JPH05302116A - Production of hardening obviated steel for hot forging - Google Patents

Abstract

PURPOSE:To manufacture hardening obviated steel having high strength and high toughness and free from heat treating cracks and strains by subjecting steel having a specified compsn. constituted of C, Si, Mn, Cn, Mo, Al, N and Fe to hot forging and thereafter executing specified cooling and tempering. CONSTITUTION:Steeel contg., by weight, 0.10 to 0.30% C, 0.05 to 1.00% Si, 0.80 to 3.00% Mn, 0.30 to 2.00% Cr, 0.05 to 1.00% Mo, 0.002 to 0.100% Al and 0.005 to 0.003% N as well as 0.5Mn +0.5Cr+Mo>=1.45 and furthermore contg., at need, one or more kinds among 0.05 to 0.50% V, 0.005 to 0.030% Ti and 0.01 to 0.30% Nb and/or one or more kinds among 0.04 to 0.12% S, 0.05 to 0.30% Pb and 0.0005 to 0.0100% C, and the balance Fe with impurity elements is subjected to hot forging. After that, this steel is cooled in the temp. range of 700 to 300 deg.C at 5 to 150 deg.C/min and is tempered at 150 to 700 deg.C. In this way, the hardening obviated steel for hot forging excellent in strength, toughness, yield ratio and fatigue ratio and free from cracks after heat treatment can be obtd.

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, cracks and strains are generated 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 SCr440, SCM440, S3
It is usually inferior in strength and toughness compared to alloy steel such as 5C and carbon steel that have been tempered. 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 the refined alloy steel and carbon steel. However, tempered alloy steel,
Compared with carbon steel, it is inferior in yield ratio and durability ratio, and yield strength and fatigue strength are low despite high tensile strength. Therefore, in order to obtain yield strength and fatigue strength equivalent to those of the steel, it is necessary to adjust to higher tensile strength, resulting in poor forgeability, machinability, etc., which is an obstacle to application. is the current situation. 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. To provide a method of manufacturing a low carbon bainite type hot forging quenching steel for hot forging which has properties equal to or higher than that of a tempered alloy steel in all properties including a ratio and can be applied to large size parts. With the goal.

[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. Further, unlike tempered alloy steel and carbon steel, quenching treatment can be omitted, so it was 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 was completed. It has come to.

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%, N: 0.005 to 0.030%, and 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≥ 1.45 with the balance being Fe and impurity elements. After hot forging steel consisting of 70
A method for manufacturing a hardened steel for hot forging, characterized by cooling in a temperature range of 0 to 300 ° C at a cooling rate of 5 to 150 ° C / min, and then tempering at a temperature of 150 to 700 ° C. Yes,
The second invention is to improve the toughness, yield ratio, and durability ratio by precipitation of carbonitride, so that V: 0.05 to 0.50%, Ti: 0.005 to 0.03
0%, Nb: 0.01 to 0.30%, one or more of them are contained, and the third and fourth inventions further improve the machinability, and therefore, the steels of the first and second inventions are further added. S: 0.04-0.12%
, Pb: 0.05 to 0.30%, Ca: 0.0005 to 0.0100%, 1
One or two or more kinds 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, forging and bainizing the structure after cooling. Mn content 0.80
If it is less than%, the hardenability is insufficient, it becomes difficult to obtain a bainite structure, and the strength and toughness are insufficient, so the lower limit is 0.80.
% However, even if the content exceeds 3.00%, the above effect is saturated and the toughness is rather decreased, so the upper limit was made 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 to 0.100% Al is an element which has a strong deoxidizing effect and also has an effect of making fine austenite crystal grains by a pinning effect in combination with N. It is necessary to contain more than%. 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%.

N: 0.005 to 0.030% N has a strong affinity with Al, V, Ti, and Nb.
, Ti, and Nb are present as carbonitrides, and they have the effect of improving the toughness by refining the austenite crystal grains due to the pinning effect. In order to obtain the above effect, it is necessary to contain 0.005% or more. However, when contained in a large amount, the carbonitride is not formed and a large amount of N 2 which is not effective in improving the toughness is present and conversely the toughness decreases, so the upper limit was made 0.030%.

V: 0.05 to 0.50%, Ti: 0.005 to 0.030%, Nb:
0.01 to 0.30% V, Ti, and Nb are carbonitrides that precipitate in the steel, and are elements effective in improving the toughness, yield ratio, and durability ratio of the steel of the present invention by refining the crystal grains and strengthening the precipitation. is there. In order to obtain the above effect, it is necessary that V content is 0.05%, Ti content is 0.005%, and Nb content is 0.01%. However, even if a large amount is contained, the effect will saturate and the cost will increase, so the upper limit of V is 0.
50%, Ti was 0.030%, 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 (%) ≧ 1.45 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.45 is a fine structure after forging and cooling. This is a necessary condition for ensuring the hardenability necessary for obtaining excellent strength and toughness by using bainite or a mixed structure of bainite and martensite. If Mn,
Insufficient Cr and Mo contents 0.5Mn (%) + 0.5Cr (%) + Mo (%) <1.
When it becomes 45, proeutectoid ferrite or pearlite is generated, or even if a bainite structure is obtained, it becomes a coarse bainite lath structure, so that excellent strength and toughness cannot be obtained. Therefore, 0.5Mn (%) + 0.5Cr (%) + Mo (%)
It is necessary to be ≧ 1.45.

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

Further, the tempering temperature is 150 ° C. or higher, 700 ° C.
Limiting to the following range is effective only at a temperature of 150 ° C or higher in order to decompose the residual stress due to transformation generated by cooling after forging and to decompose MA in the microstructure to increase the yield ratio and durability ratio. 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 20 are the steels of the present invention, steels 1 to 3 are the first invention, steels 4 to 10 are the second invention, and 11 to
Steel No. 14 corresponds to the third invention, and steel Nos. 15 to 20 correspond to the fourth invention. Steels 21 to 25 are comparative steels that do not partially satisfy the conditions of the present invention, steel 26 is a conventional ferrite / pearlite type non-heat treated steel, and steels 27 and 28 are conventional steels, respectively.
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 for 1 ~ 25 steel after forging
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 26 steel, after forging it was allowed to air-cool and used as a test material.For steel 27, 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 28 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. Further, the strain was measured by measuring the dimensions of each part and checking whether the strain was within a predetermined tolerance. Using the method described above, we evaluated 30 forged products with the shape shown in Fig. 1,
Table 2 shows the number of cracks found and the number of forged products that were out of tolerance.

[0031]

[Table 2]

As is clear from Table 2, when comparing the comparative steels and the conventional steels 21 to 28 with the steels of the present invention, since the 21 steel has a high C content, the proof stress and tensile strength are excellent. However, the impact value is inferior, and the 22 and 23 steels have a low Mn or Cr content, and the value of 0.5Mn + 0.5Cr + Mo (referred to as the following formula (1)) is less than 1.45. Insufficient permeability, ferrite and bainite structure, 0.2% proof stress, tensile strength,
The yield ratio and durability ratio are inferior, and since the 24 steel has a low Mo content and the value of the formula (1), the hardenability is insufficient like the 22 and 23 steels, and 0.2% proof stress, tensile strength, Yield ratio and durability ratio are inferior.The chemical composition of each element of 25 steel is within the scope of the present invention, but hardenability is insufficient because it does not satisfy the value of formula (1), Similar to 24 steel, it has poor tensile strength, yield ratio, and durability ratio. In addition, conventional 26 ferritic-pearlite type non-heat treated steel is inferior in all mechanical properties such as tensile strength, yield ratio, durability ratio, impact value, and 27 steel which is heat treated material of SCM440 and 28 steel, which is the forged and quenched S35C tempered material,
Although the mechanical properties are almost the same as those of the steel of the present invention, cracking and distortion occur due to quenching, and it takes a lot of time for final inspection and correction processing.

On the other hand, the steels 1 to 20 which are the subject steels of the present invention are low carbon, and the hardenability-improving elements Mn, Cr and Mo are regulated within an appropriate range and subjected to optimum cooling. Furthermore, by performing tempering treatment, it has excellent performance of 0.2% proof stress 80kgf / mm ² , tensile strength 96kgfkgf / mm ² , yield ratio 0.81 or more, durability ratio 0.51 or more, impact value 10kgfm / cm ² or more. Since the quenching process can be omitted, there are no defects due to cracking, distortion, etc. Also, regarding the machinability, the steel of the present invention
It is better than conventional steels such as SCM440 and S35C. Especially, the third and fourth inventions with added machinability elements show excellent machinability without impairing the performance such as strength, toughness and fatigue strength. Was confirmed.

Next, an example in which the influence of the change in the cooling rate after forging was investigated will be shown. Among the steels shown in Table 1, the diameters of the steels of the present invention, 2, 5, 17 and 19 and the comparative steels of 21 and 25,
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, the average cooling rate at 700 to 300 ℃ was measured at 3 to 180 ℃ / m.
The characteristics were measured and evaluated under the same test conditions as those of the above-described examples, while varying between in. The results are shown in Table 3.

[0035]

[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. Comparative steel No. 21, which has a high C content, has a low impact value at all cooling rates tested, and Steel No. 25 does not satisfy equation (1), and therefore has a range of cooling rates that provides excellent strength. 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 must be 5 ° C / min or more and 150 ° C / min or less.

Next, an example in which the influence of the change in tempering temperature was investigated is shown below. Among the steels shown in Table 1, 13 and 19 steels of the present invention, which were hot rolled steel bars having a diameter of 60 mm, were used to prepare test materials 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.

[0038]

[Table 4]

As shown in Table 4, it is understood that the tempering treatment makes it possible to eliminate the drawback of the low carbon bainite type non-heat treated steel, which had a problem in that the yield ratio and the durability ratio were low. .. However, it should be noted that the effect is insufficient when the temperature is low, and the strength decreases when the temperature is too high. 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.

[0040]

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 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, and it can be applied to large-sized parts because cracks and distortions due to heat treatment do not occur and rapid cooling is not required. Further, it has very excellent performance and 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 1]

[Table 1]

[Table 2]

[Table 2]

[Table 3]

[Table 3]

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location C22C 38/38 (72) Inventor Motohide Mori 1st Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Chiyoshi Maeda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor Shigeru Yasuda 1, Toyota Town, Toyota City, Aichi 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%, N: 0.005-0.030%, and 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.45,
After hot forging steel with the balance being Fe and impurity elements,
A method for producing a hardened steel for hot forging, which comprises cooling a temperature range of 700 to 300 ° C at a rate of 5 to 150 ° C / min, and then tempering at a temperature of 150 to 700 ° C. 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%, N: 0.005-0.030%, V: 0.05-0.50%, Ti: 0.005-0.030%, N
b: contains 0.01 to 0.30% of 1 or 2 or more,
And 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≧ 1.45, the balance is Fe
700 to 3 after hot forging steel consisting of
A method for producing a hardened steel for hot forging, which comprises cooling a temperature range of 00 ° C at a rate of 5 to 150 ° C / min, and then tempering at a temperature of 150 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%, N: 0.005-0.030%, S: 0.04-0.12%, Pb: 0.05-0.30%, C
a: A steel containing 0.0005 to 0.0100% of 1 or 2 or more, 0.5Mn (%) + 0.5Cr (%) + Mo (%) ≥1.45 and the balance of Fe and impurity elements. After hot forging, 700
Cool the temperature range of ~ 300 ℃ at a rate of 5 ~ 150 ℃ / min,
After that, tempering is performed 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%, N: 0.005-0.030%, V: 0.05-0.50%, Ti: 0.005-0.030%, N
b: 0.01 to 0.30% of 1 or 2 or more types and S: 0.04 to
0.12%, Pb: 0.05-0.30%, Ca: 0.0005-0.0100% One or more kinds are contained, and 0.5Mn (%) + 0.5Cr
(%) + Mo (%) ≥ 1.45, and after the steel is hot forged with the balance being Fe and impurity elements, the temperature range from 700 to 300 ℃ is 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 ~ 700 ° C.