Hot forged non-tempered steel and its manufacturing method

Abstract

Provided are a hot-forged untempered steel having high strength, high toughness, and favorable high-frequency quenchability, without being tempered after hot forging, and a method for producing same. The untempered steel has a predetermined component composition in which the value of Ceq (% by mass) represented by the following formula (1) is between 0.750 and 0.870 both inclusive, and which satisfies the following formula (2), and comprises a microstructure with a bainitic structure area ratio of 90% or more. (1): Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (2): 0.200 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.350 Chemical symbols in the formulae (1) and (2) indicate contents of respective elements in the steel, with 0 meaning absence in the steel.

Description

[Technical field]

[0001] The present invention relates to a hot forged non-heat treated steel and a method for producing the same. [Background technology]

[0002] 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 have been 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, the surface is further subjected to induction hardening to ensure the required characteristics.

[0003] However, these tempering processes require a huge amount of energy. Therefore, in order to meet the recent social demand for energy saving, there has been active development in recent years of non-tempered steels (hot forged non-tempered steels) that have the necessary properties in the as-formed state by hot forging, which does not require tempering.

[0004] Until now, hot forged non-tempered steels have mainly been designed to increase strength by precipitation strengthening of V carbonitrides added to the ferrite-pearlite structure. However, there is concern that simply increasing the amount of carbon or V added in a design aimed at higher strength may significantly impair toughness. Therefore, attention has been focused on hot forged non-tempered steels with a bainite structure, and development has been carried out.

[0005] For example, Patent Document 1 proposes a non-heat treated 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-heat treated steel does not require heat treatment after hot forging, and obtains excellent strength and toughness by natural cooling.

[0006] Patent Document 2 proposes a non-tempered steel that contains more than 0.30 to 0.60% C, more than 1.60 to 3.00% Mn, and has an as-hot forged structure of bainite due to the addition of V or Nb.

[0007] Patent Document 3 proposes a non-tempered steel containing 0.10 to 0.35% C and 0.30 to 0.70% V, which can obtain a bainite structure without carrying out a tempering treatment after hot forging.

[0008] Patent Document 4 proposes a non-tempered steel containing 0.25 to 0.38% C, 1.51 to 2.2% Mn, and a small amount of Mo with added V, which has a structure mainly consisting of bainite after hot forging. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 2743116 [Patent Document 2] Patent No. 3196006 [Patent Document 3] Patent No. 3241897 [Patent Document 4] Patent No. 6390685 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the non-heat treated steel described in Patent Document 1 has a problem in that high strength cannot be obtained due to its low carbon content.

[0011] Furthermore, in the component design using the precipitates described in Patent Document 2, there is a problem in that there is a large variation in performance due to differences in the forging conditions (heating temperature, processing temperature, etc.) and the cooling rate depending on the part thickness. There is also a concern that increasing the Mn content may result in material deterioration (reduction in toughness and fatigue limit) due to Mn segregation.

[0012] Furthermore, in the technology described in Patent Document 3, when strength is to be achieved with a relatively low C content, it is necessary to add a large amount of V, which has the problem of impairing toughness.

[0013] In addition, the technology described in Patent Document 4 has a problem in that a high Mn content causes segregation of Mn, resulting in deterioration of the material (reduction in toughness and fatigue limit). Also, there is a concern that the precipitation of V may deteriorate the toughness.

[0014] The present invention has been developed to solve the problems of conventional non-heat treated steels as described above, and has an object to provide a non-heat treated steel that has high strength, high toughness, and good induction hardenability even without heat treating after hot forging, together with an advantageous method for producing the same. [Means for solving the problem]

[0015] Means for Solving the Problems The inventors have conducted extensive research into hot forged non-heat treated steels with the above object in mind, and as a result have come to the following findings.

[0016] (1) A certain amount of C is necessary to ensure the induction hardening hardness, but if it 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 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, and cooling rate). Therefore, reducing the V content as much as possible leads to material stabilization. (4) By not mixing ferrite into the structure of the steel, the induction hardening depth becomes uniform within the cross section of the steel and the variation in hardness of the hardened layer is also reduced.

[0017] The present invention is based on the above findings.

[0018] That is, the gist of the present invention is as follows. 1. Steel material as hot forged, with the following mass percentages: 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% The following hot forged non-heat treated steel contains V: less than 0.050%, B: 0.0005% or less, and N: 0.0030% or more and 0.0200% or less, has a Ceq (mass%) shown in the following formula (1) of 0.750 or more and 0.870 or less, satisfies the following formula (2), has a component composition consisting of the remainder Fe and unavoidable impurities, and has an area ratio of bainite structure in the microstructure of 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.

[0019] 2. The hot forged non-heat treated steel according to 1 above, further comprising, 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.

[0020] 3. The hot forged non-heat treated steel according to 1 or 2 above, wherein the chemical composition further contains, by mass%, Sn: 0.0010% or more and 0.030% or less.

[0021] 4. In mass percentage, 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. A method for producing hot-forged non-tempered steel, comprising heating a steel material having a composition containing 0.030% or more and 0.0200% or less of C, having a Ceq (mass%) shown in the following formula (1) of 0.750 or more and 0.870 or less, satisfying the following formula (2), the balance being Fe and unavoidable impurities, to 1100 to 1300°C, 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.

[0022] 5. The method for producing hot forged non-heat treated steel according to 4 above, wherein the chemical 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.

[0023] 6. The method for producing hot forged non-heat treated steel according to 4 or 5, wherein the chemical composition further contains, by mass%, Sn: 0.0010% or more and 0.030% or less. Effect of the Invention

[0024] According to the present invention, it is possible to obtain a non-tempered steel having high strength, high toughness and excellent induction hardenability without carrying out heat treatment (tempering) such as quenching and tempering after hot forging. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention will be specifically described below.

[0026] [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 means mass % unless otherwise specified.

[0027] 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, the C content is 0.39% or more. On the other hand, if the C content exceeds 0.48%, the amount of retained austenite increases too much and the toughness decreases, so the upper limit of the C content is set to 0.48%. Preferably, the C content is 0.45% or less, and more preferably, the C content is 0.43% or less.

[0028] Silicon: 0.05 to 0.35% Silicon is useful as a deoxidizer during the steelmaking process, i.e., during the steelmaking process, and must be contained in an amount of 0.05% or more. It is preferably 0.06% or more, and more preferably 0.09% or more. On the other hand, if the silicon content exceeds 0.35%, the toughness decreases, so the upper limit of the silicon content is set at 0.35%. It is preferably 0.30% or less, and more preferably 0.28% or less.

[0029] 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, the amount of bainite structure formed is reduced, and sufficient strength and toughness cannot be obtained, so Mn is contained at 0.50% or more. It is preferably 0.55% or more, and more preferably 0.60% or more. On the other hand, if Mn is contained in excess of 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, so the upper limit of the Mn content is set to 1.20%. It is preferably 1.15% or less, and more preferably 1.10% or less.

[0030] P: 0.005~0.020% P is an element that segregates at prior austenite grain boundaries and reduces toughness. If the content exceeds 0.020%, the adverse effect on toughness is large, so the upper limit is set to 0.020%. It is preferably 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 to 0.005%.

[0031] 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 to 0.030%. Preferably, it is 0.035% or more. On the other hand, excessive addition of more than 0.070% reduces toughness because the generated MnS becomes the fracture origin. Therefore, the upper limit is set to 0.070%. Preferably, it is 0.065% or less, and more preferably, it is 0.060% or less.

[0032] Al: 0.015 to 0.050% Al is an element with a strong deoxidizing effect, but if the content is less than 0.015%, sufficient deoxidizing effect cannot be obtained, so the lower limit of the Al content is set to 0.015%. It is preferably 0.019% or more. On the other hand, if the Al content exceeds 0.050%, not only the effect of the addition is saturated, but also the fatigue limit is reduced due to excessive inclusions, so the upper limit of the Al content is set to 0.050%. It is preferably 0.045% or less, and more preferably 0.040% or less.

[0033] Cr: 0.60~1.50% Like Mn, Cr 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 expressed. On the other hand, if the Cr content exceeds 1.50%, the generation of retained austenite is promoted, and the fatigue limit decreases. Therefore, the Cr content is set to the range of 0.60 to 1.50%, preferably 0.65% or more, more preferably 0.70% or more. Also, preferably 1.40% or less, more preferably 1.30% or less.

[0034] Mo: 0.05 to 0.25% Mo is an element necessary for suppressing ferrite and pearlite transformation, transforming the structure into bainite, refining the bainite laths, improving toughness, and suppressing the amount of retained austenite. However, if the Mo content is less than 0.05%, the above effects are not sufficiently exhibited, so the lower limit of the Mo content is set to 0.05%. It is 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, resulting in a decrease in the impact value and fatigue limit, so the upper limit of the Mo content is set to 0.25%. It is preferably 0.20% or less, and more preferably 0.18% or less.

[0035] 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.

[0036] 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%, 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% to prevent impurities from being mixed into the raw material.

[0037] 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 at 0.0200%. Preferably, it is 0.0190% or less. More preferably, it is 0.0180% or less.

[0038] 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.

[0039] Moreover, both of the above formulas (1) and (2) are indices for controlling the bainite structure, and by keeping these elements within the ranges of the indices, 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.

[0040] The basic components of the present invention have been described above, but the balance 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 can further include the following components as necessary.

[0041] Specifically, the content is 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.

[0042] Cu, Ni, and Nb are elements that are effective in 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, increased manufacturing costs, and reduced toughness, so the upper limits of each addition are set at Cu: 0.30%, Ni: 0.30%, and Nb: 0.050%.

[0043] Ti forms TiN and other elements, inhibits the coarsening of grain size during heating, and improves toughness, but 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%.

[0044] Pb, Ca, Mg and Bi are all effective elements for improving machinability. When adding Pb, Ca, Mg and Bi, the amount of Pb: 0.05% or more, Ca: 0.0005% or more, Mg: 0.0005% or more and Bi: 0.05% or more must be added to obtain this effect. 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 Pb: 0.30%, Ca: 0.0050%, Mg: 0.0050% and Bi: 0.30%, respectively.

[0045] 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 is set at 0.0100%.

[0046] 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 is set at 0.030%.

[0047] In the present invention, the steel structure is defined as follows.

[0048] 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 a bainite structure in the microstructure with an area ratio of 90% or more. Furthermore, in surface hardening treatment by induction heat treatment, a bainite structure with uniform carbon distribution has the advantage that the hardness nonuniformity is smaller than that of a ferrite-pearlite structure with uneven carbon distribution. The area ratio is preferably 93% or more, more preferably 95% or more, and may be 100%.

[0049] The above-mentioned 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 a part shape. Specifically, it is necessary to satisfy both of the manufacturing conditions of the heating temperature before hot forging: 1100 to 1300°C and the average cooling rate after hot forging: 0.10 to 3.00°C / s.

[0050] 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 it is not possible to achieve an area ratio of 90% or more of the bainite structure in the microstructure. 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 higher and 1250°C or lower. This heating temperature is the center temperature of the steel material.

[0051] Average cooling rate after hot forging: 0.10~3.00℃ / s If the average cooling rate in the predetermined 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 obtained 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 predetermined temperature range means a range of 950 to 350°C at the surface temperature.

[0052] Further, manufacturing conditions other than those mentioned above for the preferred method of manufacturing the hot forged non-heat treated steel according to the present invention will be described.

[0053] Molten steel having the above-mentioned composition is melted by a conventional melting method such as a converter or an electric furnace, and made into a steel material by a conventional continuous casting or blooming method. 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, and may be appropriately determined depending on the required properties. For example, structure control such as MnS control for improving machinability may be performed so that the steel is advantageous for subsequent forging and machining for part formation.

[0054] The content of each element in steel can be determined by spark discharge optical emission spectrometry, fluorescent X-ray spectrometry, ICP optical emission spectrometry, ICP mass spectrometry, combustion method, or the like.

[0055] Other manufacturing conditions not described in this specification may be in accordance with general manufacturing methods for steel materials. EXAMPLES

[0056] 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.

[0057] [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 then 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. 。9 The average cooling rate in the temperature range from 50°C to 350°C is 0.35°C / s. Was .

[0058] 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) by the methods described below. Induction hardenability was also evaluated by the method described below.

[0059] The methods for measuring the microstructure, tensile strength, fatigue limit and impact value are as follows.

[0060] (1) The microstructure was evaluated by polishing the cross section of the steel material and etching it with nital. The exposed cross section was then observed under an optical microscope and photographed. The images were then processed to determine the bainite fraction (bainite area ratio).

[0061] Specifically, three fields of view were photographed at a magnification of 400x, and the measurement area was a total of 105,600 μm 2 (35200μm per field of view 2 The bainite phase was defined 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.

[0062] (2) The 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.

[0063] (3) The fatigue limit was measured by taking a smooth test piece of 8 mm diameter and performing a rotating bending fatigue test according to JIS Z 2274 for 10 7 The maximum stress reached without fracture was defined as the number of times.

[0064] (4) The impact value was measured by taking a Charpy test piece with a 10 mm square and a 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.

[0065] The induction hardenability was evaluated in terms of the surface hardness, the average hardened layer depth, and the standard deviation of the effective hardened layer depth after induction hardening, as follows.

[0066] At a frequency of 200 Hz, the quenching conditions were found and set for each steel under which an effective hardened layer depth exceeding 2.00 mm was obtained in one-way measurement within the cross section. After quenching under those conditions, the steel was tempered at 160°C for 1 hour.

[0067] The surface hardness was determined as the minimum value of three measurements in Rockwell hardness (HRC). Then, a cross section perpendicular to the height direction of the cylinder was 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 at 90° intervals with a Vickers hardness tester under a load of 2.94 N (300 gf), with a pitch of 0.2 mm each. The average length from the steel surface on the cross-sectional circle to the point where Hv400 was reached was calculated, and this was taken as the average hardened layer depth.

[0068] The variation in the effective case depth in the three directions was calculated by calculating the standard deviation σ of the average case depth.

[0069] The above test results and evaluation results are shown in Table 2.

[0070] [Table 1] TIFF0007597271000002.tif223100TIFF0007597271000003.tif22394TIFF0007597271000004.tif22396

[0071] [Table 2] TIFF0007597271000006.tif223131TIFF0007597271000007.tif223122TIFF0007597271000008.tif223121

[0072] As is clear from Table 2, the steels Nos. A to L, AO to AQ, AS, AU, AW, BA, BD and BF, which are examples of the invention, all have an area ratio of bainite structure in the microstructure of 90% or more, a tensile strength of 820 MPa or more and an impact value at -50°C of 25 J / cm 2 Above, fatigue strength: 550N / mm 2 As described above, the surface hardness after the high frequency treatment was HRC: 47 or more, the average hardened layer depth after the high frequency treatment was 2.00 mm or more, and the standard deviation σ of the effective hardened layer depth was 0.12 mm or less.

[0073] In the present invention, a tensile strength of 820 MPa or more was evaluated as excellent in strength.

[0074] Impact value at -50℃: 25J / cm 2 In the present invention, the above cases are Toughness It was rated as excellent.

[0075] Fatigue strength: 550N / mm 2 In the present invention, the fatigue strength was evaluated as being excellent when the above condition was satisfied.

[0076] In addition, in the present invention, a surface hardness of HRC: 47 or more after the high-frequency treatment was evaluated as being excellent in fatigue strength and wear resistance, and excellent in high-frequency hardenability.

[0077] In the present invention, a steel having an average hardened layer depth of 2.00 mm or more and an effective hardened layer depth standard deviation σ of 0.12 mm or less after induction treatment was evaluated as having excellent fatigue strength and induction hardenability.

[0078] 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 inventive 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.

[0079] [Example 2] Next, the ingots having the steel compositions shown in Table 1, Steel Nos. A, B, and C, 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.

[0080] 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. Table 3 shows these results.

[0081] [Table 3]

[0082] As is clear from Table 3, when the heating temperature before hot forging and the average cooling rate after hot forging are within the range of the present invention, the area ratio of the bainite structure in the microstructure is 90% or more, the tensile strength is 820 MPa or more, and the impact value at -50°C is 25 J / cm. 2 , Fatigue strength: 550N / mm 2 As described above, the surface hardness after the high frequency treatment was HRC: 47 or more, the average hardened layer depth after the high frequency treatment was 2.00 mm or more, and the standard deviation σ of the effective hardened layer depth was 0.12 mm or less.

[0083] In contrast, when the heating temperature or the average cooling rate after hot forging is outside the range of the present invention, any one of the tensile strength, the impact value at -50°C, the fatigue strength, the surface hardness after high-frequency treatment, the average case depth and the standard deviation σ of the effective case depth does not satisfy the desired value and is inferior.

Claims

1. 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; 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 at -50°C in a Charpy impact test of 25 J / cm 2 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.

2. 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, Bi: 0.05% or more and 0.30% or less; Sb: 0.0015% or more and 0.0100% or less The hot forged non-heat treated steel material according to claim 1, containing one or more selected from the following:

3. 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.

4. 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; 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 / cm 2 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.

5. 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, Bi: 0.05% or more and 0.30% or less; 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, comprising one or more selected from the following:

6. 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.

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