JP2001200332A - High toughness non-heattreated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel combining high toughness and free cuttability and capable of contributing to the reduction of the production cost of machine parts and the miniaturizing and lightening of the parts in bainite type non- heattreated steel. SOLUTION: This high toughness non-heattreated steel has a composition containing, by weight, 0.01 to 0.50% C, 0.05 to 2.50% Si, 1.30 to 3.50% Mn, 0.50 to 2.50% Cr, 0.05 to 0.20% S, 0.003 to 0.060% Al, 0.003 to 0.025% N, and the balance Fe with inevitable impurities and contains sulfide inclusions with a cross-sectional area of 3 μm2 or more by 200 pieces/mm2 or more in the case of being observed in the cross-section parallel to the rolling direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bainite type non-heat treated steel having excellent toughness and machinability.

[0002]

2. Description of the Related Art Conventionally, steels for machine structures such as automobile parts have been quenched and tempered (hereinafter referred to as "tempering").
The microstructure has been converted to a martensite structure by processing, and has been used with the required strength and toughness. However, since the development of non-heat treated steel that can obtain the required strength without the quenching and tempering after rolling or hot forging has been developed, there are great merits such as cost reduction and process simplification. Replacement of steel with non-heat treated steel is progressing.

Generally, when the tensile strength is 950 MPa or less, a non-heat-treated steel having a ferrite-pearlite structure is often used, and in a higher strength region, a non-heat-treated steel mainly containing a bainite structure is often used. However, there is a problem that the non-heat treated steel basically has low toughness when compared with the tempered steel having a martensitic structure at the same strength.

[0004] As a measure for improving the toughness of the non-heat treated steel, it is common practice to reduce C and to add alloys such as Si, Mn, and Cr. In addition, MnS, Ti carbonitride, or Zr carbon is used. Dispersing nitride to reduce the grain size of old austenite crystal (Japanese Unexamined Patent Publication No. 7-3386, Patent No. 2
950702, JP-A-11-269599), a method of forming a carbonitride of V or Ti on MnS inclusions to promote ferrite formation in austenite crystal grains in a ferritic / pearlite type non-heat treated steel (JP-A-Hei. 7-157
824, Japanese Patent No. 2950713). For a bainite type non-heat treated steel, a method of simultaneously achieving a high yield ratio and a high toughness by aging after hot forging (Japanese Patent Laid-Open No. 6-287)
679).

[0005] However, even if these measures are taken, the ferrite-pearlite non-heat treated steel does not reach the toughness of a medium carbon alloy tempered steel, and even if the low carbon bainite non-heat treated steel is used. However, although the toughness approaches the tempered steel level, there is a problem that the machinability is poor due to the high tensile strength. Furthermore, the method of performing the aging treatment on the bainite type non-heat treated steel reduces the merit of simplifying the process as the non-heat treated steel.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the conventional non-heat-treated steel and provides both bainite-type non-heat-treated steel with high toughness and free-cutting properties without aging treatment. It is an object of the present invention to provide steel that can reduce the cost of manufacturing mechanical parts and contribute to reducing the size and weight of parts.

[0007]

According to the first aspect of the present invention, there is provided a method for solving the above-mentioned problems.
C: 0.01 to 0.50%, Si: 0.05 to 2.50
%, Mn: 1.30 to 3.50%, Cr: 0.50 to
2.50, S: 0.05-0.20, Al: 0.003
~ 0.060, N: 0.003-0.025%, the balance consists of Fe and unavoidable impurities, and 200 sulfide-based inclusions having a cross-sectional area of 3 µm ² or more when observed on the L plane / Mm ² or more.

[0008] According to the second aspect of the present invention, C: 0.
01 to 0.50%, Si: 0.05 to 2.50%, M
n: 1.30 to 3.50%, Cr: 0.50 to 2.5
0, S: 0.05-0.20, Al: 0.003-0.
060, N: 0.003-0.025%, Mo: 0.05-1.50%, V: 0.01-0.
50%, Nb: 0.01 to 0.10%, Ti: 0.01
Of sulfide-based inclusions containing at least one of 0.5% to 0.50%, the balance being Fe and unavoidable impurities, and having a cross-sectional area of 3 μm ² or more when observed on the L plane, 200 / mm High toughness non-heat treated steel characterized by containing ² or more.

According to the third aspect of the present invention, C: 0.
01 to 0.50%, Si: 0.05 to 2.50%, M
n: 1.30 to 3.50%, Cr: 0.50 to 2.5
0, S: 0.05-0.20, Al: 0.003-0.
060, N: 0.003-0.025%, Ca: 0.0005-0.01%, Mg: 0.00
05-0.01%, Zr:. 0005-0.30%, P
b: 0.02 to 0.30%, Bi: 0.01 to 0.30
%, B: one or two or more of 0.0003 to 0.015%, the balance being Fe and unavoidable impurities, and having a cross-sectional area of 3 μm ² or more when observed on the L plane. It is a high toughness non-heat treated steel characterized by containing at least 200 inclusions / mm ² .

According to the fourth aspect of the present invention, C: 0.
01 to 0.50%, Si: 0.05 to 2.50%, M
n: 1.30 to 3.50%, Cr: 0.50 to 2.5
0, S: 0.05-0.20, Al: 0.003-0.
060, N: 0.003-0.025%, Mo: 0.05-1.50%, V: 0.01-0.
50%, Nb: 0.01 to 0.10%, Ti: 0.01
０．５0.50%, one or more of which are contained, Ca: 0.0005 to 0.01%, Mg: 0.00
05-0.01%, Zr:. 0005-0.30%, P
b: 0.02 to 0.30%, Bi: 0.01 to 0.30
%, B: one or two or more of 0.0003 to 0.015%, the balance being Fe and unavoidable impurities, and having a cross-sectional area of 3 μm ² or more when observed on the L plane. It is a high toughness non-heat treated steel characterized by containing at least 200 inclusions / mm ² .

The reasons for adding and limiting the steel components of the present invention will be described below. In addition,% shows weight%.

C: 0.01 to 0.50% C is added to secure the strength of steel. But 0.
If it is less than 01%, the effect is insufficient, and if it exceeds 0.50%, the toughness is reduced.

Si: 0.05-2.50% Si is added to secure a deoxidizing effect and strength in steel making. However, if it is less than 0.05%, the deoxidizing effect is insufficient, and if it exceeds 2.50%, the hot workability deteriorates.
0.05 to 2.50%.

Mn: 1.30 to 3.50% Mn is added to obtain a bainite structure during air cooling after hot working and to refine the structure and improve the strength. However, if it is less than 1.3%, the ferrite / pearlite structure is likely to be transformed, and if it exceeds 3.5%, the workability is reduced, so that the content is made 1.30 to 3.50%.

Cr: 0.50 to 2.50% Cr acts in the same manner as Mn, refines the structure of bainite, and improves the strength. However, if it is less than 0.50%, the effect is small, and if it exceeds 2.50%, the cost increases, so it is set to 0.50 to 2.50%.

S: 0.05 to 0.20% S forms sulfides such as MnS and suppresses austenite grain growth when heated to 1000 ° C. or more for hot working. The bainite lath is randomized by the sulfides present in the steel, and the bainite structural unit is refined to increase the toughness. To obtain the effect, at least 0.05% or more is required. However, 0.
If it exceeds 20%, the toughness is degraded by the stress concentration effect of the sulfide, so the content is made 0.05 to 0.20%.

Al: 0.003 to 0.20% Al is added for deoxidation in steel making like Si.
In addition, it generates AlN and refines austenite grains.
To obtain the effect, 0.003% or more is necessary,
Even if added in excess of 0.025%, the effect is saturated, so the content is made 0.003 to 0.20%.

N: 0.003 to 0.025% N is added for toughening. In addition, a nitride such as AlN is generated to provide an effect of making austenite grains fine. To obtain the effect, 0.003% or more is necessary,
Even if added in excess of 0.025%, the effect is saturated, so the content is made 0.003 to 0.025%.

Next, Mo, V, N as selective elements
b and Ti will be described. Mo: 0.05 to 1.50% Mo performs the same function as Cr, refines the bainite structure, and improves the strength. However, if the content is less than 0.05%, the effect is small, and if it exceeds 1.50%, the cost increases, so the content is set to 0.05 to 1.50%. V: 0.01 to 0.50% Nb: 0.01 to 0.10% Ti: 0.01 to 0.50% V, Nb, and Ti form fine carbonitrides in the steel, Precipitates refine the austenite grain size during hot working and improve toughness. There is also an effect of improving the strength by strengthening the dispersion of these precipitates. The effect is V, Nb, T
There is no effect if both i are less than 0.01%. However, if added in a large amount, the toughness is deteriorated.
Is 0.10% and Ti is 0.50%, so
V: 0.01 to 0.50%, Nb: 0.01 to 0.10
%, Ti: 0.01 to 0.50%.

Further, the selected elements Ca, Mg, Zr, P
b, Bi, and B will be described. Ca: 0.0005 to 0.01% Mg: 0.0005 to 0.01% Zr: 0.0005 to 0.30% Pb: 0.02 to 0.30% Bi: 0.01 to 0.30% B: 0.0003 to 0.015% Ca, Mg, and Zr are sulfides, Pb and Bi are present alone or in combination with other inclusions, and B is present as a nitride. Further improve. Also, Ca, Mg,
Zr is a sulfide form controlling element and has an effect of improving mechanical anisotropy. These effects are Ca, Mg,
Zr is less than 0.0005%, Pb is less than 0.02%, B
i is less than 0.01%, B is less than 0.0003%, the effect is small, Ca and Mg are more than 0.01%, Zr, Pb, B
When i is more than 0.30% and B is more than 0.015, the effect is saturated, so Ca: 0.0005 to 0.01%, Mg:
0.0005-0.01%, Zr: 0.0005-0.
30%, Pb: 0.02 to 0.30%, Bi: 0.01
~ 0.30%, B: 0.0003 ~ 0.015%.

[0021]

Embodiments of the present invention will be described below. The most important toughening in the present invention can be achieved by dispersing a large amount of sulfide (MnS) in a bainite structure. Therefore, 0.05 to 0.25% of S is added to conventional steel or more, and the cross-sectional area is 3 μm when observed on the L plane.
m ² or more MnS it is necessary to contain 200 or more. In this case, a large number of sulfides present at the hot forging or rolling temperature suppress austenite crystal grain coarsening, further promote nucleation of bainite structure transformation, randomize bainite lath and refine bainite structure unit. In addition, the fracture unit at the time of impact fracture is miniaturized, and the toughness is greatly improved.

The free-machining, ie, high machinability, of the present invention will be described. In the present invention, since a large number of sulfides of 3 μm ² or more exist in steel, the following two effects are greatly contributed. .

In the primary shearing region at the time of cutting, sulfides are a source of stress concentration and facilitate generation of chips.

Since sulfide works as a lubricating or coating agent in chip-tool steel, cutting resistance is reduced, and mechanical wear and chemical wear of the tool are reduced.

Hereinafter, embodiments will be described through examples.

[0026]

EXAMPLE A non-heat treated steel having the composition shown in Table 1 was melted in a 100 kg vacuum melting furnace. The steel ingot was forged at 1200 ° C. into a bar having a diameter of 30 mm, allowed to cool, and then subjected to the following tests. Room temperature and -50 ° C Charpy impact test (JIS2m
mU notch impact test piece) Tensile test (JIS No. 4 tensile test piece) Machinability test (φ5mm high speed drill life test, cutting speed: 25m / min, feed: 0.1mm / rev, hole depth: 15
mm, dry)

[0027]

[Table 1]

[0028]

[Table 2]

Invention steels 1 to 13 are within the scope claimed by the present invention. Since the tensile strength levels are various, it is difficult to compare them unconditionally, but the room-temperature impact value of the steel of the present invention is 40 J
/ Mm ² or more, the impact value at −50 ° C. is 15 J / mm ² or more, the drill life is 50 holes or more, and it is possible to impart high toughness and high free cutting property to the air-cooled material after forging.

In the comparative steels 14 to 19, at least one or more items in the claims of the present invention are other than the claims.
The comparative steel 14 contains a large amount of C and a small amount of Mn, and the impact value is inferior to the inventive steel. Comparative steels 15, 16 and 17
Contains a large amount of Mn, Cr or Mo, and has large Ti carbonitrides, and thus is inferior in toughness and machinability.
The comparative steel 19 contains a small amount of S and has a small number of sulfides, so that the toughness and machinability are inferior despite its relatively low strength.

[0031]

As described above, the steel of the present invention has high toughness because a large amount of sulfide (MnS) is dispersed in the bainite structure. , And facilitates the generation of chips during cutting,
It has unprecedented effects such as reducing cutting resistance and reducing mechanical and chemical wear of tools.

Claims (4)

[Claims] C .: 0.01 to 0.50% by weight,
Si: 0.05 to 2.50%, Mn: 1.30 to 3.5
0%, Cr: 0.50 to 2.50, S: 0.05 to 0.
20, Al: 0.003 to 0.060, N: 0.003
Sulfide-based inclusions having a cross-sectional area of 3 μm ² or more when observed in a cross section parallel to the rolling direction (hereinafter referred to as “L plane”). A high-toughness non-heat treated steel containing at least 200 particles / mm ² . 2. C: 0.01 to 0.50% by weight,
Si: 0.05 to 2.50%, Mn: 1.30 to 3.5
0%, Cr: 0.50 to 2.50, S: 0.05 to 0.
20, Al: 0.003 to 0.060, N: 0.003
-0.025%, and Mo: 0.05-
1.50%, V: 0.01 to 0.50%, Nb: 0.0
1 to 0.10%, Ti: 1 out of 0.01 to 0.50%
Containing Fe or two or more, the balance being Fe and unavoidable impurities, and having a cross-sectional area of 3 μm ² when observed on the L plane.
A high-toughness non-heat treated steel containing the above-mentioned sulfide-based inclusions in an amount of 200 / mm ² or more. 3. C: 0.01 to 0.50% by weight,
Si: 0.05 to 2.50%, Mn: 1.30 to 3.5
0%, Cr: 0.50 to 2.50, S: 0.05 to 0.
20, Al: 0.003 to 0.060, N: 0.003
-0.025%, and Ca: 0.0005
-0.01%, Mg: 0.0005-0.01%, Z
r:. 0005-0.30%, Pb: 0.02-0.3
0%, Bi: 0.01 to 0.30%, B: 0.0003
One or two or more of the sulfide-based inclusions having a cross-sectional area of 3 μm ² or more when observed on the L plane.
High toughness non-heat treated steel characterized by containing at least ² pieces / mm ² . 4. C: 0.01 to 0.50% by weight,
Si: 0.05 to 2.50%, Mn: 1.30 to 3.5
0%, Cr: 0.50 to 2.50, S: 0.05 to 0.
20, Al: 0.003 to 0.060, N: 0.003
-0.025%, and Mo: 0.05-
1.50%, V: 0.01 to 0.50%, Nb: 0.0
1 to 0.10%, Ti: 1 out of 0.01 to 0.50%
Or two or more species, and Ca: 0.000
5 to 0.01%, Mg: 0.0005 to 0.01%, Z
r:. 0005-0.30%, Pb: 0.02-0.3
0%, Bi: 0.01 to 0.30%, B: 0.0003
One or two or more of the sulfide-based inclusions having a cross-sectional area of 3 μm ² or more when observed on the L plane.
High toughness non-heat treated steel characterized by containing at least ² pieces / mm ² .