JP7597271B2 - Hot forged non-tempered steel and its manufacturing method - Google Patents

Description

The present invention relates to hot-forged non-tempered steel and a method for manufacturing the same.

Traditionally, high strength and toughness are required for automobile suspension parts such as steering knuckles and upper arms, and hydraulic parts for construction machinery such as rod ends. For this reason, these parts are made of carbon steel for machine construction such as S43C, S45C, and S48C, which are formed by hot forging and then subjected to heat treatment such as quenching and tempering (hereinafter also referred to as "thermal refining"), and depending on the type of part, induction hardening is further performed on the surface to ensure the required characteristics.

However, these tempering processes require enormous amounts of energy. Therefore, in order to meet the recent societal demand for energy conservation, there has been active development in recent years of non-tempered steel (hot-forged non-tempered steel) that has the necessary properties in the as-formed state by hot forging, which does not require tempering.

Until now, hot forged non-tempered steel has mainly been designed to increase strength through precipitation strengthening of V carbonitrides added to the ferrite-pearlite structure. However, in designs aiming for higher strength, there is concern that simply increasing the amount of carbon or V added could significantly impair toughness. As a result, attention has been focused on hot forged non-tempered steel with a bainite structure, and development has been carried out.

For example, Patent Document 1 proposes a non-tempered steel in which 0.05 to 0.50 mass% V is added to low carbon steel containing about 0.10 to 0.30 mass% C. This non-tempered steel does not require heat treatment after hot forging, and obtains excellent strength and toughness by natural cooling.

Patent Document 2 proposes a non-tempered steel containing more than 0.30 to 0.60% C, more than 1.60 to 3.00% Mn, and with the addition of V or Nb, in which the structure as hot forged is bainite.

Patent document 3 proposes a non-tempered steel containing 0.10 to 0.35% C and 0.30 to 0.70% V, which produces a bainite structure without the need for tempering treatment after hot forging.

Patent Document 4 proposes a non-tempered steel containing 0.25-0.38% C, 1.51-2.2% Mn, and a small amount of Mo with added V, which has a structure mainly composed of bainite after hot forging.

Patent No. 2743116 Patent No. 3196006 Patent No. 3241897 Patent No. 6390685

However, the non-tempered steel described in Patent Document 1 had the problem that it was not possible to achieve high strength due to its low carbon content.

In addition, the component design using the precipitates described in Patent Document 2 has the problem that performance varies greatly due to differences in forging conditions (heating temperature, processing temperature, etc.) and cooling rates depending on the part thickness, and there is also concern that increasing the Mn content may result in material degradation (reduced toughness and fatigue limit) due to Mn segregation.

Furthermore, the technology described in Patent Document 3 requires the addition of a large amount of V when achieving strength with a relatively low amount of C, but this poses the problem of impairing toughness.

In addition, the technology described in Patent Document 4 had the problem that a high Mn content caused segregation of Mn, resulting in deterioration of the material (reduced toughness and fatigue limit). There was also concern that the precipitation of V would deteriorate toughness.

The present invention was developed to solve the problems of conventional non-tempered steels described above, and aims to provide a non-tempered steel that has high strength, high toughness, and good induction hardenability without the need for tempering after hot forging, together with an advantageous manufacturing method.

With the above-mentioned objective in mind, the inventors conducted extensive research into hot-forged non-tempered steel and came to the following conclusions.

(1) A certain amount of C is necessary to ensure the induction hardening hardness, but if the C content is too high, the amount of retained austenite in the structure of the steel increases, causing a rapid deterioration in toughness.
(2) If the C content of low carbon bainite steel is simply increased, the toughness decreases as the strength increases, but by balancing Mn, Mo, and Cr in accordance with the increase in C content, it is possible to increase the strength while maintaining the bainite structure. Furthermore, the presence of such a bainite structure can suppress the decrease in toughness. In particular, this effect can be obtained more efficiently by suppressing the amount of V added.
(3) When V carbonitrides precipitate, they are easily affected by forging conditions (heating temperature, processing conditions, cooling rate), so reducing the V content as much as possible leads to stabilization of the material.
(4) If ferrite is not mixed into the structure of the steel material, the induction hardening depth becomes uniform within the cross section of the steel material and the variation in hardness of the hardened layer is also reduced.

The present invention is based on the above findings.

That is, the gist of the present invention is as follows.
1. A steel material as hot forged, comprising, by mass%, C: 0.35% to 0.48%, Si: 0.05% to 0.35%, Mn: 0.50% to 1.20%, P: 0.005% to 0.020%, S: 0.030% to 0.070%, Al: 0.015% to 0.050%, Cr: 0.60% to 1.50%, Mo: 0.05% to 0.25% a Ceq (mass%) shown in the following formula (1) is 0.750 or more and 0.870 or less, and satisfies the following formula (2), with the balance being Fe and unavoidable impurities; and an area ratio of bainite structure in the microstructure is 90% or more.
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14...(1)
0.200≦C-Mn/18-Cr/24-Mo/3≦0.350...(2)
The element symbols in the above formulas (1) and (2) indicate the content of the corresponding element in the steel, and if the element is not present in the steel, it is set to 0.

2. The hot forged non-tempered steel described in 1, wherein the composition further contains, in mass%, one or more of Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.005% or more and 0.025% or less, Pb: 0.05% or more and 0.30% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, Bi: 0.05% or more and 0.30% or less, and Sb: 0.0015% or more and 0.0100% or less.

3. The hot forged non-tempered steel described in 1 or 2, wherein the chemical composition further contains, by mass%, Sn: 0.0010% or more and 0.030% or less.

4. In mass%, C: 0.35% or more and 0.48% or less, Si: 0.05% or more and 0.35% or less, Mn: 0.50% or more and 1.20% or less, P: 0.005% or more and 0.020% or less, S: 0.030% or more and 0.070% or less, Al: 0.015% or more and 0.050% or less, Cr: 0.60% or more and 1.50% or less, Mo: 0.05% or more and 0.25% or less, V: less than 0.050%, B: 0.0005% or less, and N: 0.0 A method for producing a hot forged non-heat treated steel, comprising heating a steel material having a composition in which the content of C in the steel is 0.030% or more and 0.0200% or less, the Ceq (mass%) shown in the following formula (1) is 0.750 or more and 0.870 or less, the following formula (2) is satisfied, and the balance is Fe and unavoidable impurities, to 1100 to 1300°C, performing hot forging, and further cooling from 950°C to 350°C at an average cooling rate of 0.10 to 3.00°C/s.
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14...(1)
0.200≦C-Mn/18-Cr/24-Mo/3≦0.350...(2)
The element symbols in the above formulas (1) and (2) indicate the content of the corresponding element in the steel, and if the element is not present in the steel, it is set to 0.

5. A method for producing hot forged non-tempered steel as described in 4 above, wherein the composition further contains, in mass%, one or more selected from Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.005% or more and 0.025% or less, Pb: 0.05% or more and 0.30% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, Bi: 0.05% or more and 0.30% or less, and Sb: 0.0015% or more and 0.0100% or less.

6. A method for producing hot forged non-tempered steel as described in 4 or 5, wherein the chemical composition further contains, by mass%, Sn: 0.0010% or more and 0.030% or less.

According to the present invention, it is possible to obtain non-tempered steel that has high strength, high toughness and excellent induction hardenability without carrying out heat treatment (tempering) such as quenching and tempering after hot forging.

The present invention is described in detail below.

[Hot forged non-thermal steel]
First, the reason why the composition of the non-heat treated steel according to the present invention is limited to the above range will be explained. In the following explanation, the content (%) of each element is by mass unless otherwise specified. It is understood to mean %.

C: 0.35-0.48%
C is an element necessary for ensuring strength, and also acts to increase surface hardness during induction hardening. To achieve this effect, a C content of 0.35% or more is necessary. Preferably, the C content is 0.37% or more, and more preferably 0.39% or more. On the other hand, if the C content exceeds 0.48%, the amount of retained austenite increases too much, resulting in a decrease in toughness. Therefore, the upper limit of the C content is set to 0.48%, preferably 0.45% or less, and more preferably 0.43% or less.

Si: 0.05-0.35%
Silicon is useful as a deoxidizer during the steelmaking process, and must be contained in an amount of 0.05% or more. It is preferably contained in an amount of 0.06% or more, and more preferably contained in an amount of 0.09% or more. On the other hand, if the Si content exceeds 0.35%, the toughness decreases, so the upper limit of the Si content is set to 0.35%. It is preferably 0.30% or less, and more preferably 0. It is less than 28%.

Mn: 0.50-1.20%
Mn is an element useful for improving the hardenability of steel and transforming the structure into bainite. However, if the Mn content is less than 0.50%, the hardenability is insufficient and the amount of bainite structure formed is reduced. Therefore, the Mn content is set to 0.50% or more, preferably 0.55% or more, and more preferably 0.60% or more. On the other hand, if the Mn content exceeds 1.20%, the hardenability becomes too high and the formation of retained austenite is promoted, resulting in a decrease in not only toughness but also fatigue limit. The upper limit of the content of Si is set to 1.20%, preferably 1.15% or less, and more preferably 1.10% or less.

P:0.005-0.020%
P is an element that segregates at prior austenite grain boundaries and the like, lowering toughness. If the content exceeds 0.020%, it has a large adverse effect on toughness, so the upper limit is set at 0.020%. The lower limit of P is set at 0.018% or less, and more preferably 0.016% or less. On the other hand, the lower the P content, the better the toughness becomes, but the refining cost increases, so the lower limit is set at 0.005%.

S: 0.030-0.070%
S is an element useful for improving machinability, and since a content of 0.030% or more is necessary to obtain this effect, the lower limit is set at 0.030%, preferably 0.035%. On the other hand, excessive addition of more than 0.070% reduces toughness by forming MnS which acts as a fracture origin. Therefore, the upper limit is set at 0.070%. The preferable content is 0.065% or less. More preferably, it is 0.060% or less.

Al: 0.015-0.050%
Al is an element that has a strong deoxidizing effect, but if the content is less than 0.015%, a sufficient deoxidizing effect cannot be obtained, so the lower limit of the Al content is set to 0.015%. On the other hand, if the Al content exceeds 0.050%, not only does the effect of the addition become saturated, but the fatigue limit is also reduced due to the excessive inclusions. The content is preferably 0.045% or less, and more preferably 0.040% or less.

Cr:0.60~1.50%
Cr, like Mn, is an element necessary for transforming the structure into bainite. However, if the Cr content is less than 0.60%, this effect is not sufficiently exhibited. On the other hand, if the Cr content is 1. If the Cr content exceeds 50%, the generation of residual austenite is promoted, and the fatigue limit is reduced. Therefore, the Cr content is set to the range of 0.60 to 1.50%, preferably 0.65% or more, and more preferably 0.65% or more. The content is 0.70% or more. Also, the content is preferably 1.40% or less, and more preferably 1.30% or less.

Mo: 0.05-0.25%
Mo is an element necessary for suppressing ferrite and pearlite transformation, transforming the structure into bainite and refining the bainite laths to improve toughness, and suppressing the amount of retained austenite. If the Mo content is less than 10%, the above effects are not sufficiently exhibited, so the lower limit of the Mo content is set to 0.05%, preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, if the Mo content exceeds 0.25%, the cost increases and the generation of retained austenite is suppressed, lowering the impact value and fatigue limit, so the upper limit of the Mo content is set at 0.25%. The content is preferably 0.20% or less, and more preferably 0.18% or less.

V: less than 0.050% V has a strong affinity with C and N, and precipitates as carbonitrides in steel. However, this effect is small in bainite structures, and it actually reduces toughness. Therefore, the V content must be kept below 0.050%. To improve low-temperature toughness, it is preferably 0.040% or less, and more preferably 0.035% or less. On the other hand, there is no particular lower limit for the V content, but it is preferably about 0.001% to prevent impurities from being mixed into the raw materials.

B: 0.0005% or less B is an element that enhances hardenability and increases strength, but its hardenability is highly dependent on the cooling rate, which causes strength variations in the material after hot forging, so the upper limit is set to 0.0005%. It is preferably 0.0003% or less. On the other hand, the lower limit of the B content is not particularly limited, but it is preferably about 0.00005% because of impurities being mixed into the raw material.

N: 0.0030% or more and 0.0200% or less N forms nitrides in steel and has the effect of suppressing the coarsening of grain size during heating. In order to produce this effect, it is necessary to add at least 0.0030% or more. Preferably, it is 0.0050% or more. More preferably, it is 0.0060% or more. On the other hand, excessive addition of N promotes crack defects in the material and reduces toughness, so the upper limit is set to 0.0200%. Preferably, it is 0.0190% or less. More preferably, it is 0.0180% or less.

Furthermore, in the present invention, the component composition must satisfy the following formulas (1) and (2).
0.750≦Ceq (=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14)≦0.870...(1)
0.200≦C-Mn/18-Cr/24-Mo/3≦0.350...(2)
The element symbols in the above formulas (1) and (2) indicate the content of the corresponding element in the steel, and if the element is not present in the steel, it is set to 0.

In addition, both of the above formulas (1) and (2) are indicators for controlling the bainite structure, and by keeping these elements within the ranges of the indicators, it is possible to obtain a predetermined strength and excellent toughness in the bainite structure. The Ceq (mass%) shown in the above formula (1) is preferably 0.770 or more and 0.850 or less.

The basic components of the present invention have been described above, but the remainder of the hot forged non-tempered steel of the present invention other than the above components is Fe and unavoidable impurities. However, the present invention allows the addition of the following components as necessary.

Specifically, the elements are one or more selected from Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.005% or more and 0.025% or less, Pb: 0.05% or more and 0.30% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, Bi: 0.05% or more and 0.30% or less, Sb: 0.0015% or more and 0.0100% or less, and Sn: 0.0001% or more and 0.030% or less.

Cu, Ni and Nb are effective elements for increasing strength. In order to obtain this effect, when adding Cu, Ni and Nb, it is necessary to add Cu: 0.01% or more, Ni: 0.01% or more and Nb: 0.005% or more, respectively. On the other hand, excessive addition of Cu, Ni and Nb leads to deterioration of surface properties, increase in manufacturing costs and deterioration of toughness, so the upper limit of each addition is set at Cu: 0.30%, Ni: 0.30%, Nb: 0.050%.

Ti forms TiN and other elements, inhibits the coarsening of grain size during heating, and improves toughness, but in order to obtain this effect, the addition of 0.005% or more is necessary. On the other hand, excessive addition leads to a decrease in toughness and fatigue strength due to the formation of coarse precipitates, so the upper limit of addition is set at 0.025%.

Pb, Ca, Mg and Bi are all elements that are effective in improving machinability. When adding Pb, Ca, Mg and Bi, in order to obtain this effect, it is necessary to add at least 0.05% Pb, at least 0.0005% Ca, at least 0.0005% Mg and at least 0.05% Bi, respectively. On the other hand, adding large amounts of these elements not only saturates the effect, but also reduces toughness, so the upper limits of each element are set at 0.30% Pb, 0.0050% Ca, 0.0050% Mg and 0.30% Bi, respectively.

Sb has the effect of suppressing decarburization of the surface layer during high-temperature heating and increasing fatigue strength. This effect is observed at 0.0015% or more, so the lower limit is set at 0.0015%. On the other hand, excessive addition reduces toughness, so the upper limit for addition is set at 0.0100%.

Sn has the effect of suppressing decarburization of the surface layer during high-temperature heating and increasing fatigue strength. This effect is observed at 0.0010% or more, so the lower limit is set at 0.0010%. On the other hand, excessive addition reduces toughness, so the upper limit for addition is set at 0.030%.

In this invention, the steel structure is defined as follows:

Area ratio of bainite structure: 90% or more In order to obtain a predetermined strength and high toughness, the hot forged non-tempered steel of the present invention must have the above-mentioned steel components and the bainite structure in the microstructure in an area ratio of 90% or more. Furthermore, in surface hardening treatment by induction heat treatment, the bainite structure in which carbon is distributed uniformly has the advantage that the hardness nonuniformity is smaller than that of a ferrite-pearlite structure in which carbon is distributed unevenly. The area ratio is preferably 93% or more, more preferably 95% or more, and may be 100%.

The above steel structure can be obtained by adjusting the heating temperature before hot forging and the average cooling rate after hot forging when hot forging the steel material into the part shape. Specifically, it is necessary to satisfy both of the manufacturing conditions of heating temperature before hot forging: 1100-1300°C and average cooling rate after hot forging: 0.10-3.00°C/s.

Heating temperature: 1100-1300℃
If the heating temperature is low when hot forging, the above-mentioned steel structure cannot be obtained. That is, if the heating temperature before hot forging is lower than 1100°C, ferrite is likely to be generated, and the microstructure It is not possible to achieve an area ratio of 90% or more of the bainite structure. On the other hand, the higher the heating temperature, the easier it is to obtain a bainite structure, but this leads to problems such as a decrease in toughness due to coarsening of the structure, a decrease in yield due to scale loss, and an increase in energy costs. Therefore, the upper limit of the heating temperature is set to 1300° C. The heating temperature is preferably 1150° C. or more and 1250° C. or less. This heating temperature is the core temperature of the steel material.

Average cooling rate after hot forging: 0.10 to 3.00 ° C/s
If the average cooling rate in the specified temperature range after hot forging is less than 0.10°C/s, the steel structure becomes a ferrite-pearlite structure, and the strength decreases. On the other hand, if the average cooling rate exceeds 3.00°C/s, the hardness of the resulting steel becomes too high, and the toughness decreases significantly. The average cooling rate is preferably 0.30°C/s or more and 2.80°C/s or less, and more preferably 0.50°C/s or more and 2.50°C/s or less. This average cooling rate is at the surface temperature, and the above-mentioned specified temperature range means a range of 950 to 350°C at the surface temperature.

Furthermore, manufacturing conditions other than those described above for a preferred manufacturing method for hot forged non-tempered steel according to the present invention will be described.

Molten steel having the above-mentioned composition is melted using a conventional melting method such as a converter or electric furnace, and made into a steel material by conventional continuous casting or blooming. The steel material is then heated as necessary, and made into a steel bar by hot rolling such as billet rolling or bar and wire rolling. The above heating and rolling conditions are not particularly limited, but may be appropriately determined depending on the required material. For example, structure control such as MnS control to improve machinability may be performed so that it is advantageous for subsequent forging and machining for part formation.

In addition, the content of each element in steel can be determined by spark discharge optical emission spectrometry, X-ray fluorescence spectrometry, ICP optical emission spectrometry, ICP mass spectrometry, combustion method, etc.

Other manufacturing conditions not described in this specification may follow general manufacturing methods for steel materials.

Next, examples of the present invention will be described. Note that the following examples are presented to more specifically explain the present invention, and the present invention is not limited to the scope of the following examples.

[Example 1]
Ingots with the composition shown in Table 1 (balance: Fe and unavoidable impurities) were hot-rolled into round bars with a diameter of 36 mm, which were heated to 1250°C, hot-forged into round bars with a diameter of 25 mm, air-cooled to 600°C, and then slowly cooled to 100°C at a rate of 0.15°C/s. The average cooling rate in the temperature range from 950°C to 600°C was 0.50°C/s . The average cooling rate in the temperature range from 950°C to 350°C was 0.35°C/s.

The round bars after air cooling and slow cooling were used as test materials, and the microstructure, tensile strength, fatigue limit, and impact value were measured using each test material (one bar per test condition) using the methods described below. Induction hardenability was also evaluated using the method described below.

The measurement methods for microstructure, tensile strength, fatigue limit and impact value are as follows:

(1) The microstructure was determined by polishing the cross section of the steel material and etching it with nital, then observing the exposed cross section with an optical microscope and photographing it. The images obtained were then processed to determine the bainite fraction (bainite area ratio).

Specifically, three fields of view were photographed at a magnification of 400 times, and the total measurement area was 105,600 μm ² (35,200 μm ² per field of view). The bainite phase was identified as a composite phase of ferrite laths and carbides other than ferrite, pearlite, and residual γ phase, and the bainite fraction was derived by image analysis using the software ImageJ.

(2) Tensile strength was measured by taking a JIS No. 4 tensile test piece from the round bar and conducting a tensile test at a tensile speed of 1 mm/s in accordance with JIS Z 2241.

(3) The fatigue limit was determined by taking a smooth test piece of 8 mm in diameter and subjecting it to an Ono-type rotating bending fatigue test in accordance with JIS Z 2274, and the maximum stress reached without fracture up to 10 ⁷ cycles was used.

(4) The impact value was measured by taking a Charpy test specimen with a 10 mm square U-notch with a notch width of 3 mm and depth of 5 mm, cooling it to -50°C, and then conducting a Charpy impact test.

In addition, induction hardenability was evaluated in terms of surface hardness, average hardened layer depth, and standard deviation of effective hardened layer depth after induction hardening as follows.

At a frequency of 200 Hz, the hardening conditions were found and set for each steel such that an effective hardened layer depth exceeding 2.00 mm was obtained when measured in one direction within the cross section. After hardening under these conditions, the steel was tempered at 160°C for 1 hour.

The surface hardness was determined as the minimum value of three measurements in Rockwell hardness (HRC). A cross section perpendicular to the height of the cylinder was then cut out, and the hardened layer depth within this cross-sectional circle was measured from three directions toward the center of the cross-sectional circle using a Vickers hardness tester at a load of 2.94 N (300 gf) at 90° intervals, with each pitch being 0.2 mm apart. The average length from the steel surface on the cross-sectional circle at the point where the hardness reached Hv400 was then calculated, and this was taken as the average hardened layer depth.

In addition, the variation in the effective hardened layer depth in the three directions was calculated by calculating the standard deviation σ of the above-mentioned average hardened layer depth.

The above test results and evaluation results are shown in Table 2.

As is clear from Table 2, Steel Nos. A to L, AO to AQ, AS, AU, AW, BA, BD and BF, which are examples of the invention, all had an area ratio of bainite structure in the microstructure of 90% or more, a tensile strength of 820 MPa or more, an impact value at -50°C of 25 J/cm2 ^or more, a fatigue strength of 550 N/mm2 ^or more, a surface hardness HRC after high-frequency treatment of 47 or more, an average case depth after high-frequency treatment of 2.00 mm or more and an effective case depth standard deviation σ of 0.12 mm or less.

In addition, in this invention, a tensile strength of 820 MPa or more is evaluated as having excellent strength.

In the present invention, a specimen having an impact value of 25 J/ ^cm2 or more at -50°C was evaluated as having excellent toughness .

Fatigue strength: In the present invention, a fatigue strength of 550 N/ ^mm2 or more was evaluated as being excellent.

In addition, in the present invention, when the surface hardness after high-frequency treatment is HRC: 47 or more, it is evaluated as having excellent fatigue strength and wear resistance, and excellent high-frequency hardenability.

In the present invention, when the average hardened layer depth after high-frequency treatment is 2.00 mm or more and the standard deviation σ of the effective hardened layer depth is 0.12 mm or less, the material is evaluated as having excellent fatigue strength and excellent high-frequency hardenability.

In contrast, the comparative steels Nos. M to AN, AR, AT, AV, AX to AZ, BB, BC, BE and BG to BJ are inferior to the invention examples in any of the tensile strength, impact value at -50°C, fatigue strength, surface hardness after high-frequency treatment, average case depth and standard deviation σ of the effective case depth.

[Example 2]
Next, the ingots having the steel compositions shown in Steel Nos. A, B, and C in Table 1 were hot-rolled into round bars having a diameter of 36 mm, which were then hot-forged into round bars having a diameter of 25 mm. The heating temperature before the hot forging and the average cooling rate after the hot forging were set under various conditions shown in Table 3.

For the round bars thus obtained, the microstructure, tensile strength, fatigue strength, and impact value at -50°C were measured, and the induction hardenability was evaluated, in the same manner as in Example 1. The results are shown in Table 3.

As is clear from the descriptions in Table 3, when the heating temperature before hot forging and the average cooling rate after hot forging were within the range of the present invention, the area ratio of bainite structure in the microstructure was 90% or more, the tensile strength was 820 MPa or more, the impact value at -50°C was 25 J/ ^cm2 , the fatigue strength was 550 N/mm2 ^or more, the surface hardness after high-frequency treatment was HRC 47 or more, the average case depth after high-frequency treatment was 2.00 mm or more, and the standard deviation σ of the effective case depth was 0.12 mm or less.

In contrast, if the heating temperature or average cooling rate after hot forging is outside the range of the present invention, any of the tensile strength, impact value at -50°C, fatigue strength, surface hardness after high-frequency treatment, average case depth and effective case depth standard deviation σ do not meet the desired values and are inferior.

Claims (6)

A hot forged steel material,
In mass percent,
C: 0.35% or more and 0.48% or less,
Si: 0.05% or more and 0.35% or less,
Mn: 0.50% or more and 1.20% or less,
P: 0.005% or more and 0.020% or less,
S: 0.030% or more and 0.070% or less,
Al: 0.015% or more and 0.050% or less,
Cr: 0.60% or more and 1.50% or less,
Mo: 0.05% or more and 0.25% or less,
V: less than 0.050%
B: 0.0005% or less and N: 0.0030% or more and 0.0200% or less;
The composition has a Ceq of 0.750 or more and 0.870 or less as shown in the following formula (1), satisfies the following formula (2), and the balance is Fe and unavoidable impurities,
The area ratio of bainite structure in the microstructure is 90% or more,
A hot-forged non-heat-treated steel material having a tensile strength of 820 MPa or more and an impact value of 25 J/cm2 or more at -50°C in a Charpy impact ^test .
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14...(1)
0.200≦C-Mn/18-Cr/24-Mo/3≦0.350...(2)
The element symbols in the above formulas (1) and (2) indicate the content (mass%) of the corresponding element in the steel, and are set to 0 if the element is not present in the steel. The composition further comprises, in mass%,
Cu: 0.01% or more and 0.30% or less,
Ni: 0.01% or more and 0.30% or less,
Nb: 0.005% or more and 0.050% or less,
Ti: 0.005% or more and 0.025% or less,
Pb: 0.05% or more and 0.30% or less,
Ca: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less,
The hot forged non-heat treated steel material according to claim 1, containing one or more selected from Bi: 0.05% or more and 0.30% or less and Sb: 0.0015% or more and 0.0100% or less. The hot forged non-heat treated steel material according to claim 1 or 2, wherein the composition further contains, by mass%, Sn: 0.0010% or more and 0.030% or less. In mass percent,
C: 0.35% or more and 0.48% or less,
Si: 0.05% or more and 0.35% or less,
Mn: 0.50% or more and 1.20% or less,
P: 0.005% or more and 0.020% or less,
S: 0.030% or more and 0.070% or less,
Al: 0.015% or more and 0.050% or less,
Cr: 0.60% or more and 1.50% or less,
Mo: 0.05% or more and 0.25% or less,
V: less than 0.050%
B: 0.0005% or less and N: 0.0030% or more and 0.0200% or less;
A steel material having a composition in which Ceq shown in the following formula (1) is 0.750 or more and 0.870 or less, satisfies the following formula (2), and the balance is Fe and unavoidable impurities, is heated to 1100 to 1300 ° C, and then hot forged, and further cooled at an average cooling rate of 0.10 to 3.00 ° C / s between 950 ° C and 350 ° C.,
A method for producing a hot-forged non-heat-treated steel material having a microstructure with an area ratio of bainite structure of 90% or more, a tensile strength of 820 MPa or more, and an impact value at -50°C in a Charpy impact test of 25 J/cm2 or ^more .
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14...(1)
0.200≦C-Mn/18-Cr/24-Mo/3≦0.350...(2)
The element symbols in the above formulas (1) and (2) indicate the content (mass%) of the corresponding element in the steel, and are set to 0 if the element is not present in the steel. The composition further comprises, in mass%,
Cu: 0.01% or more and 0.30% or less,
Ni: 0.01% or more and 0.30% or less,
Nb: 0.005% or more and 0.050% or less,
Ti: 0.005% or more and 0.025% or less,
Pb: 0.05% or more and 0.30% or less,
Ca: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less,
The method for producing a hot forged non-heat treated steel material according to claim 4, further comprising the step of adding one or more selected from the group consisting of Bi: 0.05% or more and 0.30% or less and Sb: 0.0015% or more and 0.0100% or less. The method for producing a hot forged non-heat treated steel material according to claim 4 or 5, wherein the composition further contains, by mass%, Sn: 0.0010% or more and 0.030% or less.