JPH05302117A - Manufacturing method for quench-hardened steel for hot forging - Google Patents

Abstract

(57) [Summary] [Structure] C: 0.10 to 0.30% by weight, Si: 0.05 to 1.00%
, Mn: 0.80 to 3.00%, Cr: 0.30 to 2.00%, Mo: 0.05 to
1.00%, Al: 0.002-0.100%, Ti: 0.01-0.05%, B: 0.00
05-0.0040%, V: 0.05-0.50%, Nb:
1 to 2 out of 0.01 to 0.30%, S: 0.04 to 0.12%
, Pb: 0.05 to 0.30%, Ca: 0.0005 to 0.0100%, one or more of them are contained as necessary, and 0.5M
n (%) + 0.5Cr (%) + Mo (%) ≧ 1.20, the balance of which is Fe and impurity elements, after hot forging of steel, 700 ℃ ~ 300 ℃ 5 ~
A method for producing a hardened steel for hot forging, which comprises cooling at a rate of 150 ° C / min, and then tempering at a temperature of 150 to 700 ° C. [Effect] It solves the disadvantage of low carbon bainite type non-heat treated steel, which has low yield ratio and durability ratio, and has mechanical properties equivalent to or better than heat treated alloy steel and carbon steel. Applicable. Further, quenching can be omitted, and cracks and strains do not occur after heat treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has excellent strength, toughness and high yield ratio and durability ratio by omitting quenching after hot forging and only tempering, and the dimensions of parts and forging conditions. Has little change in strength and toughness, and has almost no cracks or strains after heat treatment, and is particularly useful as a steel used for undercarriage parts of automobiles requiring high strength and high toughness. TECHNICAL FIELD The present invention relates to a method for manufacturing quench-hardened steel for forging.

[0002]

2. Description of the Related Art Conventionally, among automobile underbody parts such as steering knuckles and upper arms, and large parts for construction machines such as rod ends, parts for which high strength and high toughness have been particularly required are used for mechanical structures. Alloy steel SCr440, SCM440
S3, which is carbon steel for machine structural use, is applied by quenching and tempering treatment after hot forging (hereinafter referred to as tempering)
5C, S45C, etc. were tempered after forging and quenching to ensure excellent performance.

However, these heat treatments require enormous energy, and since quenching treatment is indispensable, cracking and distortion occur after the heat treatment, which has been a problem. Therefore, in order to meet the social demand for energy saving, since the 1950s, the heat of hot forging has been used to develop non-heat treated steel that can obtain excellent properties by natural air cooling after forging. Has been done.

For example, a trace amount of carbonitride forming elements such as V, Nb, and Ti are added to carbon steel for machine structure specified in JIS G4051 and manganese steel for machine structure specified in JIS G4106 and manganese chromium steel. A non-heat treated steel has been developed that enables the omission of heat treatment by precipitation strengthening with the trace elements of. However, these non-heat treated steels have a coarse ferrite / pearlite structure, and are inferior in strength and toughness to those of alloy steels such as SCr440 and SCM440 and carbon steels such as S35C and S45C that have been heat treated. Is normal. Therefore, it has been difficult to apply it to parts for which strength and toughness are highly demanded, such as undercarriage parts of automobiles.

Recently, in order to solve the drawback that these ferrite-pearlite type non-heat treated steels are particularly inferior in terms of toughness, non-heat treated steels having a bainite structure have been actively researched. Compared with conventional non-heat treated steels, this non-heat treated steel has a lower carbon content, and is added with an appropriate amount of Mn, Cr, Mo, B, etc. to improve hardenability. It has a phase or a mixed structure of ferrite and bainite, and is disclosed in, for example, JP-A-61-139646 and JP-A-SHO.
61-238941, JP 62-205245, JP 62-260042
Japanese Patent Laid-Open No. Sho 63-130748 proposes steels.

[0006]

The low carbon bainite type non-heat treated steel as shown in the above-mentioned Japanese Patent Laid-Open is superior in strength and toughness to the conventional ferrite / pearlite type non-heat treated steel, It has the same tensile strength and impact value as those of high quality alloy steel. However, the yield ratio and the durability ratio are inferior to those of the heat-treated alloy steel and the carbon steel, and the yield strength and the fatigue strength are low despite the high tensile strength. Therefore, in order to obtain equivalent yield strength and fatigue strength, it is necessary to adjust to a higher tensile strength, resulting in poor forgeability, machinability, etc., which is an obstacle to application. Is. Further, when heat-treated alloy steel, carbon steel is used, cracking occurs after heat treatment as described above, strain occurs, inspection of the presence or absence of cracking and strain correction processing are required, and the manufacturing process becomes complicated, If the size of the component becomes large, the hardenability becomes insufficient and it becomes difficult to obtain excellent characteristics.

The present invention has been made in consideration of the above-mentioned problems of conventional heat-treated alloy steel, carbon steel and non-heat-treated steel. It does not cause cracks or strains after heat treatment, and has a yield ratio and durability. Aiming to provide a low carbon bainite type hot forging steel that has properties equal to or higher than those of heat-treated alloy steel and carbon steel in all properties including the ratio and can be applied to large-sized parts. To do.

[0008]

Under the above-mentioned object, the present inventor has conducted extensive studies on the cause of the low yield ratio and durability ratio of low carbon bainite type non-heat treated steel and its countermeasures,
The present invention was obtained without the following knowledge.

That is, the cause of the yield ratio and the durability ratio of the bainite type non-heat treated steel is low is that high carbon island martensite and retained austenite (hereinafter referred to as MA) existing in the microstructure of the bainite steel, It was found that this was due to the residual stress generated inside by the transformation that occurs during air cooling after hot forging. Therefore, the low carbon bainite type non-heat treated steels such as the inventions described in the above-mentioned publications have excellent toughness and have the greatest feature of complete omission of heat treatment, but have a problem in terms of structure and transformation strain. That is, the characteristics of bainite steel were not fully utilized.

Therefore, the inventor of the present invention has found that M-A in the microstructure is
As a result of further research on the method for reducing the amount and the residual stress, after the forging and cooling, the tempering treatment was performed at an appropriate temperature to remove the MA and residual stress existing in the steel. It has been found that it disappears and an excellent yield ratio and durability ratio equal to or higher than those of heat-treated alloy steel and carbon steel can be obtained.
Furthermore, unlike the above steel, since quenching treatment can be omitted, it has been confirmed that there is no deformation or cracking after heat treatment, and that excellent characteristics are obtained regardless of the size of the parts, and the present invention has been completed. Is.

The first invention of the present invention completed based on the above-mentioned idea is C: 0.10 to 0.30% in weight ratio, Si: 0.05 to.
1.00%, Mn: 0.80 to 3.00%, Cr: 0.30 to 2.00%, Mo: 0.0
5 to 1.00%, Al: 0.002 to 0.100%, Ti: 0.01 to 0.05%, B:
0.0005 to 0.0040% and 0.5Mn (%) + 0.5Cr (%) + Mo
(%) ≧ 1.20, the balance of which is 5% to 150 ° C from 700 ° C to 300 ° C after hot forging of steel consisting of Fe and impurity elements
It is a quench-excluded steel for hot forging characterized by cooling at a cooling rate of / min and then tempering at a temperature of 150 to 700 ° C. To further improve the ratio, V: 0.05-0.50%, Nb: 0.01-
One or two of 0.30% is included,
The third and fourth inventions further improve the machinability of the first and second inventions by adding S: 0.04 to 0.12%, Pb: 0.05 to 0.30%, and Ca: 0.0.
One or two or more of 005 to 0.0100% are contained.

Next, the reasons for limiting the composition of components in the method for producing a hardened steel for hot forging according to the present invention will be described below.

C: 0.10 to 0.30% C is an element necessary for securing strength, and it is necessary to contain 0.10% or more. However, if contained in a large amount, the impact value decreases, the residual stress generated during forging cooling increases, and the yield ratio and durability ratio decrease, so the upper limit is set to 0.
It was set to 30%.

Si: 0.05-1.00% Si is added for deoxidation at the time of steel making.
It is necessary to contain more than%. However, if the content exceeds 1.00%, the toughness decreases, so the upper limit was made 1.00%.

Mn: 0.80 to 3.00% Mn is an element necessary for improving the hardenability and forging and bainizing the structure after cooling. If the content of Mn is less than 0.80%, the hardenability is insufficient, it becomes difficult to obtain a bainite structure, and the strength and toughness are insufficient, so the lower limit was made 0.80%. However, even if the content exceeds 3.00%, the above effect is saturated and the toughness decreases, so the upper limit is set.
It was 3.00%.

Cr: 0.30 to 2.00% Cr is an element necessary to bainite the structure similarly to Mn, and the content of 0.30% or more is necessary. But 2.00
Even if the content is more than%, the effect will be saturated, and
Since the cost is high, the upper limit was set to 2.00%.

Mo: 0.05 to 1.00% Mo is an element necessary for improving hardenability to bainite the structure as well as Mn and Cr, and for refining bainite lath to improve strength and toughness. is there. 0.05%
If the content is less than the above, the above effect cannot be sufficiently obtained, so the lower limit was made 0.05%. However, if the content exceeds 1.00%, the above effect is saturated and the cost increases, so the upper limit was made 1.00%.

Al: 0.002-0.100% Al is an element having a strong deoxidizing effect, and it is necessary to contain 0.002% or more. However, even if the content exceeds 0.100%, the effect is saturated and the machinability is reduced, so the upper limit was made 0.100%.

Ti: 0.01 to 0.05% Ti is an element necessary for fixing N existing as an impurity in steel in order to effectively obtain the hardenability improving effect of B described later, and in order to obtain the effect. Is required to contain 0.01% or more. However, even if the content exceeds 0.05%, the effect is saturated, so the upper limit was made 0.05%.

V: 0.05 to 0.50%, Nb: 0.01 to 0.30% V, Nb refines bainite lath like Mo, and precipitates as fine carbonitrides after tempering. Ratio and yield ratio are effective elements. To obtain the above effect, V is 0.05% and Nb is
0.01% content is required. However, even if a large amount is contained, the effect will be saturated and the cost will increase, so the upper limit is
V was 0.50% and Nb was 0.30%.

S: 0.04 to 0.12%, Pb: 0.05 to 0.30%, Ca:
0.0005 to 0.0100% S, Pb, and Ca are effective elements for improving machinability, and are added as necessary. In order to obtain the above effects, it is necessary to contain 0.04%, 0.05% and 0.0005%, respectively. However, even if contained in a large amount, the effect saturates and the toughness decreases, so the upper limit is 0.12%,
It was set to 0.30% and 0.0100%.

0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.20 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.20 is a fine bainite structure after forging and cooling. Alternatively, a mixed structure of bainite and martensite is a necessary condition for ensuring the hardenability necessary for obtaining excellent strength and toughness. If Mn, Cr,
If the Mo content is insufficient and 0.5Mn (%) + 0.5Cr (%) + Mo (%) <1.20, proeutectoid ferrite or pearlite is generated, or even if a bainite structure is obtained, coarse bainite is obtained. Since it has a lath structure, excellent strength and toughness cannot be obtained. Therefore, 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.
It should be 20.

Next, the reasons for limiting the manufacturing conditions of the present invention will be described. The cooling condition after hot forging was limited to 700 to 300 ° C because when the cooling rate is 5 ° C / min or less, pro-eutectoid ferrite and pearlite are likely to be formed, or a bainite lath coarse structure is likely to occur. This is because it is difficult to secure excellent properties as a fine bainite lath structure. Also, at a cooling rate of 150 ° C / min or higher, excellent mechanical properties can be secured, but cracks occur after cooling. Or distortion may occur.

Further, the tempering temperature is 150 ° C. or higher, 700 ° C.
The temperature is limited to the following temperature. It is effective to decompose residual stress due to transformation caused by cooling after forging and MA in the microstructure to increase the yield ratio and durability ratio unless the temperature is 150 ° C or higher. In addition, at a temperature of 700 ° C. or higher, carbides are agglomerated and softened, or a reverse transformation from α to γ occurs, so that excellent strength cannot be obtained.

[0025]

EXAMPLES The features of the present invention will be described below in comparison with comparative steels and conventional steels with reference to examples. Table 1 shows the chemical components of the test materials used in the examples.

[0026]

[Table 1]

In Table 1, steels 1 to 13 are steels of the present invention, steels 1 to 3 are the first invention, steels 4 to 6 are the second invention, and steels 7 to 7 are the same.
The 10th steel corresponds to the third invention, and the 11th to 13th steels correspond to the 4th invention. Steels 14 to 19 are comparative steels that do not partially satisfy the conditions of the present invention, steel 20 is a conventional ferrite-pearlite type non-heat treated steel, and steels 21 and 22 are conventional steels.
0, S35C.

A round bar having a diameter of 60 mm manufactured by hot rolling having the composition shown in Table 1 is heated to a temperature of 1200 to 1250 ° C., and a shape as shown in FIG. 1 is set at a temperature of 1100 to 1150 ° C. Various properties were evaluated by the test described below after forging, heat treatment. Heat treatment is 700-3 after forging for 1-19 steel
Cool the temperature range of 00 ℃ at a rate of 20 ℃ / min, then 600
After heating at ℃ for 90 minutes, it was tempered by natural air cooling.
For 20 steel, after forging it was allowed to air-cool and used as the test material.For 21 steel, after forging it was naturally air-cooled to room temperature, heated to a temperature of 880 ° C, oil-quenched, and tempered at 580 ° C. It was used as a test material. The 22 steel was water-quenched immediately after forging and tempered at a temperature of 520 ° C.

Using the test material produced by the above-mentioned method,
The microstructure was observed, and 0.2% proof stress, tensile strength, yield ratio, durability ratio, impact value, machinability, presence of cracks, and strain were measured by the methods described below.

The microstructure is obtained by observing a part of the test material under an optical microscope at a magnification of 400 times. The 0.2% proof stress, tensile strength, and yield ratio are measured by making JIS14A tensile test pieces and conducting a tensile test under the conditions of a tensile speed of 1 mm / sec. The durability ratio is based on the Ono-type rotary bending fatigue test.
The fatigue strength at 10 ⁷ revolutions was calculated and the ratio to the tensile strength was taken. The machinability was evaluated by a drilling test. In the test, the drill was a straight shank with a diameter of 5 mm, the material of the drill was SKH51, and the drill rotation speed was 1710 r.p.
m., no cutting oil, load 75 kg. The measurement results are shown as an integer ratio with the perforation distance of the 27 steel, which is the conventional steel, as 100. The cracks were measured using a magnetic particle flaw detector. In addition, the strain was measured by measuring the dimensions of each part and evaluating whether or not it was within a predetermined tolerance.
30 forged products having the shape shown in FIG. 1 were evaluated by the above-described method, and the number of cracked products and those out of tolerance were shown in Table 2.

[0031]

[Table 2]

As is clear from Table 2, when comparing the comparative steels and the conventional steels 14 to 22 with the steels of the present invention, although the steel 14 has a high C content, it has excellent yield strength and tensile strength. ,
On the other hand, the impact value is inferior.
Since the content is low and the value of 0.5Mn + 0.5Cr + Mo (hereinafter referred to as the formula (1)) is less than 1.20, the hardenability is insufficient, resulting in a ferrite + bainite structure, yield strength, tensile strength, and yield. Ratio, durability ratio is inferior, 17 and 18 steels are Mo,
Since the B content is low, the hardenability is insufficient like the 15 and 16 steels, and the tensile strength, yield ratio, and durability ratio are inferior, and the 19 steels have chemical elements of each element within the range of the present invention. It's in
Since the value of the formula (1) is not satisfied, the tensile strength, yield ratio, and durability ratio are inferior as in the case of the 15-18 steel. In addition, conventional 20 steel, which is a ferrite / pearlite type non-heat treated steel, is inferior in all mechanical properties such as tensile strength, yield ratio, durability ratio and impact value, and 21 steel which is a heat treated material of SCM440 and 22 steel, which is a forged and quenched and tempered material of S35C, has almost the same mechanical properties as the steel of the present invention, but cracks and strains occur due to quenching, and it takes a lot of time for final inspection and correction processing. It costs.

On the other hand, the steels 1 to 13 which are the subject steels of the present invention are low carbon, and the hardenability-improving elements Mn, Cr and Mo are regulated to an appropriate range and subjected to optimum cooling. Furthermore, by performing tempering treatment, 0.2% proof stress 80 kgf / mm ² or more, tensile strength 96 kgfkgf / mm ² or more, yield ratio 0.81 or more, durability ratio 0.51 or more, impact value 10 kgfm / cm ² or more In addition to the above, since quenching treatment can be omitted, there are no defects due to cracking, distortion, etc.

Regarding the machinability, the steel of the present invention is SCM44.
0, S35C, etc. are better than conventional steels, and in particular, the third and fourth invention steels added with machinability elements show excellent machinability without impairing performance such as strength, toughness and fatigue strength. I was able to confirm that.

Next, an example in which the influence of the change in cooling rate after forging was investigated was shown. Diameters of the steels of the present invention 1, 5, 7, 12 and the comparative steels 14 and 15 among the steels shown in Table 1
Using a 60 mm hot rolled steel bar, various characteristics were investigated by changing only the cooling rate condition after forging with respect to the test material manufacturing conditions of the above-described examples.

In order to investigate the effect of cooling conditions, 700 to 30
The average cooling rate at 0 ° C. was changed between 3 and 180 ° C./min, and each characteristic value was measured and evaluated under the same test conditions as in the above-mentioned examples. The results are shown in Table 3.

[0037]

[Table 3]

As is clear from Table 3, the steels according to the present invention,
In both comparative steels, the higher the cooling rate, the better the tensile strength, yield ratio, durability ratio and impact value, and when the cooling rate is slower, ferrite precipitates and these mechanical properties deteriorate. The comparative steel 14 has a high C content, so the impact value is low at all the cooling rates tested, and the steel 15 does not satisfy the formula (1), so the range of cooling rates with excellent strength can be obtained. Is narrow. On the other hand, the steel of the present invention has a wider range of conditions for obtaining excellent mechanical properties than the comparative steel, but it is necessary to cool at a rate of 5 ° C / min or more.

On the other hand, when the cooling rate is increased, the mechanical properties are improved, but cracks and strains occur at about 150 ° C./min. Therefore, the cooling rate after forging is 5 ℃ / min or more, 150 ℃
It should be less than / min.

Next, an example in which the influence of the change in tempering temperature was investigated is shown below. Of the steels shown in Table 1, the steels of the present invention 3, 7 which were hot rolled steel bars having a diameter of 60 mm were prepared by the same method as in the example of Table 2 except for tempering conditions. In addition, in order to understand the effect of the tempering treatment and clarify the difference between the present invention and the low carbon bainite type non-heat treated steel that has been developed in recent years, a test material without tempering was prepared. .. Then, the structure observation, the tensile test, the impact test, the fatigue test, the crack, and the strain were measured in the same manner as in the previous example. The results are shown in Table 4.

[0041]

[Table 4]

As shown in Table 4, it is understood that the tempering treatment can eliminate the disadvantage of the low carbon bainite type non-heat treated steel having a low yield ratio and a low durability ratio.
However, when the temperature is low, the effect is insufficient,
If it is too high, the strength will decrease, so care must be taken.
Table 4 shows that in the case of the steel of the present invention, the treatment temperature should be 150 ° C or higher and 700 ° C or lower.

[0043]

EFFECTS OF THE INVENTION The method for producing a hardened steel for quenching for hot forging according to the present invention is characterized in that a low carbon bainite type non-heat treated steel is subjected to a tempering treatment so that it is more durable than a conventional low carbon bainite type non-heat treated steel. As a result of remarkably improving the ratio and the yield ratio, it has excellent properties equal to or higher than those of the heat-treated alloy steel and the carbon steel while omitting the quenching treatment. Further, since quenching can be omitted, it can contribute to energy saving, there is no cracking and distortion due to heat treatment, and rapid cooling is not required, so it can be applied to large-sized parts. Further, since the performance is also very excellent, it can be used in place of the heat-treated alloy steel and the carbon steel even for the parts that are required to have high strength and toughness.

[Brief description of drawings]

FIG. 1 is a diagram showing a shape of a forged product manufactured as an example.

[Table 3]

[Table 3]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Corporation (72) Inventor Motohide Mori 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Corporation (72) Inventor Chihori Maeda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd.

Claims (4)

[Claims] 1. A weight ratio of C: 0.10 to 0.30%, Si: 0.05
~ 1.00%, Mn: 0.80 ~ 3.00%, Cr: 0.30 ~ 2.00%, Mo:
0.05-1.00%, Al: 0.002-0.100%, Ti: 0.01-0.05%,
B: 0.0005 to 0.0040%, and 0.5Mn (%) + 0.5Cr (%) +
Mo (%) ≧ 1.20, the balance of which is Fe and impurity elements. After hot forging of steel, 5 to 150
A method for producing a hardened steel for hot forging, which comprises cooling at a rate of ℃ / min and then tempering at a temperature of 150 to 700 ℃. 2. The weight ratio of C: 0.10 to 0.30%, Si: 0.05
~ 1.00%, Mn: 0.80 ~ 3.00%, Cr: 0.30 ~ 2.00%, Mo:
0.05-1.00%, Al: 0.002-0.100%, Ti: 0.01-0.05%,
B: 0.0005 to 0.0040%, V: 0.05 to 0.50%,
Nb: contains 0.01 to 0.30% of 1 or 2 types, and
0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.20, the balance of which is Fe and impurity elements.
Cool down to 0 ° C at a rate of 5 to 150 ° C / min, then cool to 150 ° C.
A method for manufacturing a quenched and abbreviated steel for hot forging, which comprises performing tempering at a temperature of up to 700 ° C. 3. C: 0.10 to 0.30% by weight ratio, Si: 0.05
~ 1.00%, Mn: 0.80 ~ 3.00%, Cr: 0.30 ~ 2.00%, Mo:
0.05-1.00%, Al: 0.002-0.100%, Ti: 0.01-0.05%,
B: 0.0005 to 0.0040%, S: 0.04 to 0.12%,
Pb: 0.05-0.30%, Ca: 0.0005-0.0100% One or more types are contained, and 0.5Mn (%) + 0.5Cr (%) + Mo (%)
≧ 1.20, the balance of which is Fe and impurity elements, after hot forging, from 700 ℃ to 300 ℃ 5 ~ 150 ℃ / min
A method for producing a quench-excluded steel for hot forging, which comprises cooling at a rate of 1, then tempering at a temperature of 150 to 700 ° C. 4. A weight ratio of C: 0.10 to 0.30%, Si: 0.05
~ 1.00%, Mn: 0.80 ~ 3.00%, Cr: 0.30 ~ 2.00%, Mo:
0.05-1.00%, Al: 0.002-0.100%, Ti: 0.01-0.05%,
B: 0.0005 to 0.0040%, V: 0.05 to 0.50%,
Nb: 0.01 to 0.30% of 1 or 2 types and S: 0.04 to 0.12.
%, Pb: 0.05 to 0.30%, Ca: 0.0005 to 0.0100%, 1
Contains one or more species and 0.5Mn (%) + 0.5Cr (%) + Mo
(%) ≧ 1.20, the balance of which is 5% to 150 ° C from 700 ° C to 300 ° C after hot forging of steel consisting of Fe and impurity elements
A method for producing a hardened steel for hot forging, which comprises cooling at a rate of / min and then tempering at a temperature of 150 to 700 ° C.