JPH05287373A - Manufacture of high strength and high toughness hot worked non-heat-treated steel - Google Patents

Abstract

PURPOSE:To obtain a non-heat-treated steel high in tensile strength, toughness and machinability, at the time of subjecting steel to hot working into a prescribed shape and thereafter executing hardening and tempering by specifying the compsn. of the steel and the cooling velocity after the hot working. CONSTITUTION:Steel contg., by weight, 0.10 to 0.06% C, 0.05 to <0.8% Si, <=3.0% Mn, 0.001 to 0.30% S, 0.030 to 2.00% V and 0.005 to 0.060% N, contg. one or more kinds among <=3.0% Cr, <=3.0% Ni, <=1.0% Mo and <=2.0% Cu and contg. one or more kinds among 0.005 to 0.10% Al, 0.001 to 0.050% Ti and 0.005 to 0.10% Nb, and the balance Fe is hot-worked to form its shape into a prescrbed one. This molded part is cooled to a room temp. at a cooling velocity in which C2 in the formula I {where C1 denotes the formula II and CR denotes the cooling velocity ( deg.C/sec) of the steel at 500 to 800 deg.C} satisfies 4.0<=C2<=6.0 between 500 to 800 deg.C, is furthermore tempered at 200 to 600 deg.C for 5 to 300min and is thereafter cooled to a room temp. again. In this way, the objective high strength and high toughness hot cast non-heat treated steel having 85kgf/mm tensile strength and 70kfg/mm<2> yield strength can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing hot forged non-heat treated steel for machine structure having excellent yield strength, tensile strength, toughness and machinability.

[0002]

2. Description of the Related Art Conventionally, in the manufacture of mechanical parts requiring high strength and high toughness, a tempering treatment such as quenching and tempering after hot forging into a predetermined shape has been performed. However, the refining process requires many steps and requires a large amount of heat energy, which causes an increase in manufacturing cost.

Further, since bending occurs due to quenching and a straightening step is required, productivity remains a problem. For this reason, in recent years, from the viewpoint of reducing the number of steps and saving energy, steel that can be subjected to refining treatment, that is, non-refined steel has been developed.
However, the conventional non-heat treated steel has poor toughness, particularly low temperature toughness, and is insufficient as a material used for undercarriage parts of automobiles which require toughness, and also has extremely poor machinability and lacks part productivity.

[0004]

[Problems to be Solved by the Invention] Regarding the improvement of toughness,
For example, in JP-A-56-38448, Si, M
Strengthening the base iron by increasing n etc. and Ti, V, N
In addition to increasing the strength of the steel material by precipitation strengthening of b, the N content in the steel is increased to 0.29% Ti or more to increase the T content mainly of nitride.
By forming precipitates of i, V, and Nb, refining the grain size of the former austenite, and increasing the toughness of the steel material, hot forging is performed without any subsequent heat treatment, resulting in excellent tensile strength and toughness. The material that made it possible to secure is shown.

However, the use of such materials has not yet ensured high strength, high toughness and good machinability. Further, since the design strength of automobile parts is based on the yield strength, it is desired to develop a material having a high tensile strength and a high yield strength, that is, a so-called high yield ratio.

The object of the present invention is to obtain a tensile strength of 85 kgf / m.
m ² or more, sufficient toughness and excellent machinability, 70k
It is an object of the present invention to provide a method for producing a hot forged non-heat treated steel having a high yield strength of gf / mm ² or more.

[0007]

The present inventors have found that high strength,
Conducted diligent studies to provide a method for manufacturing hot forged non-heat treated steel with high toughness and further excellent machinability and a method for improving yield strength without impairing tensile strength, toughness and machinability. As a result, it is possible to manufacture non-heat treated steel with excellent tensile strength, toughness and machinability by properly combining the composition of the steel and the control of the cooling rate after hot forging. The present invention was made by obtaining new knowledge that the yield strength can be increased without deteriorating the toughness and machinability.

That is, the first aspect of the present invention, in% by weight, is C:
0.10% to 0.60%, Si: 0.05% or more and less than 0.8%, Mn: ≤ 3.0%,
S: 0.001% to 0.30%, V: 0.030% to
2.00%, N: 0.005% to 0.060%, Cr: ≤ 3.0%, Ni: ≤ 3.
0%, Mo: ≦ 1.0%, Cu: ≦ 2.0% 1
Or more than two kinds, and further Al: 0.005% ~
0.10%, Ti: 0.001% to 0.050%, N
b: 0.005% to 0.10% of one or more kinds of steel, and the balance of Fe and unavoidable impurities, hot-worked into a predetermined shape, and then 500 to 800
C2 represented by the following numerical formula 3 between 0 ° C. is cooled to room temperature at a cooling rate satisfying 4.0 ≦ C2 ≦ 6.0, and further 200 to 6
00 ° C. temperature range in 5 to 300 minutes tempering and thereafter cooled again to room temperature, a tensile strength of 85 kgf / mm ^2, yield strength 70 kgf / mm high strength and high toughness hot working microalloyed characterized by having ^two or more It is a manufacturing method of quality steel.

[0009]

[Equation 3] C2 = 3 × C1 + log ₁₀ (CR)

However, C1 = [% C] + 0.6 × [% Mn] −0.1 × [%
S] + 0.45 × ([% Cr] + [% Ni] + [% M
o] + [% Cu]) + 0.85 × [% V] CR: Steel cooling rate (° C between 500 and 800 ° C)
/ Sec) [% X]: weight% of element X

The second aspect of the present invention is, in% by weight, C: 0.
10% to 0.60%, Si: 0.05% or more 0.
Less than 8%, Mn: ≤ 3.0%, S:
0.001% to 0.30%, V: 0.030% to 2.0
0%, N: 0.005% to 0.060%, Cr: ≤ 3.0%, Ni: ≤ 3.0
%, Mo: ≦ 1.0%, Cu: ≦ 2.0%, and one or more kinds of Al, 0.005% to
0.10%, Ti: 0.001% to 0.050%, N
b: 0.005% to 0.10% of 1 type or 2 or more types, and Pb: 0.005% to 0.50%, C
a: 0.001% to 0.050%, Te: 0.001%
~ 0.20%, Se: 0.010% to 0.50%, B
Steel containing i: 0.010% to 0.50% and the balance being Fe and unavoidable impurities is hot-worked into a predetermined shape, and then a temperature of 500 to 800 ° C. is represented by the following numerical formula 4. C2 is cooled to room temperature at a cooling rate satisfying 4.0 ≦ C2 ≦ 6.0, and further 5 to 3 in a temperature range of 200 to 600 ° C.
00 minutes tempering and thereafter cooled again to room temperature, a tensile strength of 85 kgf / mm ^2, yield strength 70 kgf / mm and having ^two or more, characterized in that excellent machinability high strength and high toughness hot working non-heat treated It is a method of manufacturing steel.

[0012]

[Equation 4] C2 = 3 × C1 + log ₁₀ (CR)

However, C1 = [% C] + 0.6 × [% Mn] −0.1 × [%
S] + 0.45 × ([% Cr] + [% Ni] + [% M
o] + [% Cu]) + 0.85 × [% V] CR: Steel cooling rate (° C between 500 and 800 ° C)
/ Sec) [% X]: weight% of element X

[0014]

The present invention will be described in detail below.

First, C is an element effective for increasing the strength of the forged product, but if it is less than 0.10%, the strength is insufficient, and if it exceeds 0.60%, the toughness is deteriorated. The content was 0.10 to 0.60%.

Next, Si is an element effective in increasing strength by deoxidizing element and solid solution hardening, but if less than 0.05%, its effect is insufficient, while if over 0.8%, its effect is high. Is saturated and rather causes deterioration in toughness, so the content was made 0.05 to 0.8%.

Further, Mn and S exist as MnS in steel and contribute to the refinement of the structure, but if S: less than 0.001%, the effect is insufficient. Further, Mn: more than 3.0%,
If the content of S: 0.30% is exceeded, the effect is saturated, and rather the toughness is deteriorated. Therefore, the contents of Mn and S are respectively Mn:
3.0% or less and S: 0.001 to 0.30%.

Further, V and N contribute to the refinement of the structure through the precipitation behavior of VN, but V: less than 0.030%,
If N: less than 0.005%, the effect is insufficient. On the other hand, if V: more than 2.00% and N: more than 0.060%, the effect saturates and rather deteriorates toughness. To V: 0.030 to 0.30%, N: 0.005 to 0.0
It was set to 60%.

In addition, Cr, Mo, Ni and Cu are effective elements for increasing the strength of the forged product, but from the economical point of view, the content is Cr: 3.0% or less, Mo: 1.0.
%, Ni: 3.0% or less, Cu: 2.0% or less.

In addition to this, in the steel according to the present manufacturing method, one or more kinds of Al, Ti and Nb are added for the purpose of adjusting the grain size. However, if Al: less than 0.005%, Ti: less than 0.001%, Nb: less than 0.005%, the effect is insufficient, while Al: 0.10%.
If the content exceeds 0.005%, Ti exceeds 0.050%, and Nb exceeds 0.10%, the effect saturates and rather deteriorates toughness.
0.005-0.10%, Ti: 0.001-0.05
0%, Nb: 0.005 to 0.010%.

Further, in the non-heat treated steel according to the method of the present invention, one kind or two or more kinds of Pb, Ca, Te, Se and Bi are added, but these are for the purpose of improving machinability. Is. However, Pb: less than 0.005%, Ca:
Less than 0.001%, Te: less than 0.001%, Se:
If less than 0.010% and Bi: less than 0.010%, the effect is insufficient, Pb: more than 0.50%, Ca: 0.0
Over 50%, Te: over 0.20%, Se: over 0.50%,
If the Bi content exceeds 0.50%, the effect is saturated and the toughness is rather deteriorated. Therefore, the Pb content is 0.005 to 0.005.
0.50%, Ca: 0.001 to 0.050%, Te:
0.001-0.20%, Se: 0.010-0.50
%, Bi: 0.010 to 0.50%.

The above is the basic composition of steel according to the manufacturing method of the present invention. Hot forging was performed using steel materials having these compositions, and cooling conditions for obtaining a structure in which bainite mainly and a small amount of martensite and austenite were mixed were specified as shown below. That is, as shown in the following equation 5,

[0023]

(5) 4.0 ≦ C2 ≦ 6.0

However, C2 = 3 × C1 + Log ₁₀ (CR) C1 = [% C] + 0.6 × [% Mn] -0.1 × [%
S] + 0.45 × ([% Cr] + [% Ni] + [% M
o] + [% Cu]) + 0.85 × [% V] CR: cooling rate at 500 to 800 ° C. (° C./se)
c) [% X]: weight% of element X

When cooled to room temperature according to these conditions, there is no problem in terms of tensile strength and toughness, but the yield strength is insufficient as compared with the heat-treated steel. Therefore, we focused on increasing the yield strength by applying tempering. However, if the tempering temperature is less than 200 ° C, the effect is insufficient, and 600
When the temperature exceeds ℃, the effect is saturated and rather the tensile strength is rather lowered, so the tempering temperature was set to 200 ℃ to 600 ℃.

Regarding the holding time during tempering, if it is less than 5 minutes, the yield strength is not sufficiently improved, and if it exceeds 300 minutes, the effect is saturated, and rather the tensile strength is remarkably lowered, so that it is 5 to 300 minutes. And By complying with the above conditions, tensile strength of 85 kgf / mm ² or more, yield strength of 70
It is possible to manufacture a hot forged non-heat treated steel having a high strength, a high toughness, and an excellent machinability, which has a kgf / mm ² or more.

[0027]

EXAMPLES Examples of a method for producing a non-heat treated steel excellent in high strength, high toughness and machinability according to the present invention will be shown below.

Steels having compositions of symbols 1 to 5 and 6 to 10 shown in Table 1 were melted in a high frequency furnace, cast, and then rolled into a round bar having a diameter of 50 mm. After heating these at 1250 ° C., they were hot forged into I-beams and cooled to room temperature at various cooling rates.
Further, tempering was performed under various conditions.

JIS No. 4 tensile test pieces and JIS No. 3 impact test pieces were sampled in the longitudinal direction from the central portion of these I-beams, and the tensile strength and the Charpy impact value at -50 ° C and 20 ° C were determined.

On the other hand, a part of the slab was heated to 1250 ° C.,
After rolling to a steel plate with a thickness of 30 mm, the cooling rate is 0.3 ° C / s
After cooling to room temperature by ec, the difference in machinability between with and without tempering was compared for this steel sheet. As a measure of machinability, the drill life when a blind hole with a depth of 20 mm is drilled with a SKH9 (φ5) drill at an initial speed of 0.1 mm / rev without cutting oil and the total depth of the holes is 5000 m
The cutting speed (m / min) when m was used.

Table 2 shows that after hot forging, 4.0≤C2≤6.
The mechanical properties when tempered at 300 ° C. for 30 minutes after cooling under the condition of satisfying 0 are shown. Symbols 1 and 2 are first
Is a non-heat treated steel having high strength and high toughness of the invention example of No. 6, and symbols 6 and 7 are comparative steels. Further, symbols 3, 4, and 5 are non-heat treated steels having high strength and high toughness of the invention example according to the second invention, and symbols 8, 9, 10
Are steels of these comparative examples.

[0032]

[Table 1]

[0033]

[Table 2]

As is clear from Table 2, steels 1 to 1 of the present invention are
No. 5 is 85 regardless of tempering after hot forging
Has a tensile strength of more than kgf / mm ² and 3.0 kgf ・
Low temperature (-50 ° C) toughness of m / cm ² or more, 6.5 kgf
-It is found that it has a room temperature (20 ° C) toughness of m / cm ² or more. However, the yield strength is insufficient without tempering. On the other hand, when tempering is performed, the yield strength is improved without significantly impairing the tensile strength and toughness.
gf / mm ² or more.

On the other hand, the steel 6 of the comparative example is an example in which the content of C is below the range of the present invention, and there is no problem in toughness, but the tensile strength and the yield strength are both with and without tempering. It is insufficient.

On the other hand, the steels 7 to 10 of the comparative examples are S
This is an example in which the contents of i, S, V, and N are below the range of the present invention, and there is no problem in strength, but low temperature toughness and room temperature toughness are insufficient.

As for the machinability, as is clear from Table 2, the machinability elements (Pb, Ca, Te, Se, Bi) -containing 3, 4, and 5 are more preferable than the non-comprising ones. It can be seen that the machinability is excellent. Further, these steel types have almost the same machinability regardless of the presence or absence of tempering.

Table 3 shows the effect of the cooling rate after hot forging on the mechanical properties of Steel 1 of the invention, as an example. Cooling satisfying the formula of 4.0≤C2≤6.0. When cooled at a speed, the tensile strength is ≧ 85 kg before tempering.
It satisfies f / mm ² and has excellent toughness. Further 300
By tempering at ℃ × 30 minutes, yield strength ≧ 70 kgf / mm without significantly impairing tensile strength and toughness.
It is possible to achieve ² .

[0039]

[Table 3]

On the other hand, when cooled under cooling conditions such that C2 <4.0 or C2> 6.0, the toughness is inferior, and even if tempered, the low temperature toughness is 3.0 kgf · m / cm ² or more. It is not possible to achieve room temperature toughness of 6.5 kgf · m / cm ² or more.

Table 4 shows the effects of tempering temperature and time on the mechanical properties of Steel 1 of the invention. Tempering temperature: 200-600 ℃, and holding time: 5
Tensile strength ≧ 85 kgf /
It is possible to satisfy mm ² and yield strength ≧ 70 kgf / mm ² .

[0042]

[Table 4]

[0043]

As described above, by using the manufacturing method according to the present invention, a hot worked non-heat treated steel having a high tensile strength of 85 kgf / mm ² or more, high toughness and excellent machinability. Of which yield strength of 70 can be obtained by tempering at a temperature of 200 to 600 ° C. for 5 to 300 minutes.
It is possible to obtain a high yield strength of not less than kgf / mm ² . As a result, the steel refining step and the straightening step after refining, which are conventionally required, can be omitted, and the manufacturing cost can be reduced. In addition, the increase in yield strength contributes to the improvement in design strength, which makes it possible to reduce the weight of automobiles and the like and to improve the productivity of parts, and the industrial effect is extremely large.

Claims (2)

[Claims] 1. C: 0.10% to 0.60 in% by weight.
%, Si: 0.05% or more and less than 0.8%, Mn:
≦ 3.0%, S: 0.001% to 0.
30%, V: 0.030% to 2.00%, N: 0.
005% to 0.060%, and Cr: ≤
3.0%, Ni: ≤ 3.0%, Mo: ≤
1.0%, Cu: ≤ 2.0%, one or more kinds are included, and Al: 0.005% to 0.10%, Ti:
0.001% to 0.050%, Nb: 0.005% to
0.10% of 1 type or 2 types or more, with the balance being Fe
After hot working the steel consisting of unavoidable impurities and forming it into a predetermined shape, the C2 shown by the following mathematical formula 1 at a cooling rate satisfying 4.0 ≦ C2 ≦ 6.0 between 500 and 800 ° C. Cool to room temperature, and then in the temperature range of 200-600 ° C for 5
~ 300 minutes tempering, then cooled to room temperature again, tensile strength 85kgf / mm ² , yield strength 70kgf / mm
^A method for producing a high-strength, high-toughness hot-worked non-heat treated steel characterized by having ^two or more. ## EQU1 ## C2 = 3 × C1 + log ₁₀ (CR) where C1 = [% C] + 0.6 × [% Mn] −0.1 × [%
S] + 0.45 × ([% Cr] + [% Ni] + [% M
o] + [% Cu]) + 0.85 × [% V] CR: Steel cooling rate (° C between 500 and 800 ° C)
/ Sec) [% X]: weight% of element X 2. C: 0.10% to 0.60 in% by weight.
%, Si: 0.05% or more and less than 0.8%, Mn:
≦ 3.0%, S: 0.001% to 0.
30%, V: 0.030% to 2.00%, N: 0.
005% to 0.060%, and Cr: ≤
3.0%, Ni: ≤ 3.0%, Mo: ≤
1.0%, Cu: ≤ 2.0%, one or more kinds are included, and Al: 0.005% to 0.10%, Ti:
0.001% to 0.050%, Nb: 0.005% to
0.1% of 1 type or 2 types or more, and P
b: 0.005% to 0.50%, Ca: 0.001%
~ 0.050%, Te: 0.001% to 0.20%,
Se: 0.010% to 0.50%, Bi: 0.010%
Steel containing 0.5% to 0.50% and the balance being Fe and unavoidable impurities is hot-worked into a predetermined shape, and then 5
C2 shown by the following mathematical formula 2 between 00 and 800 ° C. is 4.0 ≦ C
Cool to room temperature at a cooling rate satisfying 2 ≦ 6.0, and further temper in a temperature range of 200 to 600 ° C. for 5 to 300 minutes,
Then, it is cooled to room temperature again, and the tensile strength is 85 kgf / m.
A method for producing a high-strength, high-toughness hot-worked non-heat treated steel having a m ² and a yield strength of 70 kgf / mm ² or more and excellent machinability. [Expression 2] C2 = 3 × C1 + log ₁₀ (CR) where C1 = [% C] + 0.6 × [% Mn] −0.1 × [%
S] + 0.45 × ([% Cr] + [% Ni] + [% M
o] + [% Cu]) + 0.85 × [% V] CR: Steel cooling rate (° C between 500 and 800 ° C)
/ Sec) [% X]: weight% of element X