JP2013151719A - Rolled steel bar or wire rod for hot forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled steel bar or wire rod for hot forging in which bending fatigue strength and surface fatigue strength are high and heat treatment strain is reduced after carburizing and quenching or carbonitriding quenching.SOLUTION: A rolled steel bar or wire rod for hot forging has a chemical composition including 0.1 to 0.3% C, 0.01 to 0.6% Si, 0.4 to 1.0% Mn, 0.003 to 0.05% S, 0.80 to 2.00% Cr, ≤0.50% (including 0%) Mo, 0.01 to 0.05% Al, 0.010 to 0.025% N, and the balance comprising Fe and impurities, in which ≤0.025 P, ≤0.003% Ti and ≤0.002% O are satisfied in the impurities, a difference between the maximum value and the minimum value of [550-361×C-39×Mn-20×Cr-5×Mo] value in specified 17 parts in the cross-sectional face is ≤10, microstructure is made of ferrite-pearlite, ferrite-pearlite-bainite or ferrite-bainite, and a ratio between the maximum value and the minimum value of ferrite average grain sizes upon 15 visual field observation measurement at random in an area of 62,500 μmper visual field in the cross-sectional face is ≤2.0.

Description

  The present invention relates to a rolled steel bar or wire rod for hot forging. More specifically, the bending fatigue strength and surface after carburizing quenching or carbonitriding quenching are small, as the material of parts formed by hot forging such as gears and shafts, and after heat treatment after carburizing quenching or carbonitriding. The present invention relates to a rolled steel bar or wire rod for hot forging having excellent fatigue strength.

  Steel parts such as gears and shafts used in automobiles and industrial machines are manufactured by, for example, the following manufacturing method using “mechanical structural steels” such as JIS standard SCr420, SCM420, and SNCM420.

<1> A rolled steel bar or wire made of alloy steel for machine structure is roughly formed by hot forging or cold forging to obtain an intermediate product. If necessary, normalize the intermediate product.
<2> Cutting the intermediate product.
<3> Carburizing quenching or carbonitriding quenching is performed on the cut intermediate product, and then tempering at 200 ° C. or less is performed.
<4> A shot peening treatment is performed on the quenched and tempered intermediate product as necessary.

  Through the above manufacturing process, steel parts having excellent characteristics such as contact fatigue strength, bending fatigue strength, and wear resistance are manufactured.

  In recent years, parts have been made lighter and smaller in order to improve automobile fuel efficiency and engine output. Along with this, the load on the components tends to increase. Therefore, steel parts are required to have high bending fatigue strength and contact fatigue strength.

  Here, “contact fatigue” includes “face fatigue”, “line fatigue”, and “point fatigue”, but actually, there is almost no line contact or point contact in a steel part. For this reason, "surface fatigue strength" is handled as contact fatigue strength.

  The form of damage due to surface fatigue in the tooth surface of the gear and the shaft is mainly pitching. For this reason, high surface fatigue strength means high pitching strength.

  On the other hand, there is a great demand for omitting an additional surface treatment step such as shot peening for cost reduction.

  Furthermore, when the heat treatment distortion after carburizing and carbonitriding is large, cutting or polishing is necessary after tempering at 200 ° C. or lower after quenching, which causes problems such as an increase in cost and a decrease in yield. Therefore, there is a great demand for reducing heat treatment strain.

  As described above, rolled steel bars or wire rods for steel parts are required to have both high levels of improvement in bending fatigue strength and surface fatigue strength and reduction of heat treatment strain after carburizing and carbonitriding. Yes.

  Thus, for example, various techniques are proposed in Patent Documents 1 to 5.

  Specifically, Patent Document 1 includes a specific amount of elements such as C, Si, Mn, S, Cr, Al, and N, and further has values of [Mn / S] and [Cr / (Si + 2Mn)]. “Skin-hardened steel” is disclosed which is excellent in bending fatigue strength and pitting resistance and is suitable for use as a material for carburized parts such as automobile gears.

  Patent Document 2 includes specific amounts of elements such as C, Si, Mn, S, Cr, Mo, Al, and N, and further, the values of [Mn / S] and [Cr / (Si + 2Mn)] are within a specific range. “Skin-hardened steel” is disclosed which is excellent in bending fatigue strength and pitting resistance and is suitable for use as a material for carburized parts such as automobile gears.

  Patent Document 3 contains a specific amount of elements such as C, Si, Mn, and Cr, and the surface when quenching and tempering is performed after carbonitriding or carbonitriding when the value of [Si + Cr] is in a specific range A “steel for gears having excellent surface fatigue strength” with a specified amount of (C + N) is disclosed.

  Patent Document 4 stipulates that the area occupied by equiaxed crystals is 30% or less in terms of the area ratio in the cross section of the rod-shaped rolled material, and does not require processing such as correcting the shape after carburizing and quenching. “Skin-hardened steel with less heat treatment distortion” is disclosed.

  In Patent Document 5, in the slab manufacturing method by continuous casting of carburizing steel, the variation in the heat treatment dimensional change is suppressed by defining the average steel flow velocity in the mold during continuous casting as 2 cm / s to 15 cm / s. In addition, “a slab manufacturing method and slab for carburizing steel with small variation in heat treatment strain”, which has good dimensional accuracy and can omit shape correction by cutting or polishing after heat treatment, is disclosed.

JP 2009-249865 A JP 2009-249684 A Japanese Patent Laid-Open No. 9-296250 JP-A-11-131184 JP 2003-320439 A

  The case-hardened steels proposed in Patent Documents 1 and 2 do not consider reduction of heat treatment strain.

  Although the gear steel proposed in Patent Document 3 is excellent in pitting resistance, no consideration is given to the reduction of heat treatment strain.

  The case-hardened steel proposed in Patent Document 4 is different in the content of component elements at each position in the cross section with respect to the heat treatment strain after carburizing and quenching, in particular, C, Mn, Cr and Mo which are easily segregated elements. The difference in Ms point at each position due to the difference in the content of is not considered. For this reason, it is difficult to stably reduce the heat treatment strain.

  The technique proposed in Patent Document 5 also relates to the heat treatment strain after carburizing and quenching, the difference in the content of component elements at each position of the cross section, in particular, Mn, Cr excluding C among elements that easily segregate. The difference in Ms point at each position due to the difference in the Mo and Mo contents is not taken into consideration. For this reason, it is difficult to stably reduce the heat treatment strain.

  The present invention has been made in view of the above-described situation, and after carburizing and quenching or carbonitriding and quenching, the bending fatigue strength and the surface fatigue strength are high and the heat treatment distortion is small. It aims at providing the rolled steel bar or wire for hot forging suitable as a raw material.

  As disclosed in Patent Documents 1 to 3, the technology for obtaining a steel material having excellent bending fatigue strength and surface fatigue strength after carburizing quenching or carbonitriding quenching by adjusting the Si and Cr contents has been known so far. It was done. However, as described above, in these patent documents, no consideration is given to the reduction of heat treatment strain.

  Further, as disclosed in Patent Documents 4 and 5, a technique for obtaining a steel material with a small heat treatment strain by reducing the proportion of equiaxed crystals or by reducing variation in C concentration has been known. However, as described above, the heat treatment strain cannot be stably reduced only by reducing the equiaxed crystal region and the variation in C concentration.

  Therefore, the present inventors have repeated investigations and studies in order to achieve both a high level of improvement in bending fatigue strength and surface fatigue strength and a reduction in heat treatment strain after carburizing and quenching and carbonitriding. As a result, the following findings (a) to (c) were obtained.

  (A) In order to increase the bending fatigue strength and the surface fatigue strength after carburizing and quenching or carbonitriding, it is effective to suppress the unevenness of the crystal grain size. The nonuniformity of the crystal grain size can be evaluated by the ferrite grain size.

  (B) If a martensitic transformation occurs non-uniformly when carburizing or carbonitriding and quenching an intermediate product made of rolled steel for hot forging or wire, a non-uniform volume change occurs due to phase transformation. Heat treatment distortion occurs.

  (C) If the variation in the martensitic transformation point (Ms point) in the cross section perpendicular to the longitudinal direction of the hot forging steel bar or wire (hereinafter sometimes referred to as “cross section”) is small, the steel bar Or since the generation | occurrence | production of the non-uniform martensitic transformation when carburizing quenching or carbonitriding quenching of the intermediate product which uses a wire as a raw material can be suppressed, the heat treatment distortion after carburizing quenching or carbonitriding quenching becomes small.

  This invention is completed based on said knowledge, The summary exists in the rolled steel bar or wire for hot forging shown to following (1)-(3).

(1) Hot-rolled steel bar or wire,
C: 0.1 to 0.3%
Si: 0.01 to 0.6%,
Mn: 0.4 to 1.0%,
S: 0.003-0.05%,
Cr: 0.80 to 2.00%,
Mo: 0 to 0.50%,
Al: 0.01 to 0.05% and N: 0.010 to 0.025%,
The balance consists of Fe and impurities,
P, Ti and O in the impurities are each
P: 0.025% or less,
Having a chemical composition of Ti: 0.003% or less and O: 0.002% or less;
In a cross section perpendicular to the longitudinal direction of the steel bar or wire having a radius R, the center position of the cross section and a circle with a radius (1/3) R centered on the center position every 45 ° of the central angle The eight first measurement positions are arranged on a circle having a radius (2/3) R with the center position as the center at every 45 ° central angle, and the center position and the first measurement position are The difference between the maximum value and the minimum value of the Ms value obtained from the equation (1) is 10 or less for a total of 17 points with the 8 second measurement positions on the straight line including,
Furthermore, the microstructure is composed of a ferrite pearlite structure, a ferrite pearlite bainite structure, or a ferrite bainite structure,
In the cross section, the ratio of the maximum value and the minimum value of the average ferrite particle diameter is 2.0 or less when 15 fields of observation are randomly measured at an area of 62500 μm ² per field.
A rolled steel bar or wire rod for hot forging characterized by the above.
Ms = 550-361 × C-39 × Mn-20 × Cr-5 × Mo (1)
However, the element symbol in Formula (1) represents content in the mass% in the said each measurement position of the element.

(2) Instead of a part of Fe, in mass%,
Nb: The rolled steel bar or wire for hot forging according to (1) above, containing 0.08% or less.

(3) Instead of part of Fe, in mass%,
The rolled steel bar for hot forging as described in (1) or (2) above, containing at least one selected from Cu: 0.40% or less and Ni: 0.80% or less Or wire rod.

  The “impurities” in the remaining “Fe and impurities” refers to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  “Ferrite and pearlite structure” means a two-phase structure composed of ferrite and pearlite, “Ferrite and pearlite and bainite structure” means a three-phase structure composed of ferrite, pearlite and bainite, and “ferrite and bainite structure”. Refers to a two-phase structure composed of ferrite and bainite.

  Since the rolled steel bar or wire rod for hot forging of the present invention has high bending fatigue strength and surface fatigue strength and low heat treatment strain after carburizing and quenching or carbonitriding, it is suitable for parts formed by hot forging such as gears and shafts. It can be suitably used as a material.

In a cross section of a steel bar or wire having a radius R, a total of 17 positions for obtaining the Ms value from the formula (1) (the central position of the cross section, the central angle on a circle with a radius (1/3) R centered on the central position) Eight first measurement positions arranged every 45 ° and a circle having a radius (2/3) R centered on the central position are arranged at central angles of 45 °, and the central position and the first It is a figure explaining 8 second measurement positions on the straight line containing a measurement position. It is a figure explaining the shape of the roller pitching small roller test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure explaining the shape of the Noto-type rotary bending fatigue test piece with a notch used in the Example. The unit of the dimension in the figure is “mm”. It is a figure explaining the heat pattern of the "carburization quenching" performed to the various test pieces used in the Example. In the figure, “930 ° C.” and “850 ° C.” indicate “carburizing temperature” and “heating temperature for quenching”, respectively, and “Cp” indicates “carbon potential”. “Oil cooling” indicates that the oil was immersed in an oil having a temperature of 90 ° C. and quenched. It is a figure explaining the shape of the roller pitching large roller test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure explaining the shape of the ring type test piece used for the heat processing distortion measurement in the Example.

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

(A) Chemical composition C: 0.1 to 0.3%
C is an essential element for securing the core strength of the parts when carburizing and quenching or carbonitriding and quenching. If the C content is less than 0.1%, the strength is insufficient. On the other hand, when the content of C exceeds 0.3%, an increase in the amount of deformation of the parts becomes significant when carburizing or carbonitriding. Therefore, the content of C is set to 0.1 to 0.3%. The C content is preferably 0.13% or more, and more preferably 0.25% or less.

Si: 0.01 to 0.6%
Si has a deoxidizing action. If the Si content is less than 0.01%, the effect is insufficient. On the other hand, if the Si content exceeds 0.6%, the strength after hot forging or normalizing of the steel bar or wire becomes too high, and the machinability is greatly reduced. Therefore, the Si content is set to 0.01 to 0.6%. The Si content is desirably 0.10% or more, and desirably 0.55% or less.

Mn: 0.4 to 1.0%
Mn has a large effect of improving hardenability, and is an essential element for securing the core strength of parts when carburized or carbonitrided. If the Mn content is less than 0.4%, the above effect is insufficient. On the other hand, if the content of Mn exceeds 1.0%, not only the effect is saturated, but also machinability after hot forging or normalization becomes remarkable. Furthermore, the amount of deformation of the parts increases when carburizing or carbonitriding. Therefore, the Mn content is set to 0.4 to 1.0%. The Mn content is desirably 0.5% or more, and desirably 0.9% or less.

S: 0.003-0.05%
S combines with Mn to form MnS and improves machinability. However, when the S content is less than 0.003%, the above effects are insufficient. On the other hand, when the S content is excessively large, coarse MnS is easily generated, and the bending fatigue strength and the surface fatigue strength are reduced. Particularly when the S content exceeds 0.05%, the fatigue strength is increased. The decrease becomes noticeable. Therefore, the content of S is set to 0.003 to 0.05%. The S content is desirably 0.01% or more, and desirably 0.04% or less.

Cr: 0.80 to 2.00%
Cr has a great effect of enhancing hardenability and temper softening resistance, and is an element effective for improving bending fatigue strength and surface fatigue strength. However, when the Cr content is less than 0.80%, the above effects are insufficient. On the other hand, if the content of Cr exceeds 2.00%, the machinability after hot forging or after normalizing decreases. Furthermore, since Cr is an element that easily segregates, when its content exceeds 2.00%, the Ms value variation defined by the formula (1) becomes noticeable due to macrosegregation, and carburizing or quenching or Heat treatment distortion after carbonitriding is increased. Therefore, the Cr content is set to 0.80 to 2.00%. The Cr content is desirably 0.90% or more, and desirably 1.95% or less.

Mo: 0 to 0.50%
The content of Mo may be 0% (that is, it may not be contained). However, Mo has a large effect of enhancing hardenability and temper softening resistance, and is an element effective for improving bending fatigue strength and surface fatigue strength. For this reason, you may contain Mo. However, if the Mo content exceeds 0.50%, the machinability after hot forging or after normalization decreases. Furthermore, since Mo is an element that easily segregates, if its content exceeds 0.50%, the variation in Ms value defined by the above formula (1) becomes significant due to macrosegregation, and carburizing or quenching or Heat treatment distortion after carbonitriding is increased. Therefore, the Mo content is set to 0 to 0.50%. The Mo content is desirably 0.02% or more, and is desirably 0.45% or less.

Al: 0.01 to 0.05%
Al is an element having a deoxidizing action. Al is an element effective for preventing austenite grain coarsening during carburizing or carbonitriding heating because Al is easily formed by bonding with N. However, if the Al content is less than 0.01%, the austenite grains cannot be stably coarsened, and if the coarsening occurs, the bending fatigue strength decreases. On the other hand, when the Al content exceeds 0.05%, it becomes easy to form a coarse oxide, and the bending fatigue strength decreases. Therefore, the Al content is set to 0.01 to 0.05%. The Al content is preferably 0.02% or more, and is preferably 0.04% or less.

N: 0.010 to 0.025%
N is an element effective in preventing austenite grain coarsening during carburizing or carbonitriding heating because NN is easily bonded to Al and Nb to form AlN and NbN. However, if the N content is less than 0.010%, the austenite grains cannot be stably coarsened, and if the coarsening occurs, the bending fatigue strength decreases. On the other hand, when the content of N exceeds 0.025%, it is difficult to stably manufacture by mass production in the steel making process. Therefore, the N content is set to 0.010 to 0.025%. The N content is desirably 0.013% or more, and desirably 0.02% or less.

  One of the chemical compositions of the rolled steel bar or wire rod for hot forging according to the present invention is that, in addition to the above elements, the balance is Fe and impurities, and P, Ti, and O (oxygen) in the impurities are P: 0. 0.025% or less, Ti: 0.003% or less, and O: 0.002% or less.

  Hereinafter, P, Ti, and O in the impurities will be described.

P: 0.025% or less P is an element that segregates at the grain boundary and easily embrittles the grain boundary and lowers the bending fatigue strength. In particular, when the P content exceeds 0.025%, the bending fatigue strength is significantly reduced. Therefore, the content of P in the impurities is set to 0.025% or less. The content of P in the impurities is preferably 0.020% or less.

Ti: 0.003% or less Ti is an element that is easily bonded to N to form hard and coarse TiN, and TiN reduces bending fatigue strength. In particular, when the Ti content exceeds 0.003%, the bending fatigue strength is significantly reduced. Therefore, the content of Ti in the impurities is set to 0.003% or less. The content of Ti in the impurities is preferably 0.002% or less.

O: 0.002% or less O (oxygen) combines with Al to form hard oxide inclusions and lowers bending fatigue strength. In particular, when the O content exceeds 0.002%, the bending fatigue strength is significantly reduced. Therefore, the O content in the impurities is set to 0.002% or less. The content of O in the impurities is preferably 0.001% or less.

  Another one of the chemical compositions of the hot forging rolled steel bar or wire according to the present invention contains one or more elements of Nb, Cu and Ni instead of a part of Fe.

  Hereinafter, the operational effects of the above-mentioned optional elements Nb, Cu and Ni and the reasons for limiting the content will be described.

Nb: 0.08% or less Nb easily forms NbC, NbN, Nb (C, N) by combining with C and N, and supplements the above-described suppression of austenite grain coarsening during carburizing or carbonitriding heating with AlN. It is an effective element to do. Therefore, you may contain Nb. However, if the content of Nb exceeds 0.08%, the effect of preventing austenite grain coarsening is reduced. For this reason, the amount of Nb in the case of inclusion is set to 0.08% or less. In addition, it is preferable to make the quantity of Nb in the case of containing 0.07% or less.

  On the other hand, in order to stably obtain the above-described effect of Nb, the amount of Nb when contained is preferably 0.01% or more.

Cu: 0.40% or less Cu has a large effect of enhancing hardenability, and is an element effective for improving bending fatigue strength and surface fatigue strength. Therefore, Cu may be contained. However, if the Cu content exceeds 0.40%, the machinability after hot forging or after normalization decreases. For this reason, the quantity of Cu in the case of making it contain was 0.40% or less. In addition, it is preferable that the quantity of Cu in the case of making it contain is 0.30% or less.

  On the other hand, in order to stably obtain the effect of Cu described above, the amount of Cu in the case of inclusion is preferably 0.10% or more.

Ni: 0.80% or less Ni is an element that has a large effect of improving hardenability and is effective in improving bending fatigue strength and surface fatigue strength. Therefore, Ni may be included. However, if the Ni content exceeds 0.80%, the machinability after hot forging or after normalization decreases. For this reason, the amount of Ni in the case of inclusion is set to 0.80% or less. In addition, when Ni is contained, the amount of Ni is preferably 0.60% or less.

  On the other hand, in order to stably obtain the effect of Ni described above, the amount of Ni in the case of inclusion is preferably 0.10% or more.

  Said Cu and Ni can be contained only in any 1 type or 2 types of composites. The total amount when Cu and Ni are combined and contained may be 1.20% or less, but is preferably 0.90% or less.

(B) Difference between maximum value and minimum value of Ms value obtained from equation (1) The rolled steel bar or wire rod for hot forging of the present invention has a total of 17 locations shown in FIG. 1, that is, the longitudinal direction of the steel bar or wire rod. In the cross section perpendicular to the cross section, that is, in the transverse cross section, the center position of the cross section and eight points arranged at a central angle of 45 ° on a circle having a radius (1/3) R centered on the central position. A first measurement position and a circle having a radius (2/3) R centered on the center position are arranged at a central angle of 45 ° and are on a straight line including the center position and the first measurement position. The difference between the maximum value and the minimum value of the Ms values obtained from the expression (1) must be 10 or less for a total of 17 positions with the second measurement positions.
Ms = 550-361 × C-39 × Mn-20 × Cr-5 × Mo (1)
However, the element symbol in Formula (1) represents content in the mass% in the said each measurement position of the element.

  Ms represented by the above formula (1) is a parameter having a correlation with the martensite transformation point, and the difference between the maximum value and the minimum value of the Ms value at the 17 positions in the steel bar or wire as the material is 10 If it is below, the variation of the martensitic transformation point in the cross section is sufficiently small. For this reason, it is possible to suppress the occurrence of non-uniform martensitic transformation when the intermediate product made of the steel bar or wire is carburized or carbonitrided. In other words, the variation in the timing of the volume change accompanying the phase transformation in the intermediate product is small. Accordingly, the heat treatment strain after carburizing and quenching or after carbonitriding and quenching is reduced.

  On the other hand, if the difference between the maximum value and the minimum value of the Ms values at the 17 locations exceeds 10, the heat treatment strain after carburizing and quenching or carbonitriding and quenching increases, so that cutting or polishing is necessary. End up.

  The difference between the maximum value and the minimum value of the Ms values at the 17 locations is preferably as small as possible, and is most preferably 0.

  The Ms value at the 17 locations can be obtained, for example, by the following method.

  First, a steel bar or wire is cut perpendicular to its longitudinal direction. Next, a sample for chemical composition analysis, for example, a chip, from a circular region having a diameter (5/100) R to (10/100) R centering on each of the measurement positions of the above 17 by an appropriate method such as drilling. Collect. A chemical composition analysis is performed using the sample, and the Ms value is obtained from the contents of C, Mn, Cr, and Mo at each position by the equation (1).

(C) Microstructure (C-1) Phase When the rolled steel bar or wire rod for hot forging of the present invention contains martensite, the martensite is hard and has low ductility. Thus, cracks are likely to occur during the correction and transportation of hot-rolled steel bars or wires, so that a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure was adopted.

The above “phase” is obtained by, for example, cutting a section perpendicular to the longitudinal direction of the rolled steel bar or wire rod for hot forging and including the center portion, and mirror-polishing so that the section becomes a test surface. It can be identified by corroding and revealing a microstructure, and then observing at random 15 fields using an optical microscope at a magnification of 400 times and a field size of 250 μm × 250 μm (= 62500 μm ² ). .

(C-2) Ferrite average grain size Non-uniformity in the grain size of steel bars or wire rods that have been hot-rolled is also inherited as a tendency after hot forging and further after carburizing quenching or carbonitriding quenching. Affects fatigue strength and surface fatigue strength.

Accordingly, in the rolled steel bar or wire rod for hot forging of the present invention, the crystal grain size non-uniformity is randomly determined in a cross section perpendicular to the longitudinal direction, that is, in a cross section, with an area of 62500 μm ² per field of view. Evaluation is made using the ratio of the maximum value and the minimum value of the average ferrite grain size (hereinafter referred to as “the maximum value / minimum value of the ferrite average grain size”) as an index when 15 fields of view are measured. However, the 15 visual fields for observing and measuring the ferrite average particle diameter do not include the surface layer region where the decarburized layer generated during hot rolling is present.

  The ferrite grain size was used as an index because, compared to pearlite and bainite, ferrite can easily observe grain boundaries due to corrosion by nital, and if the ferrite grain size is used, it is easier to evaluate the uniformity of the structure. It is. Also, the [maximum value / minimum value] of the average ferrite grain size was used as an index because fatigue fracture occurs starting from the portion with the lowest strength. This is because I thought.

When the hot-rolled steel bar or wire microstructure is composed of the ferrite / pearlite structure, ferrite / pearlite / bainite structure, or ferrite / bainite structure described in the above (C-1), If the [maximum value / minimum value] of the average ferrite grain size is 2.0 or less when the observation area is randomly measured at 15500 fields per area of 62500 μm ² , the crystal grain size is uniform. Bending fatigue strength and surface fatigue strength are higher after carbonitriding.

  On the other hand, even if the hot-rolled steel bar or wire microstructure is the ferrite pearlite structure, the ferrite pearlite bainite structure, or the ferrite bainite structure described in the section (C-1), When the [maximum value / minimum value] of the ferrite average grain size exceeds 2.0, the bending fatigue strength and the surface fatigue strength after carburizing quenching or carbonitriding quenching are caused by non-uniformity of the crystal grain size. descend.

  The more uniform the grain size of the hot-rolled steel bar or wire, the higher the bending fatigue strength and the surface fatigue strength after carburizing quenching or carbonitriding and quenching are preferred. Therefore, the [maximum value / minimum value] of the average ferrite particle diameter is preferably 1.6 or less, and most preferably 1.

The above-mentioned “ferrite average particle diameter” is obtained by, for example, cutting a cross section including the center of a rolled steel bar or wire rod for hot forging, mirror-polishing so that the cross section becomes a test surface, and then corroding with nital. A microscopic structure was revealed, and then a photograph taken by observing 15 visual fields at random using a post-mortem microscope at a magnification of 400 times and a visual field size of 250 μm × 250 μm (= 62,500 μm ² ) It can be measured by image processing. Next, the [maximum value / minimum value] can be obtained from the maximum value and the minimum value of the average ferrite grain diameter thus measured.

  The rolled steel bar or wire rod for hot forging of the present invention can be produced, for example, by the method described below.

  First, steel having the chemical composition described in the above section (A) is melted and a slab is manufactured by a continuous casting method.

  In the continuous casting method, it is preferable to carry out electromagnetic stirring in the mold using a continuous casting machine having a vertical portion having a length of 1.5 m or more below the mold.

  In the continuous casting method, it is preferable to reduce the slab during solidification.

  Next, the manufactured slab is charged into a heating furnace, heated at a heating temperature of 1250 to 1300 ° C. for 10 hours or more, and then subjected to ingot rolling to manufacture a steel slab. In addition, said heating temperature means the average temperature in a furnace, and heating time means in-furnace time.

  The steel slab thus obtained was charged into a heating furnace, heated at a heating temperature of 1250 to 1300 ° C. for 1.5 hours or more, then hot-rolled at a finishing temperature of 900 to 1100 ° C., and subjected to finish rolling. After that, it is cooled in the atmosphere under the condition that the cooling rate is not more than the cooling rate.

  After the finish rolling, it may be cooled to room temperature under the condition that the cooling rate is equal to or less than the above-described cooling, but in order to increase productivity, air cooling is performed when the temperature reaches 600 ° C. It is preferable to cool by appropriate means such as mist cooling and water cooling.

  In addition, said heating temperature and heating time also mean the average temperature in a furnace, and in-furnace time, respectively. Moreover, the finishing temperature of hot rolling means the surface temperature of the bar or wire at the final stand outlet of a rolling mill having a plurality of stands. The cooling rate after finish rolling refers to the cooling rate at the surface of the steel bar or wire.

When the steel slab is processed into a steel bar or a wire by hot rolling, the area reduction rate (RD) represented by the following formula (2) is preferably 87.5% or more.
RD = {1- (cross-sectional area of steel bar or wire / cross-sectional area of steel slab)} × 100 (2)
In addition, said cross-sectional area means the area in a cross section perpendicular | vertical with respect to a longitudinal direction, ie, the area of a cross section.

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

Example 1
After steel A having the chemical composition shown in Table 1 was adjusted in a 70-ton converter, continuous casting was performed under the conditions shown in Table 2 to produce a 400 mm × 300 mm square slab and cooled to 600 ° C. . Note that reduction was applied during the solidification of continuous casting. The steel A is a steel that does not contain Mo among the steels having a chemical composition within the range defined by the present invention, and is a steel corresponding to JIS standard SCr420H, which is a general-purpose steel type.

  The slab thus produced was heated again from the above 600 ° C. to 1200 ° C. or 1280 ° C., and then rolled into pieces to produce a 180 mm × 180 mm square steel slab and cooled to room temperature. Furthermore, after heating the above 180 mm × 180 mm square steel pieces, hot rolling was performed to obtain steel bars having diameters of 50 mm and 70 mm.

  Table 2 shows production conditions <1> to <5>, continuous casting conditions, and slab heating conditions for split rolling when finishing a steel bar having a diameter of 50 mm and 70 mm from a 400 mm × 300 mm slab, The details of the billet heating condition for rolling and the finishing condition at the time of bar rolling are shown. In addition, the finishing conditions at the time of steel bar rolling showed only the presence or absence of water cooling before finishing rolling, the finishing temperature, and the cooling conditions after finishing rolling.

  The production condition <1> is a standard production condition when rolling a steel bar from a slab using JIS standard SCr420H which is a general-purpose steel type.

  Of the rolled steel bars obtained as described above, a steel sheet having a diameter of 50 mm was used to investigate the microstructure, and using a steel bar having a diameter of 70 mm, the above-mentioned formula (1) in 17 locations shown in FIG. A survey of defined Ms values was conducted.

  That is, after cutting a section (transverse section) perpendicular to the longitudinal direction of a rolled steel bar having a diameter of 50 mm and including the center portion, a region excluding the decarburized layer on the surface layer of the test piece that was mirror-polished and corroded with nital At a magnification of 400 times, each 15 visual fields were randomly observed to examine the microstructure (phase). The size of each field of view was 250 μm × 250 μm. Next, image analysis was performed on each of the above visual fields by a normal method to obtain the average particle diameter of the ferrite, and the ratio between the maximum value and the minimum value of the ferrite average particle diameter was calculated therefrom.

Further, in a cross section of a rolled steel bar having a diameter of 70 mm, a total of 17 locations (center position of the cross section, arranged at a central angle of 45 ° on a circle of １ ／ of the bar steel radius centered on the central position) shown in FIG. 8 places, and a center angle of 45 ° on a circle of 2/3 of the steel bar radius centered on the center position, and includes the center position and a position on a circle of 1/3 of the steel bar radius From 8 points on the straight line), chips were collected with a drill having a diameter of 5 mm, and chemical composition analysis was performed. Next, from the contents of C, Mn and Cr at each position,
Ms = 550-361 × C-39 × Mn-20 × Cr-5 × Mo (1)
(However, the element symbol in the formula (1) represents the content of the element in mass% at each measurement position.)
The Ms value was obtained by the above, and the difference between the maximum value and the minimum value of the Ms value was calculated therefrom. In addition, since steel A is steel which does not contain Mo, Mo was not detected.

  Further, a rolled steel bar having a diameter of 50 mm was heated at 1200 ° C. for 30 minutes, and hot forged so that the finishing temperature was 950 ° C. or higher to obtain a round bar having a diameter of 35 mm. Next, a roller pitching small roller test piece shown in FIG. 2 and an Ono-type rotary bending fatigue test piece with a notch having a shape shown in FIG. 3 were prepared from the center of each round bar having a diameter of 35 mm by machining. .

  Each test piece was carburized and quenched under the conditions shown in FIG. 4 using a gas carburizing furnace, and then tempered at 170 ° C. for 1.5 hours.

  The above-mentioned carburizing quenching-tempering roller pitching small roller test piece and notched Ono type rotary bending fatigue test piece are finished for the grip part for the purpose of eliminating heat treatment strain, and each of the following [1] The Ono rotary bending fatigue test shown in FIG. 2 and the roller pitching test shown in [2] were used.

[1] Ono-type rotating bending fatigue test Using the Ono-type rotating bending fatigue test piece with a notch finished, an Ono-type rotating bending fatigue test was performed under the following test conditions, and the number of repetitions was 1.0 × 10 ^4. of that I did not break up times and 1.0 × 10 ⁷ times, respectively the highest stress, and "medium-cycle rotary bending fatigue strength" and "high cycle rotating bending fatigue strength."

  The target value of the rotational bending fatigue strength described above is obtained when the medium cycle rotational bending fatigue strength and the high cycle rotational bending fatigue strength of test number 1 manufactured under the manufacturing condition <1> are normalized as “100”, respectively. , Both were over 10%, that is, over 110%.

・ Number of tests: 8,
・ Temperature: Room temperature,
・ Atmosphere: In the air
-Number of rotations: 3000 rpm.

[2] Roller pitching test The roller pitching test was performed under the conditions shown in Table 3 with a combination of a finished roller pitching small roller test piece and a roller pitching large roller test piece having the shape shown in FIG. In addition, it carried out by ejecting lubricating oil to the contact part of a roller pitching small roller test piece and a roller pitching large roller test piece.

The “slip rate” in Table 3 refers to a value calculated by the following equation, where V1 is the peripheral speed of the small roller test piece and V2 is the peripheral speed of the large roller test piece.
{(V1-V2) / V1} × 100.

  The above roller pitching large roller test piece is a general manufacturing process using JIS SCM420H, that is, "normalization, test piece processing, eutectoid carburization by gas carburizing furnace, low temperature tempering and polishing" It was produced by.

For each test number, the number of tests in the roller pitching test was set to 6, an SN graph was prepared with the surface pressure on the vertical axis and the number of repetitions until the occurrence of pitching on the horizontal axis, and the number of repetitions was 2.0 × 10 ^7. The highest surface pressure was defined as “surface fatigue strength” among the cases where no pitting occurred.

In addition, when the area of the largest thing became 1 mm < ² > or more among the locations where the surface of the test part of a roller pitching small roller was damaged, it was set as pitching generation | occurrence | production.

  The target value of the above-mentioned surface fatigue strength exceeds 15% or more, that is, 115 or more when the surface fatigue strength of test number 1 manufactured under the manufacturing condition <1> is normalized as “100”. did.

  Further, the rolled steel bar having a diameter of 70 mm is subjected to normalization that is heated at 925 ° C. for 1 hour and then allowed to cool in the atmosphere, and then machined from the center of each rolled steel bar having a diameter of 70 mm. A ring-type test piece having an outer diameter of 60 mm, an inner diameter of 20 mm, and a thickness of 25 mm as shown in FIG.

  Next, carburizing and quenching was performed on the above-described ring-type test piece using a gas carburizing furnace under the conditions shown in FIG. 4, and then tempering was performed at 170 ° C. for 1.5 hours, and then the heat treatment strain was investigated. It was used for. The heat treatment strain was investigated by measuring the outer diameter roundness (difference between major and minor diameters) of the end face.

  The target value of the heat treatment strain is 30% or less, that is, 70 or less, when the outer diameter roundness of the end surface of the test number 1 manufactured under the manufacturing condition <1> is normalized as “100”. It was decided to become.

  Table 4 summarizes the results of the above investigations together with the manufacturing conditions for the steel bars. The manufacturing condition symbols in Table 4 correspond to the manufacturing condition symbols described in Table 2 above. “ΔMs” in the Ms value column of Table 4 represents the difference between the maximum value and the minimum value of the Ms value.

  From Table 4, in the case of “examples of the present invention” of test numbers 2 and 3 that satisfy the conditions specified in the present invention, the bending fatigue strength (medium cycle rotational bending fatigue strength and high cycle rotational bending fatigue strength) and the surface fatigue strength are It is clear that the outer surface has a good outer diameter roundness and a small heat treatment strain.

  On the other hand, Test Nos. 4 and 5 of “Comparative Examples” that do not satisfy the conditions defined in the present invention are inferior in all of bending fatigue strength, surface fatigue strength, and heat treatment strain characteristics.

(Example 2)
Components of steels B to H having chemical compositions shown in Table 5 were adjusted in a 70-ton converter, and then continuous casting was performed to produce a 400 mm × 300 mm square slab and cooled to 600 ° C. Note that reduction was applied during the solidification of continuous casting.

  Steels B to E and Steel H in Table 5 are all steels whose chemical compositions are within the range defined by the present invention. On the other hand, Steel F and Steel G are comparative steels whose chemical compositions deviate from the conditions defined in the present invention.

  The slab thus produced was heated again from the above 600 ° C. to 1200 ° C. or 1280 ° C., and then rolled into pieces to produce a 180 mm × 180 mm square steel slab and cooled to room temperature. Furthermore, after heating the above 180 mm × 180 mm square steel pieces, hot rolling was performed to obtain steel bars having diameters of 50 mm and 70 mm.

  Using the rolled steel bar obtained as described above, the microstructure, Ms value, Ono-type rotary bending fatigue test, roller pitching test, and heat treatment were conducted in the same manner as in “(Example 1)”. The distortion was investigated.

  The target values for rotational bending fatigue strength (medium cycle rotational bending fatigue strength and high cycle rotational bending fatigue strength), surface fatigue strength, and heat treatment strain were set to the target values in “(Example 1)”, respectively. .

  Table 6 shows the results of the above surveys together with the manufacturing conditions of the steel bars. In Table 6, the test number 1 shown in Table 4 of the “(Example 1)”, which is a target value evaluation standard for each of the above characteristics, is also shown.

  The manufacturing condition symbols in Table 6 correspond to the manufacturing condition symbols described in Table 2 of the “(Example 1)”. Further, “ΔMs” in the Ms value column of Table 6 also represents the difference between the maximum value and the minimum value of the Ms value.

  From Table 6, in the case of “examples of the present invention” of test numbers 6, 9, 11, 13, and 20 that satisfy the conditions specified in the present invention, bending fatigue strength (medium cycle rotational bending fatigue strength and high cycle rotational bending fatigue strength) It is clear that the surface fatigue strength is high, the outer surface has a good outer diameter roundness and a small heat treatment strain.

  On the other hand, the test numbers 7, 8, 10, 12, 14 to 19 and 21 of “Comparative Examples” that do not satisfy the conditions specified in the present invention are at least of bending fatigue strength, surface fatigue strength, and heat treatment strain characteristics. Either property is inferior.

  Since the rolled steel bar or wire rod for hot forging of the present invention has high bending fatigue strength and surface fatigue strength and low heat treatment distortion after carburizing and quenching or carbonitriding, it is suitable for parts formed by hot forging such as gears and shafts. It can be suitably used as a material.

Claims (3)

Hot-rolled steel bar or wire,
C: 0.1 to 0.3%
Si: 0.01 to 0.6%,
Mn: 0.4 to 1.0%,
S: 0.003-0.05%,
Cr: 0.80 to 2.00%,
Mo: 0 to 0.50%,
Al: 0.01 to 0.05% and N: 0.010 to 0.025%,
The balance consists of Fe and impurities,
P, Ti and O in the impurities are each
P: 0.025% or less,
Having a chemical composition of Ti: 0.003% or less and O: 0.002% or less;
In a cross section perpendicular to the longitudinal direction of the steel bar or wire having a radius R, the center position of the cross section and a circle with a radius (1/3) R centered on the center position every 45 ° of the central angle The eight first measurement positions are arranged on a circle having a radius (2/3) R with the center position as the center at every 45 ° central angle, and the center position and the first measurement position are The difference between the maximum value and the minimum value of the Ms value obtained from the equation (1) is 10 or less for a total of 17 points with the 8 second measurement positions on the straight line including,
Furthermore, the microstructure is composed of a ferrite pearlite structure, a ferrite pearlite bainite structure, or a ferrite bainite structure,
In the cross section, the ratio of the maximum value and the minimum value of the average ferrite particle diameter is 2.0 or less when 15 fields of observation are randomly measured at an area of 62500 μm ² per field.
A rolled steel bar or wire rod for hot forging characterized by the above.
Ms = 550-361 × C-39 × Mn-20 × Cr-5 × Mo (1)
However, the element symbol in Formula (1) represents content in the mass% in the said each measurement position of the element. Instead of part of Fe, in mass%,
The rolled steel bar or wire rod for hot forging according to claim 1, comprising Nb: 0.08% or less. Instead of part of Fe, in mass%,
The rolled steel bar or wire rod for hot forging according to claim 1 or 2, comprising at least one selected from Cu: 0.40% or less and Ni: 0.80% or less.

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