JPH07316737A - Air-cooled type martensitic strength non-refining steel for hot forging - Google Patents

Abstract

PURPOSE:To produce a non-refining steel capable of obtaining high strength and high toughness with the amt. of expensive alloy components reduced and without executing heat treatment. CONSTITUTION:This air-cooled type martensitic high strength non-refining steel for hot forging is the one having a compsn. contg. 0.04 to 0.25% C, 0.01 to 0.50% Si, 2.00 to 3.50% Mn, 0.001 to 0.030% P, 0.05 to 1.50% Cr, 0.01 to 0.50% Mo and 0.010 to 0.100% Al, contg., at need, at least one component selected from the group of 0.003 to 0.050% Nb, 0.003 to 0.030% Ti, 0.01 to 0.30% V and 0.0002 to 0.0030% B and/or at least one component selected from the group of <=0.10% S, 0.05 to 0.30% Pb and 0.0010 to 0.0100% Ca in addition, and the balance Fe with inevitable impurities, and contg. >=30% martensite by air cooling. If desired, the chemical componental compsn. of the non-refining steel having the same constitution furthermore satisfies the inequality; 15<=3Cr+8Mo+4.5Mn<=16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated steel for hot forging, which is applied to mechanical structural parts which require toughness.

[0002]

2. Description of the Related Art Steel bars, which are used as materials for automobiles and parts for machine structures used in industries, civil engineering, construction machinery, etc., are required to have certain strength and toughness. Conventionally, such strength and toughness (referred to as toughness) are given to a steel bar by hot forging, followed by quenching and tempering. However, in recent years
Non-heat treated steel for hot forging has been developed for the purpose of reducing the production cost by omitting the manufacturing process. According to such a non-heat treated steel for hot forging, the heat treatment step can be omitted and the production cost can be reduced.

A conventional non-heat treated steel for hot forging is made of carbon steel having a ferrite / pearlite structure. For example, as disclosed in JP-A-59-9122, V alone is used. Addition or combined addition of V and Ti and rational use of alloying elements such as C and Mn have been made to improve the reduction of strength and toughness due to omission of heat treatment (hereinafter referred to as prior art).

[0004]

However, in response to recent global environmental protection measures, fuel consumption has been reduced to reduce the weight of automobiles, and the application of non-heat treated steel for hot forging to machine structural parts has been expanded. For non-heat treated steel for hot forging,
It exceeds the performance range of conventional non-heat treated steel, for example, has a tensile strength of 880 N / mm ² or more and a Charpy impact value (JI
No. S3 test piece, the same hereinafter) is 60 J / cm ² at 20 ° C.
The above high toughness has come to be required. However, in the prior art, the toughness deteriorates significantly as the strength increases, and the Charpy impact value at room temperature is 50 J at 20 ° C.
/ Cm ² is low. Therefore, depending on the prior art, for example, high tensile strength of 880 N / mm ² or more and high toughness of 60 J / cm ² or more at an impact value of 20 ° C.
It is difficult to satisfy both.

Therefore, an object of the present invention is to solve the above-mentioned problems and to have a tensile strength of 880 N / mm ² or more,
Moreover, the impact value is 60 J / cm ² or more at 20 ° C,
It is to provide a non-heat treated steel for hot forging having high strength and high toughness.

[0006]

From the above-mentioned viewpoint, the present inventors have proposed a non-heat treated steel for hot forging using martensite as a means for achieving high strength and high toughness of structural parts. As a result of earnest studies for development, the following points were found to be important in the production of martensitic non-heat treated steel for hot forging. (1) To reduce the carbon content in order to achieve the toughness of martensite. (2) In order to reduce quenching strain, a predetermined amount of martensite necessary for achieving toughening is generated by air cooling.

(3) The amount of martensite required to achieve toughening is 30%, and the amount of alloy added may be smaller than the amount of alloying components required to obtain 100% martensite. .

The present invention was made on the basis of the above findings, and the air-cooled martensitic toughness non-heat treated steel for hot forging of the present invention has carbon (C): 0.04 to 0.04. 25 wt%, silicon (Si): 0.01 to 0.50 wt%, manganese (Mn): 2.00 to 3.50 wt%, phosphorus (P): 0.001 to 0.030 wt%, chromium ( Cr): 0.05 to 1.50 wt%, molybdenum (Mo): 0.01 to 0.50 wt%, and aluminum (Al): 0.010 to 0.100 wt%, and if necessary. , Niobium (Nb): 0.003 to 0.050 wt%, titanium (Ti): 0.003 to 0.030 wt%, vanadium (V): 0.01 to 0.30 wt%, and boron (B): 0.0002-0.0030wt% At least one component selected from the group consisting of
Alternatively, at least one selected from the group consisting of sulfur (S): 0.10 wt% or less, lead (Pb): 0.05 to 0.30 wt%, and calcium (Ca): 0.0010 to 0.0100 wt%. One component is additionally contained, the balance consists of iron (Fe) and inevitable impurities, and after hot forging, 30% or more of martensite is contained by air cooling.

A more desirable air-cooled martensitic toughness non-heat treated steel for hot forging according to the present invention is the same as in each of the above-mentioned inventions, except that the chemical composition of the non-heat treated steel is
It is characterized by further satisfying the formula: 15 ≦ 3 × Cr + 8 × Mo + 4.5 × Mn ≦ 16 (1).

[0010]

[Action]

(1) Carbon (C): Carbon is an element that enhances hardenability, promotes the production of martensite, and determines the strength of martensite. If the carbon content is less than 0.04 wt%, it is impossible to secure a tensile strength of 880 N / mm ² or more. On the other hand, if the carbon content exceeds 0.25 wt%, the toughness of martensite itself deteriorates, and it is not possible to achieve high toughness. Therefore, the carbon content is
It should be limited to the range of 0.04 to 0.25 wt%.

(2) Silicon (Si): Silicon is an element having a deoxidizing action. If the silicon content is less than 0.01 wt%, the desired effect cannot be obtained. On the other hand, the silicon content is 0.50
If it exceeds wt%, the toughness deteriorates. Therefore, the silicon content should be limited to the range of 0.01 to 0.50 wt%.

(3) Manganese (Mn): Manganese is an element most effective in the production of martensite, and is an important component of the steel of the present invention that efficiently moves the martensite formation critical cooling rate to the low speed side. is there. If the manganese content is less than 2.00 wt%, martensite formation by air cooling will be insufficient. On the other hand, the manganese content is 3.50 wt%
If it exceeds, the martensite structure will be perfect, and further addition will have no effect and the manufacturing cost will increase.
Therefore, the manganese content is 2.00 to 3.50 wt%
Should be limited to within the range.

(4) Phosphorus (P): Phosphorus content of 0.030
Since the toughness is deteriorated when the content exceeds wt%, the phosphorus content should be as low as possible. However, the lower limit of the phosphorus content that can be industrially achieved is 0.001 wt%. Therefore, the phosphorus content should be limited to the range of 0.001 to 0.030 wt%.

(5) Chromium (Cr): Chromium is an element effective for improving hardenability and ensuring strength.
It is an important component of the present invention. Chromium content is 0.05wt
If it is less than%, the desired tensile strength cannot be obtained. On the other hand, if the chromium content exceeds 1.50 wt%, the toughness deteriorates. Therefore, the chromium content is 0.05-1.50 wt.
It should be limited to the range of%.

(6) Molybdenum (Mo): Molybdenum not only enhances hardenability but also strengthens martensite.
It is an important component of the steel of the present invention in order to improve the toughness balance. However, the molybdenum content is 0.01 w
If it is less than t%, the desired tensile strength cannot be obtained. On the other hand, if the content exceeds 0.50 wt%, the toughness deteriorates. Therefore, the molybdenum content is 0.01 to 0.50.
It should be limited to the range of wt%.

(7) Aluminum (Al): Aluminum has a deoxidizing action. Aluminum content is 0.0
If it is less than 10% by weight, the desired effect cannot be obtained in the above-mentioned operation. On the other hand, the aluminum content is 0.100 wt%
If it exceeds, the toughness deteriorates. Therefore, the aluminum content should be limited to within the range of 0.010 to 0.100 wt%.

(8) Niobium (Nb): Niobium is a carbonitride forming element, but during heating of the steel material before hot forging,
It forms a solid solution in austenite, and this solid solution niobium has a function of enhancing hardenability. However, if the niobium content is less than 0.003 wt%, such an effect cannot be obtained. On the other hand, when the niobium content exceeds 0.050 wt%, the effect of improving the hardenability by the solid solution Nb is saturated, and conversely, undissolved Nb carbonitride is generated during the heating, and the hardenability is deteriorated. . Therefore, the niobium content is 0.003 to
It should be limited to the range of 0.050 wt%.

(9) Titanium (Ti): Titanium does not form a solid solution in austenite during heating of the steel material before hot forging, suppresses the growth of crystal grains, and has a martensite structure formed during subsequent air cooling. Miniaturize. However, if the titanium content is less than 0.003 wt%, the desired effect cannot be obtained in the above-described action. On the other hand, its content is 0.030wt
%, The toughness deteriorates due to the excessive precipitates (TiC, TiN) of coarse titanium formed in the melting stage. Therefore, the titanium content is 0.003 to 0.03.
It should be limited to the range of 0 wt%.

(10) Vanadium (V): Vanadium is an element that improves hardenability. Therefore, vanadium is additionally contained if necessary. However, if the vanadium content is less than 0.01 wt% or more than 0.30 wt%, the desired effect cannot be obtained in the above-described action. Therefore, the vanadium content is 0.01 to
It should be limited to the range of 0.30 wt%.

(11) Boron (B): Boron is an element that improves the hardenability when it forms a solid solution in steel. Therefore, if necessary, boron is additionally contained. However, the boron content is 0.0002 wt%
If the amount is less than the above, a sufficient amount of solid solution B cannot be obtained, and the desired effect cannot be obtained in the above-described action. On the other hand, the boron content is 0.0
If it exceeds 030 wt%, boron precipitates as carbon boride, and conversely the hardenability deteriorates. Therefore, the boron content should be limited to the range of 0.0002 to 0.0030 wt%.

(12) Sulfur (S), lead (Pb) and calcium (Ca): Sulfur, lead and calcium have a function of imparting free-cutting property. Therefore, when machinability is required in the process of processing into a component, at least one element of sulfur, lead and calcium is additionally contained, if necessary. Lead content 0.0
Less than 5 wt%, calcium content 0.0010 wt%
If it is less than 1, the free-cutting property cannot be improved. on the other hand,
Sulfur content is over 0.10wt%, lead content is 0.30w
More than t% or calcium content 0.0100wt
If it exceeds%, the toughness is significantly deteriorated. Therefore, the sulfur content is 0.10 wt% or less, and the lead content is 0.05-0.
It should be limited to the range of 30 wt% and the calcium content to the range of 0.0010 to 0.0100 wt% respectively.

(13) Relationship between the respective contents of Cr, Mo and Mn: Cr, Mo and Mn all have a hardenability improving action and enhance strength. However, even if these chemical component compositions are individually within the scope of the present invention, if the sum of each is too large, the toughness may be deteriorated. Each content of Cr, Mo and Mn is as follows (1)
When the formula: 15 ≦ 3 × Cr + 8 × Mo + 4.5 × Mn ≦ 16 (1) is satisfied, it is possible to stably manufacture a steel material having excellent strength without causing deterioration in toughness. . Therefore, the contents of Cr, Mo and Mn are the same as those in (1) above.
It is desirable to have a relationship that satisfies the formula.

(14) Martensite The martensite content needs to be 30% or more. If the martensite content is less than 30 wt%,
A non-heat treated steel for hot forging having a tensile strength of 880 N / mm ² or more and excellent toughness cannot be obtained.

As described above, according to the present invention, in order to contain martensite in an amount of 30% or more, an appropriate amount of C, Mn, Cr, Mo and, if necessary, Nb, V and a trace amount of B are added. As a result, the hardenability is improved. Further, by adding Ti, by refining the austenite grains to make the martensite fine, hot forging excellent in strength and toughness containing 30% or more of tough martensite by air cooling. Non-heat treated steel can be obtained.

The air cooling is mainly carried out by cooling in the atmosphere, but it may be carried out by the cooling with an air blow using a fan or the like. The steel of the present invention is not limited to hot forging,
It is also applicable to steel materials that are air-cooled after other hot working such as rolling, for example, non-heat treated steel for direct cutting. Further, after hot forging, it can be applied even when subjected to strong cooling such as water cooling or oil cooling, because it shows sufficient hardenability, and it can be applied after hot forging, air cooling, and tempering. It is also possible to improve performance.

[0026]

EXAMPLES Next, the non-heat treated steel of the present invention will be described in more detail by way of examples, in comparison with comparative steels outside the scope of the present invention. After smelting the present invention steels No. 1 to 16 having the chemical composition within the scope of the present invention and the comparative steels No. 1 to 5 having the chemical composition outside the scope of the present invention shown in Table 1. By rolling, diameter 30mm or 72mm
It was finished into a round bar shape and was used as the material for hot forging. After heating the obtained material to 1200 ° C, it was hot forged by a swaging machine to form a round bar having a surface reduction rate of 30%, and then allowed to cool. Thus, the diameter is 25 mm or 6
A 0 mm round bar was prepared.

[0027]

[Table 1]

The steel No. 1 of the present invention thus prepared
In order to investigate the mechanical properties of the round bar made of ~ 16 and the round bar made of comparative steel No. 1-5, tensile test and Charpy impact test (processed into JIS No. 3 test piece,
The test was conducted at 20 ° C. Table 2 shows the martensite content and mechanical properties of the inventive steel and the comparative steel, and the diameter of the round bar.

[0029]

[Table 2]

As is apparent from Tables 1 and 2, the steels No. 1 to 16 of the present invention having the chemical composition within the scope of the present invention.
Has a tensile strength of 880 N / mm after hot forging.
^It had a strength of ² or more and an absorbed energy at 20 ° C., that is, an impact value of 60 J / cm ² or more, and had high strength and high toughness mechanical properties.

On the other hand, the comparative steel No. 1 has a C content of 0.
Since it is less than 04 wt%, the Mn content is less than 2.00 wt%, and the value of the formula (1) is as low as 11.0, the desired martensite hardness cannot be obtained, and the martensite is not obtained. The content was 25%, which was low outside the range of the present invention, and the tensile strength was 860 N / mm ² , and the desired tensile strength could not be obtained.

Comparative steel No. 2 has a C content of 0.27 wt%,
Since the amount of Mn is 3.65 wt%, which is outside the range of the present invention, and the value of the formula (1) is as high as 18,
The impact value was as low as 25 J / cm ^2, and the desired toughness could not be obtained.

Comparative steel No. 3 has a Si content of 0.55 wt.
%, Cr amount is 1.56 wt%, Mo amount is 0.60 wt%
However, the impact value is 40 J / c because there are many values outside the range of the present invention and the value of the formula (1) is as high as 22.6.
m ² was so low that the desired toughness could not be obtained.

Comparative steel No. 4 had a large deoxidizing element Al content of 0.105 wt%, which was outside the range of the present invention, and thus had a low impact value of 35 J / cm ² and only a low toughness. I couldn't get it.

On the contrary, in the comparative steel No. 5, the Nb amount is 0.055 wt% and the Ti amount is 0.
Since the amount was 035 wt%, which was outside the range of the present invention, the impact value was as low as 35 J / cm ^2, and only low toughness could be obtained.

[0036]

As described above, according to the present invention,
High tensile strength with a tensile strength of 880 N / mm ² or more and a Charpy impact value of 60 J / cm ² or more at 20 ° C., with less addition of expensive alloy components and without heat treatment. An air-cooled martensitic toughness non-heat treated steel for hot forging having a toughness is obtained, and an industrially useful effect that can contribute to cost reduction is brought about.

Claims (5)

[Claims] 1. Carbon (C): 0.04 to 0.25 wt%, silicon (Si): 0.01 to 0.50 wt%, manganese (Mn): 2.00 to 3.50 wt%, Phosphorus (P): 0.001 to 0.030 wt%, chromium (Cr): 0.05 to 1.50 wt%, molybdenum (Mo): 0.01 to 0.50 wt%, and aluminum (Al): An air-cooled type which contains 0.010 to 0.100 wt% and the balance is iron (Fe) and unavoidable impurities, and contains 30% or more of martensite by air cooling after hot forging. Martensitic toughness non-heat treated steel for hot forging. 2. Niobium (Nb): 0.003 to 0.050 wt%, Titanium (Ti): 0.003 to 0.030 wt%, Vanadium (V): 0.01 to 0.30 wt%, and boron. (B): The air-cooled martensitic toughness non-heat treated steel for hot forging according to claim 1, which additionally contains at least one component selected from the group consisting of 0.0002 to 0.0030 wt%. 3. Sulfur (S): 0.10 wt% or less, lead (Pb): 0.05 to 0.30 wt%, and calcium (Ca): 0.0010 to 0.0100 wt% The air-cooled martensitic toughness non-heat treated steel for hot forging according to claim 1, which additionally contains at least one component. 4. Niobium (Nb): 0.003 to 0.050 wt%, Titanium (Ti): 0.003 to 0.030 wt%, Vanadium (V): 0.01 to 0.30 wt%, and boron. (B): at least one component selected from the group consisting of 0.0002 to 0.0030 wt%, and further, sulfur (S): 0.10 wt% or less, lead (Pb): 0.05 to 0.30 wt% And calcium (Ca): at least one component selected from the group consisting of 0.0010 to 0.0100 wt%, for air-cooled martensite toughness hot forging according to claim 1. Non-heat treated steel. 5. The chemical composition of the non-heat treated steel further satisfies the following formula (1): 15 ≦ 3 × Cr + 8 × Mo + 4.5 × Mn ≦ 16 (1) The air-cooled martensitic toughness non-heat treated steel for hot forging according to any one of claims 1, 2, 3, and 4.