JP3089424B2 - Manufacturing method of tough non-heat treated steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is not subjected to heat treatment such as quenching and tempering after hot forging, and has a tensile strength of 90 kgf / mm ² or more without being tempered, and a Charpy impact value. The present invention relates to a method for producing a tough non-heat treated steel having high strength and high toughness of 8 kgf / mm ² or more, and particularly useful as a steel used for undercarriage parts of automobiles requiring high strength and high toughness.

[Prior Art] Conventionally, high strength and high toughness are required for steel used for undercarriage parts of automobiles such as steering knuckles and upper arms or large parts such as construction machines, and are alloy steels for machine manufacturing. SCM440 or SCr440 was used, and after forming by hot forging, heat treatment such as quenching and tempering (hereinafter referred to as tempering) was performed to impart high strength and high toughness.

However, these heat treatment steps are quite expensive, and if the heat treatment steps can be omitted, a significant cost reduction can be achieved and social demands for energy saving can be met. Therefore, development of non-heat treated steel that can be used as hot forged has been actively performed in recent years.

For example, a medium-carbon steel containing 0.30 to 0.50% of C, or a ferrite-pearlite non-heat treated steel obtained by adding 0.03 to 0.20% of V to Mn steel has been proposed. In the non-heat-treated steel, carbonitrides of V precipitate during the cooling process after hot forging.
Carbonitride strengthens the ferrite material.

[Problems to be Solved by the Invention] However, the conventionally developed non-heat treated steel has a coarse ferrite / pearlite structure, and the toughness is equivalent to that of a medium carbon steel heat treated material. It is far from the performance of SCM440 and others. On the other hand, a bainite type non-heat treated steel in which the C content is reduced and the alloy element is increased has been proposed, but the strength and toughness are insufficient, and only insufficient results have been obtained. In particular, although toughness can be ensured for small parts, toughness is insufficient for large parts, so that they have not yet been put to practical use.

The present invention has been made in view of the above-mentioned problems of conventional non-heat-treated steel, and can provide a tensile strength and impact value of SCM440 or SCr440 or more without heat-treatment, and can provide properties such as strength and toughness. It is an object of the present invention to provide a method for producing a tough non-heat treated steel having a high hardness.

[Means for Solving the Problems] The inventors have conducted intensive studies on the reason why the current bainite type non-heat treated steel lacks strength and toughness. As a result, in the case of bainite steel, if the cooling rate is sufficiently fast, a small part with excellent strength and toughness can be obtained, but if the part becomes large and the cooling rate becomes slow, the interval between the bainite laths will increase. It was found that the structure was large and coarse, and sufficient strength and toughness could not be obtained.

Therefore, the research was repeated on various measures for making the bainite structure having a large bainite lath interval into a fine structure. This coarse structure is composed of a mixed structure of bainite lath and untransformed austenite, and the inventors control the cooling rate from 700 ° C to 500 ° C after hot forging and from 500 ° C to 300 ° C, This untransformed austenite is easily decomposed into fine carbides and ferrite,
It was found that a structure with a finely packed bainite lath interval was obtained, and the strength and impact value could be significantly improved.

The characteristics of the steel used in the method of the present invention are as follows: first, the toughness is improved by low carbonization;
Hardening properties were improved by the addition of Cr and Mo, and a bainite structure having excellent strength and toughness was obtained only by cooling after hot forging. Third, when the cooling rate after heating changes, the bainite structure generally changes in strength and toughness and is difficult to handle. However, by adding V to the structure, the strength and the strength against the change in cooling rate are increased. That is, the change in toughness was reduced.

The method for producing a tough non-heat treated steel of the present invention has been completed based on the above findings, and as a first invention, C: 0.05 to 0.20%, Si; 0.10 to 0.50%, Mn; 1.50-2.
50%, Cr; 0.30-1.50%, Mo; 0.50-0.30%, V; 0.05-0.
After hot forging, steel containing 30%, the balance consisting of Fe and impurity elements, is cooled from 700 ° C to 500 ° C at a cooling rate of 10 ° C / min or more. 200 ℃ /
The gist is that cooling is performed at a cooling rate of one minute.

Further, in the second invention, in order to improve the machinability of the first invention, S: 0.04 to 0.12%, Pb; 0.05 to 0.30%, Ca; 0.00
The third invention uses steel containing 0.20 to 2.0% of Ni in order to further improve the toughness of the first invention. In order to improve the toughness in combination with the machinability of the first invention, Ni: 0.2
0-2.0%, S; 0.04-0.12%, Pb; 0.05-0.30%, Ca;
After hot forging, steel containing one or more of 0005 to 0.0100% is cooled from 700 ° C. to 500 ° C. at a cooling rate of 10 ° C./min or more. ° C ~
The main point is to cool at a cooling rate of 200 ° C./min.

[Action] In the method for producing a tough non-heat treated steel of the present invention, the cooling rate from 700 ° C. to 500 ° C. after hot forging is set to 10 ° C./min or more, thereby preventing the precipitation of polygonal ferrite. Can be. The reason why the cooling rate from 700 ° C. to 500 ° C. is 10 ° C./min or more is that when the temperature is lower than 10 ° C., polygonal ferrite precipitates and the strength is reduced.

Also, the cooling rate from 500 ° C to 300 ° C is 10 to 200 ° C /
Because of this, untransformed austenite is decomposed into fine carbides and ferrite, and a structure in which the bainite lath interval is finely packed is obtained, and the strength and impact value can be significantly improved. The reason why the cooling rate from 500 ° C. to 300 ° C. is 10 to 200 ° C./min is that if the cooling rate is less than 10 ° C./min, the bainite lath cannot have a sufficiently fine structure. If the temperature exceeds 200 ° C./min, martensite will be formed.

Next, the reasons for limiting the component composition of the tough non-heat treated steel according to the present invention will be described.

C: 0.05 to 0.20% C is an element necessary for securing the strength, and if it is less than 0.05%, the strength is insufficient, so the lower limit was made 0.05%. When C exceeds 0.20%, the toughness decreases.
0.20%.

Si: 0.10 to 0.50% Si is added as a deoxidizing agent during steel making.
10% is needed. However, if it exceeds 0.50%, the toughness decreases, so the upper limit was made 0.50%.

Mn; 0.50 to 2.50% Mn is an element necessary for improving hardenability and turning the structure into bainite. If Mn is less than 1.50%, the hardenability is insufficient, the formation of bainite is insufficient, and the strength is insufficient, so the lower limit was set to 1.50%. However, when the content exceeds 2.5%, the hardenability is excessively improved, martensite is generated, and the toughness is reduced. Therefore, the upper limit is set to 2.50%.

Cr; 0.30 to 1.50% Cr is an element necessary for turning a structure into bainite.
When the content is less than 0.30%, the effect is insufficient, so the lower limit is set to 0.30%. However, when the content exceeds 0.50%, the effect is saturated and the cost is increased.
50%.

Mo: 0.05 to 0.30% Mo is an element necessary for turning a structure into bainite. If Mo is less than 0.05%, bainitization is insufficient, so the lower limit was set to 0.50%. Mo is an expensive element,
If it exceeds 0.30%, the above effect is saturated and the cost increases, so the upper limit was made 0.30%.

V: 0.05 to 0.30% V is an element necessary for precipitating fine carbides with Mo and imparting strength and toughness, and has an effect of stabilizing the strength against a change in cooling rate after hot forging. If it is less than 0.05%, the effect is insufficient, so the lower limit was made 0.05%.
However, if the content exceeds 0.30%, the effect is saturated and the cost increases, so the upper limit is set to 0.30%.

S: 0.04 to 0.12% S is an element effective for further improving machinability, and 0.04% or more is necessary to obtain the effect. However, if the content exceeds 0.12%, the effect is saturated and the toughness is reduced. Therefore, the upper limit is set to 0.12%.

Pb; 0.05 to 0.30% Pb is an effective element for further improving machinability, and 0.05% or more is required to obtain the effect. However, if the content exceeds 0.30%, the effect of improving the machinability is reduced, so the upper limit is set to 0.30%.

Ca; 0.0005 to 0.0100% Ca is an effective element for further improving the machinability, and 0.0005% or more is required to obtain the effect. However, even if the content exceeds 0.0100%, the effect of improving the machinability is reduced, so the upper limit is set to 0.0100%.

Ni: 0.2 to 2.0% or less Ni is an element effective for further improving hardenability and improving toughness. At least 0.20% to achieve said effect
It is necessary to add above. However, even if the content exceeds 2.0%, the effect is saturated and the cost becomes high.
2.0%.

[Examples] Examples of the present invention will be described in comparison with comparative steels and conventional steels to clarify the features of the present invention.

(Example 1) Table 1 shows the chemical components of the steel of the present invention, the comparative steel and the conventional steel. In Table 1, Nos. 1 to 16 are steels of the present invention, Nos. 1 to 4 are first inventions, Nos. 5 to 8 are second inventions, Nos. 9 to 11 are third inventions, and No. 12 No. to No. 16 are the fourth invention. Nos. 17 to 20 are comparative steels, No. 17 is a comparative steel in which C is higher than the composition range of the present invention, and No. 18 is a comparative steel in which other elements are in the composition range of the present invention but do not contain Mo. No. 19 is a comparative steel having Mn lower than the composition range of the present invention, and No. 20 is a comparative steel containing other elements in the composition range of the present invention but containing no V. No. 21 is conventional steel, and its composition corresponds to SCM445.

The invention steels and comparative steels in Table 1 were 200 mm diameter steel bars, which were heated to 1250 ° C., and then hot forged at about 1100 ° C., thereby forming 30 mm diameter 100 mm diameter and 150 mm diameter steel bars, After that, as shown in Table 2,
Natural or forced cooling between 00 ℃, 500 ℃ to 300 ℃
The air was cooled naturally or forcibly. A JIS No. 4 test piece and a Charpy JIS No. 3 test piece were collected from the center of each test material and subjected to the test. Also, No. 21 of conventional steel is 30 mm diameter, 1 mm
After forming into steel bars having a diameter of 00 mm and 150 mm, oil quenching was performed at 850 ° C., and then tempering was performed at 520 ° C. Similarly, test pieces were manufactured, and tensile strength and impact value were measured. The measured results are shown in Table 2. In Table 2, 700 ~
(A) is the case where the temperature is naturally cooled from 700 ° C to 500 ° C.
0 to (strong) from 700 to 500 ° C when forced cooling, 500 to (self) from 500 to 300 ° C natural cooling, 500 to (strong) from 500 to 300 ° C Shows the case of forced cooling.

As can be seen from Table 2, No. 17 had a high C content, so that the impact value was low and the toughness was insufficient. No.1
8 is inferior in strength and toughness because it does not contain Mo. No. 19 has a low Mn content, and similarly has poor strength and toughness. No. 20 was inferior in toughness because the content of V was low.

Further, even in the case of the steel of the present invention, those whose cooling rates were out of the range of the method of the present invention, that is, those naturally cooled at a diameter of 100 mm, those naturally cooled at a diameter of 150 mm, and those cooled at a diameter of 150 mm,
When only either from 0 ° C or 500 ° C was forcibly cooled, the desired tensile strength and impact value were not obtained.

On the other hand, the cooling rate of the method of the present invention using the steel of the present invention, that is, the cooling rate from 700 ° C.
C / min or more, and the cooling rate from 500 ° C. to 300 ° C. was 10 ° C. to 200 ° C./min, the tensile strength was 90 kgf / mm ² or more, and the impact value was 8.0 kgfm / cm ² or more, confirming the effect of the method of the present invention.

(Example 2) The same quenching and tempering as in Example 1 was performed for the conventional steel R steel in the state where the forged steel of Example 1 was applied to the invention steel and the comparative steel shown in Table 1. In this state, a drilling test was performed. The drill material is SKH9, the drill speed is 1710rpm, no cutting oil, load is 75kg, drill is 5mm
φ straight shank was used. Table 2 shows the measurement results.
The perforation distance was indicated by an integer ratio with 100. The results obtained are shown in Table 3.

As shown in Table 3, the machinability of conventional steel No. 21
In comparison with the steel of the present invention, the steel of the present invention is very good. In particular, the effects of the second and fourth inventions to which S and Pb are added are greatly exhibited.

[Effects of the Invention] As described above, the method for producing a tough non-heat treated steel according to the present invention reduces the carbon in order to increase the toughness, improves the hardenability, obtains a bainite structure, and reduces the decrease in strength with respect to the cooling rate. By using V-added steel to increase the cooling rate from 700 ° C to 500 ° C at 10 ° C / min or more, the precipitation of ferrite is delayed, and the cooling rate from 500 ° C to 300 ° C is increased by 10%.
With to 200 DEG ° C. / min, to obtain a finely jammed tissue bainite laths intervals, be one obtained by significantly improving the strength and impact value, tensile strength 90 kgf / mm ² or more, impact value 8.0kgfm Large parts of / cm ² can be obtained without tempering, and are extremely useful for manufacturing large parts such as undercarriage parts of automobiles and construction machines.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 6/00 C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. A weight ratio of C: 0.05 to 0.20%, Si: 0.10 to 0.50
%, Mn; 1.50 to 2.50%, Cr; 0.30 to 1.50%, Mo; 0.50 to 0.3
0%, V; steel containing 0.05 to 0.30%, with the balance being Fe and impurity elements.
Cool at a cooling rate of at least ℃ / min.
A method for producing a tough non-heat treated steel, comprising cooling at a cooling rate of 10 ° C to 200 ° C / min. 2. C: 0.05 to 0.20% by weight, Si: 0.10 to 0.50 by weight ratio
%, Mn; 1.50 to 2.50%, Cr; 0.30 to 1.50%, Mo; 0.50 to 0.3
0%, V; 0.05-0.30%, and S; 0.04-0.12
%, Pb; 0.05 to 0.30%, Ca; 0.0005 to 0.0100%, steel containing at least one of the following elements, the balance being Fe and impurity elements. Cool at a cooling rate of at least ℃ / min.
A method for producing a tough non-heat treated steel, characterized by cooling at a cooling rate of 200 ° C / min. 3. A weight ratio of C: 0.05 to 0.20%, Si: 0.10 to 0.50
%, Mn; 1.50 to 2.50%, Cr; 0.30 to 1.50%, Mo; 0.50 to 0.3
0%, V; 0.05-0.30%, Ni; 0.20-2.0%, the balance consisting of Fe and impurity elements, after hot forging 700 ° C
From 500 to 500 ° C at a cooling rate of 10 ° C / min or more.
A method for producing a tough non-heat treated steel, comprising cooling at a cooling rate of 10 ° C to 200 ° C / min from 0 ° C to 300 ° C. 4. A weight ratio of C: 0.05 to 0.20%, Si: 0.10 to 0.50
%, Mn; 1.50 to 2.50%, Cr; 0.30 to 1.50%, Mo; 0.50 to 0.3
0%, V; 0.05 to 0.30%, Ni; 0.20 to 2.0%, and further S: 0.04 to 0.12%, Pb; 0.05 to 0.30%, Ca; 0.0005 to 0.01
A steel containing one or two or more of 00%, with the balance being Fe and impurity elements, after hot forging at 700 ° C to 500 ° C
Up to 10 ° C / min.
A method for producing a tough non-heat treated steel, characterized in that the steel is cooled to 10 ° C. at a cooling rate of 10 ° C. to 200 ° C./min.