Non-tempered steel for crankshafts and crankshafts using said non-tempered steel

Abstract

This untempered steel for crankshafts is mainly iron (Fe) but contains 0.37–0.43 mass% of carbon (C), 0.15–0.35 mass% of silicon (Si), 0.90–1.30 mass% of manganese (Mn), 0.08–0.15 mass% of vanadium (V), no more than 0.030 mass% of phosphorus (P), no more than 0.300 mass% of copper (Cu), no more than 0.30 mass% of nickel (Ni), and no more than 0.35 mass% of chromium (Cr).　The untempered steel also contains 0.010–0.035 mass% and 0.02–0.05 mass%, respectively, of the machinability-improving elements sulfur (S) and bismuth (Bi) and, as a result, has high fatigue strength and yield strength and excellent machinability.

Description

[Technical field]

[0001] The present invention relates to a non-heat treated steel for crankshafts and a crankshaft using the non-heat treated steel. [Background technology]

[0002] Many mechanical parts are made by heat treatment, in which carbon steel or alloy steel formed into the shape of the part is heated to the austenite phase region, then rapidly cooled and quenched to form the martensite phase, and then tempered to adjust the mechanical properties.

[0003] There is a type of non-tempered steel that can achieve the desired strength without this refining heat treatment. This non-tempered steel does not require refining heat treatment after hot working by adding small amounts of elements such as vanadium (V), niobium (Nb), and titanium (Ti) that promote precipitation strengthening, and by controlling the composition of the steel.

[0004] Many automotive parts must be able to withstand fluctuating loads over long periods of time, and are therefore required to have high fatigue strength.

[0005] In particular, the crankshaft is subjected to sudden changes in load due to the explosive force applied to the piston, so it is necessary to ensure yield strength in addition to fatigue strength.

[0006] If a high hardness steel material is used for the crankshaft, the fatigue strength and yield strength of the crankshaft can be improved, but a steel material that is simply high hardness has poor machinability.

[0007] Because crankshafts are used in conjunction with connecting rods, they require high surface precision and must be made from steel that has excellent machinability. It is difficult to make crankshafts simply using steel that has high hardness.

[0008] Although not related to steel materials for crankshafts, Patent Document 1 discloses that machinability can be improved by adding bismuth (Bi) and sulfur (S) as inclusion-forming elements. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2005-154886 Summary of the Invention [Problem to be solved by the invention]

[0010] However, crankshafts are large components that require a large amount of machining, which places a high load on cutting tools and lengthens the machining cycle time.

[0011] The present invention has been made in consideration of the problems associated with the conventional technology, and an object of the present invention is to provide a non-heat treated steel for crankshafts that has improved fatigue strength and yield strength while also having improved machinability, and a crankshaft using the non-heat treated steel. [Means for solving the problem]

[0012] As a result of extensive research into achieving the above object, the inventors have discovered that the above object can be achieved by designing the components in a way that takes into consideration the relationship between elements that improve strength and elements that improve machinability, and have thus completed the present invention.

[0013] That is, the non-heat treated steel for crankshafts of the present invention has the following features: Carbon (C) is 0.39 to 0.42 mass%, Silicon (Si) 0.15 to 0.35 mass% Manganese (Mn) 0.90 to 1.30 mass%; Contains 0.08 to 0.15 mass% vanadium (V), Phosphorus (P) content is 0.030% by mass or less, Copper (Cu) content is 0.300 mass% or less, Nickel (Ni) content is more than 0 and 0.30 mass% or less, The chromium (Cr) content is 0.35 mass% or less. the law of nature, The balance is iron (Fe) and unavoidable impurities. A non-tempered steel for crankshafts, moreover, Sulfur (S) 0.010 to 0.035 mass%, Contains 0.02 to 0.05 mass% bismuth (Bi). death, The carbon equivalent (Ceq) represented by the following formula (1) is 0.95 to 0.99. Let us say; Ceq=[C]+0.167[Si]+0.222[Mn]+0.067[Cu]+0.067[Ni]+0.25[Cr]+0.4[Mo]+1.8[V]...Formula (1) In formula (1), the brackets indicate the mass percentage of the element shown in each bracket.

[0014] The crankshaft of the present invention is characterized in that it uses the above-mentioned non-heat treated steel for crankshafts. Effect of the Invention

[0015] According to the present invention, in addition to components that fully satisfy the strength required for crankshafts, components that improve machinability are added and strict component adjustment is performed taking into account the interrelationships of these components. As a result, it is possible to provide a non-heat treated steel for crankshafts that has improved fatigue strength and yield strength as well as improved machinability, and a crankshaft using this non-heat treated steel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The non-heat treated steel for crankshafts of the present invention (hereinafter sometimes simply referred to as "non-heat treated steel") is a non-heat treated steel that contains vanadium (V) as an element that promotes precipitation strengthening.

[0017] The above-mentioned vanadium (V) is an element that promotes precipitation strengthening. It precipitates as fine carbonitrides in the ferrite-pearlite structure that is produced by transformation during the cooling process after hot working, strengthening the structure and eliminating the need for thermal refining treatment after hot working.

[0018] In particular, vanadium (V) has a lower solid solution temperature in the austenite structure than other elements that promote precipitation strengthening, such as niobium (Nb) and titanium (Ti), and is easily dissolved at the heating temperatures used in hot working. This allows hot working to be performed at low temperatures, and the structure after hot working can be refined, making it possible to achieve even higher yield strength.

[0019] Furthermore, the part of the crankshaft that receives the highest stress due to bending deformation is the fillet, and by strengthening this fillet by cold rolling, a highly reliable crankshaft can be manufactured.

[0020] The cold rolling is a surface processing method in which pressure is applied to the surface of a crankshaft while it is rotating with rolls, generating residual compressive stress and improving fatigue strength. Because the processing is carried out at high pressure, the processed material itself needs to be a steel material with high yield strength.

[0021] The vanadium (V)-containing non-tempered steel has high hardness and yield strength due to the combination of vanadium carbonitrides and a fine structure, and it is possible to impart residual compressive stress by cold rolling, but it generally has poor machinability.

[0022] The non-tempered steel of the present invention contains sulfur (S) and bismuth (Bi) as elements that improve machinability. By strictly controlling the composition of not only these elements but also other elements, it is possible to achieve both improved fatigue strength and yield strength required for crankshafts and machinability.

[0023] The non-tempered steel for crankshafts of the present invention is The main component is iron (Fe), and the content is 0.37 to 0.43 mass% of carbon (C), 0.15 to 0.35 mass% of silicon (Si), 0.90 to 1.30 mass% of manganese (Mn), and 0.08 to 0.15 mass% of vanadium (V), The phosphorus (P) content is 0.030 mass% or less, the copper (Cu) content is 0.300 mass% or less, the nickel (Ni) content is 0.30 mass% or less, and the chromium (Cr) content is 0.35 mass% or less. The alloy contains 0.010 to 0.035 mass % of sulfur (S) and 0.02 to 0.05 mass % of bismuth (Bi), which are elements that improve machinability.

[0024] Each component of the non-heat treated steel will now be described. The vanadium (V) content is 0.08 to 0.15 mass % (not less than 0.08 mass % and not more than 0.15 mass %).

[0025] As described above, vanadium (V) is an element that promotes precipitation strengthening, and as its content increases, machinability decreases. However, by keeping the content within the above range, in combination with sulfur (S) and bismuth (Bi) described below, it is possible to improve machinability and fatigue strength by imparting residual compressive stress.

[0026] A low vanadium (V) content tends to reduce strength, while a high vanadium (V) content reduces machinability, so the vanadium (V) content is preferably 0.09 to 0.14 mass%, and more preferably 0.10 to 0.13 mass%.

[0027] The sulfur (S) content is 0.010 to 0.035 mass %. The above-mentioned non-tempered steel containing vanadium (V) does not require thermal treatment for refining after hot working, so it forms a ferrite-pearlite structure by cooling, and as the proportion of the relatively soft ferrite structure increases, the machinability improves.

[0028] Sulfur (S) combines with manganese (Mn) to form manganese sulfide (MnS). This manganese sulfide (MnS) acts as a stress concentration source in the shear zone during cutting, inducing minute internal cracks, reducing cutting resistance and improving chip breakability. It also acts as a nucleus for the ferrite structure, increasing the proportion of the ferrite structure and improving machinability.

[0029] Furthermore, if too much sulfur (S) is added, it generates FeS, which may reduce the strength of the steel material and promote the occurrence of cracks during manufacturing. However, when manganese (Mn) is present in sufficient amounts, it becomes manganese sulfide (MnS) as described above and exists in the non-tempered steel, preventing the strength of the steel from decreasing and cracking during manufacturing. Therefore, the sulfur (S) content must always be less than that of manganese (Mn).

[0030] In addition, manganese sulfide (MnS) precipitates bismuth (Bi) around itself, preventing the bismuth (Bi) from coarsening and dispersing it finely, thereby preventing the fatigue strength from decreasing due to the bismuth (Bi). Therefore, the content of sulfur (S) is preferably 0.015 to 0.030 mass%, more preferably 0.017 to 0.028 mass%, and even more preferably 0.020 to 0.028 mass%.

[0031] The content of bismuth (Bi) is 0.02 to 0.05 mass %. Bismuth (Bi) has a melting point of about 271°C and is easily melted by the frictional heat generated during cutting. Therefore, by finely dispersing bismuth (Bi), the friction surface between the non-tempered steel and the cutting tool is liquid lubricated by the molten bismuth (Bi), reducing the cutting resistance.

[0032] In addition, the molten bismuth (Bi) embrittles and breaks the chips, reducing the load on the cutting edge of the cutting tool and extending the life of the cutting tool. The content of bismuth (Bi) is more preferably 0.02 to 0.04 mass%. As the content of bismuth (Bi) increases, the maximum area of ​​inclusions containing bismuth (Bi) tends to increase.

[0033] Bismuth (Bi) exists as an inclusion in the structure of non-tempered steel, and if it coarsens it can easily become the initiation point of fatigue fracture. However, in the present invention, since it is finely dispersed by manganese sulfide (MnS) as described above, it does not become the initiation point of fatigue fracture.

[0034] The maximum area of ​​the inclusions containing bismuth (Bi) dispersed in the structure of non-tempered steel, i.e., the inclusions containing bismuth (Bi) alone and the inclusions containing bismuth (Bi) in contact with manganese sulfide (MnS) and the manganese sulfide combined, is 2300 μm 2 It is preferable that the maximum area of ​​the inclusion is 2300 μm or less. 2 By keeping the temperature below 100° C., it is possible to prevent a decrease in fatigue strength.

[0035] The carbon (C) content is 0.37 to 0.43 mass %. Carbon (C) increases the proportion of pearlite in the ferrite-pearlite structure, thereby improving strength.

[0036] A low carbon (C) content tends to reduce strength, while a high carbon (C) content reduces machinability. Therefore, the carbon (C) content is preferably 0.38 to 0.43 mass%, more preferably 0.38 to 0.42 mass%, and even more preferably 0.39 to 0.42 mass%.

[0037] The manganese (Mn) content is 0.90 to 1.30 mass %. Manganese (Mn) increases the proportion of pearlite structure and improves strength. It also combines with sulfur (S) to form MnS, which prevents the reduction in strength of steel due to the formation of FeS, and also prevents cracks during steel manufacturing.

[0038] A low manganese (Mn) content tends to reduce strength, while a high manganese (Mn) content reduces machinability, so the manganese (Mn) content is preferably 0.96 to 1.28 mass%, more preferably 1.00 to 1.20 mass%, and even more preferably 1.01 to 1.17 mass%.

[0039] The silicon (Si) content is 0.15 to 0.35 mass %. Silicon (Si) dissolves in the structure of non-tempered steel and improves strength through solid solution strengthening. When the silicon (Si) content is low, the strength is likely to decrease, and when the content is high, not only the machinability but also the hot workability decreases. Therefore, the silicon (Si) content is preferably 0.19 to 0.30 mass%, more preferably 0.20 to 0.30 mass%, and even more preferably 0.20 to 0.26 mass%.

[0040] Phosphorus (P) easily segregates at grain boundaries and reduces toughness, so its content is 0.030 mass% or less, more preferably more than 0 and 0.030 mass% or less, and even more preferably more than 0 and 0.023 mass% or less. Copper (Cu) easily causes cracks during hot working, so its content is 0.30 mass% or less, more preferably more than 0 and 0.30 mass% or less, and even more preferably more than 0 and 0.21 mass% or less. Nickel (Ni) promotes the formation of bainite, so its content is 0.30 mass% or less, more preferably more than 0 and 0.30 mass% or less, even more preferably more than 0 and 0.20 mass% or less, and even more preferably more than 0 and 0.10 mass% or less. Chromium (Cr) promotes the formation of bainite, so its content is 0.35 mass% or less, more preferably more than 0 and 0.35 mass% or less, and even more preferably more than 0 and 0.20 mass% or less. It is preferable that the content of inevitable impurities other than the above elements is as small as possible, but their presence is not completely denied. In addition, iron (Fe) is the main component, and the remainder other than the above elements and inevitable impurities is substantially composed of iron (Fe).

[0041] The non-heat treated steel preferably has a carbon equivalent (Ceq) represented by the following formula (1) of 0.95 to 0.99. Ceq=[C]+0.167[Si]+0.222[Mn]+0.067[Cu]+0.067[Ni]+0.25[Cr]+0.4[Mo]+1.8[V]...Formula (1) In formula (1), the brackets indicate the mass percentage of the element shown in each bracket.

[0042] By satisfying the above range of carbon equivalent (Ceq), it is possible to achieve both the mechanical properties and machinability required for a crankshaft. In addition, the residual compressive stress in the fillets of the crank pin and journal is 500M from the viewpoint of fatigue strength. P a or more, and more preferably 600M P a or more, and more preferably 626M P is greater than or equal to a. EXAMPLES

[0043] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0044] A non-tempered steel for crankshafts having the composition shown in Table 1 below was heated at 1250°C, hot forged, and then machined to form the crank pin and crank journal parts. By hot forging at a temperature between 750 and 1300°C, i.e., in the austenite region, the crankshaft has a ferrite and pearlite structure, and the hardness and yield strength required for the crankshaft can be obtained. Then, the R-sections of the crank pin and the crank journal were fillet rolled (cold rolled) to impart residual compressive stress and harden them to obtain the crankshaft. The fillet roll process was performed so that the indentation depth was the same, and the fillet roll load was set to 700 to 1300. kgf was controlled between In both the examples and comparative examples, hot forging and fillet rolling were carried out under the same conditions.

[0045] <Evaluation> The above crankshaft was evaluated by the following method. The evaluation results are shown in Table 1 together with the composition of the non-heat treated steel.

[0046] (hardness) The internal hardness of the crank pin (part not affected by the fillet rolling) was measured at six points using a Rockwell hardness tester, and the average value was calculated.

[0047] (yield strength) Clan Kupi A test piece was cut out from the pin portion, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 to measure the yield strength.

[0048] (Fatigue strength) A load of 10 was applied to the crank pin. 7 After the load was applied 100 times, the stress at which the fillet of the crank pin, which is a stress concentrated portion, was broken was measured. Comparative Example 4, which had the weakest fatigue strength, was assigned a value of 1 and the fatigue strength was expressed as a relative value.

[0049] (Area of ​​bismuth and bismuth and manganese sulfide inclusions combined) The cross section of the crank pin was observed with a scanning electron microscope (SEM), and the area of ​​the precipitates was calculated by extreme value statistics.

[0050] (cutting workability) The evaluation was based on the machining cycle time and cutting resistance when cutting the crankshaft. Comparative Example 3, which had the lowest machinability, was given a score of 1, and the machinability was shown relative to the comparative example.

[0051] (residual stress) Using an X-ray diffraction measurement device, a 1.0 mm deep section from the surface of the fillet of the crank pin was The peak position of X-ray diffraction was detected in the range from 0.1 to 1.0 mm, and the diffraction angle was calculated based on the detected position. The residual stress was measured from the diffraction angle or the change in the diffraction angle, and the maximum value was calculated.

[0052] [Table 1]

[0053] The results in Table 1 show that the non-heat treated steel for crankshafts of the present invention has high fatigue strength and yield strength, and is also excellent in machinability.

Claims

1. Carbon (C) is 0.39 to 0.42 mass%, Silicon (Si) 0.15 to 0.35 mass%; Manganese (Mn) 0.90 to 1.30 mass%; Contains 0.08 to 0.15 mass% vanadium (V), The phosphorus (P) content is 0.030% by mass or less, The copper (Cu) content is 0.300 mass% or less, The content of nickel (Ni) is more than 0 and 0.30 mass% or less, The chromium (Cr) content is 0.35 mass% or less, A non-tempered steel for crankshafts, the balance of which is iron (Fe) and unavoidable impurities, moreover, Sulfur (S) 0.010 to 0.035% by mass, Contains 0.02 to 0.05 mass% bismuth (Bi), The non-heat treated steel for crankshafts is characterized in that the carbon equivalent (Ceq) represented by the following formula (1) is 0.95 to 0.

99. Ceq=[C]+0.167[Si]+0.222[Mn]+0.067[Cu]+0.067[Ni]+0.25[Cr]+0.4[Mo]+1.8[V]...Formula (1) In formula (1), the brackets indicate the mass percentage of the element shown in each bracket.

2. Carbon (C) is 0.37 to 0.43 mass%, Silicon (Si) 0.20 to 0.30 mass%; Manganese (Mn) 0.90 to 1.30 mass%; Contains 0.08 to 0.15 mass% vanadium (V), The phosphorus (P) content is 0.030% by mass or less, The copper (Cu) content is 0.300 mass% or less, The content of nickel (Ni) is more than 0 and 0.30 mass% or less, The chromium (Cr) content is 0.35 mass% or less, A non-tempered steel for crankshafts, the balance of which is iron (Fe) and unavoidable impurities, moreover, Sulfur (S) 0.010 to 0.035% by mass, Contains 0.02 to 0.05 mass% bismuth (Bi), The non-heat treated steel for crankshafts is characterized in that the carbon equivalent (Ceq) represented by the following formula (1) is 0.95 to 0.

99. Ceq=[C]+0.167[Si]+0.222[Mn]+0.067[Cu]+0.067[Ni]+0.25[Cr]+0.4[Mo]+1.8[V]...Formula (1) In formula (1), the brackets indicate the mass percentage of the element shown in each bracket.

3. 3. The non-heat treated steel for crankshafts according to claim 1, characterized in that the manganese (Mn) content is 1.00 to 1.20 mass %.

4. 3. The non-heat treated steel for crankshafts according to claim 1, characterized in that the vanadium (V) content is 0.10 to 0.13 mass %.

5. The non-tempered steel for crankshafts as set forth in claim 1 or 2, characterized in that the maximum area of ​​inclusions of bismuth (Bi) alone and composite inclusions of bismuth (Bi) in contact with manganese sulfide (MnS) and said manganese sulfide together is 2300 μm 2 or less.

6. A crankshaft using the non-tempered steel for crankshafts described in claim 1 or 2.

7. A crankshaft as described in claim 6, characterized in that the crank pin and the fillet portions of the journal have residual compressive stresses.

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