WO2019182054A1

WO2019182054A1 - Steel material

Info

Publication number: WO2019182054A1
Application number: PCT/JP2019/011847
Authority: WO
Inventors: 達也小山; 根石　豊
Original assignee: 日本製鉄株式会社
Priority date: 2018-03-23
Filing date: 2019-03-20
Publication date: 2019-09-26
Also published as: JPWO2019182054A1; CN111868281B; JP6919762B2; EP3770291A1; EP3770291B1; KR102463278B1; CN111868281A; KR20200118854A; US20200407815A1; EP3770291A4

Abstract

The steel material according to an embodiment of the present invention has a predetermined chemical composition, and includes, in a region in which a distance r from the center of a cross-section thereof perpendicular to the length direction thereof satisfies 0.7R≤r≤0.9R, a structure that contains ferrite and bainite and in which the average fraction of the ferrite is in the range of 40-70% by area percentage, the average value of the total of the average fractions of structures other than the ferrite and the bainite is 0-3%, and the balance is bainite, and the standard deviation of the fraction of the ferrite in the region is 4% or less.

Description

Steel

The present invention relates to a steel material.
This application claims priority on March 23, 2018 based on Japanese Patent Application No. 2018-056867 filed in Japan, the contents of which are incorporated herein by reference.

Gears used in automobiles, construction machinery, industrial machinery, etc. are generally used after carburizing and quenching after machining in order to achieve both precise dimensional accuracy and strength. In recent years, quietness during operation has been strongly demanded compared to the conventional case, and improvement in dimensional accuracy of gears, in particular, dimensional accuracy of tooth portions has been demanded. The dimensional accuracy of the gear teeth is caused by deformation (hereinafter referred to as heat treatment strain) associated with heat treatment during carburizing and quenching. Since this heat treatment strain differs for each gear tooth portion and is not stable, vibrations are generated during use by deviating from a symmetrical shape within the same gear, and silence is lost. For this reason, it is calculated | required to stabilize the heat processing distortion of the gear tooth part so that it may become a symmetrical shape.

Regarding the technical development of conventional steel for carburized gears, Patent Document 1 describes a technique for providing a steel material excellent in cold forgeability and temper softening resistance. However, Patent Document 1 does not provide a technique for stabilizing the heat treatment distortion of the tooth portion of the gear during carburizing and quenching, which is a problem to be solved by the present invention.
According to Patent Document 2, the ferrite is composed of a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure, and is measured at 15 visual fields at random with an area per visual field of 62500 μm ^2. When the standard deviation of the fraction is 0.10 or less, and a region from the surface to 1/5 of the radius and a region from the center to 1/5 of the radius are observed in the cross section, in each region, as AlN Disclosed is a technique for providing a hot rolled steel bar or wire, wherein the amount of precipitated Al is 0.005% or less and the number density of AlN having a diameter of 100 nm or more is 5 pieces / 100 μm ² or less. Has been. However, in view of the examples disclosed in Patent Document 2, it is presumed that the technique of Patent Document 2 uses a pearlite structure to suppress the standard deviation of the ferrite fraction. That is, according to the technique of Patent Document 2, it is not possible to sufficiently reduce the standard deviation of the ferrite fraction while controlling the structure so as not to substantially contain pearlite.

International Publication No. 2014/171472 International Publication No. 2011/055651

An object of the present invention is to provide a steel material that stabilizes the heat treatment distortion of the gear teeth during carburizing and quenching.

The gist of the present invention is as follows.
(1) The steel material according to one embodiment of the present invention is, in mass%, C: 0.17 to 0.21%, Si: 0.40 to 0.60%, Mn: 0.25 to 0.50%, Cr: 1.35 to 1.55%, Mo: 0.20 to 0.40%, S: 0.010 to 0.05%, N: 0.005 to 0.020%, Al: 0.001% To 0.100%, Nb: 0.001 to 0.030%, Ni: 0 to 3.0%, Cu: 0 to 1.0%, Co: 0 to 3.0%, W: 0 to 1.0 %, V: 0-0.3%, Ti: 0-0.3%, B: 0-0.005% O: 0.005% or less, P: 0.03% or less, Pb: 0-0. 5%, Bi: 0 to 0.5%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Zr: 0 to 0.05%, Te: 0 to 0.1%, rare earth elements : 0-0.005% balance is composed of Fe and impurities, In a region where the distance r from the center of the cross section perpendicular to the vertical direction satisfies the following formula, the structure contains ferrite and bainite, and the average fraction of the ferrite in terms of area ratio is in the range of 40 to 70%, The sum of the average fractions of the structures other than ferrite and bainite is 0% or more and 3% or less on average, the remainder is a structure composed of bainite, and the standard deviation of the fraction of ferrite in the region is 4 % Or less.
0.7R ≦ r ≦ 0.9R
However, R represents a circle-equivalent radius of the steel material.
(2) The steel material according to the above (1) is in mass%, Ni: 0.01 to 3.0%, Cu: 0.01 to 1.0%, Co: 0.01 to 3.0%, One or more of W: 0.01 to 1.0%, V: 0.01 to 0.3%, Ti: 0.001 to 0.3%, B: 0.0001 to 0.005% It may contain.
(3) The steel material described in the above (1) or (2) is in mass%, Pb: 0.01 to 0.5%, Bi: 0.0001 to 0.5%, Ca: 0.0001 to 0 .01%, Mg: 0.0001 to 0.01%, Zr: 0.0001 to 0.05%, Te: 0.0001 to 0.1%, rare earth element: 0.0001 to 0.005% You may contain a seed or two or more sorts.

It is possible to stabilize the heat treatment distortion of the gear teeth produced by carburizing and quenching using the steel material of the present invention.

It is a cross-sectional schematic diagram of the steel materials explaining the measurement position of the average fraction of ferrite and the standard deviation of ferrite fraction.

Hereinafter, embodiments for carrying out the present invention will be described in detail.
First, the background to the present invention will be described.
The inventors of the present invention conducted intensive studies on a method for stabilizing the heat treatment strain of the gear teeth after carburizing and quenching. As a result, it was found that the heat treatment strain is stabilized by improving the uniformity of the structure in the region of the steel material that becomes the tooth after machining. Therefore, the present inventors further investigated the influence of changing the chemical composition of the steel material and the manufacturing method on the method of homogenizing the structure of the region corresponding to the tooth portion of the gear in the steel material. As a result, it was found that the structure of the region corresponding to the tooth portion of the gear in the steel material can be made uniform by controlling the casting method and the cooling rate after rolling after setting the steel material component within a predetermined range. Regarding the control of the casting method, the control is performed by combining the cross-sectional area of casting and the casting speed and the average cooling rate from the casting start to the correction point on the surface. This makes it possible to homogenize the cast structure in the region of the slab that will ultimately become the gear teeth. Furthermore, regarding the control of the cooling rate after rolling, the cooling rate of the steel material surface is controlled. Thereby, the structure | tissue of the area | region equivalent to the gear tooth part in steel materials can be homogenized.

Next, the reason for limiting the chemical composition of the steel material according to this embodiment will be described. Hereinafter, “mass%”, which is a unit related to the content of alloy elements, is simply referred to as “%”.

C: 0.17 to 0.21%
The C content affects the hardness of the non-carburized portion of the gear. In order to ensure the required hardness, the C content is 0.17% or more. On the other hand, if the C content is too high, the non-carburized part hardness after carburizing increases and the strength against impact decreases, so the C content is set to 0.21% or less. A preferable lower limit of the C content is 0.175%, 0.18%, 0.185%, or 0.19%. The upper limit with preferable C content is 0.205%, 0.200%, 0.195%, or 0.19%.

Si: 0.40 to 0.60%
Si is an element whose content needs to be strictly limited in order to homogenize the structure of the region corresponding to the tooth portion of the gear steel after machining in the steel material. However, if the Si content is too high, the amount of ferrite in the steel material is insufficient, the amount of bainite and the like is increased, and workability is impaired. In order to obtain the above effects, the Si content needs to be in the range of 0.40 to 0.60%. A preferable lower limit of the Si content is 0.42%, 0.45%, 0.48%, or 0.50%. A preferable upper limit of Si content is 0.58%, 0.55%, 0.53%, or 0.51%.

Mn: 0.25 to 0.50%
Mn is an element whose content needs to be strictly limited in order to homogenize the structure of the region corresponding to the tooth portion of the gear steel after machining in the steel material. In order to acquire the above-mentioned effect, it is necessary to make Mn content 0.25% or more. However, if the Mn content is too high, the amount of ferrite in the steel material is insufficient, the amount of bainite and the like is increased, and workability is impaired. Therefore, the Mn content is 0.50% or less. A preferable lower limit of the Mn content is 0.27%, 0.30%, 0.32%, or 0.35%. The upper limit with preferable Mn content is 0.48%, 0.45%, 0.42%, or 0.40%.

Cr: 1.35 to 1.55%
Cr is an element whose content needs to be strictly limited in order to homogenize the structure of the region corresponding to the tooth portion of the gear steel after machining in the steel material. However, if the Cr content is too high, the amount of ferrite in the steel material is insufficient, the amount of bainite and the like is increased, and workability is impaired. In order to obtain the above effect, the Cr content needs to be in the range of 1.35 to 1.55%. A preferable lower limit of the Cr content is 1.37%, 1.40%, 1.42%, or 1.45%. The upper limit with preferable Cr content is 1.53%, 1.50%, 1.49%, or 1.47%.

Mo: 0.20 to 0.40%
Mo is an element whose content needs to be strictly limited in order to homogenize the structure of the region corresponding to the tooth portion of the gear steel after machining in the steel material. Moreover, when Mo is contained in a steel material together with Nb described later, the hardenability of the steel material is increased, pearlite transformation is suppressed, and austenite crystal grain coarsening during heating of the steel material is suppressed. Thereby, it becomes possible to control hardenability moderately and to suppress a martensitic transformation and to obtain a desired bainite structure. However, when the Mo content is too high, the amount of ferrite in the steel material is insufficient, the amount of bainite and the like is increased, and workability is impaired. In order to obtain the above effect, the Mo content needs to be in the range of 0.20 to 0.40%. A preferable lower limit of the Mo content is 0.22%, 0.25%, 0.28%, or 0.30%. The upper limit with preferable Mo content is 0.38%, 0.35%, 0.32%, or 0.30%.

S: 0.010 to 0.05%
S forms MnS in the steel, thereby improving the machinability of the steel. In order to obtain a machinability at a level that enables machining of a part, an S content equivalent to that of a general machine structural steel is required. For these reasons, the S content needs to be in the range of 0.010 to 0.05%. A preferable lower limit of the S content is 0.012%, 0.014%, 0.020%, or 0.022%. The upper limit with preferable S content is 0.035%, 0.030%, 0.028%, or 0.025%.

N: 0.005 to 0.020%
N has a crystal grain refining effect by forming a compound with Al, Ti, V, Cr, or the like, and therefore needs to be contained in an amount of 0.005% or more. However, if it exceeds 0.020%, the compound becomes coarse and the effect of crystal grain refinement cannot be obtained. For the above reasons, the N content needs to be in the range of 0.005 to 0.020%. The preferable lower limit of the N content is 0.0055%, 0.0060%, 0.007%, or 0.010%. The upper limit with preferable N content is 0.018%, 0.015%, 0.012%, or 0.010%.

Al: 0.001% to 0.100%
Al is an element effective for deoxidation of steel, and is an element that combines with N to form a nitride to refine crystal grains. If the Al content is less than 0.001%, this effect is insufficient. On the other hand, if the Al content exceeds 0.100%, the nitride becomes coarse and embrittles. A preferable lower limit of the Al content is 0.004%, 0.007%, 0.010%, or 0.020%. The upper limit with preferable Al content is 0.080%, 0.050%, 0.040%, or 0.030%.

Nb: 0.001 to 0.030%
Nb is an element that produces a fine compound of C and N in steel and brings about a grain refinement effect. Moreover, Nb exhibits the synergistic effect (suppression effect of pearlite transformation and martensitic transformation) described above when contained in steel together with Mo. If the Nb content is less than 0.001%, this effect is insufficient. If the Nb content exceeds 0.030%, the carbonitride becomes coarse and this effect cannot be sufficiently obtained. For the above reasons, the Nb content needs to be 0.001 to 0.030%. The minimum with preferable Nb content is 0.005%, 0.010%, 0.012%, or 0.015%. The upper limit with preferable Nb content is 0.028%, 0.025%, 0.022%, or 0.020%.

O: 0.005% or less O forms an oxide in steel and acts as an inclusion to reduce fatigue strength. Therefore, the O content is preferably limited to 0.005% or less. The upper limit with preferable O content is 0.003%, 0.002%, 0.0015%, or 0.001%. Since it is preferable that the O content is small, the lower limit of the O content is 0%. However, if O is removed more than necessary, the manufacturing cost increases. Accordingly, the lower limit value of the O content may be 0.0001%, 0.0002%, 0.0005%, or 0.0008%.

P: 0.03% or less P segregates at austenite grain boundaries during heating before quenching, thereby reducing fatigue strength. Therefore, it is preferable to limit the P content to 0.03% or less. The upper limit with preferable P content is 0.025%, 0.023%, 0.020%, or 0.015%. Since it is preferable that the P content is small, the lower limit of the P content is 0%. However, if P is removed more than necessary, the manufacturing cost increases. Therefore, the substantial lower limit of the P content is usually about 0.004% or more. The lower limit value of the P content may be 0.005%, 0.007%, 0.010%, or 0.012%.

The steel according to the present embodiment is further selected from the group consisting of Ni, Cu, Co, W, V, Ti, and B instead of a part of Fe in order to enhance the hardenability or grain refinement effect. 1 type (s) or 2 or more types may be contained. The lower limit in the case of not containing these elements is 0%.

Ni: 0 to 3.0%
Ni is an effective element for imparting the necessary hardenability to the steel. In order to obtain this effect, the Ni content is preferably set to 0.01% or more. If the Ni content exceeds 3.0%, the retained austenite becomes large after quenching, and the hardness decreases. For these reasons, the Ni content is set to 3.0% or less, more preferably 0.01 to 3.0%. The upper limit of the Ni content is more preferably 2.0%, still more preferably 1.8%. The lower limit of the Ni content is more preferably 0.1%, still more preferably 0.3%.

Cu: 0 to 1.0%
Cu is an element effective for improving the hardenability of steel. In order to obtain this effect, the Cu content is preferably set to 0.01% or more. Moreover, when Cu content exceeds 1.0%, hot ductility will fall. Therefore, the Cu content is 1.0% or less, more preferably 0.01 to 1.0%. When Cu is contained to obtain the above-described effect, the more preferable lower limit of the Cu content is 0.05%, and more preferably 0.1%.

Co: 0 to 3.0%
Co is an element effective for improving the hardenability of steel. In order to obtain this effect, the Co content is preferably 0.01% or more. When the Co content exceeds 3.0%, the effect is saturated. Therefore, the Co content is 3.0% or less, more preferably 0.01 to 3.0%. When Co is contained to obtain the above-described effect, the more preferable lower limit of the Co content is 0.05%, and more preferably 0.1%.

W: 0 to 1.0%
W is an element effective for improving the hardenability of steel. In order to obtain this effect, the W content is preferably set to 0.01% or more. If the W content exceeds 1.0%, the effect is saturated. Therefore, the W content is 1.0% or less, and more preferably 0.01 to 1.0%. When W is contained to obtain the above-described effect, the lower limit of the W content is more preferably 0.05%, and still more preferably 0.1%.

V: 0 to 0.3%
V is an element that forms a fine compound with C and N in steel and brings about a grain refinement effect. In order to obtain this effect, the V content is preferably 0.01% or more. If the V content exceeds 0.3%, the compound becomes coarse and the effect of crystal grain refinement cannot be obtained. Therefore, the V content is 0.3% or less, and more preferably 0.01 to 0.3%. When V is contained and the above-mentioned effect is obtained, the more preferable lower limit of the V content is 0.1%, more preferably 0.15%.

Ti: 0 to 0.3%
Ti is an element that produces a fine compound of C and N in steel and brings about a grain refinement effect. In order to obtain this effect, the Ti content is preferably 0.001% or more. When the Ti content exceeds 0.3%, the effect is saturated. For the above reasons, the Ti content is set to 0.3% or less, more preferably 0.001 to 0.3%. The upper limit with more preferable Ti content is 0.25%, More preferably, it is 0.2%.

B: 0 to 0.005%
B has a function of suppressing grain boundary segregation of P. B also has an effect of improving grain boundary strength and intragranular strength, and an effect of improving hardenability, and these effects improve the fatigue strength of steel. In order to obtain this effect, the B content is preferably 0.0001% or more. When the B content exceeds 0.005%, the effect is saturated. For these reasons, the B content is set to 0.005% or less, preferably 0.0001 to 0.005%. The upper limit with more preferable B content is 0.0045%, More preferably, it is 0.004%.

The chemical composition of the steel according to the present embodiment further includes one or more selected from the group consisting of Pb, Bi, Ca, Mg, Zr, Te and rare earth elements (REM) instead of a part of Fe. You may contain. The lower limit in the case of not containing these elements is 0%.

Pb: 0 to 0.5%
Pb is an element that improves machinability by melting and embrittlement during cutting. In order to obtain this effect, the Pb content is preferably 0.01% or more. On the other hand, when it contains excessively, manufacturability will fall. Therefore, the Pb content is 0.5% or less, preferably 0.01 to 0.5%. When Pb is contained to obtain the above-described effects, the more preferable lower limit of the Pb content is 0.05%, and more preferably 0.1%. The upper limit with preferable Pb is 0.4%, More preferably, it is 0.3%.

Bi: 0 to 0.5%
Bi is an element that improves machinability by finely dispersing sulfides. In order to obtain this effect, the Bi content is preferably 0.0001% or more. On the other hand, if excessively contained, the hot workability of the steel deteriorates and hot rolling becomes difficult, so the Bi content is set to 0.5%, more preferably 0.0001 to 0.5%. When Bi is contained to obtain the above-described effect, the preferable lower limit is 0.0001%, and more preferably 0.001%. The upper limit with preferable Bi is 0.4%, More preferably, it is 0.3%.

Ca: 0 to 0.01%
Ca is an element that is effective for deoxidation of steel and reduces the content of Al ₂ O ₃ in the oxide. In order to obtain this effect, the Ca content is preferably 0.0001% or more. If the Ca content exceeds 0.01%, a large amount of coarse oxides containing Ca appears, which causes a reduction in rolling fatigue life. For these reasons, the Ca content needs to be in the range of 0.0001 to 0.01%. The minimum with preferable Ca content is 0.0003%, More preferably, it is 0.0005%. The upper limit with preferable Ca content is 0.008%, More preferably, it is 0.006%.

Mg: 0 to 0.01%
Mg is a deoxidizing element and generates an oxide in steel. Further, the Mg-based oxide formed by Mg tends to be a nucleus of MnS crystallization and / or precipitation. Further, the Mg sulfide becomes a composite sulfide of Mn and Mg, thereby spheroidizing MnS. Thus, Mg is an effective element for controlling the dispersion of MnS and improving machinability. In order to obtain this effect, the Mg content is preferably 0.0001% or more. However, if the Mg content exceeds 0.01%, a large amount of MgS is generated and the machinability of the steel is lowered. Therefore, when Mg is contained to obtain the above effect, the Mg content is reduced to 0. .01% or less is necessary. The upper limit with preferable Mg content is 0.008%, More preferably, it is 0.006%. The minimum with preferable Mg content is 0.0005%, More preferably, it is 0.001%.

Zr: 0 to 0.05%
Zr is a deoxidizing element and generates an oxide. Furthermore, the Zr-based oxide formed by Zr tends to be a nucleus of MnS crystallization and / or precipitation. Thus, Zr is an effective element for controlling the dispersion of MnS and improving machinability. In order to obtain this effect, the Zr content is preferably 0.0001% or more. However, if the amount of Zr exceeds 0.05%, the effect is saturated. Therefore, when the above-described effect is obtained by containing Zr, the Zr content is set to 0.05% or less, more preferably 0.8%. 0001 to 0.05%. The upper limit with preferable Zr content is 0.04%, More preferably, it is 0.03%. The minimum with preferable Zr content is 0.0005%, More preferably, it is 0.001%.

Te: 0 to 0.1%
Te promotes the spheroidization of MnS and improves the machinability of the steel. In order to obtain this effect, the Te content is preferably 0.0001% or more. When the Te content exceeds 0.1%, the effect is saturated. Therefore, the Te content is 0.1% or less, more preferably 0.0001 to 0.1%. The upper limit with preferable Te content is 0.08%, More preferably, it is 0.06%. The minimum with preferable Te content is 0.0005%, More preferably, it is 0.001%.

Rare earth elements: 0 to 0.005%
The rare earth element is an element that promotes the production of MnS by producing sulfides in the steel and these sulfides become MnS precipitation nuclei, and improves the machinability of the steel. In order to obtain this effect, the total content of rare earth elements is preferably 0.0001% or more. However, if the total content of rare earth elements exceeds 0.005%, the sulfide becomes coarse and the fatigue strength of the steel is lowered. Therefore, the total content of rare earth elements is set to 0.005% or less, more preferably 0.0001 to 0.005%. The upper limit with preferable total content of rare earth elements is 0.004%, More preferably, it is 0.003% or less. The minimum with preferable total content of rare earth elements is 0.0005%, More preferably, it is 0.001%.

The rare earth elements referred to in this specification are 15 elements from lanthanum (La) having atomic number 57 to lutetium (Lu) having atomic number 71 in the periodic table, plus yttrium (Y) and scandium (Sc) 17 A general term for elements. The rare earth element content means the total content of one or more of these elements.

The steel according to this embodiment contains the above-described alloy components, and the balance contains Fe and impurities. It is permissible for elements other than the above-mentioned alloy components to be mixed into the steel as impurities from the raw materials and production equipment as long as the mixed amount is at a level that does not affect the properties of the steel.

Next, the uniformity of the structure of the steel material will be described.
As described above, in order to improve the heat treatment distortion of the gear teeth, it is necessary to improve the uniformity of the structure in the region corresponding to the gear teeth in the steel material. Here, the region corresponding to the gear teeth of the steel material is a region containing from the tooth tip to the tooth root of the gear after forging or cutting, and 0.7R ≦ r ≦ 0. 9R region. r is a distance from the center of the steel material cross section perpendicular to the length direction, and R is a circle-equivalent radius in the steel material cross section perpendicular to the length direction of the steel material.

As a result of investigations by the inventors, it has been clarified that the uniform structure suitable for improving the heat treatment strain is a structure containing ferrite and bainite, and the structure fraction is in an appropriate range. As a result of investigating the relationship between the structure fraction and the heat treatment strain, the ferrite fraction average value (average fraction) in the area of 0.7R ≦ r ≦ 0.9R was 40 to 40 by the measurement method described later. 70% of the range, the sum of the average fractions of structures other than ferrite and bainite is 0% or more and 3% or less on average, and the balance is a structure composed of bainite, and 0.7R ≦ r ≦ 0.9R The heat treatment strain was stabilized when the standard deviation of the average fraction of ferrite in the range was 4% or less. When the structure fraction exceeded the above range, the heat treatment strain increased. Hereinafter, when it is simply described as “fraction” with respect to the metal structure, it means an average value of the structure fraction (unit: area%) in the cross section of the steel material obtained by means described later. However, in the description of “standard deviation of fraction”, as will be described later, “fraction” is not an average value in the entire cross section but means a fraction in each measurement visual field.
The minimum with a preferable ferrite fraction is 42%, More preferably, it is 45%. The upper limit with a preferable ferrite fraction is 68%, More preferably, it is 65%. The lower the standard deviation of the ferrite fraction in the range of 0.7R ≦ r ≦ 0.9R, the better. Therefore, the lower limit is 0%. The upper limit of the standard deviation of the ferrite fraction in the region of 0.7R ≦ r ≦ 0.9R is preferably 3.5%, more preferably 3%.
In the steel material according to the present embodiment, “bainite” refers to a ferrite structure, a pearlite structure, and martensite among the structures obtained by heating the steel material to an austenite single-phase structure and then cooling to room temperature by continuous cooling. It means a structure excluding a structure, and it is a general term for an upper bainite structure, a lower bainite structure, or a mixed structure of an upper bainite structure and a lower bainite structure.
In addition, it is not preferable that pearlite is included in the structure of the steel material according to the present embodiment because the carburizing and hardenability is impaired. For example, when a steel material composed of a mixed structure of ferrite, pearlite, and bainite is carburized and quenched, the austenite grain structure in the region corresponding to the tooth portion becomes nonuniform during heating. Due to this, deformation after carburizing and quenching, that is, heat treatment distortion increases. Therefore, the area ratio of pearlite needs to be limited as much as possible. In view of this situation, the total of structures other than ferrite and bainite is defined as 0% or more and 3% or less. In general, a structure in which the total of structures other than ferrite and bainite is 0% or more and 3% or less is referred to as a “ferrite / bainite structure”. In other words, the steel material according to the present embodiment is a steel material having a ferrite bainite structure.

Next, a method for measuring the tissue fraction will be described.
As shown in Fig. 1, the circumference is 0.7R + 0.25mm, 0.8R, 0.9R-0.25mm with respect to a straight line that divides the cross section radially into 8 (center angle 45 degrees) from the center of the steel material A point intersecting the line was taken as a measurement point, and a rectangular area of 0.5 mm × 1 mm = 0.5 mm ² was taken as a measurement region so that each measurement point was at the center of the rectangle. There are 24 measurement areas. The ferrite fraction in the range of 0.7R ≦ r ≦ 0.9R and the standard deviation of the ferrite fraction were calculated by observation using a light microscope on a sample that had been subjected to nital corrosion after mirror polishing of the steel material cross section. Since MnS or the like may exist as a structure other than ferrite and bainite, each measurement area is visually observed with respect to the sample after nital corrosion, and is photographed at an observation magnification of 100 times in each measurement area (structure boundaries are unclear) , The area ratio of the bright area when binarizing ferrite and bainite as bright areas using image processing software Winroof 2015 for 0.5 mm ² in the image taken at an observation magnification of 400 times) The ferrite fraction and bainite fraction for each measurement region were determined. In calculating the area ratio, the area obtained by removing the area of the non-metallic structure such as MnS from the test area is set as the evaluation area, and the ratio of the area of the ferrite structure and the bainite structure to the evaluation area is set as the area ratio of the ferrite structure and The area ratio of the bainite structure was used. And the average value of the ferrite fraction of 24 measurement areas was made into the ferrite fraction, and the average value of the bainite fraction of 24 measurement areas was made into the bainite fraction. The area ratio of the structure other than ferrite and bainite was calculated by 100− (ferrite fraction + bainite fraction). The standard deviation of the ferrite fraction at 24 measurement points was defined as the standard deviation of the ferrite fraction in the range of 0.7R ≦ r ≦ 0.9R.

Next, the cross-sectional area and casting speed during casting, the cooling rate from casting to the correction point, and the cooling rate after rolling will be described.

In order to improve the heat treatment distortion of the gear teeth, the inventors have strictly determined the component ranges of Si, Cr, Mn, and Mo as described above, and then controlled the casting method and the cooling method during rolling. Need to do. Regarding the casting method, it is important to control the temperature change in the region corresponding to the gear teeth during casting. When the casting size changes, the temperature and cooling rate in this region change even at the same casting rate and the same cooling rate. Therefore, as a result of examining the casting size and the temperature change inside the slab, it was clarified that the degree of segregation in the region corresponding to the gear teeth can be controlled by controlling V × A ^0.5 / C. However, V is a casting speed here and a unit is m / min. A is the casting size (cross-sectional area of the slab), and the unit is mm ² . C is the average cooling rate of the slab from immediately after casting to the bending correction point. The average cooling rate of the slab is a value obtained by dividing the temperature difference between the casting temperature of the molten steel and the surface temperature of the slab at the bending correction point by the time required to reach the correction point from directly below the mold. The unit is ° C./min. The bending correction point is a position where the shape of the slab is corrected from a curved shape to a straight shape in curved continuous casting.

In order to appropriately control the degree of segregation in the region corresponding to the gear teeth, it is necessary to control the range of V × A ^0.5 / C to 6.0 to 20.0. A preferable lower limit is 6.2 or more, and more preferably 6.5 or more. A preferable upper limit is 19.0 or less, More preferably, it is 18.0 or less. Although it is impossible to actually measure the internal temperature during casting, it is possible to estimate by considering the items that can be actually measured and the casting size by using this formula. It is possible to control casting in a region corresponding to a tooth.

Also, regarding cooling after rolling, it is important to control the average cooling rate when the surface temperature of the steel during cooling is between 800 ° C and 300 ° C. A uniform structure can be obtained by controlling the average cooling rate between 0.1 ° C. and 1.0 ° C./sec when the steel surface temperature is between 800 ° C. and 300 ° C., and the ferrite fraction should be within the specified range. Can do. If this range is exceeded, a uniform structure cannot be obtained, and heat treatment strain increases. The minimum with the preferable cooling rate after rolling is 0.15 degree-C / sec or more, More preferably, it is 0.2 degree-C / second or more. A preferable upper limit of the cooling rate after rolling is 0.9 ° C./second or less, and more preferably 0.8 ° C./second or less.

A preferable manufacturing condition of the steel material according to the present embodiment will be described.
The molten steel whose chemical composition is adjusted in the refining process is cast using a curved continuous casting machine (casting process). During casting, the mold size, casting speed, and cooling rate are controlled as described above, but are preferably in the following ranges from the viewpoint of productivity. The mold size is 30000 mm ² or more and 400000 mm ² or less, the casting speed is 0.2 m / min or more and 3.0 m / min or less, and the cooling rate between casting and the correction point is 4.0 ° C./min or more and 100 ° C./min or less. .
The slab obtained by the above casting process is subjected to partial rolling to obtain a steel slab (partial rolling process). The heating temperature at the time of the block rolling is desirably 1100 ° C. or higher in order to surely form a solution of the Nb compound. A more preferable heating temperature is 1200 ° C. or higher. On the other hand, if the heating temperature is too high, the crystal grains become coarse, so the upper limit of the heating temperature is preferably 1280 ° C. The area reduction rate of the block rolling is desirably 30% or more. More preferably, it is 40% or more.

In order to use the steel slab as a steel material for a carburized gear (a steel bar or a wire having a round cross section), bar wire rolling or wire rod rolling is performed. It is desirable that the heating temperature of the bar wire rolling or the wire rod rolling is 1100 ° C. or higher in order to surely form a solution of the Nb compound. A more preferable heating temperature is 1150 ° C. or higher. On the other hand, if the heating temperature is too high, the crystal grains become coarse, so the upper limit of the heating temperature is preferably 1250 ° C. As described above, the cooling rate after rolling is such that the average cooling rate when the surface temperature of the steel material is between 800 ° C. and 300 ° C. is 0.1 to 1.0 ° C./second.

A carburized gear can be obtained by machining the above steel material to form a gear shape and then carburizing, quenching and tempering. Here, as a method of creating the gear shape, hot forging, cold forging, cutting, or processing with a grindstone may be performed. Moreover, in order to improve workability, normalization and annealing may be performed. Moreover, you may combine these. Carburizing and quenching may be performed by any carburizing method such as gas carburizing or vacuum carburizing. Carbonitriding may also be performed. The gear to be created may be any type of gear, such as a spur gear, a helical gear, a bevel gear, an external tooth, and an internal tooth.

The following examples further illustrate the present invention.
For molten steel having chemical components of steel numbers 1 to 23, 25 and 26 shown in Table 1, No. Casting was performed under the conditions shown in No. 1 to obtain a slab. The balance of the chemical components disclosed in Table 1 is iron and impurities, and the blank indicates that it is not intentionally contained. Thereafter, the slab was heated to 1250 ° C. and subjected to ingot rolling to obtain a 162 mm square steel slab. These steel slabs were heated to 1200 ° C. and rolled into a steel bar to make the diameter 40 mm. Cooling was performed under the conditions shown in No. 1 to obtain steel materials 1 to 23, 33 and 34. With respect to these steel materials, the structure fraction such as the ferrite fraction and the standard deviation of the ferrite fraction (variation of ferrite fraction (%)) were calculated by the method described above. The results are shown in Table 3.

Then, in order to evaluate the heat treatment distortion of the gear, a spur gear with a module 2, the number of teeth of 16, and an inner diameter of φ18 mm and a width of 30 mm was prepared by cutting. Gas carburization was held at 925 ° C. for 2 hours in an atmosphere where the carbon potential CP was 0.8, and then oil quenching was performed at 130 ° C. Thereafter, tempering was performed at 150 ° C. After that, the gear shape measuring machine is used to measure the shape of the tooth trace direction at 90 ° pitch with 4 teeth per gear, and the maximum and minimum values of the tooth trace error thus obtained are measured. The difference between these was defined as the variation in the tooth trace error. When the variation in the tooth trace error was 15 μm or less, the heat treatment strain was judged to be good. The results are shown in Test No. 3 of Table 3. 1 to 23, 33, and 34.

Test No. of Invention Example Nos. 1 to 19 had good heat treatment strain. Test No. of the comparative example. For 20 to 23, 33, and 34, the range of chemical components was outside the range of the present invention, so that good heat treatment strain could not be obtained.
Specifically, Test No. In No. 20, the ferrite fraction was insufficient and the variation in ferrite fraction was excessive. This is presumed to be because the amount of Si was too large.
Test No. In No. 21, the ferrite fraction was insufficient and the variation in ferrite fraction was excessive. This is presumably because the amount of Mn was too large.
Test No. In No. 22, the ferrite fraction was insufficient and the variation of the ferrite fraction was excessive. This is presumed to be because the amount of Cr was too large.
Test No. In No. 23, the ferrite fraction was insufficient, and the variation in the ferrite fraction was excessive. This is presumed to be because the amount of Mo was too large.
Test No. In No. 33, the ferrite fraction was insufficient, and the fraction of the structure other than ferrite and bainite was excessive. This is presumed to be because one of Nb and Mo was not included in the steel material, and thus the effect of suppressing the pearlite generation of Nb and Mo could not be obtained.
Test No. In No. 34, the fraction of the structure other than ferrite and bainite became excessive. This is presumed to be because one of Nb and Mo was not included in the steel material, and thus the effect of suppressing the pearlite generation of Nb and Mo could not be obtained.
Test No. described above. In Nos. 20 to 23, 33, and 34, any one or more of the ferrite fraction, the fraction of the structure other than ferrite and bainite, and the variation of the ferrite fraction were out of the scope of the invention. The variation could not be suppressed.

Next, the molten steel having the chemical components shown in steel numbers 1, 3 and 24 in Table 1 was cast under the conditions shown in Manufacturing Conditions 1 to 12 in Table 2 to obtain slabs. Thereafter, the slab was heated to 1250 ° C. and subjected to ingot rolling to obtain a 162 mm square steel slab. These steel slabs were heated to 1200 ° C., rolled into a shape (diameter after rolling) shown in production conditions 1 to 12 in Table 2, and cooled under the cooling conditions shown in the same table. 24-32, 35, and 36 were obtained. With respect to these steel materials, the structure fraction such as the ferrite fraction, the standard deviation of the ferrite fraction (variation of ferrite fraction (%)), and the variation of tooth trace error were evaluated by the above-described methods. The results are shown in Test No. 3 of Table 3. 1, 24 to 32, 35, and 36. In addition, Test No. 32 is a production number of International Publication No. 2014/171472. This is a test example corresponding to 1.

Test No. of Invention Example Nos. 1, 24 to 28 had good heat treatment strain. On the other hand, test No. of the comparative example. For 29-32, 35, and 36, since the manufacturing conditions were not desirable, good heat treatment strain could not be obtained.
Specifically, Test No. No. 29 had an excessive variation in ferrite fraction. This is presumed to be because segregation could not be resolved because V × A ^0.5 / C was too large. For this reason, test no. In No. 29, the variation in the tooth trace error could not be suppressed.
Test No. No. 30 had an excessive variation in the ferrite fraction. This is presumed to be because segregation could not be resolved because V × A ^0.5 / C was too small. For this reason, test no. In 30, it was not possible to suppress the variation in the tooth trace error.
Test No. No. 31 lacked the ferrite fraction. This is presumably because most of the structure became bainite because the cooling rate after rolling was too high. For this reason, test no. No. 31 could not suppress the variation of the tooth trace error.
Test No. No. 32 had an excessive variation in the ferrite fraction. This is presumed to be because segregation could not be resolved because V × A ^0.5 / C was too large. For this reason, test no. In 32, the variation of the tooth trace error could not be suppressed.
Test No. No. 35 had an excessive variation in ferrite fraction. This is presumed to be because the cooling rate after rolling was too high, so that the homogenization of the structure could not be achieved. For this reason, test no. In 35, the variation of the tooth trace error could not be suppressed.
Test No. In 36, the fraction of the structure other than ferrite and bainite was excessive. The structure other than ferrite and bainite was pearlite. This is presumed to be because V × A ^0.5 / C was too small, so that segregation could not be eliminated and the cooling rate after rolling was too small. For this reason, test no. In 36, the variation of the tooth trace error could not be suppressed. In addition, Test No. In 36, the variation in the ferrite fraction is suppressed even though V × A ^0.5 / C is too small. This is thought to be because the tissue contained perlite. However, since pearlite also increases the variation in tooth trace error, test no. The steel material of 36 was not a steel material that stabilizes heat treatment strain.

Claims

% By mass
C: 0.17 to 0.21%,
Si: 0.40 to 0.60%,
Mn: 0.25 to 0.50%,
Cr: 1.35 to 1.55%,
Mo: 0.20 to 0.40%,
S: 0.010 to 0.05%,
N: 0.005 to 0.020%,
Al: 0.001% to 0.100%,
Nb: 0.001 to 0.030%
Ni: 0 to 3.0%,
Cu: 0 to 1.0%,
Co: 0 to 3.0%,
W: 0 to 1.0%
V: 0 to 0.3%,
Ti: 0 to 0.3%,
B: 0 to 0.005%
O: 0.005% or less,
P: 0.03% or less,
Pb: 0 to 0.5%,
Bi: 0 to 0.5%
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
Zr: 0 to 0.05%,
Te: 0 to 0.1%,
Rare earth elements: 0 to 0.005%
The balance consists of Fe and impurities,
In a region where the distance r from the center of the cross section perpendicular to the length direction satisfies the following formula, the structure contains ferrite and bainite, and the average fraction of the ferrite in terms of area ratio is in the range of 40 to 70%. The sum of the average fraction of the structure other than the ferrite and the bainite is an average value of 0% or more and 3% or less, and the balance is a structure composed of bainite,
A steel material characterized in that a standard deviation of a fraction of the ferrite in the region is 4% or less.
0.7R ≦ r ≦ 0.9R
However, R represents a circle-equivalent radius of the steel material.
% By mass
Ni: 0.01 to 3.0%,
Cu: 0.01 to 1.0%,
Co: 0.01 to 3.0%,
W: 0.01 to 1.0%,
V: 0.01 to 0.3%,
Ti: 0.001 to 0.3%,
B: 0.0001 to 0.005%
1 type or 2 types or more of these are contained, The steel materials of Claim 1 characterized by the above-mentioned.
% By mass
Pb: 0.01 to 0.5%,
Bi: 0.0001 to 0.5%,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%,
Zr: 0.0001 to 0.05%,
Te: 0.0001 to 0.1%,
Rare earth elements: 0.0001 to 0.005%
The steel material according to claim 1 or 2, comprising one or more of the following.