JP4784103B2 - Non-tempered steel with high yield strength ratio - Google Patents

Description

The present invention relates to a non-heat treated steel having a high yield strength ratio as a material for machine parts and a method for producing machine parts using the same. Here, “bearing ratio” means a ratio of (0.2% yield strength / tensile strength).

Needless to say, material strength is an important factor in the design and manufacture of machine parts. On the other hand, machinability affects manufacturing costs and component accuracy, which is also an important factor. In general, machinability decreases with increasing tensile strength. Therefore, in the case of a component with a proof stress design, a high proof stress ratio means that a component having a higher strength can be manufactured from a material having equivalent machinability.

Conventionally, when parts such as connecting rods are manufactured using non-tempered steel, the yield strength ratio is the upper limit of 0.75 level under the forging / air cooling manufacturing conditions at a normal heating temperature of around 1200 ° C. Select the manufacturing conditions of low temperature forging and air cooling at around 1050 ° C, use of blast cooling that responds to the need to increase the cooling rate after forging, which may reduce the life of the forging die, cause the product to be thin. Even if non-refined steel parts are manufactured with an increase in manufacturing cost, etc., the improvement that can be obtained is slight, and the yield ratio that can be realized is limited to around 0.80, and a greater yield ratio is required. In some cases, tempered steel was generally used.

Regarding the manufacture of connecting rods, improvements have been implemented in which the cap portion is configured not by mechanical cutting but by brittle fracture. A low-ductility non-heat treated steel suitable for this purpose has been proposed with high strength and high yield ratio (Patent Document 1). The steel is C: 0.20-0.65%, Si: 0.1-1.5%, Mn: 0.3-2.0, P: 0.02-0.15%, S: 0 0.1 to 0.105%, Cr: 0.02 to 1.50%, V: 0.05 to 0.50%, Nb: 0 to 0.10%, Ti: 0 to 0.20%, Al: 0 to 0.100% and N: 0 to 0.02%, consisting of the balance Fe and inevitable impurities, Si (%) + 2V (%) + 5P (%) − 0.8 ≧ 0 and C (%) It is characterized by an alloy composition of + Si (%) / 10 + Mn (%) / 5 + 5Cr (%) / 22 + 1.65 V (%)-5S (%) / 7-0.8 ≧ 0. The yield ratio level secured by the present invention is “0.7 or more”. (Since no definition of “yield ratio” is given, the relationship with the intended strength ratio is not clear, but similar physical properties were understood.)
JP 9-1111412 A

The inventors have obtained the following findings as a result of various studies with the intention of realizing a non-heat treated steel that reliably shows a proof steel level proof stress ratio of 0.80 or more.
-Si and P for strengthening ferrite by solid solution strengthening and V for strengthening ferrite by precipitation strengthening should be positively added.
-In order to effectively exhibit these strengthening mechanisms, it is preferable to add appropriate amounts of S and P so that the ferrite to be produced is refined together with the amount of ferrite. A high yield strength ratio is obtained by the combined effect of P and S.
-When the alloy composition of steel is in a specific range, if the condition of Mn + Cr + 2Mo: 1.8 or less is satisfied, generation of a bainite structure that is harmful to machinability can be suppressed.
In such steel, if the condition that the value of carbon equivalent Ceq represented by C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + 0.4Mo + V is satisfied is 0.80 to 1.15 or more, normal forging -A ferrite-pearlite type non-heat treated steel part having a hardness of 20 HRC or more generally required for parts is obtained by air cooling.
-The cooling rate after forging should be higher within the range where no bainite structure is generated. It is desirable to cool the temperature range of at least 800 to 500 ° C. at an average value of 0.8 ° C./sec or more, thereby further improving the yield ratio. For this, blast cooling is effective.

The purpose of the present invention is to always use the new knowledge obtained by the above-described inventors, even when performing normal forging at around 1200 ° C. or forging at a high temperature of about 1250 ° C. An object of the present invention is to provide a non-heat treated steel with a ratio of 0.80 or more and a method of manufacturing a machine part using the same. Accordingly, the final purpose is to make the machine parts compact and lightweight, and to suppress an increase in manufacturing cost.

In a basic aspect, the high yield strength non-heat treated steel of the present invention is in mass % (hereinafter the same) , C: 0.10 to 0.35%, Si: 0.4 to 1.0%, Mn : 0.6 to 1.5%, P: 0.06 to 0.15%, S: 0.07 to 0.15%, Cu: 0.05 to 0.50%, Ni: 0.05 to 0 .50%, Cr: 0.05 to 0.50%, Mo: (including 0%) 0.05% or less, Al : 0.001 to 0.050% , V: 0.20 to 0.45% And N: 0.008 to 0.025%, and having an alloy composition consisting of the balance Fe and inevitable impurities,
P + S: 0.14% or more,
Mn + Cr + 2Mo: 1.8% or less,
Ceq = C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + 0.4Mo + V: 0.80 to 1.15
And, the structure after forging mainly consists of (ferrite + pearlite) , and is a non-tempered steel showing a proof stress ratio of 0.80 or more.

The high yield strength untempered steel of the present invention reliably achieves a high yield ratio of 0.8 or more without degrading the machinability of the existing untempered steel by selecting an appropriate alloy composition. Therefore, it is possible to manufacture mechanical parts with high yield strength with the same processability as before. This high yield strength ratio is always obtained even when low-temperature forging is not selected and normal forging at around 1200 ° C. or high-temperature forging at about 1250 ° C. is performed.

As a result, the machine parts can be reduced in size and weight. This not only has the effect of improving the performance of the component itself and reducing the manufacturing cost, but also contributes to reducing the size and weight of the machine in which the component is incorporated. In forging, normal heating temperature or higher heating temperature can be adopted, and it is not necessary to perform low-temperature forging, so that a long forging die life can be enjoyed, which also has an advantageous effect on component manufacturing costs. To do.

The high yield strength non-heat treated steel of the present invention can contain one or two or more optional additional elements belonging to the following groups, in addition to the basic alloy components described above.
(I) One or two elements of Nb: 0.001 to 0.050% and Ti: 0.001 to 0.050% These elements have a grain refining action, so the forging temperature is 1250 ° C or higher When the temperature is higher than the above, it is an optional additive component that is recommended when it is desired to avoid a situation in which the structure of the molded product is coarse and a desired yield strength ratio cannot be obtained.
(II) One or two of Ca: 0.0001 to 0.0100 % and O: 0.001 to 0.010 %. These serve to control the form of inclusions. It may be added when the machinability is low depending on the alloy composition or when high machinability is desired.

Hereinafter, the basic alloy composition and the modification mode will be described for the function of each alloy component in the high yield strength non-heat treated steel of the present invention and the reason for limiting the alloy composition as described above.

C: 0.10 to 0.35%
C is necessary for securing the strength, and at least 0.10% is present. When added in a large amount, the yield ratio decreases, so the upper limit was made 0.35%.

Si: 0.4 to 1.0%
Si acts as a deoxidizer when steel is melted, and solid-dissolves in ferrite to strengthen it, thereby improving proof stress and strength. In order to clearly obtain such an effect, it is necessary to contain 0.4% or more of Si. If added in a large amount, the life of the forging die is shortened, so the addition is limited to 1.0%.

Mn: 0.6 to 1.5%
Mn is not only necessary for deoxidation, but also has the effect of improving strength. This effect cannot be obtained with a small content of less than 0.6%. On the other hand, if the content is too large, bainite is generated and the machinability is lowered, so an addition amount of 1.5% or less is selected.

P: 0.06-0.15%
S: 0.07 to 0.15%
However, P + S: 0.14% or more P is an element effective in improving the yield ratio, and in order to exert its effect, it is necessary to contain 0.06% or more, but it is contained in a large amount. Then, since hot workability becomes low, the upper limit was made 0.15%. S is also an element effective for improving the yield ratio, and in order to exert its effect, it is necessary to contain 0.07% or more. However, if it is contained in a large amount, the yield ratio is lowered. To do. In order to ensure the yield strength ratio of 0.8 or more intended in the present invention, the appropriate upper limit is 0.15% for the amount of S. The total amount of P + S should be 0.14% or more in order to obtain a yield ratio of 0.8 or more.

Cu: 0.05 to 0.50%
Ni: 0.05 to 0.50%
Both Cu and Ni are effective components for improving the proof stress ratio, as is the case with P, and the significance of their existence is found at a content of 0.05% or more, but adding a large amount is economically disadvantageous. From the range up to 0.50% selected as the upper limit of each, the addition amount is selected.

Cr: 0.05 to 0.50%
Cr has an effect of improving strength. However, if the content is less than 0.05%, the desired effect cannot be obtained, and if it is too much, bainite is generated and the machinability is impaired, so the addition amount in the range of 0.05 to 0.50% Is appropriate.

Mo: 0.05% or less Mo is an element that improves the strength. However, if it is too much, bainite is formed, which adversely affects machinability, so 0.05% was made the upper limit of addition.

V: 0.20 to 0.45%
V, like P and Cu and Ni, is an effective component for improving the yield ratio. In order to stably obtain a yield ratio of 0.8 or more, V of 0.2% or more is added. Even if it is added in a large amount, its effect is saturated and disadvantageous in terms of cost, so it is advantageous to add up to 0.45%.

N: 0.008 to 0.025%
N is useful in that it produces nitrides or carbonitrides with V, Al, Ti, Nb, etc., thereby refining the crystal grains and consequently improving the yield strength. In order to obtain this effect, N of 0.008% or more is necessary. If an attempt is made to add a large amount, blow holes may occur and a healthy steel ingot may not be obtained, so the upper limit was made 0.025%.

Mn + Cr + 2Mo: 1.8% or less If this value exceeds 1.8, a large amount of bainite precipitates, worsening the machinability and lowering the yield ratio, so Mn + Cr + 2Mo is limited to this limit.

Ceq = C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + 0.4Mo + V: 0.80 to 1.15
From the viewpoint of ensuring strength, most of the non-heat treated steel parts need to have a hardness of 20 HRC or more, while from the viewpoint of machinability, it is necessary that the hardness is up to about 25 HRC. . Aside from large parts such as truck crankshafts, ordinary non-heat treated steel parts have an average cooling rate in the temperature range of 800-500 ° C after forging of about 0.7 ° C / sec (equivalent steel material diameter). Is about 35 mm) or more. Under such conditions, in order to obtain a desired hardness of 20 HRC or more, the value of the carbon equivalent Ceq needs to be 0.80 or more. When Ceq exceeds 1.15, the hardness becomes too high and the machinability deteriorates. Therefore, the range of Ceq is set to the above 0.80 to 1.15.

One or two types of Nb: 0.001 to 0.050% and Ti: 0.001 to 0.050% Nb and Ti produce nitrides and carbonitrides, and refine crystal grains after forging However, when added in a large amount, the strength decreases. The appropriate addition amount is 0.001 to 0.050% for all.

One or two of Ca: 0.0001 to 0.0100% and O: 0.001 to 0.010% Ca and 0 have an action of controlling the form of MnS inclusions, thereby suppressing tool wear. And has the effect of improving machinability. In order to obtain this effect, Ca: 0.0001% or more and O: 0.001% or more are required. When added in a large amount, both lower the hot workability, so the upper limit was made 0.010% for both.

Steels having the alloy compositions (% by weight, balance Fe) shown in Table 1 (Examples) and Table 2 (Comparative Examples) are vacuum-melted into steel pieces according to a conventional method, and then heated to 1200 ° C. or higher. The steel was hot forged in a temperature range of 1200 to 1000 ° C. to form a round bar with a diameter of 25 mm, and then air-cooled to room temperature. At this time, the average cooling rate in the temperature range of 800 to 500 ° C. was 0.8 ° C./sec.

A JlS4 test piece was cut out from the hot forged / air-cooled material and subjected to a tensile test at room temperature. At the same time, samples for hardness measurement and microstructure were collected from the portion adjacent to the position where the tensile test piece was collected, and the hardness measurement and microstructure were observed. The results are shown in Table 3 (Examples) and Table 4 (Comparative Examples) together with the values of P + S and Mn + Cr + 2Mo.

The high yield strength non-heat treated steel of the present invention is typically optimal as a material for producing parts such as a crankshaft of an automobile engine and a connecting rod because of its high yield strength and low production cost. In addition, it is useful as a material for various machine parts that are required to be small and lightweight.

Claims (4)

% By mass (hereinafter the same) , C: 0.10 to 0.35%, Si: 0.4 to 1.0%, Mn: 0.6 to 1.5%, P: 0.06 to 0.15 %, S: 0.07 to 0.15%, Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Mo: (0 % hints) 0.05% or less, Al: 0.001 to 0.050%, V: 0.20 to 0.45% and N: containing 0.008 to 0.025%, the balance Fe and unavoidable Having an alloy composition of mechanical impurities, provided that
P + S: 0.14% or more,
Mn + Cr + 2Mo: 1.8% or less,
Ceq = C + 0.07Si + 0.16Mn + 0.61P + 0.19Cu + 0.17Ni + 0.2Cr + 0.4Mo + V: 0.80 to 1.15
And the structure after forging mainly consists of (ferrite + pearlite), non-heat treated steel showing a proof stress ratio of 0.80 or more. The alloy according to claim 1, wherein the alloy contains one or two of Nb: 0.001 to 0.050% and Ti: 0.001 to 0.050% in addition to the alloy components according to claim 1. Yield ratio non-heat treated steel. The alloy contains one or two of Ca: 0.0001 to 0.0100% and O: 0.001 to 0.010% in addition to the alloy components according to claim 1 or 2. Or high proof ratio non-heat treated steel of 2. The non-tempered steel having the alloy composition according to any one of claims 1 to 3 is formed into a machine part shape by hot working, and a temperature range of 800 to 500 ° C is averaged to 0.8 ° C / sec or more. A method for manufacturing a machine part exhibiting a high yield strength ratio comprising cooling at a speed of 5 mm.