JPH076033B2 - Non-heat treated steel for induction hardening - Google Patents

Description

TECHNICAL FIELD The present invention relates to a non-heat treated steel, and more particularly to a non-heat treated steel suitable for induction hardening.

(Conventional technology and problems) Non-heat treated steel is subjected to heat treatment after hot working to obtain desired strength,
Compared with heat-treated steel that obtains toughness, it is used until hot working such as rolling and forging, and has the advantage of reducing its manufacturing cost. Therefore, for example, mechanical parts that require surface hardness and core strength such as steering knuck, pinion, knuckle arm, knuckle spindle, etc. are often manufactured by subjecting non-heat treated steel to induction hardening.

Conventionally, non-heat treated steel, which is a material for such induction hardening, is manufactured until hot working such as hot forging, so ferrite + pearlite or ferrite + pearlite +
It has a mixed structure of bainite, and when induction hardening is applied to this, residual ferrite comes out and the depth of the hardened layer becomes shallow. Therefore, in order to prevent this, there is an attempt to carry out induction hardening for a long time, but the hardening layer depth increases but quenching strain increases, and further, crystal grains become coarse and core strength decreases. There is a problem. In this respect, according to the method of induction hardening the heat-treated steel, there is no such problem,
On the contrary, there is a drawback that the manufacturing cost becomes high.

(Object of the Invention) The present invention provides a non-heat treated steel for induction hardening which solves the above-mentioned drawbacks of the prior art, can easily obtain a desired hardened layer depth, and can improve toughness and core strength. That is the purpose.

(Structure of the invention) In order to achieve the above-mentioned object, the present inventor has studied the chemical composition of non-heat treated steel, the manufacturing process conditions, etc. Ah.

That is, the non-heat treated steel for induction hardening according to the present invention, C: 0.
30-0.60%, Si: 0.03-1.0% and Mn: 0.5-2.0%, Mo: 0.05-0.5% and Nb: 0.01-0.3%, one or two kinds, and if necessary, Cr : 0.2-0.9% and Ni: 0.
It is characterized by having a structure containing 7 to 1.6% of one or two kinds, the balance being substantially Fe, and having a volume ratio of bainite of 75% or more.

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

First, the reasons for limiting the chemical composition of the steel of the present invention will be shown.

C requires 0.30% or more to secure the induction hardening hardness, but if it is too much, it causes quench cracking during induction hardening or causes deterioration in toughness, so the upper limit is made 0.60%.

Si is a deoxidizing element, and it is necessary to add 0.03% or more, but if it is 1.0% or less, the deoxidizing effect is sufficiently obtained, and if added in a larger amount, the toughness decreases, so the range is 0.03 to 1.0%. To do.

It is necessary to add 0.5% or more of Mn to secure the core strength, but if it is too much, the machinability deteriorates, so 2.0%
Is the upper limit.

Mo and Nb are elements necessary for obtaining an appropriate amount of bainite structure (described later) by cooling after hot working such as hot forging. For that purpose, at least one of these is Mo: 0.05-
0.5%, Nb: added in the range of 0.01 to 0.3%. It should be noted that if each element is added in excess of the upper limit, the toughness decreases and the machinability deteriorates, which is not preferable.

Although each of the above elements is an essential component, one or two of Cr and Ni can be added if necessary. Cr and Ni are effective in improving strength, so when added, Cr: 0.2 to 0.
9%, Ni: 0.7 to 1.6%. It is not preferable to add a large amount of each element in excess of the upper limit, because a large amount of martensite is produced and the element is hardened significantly.

Like the conventional non-heat treated steel, the steel of the present invention having the above chemical composition is subjected to induction hardening before hot working such as hot forging. However, in the present invention, it is important to obtain a structure having an appropriate amount of bainite as the structure of the hot-worked material. That is, in the ferrite + pearlite structure or the ferrite + pearlite + bainite structure or the bainite + martensite structure in which the volume ratio of bainite is less than 75%, residual ferrite appears due to induction hardening, or the martensite is too much and hardens. Therefore, it is not preferable for induction hardening for obtaining an appropriate hardened layer depth and core strength.

For that purpose, it is desirable to obtain a predetermined amount of bainite by controlling and cooling the steel of the present invention having the above chemical composition after hot forging, and according to the experiment of the present inventor, after hot forging 700- Substantial cooling rate between 300 ℃ is 150 ~ 10 ℃ / mi
By cooling with n, it is possible to easily obtain a structure having a bainite with a volume ratio of 75% or more. Of course, it goes without saying that it is possible to use other bainite-friendly methods.

(Example) Next, the Example of this invention is shown.

Example 1 A test steel round bar 32 mmφ having the chemical composition (wt%) shown in Table 1 was used.
It was hot forged to 25 mmφ and controlled-cooled at various cooling rates shown in Table 2. As a result, as shown in the table, the cooling rate
At 25.0 ° C / min or more, a structure having a bainite with a volume ratio of 75% or more was obtained, but at 7.0 ° C / min or less, the bainite content was low as in the conventional ferrite + pearlite + bainite or ferrite + pearlite. It was a mixed tissue.

Next, after induction hardening of each forging, tempering (160
X 1 hr, AC) was applied to adjust the surface hardness shown in the same table, and the depth of the hardened layer was examined.The example of the present invention having a controlled cooling rate with a large amount of bainite showed a sufficient hardened layer depth. In contrast, in the comparative example, the depth of the hardened layer was shallow and residual ferrite was observed.

Example 2 A test steel round bar 70 mmφ having the chemical composition (wt%) shown in Table 3 was used.
It was hot forged to 40 mmφ and controlled-cooled at a cooling rate of about 25 ° C / min. As a result of performing a structural quantification on each of the obtained forged materials, it was found that the forged materials had the structures shown in the table. As a result, a structure having bainite with a volume ratio of 75% or more can be obtained unless the chemical composition is within the scope of the present invention, even if the cooling rate is increased and the control cooling is performed after hot forging as shown in Example 1. I know there isn't.

(Effects of the Invention) As described in detail above, according to the present invention, the chemical composition of the non-heat treated steel is adjusted and the structure having bainite with a volume ratio of 75% or more is used. With no residual ferrite, the depth of the hardened layer is the same as that of conventional tempered steel,
The core strength can be secured by induction hardening for a short time and can be provided at low cost.

Claims (2)

[Claims] 1. Weight% (hereinafter the same), C: 0.30 to 0.60%,
Si: 0.03 to 1.0% and Mn: 0.5 to 2.0%, and Mo: 0.05
~ 0.5% and Nb: 0.01 to 0.3% of 1 or 2 types, the balance consists essentially of Fe, and the volume ratio occupied by bainite is
Non-heat treated steel for induction hardening characterized by having a structure of 75% or more. 2. C: 0.30 to 0.60%, Si: 0.03 to 1.0% and Mn:
0.5-2.0%, further Mo: 0.05-0.5% and Nb: 0.01-
0.3% 1 type or 2 types, Cr: 0.2 to 0.9% and Ni: 0.7 to 1.
A non-heat treated steel for induction hardening, which has a structure containing 1% or 2% of 6%, the balance being substantially Fe, and having a volume ratio of bainite of 75% or more.