Carburizing round steel bars

Abstract

This round steel bar material to be carburized: has a chemical component composition comprising, by mass%, 0.17 to 0.28% of C, 0.15 to 0.25% of Si, 0.70 to 1.00% of Mn, 0.030% or less of P, 0.015 to 0.035% of S, 1.35 to 1.75% of Cr, 0.020 to 0.040% of Al, 0.0100 to 0.0200% of N, 0.01 to 0.10% of Nb as an optional element, 0.01 to 0.10% of Ti as an optional element, 0.0010 to 0.0040% of B as an optional element, and the balance Fe and inevitable impurities; and satisfies expression 1: (5 Si + 1.4 Mn + Cr) / C ≤ 18.6, expression 2: 42 Si + 11 Mn + 18 (Cu + Ni) + 24 Mo + 600 N - 200 Al ≥ 18.3, and expression 3: Mn / S ≥ 28. In a round cross-section along the diameter D of the round steel bar material to be carburized, the concentration of P at a D / 4 deep position from the surface is 0.15% or less.

Description

Technical Field

[0001] The present invention relates to carburizing round bar steel materials.

Background Art

[0002] As case-hardened steel having high strength, alloy steels such as SCM420 are known. By adding elements such as Mo, these have high strength. However, Mo is a rare metal, and there are problems with price fluctuations and the drawback of high addition costs. Therefore, hitherto, in order to reduce the addition cost, development of steel materials having the same strength as SCM420 without actively adding Mo has been promoted. For example, as described in Patent Documents 1 and 2, in order to maintain the same strength as SCM420, steel materials with Si reduced to 0.15% or less have been developed aiming at reducing the abnormal carburized layer.

[0003] However, when the Si content rate is low, although it is effective for reducing the abnormal carburized layer, the ferrite hardness of the ferrite + pearlite structure, which is the structure during cutting, decreases, the chips tend to become long, the contact frequency between the chips and the tool increases, and it has been found that a phenomenon (boundary wear) occurs that deteriorates tool wear. Also, even when the Si content rate is high, when the S content rate is at the conventional lower limit level, it has been found that the formation of MnS decreases and tool wear deteriorates in the same manner as above. Thus, new problems with respect to machinability have been found when Si is reduced or S is low.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] This invention was made in view of the above background, and aims to provide a carburizing round steel bar material that can secure a strength equivalent to that of SCM420 without actively adding Mo, and that has excellent machinability. [Means for solving the problem]

[0006] One aspect of the present invention has a chemical composition in mass%, containing C: 0.17-0.28%, Si: 0.15-0.25%, Mn: 0.70-1.00%, P: 0.030% or less, S: 0.015-0.035%, Cr: 1.35-1.75%, Al: 0.020-0.040%, N: 0.0100-0.0200%, with optional elements Nb: 0.01-0.10%, optional element Ti: 0.01-0.10%, optional element B: 0.0010-0.0040%, with the remainder being Fe and unavoidable impurities. The Cu, Ni, and Mo content in the above unavoidable impurities is as follows: Cu: 0.150% or less, Ni: 0.150% or less, and Mo: 0.06% or less. The following equations 1 to 3 must be satisfied, Formula 1: (5Si+1.4Mn+Cr) / C≦18.6, Formula 2: 42Si+11Mn+18(Cu+Ni)+24Mo+600N-200Al≧18.3, Formula 3: Mn / S≧28, (However, the element symbols in equations 1 to 3 above represent the mass percentage of each element.) Furthermore, the carburizing round steel bar has a maximum P concentration of 0.15% or less, as determined by surface analysis at a depth of D / 4 from the surface of a circular cross-section with diameter D. [Effects of the Invention]

[0007] To solve the above problems, compared to the low-mo steels developed in the aforementioned Patent Documents 1 and 2, it is effective to optimize the chip length, improve ferrite hardness by increasing ferrite solid solution strengthening elements such as Si, and improve chip fragmentation by increasing MnS due to increased S addition. However, simply increasing Si makes it easier for a low-hardness abnormal carburization layer to form during carburization, affecting the strength after carburization. Therefore, it is necessary to adjust while maintaining a balance with other elements.

[0008] For example, increasing the amount of Mn and Cr added tends to increase the amount of carburized abnormal layer, so the amounts of Mn and Cr added need to be limited to an appropriate range. On the other hand, increasing the C concentration can shorten the carburizing treatment time, which in turn can make the carburized abnormal layer shallower, so the amount of C added also needs to be adjusted to an appropriate range.

[0009] The above-mentioned round steel bar for carburizing is primarily based on restricting its chemical composition to the specific range described above. Furthermore, by adjusting the balance of Si, Mn, Cr, and C additions to satisfy Equation 1, at least the abnormal carburization layer can be optimized. Moreover, by adjusting the balance of Si, Mn, Cu, Ni, Mo, N, and Al additions to satisfy Equation 2, the ferrite hardness can be set to an appropriate range, the chip length can be optimized, and machinability can be improved.

[0010] In particular, for carburizing steel, Cu and Ni are elements that may be present in small amounts as impurities. However, even small amounts have a significant impact on ferrite hardness, so manufacturing adjustments are necessary to ensure that the value in Equation 2, including these impurity elements, falls within the specified range. The upper limit for the presence of Cu and Ni as unavoidable impurities is approximately 0.15%. As mentioned above, Mo is not actively added, but it may be present as an unavoidable impurity up to approximately 0.06%.

[0011] Furthermore, increasing the amount of sulfur (S) added lowers the solidus temperature during casting, expanding the solid-liquid coexistence area. This causes concentrated molten steel to be trapped between dendrites, leading to casting segregation. When casting segregation becomes significant and phosphorus (P) is concentrated, hot workability decreases, cracking becomes more likely during hot working, and manufacturability deteriorates. Simultaneously, cold workability also decreases. To suppress casting segregation, it is effective to further adjust the balance between Mn and S to satisfy Equation 3 and then increase the cooling rate during casting. Increasing the cooling rate reduces the solid-liquid coexistence area and suppresses segregation.

[0012] While it is well known that applying light reduction to a cast slab with an unsolidified layer inside during the final stages of solidification is a technique to suppress casting segregation, and this invention naturally includes the application of such techniques, the inventors' detailed investigation revealed that even with steel containing only an average impurity level of about 0.015-0.020% phosphorus, conventional segregation reduction measures alone can result in a maximum phosphorus concentration exceeding 0.15%. Therefore, this invention is characterized by its ability to reliably achieve a maximum phosphorus concentration of 0.15% or less by intentionally controlling the cooling rate during casting.

[0013] As a result, the maximum P concentration measured by surface analysis at a depth of D / 4 from the surface in a circular cross-section of diameter D is 0.15% or less. This allows for control to prevent P segregation from deteriorating to problematic levels, resulting in excellent hot and cold workability.

[0014] As described above, the above-mentioned round steel bar material for carburizing is based on restricting the chemical composition to a specific range, and further, by adjusting the composition to satisfy all of Equations 1 to 3, it is possible to achieve excellent machinability without sacrificing hot workability or cold workability, and to suppress abnormal carburization layers. [Modes for carrying out the invention]

[0015] The above carburizing round bar steel has, as its basic chemical composition, in terms of mass%, C: 0.17 - 0.28%, Si: 0.15 - 0.25%, Mn: 0.70 - 1.00%, P: 0.030% or less, S: 0.015 - 0.035%, Cr: 1.35 - 1.75%, Al: 0.020 - 0.040%, N: 0.0100 - 0.0200%, contains, as optional elements, Nb: 0.01 - 0.10%, Ti: 0.01 - 0.10% as an optional element, B: 0.0010 - 0.0040% as an optional element, and has a chemical composition with the balance being Fe and inevitable impurities.

[0016] C: 0.17 - 0.28%; C (carbon) is contained at 0.17% or more for ensuring the internal hardness after quenching and improving the bending fatigue strength. On the other hand, if the C content rate is too high, there is a risk that the hardness after annealing will increase and the machinability will decrease, so it is set to 0.28% or less.

[0017] Si: 0.15 - 0.25%; Si (silicon) is an element effective for ensuring machinability. If the Si content rate is too low, the ferrite hardness will be low, the chip breakability will decrease, and there is a risk of promoting tool wear, so it is contained at 0.15% or more, preferably 0.16% or more. On the other hand, if the Si content rate is too high, it may assist in the formation of abnormal carburized layers and there is a risk of reducing the strength, so it is set to 0.25% or less, preferably 0.20% or less.

[0018] Mn: 0.70 - 1.00%; Mn (manganese) is contained at 0.70% or more to improve hardenability and ensure internal hardness strength. On the other hand, if the Mn content rate is too high, there is a risk that the hardness after annealing will increase too much and the machining performance will decrease, so it is set to 1.00% or less.

[0019] P: 0.030% or less; If the content rate of P (phosphorus) is too high, it segregates at grain boundaries and causes a decrease in fatigue strength, so it should be 0.030% or less. In addition, for the reduction of the maximum P concentration analyzed by surface analysis at a depth of D / 4 from the surface in a circular cross-section with a diameter D, not only the total content rate of P but also the ingenuity of the manufacturing method as described later is necessary.

[0020] S: 0.015 - 0.035%; S (sulfur) is contained at 0.015% or more, preferably 0.020% or more, in order to ensure chip breakability during cutting and suppress tool wear. On the other hand, if the content rate of S is too high, it may promote casting segregation and deteriorate hot and cold workability, so it should be 0.035% or less. Preferably, it should be 0.030% or less.

[0021] Cr: 1.35 - 1.75%; Cr (chromium) is contained at 1.35% or more because it is effective in ensuring internal hardness by improving hardenability. On the other hand, if the Cr content rate is too high, the hardness after annealing may increase and the machinability may decrease, so it should be 1.75% or less.

[0022] Al: 0.020 - 0.040%; Al (aluminum) is contained at 0.020% or more, preferably 0.023% or more, because it is effective in refining crystal grains. On the other hand, if the Al content rate is too high, there is a risk that alumina will be generated in the steel and the strength will decrease, so it should be 0.040% or less, preferably 0.037% or less.

[0023] N: 0.0100 - 0.0200%; N (nitrogen) is contained at 0.0100% or more, preferably 0.0120% or more, because it has the effect of suppressing the coarsening of crystal grains. On the other hand, if the N content rate is too high, it will lead to an increase in manufacturing cost, so it should be 0.0200% or less.

[0024] Nb as an optional element: 0.01 - 0.10%; Niobium (Nb) is an optional additive and does not need to be actively included, but including it at a concentration of 0.01% or more can suppress grain coarsening. On the other hand, if the Nb content is too high, the processability may deteriorate, so it should be limited to 0.10% or less.

[0025] Ti as an arbitrary element: 0.01~0.10%; Titanium (Ti) is an optional additive and does not need to be actively included, but including it at a concentration of 0.01% or more can suppress grain coarsening. On the other hand, if the Ti content is too high, the processability may deteriorate, so it should be kept below 0.10%.

[0026] B as an arbitrary element: 0.0010~0.0040%; Boron (B) is an optional additive and does not need to be actively included, but including it at a concentration of 0.0010% or more can improve hardenability. However, if the B content is too high, the material cost will increase, so it should be kept below 0.0040%.

[0027] Next, assuming that the basic chemical composition described above is present, it is important to adjust the chemical composition so that it satisfies the following equations 1 to 3.

[0028] Next, assuming that the basic chemical composition described above is present, it is important to adjust the chemical composition so that it satisfies the following equations 1 to 3.

[0029] Formula 1: (5Si+1.4Mn+Cr) / C≦18.6; This equation relates to the depth of the carburized abnormal layer. While the carburized abnormal layer tends to increase with higher concentrations of Si, Mn, and Cr, a higher concentration of C allows for a reduction in carburizing time and thus a reduction in the abnormal layer. This equation was determined experimentally, taking these factors into consideration. By satisfying Equation 1, the depth of the carburized abnormal layer can be reduced to approximately 30 μm or less.

[0030] Formula 2:42Si+11Mn+18(Cu+Ni)+24Mo+600N-200Al≧18.3; This formula is an index indicating the amount of solid solution strengthening in ferrite; the higher this value, the higher the ferrite hardness and the improved chip evacuation performance. Therefore, by adjusting the chemical composition so that the value of formula 2 is 18.3 or higher, it is possible to improve machinability.

[0031] Formula 3: Mn / S≧28; This equation is a relation that affects the solidus temperature. The smaller this value, the easier it is for sulfur to concentrate in the liquid phase, the lower the solidus temperature, and the easier it is for phosphorus to solidify and segregate. Therefore, by setting the value of equation 3 to 28 or higher, it is possible to prevent the solidus temperature from dropping too low and to suppress the casting segregation of phosphorus.

[0032] Furthermore, for the above-mentioned round steel bar material for carburizing, it is essential that the maximum P concentration measured by surface analysis at a depth of D / 4 from the surface of a circular cross-section with a diameter D is 0.15% or less. In order to meet this requirement, in addition to meeting the chemical composition requirements described above, it is also necessary to increase the cooling rate during casting as a manufacturing condition. Specifically, for example, in the case of continuous casting, it is necessary to increase the amount of cooling water during continuous casting, increase the area over which the cooling water is applied, or increase the time the material is exposed to the cooling water. As a specific numerical value for the cooling rate, it is effective to set the cooling rate to 3°C / min or higher until solidification is completed at a position 3H / 4 from the bottom surface with respect to the height H of the slab cross-section.

[0033] Here, the maximum P concentration obtained by surface analysis at a depth of D / 4 from the surface of a circular cross-section of a carburizing round steel bar with a diameter D is measured using the method shown in the examples described later. [Examples]

[0034] (Example 1) This section describes an example relating to round steel bars for carburizing. In this example, as shown in Table 1, carburizing round steel bars were prepared using seven types of steel materials with different chemical compositions (Tests A1-A7), and various evaluations were conducted. Although Cu, Ni, and Mo are not actively added elements, they were present as impurities, so their analytical values ​​are shown.

[0035] [Table 1]

[0036] The base material for each round steel bar was prepared by manufacturing ingots using a continuous casting machine, which is a mass production facility. During this process, the cooling rate of the ingots was adjusted. The cooling rate was adjusted by changing the amount of cooling water during continuous casting. Based on the measured surface temperature change of the ingots, the cooling rate at a position 3 / 4H from the bottom (average cooling rate in the range from liquidus temperature to solidus temperature; the liquidus and solidus temperatures were estimated from the composition using known empirical formulas) was calculated using solidification simulation.

[0037] The round steel bar in this example was produced by rolling an ingot obtained by the casting method described above to a diameter of φ70 mm. Test specimens were taken from this round steel bar and evaluated as described below.

[0038] <Measurement of maximum P concentration> For a round steel bar with a diameter D (φ70 mm), a 2 mm × 2 mm rectangular area was analyzed using EPMA (beam diameter 2 μm) in a cross-section at a position D / 4 from the outer surface. The average P concentration in a 50 μm × 50 μm rectangular area, centered on the location with the highest P concentration within that area, was measured for 10 different cross-sections, and the maximum value among these 10 data points was defined as the maximum P concentration. The measurement results are shown in Table 2.

[0039] <Measurement of abnormally carburized layers> A φ20mm test specimen for carburizing was prepared from a φ70mm round steel bar by machining, and then subjected to gas carburizing treatment. The carburizing conditions were as follows: carburizing temperature: 950℃ for 2.5 hours, Cp: 0.85, followed by oil cooling and quenching, and then tempering treatment at 150℃ for 1 hour.

[0040] The cross-section of the test specimen after carburizing treatment was observed, and the depth of the abnormal carburizing layer on the surface was measured at three locations in the circumferential direction. The average value was taken as the value of the abnormal carburizing layer. The measurement results are shown in Table 2.

[0041] <Measurement of boundary wear in cutting tests> A φ70mm round steel bar was annealed by holding it at 900°C for 1 hour, then cooling it to 600°C in 4 hours, followed by air cooling. After this annealing treatment, a 60mmφ test piece was prepared for cutting tests, and cutting tests were performed using a lathe. A tool suitable for cutting P-type steel was used as the cutting tool. Cutting was performed under the test conditions of cutting speed: 250m / min, feed rate: 0.4mm / rev, and depth of cut: 0.8mm. After the test, the amount of wear on the cutting tool was measured. The measurement was taken at the boundary where the lateral relief face of the cutting tool no longer contacts the test piece (boundary wear). A measured amount of wear of 0.20mm or less was considered acceptable.

[0042] <Measuring the aperture value of the Greeble> A φ8mm × 55mm length greebble test specimen was cut from a φ70mm round steel bar with its longitudinal direction perpendicular to the rolling direction to prepare the greebble test specimen. Five of these specimens were prepared from five different cross-sections of the φ70mm round steel bar. Hot tensile tests were then performed using a greebble testing machine with a test number of N=5. The area of ​​the fracture surface of the test face was measured, and the ratio of the area to the area before the test was taken. The lowest value among the five tests was defined as the greebble reduction of area.

[0043] [Table 2]

[0044] As can be seen from Table 2, tests A1 to A4 consisted of chemical components that satisfied the basic chemical composition and all of Equations 1 to 3, and the desired requirements for maximum P concentration were also met by optimizing the casting conditions. As a result, it was confirmed that the depth of the abnormal carburization layer in tests A1 to A4 was within the appropriate range of 30 μm or less, and that they exhibited excellent machinability.

[0045] On the other hand, in tests A5 and A7, although there were no problems with the chemical composition, the slow cooling rate during casting resulted in the maximum P concentration of D / 4 from the surface exceeding 0.15%, which lowered the grease depth and reduced the hot workability.

[0046] Furthermore, although Test A6 satisfied the ranges for the individual components, it did not possess Equation 3, and the balance of Mn and S content was not appropriate. As a result, the maximum P concentration of D / 4 from the surface exceeded 0.15%, which led to a lower reduction in the grible's drawing value and a decrease in its hot workability.

[0047] From the results of tests A1 to A6 in Example 1, it can be seen that by appropriately selecting the chemical composition, including the conditions of the three formulas (especially formula 3), and by optimizing the manufacturing conditions so that the maximum P concentration of D / 4 from the surface is 0.15% or less, a steel material can be obtained that has a sufficiently high grible reduction value, excellent hot workability, and excellent machinability.

[0048] (Example 2) In Example 1, mass production equipment was used for evaluation, and the effect of P segregation due to the cooling rate was investigated in particular. However, since it is difficult to investigate the effects of differences in composition in detail using mass production equipment, an example in which a small-scale electric furnace was used to melt and produce various types of steel by changing the chemical composition is described below.

[0049] In this example, as shown in Table 3, carburizing round steel bars were prepared using 16 types of steel materials with different chemical compositions (Tests B1 to B16). In addition to the evaluations performed in Example 1, surface fatigue strength and bending fatigue strength evaluations were added to investigate the effect of the differences in composition on the strength after carburizing. However, since the steel materials were not prepared by continuous casting, the maximum P concentration and greebre test, which are thought to be greatly affected by the cooling rate during casting, were not performed. The analytical values ​​for Cu, Ni, and Mo, which were present as impurities, are shown in Table 3, as in Table 1.

[0050] [Table 3]

[0051] <Measurement of abnormal carburized layers and measurement of boundary wear in cutting tests> The round steel bar used in this example was prepared by forging an ingot obtained by the casting method described above to a diameter of φ70 mm. Test specimens were taken from this round steel bar, and measurements of the abnormal carburization layer and boundary wear in cutting tests were performed. The method for preparing and evaluating the test specimens was the same as in Example 1.

[0052] <Evaluation of surface fatigue strength> Using the ingots obtained by the above casting method, two types of round steel bars with diameters of φ32 mm and φ140 mm were prepared by forging. From the φ32 mm round steel bar, a small roller-shaped test piece (small roller) with a cylindrical section having a diameter of 26 mm and a width (axial length) of 28 mm was manufactured by machining. Furthermore, from the φ140 mm round steel bar, a large roller-shaped test piece (large roller) with a cylindrical section having a diameter of 130 mm and a width (axial length) of 18 mm was manufactured. Subsequently, these test pieces were subjected to the gas carburizing treatment described later, and then the parts excluding the test surface were finished.

[0053] Each test specimen was subjected to gas carburizing treatment under the same conditions as for measuring abnormal carburizing layers, and was prepared as a test specimen for roller pitting testing. The roller pitting test was performed using a roller pitting test machine manufactured by Nikko Create Co., Ltd., with the small and large rollers prepared as described above set between them with a predetermined load stress. The surface fatigue limit was determined when the rotation of the small roller reached 10° 7 The maximum load stress that could be withstood without fracture at the point of reaching the specified number of cycles was defined. The test conditions were: rotational speed (small roller): 2000 rpm, peripheral speed difference: -40%, lubricant: automatic transmission oil, oil temperature: 120°C. Compared with conventional gas-carburized SCM420, a test was judged as passing (○) if the surface fatigue strength was equal to or greater than that of conventional products, and failing (×) if it was less. The evaluation results are shown in Table 4 below.

[0054] <Bending fatigue strength> Using the ingot obtained by the above casting method, a round steel bar with a diameter of φ15 mm was prepared by forging. A test piece with a parallel section diameter of φ10 mm was taken from this round steel bar, and a notch with a depth of 1 mm perpendicular to the parallel section (notch coefficient: 1.78) was made around the entire circumference to create a rotary bending fatigue test piece. Subsequently, each test piece was subjected to gas carburizing treatment under the same conditions as for measuring the abnormal carburizing layer to create a rotary bending fatigue test piece.

[0055] The rotary bending fatigue test was performed by setting the rotary bending test specimen prepared as described above into an Ono-type rotary bending fatigue testing apparatus (model number: H6) manufactured by Shimadzu Corporation, and repeatedly applying bending stress at a rotation speed of 3600 rpm. The bending fatigue limit was 10 cycles. 7 Fatigue in the round The fatigue limit was determined according to the standards of JIS Z2274. Compared to conventional gas-carburized SCM420, a product was judged as passing (○) if its bending fatigue strength was equal to or greater than that of conventional products, and failing (×) if it was less. The evaluation results are shown in Table 4 below.

[0056] [Table 4]

[0057] As can be seen from Table 4, tests B1 to B11 had appropriate chemical composition and satisfied all of Equations 1, 2, and 3, resulting in favorable evaluation results for all tests.

[0058] On the other hand, although test B12 had the basic chemical composition, it did not satisfy Equation 1, resulting in a deep carburization abnormality layer and worsened surface fatigue strength and bending fatigue strength compared to conventional tests.

[0059] Test B13 showed that the Si content was too high, it did not satisfy Equation 1, the carburized abnormal layer was deep, and the surface fatigue strength and bending fatigue strength were worse than conventional tests.

[0060] Test B14 had too low a Si content and did not satisfy Equation 2, resulting in increased boundary wear in the machinability test.

[0061] Although test B15 possessed the basic chemical composition, it did not satisfy Equation 2, resulting in increased boundary wear in the machinability test.

[0062] In test B16, the low sulfur content resulted in increased boundary wear in the machinability test.

Claims

1. The chemical composition is as follows, in mass%, C: 0.17-0.28%, Si: 0.15-0.25%, Mn: 0.70-1.00%, P: 0.030% or less, S: 0.015-0.035%, Cr: 1.35-1.75%, Al: 0.020-0.040%, N: 0.0100-0.0200%, with optional elements Nb: 0.01-0.10%, optional Ti: 0.01-0.10%, optional B: 0.0010-0.0040%, and the remainder being Fe and unavoidable impurities. The Cu, Ni, and Mo content in the above unavoidable impurities is as follows: Cu: 0.150% or less, Ni: 0.150% or less, and Mo: 0.06% or less. The following equations 1 to 3 are satisfied, Formula 1: (5Si+1.4Mn+Cr) / C≦18.6, Formula 2: 42Si+11Mn+18(Cu+Ni)+24Mo+600N-200Al≧18.3, Formula 3: Mn / S≧28, (However, the element symbols in formulas 1 to 3 above represent the mass percentage of each element.) Furthermore, the maximum P concentration measured by surface analysis at a depth of D / 4 from the surface of a circular cross-section with diameter D is 0.15% or less, and is a round steel bar for carburizing.

2. A method for producing a carburizing round steel bar according to Claim 1, wherein the ingot having the above chemical composition is produced by continuous casting, and the method is performed under conditions that the average cooling rate in the range from the liquidus temperature to the solidus temperature at a position 3 / 4 of the way from the bottom surface of the ingot is 3°C / min or more.

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