Non-quenched and tempered steel

Abstract

This invention provides a ferrite-pearlite type non-heat-treated steel material that is free of Pb and Bi additives, yet possesses excellent machinability and high strength. [Solution] The non-heat-treated steel contains, by mass%, C: 0.35% to 0.45%, Si: 0.40% to 0.80%, Mn: 0.40% to 0.90%, P: 0.020% to 0.100%, S: 0.040% to 0.070%, Cr: greater than 0.25% to 0.70%, and V: 0.30% to 0.45%, and satisfies the following conditions: the value of P1 in formula (1) is P1, the value of P2 in formula (2) is 1.00, and the value of P4 in formula (4) is 0.24, and the metal structure is a ferrite-pearlite structure with a ferrite area ratio of 42% or more to the total structure. P1=-175[C]-16[Si]-12[Mn]+51[V]+117 ...Equation (1), P2=[C]+0.07[Si]+0.16[Mn]+0.61[P]+0.19[Cu]+0.17[Ni]+0.20[Cr]+[V] ...Equation (2), P3=[Mn]+0.49[Cu]+0.89[Ni]+0.40[Cr]-0.30[Si] ...Equation (3), P4=P3×[V] / P2 ...Equation (4)

Description

Technical Field

[0001] This invention relates to a ferrite - pearlite type non - quenched steel material suitably used for manufacturing automotive parts and the like.

Background Art

[0002] Conventionally, automotive parts and other mechanical structural parts have been used in a state where the required mechanical strength is imparted by performing quenching and tempering treatment (tempering treatment) after hot - forging the material. However, in this case, due to the cost of the tempering treatment, in recent years, non - quenched steel materials that can exhibit the required strength as they are after forging without performing such tempering treatment have come to be used.

[0003] For example, regarding non - quenched steel materials used for manufacturing engine parts such as crankshafts and connecting rods (hereinafter referred to as conrods), high strength (especially high yield stress) that enables weight reduction of parts and good machinability are required. On the other hand, in conventional non - quenched steel materials, as shown in the following patent documents, a method has been used in which the strength is improved by precipitating V carbides and Ti carbides, and the machinability is ensured by adding Pb and Bi, which are free - cutting elements. However, since Pb has a high environmental load and Bi is an expensive rare metal, it causes an increase in material cost.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Based on the above circumstances, an object of the present invention is to provide a ferrite - pearlite type non - quenched steel material that is excellent in machinability and has high strength without adding Pb and Bi. [Means for solving the problem]

[0006] The inventors of this invention have conducted extensive research to solve the above problems and have found that machinability can be ensured by maintaining a high ferrite area ratio even without the addition of Pb and Bi, and that by appropriately balancing the amount of elements that affect the ferrite area ratio with the amount of elements that affect strength, it is possible to achieve both machinability and strength in non-heat-treated steel materials as an overall effect. This invention was made based on these findings.

[0007] The gist of this invention is as follows:

[0008] [1] Contains, by mass%, C: 0.35% to 0.45%, Si: 0.40% to 0.80%, Mn: 0.40% to 0.90%, P: 0.020% to 0.100%, S: 0.040% to 0.070%, Cr: greater than 0.25% to 0.70%, V: 0.30% to 0.45%, It further contains at least one element selected from Cu: 0.40% or less, Ni: 0.30% or less, Mo: 0.10% or less, Ca: 0.010% or less, s-Al: 0.050% or less, and N: 0.030% or less, with the remainder being Fe and unavoidable impurities. The value of P1 obtained by equation (1) below satisfies P1 ≥ 42, the value of P2 obtained by equation (2) below satisfies 1.00 ≤ P2 ≤ 1.09, and the value of P4 obtained by equation (4) below satisfies 0.24 ≤ P4 ≤ 0.30. Non-heat-treated steel material having a ferrite-pearlite structure after hot forging, with a ferrite area ratio of 42% or more of the total structure, and a 0.2% yield strength of 800 MPa or more. P1=-175[C]-16[Si]-12[Mn]+51[V]+117...Formula (1) P2=[C]+0.07[Si]+0.16[Mn]+0.61[P]+0.19[Cu]+0.17[Ni]+0.20[Cr]+[V]...Formula (2) P3=[Mn]+0.49[Cu]+0.89[Ni]+0.40[Cr]-0.30[Si]...Formula (3) P4=P3×[V] / P2…Equation (4) However, in the formula, [ ] represents the mass percentage of the element contained within [ ]. [Modes for carrying out the invention]

[0009] The reasons for the limitations on the chemical components, etc., of the non-heat-treated steel material in this embodiment are described in detail below. In the following explanation, unless otherwise specified, "%" means "mass%".

[0010] C: 0.35% or more and 0.45% or less Carbon (C) is an important element for improving the strength of non-heat-treated steel. In this embodiment, to obtain the required strength, the C content is 0.35% or more. Preferably, it is 0.38% or more. However, excessive addition leads to a decrease in the ferrite area ratio and a decrease in the yield ratio. For this reason, the C content is 0.45% or less. Preferably, it is 0.42% or less.

[0011] Si: 0.40% or more and 0.80% or less Si is an element that has deoxidizing and desulfurizing properties during steel production. Furthermore, Si is an element that contributes to improving the strength of steel by solid-solving in steel without making it easier for bainite to form. To obtain this effect, in this embodiment, Si is included at a concentration of 0.40% or more. Preferably, it is 0.50% or more. However, if the amount of Si is excessive, a hard, difficult-to-peel oxide scale will form on the surface of the steel during hot working, causing wear on the die used during hot forging. For this reason, the Si content is 0.80% or less. Preferably, it is 0.70% or less.

[0012] Mn: 0.40% or more and 0.90% or less Mn promotes the fine precipitation of V-type carbides in steel, thereby improving the strength of the steel material. Furthermore, Mn forms MnS, improving machinability. To obtain these effects, this embodiment includes 0.40% or more Mn, preferably 0.50% or more. However, excessive addition can easily lead to bainite formation, resulting in a decrease in ferrite area ratio and strength. Therefore, the Mn content is 0.90% or less, preferably 0.70% or less.

[0013] P: 0.020% or more and 0.100% or less P is an effective element for increasing steel strength without making bainite formation more likely. Furthermore, when fracture separation occurs at the big end of a connecting rod, it is an effective element for increasing the brittle fracture surface and improving the adhesion of the fracture surface. To obtain these effects, this embodiment includes 0.020% or more of P. Preferably, 0.060% or more. However, excessive addition reduces the surface irregularities of the fracture and decreases the fit during assembly, so the P content should be 0.100% or less.

[0014] S: 0.040% or more and 0.070% or less S forms sulfides with Mn, improving machinability. To achieve this effect, the S content in this embodiment is set to 0.040% or more. However, excessive addition will worsen hot workability, so the S content is set to 0.070% or less.

[0015] Cr: more than 0.25% and less than 0.70% Like manganese, Cr promotes the fine precipitation of V-type carbides in steel, thereby improving the strength of the steel material. To achieve this effect, the Cr content in this embodiment is set to more than 0.25%, preferably 0.28% or more. However, excessive addition leads to increased material costs and makes bainite formation more likely, resulting in a decrease in the ferrite area ratio. For this reason, the Cr content is set to 0.70% or less, preferably 0.50% or less.

[0016] V: 0.30% or more and 0.45% or less V has the effect of improving the strength of steel by precipitating as fine carbides. V also has the effect of increasing the ferrite area ratio. In this embodiment, the V content is set to 0.30% or more in order to obtain these effects. Preferably, it is 0.35% or more. However, excessive addition causes an increase in the VC solid solution temperature, and as a result, coarser VC carbides remain after hot forging, reducing the strength of the steel. Therefore, the V content is set to 0.45% or less. Preferably, it is 0.40% or less.

[0017] Cu: 0.40% or less Ni: 0.30% or less Cu and Ni may not be contained, but they are elements that are inevitably contained in the steel due to the raw materials. Cu and Ni have the effect of finely precipitating V-based carbides in the steel and improving the strength of the steel. In order to obtain such an effect highly, the Cu content may be 0.06% or more and the Ni content may be 0.03% or more. However, Cu and Ni promote the formation of bainite. When a large amount of bainite is formed, it causes a decrease in strength due to a decrease in the yield ratio of the steel. In addition, Cu and Ni are expensive elements, and when added in large amounts, the cost of the steel increases. From the viewpoint of avoiding these phenomena, the Cu content is set to 0.40% or less and the Ni content is set to 0.30% or less. Preferably, the Cu content is 0.30% or less and the Ni content is 0.20% or less.

[0018] Mo: 0.10% or less Mo may not be contained, but it is an element that is inevitably contained in the steel due to the raw materials. Similar to Cu and Ni, it has the effect of increasing the strength of the steel, but excessive content makes it easier for bainite to be formed. Therefore, the Mo content is set to 0.10% or less.

[0019] Ca: 0.010% or less Ca is an element effective for improving machinability and can be contained as needed. However, if it is contained more than necessary, the effect saturates, and furthermore, it causes deterioration of hot workability (hot rolling property), so the Ca content is set to 0.010% or less.

[0020] s - Al: less than or equal to 0.050% Acid-soluble Al forms nitrides with N in the steel and is finely dispersed to suppress the grain growth during hot working. Therefore, it can be contained as needed. However, even if it is present in a large amount, the ductility of the material improves due to grain refinement, leading to a decrease in adhesion after fracture separation. Therefore, the s - Al content is set to 0.050% or less.

[0021] N: less than or equal to 0.030% N forms nitrides with s - Al and has the effect of being finely dispersed to suppress grain growth during hot working. On the other hand, it generates coarse nitride - based inclusions in the steel, leading to a decrease in fatigue strength. Therefore, from the perspective of suppressing the generation of nitride - based inclusions, the N content is set to 0.030% or less.

[0022] Regarding each element of C, Si, Mn, P, Cu, Ni, Cr, V contained in the non - tempered steel material of this embodiment, the content of each element is defined as described above. In addition, it is necessary to balance the content of each element so that the following parameters P1, P2, P4 satisfy a predetermined numerical range. In the definition formulas of the following parameters P1, P2, P4, [ ] represents the mass percentage of the element within [ ].

[0023] P1 ≧ 42 (However, the parameter P1 is obtained by the following formula (1)) P1 = - 175[C] - 16[Si] - 12[Mn] + 51[V] + 117 … Formula (1) The parameter P1 defined by formula (1) indicates an index for the ferrite area ratio. C, Si, Mn have the effect of reducing the ferrite area ratio in the steel, and V has the effect of increasing the ferrite area ratio. The coefficients of C, Si, Mn, V in formula (1) represent the contribution degrees to the ferrite area ratio respectively. When the value of parameter P1 is less than 42, the ferrite area ratio for the entire structure may be less than 42%, and in some cases, the required machinability may not be obtained. Therefore, in this embodiment, the value of parameter P1 is set to 42 or more so as to surely obtain a ferrite area ratio of 42% or more.

[0024] 1.00 ≤ P2 ≤ 1.09 (where the parameter P2 is calculated by equation (2) below) P2=[C]+0.07[Si]+0.16[Mn]+0.61[P]+0.19[Cu]+0.17[Ni]+0.20[Cr]+[V]...Formula (2) The parameter P2 defined in equation (2) is expressed in terms of carbon equivalent Ceq, and is calculated by converting the hardening ability of each alloying element to its respective carbon content and summing them up. In this embodiment, the value of parameter P2 must be 1.00 or higher to ensure the required strength. However, since excessive hardness would worsen machinability, the value of parameter P2 is set to 1.09 or lower.

[0025] 0.24 ≤ P4 ≤ 0.30 (where the parameter P4 is determined by equations (3) and (4) below) P3=[Mn]+0.49[Cu]+0.89[Ni]+0.40[Cr]-0.30[Si]...Formula (3) P4=P3×[V] / P2…Equation (4) The parameter P3, defined in equation (3), is an index indicating the degree of promotion of fine precipitation of V-type precipitates. Mn, Cu, Ni, and Cr, included in equation (3), are elements that promote fine precipitation of V-type precipitates by lowering the ferrite transformation temperature, while Si is an element that hinders fine precipitation of V-type precipitates. The coefficients of each element included in equation (3) represent their respective contributions to the promotion of fine precipitation of V-type precipitates.

[0026] The parameter P4 defined in equation (4) is an index that shows the degree of improvement in the yield ratio, with the above parameter P2 as the denominator and parameter P3 as the numerator. Here, the yield ratio is a value calculated as yield strength / tensile strength, and the yield strength is assumed to be 0.2% proof stress. As the value of P4 increases, the effect of increasing the 0.2% yield strength of the steel material by promoting the fine precipitation of V-type precipitates becomes greater. In this embodiment, the target 0.2% yield strength (800 MPa) can be secured by setting the value of P4 to 0.24 or higher. However, if the value of P4 becomes excessively high, the machinability deteriorates, so in this embodiment, the value of P4 is set to 0.30 or lower.

[0027] The metal structure is ferrite-pearlite, and the ferrite area ratio to the total structure is 42% or more. Under the alloy composition specified above, the non-heat-treated steel of this embodiment undergoes a pearlite transformation following a ferrite transformation during the cooling process in hot forging, resulting in the formation of a ferrite-pearlite structure as the metal structure. By creating a ferrite-pearlite structure, the yield ratio of the steel can be increased compared to, for example, a bainite structure. Furthermore, by setting the ferrite area ratio to the total structure to 42% or more, the machinability of the non-heat-treated steel without Pb and Bi additives can be improved. In this case, the metallic structure is preferably composed almost entirely of pearlite, with the exception of ferrite, and no other structures such as bainite are present. However, structures other than ferrite and pearlite, such as bainite and martensite, may be included as long as their area ratio is less than 5% of the total structure.

[0028] The conditions for hot forging of non-heat-treated steel materials in this embodiment are described below. The heating temperature during hot forging should be 1100°C or higher. This is to allow the V-type precipitates that had precipitated before hot forging to solidify in the steel. Preferably, the temperature is 1200-1300°C. Then, by hot forging at a processing termination temperature of 950°C or higher and then cooling, a ferrite-pearlite structure with a ferrite area ratio of 42% or more, in which V-type precipitates are finely precipitated, can be obtained.

[0029] 0.2% yield strength of 800 MPa or higher In this embodiment, the non-heat-treated steel material has a 0.2% yield strength of 800 MPa or higher. By ensuring this strength, it becomes possible to achieve weight reduction through increased strength in engine parts such as connecting rods. [Examples]

[0030] Next, embodiments of the present invention will be described below. Here, test materials with the alloy compositions of the embodiments and comparative examples shown in Tables 1 and 2 below were prepared, and the ferrite area ratio, 0.2% yield strength, and machinability were evaluated.

[0031] [Table 1]

[0032] [Table 2]

[0033] 1. Preparation of test materials Alloys with the compositions shown in Tables 1 and 2 were subjected to vacuum induction melting, and a 50 kg ingot was obtained by ingot casting. These were then forged into φ72 mm round bars, which were hot-forged to produce test materials. Specifically, the materials were heated and held at 1200°C for 60 minutes, followed by hot forging at a processing termination temperature of 950°C or higher (to reduce the size from φ72 mm to φ32 mm). Afterward, cooling was carried out so that the average cooling rate between 800°C and 600°C was 1.0 to 1.5°C / s. The test materials thus obtained were processed into test pieces suitable for each evaluation test, and various evaluations were performed.

[0034] 2. Evaluation 2-1. Ferrite area ratio The surface of the test material after hot forging was mirror-polished to allow cross-sectional observation at an intermediate position between the surface and the center (corresponding to half the radius), and then etching was performed with Picral. Three arbitrary fields of view were selected as observation surfaces, and the area percentage occupied by the ferrite structure was determined using an image analysis device with optical microscope images taken at an observation magnification of 100x. The average value of the three fields of view was evaluated as the ferrite area percentage. The evaluation results are shown in Tables 1 and 2.

[0035] 2-2.0.2% load-bearing capacity evaluation After hot forging, the test material was precision-machined to obtain a test specimen of type 14A as specified in JIS Z 2241. Subsequently, a tensile test was conducted in accordance with JIS Z 2241. The target 0.2% yield strength was 800 MPa. The evaluation results are shown in Tables 1 and 2.

[0036] 2-3. Machinability Evaluation Machinability was evaluated by measuring the amount of wear on the cutting tool when the above-mentioned test material was cut on a lathe. Carbide was used as the cutting tool. The cutting conditions were: cutting speed: 220 m / min, feed rate: 0.15 mm / rev, depth of cut: 0.65 mm. After a 2500 m cutting test, the average wear on the flank face of the cutting tool was measured, and if the value was 80 mm or less, it was judged as a pass (○), otherwise it was judged as a fail (×). The evaluation results are shown in Tables 1 and 2.

[0037] Based on the evaluation results shown in Tables 1 and 2, the following can be concluded.

[0038] Comparative Example 1 has a low C content and the value of parameter P2 is below the lower limit of this embodiment, so the target 0.2% yield strength is not achieved.

[0039] In Comparative Example 2, the value of parameter P2 is below the lower limit of this embodiment, and the target 0.2% yield strength is not achieved. Comparative Example 3 has a low V content, and the values ​​of parameters P2 and P4 are below the lower limit of this embodiment, so the target 0.2% yield strength is not achieved. Also, the value of parameter P1 is below the lower limit of this embodiment, and the ferrite area ratio is lower than 42%, so the target machinability is not achieved.

[0040] In Comparative Example 4, the values ​​of parameters P2 and P4 exceed the upper limit of this embodiment, and the target machinability is not achieved. Comparative Example 5 has high amounts of Mn and V, and the values ​​of parameters P2 and P4 exceed the upper limit of this embodiment, so the target machinability is not achieved.

[0041] In Comparative Example 6, the values ​​of parameters P2 and P4 exceed the upper limit of this embodiment, and the target machinability is not achieved. Comparative Example 7 has a high carbon content, the value of parameter P1 is below the lower limit of this embodiment, the ferrite area ratio is lower than 42%, and the value of parameter P2 is above the upper limit of this embodiment, so the target machinability is not achieved.

[0042] In Comparative Example 8, the amount of V is large, and the values ​​of parameters P2 and P4 exceed the upper limit of this embodiment, so the target machinability is not achieved.

[0043] In Comparative Example 9, the value of parameter P1 is below the lower limit of this embodiment, the ferrite area ratio is lower than 42%, and the target machinability is not achieved. In Comparative Example 10, the value of parameter P4 is below the lower limit of this embodiment, and the target 0.2% yield strength is not achieved.

[0044] As described above, in all of the comparative examples, the evaluation of either the 0.2% yield strength or machinability fell short of the target.

[0045] In contrast, Examples 1 to 37, whose alloy compositions including parameters P1, P2, and P4 fall within the range of this embodiment, achieve the target 0.2% yield strength and machinability. This demonstrates that, according to the non-heat-treated steel of this embodiment, high-strength parts can be manufactured without heat treatment while ensuring good machinability during part manufacturing.

[0046] Although embodiments and examples of the present invention have been described in detail above, the present invention is not limited to the above embodiments and examples, and various modifications are possible without departing from the spirit of the invention.

Claims

1. by mass C: 0.35% or more and 0.45% or less Si: 0.40% or more and 0.80% or less Mn: 0.40% or more and 0.90% or less P: 0.020% or more and 0.100% or less S: 0.040% or more and 0.070% or less Cr: more than 0.25% but not more than 0.70% V: 0.30% or more and 0.45% or less It contains, Cu: 0.40% or less Ni: 0.30% or less Mo: 0.10% or less Ca: 0.010% or less s-Al: 0.050% or less N: 0.030% or less It further contains at least one selected from, with the remainder being Fe and unavoidable impurities. The value of P1 obtained by the following formula (1) satisfies P1 ≥ 42, the value of P2 obtained by the following formula (2) satisfies 1.00 ≤ P2 ≤ 1.09, and the value of P4 obtained by the following formula (4) satisfies 0.24 ≤ P4 ≤ 0.

30. Non-heat-treated steel material having a ferrite-pearlite structure after hot forging, with a ferrite area ratio of 42% or more of the total structure, and a 0.2% yield strength of 800 MPa or more. P1=-175[C]-16[Si]-12[Mn]+51[V]+117...Formula (1) P2=[C]+0.07[Si]+0.16[Mn]+0.61[P]+0.19[Cu]+0.17[Ni]+0.20[Cr]+[V]...Formula (2) P3=[Mn]+0.49[Cu]+0.89[Ni]+0.40[Cr]-0.30[Si]...Formula (3) P4=P3×[V] / P2...Equation (4) However, in the formula, [ ] represents the mass percentage of the element contained in [ ].

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