JPH06287677A - High strength non-refining steel for hot forging - Google Patents

Abstract

PURPOSE:To stably maintain high tensile strength in non-refining steel and to miniaturize parts by specifying the content of C, Si, Mn, Cr, S, V, N, Al and Ti in steel and specifying the carbon equivalent and bainitic transformation index. CONSTITUTION:The compsn. of the non-refining steel is constituted of, by weight, 0.25 to 0.50% C, 0.40 to 2.00% Si, 0.50 to 2.50% Mn, 0.10 to 1.00% Cr, 0.03 to 0.10% S, 0.05 to 0.30% V, 0.0050 to 0.0200% N and one or two kinds of 0.005 to 0.050% Al and 0.002 to 0.050% Ti, and the balance Fe with inevitable impurities. Moreover, the carbon equivalent expressed by Ceq%=%C +(%Si)/20+(%Mn)/5+(%Cr)/9+1.54(%V) is regulated to 0.83 to 1.23. Furthermore, the value of the bainitic transformation index expressed by Bt=31.2-100(%C)-6.7(%Si)+9.0(%Mn)+4.9(%Cr)-81(%V) is regulated to <=0. In this way, >=900MPa tensile strength can be realized in a hot forging and non-refining state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material which is machined into machine parts such as automobiles and industrial machines, and which has high strength until hot working, especially after being processed by hot forging. The present invention relates to non-heat treated steel for high strength hot forging.

[0002]

2. Description of the Related Art In many automobile and industrial machine parts, a material steel bar is hot-worked and then hardened and tempered (refining treatment) to make the structure finer to increase strength and toughness. In recent years, non-tempered hot forgings, which are used without tempering treatment, are rapidly becoming popular for cost reduction.

Recently, in order to protect the global environment, it is required to reduce the fuel consumption of automobiles, and one of the effective methods for achieving the fuel consumption reduction of automobiles is to reduce the weight of the automobile.
The aim is to reduce the size and weight of parts by increasing their strength.

Various non-heat treated steels have been used so far in order to meet the requirements for non-heat treated and high strength automobile parts. For example, general non-heat treated steel is added with V and Nb, and V and Nb are precipitated as carbonitrides in the cooling process after hot working to strengthen the ferrite pearlite structure. The hot-forged tensile strength of this type of non-heat treated steel is about 800 MPa, and since the as-hot-forged structure is very coarse, its toughness is low. Therefore, in recent years, as disclosed in JP-A-1-198450, a steel has been developed in which the structure is refined while hot forging and the toughness is enhanced while achieving high strength. However, the tensile strength of the steel described in JP-A-1-198450 as hot forging is limited to about 1000 MPa.

[0005]

Ferrite pearlite steel is characterized by a high yield ratio and excellent machinability, so that it can stably maintain a tensile strength of 900-1200 MPa in a hot forged non-tempered state. If it can be realized, there will be great industrial benefits such as improvement in fuel efficiency due to reduction in size and weight of automobile parts.

[0006] Therefore, the present invention is 900M as hot forged
It is intended to provide a ferrite-pearlite-type high-strength non-heat treated steel for hot forging which stably realizes a tensile strength of Pa or more.

[0007]

[Means for Solving the Problems] The tensile strength of a conventional ferrite perlite type non-heat treated product is 900 to 1000.
The reason why it was not possible to exceed MPa was that if a large amount of alloying elements were added to increase the strength, a bainite structure is likely to occur in ordinary air cooling, and the quality cannot be guaranteed. However, the effect of alloying elements on the generation of bainite in ferrite-pearlite type non-heat treated steel has not been sufficiently investigated.

[0008] Therefore, the inventors of the present invention have conducted intensive studies to clarify the influence of alloying elements on the generation of bainite and to realize a high-strength ferritic pearlite steel in a hot forged non-tempered state. As a result, although the generation of bainite structure in the hot forged non-heat treated state is promoted by the addition of Mn and Cr, it can be suppressed by the addition of C, Si and V. The effect of strength was relatively small. Therefore, it has been found that a non-heat treated high-strength ferrite pearlite of 900 MPa or more can be realized by optimally designing the components.

Therefore, the bainite transformation index Bt for predicting the bainite transformation was obtained by multiple regression from the relationship between the composition of the steel and the microstructure of the hot forged state.

Bt = 31.2-100 (% C) -6.7
(% Si) +9.0 (% Mn) +4.9 (% Cr) -8
1 (% V), and when Bt is 0 or less, the bainite fraction is 0%
Is. In steel having a 100% ferrite pearlite structure, carbon equivalent Ceq. (%) =% C + (% Si) / 20 (%
Mn) / 5 + (% Cr) /9+1.54 (% V), tensile strength (MPa) = 759 × Ceq. (%) +26
Could be represented as 7.

Bainite transformation index Bt and carbon equivalent Ceq. FIG. 1 shows the relationship. 1 to Ceq. > 1.23% is bainite, and Ceq.
It can be seen that bainite is generated when Bt is larger than 0 when ≦ 1.23%.

As described above, Ceq. The present invention has been completed based on the finding that by controlling Bt and Bt within a certain range, high tensile strength can be realized while preventing the generation of bainite. That is, according to the present invention, as defined in the claims, C: 0.25-0.50%, Si: 0.
40-2.00%, Mn: 0.50-2.50%, C
r: 0.10-1.00%, S: 0.03-0.10
%, V: 0.05-0.30%, N: 0.0050-
0.0200% Al: 0.005-0.050
%, Ti: 0.002-0.050%, one or two
Seeds, the balance consisting of Fe and unavoidable impurities, and the carbon equivalent Ceq. (%) Is 0.83%-
High-strength hot-forging non-heat treated steel Ceq. With 1.23% and bainite transformation index Bt of 0 or less. (%) =% C + (% Si) / 20 + (% Mn)
/5+(%Cr)/9+1.54(%V) Bt = 31.2-100 (% C) -6.7 (% Si) +
9.0 (% Mn) +4.9 (% Cr) -81 (% V) Further, C: 0.25-0.50% by weight%, Si:
0.40-2.00%, Mn: 0.50-2.50%,
Cr: 0.10-1.00%, S: 0.03-0.10
%, V: 0.05-0.30%, N: 0.0050-
0.0200% Ca: 0.0004-0.0050% included, and further A
1: 0.001-0.010%, Ti: 0.005-
0.020% of 1 type or 2 types or more, with the balance being Fe
And a carbon equivalent Ceq. (%) Is 0.83% to 1.23%, and bainite transformation index Bt is 0 or less. High-strength hot forging non-heat treated steel Ceq. (%) =% C + (% Si) / 20 + (% Mn)
/5+(%Cr)/9+1.54(%V) Bt = 31.2-100 (% C) -6.7 (% Si) +
It is 9.0 (% Mn) +4.9 (% Cr) -81 (% V).

The reasons for limiting the invention will be described below.

C: C is required to be 0.25% or more in order to strengthen the steel. If it is less than 0.25%, the amount of other alloy elements increases, so that bainite is likely to be generated during hot forging. On the other hand, when a large amount of C is added, the ductility becomes extremely low, so the upper limit is made 0.50%.

Si: Si strengthens the steel as a solid solution strengthening element and also serves to suppress the transformation of bainite. 0.40% or more is required for strengthening and transformation control, but if it exceeds 2.0%, ductility deteriorates.

Mn: Mn is an element useful for strengthening steel without relatively deteriorating ductility, and at least 0.50% is required for strengthening. On the other hand, addition of a large amount exceeding 2.5% causes generation of bainite.

Cr: Cr is also required to be 0.1% or more for strengthening the steel, but a large amount of addition causes 1.00 as well as Mn to generate bainite, so the content is made 1.00% or less.

S: S crystallizes as MnS and has a function of transforming ferrite in the former austenite grain boundaries, and improves ductility and toughness. To improve ductility and toughness, it is necessary to add 0.03% or more. However, if added in a large amount, mechanical properties become anisotropic, so the upper limit is made 0.10%. Further, S improves machinability.

N: N precipitates as VN, TiN or NbN, prevents coarsening of the austenite structure, promotes transformation of intragranular ferrite, and improves ductility. If it is less than 0.0050%, these effects are small and the effect of improving ductility cannot be expected. Further, the effect is saturated even if added over 0.0200%.

V: V precipitates as VN to promote intragranular ferrite transformation, and at the same time as VC finely precipitates in ferrite to strengthen the steel. 0.05% for strengthening
The above is required, but if it exceeds 0.30%, the toughness deteriorates. V addition is effective in strengthening while suppressing bainite transformation.

Al: Al is an element added as a deoxidizer. In order to expect a sufficient deoxidizing effect in claim 1, 0.005% or more is necessary, but 0.050%
Addition of more than 10 reduces machinability.

Further, particularly in the case of adding Ca for improving the machinability, it is necessary to add 0.010% or less because it is necessary to generate Ca oxide. It is necessary to add 0.001% or more of a small amount of Al to form a low melting point oxide and improve machinability.

Ti: Ti is added as a deoxidizer,
Prevents coarsening of the austenite structure during forging heating,
It is also effective in refining the structure while improving the ductility and toughness while forging and cooling. In claim 1, 0.002% or more is required to be added in order to achieve these effects.
Addition in excess of 0% deteriorates machinability. Claim 2
In order to improve the machinability with Ca oxide,
It should be 0.020% or less, but if it is less than 0.005%, the effect of coarsening the austenite structure is not expected.

Ca: In order to improve the machinability particularly in the cutting using a cemented carbide tool, Ca0.0004-0.0
Addition of 050% is effective. If it is less than 0.0004%, there is no effect, and if it is added more than 0.0050%, the machinability is rather lowered. When it is desired to improve the machinability in the steel of claim 1, addition of the same amount of Ca is effective, but when Al and Ti are added in excess of the upper limit of claim 2, the machinability is increased. The effect of improving sex is reduced.

Ceq. : Tensile strength as forged and cooled is carbon equivalent Ceq. It can be expressed by a linear expression. In high strength ferritic pearlite steels such as the steel of the present invention,
Tensile strength (MPa) = 759 × Ceq. (%) +26
7 and Ceq. Is 0.83%, the tensile strength is 9
It is 00 MPa. Therefore, in order to obtain a tensile strength of 900 MPa or more, Ceq. Is limited to 0.83% or more. However, Ceq. Exceeds 1.23%, bainite transformation occurs, so the upper limit is made 1.23%.

Ceq. (%) =% C + (% Si) / 20
+ (% Mn) / 5 + (% Cr) /9+1.54 (% V) Bt: In the present invention, the bainite transformation index Bt for predicting bainite transformation is because the steel structure after forging and cooling is 100% ferrite pearlite. Is extremely important to. Bainite transformation is promoted by the addition of Mn and Cr, and C,
It is suppressed by the addition of Si and V. In the cooling rate range after normal forging (the average cooling rate between 1100-700K is 0.5-2.0K / S), Bt = 312-100
(% C) -67 (% Si) +90 (% Mn) +49 (%
When Cr) -810 (% V) is 0 or less, a 100% ferrite pearlite structure is formed, and when it exceeds 0, bainite transformation occurs.

In addition to the above elements, if Pb, Bi, Te, and Se, which are generally known as free-cutting elements, are added in appropriate amounts, the machinability of the present invention steel is naturally improved. For example, when Pb and Bi are added in an amount of 0.05% or more, they are dispersed in steel as a low melting point metal to improve machinability. However, the upper limit is preferably 0.30% so as not to deteriorate the hot workability.

Te and Se of 0.02% or more form sulfides like S and improve the machinability. Addition of 0.10% or less does not cause anisotropy of mechanical properties.

[0029]

EXAMPLE Steels having various compositions shown in Table 1 were melted in a vacuum melting furnace of 150 kg and made of steel bars molded into a diameter of 30 mm. These steels were heated at 1525 K and 1200 S,
After allowing to cool to room temperature, the structure was observed and a tensile test was performed. A JIS No. 4 test piece was used as the tensile test piece.

Further, in Table 1, No. With respect to Nos. 26-31, after the above-mentioned heat-cooling, the peripheral flank turning was carried out by a cemented carbide tool in which JIS-P20 was coated with TiN, and the tool flank wear after machining for 5 minutes was measured. Cutting conditions are cutting speed 15
0 m / min. Feed 0.2 mm / rev. Notch 2.0
mm and processed by dry method.

As shown in Table 1, the steel of the present invention is a 100% ferrite pearlite steel having a tensile strength of 900 MPa or more as hot worked. Generally, as the tensile strength (TS) increases, the yield ratio (YR) tends to increase and the aperture value (RA) tends to decrease, but when comparing the same tensile strength, As for the yield ratio and the drawing value of the steel of the present invention, the comparative steel No. Better than 4,8,12,13. Comparative steel No. 5,9,15,18,21,22,23,24
Although it is a ferritic pearlitic steel of 900 MPa or more, the yield ratio and the drawing value are low as compared with the steel of the present invention having the same tensile strength. Comparative steel No. 25 has a Bt of 0 or less, but Ceq. Is more than 1.23%, so that it has a bainite structure.

Further, the steel No. 2 of the present invention according to claim 2 26, 2
Cemented carbide tool wear in Nos. 7, 29, and 30 is comparative steel No.
Compared with 28 and 31, it is 30-40 μm smaller.

[0033]

[Table 1]

[0034]

As described above, according to claims 1 and 2 of the present invention, the steel material according to the present invention is a steel material having a ferrite pearlite structure and a tensile strength of 900 MPa or more in the hot forging ratio temper state. Is used for automobile parts,
Not only the heat treatment cost is reduced, but also the parts can be made smaller and lighter, which contributes to the improvement of fuel consumption and further to the protection of the global environment by the improvement of fuel consumption.

The steel according to claim 2 is particularly excellent in machinability using a cemented carbide tool, and contributes to further cost reduction.

[Brief description of drawings]

FIG. 1 shows bainite transformation index Bt and carbon equivalent Ceq. Shows the relationship.

Claims (2)

[Claims] 1. C: 0.25-0.50%, Si: 0.40-2.00%, Mn: 0.50-2.50%, Cr: 0.10-1.00% by weight. %, S: 0.03-0.10%, V: 0.05-0.30%, N: 0.0050-0.0200% Further Al: 0.005-0.050%, Ti: 0. 002-
0.050% of one or two kinds, the balance consisting of Fe and inevitable impurities, and having a carbon equivalent Ceq. (%) Is 0.83% to 1.23%, and bainite transformation index Bt is 0 or less. High-strength hot forging non-heat treated steel Ceq. (%) =% C + (% Si) / 20 + (% Mn)
/5+(%Cr)/9+1.54(%V) Bt = 31.2-100 (% C) -6.7 (% Si) +
9.0 (% Mn) +4.9 (% Cr) -81 (% V) 2. C: 0.25 to 0.50% by weight, Si: 0.40 to 2.00%, Mn: 0.50 to 2.50%, Cr: 0.10 to 1.00. %, S: 0.03-0.10%, V: 0.05-0.30%, N: 0.0050-0.0200% Ca: 0.0004-0.0050%, and further Al: 0.001-0.010%, Ti: 0.005-
0.020% of 1 type or 2 types or more, with the balance being Fe
And a carbon equivalent Ceq. (%) Is 0.83% to 1.23%, and bainite transformation index Bt is 0 or less. High-strength hot forging non-heat treated steel Ceq. (%) =% C + (% Si) / 20 + (% Mn)
/5+(%Cr)/9+1.54(%V) Bt = 31.2-100 (% C) -6.7 (% Si) +
9.0 (% Mn) +4.9 (% Cr) -81 (% V)