Steel material for cutting and carburizing and method for manufacturing the steel material for cutting and carburizing

Abstract

To provide a steel for cutting and carburizing that can be cut and subsequently carburized without normalizing after hot forging, and a method for producing the same.SOLUTION: A steel for cutting and carburizing comprises, in mass%, C: 0.30% or less, Si: 1.00% or less, Mn: 2.00% or less, S: more than 0.005% to 0.100% or less, Al: 0.100% or less, Nb: 0.100% or less, Ti: more than 0.010% to 0.100% or less, and N: 0.0250% or less, with the balance being Fe and inevitable impurities. The steel structure consists of ferrite, perlite and bainite, where the bainite has an area fraction of more than 10.0% to 50.0% or less; and the number density of precipitates, which include at least one of Ti or Nb with a spherical equivalent radius of 1 nm or more and 20 nm or less, is at least 10.0 / μm2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel material for cutting and carburizing and a method for manufacturing the steel material for cutting and carburizing. [Background technology]

[0002] Mechanical structural parts used in automobiles and other vehicles are manufactured from case-hardened steel, which is forged and machined to form the part shape, and then carburized, quenched, and tempered (hereinafter simply referred to as carburizing) to improve fatigue strength.

[0003] The forging process used to shape steel into parts can be broadly divided into hot forging and cold forging depending on the temperature at which the forging process is carried out, but hot forging is the mainstream method for manufacturing parts due to the material's resistance to deformation.

[0004] After hot forging, parts are often cooled by natural cooling. However, if parts made of case-hardened steel, which has a relatively high hardenability, are allowed to cool naturally, they will develop a hard metal structure consisting mainly of bainite, which will cause problems during subsequent cutting.

[0005] For this reason, in the usual process, parts after hot forging are heated again to the austenite single phase region, held at that temperature for an appropriate time, and then slowly cooled, a heat treatment known as normalizing. Normalizing transforms the material structure into a soft ferrite-pearlite structure, improving machinability.

[0006] Furthermore, it is known that the material structure after normalizing (relatively coarse ferrite-pearlite structure) is less susceptible to grain coarsening during carburizing compared to the structure naturally cooled after hot forging (a mixed structure of ferrite, pearlite, and bainite) (see, for example, Non-Patent Document 1).

[0007] Thus, normalizing treatment after hot forging is effective both in ensuring the machinability of the steel material and in preventing the grains from becoming coarse during the subsequent carburizing treatment.

[0008] On the other hand, from the viewpoint of reducing the cost of heat treatment, shortening the lead time involved in heat treatment, and further reducing the energy used in heat treatment, it is desirable to omit the normalizing treatment.

[0009] In light of this background, there is a need for the development of a steel material that can be easily cut even when the normalizing treatment after hot forging is omitted, and that has excellent properties in preventing the coarsening of crystal grains during the subsequent carburizing treatment.

[0010] For example, Patent Document 1 discloses a case-hardened steel that has excellent machinability in the as-hot-forged state and excellent high-temperature carburization properties, by adding a large amount of Ti to finely disperse Ti sulfides, Ti carbosulfides, Ti carbides, or Ti carbonitrides in the steel.

[0011] Furthermore, Patent Document 2 discloses a hot forging base material in which the precipitation state of AlN and Nb(C,N) is controlled by controlling the cooling rate after hot forging, which eliminates the need for normalizing treatment after hot forging and has excellent properties that prevent coarsening of crystal grains during carburizing treatment, and a method for manufacturing the same. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-101566 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-303174 [Non-patent literature]

[0013] [Non-Patent Document 1] Tamaya et al.: Heat Treatment, Vol. 37, No. 6 (1997) 356-361. Summary of the Invention [Problem to be solved by the invention]

[0014] However, the steel described in Patent Document 1 contains a large amount of Ti, which poses problems in terms of manufacturability and economy. In addition, in the steel described in Patent Document 2, the precipitation states of AlN and Nb(C,N) are controlled, but in order to suppress grain coarsening during carburizing, it is necessary not only to control these but also to suppress the bainite area fraction to 10% or less.

[0015] The present invention has been developed in view of the above circumstances, and aims to provide a steel material for cutting and carburizing, which can be well cut in the as-hot-forged state without normalizing after hot forging, and which has excellent properties for preventing coarsening of crystal grains during the subsequent carburizing treatment, together with a manufacturing method thereof. [Means for solving the problem]

[0016] In order to solve the above problems, the present inventors focused on Ti and Nb, which are precipitate-forming elements added to case-hardened steel, and studied the effects these have on hardness after hot forging. As a result, they found that if the contents of these elements are controlled and the precipitates containing at least one of Ti and Nb are refined in the steel structure, then, as long as the area fraction of bainite is greater than 10% but not more than 50%, it is possible to obtain a steel material for cutting carburization that is excellent in the property of preventing coarsening of crystal grains during the subsequent carburization treatment, and that cutting carburization can be performed well in the as-hot-forged state without normalizing after hot forging. The present inventors have also found that in order to obtain such a steel material for cutting and carburizing, it is important to control the cooling rate after hot forging within an appropriate range.

[0017] Based on these findings, the present inventors have completed the present invention. The gist of the steel material for cutting and carburizing of the present invention is as follows. [1] In mass%, C: 0.30% or less, Si: 1.00% or less, Mn: 2.00% or less, S: more than 0.005% and less than 0.100%, Al: 0.100% or less, Nb: 0.100% or less, Ti: more than 0.010% and not more than 0.100% N: 0.0250% or less Contains The balance has a composition consisting of Fe and unavoidable impurities, The alloy consists of ferrite, pearlite, and bainite, the area fraction of bainite is more than 10.0% and not more than 50.0%, and the number density of precipitates containing at least one of Ti and Nb with a circle equivalent radius of 1 nm or more and 20 nm or less is 10.0 precipitates / μm 2 A steel material for cutting and carburizing having the above steel structure. [2] The component composition further includes: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 2.00% or less and Mo: 0.50% or less Contains one or more selected from the group consisting of: [1] Steel for cutting and carburizing.

[0018] The gist of the method for producing a steel material for cutting and carburizing according to the present invention is as follows. [3] In mass%, C: 0.30% or less, Si: 1.00% or less, Mn: 2.00% or less, S: more than 0.005% and less than 0.100%, Al: 0.100% or less, Nb: 0.100% or less, Ti: more than 0.010% and not more than 0.100% N: Contains 0.0250% or less, A method for producing a steel material for cutting carburizing, which includes a step of hot forging a steel material having a chemical composition with the balance consisting of Fe and unavoidable impurities, wherein the average cooling rate between 800°C and 500°C during cooling after hot forging is set to be equal to or higher than VC1 obtained by the following formula (1) and equal to or lower than VC2 obtained by the following formula (2), in units of °C / s. Note VC1=0.3-0.5×[Nb]+2×[Ti]...Equation (1) VC2=1-5×[Nb]+10×[Ti] ···(2) formula Here, [Nb] and [Ti] represent the Nb concentration and Ti concentration in the steel material, respectively, in units of mass %, and [Nb] may be 0. [4] The component composition further includes: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 2.00% or less and Mo: 0.50% or less Contains one or more selected from the group consisting of: [3] A manufacturing method for steel for cutting and carburizing. [Effects of the Invention]

[0019] According to the present invention, a steel material for cutting and carburizing, which can be well cut without normalizing treatment after hot forging and has excellent properties of preventing coarsening of crystal grains during the subsequent carburizing treatment, is provided together with a manufacturing method thereof. According to the present invention, it is possible to omit the normalizing treatment that has conventionally been carried out after hot forging. DETAILED DESCRIPTION OF THE INVENTION

[0020] The reasons for limiting the range of the component composition in the present invention will be explained below for each element. Note that "%" representing the component composition below means "% by mass" unless otherwise specified.

[0021] C: 0.30% or less C is an element necessary for ensuring the strength of steel, but excessive addition increases the hardness of the steel, worsening its machinability and reducing the toughness of the core of the component after carburizing, so the C content must be kept to 0.30% or less. For this reason, the upper limit of the C content is set to 0.30%. The C content is preferably 0.25% or less. There is no particular restriction on the lower limit of the C content, and it may be 0%, but from the viewpoint of steel strength, it is preferably 0.10% or more.

[0022] Si: 1.00% or less Si is an element used as a deoxidizer during steel refining, and also has the effect of improving the strength and hardenability of steel. However, addition of more than 1.00% increases the hardness of the ferrite phase through solid solution strengthening, deteriorating the workability and machinability of the steel. For this reason, the upper limit of the Si content is set to 1.00%. The Si content is preferably 0.80% or less, and more preferably 0.50% or less. There is no particular restriction on the lower limit of the Si content, and it may be 0%, but from the viewpoint of steel strength, it is preferably 0.01% or more.

[0023] Mn: 2.00% or less Mn is an element that improves the hardenability and strength of steel. Furthermore, by combining with S to form MnS, it also improves the machinability of steel. However, adding more than 2.00% of Mn makes the steel too hard, adversely affecting the machinability of the steel. For this reason, the upper limit of the Mn content is set at 2.00%. The Mn content is preferably 1.00% or less, and more preferably 0.90% or less. Furthermore, there is no particular restriction on the lower limit of the Mn content, and it may be 0%, but from the viewpoint of the strength of the steel, it is preferably 0.30% or more.

[0024] S: More than 0.005% and less than 0.100% S is an element that combines with Mn in steel to form MnS, improving the machinability of steel. Generally, hard bainite structures have poor machinability, but adding more than 0.005% S improves machinability. For this reason, the lower limit of the S content is set at 0.005%. While adding a large amount of S is effective from the standpoint of machinability, adding more than 0.100% S causes grain boundary segregation of S, embrittling the steel. For this reason, the upper limit of the S content is set at 0.100%. The S content is preferably 0.050% or less.

[0025] Al: 0.100% or less Al is an element used as a deoxidizing element for steel. It also combines with N in steel to form AlN precipitates, contributing to the suppression of grain coarsening during carburization. However, adding more than 0.100% Al increases the amount of Al oxides, reducing the fatigue strength of the steel. Furthermore, coarse AlN precipitates are more likely to form, making it impossible to obtain fine AlN precipitates, which are effective in suppressing grain coarsening during carburization. For this reason, the upper limit of the Al content is set to 0.100%. The Al content is preferably 0.050% or less. The lower limit of the Al content is not particularly limited and may be 0%, but is preferably 0.003% or more from the viewpoint of deoxidation of the steel.

[0026] Nb: 0.100% or less Nb is an element that combines with C in steel to form Nb-based precipitates, contributing to the suppression of grain coarsening. However, adding more than 0.100% Nb saturates the effect of suppressing grain coarsening, and the hardenability of the steel material increases excessively, increasing the bainite area fraction after hot forging, resulting in increased hardness and reduced machinability. For this reason, the upper limit of the Nb content is set to 0.100%. The Nb content is preferably 0.050% or less. The lower limit of the Nb content is not particularly limited and may be 0%, but from the viewpoint of suppressing grain coarsening, it can be set to 0.001% or more, preferably 0.01% or more. The Nb-based precipitates include Nb carbides, Nb nitrides, and Nb carbonitrides, and may also be composite precipitates containing Ti.

[0027] Ti: more than 0.010% and less than 0.100% Ti combines with C and N in steel to form Ti-based precipitates, which contribute to the suppression of grain coarsening. Furthermore, Ti-based precipitates have good lattice matching with ferrite and promote ferrite transformation during cooling of the steel. Therefore, a ferrite structure can be obtained even when the cooling rate after hot forging is high. If the Ti content is 0.010% or less, the fraction of Ti-based precipitates in the steel is low, and the aforementioned effects of promoting ferrite transformation and suppressing grain coarsening cannot be achieved. On the other hand, adding Ti in excess of 0.100% reduces the fatigue strength of the steel by forming coarse Ti-based precipitates, and also reduces the number density of fine Ti-based precipitates, which are effective in suppressing grain coarsening during carburizing, thereby being detrimental to the suppression of grain coarsening during carburizing. For these reasons, the Ti content is limited to more than 0.010% but not more than 0.100%. The Ti content is preferably 0.050% or less. The Ti-based precipitates include Ti carbides, Ti nitrides, and Ti carbonitrides, and may be composite precipitates further containing Nb.

[0028] N: 0.0250% or less N is an element that combines with Al and Ti in steel to form AlN precipitates and TiN precipitates, thereby contributing to the suppression of grain coarsening. If the N content exceeds 0.0250%, blowholes are formed, the properties of the steel deteriorate, and coarse AlN and TiN precipitates are more likely to form, reducing the amount of fine precipitates that are effective in preventing grain coarsening. For this reason, the upper limit of the N content is set to 0.0250%. The N content is preferably 0.0200% or less. There is no particular restriction on the lower limit of the N content, and it may be 0%, but an excessively low N content increases manufacturing costs, so it is preferably 0.0030% or more.

[0029] In addition to the above elements, the composition may contain one or more elements selected from the group consisting of Cu: 1.00% or less, Ni: 1.00% or less, Cr: 2.00% or less, and Mo: 0.50% or less.

[0030] Cu: 1.00% or less Cu is a useful element that improves the hardenability of steel and can be added. However, if added in an amount exceeding 1.00%, cracks are more likely to occur during hot working, reducing the manufacturability of the steel. Therefore, when Cu is added, the Cu content is set to 1.00% or less, preferably 0.50% or less. Furthermore, when Cu is added, the Cu content is preferably 0.01% or more in order to fully obtain the above effects.

[0031] Ni: 1.00% or less Ni is a useful element that improves the hardenability and toughness of steel materials and can be added. However, Ni is an expensive element, and excessive addition increases alloy costs. Therefore, when Ni is added, the Ni content is set to 1.00% or less, preferably 0.50% or less. Furthermore, when Ni is added, the Ni content is preferably 0.01% or more in order to fully obtain the above effects.

[0032] Cr:2.00% or less Cr is an element that improves the hardenability of steel and can be added. However, adding more than 2.00% of Cr increases the hardness of the steel and reduces the workability and machinability of the steel. Therefore, when Cr is added, the Cr content is set to 2.00% or less, and preferably 1.50% or less. Furthermore, when Cr is added, the Cr content is preferably 0.01% or more in order to fully obtain the above effects.

[0033] Mo: 0.50% or less Mo is an element that can significantly improve the hardenability of steel materials with a small amount of addition, and can be added. However, Mo is an expensive element, and excessive addition increases alloy costs. Therefore, when Mo is added, the Mo content is set to 0.50% or less, preferably 0.35% or less, and more preferably 0.25% or less. Furthermore, when Mo is added, in order to fully obtain the above effects, the Mo content is preferably 0.01% or more.

[0034] The balance of the composition is Fe and unavoidable impurities.

[0035] The method for producing a steel material for cutting and carburizing of the present invention includes a step of hot forging a steel material having the above-mentioned composition, and in the cooling after hot forging, the average cooling rate between 800°C and 500°C is set by the following formula (1): The temperature is characterized by being equal to or higher than VC1 (unit: °C / s) obtained by the above formula and equal to or lower than VC2 (unit: °C / s) obtained by the following formula (2). VC1=0.3-0.5×[Nb]+2×[Ti]...Equation (1) VC2=1-5×[Nb]+10×[Ti] ···(2) formula Here, [Nb] and [Ti] represent the Nb concentration and Ti concentration in the steel material, respectively, in units of mass %, and the Nb concentration may be 0.

[0036] As described above, by controlling the cooling rate after hot forging in accordance with the steel composition, it is possible to omit the normalizing treatment. Generally, when steel is cooled by natural cooling after hot forging, the steel exhibits a metal structure mainly consisting of bainite, which makes it very hard and results in poor machinability. Furthermore, when the steel structure contains bainite, grain coarsening is more likely to occur during subsequent carburizing than when the structure is ferrite-pearlite (equivalent to the steel structure after normalizing). According to the findings of the present inventors, by controlling the cooling rate after hot forging within a certain range, it is possible to ensure the machinability of the steel material while preventing the coarsening of crystal grains during carburizing. The appropriate cooling rate range varies depending on the type and amount of precipitate-forming elements added, so it is important to cool the steel so as to satisfy formulas (1) and (2). The Ti-based precipitates formed by the addition of Ti have good lattice matching with ferrite, and the presence of these precipitates in steel promotes ferrite transformation. For this reason, Ti-added steel can obtain a soft structure even if the cooling rate after hot forging is increased depending on the amount of Ti added. Furthermore, Nb mainly forms Nb carbides and contributes to suppressing grain coarsening, while trace amounts of dissolved Nb improve the hardenability of steel. Therefore, in order to obtain a soft structure in Nb-added steel, it is effective to reduce the cooling rate after hot forging depending on the amount added. Therefore, the inventors have investigated steel materials with various amounts of Ti and Nb, and as a result, have confirmed that machinability during subsequent cutting can be ensured by setting the cooling rate (unit: °C / s) after hot forging to VC2 or less, which is obtained by equation (2). Furthermore, the lower the cooling rate after hot forging, the lower the hardness of the steel, so a slow cooling rate is desirable from the perspective of machinability. However, if the cooling rate is reduced too much, the steel will be held at high temperatures for a longer period of time, which will lead to the aggregation and coarsening of precipitates containing at least one of Ti and Nb, which contribute to grain refinement, and this may degrade the ability to prevent grain coarsening during subsequent carburizing. The inventors have confirmed that in order to ensure machinability after cooling while also preventing grain coarsening during carburizing, it is effective to set the cooling rate (unit: °C / s) to VC1 or higher, as calculated by equation (1).

[0037] In the method for producing a steel material for cutting and carburizing according to the present invention, it is preferable to heat a steel material having the above-mentioned composition prior to hot forging. The shape of the steel material is not particularly limited, and for example, a steel bar, a bar in coil, etc. can be used. The heating temperature of the steel material can be 950°C or higher and 1250°C or lower. Within this range, it is possible to easily achieve both the machinability of the steel material and the suppression of grain coarsening during carburizing, as described above. The heating temperature is preferably 1000°C or higher and 1200°C or lower.

[0038] The heated steel material can be hot forged. The hot forging method is not particularly limited, but the average cooling rate between 800°C and 500°C after hot forging is set to be equal to or higher than the above-mentioned VC1 (unit: °C / s) and equal to or lower than the above-mentioned VC2 (unit: °C / s). The method for controlling the cooling rate is not particularly limited, but examples include a method in which a furnace maintained at an isothermal temperature is installed downstream of the hot forging line, and a method in which hot forged parts are collected in a highly insulating container and slowly cooled. The cooling rate in the temperature range below 500°C is not particularly limited, and for example, air cooling can be used.

[0039] The present invention relates to a steel material for cutting and carburizing, which has the above-mentioned chemical composition and a steel structure consisting of ferrite, pearlite, and bainite, in which the area fraction of bainite is more than 10.0% and not more than 50.0%, and in which the number density of precipitates containing at least one of Ti and Nb and having a circle-equivalent radius of 1 nm or more and 20 nm or less is 10.0 precipitates / μm2 or more.

[0040] When carburizing steel is allowed to cool after hot forging, it generally exhibits a steel structure mainly composed of bainite. However, a structure mainly composed of bainite is hard and has poor machinability, and therefore, in the steel material for cutting carburizing of the present invention, the area fraction of bainite in the steel structure is controlled within a certain range. If the bainite area fraction exceeds 50.0%, the hardness increases and machinability deteriorates. If the bainite area fraction is 10.0% or less, the hardness of the steel is low enough that machinability is not an issue. However, to achieve this structure without normalizing, the cooling rate after hot forging must be significantly reduced. However, if the cooling rate is reduced too much, the steel is held at high temperatures for a long time, which leads to the aggregation and coarsening of precipitates containing at least one of Ti and Nb, which contribute to grain refinement. This deteriorates the ability to prevent grain coarsening during subsequent carburizing. For this reason, the bainite area fraction in the steel structure must be greater than 10% and less than 50%, and the number density of precipitates containing at least one of Ti and Nb with a circle-equivalent radius of 1 nm to 20 nm must be 10.0 particles / μm2 or more. Here, the precipitates containing at least one of Ti and Nb are Ti carbide, Ti nitride, Ti carbonitride, Nb carbide, Nb nitride, Nb carbonitride, Nb and Ti composite carbide, Nb and Ti composite nitride, and Nb and Ti composite carbonitride, and at least one type may be present, but two or more types may also be present. For example, when the component composition is substantially free of Nb, the precipitates containing at least one of Ti and Nb can consist of Ti carbide, Ti nitride, and Ti carbonitride. The area fraction of bainite is preferably 20.0% or more and 50.0% or less. The number density of precipitates containing at least one of Ti and Nb and having a circle-equivalent radius of 1 nm to 20 nm is preferably 20.0 particles / μm 2From the viewpoint of steel hardness, the number of particles is 50.0 / μm. 2 The following is preferred: The steel structure and precipitates containing at least one of Ti and Nb can be measured by the method described in the examples below.

[0041] The steel material for cutting and carburizing of the present invention can be cut to a desired shape, and then carburized or carbonitrided to obtain a steel part.

[0042] The cutting method is not particularly limited, and examples thereof include drilling and turning.

[0043] The method of carburizing or carbonitriding is not particularly limited. Carburizing treatment may involve, for example, quenching from a temperature of 900°C or higher into oil at 60 to 180°C in a carburizing atmosphere with a carbon potential of 0.8 to 1.3%, followed by tempering at 120°C or higher. Examples of carbonitriding treatment include quenching from a temperature of 900°C or higher in oil at 60 to 180°C in an atmosphere with a carbon potential of 0.8 to 1.3% and a nitrogen potential of 0.2 to 0.6%, followed by tempering at 120°C or higher.

[0044] The steel material for cutting and carburizing of the present invention can be suitably used in the manufacture of machine structural parts used in the fields of construction and industrial machinery and automobiles. In the field of construction and industrial machinery, examples of parts for which the steel material for cutting and carburizing of the present invention can be used include gears for travel reducers (such as planetary gears and sun gears), gears for large reducers, valve plates for hydraulic pumps, nuts for ball screws, curved plates for cyclone reducers, pins, and blocks for linear bearings. In the field of automobiles, examples of parts for which the steel material for cutting and carburizing of the present invention can be used include various bearings, piston pins for engines, camshafts and timing gears, gears for transmissions (such as missing gears, ring gears, sun gears, and planetary gears), differential bevel gears, tripods, inner gears, and balls for drive systems. In addition to the fields of construction and industrial machinery and automobiles, the steel material can also be suitably used in bearings and reduction gears for wind turbine generators in the field of electrical equipment. [Example]

[0045] The following examples are provided to more specifically explain the configuration and effects of the present invention, but the present invention is not limited to these examples and may be modified as appropriate within the scope of the present invention, and all such modifications are within the scope of the present invention.

[0046] Steel having the chemical composition shown in Table 1 below (the balance being Fe and unavoidable impurities) was melted, heated to 1200°C, and then hot forged to produce a steel bar material with a diameter of 40 mm. This steel bar material was subjected to the hot forging simulation test described below.

[0047] The hot forging simulation test was conducted as follows. Specifically, the 40 mm diameter steel bar material prepared in the previous section was heated to 1200°C, held at that temperature for 30 minutes, and then reduced to a diameter of 32 mm by hot swaging. The hot swaged steel bar was then cooled to room temperature at various cooling rates to obtain the hot forging simulation test material. Three cooling rates (unit: °C / s) were used for each steel type. These were: a condition in which the average cooling rate between 800°C and 500°C was controlled within the range of VC1 or higher and VC2 or lower, as calculated by equations (1) and (2); and a condition in which the average cooling rate between 800°C and 500°C was outside this range (below the upper or lower limit). Table 2 shows the conditions used for each example.

[0048] The steel structure of the hot forged simulated test material was observed. The specimen was cut at the D / 4 position (D: diameter of the hot forged simulated test material) so that the longitudinal direction was the observation surface. After mirror polishing, the specimen was subjected to microstructural observation. An optical microscope was used for microstructural observation, and five randomly selected fields were observed at 100x magnification. Each field measured 600 μm × 800 μm. The microstructural photographs were analyzed to identify the steel structure and calculate the bainite area fraction. The average of the bainite area fractions determined from the five fields was used as the bainite area fraction for that steel type. Steels with a bainite area fraction greater than 10% and less than 50% were deemed acceptable. The bainite area fraction was calculated using the image analysis software ImageJ. Table 2 shows the results for each example.

[0049] The Vickers hardness of the hot forged simulated test material was also measured. The load during hardness measurement was 1 kgf, and the measurement positions were one point at the center of the test piece and four points 1 mm away from the center, above, below, left and right, for a total of five points, and the average value was taken. Table 2 shows the results for each example.

[0050] The machinability of the hot forged simulated test material was evaluated in a peripheral cutting test. 1 mm of the outer periphery was cut off to remove the scale and decarburized layer on the surface of the hot forged simulated test material, and then the material was subjected to the peripheral cutting test. The cutting tool used in the peripheral turning test was a P20 type, and the cutting conditions were a depth of cut of 1 mm, cutting speed of 200 mm / min, feed rate of 0.20 mm / rev, and no lubrication. After 300 seconds of cutting, the tool flank was observed with a stereomicroscope and the amount of tool wear was measured. A wear amount of 150 μm or less was judged to be acceptable.

[0051] The number density of precipitates containing at least one of Ti and Nb with a circle equivalent radius of 1 nm to 20 nm present in the hot forging simulated test material was calculated using the following procedure. Extraction replica samples were taken from the hot forging simulated test material and observed in 10 fields of view at a magnification of 200,000 times using a transmission electron microscope (TEM). The observation area per field of view was 5 μm × 5 μm. The composition of the observed precipitates was calculated as T The precipitates were confirmed using an energy dispersive X-ray spectrometer (TEM-EDX) attached to the EM, and the precipitate species were identified based on whether Ti, Nb, C, or N was detected. The circle-equivalent radius of precipitates in which at least one of Ti and Nb was detected was calculated by image analysis. The number of precipitates in which at least one of Ti and Nb was detected and had a circle-equivalent radius of 1 nm to 20 nm was counted and divided by the total observation area to calculate the number density of precipitates containing at least one of Ti and Nb. Table 2 shows the results for each example.

[0052] Furthermore, the hot-forged simulated test material was subjected to a pseudo-carburizing process (a heat treatment that mirrored only the temperature history of the actual carburizing process) to check for the occurrence of grain coarsening. The pseudo-carburizing process consisted of holding the test specimen at 950°C for three hours, followed by oil quenching. After the pseudo-carburizing process, the test specimens were polished and corroded, and the entire 32 mm diameter cross section was observed (segmented photographs) using an optical microscope at a magnification (100x) sufficient to observe grains of approximately size number 5. The presence of coarse grains was confirmed. Samples with no visible coarse grains were judged to pass, while samples with even one visible coarse grain were judged to fail. Table 2 shows the results for each example.

[0053] [Table 1]

[0054] [Table 2]

[0055] In the examples in which the steel materials Nos. 10 to 20 having chemical compositions within the range of the present invention were used and the average cooling rate between 800°C and 500°C after the hot forging simulation test was controlled to be within the range of the present invention, the bainite area fraction was in the range of more than 10.0% and not more than 50.0%, and the number density of precipitates containing at least one of Ti and Nb with a circle equivalent radius of 1 nm to 20 nm was also 10.0 particles / μm 2 The tool wear after the peripheral turning test was 150 μm or less. Furthermore, after the subsequent pseudo-carburizing treatment, no coarse crystal grains below grain size number 5 were observed, resulting in a passing result. On the other hand, even when steel materials No. 10 to No. 20 were used, in examples in which the average cooling rate after the hot forging simulation test was outside the range of the present invention, at least one of the bainite area fraction and the number density of the specified precipitates was outside the range of the present invention, and the tool wear amount after the peripheral turning test exceeded 150 μm and / or coarse crystal grains were observed after the pseudo-carburizing treatment, resulting in failure. Furthermore, in the examples using steel materials Nos. 1 to 9 having chemical compositions outside the range of the present invention, regardless of whether the average cooling rate after the hot forging simulation test was within the range of the present invention or not, at least one of the bainite area fraction and the number density of the specified precipitates was outside the range of the present invention, and the tool wear amount exceeded 150 μm and / or coarse crystal grains were observed in the pseudo-carburizing treatment, resulting in unacceptable results.

[0056] Among these, steels No. 4, No. 6, and No. 8 exceeded the ranges of the present invention for Al, Ti, and N, respectively. When these steels were used, the bainite area fraction exceeded 50.0% when the average cooling rate after the hot forging simulation test was faster than the range of the present invention. However, when the average cooling rate was within the range of the present invention, the bainite area fraction was within the range of the present invention, and the tool wear was 150 μm or less. However, coarse grains were observed after quasi-carburizing, resulting in a rejection result. When these steels were used, AlN precipitates or TiN precipitates are thought to be present in the steel. However, because the Al, Ti, or N contents exceeded the ranges of the present invention, the number density of fine precipitates, which are effective in preventing grain coarsening, was below the range of the present invention, and therefore grain coarsening during quasi-carburizing could not be suppressed.

[0057] Furthermore, Steel No. 5 is a steel material containing Nb exceeding the range of the present invention. Even though the average cooling rate after the hot forging simulation test was within the range of the present invention, the bainite area fraction exceeded the range of the present invention, and the tool wear amount after the peripheral turning test exceeded 150 μm. It is believed that the addition of a large amount of Nb resulted in excessive hardenability, causing the bainite area fraction to exceed the range of the present invention. It is also believed that when the cooling rate is below the range of the present invention, the number density of fine precipitates decreases, making it impossible to suppress grain coarsening.

[0058] Steel No. 7 is a steel material with a Ti content below the range of the present invention, and if the average cooling rate after the hot forging simulation test was within the range of the present invention, the bainite area fraction would be within the range of the present invention, and the tool wear after the peripheral turning test would also be 150 μm or less, but coarse crystal grains with a grain size number of 5 or less were observed after the quasi-carburizing treatment. It is thought that because the amount of Ti added was below the range of the present invention and the number density of fine precipitates was below the range of the present invention, coarsening of the crystal grains could not be prevented.

[0059] Steel No. 9 is a steel material with an S content below the range of the present invention. Because of the low S content, this steel had poor machinability, and even though the bainite area fraction was within the range of the present invention, the tool wear amount after a peripheral turning test was 150 μm or more.

Claims

1. In mass%, C: 0.30% or less, Si: 1.00% or less, Mn: 2.00% or less, S: more than 0.005% and less than 0.100%, Al: 0.100% or less, Nb: 0.100% or less, Ti: More than 0.010% and not more than 0.100% N: 0.0250% or less Contains The balance has a composition consisting of Fe and unavoidable impurities, The alloy consists of ferrite, pearlite, and bainite, the area fraction of bainite is more than 10.0% and not more than 50.0%, and the number density of precipitates containing at least one of Ti and Nb with a circle equivalent radius of 1 nm or more and 20 nm or less is 10.0 precipitates / μm 2 A steel material for cutting and carburizing having the above steel structure.

2. The component composition is further Cu: 1.00% or less, Ni: 1.00% or less, Cr: 2.00% or less and Mo: 0.50% or less Contains one or more selected from the group consisting of: The steel material for cutting and carburizing according to claim 1.

3. In mass%, C: 0.30% or less, Si: 1.00% or less, Mn: 2.00% or less, S: more than 0.005% and less than 0.100%, Al: 0.100% or less, Nb: 0.100% or less, Ti: More than 0.010% and not more than 0.100% N: Contains 0.0250% or less, 2. A method for producing a steel material for cutting and carburizing according to claim 1, comprising a step of hot forging a steel material having a composition with the balance being Fe and unavoidable impurities, wherein the average cooling rate between 800°C and 500°C in cooling after hot forging is expressed as VC obtained by the following formula (1), where VC is the average cooling rate in °C / s between 800°C and 500°C. 1 The above is true, and VC is obtained by the following equation (2) 2 A method for manufacturing a steel material for cutting and carburizing, comprising: Note VC 1 =0.3-0.5×[Nb]+2×[Ti] ・・・(1) VC 2 =1-5×[Nb]+10×[Ti] ・・・(2) Here, [Nb] and [Ti] represent the Nb concentration and Ti concentration in the steel material, respectively, in units of mass %, and [Nb] may be 0.

4. The component composition is further Cu: 1.00% or less, Ni: 1.00% or less, Cr: 2.00% or less and Mo: 0.50% or less 4. The method for producing a steel material for cutting and carburizing according to claim 3, which is the method for producing a steel material for cutting and carburizing according to claim 2, which comprises one or more selected from the group consisting of:

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