JPS61279656A - Non-heattreated steel for hot forging - Google Patents

Abstract

PURPOSE:To provide a non-heattreated steel with a high impact value by subjecting a steel containing specific amounts of C, Si, Mn, V, Nb, Al and N to hot forging and then to cooling so as to form a mixed structure of ferrite and bainite. CONSTITUTION:The steel consisting of, by weight, 0.20-0.50% C, 0.10-1.00% Si, 1.0-2.0% Mn, 0.05-0.30% V, 0.01-0.06% Nb, 0.01-0.06% sol Al, 0.01-0.03% N and the balance Fe with inevitable impurities is refined, which is cast into a steel ingot and then rolled. On subjecting to hot forging and then to air cooling, this stock is formed into the mixed structure of ferrite and bainite or a single phase structure of bainite. If necessary, >=1 kind among 0.30-1.0% Cr, 0.3-2.0% Ni and 0.05-0.50% Mo or further proper amounts of S, Pb, Te, Bi, Ca, etc., are incorporated to the above steel. Owing to the above composition of the non-heattreated steel, deterioration in toughness in the high temp. region can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to non-thermal treated steel, and particularly to non-thermal treated steel for hot forging which can improve toughness.

(Conventional technology and problems) Non-tempered steel is heat treated after hot working to achieve the desired strength and
Compared to tempered steel that obtains toughness, it is provided through hot processing such as rolling and forging, and has the advantage of reducing manufacturing costs. In particular, non-tempered steel for hot forging is frequently used in the manufacture of large parts, and research is being actively conducted to improve its mechanical properties.

Conventional non-tempered steel for hot forging is made by adding V, N, etc. to precipitate V carbonitride to obtain a mixed structure of ferrite + pearlite or ferrite + pearlite + bainite.
Efforts are being made to increase strength and toughness. However, although a high level of strength of approximately 90 kgf/mm'' or more can be obtained, there is a problem in that the toughness decreases. This is a common drawback of non-tempered steels, which attempt to obtain mechanical properties only by adding strength-enhancing elements under natural cooling after forging, and its application is difficult due to its low impact value, especially in the high strength range. There were naturally limitations on parts.

(Object of the Invention) The present invention has been made in order to eliminate the drawbacks of the above-mentioned prior art, and its purpose is to improve The object of the present invention is to provide a non-tempered steel for hot forging that can obtain a high impact value and expand the range of parts to which it can be applied.

(Structure of the Invention) In order to achieve the above object, the present invention has been made possible by adjusting the additive elements and improving the structure.

That is, the non-temperature steel for hot forging according to the present invention has a C:0
．． 20-0.50%, Si: 0.10-1.00%, M
n: 1.0-2.0%, V: 0.05-0.30%, N
b: 0.01-0.06%, 5oQAQ: 0.01-0
．． The basic composition includes 0.06% and N: 0.01~0.03%, and if necessary, Cr: 0.30~0.30%.
1.0%, Ni: 0.3-2.0%, MO=0.05-
One or more of 0.50% and/or as stiffness improving elements S: 0.2% or less, Pb: 0.4%
Below, Te: 0.1% or less, Bi: 0.1% or less, and C
a: One or more of 0.01% or less is added, and the remainder has a composition consisting essentially of Fe, and by cooling after hot forging, a mixed structure of ferrite + bainite or a single bainite phase is formed. It is characterized by having an organization consisting of tissues.

The present invention will be explained in detail below based on examples.

First, the reasons for limiting the upper and lower limits of the additive elements in the steel of the present invention are shown below.

C must be contained in an amount of 0.20% or more to ensure strength. However, if it exceeds 0.50%, the toughness deteriorates, so the upper limit is set at 0.50%.

Lower 1 Si is a deoxidizing element, and it is necessary to add 0.10% or more, but a sufficient deoxidizing effect can be obtained with 1.00% or less.
If added in a larger amount, the machinability will deteriorate, so the upper limit is set at 1.00%.

It is necessary to add an appropriate amount of Mn to increase the strength.
In particular, in the present invention, ferrite +
It is added in an amount of 1.0% or more in order to obtain a bainite mixed structure or a bainite single-phase structure and to prevent the formation of pearlite. However, if added in a large amount, martensite will be formed, so the upper limit is set at 2.0%.

■■ Carbonitrides precipitate during cooling after hot forging and can strengthen precipitation, so it is necessary to add 0.05% or more, but adding too much will actually reduce toughness. The upper limit is 0.30%.

Nb, 5oQAQ, N Nb, so Q A Q and N are effective elements for refining crystal grains and improving toughness, and these three
It is necessary to add various types of elements in a well-balanced manner. To achieve this, each element must be added in an amount of 0.01% or more, but adding too much will disrupt the balance.

On the contrary, it causes toughness deterioration, so Nb and 5oQAQ each have an upper limit of 0.06%, and N has an upper limit of 0003%.

Cr, Ni, M.

Cr, Ni, and Mo, like Mn, are pearlite-free elements that prevent pearlite from being formed by cooling after hot forging, but since Mn is added as an essential component, Cr, Ni, and Mo can be added as necessary. When added, Cr: 0.30%
Above, in the case of Ni, it is 0.3% or more, and in the case of Mo, it is 0.3% or more.
The above effect cannot be expected unless it is 0.5% or more, however,
If Cr exceeds 1.0%, Ni2゜0%, and Mo exceeds 0.5%, martensite will be formed, so Cr:0
．． 30-1.0%, Ni: 0.30-2.0%, Mo:
0.05" 6S, Pb, Te% Bi, Ca S, Pb, Te, Bi, and Ca added one or more in the range of 0.50% are elements that significantly improve machinability. Therefore, it can be added as necessary.If added, one or more of these elements □ may be added, but adding a large amount of each element will lead to deterioration of toughness. S is 0.2%,
pb is 0.40%, Te is 0.1%, Bi is 0.1%,
The upper limit of Ca is 0.01%.

After hot forging, the steel having the above-mentioned composition is cooled by air cooling or breeze cooling to obtain a mixed structure of ferrite + bainite or a single phase structure of bainite while preventing the formation of pearlite. At that time, the cooling rate is determined as appropriate depending on the amount of Mn, Cr, Ni, and Mo and the mass effect of the applied parts.

Thus, the steel of the present invention can have a ferritic-bainite mixed structure or a bainite single-phase structure by adjusting the composition as described above and cooling after hot forging.
It is possible to obtain a high impact value of 5 kgt-m/ca+" (JIS No. 3 test piece, room temperature) in a high strength range of 5 kgt-m/ca+" (equivalent to a tensile strength of about 89 kgf/mn+2) or more.

(Example) Steel having the composition (wt%) shown in Table 1 is melted by electric furnace melting, after which it is refined outside the furnace, and then cast to obtain an ingot, which is then decomposed and rolled to obtain a forging ingot. A material (80 mmφ) was manufactured. Next, these were hot forged at 1050° C. to produce a crankshaft with a journal diameter of 40 mmφ, and after the hot forging, the crankshaft was cooled in a breeze (100° C./m1n). The mechanical properties and structure of the obtained forged material were investigated. The results are shown in Table 2. In addition, the impact value is JISa than forged material.
A No. Charpy test piece was taken and a Charpy impact test was conducted at room temperature.The strength was evaluated by hardness HRC25, which has a tensile strength of approximately 89kgf/mad2.
higher hardness indicates higher tensile strength.

[Margin below c] As is clear from Table 2, the conventional steel, which has been increased in strength by adding ■, has an impact value of 2 at a high strength of HRC25.
kgf/cm", whereas all of the steels of the present invention have extremely excellent VR impact values and are also high in strength.

Furthermore, among the steels of the present invention, those to which Cr, Ni, and Mo were added had a bainitic structure and exhibited higher strength, but the impact value remained at a high level considering the increased strength. In addition, it was confirmed that the material to which machinability-improving elements were added maintains toughness in the high strength range and has excellent machinability.

(Effects of the Invention) As detailed above, the non-tempered steel for hot forging of the present invention has the following features:
A mixed structure of ferrite + bainite or a single phase structure of bainite is obtained by adjusting the composition of the additive components, especially adding pearlite-free elements such as Mn, and adding grain-refining elements such as Nb, soΩAQ, and N in a well-balanced manner,
Since pearlite is not formed, it is possible to improve toughness deterioration in the high strength range, which was a drawback of conventional steel, and it is possible to provide excellent toughness, and it is also possible to significantly improve stiffness. Therefore, the cost reduction of non-thermal treated steel can be fully utilized and the application of hot forged parts can be expanded.

1・ 1゜

Claims (1)

[Claims] 1% by weight (the same applies hereinafter), C: 0.20 to 0.50
%, Si: 0.10-1.00%, Mn: 1.0-2.
0%, V: 0.05-0.30%, Nb: 0.01-0
．． 06%, solAl: 0.01-0.06% and N:
For hot forging, characterized by containing 0.01 to 0.03%, the remainder consisting of Fe and unavoidable impurities, and having a mixed structure of ferrite + bainite or a single phase structure of bainite by cooling after hot forging. Non-tempered steel. 2C: 0.20-0.50%, Si: 0.10-1.
00%, Mn: 1.0-2.0%, V: 0.05-0.
30%, Nb: 0.01-0.06%, solAl: 0
．． 01 to 0.06% and N: 0.01 to 0.03%, further Cr: 0.30 to 1.0%, Ni: 0.30 to
2.0%, Mo: 0.05 to 0.50%, the remainder consists of Fe and unavoidable impurities, and a mixed structure of ferrite + bainite is formed by cooling after hot forging. Or a non-temperature steel for hot forging characterized by having a bainitic single phase structure. 3C: 0.20-0.50%, Si: 0.10-1.
00%, Mn: 1.0-2.0%, V: 0.05-0.
30%, Nb: 0.01-0.06%, solAl: 0
．． 01 to 0.06% and N: 0.01 to 0.03%, further S: 0.2% or less, Pb: 0.40% or less, T
e: 0.1% or less, Bi: 0.1% or less, and Ca: 0.
01% or less, and the remainder is F.
1. A non-temperature steel for hot forging, characterized in that it is composed of E and unavoidable impurities and has a mixed structure of ferrite + bainite or a single phase structure of bainite by cooling after hot forging. 4C: 0.20-0.50%, Si: 0.10-1.
00%, Mn: 1.0-2.0%, V: 0.05-0.
30%, Nb: 0.01-0.06%, solAl: 0
．． 01 to 0.06% and N: 0.01 to 0.03%, further Cr: 0.30 to 1.0%, Ni: 0.30 to
2.0%, Mo: one or more of 0.05 to 0.50%, S: 0.2% or less, Pb:
0.4% or less, Te: 0.1% or less, Bi: 0.1% or less, and Ca: 0.01% or less, and the remainder consists of Fe and inevitable impurities. A non-temperature steel for hot forging, characterized in that it has a mixed structure of ferrite + bainite or a single phase structure of bainite due to cooling after hot forging.