferrite. However, simply by hot rolling in the two-phase temperature range of austenite and ferrite, the austenite remaining at the end of hot rolling forms a pearlite structure during cooling, making it difficult to suppress layered cementite. is there. For this reason, the target toughness level cannot be obtained.

(C) In order to suppress layered cementite generated from austenite existing at the end of hot rolling in the two-phase temperature region of austenite and ferrite,
・ When heating to the two-phase temperature range of austenite and ferrite before rolling, the cementite is not completely dissolved as in the prior art, but remains so that there are three phases of austenite, ferrite and cementite. To
-At the end of hot rolling, make the processed austenite stretched as much as possible, or make fine austenite exist,
・ Cementite remains in the austenite.
It is necessary to satisfy the condition. That is, if the three-phase state of ferrite, austenite, and cementite can be satisfied at the end of the hot rolling, the formation of layered cementite constituting the pearlite structure can be suppressed after cooling, and spherical cementite can be obtained.

  (D) In order to realize a three-phase state of ferrite, austenite, and cementite at the end of hot rolling, the Cr content in the steel component is adjusted and at a temperature at which Cr can be concentrated in cementite. It is necessary to perform heating and hot rolling, and thereby, it is possible to suppress the layered cementite constituting the pearlite structure and to form spherical cementite.

  (E) When the structure of the steel material is mainly composed of fine ferrite and spherical cementite, both high strength and excellent toughness can be achieved.

  (F) In the steel material having the structure obtained by the above-described manufacturing method, a large amount of Cr-concentrated spherical cementite is precipitated, and in the case of short-time heating such as high-frequency heating, this Cr is concentrated. Spherical cementite is difficult to dissolve in austenite. For this reason, the above-mentioned spherical cementite as pinning particles together with AlN suppresses the growth of austenite grains during induction heating, thereby increasing the toughness of the induction hardening layer and suppressing the occurrence of cracks in the induction hardening layer. it can.

(G) 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 600 MPa or more by achieving appropriate strengthening by solid solution strengthening and precipitation strengthening. The impact value at 25 ° C. is 150 J / cm ² or more, and the crack generation strength of the hardened layer by the test method described later can achieve the target characteristic of a load of 10 kN or more in a three-point bending test.

  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 % Or less, Cr: 0.10 to 2.0%, Al: 0.010 to 0.10% and N: 0.004 to 0.03%, with the balance being Fe and impurities, 1) It has a chemical component satisfying a Ceq value of 1.20 or less and a total content of Si, Mn and Cr of 1.2 to 3.5%, and the microstructure is ferrite, lamellar pearlite and It consists of spherical cementite, the average grain size of the ferrite is 10 μm or less, the area ratio of the lamellar pearlite in the microstructure is 20% or less (including 0%), and the number of spherical cementite is 6 × 10 ⁵ pieces / mm ² or more. Rolled steel for induction hardening, characterized by .
Ceq = 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] The chemical component is, by mass%, further containing one or more elements of Cu: 1.0% or less, Ni: 3.0% or less, and Mo: 0.50% or less. The rolled steel material for induction hardening according to the above [1].

  [3] The chemical component further includes at least one element selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.30% or less in terms of mass%. The rolled steel material for induction hardening according to [1] or [2] above.

[4] All-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 810 ° C. A method for producing a rolled steel for induction hardening, in which rolling is performed by a hot rolling method, and after the rolling in the final rolling step is finished, a temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or less. An all-continuous hot rolling method satisfies the following [1] and [2]: A method for producing a rolled steel for induction hardening, characterized in that:
[1] The surface temperature of the material to be rolled during each rolling step is within a temperature range of 650 to 810 ° C.
[2] The total area reduction rate is 30% or more.

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

  The “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.

  Further, the “all continuous hot rolling method” is, 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 comprising the rolling mill rows, a method in which the material to be rolled cannot be left between the rolling mill rows is indicated. 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 (%).

The rolled steel material for induction hardening according to the present invention does not necessarily contain expensive V, and even without tempering treatment, a tensile strength of 600 MPa or more and a 2 mm U notch Charpy impact test piece in the state of the rolled steel material. In the used Charpy impact test, the impact value at a test temperature of 25 ° C. is 150 J / cm ² or more. Further, since the grain growth hardly occurs during induction hardening, the hardened layer is excellent in 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 which arranges and shows the relation of tensile strength (MPa) and total content of Si, Mn, and Cr using test numbers 1-12 and test number 22 among the test numbers of an example. In FIG. 1, the total content of Si, Mn and Cr is represented as “Si + Mn + Cr”. 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. Note that. In the following description, “%” notation of the content of each element means “mass%”.

1. Chemical composition:
C: 0.35-0.55%
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 due to the above action 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. Further, the C content is preferably 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. The Si content is preferably 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 content of Mn is less than 0.20%, the desired effect due to the above action cannot be obtained. On the other hand, even if it contains Mn exceeding 2.0%, the said effect will be saturated and cost will increase. Furthermore, the base material toughness is deteriorated. 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 preferably 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%. Note that the Cr content is preferably 0.20% or more, and more preferably 1.8% or less.

Al: 0.010 to 0.10%
Al combines with N in steel to form AlN, and has the effect of suppressing the coarsening of crystal grains during induction hardening and improving the toughness of the hardened layer produced by induction hardening. However, such an effect cannot be obtained when the Al content is less than 0.010%. On the other hand, Al is an element having a deoxidizing action like Si, but raises the A ₃ transformation point and causes induction hardenability to be lowered. 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 content of Al is set to 0.010 to 0.10%. The Al content is preferably 0.08% or less. On the other hand, from the viewpoint of preventing crystal grain coarsening during induction hardening, the Al content is preferably 0.015% or more for the reliable formation of the above-described AlN, and 0.020% or more. Even more preferred.

N: 0.004 to 0.03%
N combines with Al in steel to form AlN, suppresses the coarsening of crystal grains during induction hardening, and has the effect of improving the toughness of a hardened layer generated by induction hardening by refining crystal grains . However, when the N content is less than 0.004%, the effect is insufficient. On the other hand, when the N content exceeds 0.03%, the toughness of the base material is lowered. Therefore, the content of N is set to 0.004 to 0.03%. The N content is preferably 0.0045% or more, and is preferably 0.02% or less.

Ceq value: 1.20 or less In the present invention, the value of Ceq, that is,
Ceq = 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. The rolled steel for induction hardening according to the present invention is excellent in having a microstructure composed of “fine ferrite, lamellar pearlite and spherical cementite having an area ratio of 20% or less (including 0%)” as described later. “Strength-Toughness Balance” is ensured, but even if such a microstructure can be obtained, if the value of Ceq exceeds 1.20, the target base material toughness (2 mm U notch Charpy impact test piece The impact value at a test temperature of 25 ° C. in the Charpy impact test used is 150 J / cm ² or more). Therefore, the value of Ceq represented by the above formula (1) is set to 1.20 or less.

  The Ceq value is preferably 1.00 or less. Further, the value of Ceq is preferably 0.60 or more in order to ensure strength.

Total content of Si, Mn and Cr: 1.2 to 3.5%
Even if the Ceq value satisfies the above range, the microstructure of simple “fine ferrite, lamellar pearlite and spherical cementite with an area ratio of 20% or less (including 0%)” is mainly composed of a soft ferrite phase. The target strength cannot be obtained.

  In other words, in order to increase the strength of the microstructure of “fine ferrite, lamellar pearlite and spherical cementite with an area ratio of 20% or less (including 0%)”, the soft phase constituting the main body of the microstructure is used. Some ferrite needs to be strengthened.

  The rolled steel material for induction hardening of the present invention has fine ferrite phase grains, but it cannot reach the desired strength level only by refining the grains.

  The rolled steel for induction hardening according to the present invention has a target strength by solid solution strengthening of ferrite phase with Si and Mn and precipitation strengthening with fine spherical cementite enriched with Cr in addition to grain refinement of ferrite phase. Can be obtained.

  And Cr in the steel component contributes to fine dispersion of spherical cementite.

  Among the test numbers of Examples described later, as shown in FIG. 1 arranged using Test Nos. 1 to 12 of the present invention and Comparative Example 22 having a small total content of Si, Mn and Cr, By setting the total content of Mn and Cr to 1.2% or more, a tensile strength of 600 MPa can be obtained. In FIG. 1, the total content of Si, Mn and Cr is represented as “Si + Mn + Cr”.

  If the total content of Si, Mn and Cr is less than 1.2%, the target strength cannot be obtained, and if it exceeds 3.5%, the strength can be improved, but the base material toughness is reduced. Invite. Therefore, the total content of Si, Mn and Cr is set to 1.2 to 3.5%.

  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.

  The chemical component of the rolled steel for induction hardening according to the present invention may further contain one or more elements selected from the following first group and second group as necessary.

First group: Cu: 1.0% or less, Ni: 3.0% or less and Mo: 0.50% or less Second group: Ti: 0.10% or less, Nb: 0.10% or less and V: 0 .30% or less That is, it may be a chemical component containing one or more elements of the first group and the second group as optional elements.

  Hereinafter, the above optional elements will be described.

  Cu, Ni, and Mo, which are elements of the first group, have the effects of improving the induction hardenability and increasing the strength, so that the above elements may be included in order to obtain this effect. 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.

  In addition, said Cu, Ni, and Mo can be contained only in any 1 type in 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.

  Next, Ti, Nb and V, which are elements of the second group, have a crystal grain refining action, so that the above elements may be included in order to obtain this effect. Hereinafter, the second group of elements will be described in detail.

Ti: 0.10% or less Ti combines with carbon or nitrogen in steel to form carbides, nitrides or carbonitrides, and has the effect of refining crystal grains during hot rolling or induction hardening. Therefore, Ti may be contained for crystal grain refinement. However, when Ti is contained in an amount exceeding 0.10%, the effect of refining crystal grains can be expected, but the toughness is reduced. Therefore, when Ti is included, the amount of Ti is set to 0.10% or less. In addition, from the viewpoint of suppressing toughness reduction, the Ti content when contained is preferably 0.08% or less.

  On the other hand, the Ti content is preferably 0.010% or more in order to reliably exhibit the Ti crystal grain refining effect, in particular, the crystal grain refining effect during induction hardening. From the viewpoint of suppression, 0.015% or more is more preferable.

Nb: 0.10% or less Nb combines with carbon or nitrogen in steel to form carbides or carbonitrides and has the effect of 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 0.080% 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 a function of forming carbides or carbonitrides by combining with carbon or nitrogen in steel to refine 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.

  In addition, said Ti, Nb, and V can be contained only in any 1 type in them, or 2 or more types of composites. In addition, although the total amount of these elements in the case of making it contain may be 0.50% or less, it is preferable to set it as 0.41% or less.

2. Microstructure:
The microstructure of the rolled steel for induction hardening 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 area occupied by the lamellar pearlite microstructure. The ratio should be 20% or less (including 0%) and the number of spherical cementite should be 6 × 10 ⁵ pieces / 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 for induction hardening of the present invention, the smaller the lamellar pearlite in the microstructure, the better. However, if the lamellar pearlite is 20% or less in area ratio, the target performance can be obtained. On the other hand, when lamellar pearlite exceeds 20% in area ratio, the base material toughness is reduced. Therefore, the area ratio of the lamellar pearlite in the microstructure is defined as 20% or less (including 0%).

  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 average crystal grain size of lamellar pearlite and ferrite occupying the microstructure satisfies the above conditions, spherical cementite per unit area affects the strength and toughness of the hardened layer portion during induction hardening.

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 6 × 10 ⁵ pieces / mm ² , the target strength cannot be obtained, and the effect of suppressing the growth of crystal grains in the hardened layer during induction hardening cannot be sufficiently exhibited. The target toughness of the hardened layer cannot be obtained. Therefore, the number of spherical cementite was defined as 6 × 10 ⁵ pieces / mm ² or more. The larger the number of spherical cementite, the better. However, the upper limit is substantially 1 × 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 10% AA-based electrolyte (10% acetylacetone, 1% tetraammonium chloride / methanol), and the 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 is based on the rolling by the above-mentioned all continuous hot rolling method (hereinafter, simply 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 810 ° C. in which the main constituent phases are austenite and ferrite, and then all continuous hot rolling is started.

  By this heating, the cementite in the pearlite that was present in the material to be rolled, that is, the steel material before being processed into a predetermined shape by full continuous hot rolling, can be adjusted by adjusting the steel components, especially by optimizing the Cr content. In addition to being able to stabilize, the cementite can remain in the austenite matrix without being completely dissolved.

  The cementite remaining in the austenite becomes a precipitation site of work-induced cementite in the all-continuous hot rolling process, and at the end of the all-continuous hot rolling, fine cementite or It exists as spherical cementite. The presence of fine granular or spherical cementite at the end of the all-continuous hot rolling suppresses the formation of layered cementite during the cooling process and ultimately leads to improved toughness.

  Accordingly, the material to be rolled having the chemical components already described is heated to a temperature range of 670 to 810 ° C. in which austenite and ferrite are the main constituent phases, and then all continuous hot rolling is started.

  In addition, in the heating in the temperature range of 670 to 810 ° C. performed before hot rolling, the temperature of the material to be rolled (raw material) is not only increased to a predetermined region, but 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 810 ° C.
[2] The total area reduction rate is 30% or more.

  This is a heating step before all-continuous hot rolling, and the cementite in the pearlite that was present in the material to be rolled, which is the rolling material, is not completely dissolved in the matrix, In the process of all-continuous hot rolling, the above-mentioned remaining cementite is utilized as a precipitation site for cementite that precipitates during rolling, and the prior austenite grain boundaries and This is because the cementite is processed and precipitated in the prior austenite grains and further grown, whereby spherical cementite can be present in the austenite at the end of the hot rolling.

  In other words, by controlling the temperature in all-continuous hot rolling, in addition to making the total area reduction rate by the rolling equal to or higher than a specific value and promoting processing-induced precipitation, the fineness remaining at the heating stage described above can be obtained. This is because it is possible to prevent solid granular or spherical cementite, and further, cementite that has been induced by processing in the hot rolling stage from being dissolved in austenite.

  That is, the all-continuous hot rolling temperature maintains a two-phase structure of austenite and ferrite and a three-phase structure of non-equilibrium cementite, and the cementite does not dissolve during the hot rolling, but rather is process-induced precipitation. For this purpose, the material to be rolled is heated to the above-mentioned temperature range of 670 to 810 ° C. before the hot rolling to obtain a structure of austenite, ferrite and residual cementite, It is preferable to manage the temperature of hot rolling on the low temperature side of the temperature range in which austenite and ferrite can exist as main constituent phases.

  Here, setting the total continuous hot rolling temperature to the low temperature side of the temperature range in which austenite and ferrite can exist as the main constituent phases allows introduction of many dislocations and is introduced into the ferrite phase. The dislocations are used for dynamic recrystallization of ferrite, and the refinement of ferrite grains is promoted. The dislocations introduced into austenite are not easily lost and remain in the prior austenite grain boundaries and the prior austenite grains. Because it accumulates in the vicinity of cementite, the dislocation density increases, and cementite precipitates preferentially from austenite in the vicinity of the residual cementite, that is, it is used for work-induced precipitation. .

  And the above-mentioned effect can be expressed in a state where austenite and ferrite are held as main constituent phases. For this purpose, first, the upper limit of the surface temperature of the material to be rolled during each rolling step in all-continuous hot rolling. Is preferably 810 ° C.

  That is, when the surface temperature of the material to be rolled during each rolling process exceeds 810 ° C., the dislocations introduced in the hot rolling easily disappear with the recovery and recrystallization of austenite. It is not possible to sufficiently induce processing-induced precipitation, making it difficult to obtain the above effect.

  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. When pearlite generated by transformation is rolled, a part of the layered cementite constituting the pearlite structure is slightly divided, but the cementite aspect ratio is not so small. In addition, since the deformation resistance of pearlite is extremely large, the mill load is extremely increased. Therefore, the lower limit of 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 all-continuous hot rolling satisfies the above condition [1], that is, “within a range of 650 to 810 ° C.”.

  Even when the surface temperature of the material to be rolled during each rolling step is in the temperature range of 650 to 810 ° 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 790 ° C.

  In order to make ferrite crystal grains finer, and more stably to disperse spherical cementite and to suppress lamellar cementite, during the rolling process in the latter rolling mill row in the above two or more rolling mill rows. The surface temperature of the material to be rolled is more preferably 650 to 770 ° C.

  Even if the above-mentioned 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 the prior austenite grain boundaries In some cases, fine cementite cannot be processed and precipitated in the prior austenite grains.

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

  The total area reduction rate in all-continuous hot rolling is preferably 60% or more, because fine cementite is stably precipitated from austenite by processing-induced precipitation. 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 a continuous hot rolling under the conditions described in the section" 2 "to form a predetermined shape, the temperature range up to 400 ° C should be finally cooled at a cooling rate of 5 ° C / s or less. .

  When the final cooling rate in the temperature range up to 400 ° C exceeds 5 ° C / s after completion of all-continuous hot rolling, that is, after rolling in the final rolling step, pearlite transformation is suppressed during the cooling. In some cases, the area ratio of lamellar pearlite in the microstructure exceeds 20%. In addition, when the final cooling rate becomes extremely large, bainite transformation and martensite transformation occur instead of pearlite transformation, so that precipitation of layered cementite can be suppressed, but the hardness of the rolled material becomes too high, so that toughness Will be reduced. Therefore, after the rolling to a predetermined shape is completed, the temperature range up to 400 ° C. is preferably finally cooled under the condition that the cooling rate is 5 ° C./s or less.

  The temperature range for final cooling at the cooling rate of 5 ° C./s or less is sufficient up to 400 ° C. after rolling, and the temperature range below 400 ° C. is not particularly specified. 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 and productivity.

  In addition, the lower limit of the final cooling rate in the temperature range up to 400 ° C. is that if the cooling rate is slowed down, the effect of suppressing pearlite increases, but a temperature control facility for slowing down the cooling rate is necessary, and as a result Since it causes an increase in manufacturing cost, it is preferably set to 1 ° C./min.

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

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

  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 the end of rolling by the second rolling mill in the finish rolling mill row, the cooling rate is controlled by changing the cooling medium such as air cooling or air cooling, Final cooling to 400 ° C. The subsequent cooling was allowed to cool in the atmosphere.

  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”.

  Note that the rolling start temperature, entry side temperature, exit side temperature, and rolling end temperature described in Table 2 are the surface temperature of the material to be rolled measured using a radiation thermometer, up to 400 ° C. after the end of continuous rolling. The cooling rate was calculated | required with the surface temperature of the to-be-rolled material measured using the radiation thermometer, and the cooling time to 400 degreeC. In the intermediate cooling step, the time Δt from the start of cooling until the end of cooling until the surface temperature of the material to be reheated to 650 ° C. or higher is the surface temperature measurement value measured with a radiation thermometer, the cooling conditions in each intermediate cooling facility, It is calculated and described from the temperature history of the surface of the material to be rolled obtained by numerical analysis by the difference method in consideration of the cooling conditions and rolling conditions in the air after leaving each intermediate cooling facility.

  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 is cut out from each steel bar having a diameter of 40 mm, embedded in a resin so that the longitudinal section thereof becomes the test surface through the central axis of these test pieces, and after mirror polishing, 3 It was corroded with% nitric acid alcohol (nitral solution) to reveal a microstructure, and an optical microscope or SEM observation was performed to identify phases constituting the microstructure.

  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.

  Moreover, the presence or absence of the production | generation of decarburization of a test piece surface layer part was confirmed using the same test piece. That is, after corroding each test piece with a night liquid, using an optical microscope, observing magnification was set to 400 times, observing the surface area of the test piece with 8 visual fields, and measuring the total decarburization depth DM-T, The total decarburized layer depth DM-T was evaluated by an arithmetic average value. It was determined that decarburization occurred when DM-T was 0.2 mm or more.

Next, using the above photographed image, the area ratio of lamellar pearlite occupying the microstructure and the average crystal grain size of ferrite are obtained by image processing software, and the major axis L and minor axis W of each cementite are individually measured. The number of cementite having L / W of 2.0 or less, that is, the number of spherical cementite was counted, and the number of spherical cementite per 1 mm ² area (pieces / mm ² ) was finally calculated.

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

  The tensile property is a 14A test piece as defined in JIS Z 2201 (1998) (where the diameter of the parallel part is 7 mm) such that a portion in the radial direction of each steel bar having a diameter of 40 mm is the central axis of the test piece. ) Was taken and a tensile test was carried out 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 central portion of the test piece had a radial half of each steel bar having a diameter of 40 mm, and a Charpy impact was obtained at 25 ° C. The test was carried out to determine the impact value (J / cm ² ).

  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 fulcrums 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 600 MPa or more, an impact value of 150 J / cm ² or more, and a crack generation load of 10 kN. That's it.

  Tables 3 and 4 show the results of the above investigations. In Tables 3 and 4, 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 “x” mark indicates that even one of the above goals is not satisfied.

  Moreover, in FIG. 1, among the test numbers of the examples, the test numbers 1 to 12 and the test number 22 are used to organize the relationship between the tensile strength (MPa) and the total content of Si, Mn, and Cr. Show. In FIG. 1, the total content of Si, Mn and Cr is represented as “Si + Mn + Cr”.

From Table 3, in the case of the steel bars of test numbers 1 to 12 that satisfy the conditions of the 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 600 MPa or more, and the mechanical properties at a test temperature of 25 ° C. in a Charpy impact test using a 2 mm U-notch Charpy impact test piece were 150 J / cm ² or more. Further, the tensile strength was generated by induction hardening. It is apparent that a rolled steel material excellent in the toughness of the hardened layer with a crack generation load of the hardened layer of 10 kN or more can be stably obtained at a low cost.

  On the other hand, from Table 4, in the case of the steel bars of test numbers 13 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 13, the C content of the steel M used is as low as 0.35%, which is lower than the value specified in the present invention. For this reason, the strength of the hardened layer generated by induction hardening is low, and the crack generation load is as low as 8 kN.

In the case of test number 14, the C content of the steel N used is as high as 0.58%, which exceeds the value specified in the present invention. For this reason, the Charpy impact value of the base material is as low as 140.0 J / cm ^2, and the toughness of the hardened layer generated by induction hardening is reduced, and the crack generation load is as low as 6 kN.

In the case of test number 15, the Si content of the steel O used is as high as 1.20%, which exceeds the value specified in the present invention. For this reason, the A ₃ transformation point rises, and ferrite remains in the hardened layer generated by induction hardening, so that the strength is reduced and the crack initiation load is as low as 5 kN. Moreover, since the Si content was high, decarburization occurred on the surface of the rolled steel material.

  In the case of test number 16, the Mn content of the steel P used is as low as 0.10%, which is lower than the value specified in the present invention. For this reason, induction hardenability is low, a fine pearlite structure exists in the hardened layer produced by induction hardening, the toughness of the hardened layer is lowered, and the crack generation load is as low as 4 kN.

In the case of the test number 17 and the test number 21, the Ceq values of the steel Q and the steel U used are as high as 1.26 and 1.34, respectively, exceeding the values specified in the present invention. Therefore, each of the Charpy impact value of the base material, a low and 110.0J / cm ² and 125.0J / cm ^2.

  In the case of test number 18, the N content of the steel R used is as low as 0.002%, which is lower than the value specified in the present invention. For this reason, the formation amount of AlN is small, the effect of suppressing the coarsening of crystal grains during induction hardening becomes insufficient, the toughness of the hardened layer generated by induction hardening is reduced, and the crack generation load is as low as 5 kN.

In the case of test number 19, the Cr content of the steel S 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 area ratio of lamellar pearlite in the microstructure of the austenite easily transformed into pearlite during the cooling process after rolling is 25%. The ratio specified in the invention is exceeded, and the Charpy impact value of the base material is as low as 70.0 J / cm ² . Further, since the number of spherical cementite enriched with Cr is as small as 4.5 × 10 ⁵ / mm ^{2, which} is less than the amount specified in the present invention, the toughness of the hardened layer generated by induction hardening is reduced, and cracks are generated. The generated load is as low as 6 kN.

In the case of test number 20, the Cr content of the steel T 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 Charpy impact value of the base material is as low as 140.0 J / cm ² .

  In the case of the test number 22, the total content of Si, Mn and Cr of the steel V used is as low as 1.07%, which is lower than the value specified in the present invention. For this reason, the tensile strength of the base material is as low as 580 MPa.

In the case of Test No. 23 and Test No. 24, the chemical components of Steel A used satisfy the conditions defined in the present invention, but both are out of the microstructure conditions. That is, for these test numbers, the average grain size of ferrite and the area ratio of the lamellar pearlite in the microstructure exceed the conditions defined in the present invention, while the number of spherical cementite is defined in the present invention. The condition is below. Therefore, each of the Charpy impact value of the base material, 90.0J / cm ² and 55.0J / cm ² and lower, further, reduces the toughness of the cured layer produced by induction hardening, crack initiation load, respectively As low as 8 kN and 5 kN.

In the case of Test No. 25, the chemical composition of Steel A used satisfies the conditions specified in the present invention, but the area ratio of the lamellar pearlite in the microstructure exceeds the conditions specified in the present invention, while the number of spherical cementite Is below the conditions specified in the present invention. For this reason, the Charpy impact value of the base material is as low as 45.0 J / cm ² , the toughness of the hardened layer generated by induction hardening is reduced, and the crack generation load is as low as 6 kN.

In the case of test number 26, the chemical composition of steel B used satisfies the conditions specified in the present invention, but the area ratio of lamellar pearlite in the microstructure is 45%, which exceeds the conditions specified in the present invention, while spherical cementite Is less than the conditions defined in the present invention. For this reason, the Charpy impact value of the base material is as low as 55.0 J / cm ² , the toughness of the hardened layer generated by induction hardening is lowered, and the crack generation load is as low as 8 kN.

In the case of Test No. 27, the chemical component of Steel C used satisfies the conditions specified in the present invention, but pearlite transformation was not performed, so that bainite which is a hard phase was generated. For this reason, the Charpy impact value of the base material is as low as 105.0 J / cm ² , the toughness of the hardened layer generated by induction hardening is lowered, and the crack generation load is as low as 5 kN.

  In the case of test number 28, the C content of the steel W used is as low as 0.35%, which is lower than the value specified in the present invention. For this reason, the strength of the hardened layer generated by induction hardening is low, and the crack generation load is as low as 5 kN.

The rolled steel material for induction hardening according to the present invention does not necessarily contain expensive V, and even without tempering treatment, a tensile strength of 600 MPa or more and a 2 mm U notch Charpy impact test piece in the state of the rolled steel material. In the used Charpy impact test, the impact value at a test temperature of 25 ° C. is 150 J / cm ² or more. Further, since the grain growth hardly occurs during induction hardening, the hardened layer is excellent in 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: 0.010 to 0.10%, and N: 0.004 to 0.03%, with the balance being Fe and impurities, the following formula (1) A chemical component satisfying a Ceq value of 1.20 or less and a total content of Si, Mn, and Cr of 1.2 to 3.5%, and a microstructure of ferrite, lamellar pearlite, and spherical cementite. The average grain size of the ferrite is 10 μm or less, the area ratio of the lamellar pearlite in the microstructure is 20% or less (including 0%), and the number of spherical cementite is 6 × 10 ⁵ pieces / mm ² or more. Rolled steel for induction hardening characterized by
Ceq = 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 further contains at least one element selected from the group consisting of Cu: 1.0% or less, Ni: 3.0% or less, and Mo: 0.50% or less in terms of mass%. Item 2. Rolled steel for induction hardening according to Item 1.   The chemical component further contains at least one element selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.30% or less in terms of mass%. Claim | item 1 or the rolling steel materials for induction hardening of Claim 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 810 ° 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, a method of producing a rolled steel material for induction hardening in which a temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or less, the all-continuous hot rolling The method satisfies the following [1] and [2]: A method for producing a rolled steel material for induction hardening.
[1] The surface temperature of the material to be rolled during each rolling step is within a temperature range of 650 to 810 ° C. [2] The total area reduction is 30% or more.

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