JPH10237589A - Martensitic non-heat treated steel excellent in machinability and having high strength and high toughness, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a martensitic non-heat treated steel excellent in machinability and having high strength and high toughness by forming a steel, containing specific weight presents of C, Mn, Cr, Mo, etc., and having the balance essentially Fe, into martensite under specific conditions. SOLUTION: The martensitic non-heat treated steel has a composition consisting of, by weight, 0.04-0.10% C, 0.10-1.0% Si, 1.5-3.5% Mn, <=0.04% P, 0.5-2% Cr, <=0.1% Al, <=0.03% N, 0.015-0.06% Ti, and the balance essentially Fe and satisfying inequality I. In the inequality, Ni<=1.0%, Cu<=0.5% Mo<=1.0%, and V<=0.3% are satisfied, and further, when 0.0005<=B<=0.01% and 2.5×B<=Ti are satisfied, XB is regulated to 0.5. Further, as free cutting components, one or >=2 elements among S, Pb, Bi, Ca and Te are incorporated. This steel is austenitized. plastic-worked at 550-900 deg.C, and then cooled and formed into martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cutting rod which can be suitably applied to parts such as connecting rods, wheel hubs, spindle-related parts, suspension-related parts, transportation equipment such as automobiles such as arms, construction equipment, and industrial machinery. TECHNICAL FIELD The present invention relates to a high-strength and high-toughness martensitic non-heat-treated steel having excellent heat resistance and a method for producing the same.

[0002]

2. Description of the Related Art Increasing the strength of components is an indispensable technology for weight reduction, and is one of the most important technologies that have been developed and studied from all directions. Conventionally, for parts used in transportation equipment such as automobiles, construction equipment, industrial machines, etc., which require strength and toughness, JIS-
Hardened and tempered tough steels such as SCR420 are used.

[0003] Although this tough steel is excellent in mechanical properties, it has a problem in that the quenching-tempering heat treatment increases the lead time and the production cost.

[0004] On the other hand, non-heat treated steel, which has been widely used in recent years, does not require heat treatment, and is therefore excellent in terms of lead time and manufacturing cost. However, the balance between strength and toughness of a non-heat treated steel is low, and mechanical properties are hardly sufficient.

[0005] Usually, a part which has been subjected to plastic working such as forging is cut and finished in most cases. Therefore, except for some important parts and cases where the machinability does not matter much, steel materials to which a free-cutting component is added are used for the purpose of improving machinability in many cases.

Needless to say, the addition of the free-cutting component lowers the mechanical properties, and the tendency becomes more pronounced as the strength is increased. Therefore, in order to balance machinability and mechanical properties, it has to be used with a reduced strength level, and at present, the high strength of components is limited to about 1000 MPa.

[0007]

Therefore, in the present invention, 1
An object of the present invention is to provide a martensitic non-heat-treated steel having excellent machinability and sufficient toughness while having high strength exceeding 000 MPa and a method for producing the same.

[0008]

The martensitic non-heat treated steel of claim 1 of the present invention has a C content of 0.04% by weight.
0.10%, Si: 0.10 to 1.0%, Mn: 1.10%
5 to 3.5%, P: ≦ 0.04%, Cr: 0.5 to 2.
0%, Al: ≦ 0.1%, N: ≦ 0.03%, Ti:
0.015 to 0.060% and Mn + Cr + Mo +
V + Cu + Ni / 2 + XB ≧ 3.5 (where Ni: ≦ 1.
0%, Cu: ≤ 0.5%, Mo: ≤ 1.0%, V: ≤
0.3% and 0.0005 ≦ B ≦ 0.01%, 2.5 ×
XB = 0.5 when B ≦ Ti), and the balance substantially consists of Fe.

[0009] In a second aspect of the present invention, in the first aspect, one or more of S, Pb, Bi, Ca, and Te are used as free-cutting components, and S: 0.03 to 0.20%. ,
Pb: ≦ 0.30%, Bi: ≦ 0.15%, Ca: ≦
0.05%, Te: ≤0.1%.

A third aspect of the present invention relates to a method of manufacturing a high-strength, high-toughness martensitic non-heat-treated steel having excellent machinability. After heating to 860 ° C. or more to austenitize, cooling and cooling to 550 to 900 ° C., plastic working is performed in that temperature range, and then the Mf point at an average cooling rate of 100 ° C./min or more. It is characterized in that it is cooled to 300 ° C. or less to form martensite.

Further, in the manufacturing method according to the present invention, in the method according to the present invention, after the plastic working, the steel sheet is subjected to a reheating treatment for 5 minutes or more in a temperature range of 200 to 600 ° C. after the martensitization. .

[0012]

[Functions and Effects of the Invention] Conventionally, two types of forging and quenching and ausforming are used for the machining heat treatment for directly quenching after forging.
There is a method. Forging and quenching is performed by hot forging (usually 1100 ° C
This is a method of directly quenching after forging), and is often applied mainly to carbon steel or the like having a low quenching property because the quenching property is improved.

The feature of forging and quenching is that hardenability is improved as described above. However, since inexpensive carbon steel often provides a complete martensitic structure, it is a frequently used technique. However, forging and quenching is characterized only by the improvement of the quenchability, so that when used for alloy steel or the like, a sufficient effect cannot be obtained.

Ausforming, on the other hand, is a method of quenching after working (warm zone) with metastable austenite, and is an attractive method in which mechanical properties such as strength and toughness are greatly increased. However, ausforming is a major problem in that ferrite tends to appear during processing, and the forging temperature is lower than in hot forging, so that deformation resistance is high and forging is difficult.

The present invention is based on a low C steel having a low deformation resistance at the time of metastable austenite to enhance the hardenability, and is applicable to ausforming, that is, plastic working in a metastable austenite state is possible and thereafter. (Claim 1), the martensitic non-heat treated steel of the present invention has good machinability because the C content is sufficiently low. Thus, the deformation resistance during plastic working such as forging is small, and good plastic working can be performed in a metastable austenite state, that is, even at a low working temperature. The strength and toughness can be effectively increased by the ausforming.

In the case of ultra-low C martensitic steel, the machinability is good due to the low carbon content, but the strength is at most about 900 to 1200 MPa, but the strength is increased by ausforming. 1200-140
It can be increased to about 0 MPa.

Moreover, the mechanism of increasing the strength is mainly due to the strain and the dislocation being fixed during the processing and the refinement of the structure, so that only the strength is effectively increased and the machinability is not impaired. Such advantages can be obtained.

For example, in ferrite + pearlite steel, cementite having a hard structure and soft ferrite are mixed, and the hard cementite damages a tool such as a drill during cutting. That is, the tool is easily damaged.

On the other hand, in the case of a tough steel, that is, a martensitic type tough steel used after being subjected to quenching and tempering treatment, it contains a large amount of C, so that C is precipitated as hard carbide, and this precipitates in a soft matrix. It becomes a dispersed state. Therefore, also in this case, the tool is damaged during cutting.

On the other hand, in the case of ultra-low C martensitic steel, since the amount of C is small, the C gives a strain to the matrix and exerts a strain on the matrix due to its strain effect, rather than hardening the structure as a precipitate. Tend to be hard.

In the physical phenomenon of cutting, it is important how fine and uniform the structure is, and in that sense, a very low C martensitic steel is excellent in machinability.

The martensitic non-heat-treated steel according to the first aspect of the present invention has a good workability compared to a conventional tough steel even if it is cooled to martensite while hot forging without applying ausforming. It is confirmed that it has the property.

[0023] In this sense, the martensitic non-heat treated steel of the present invention has a specific meaning even if it is not necessarily linked to ausforming.

In addition to this, when working by ausforming, high strength can be achieved by the above-described mechanism.
Although the toughness is achieved, the mechanism is not due to the non-uniform structure, that is, the precipitation of coarse carbides. Therefore, only the high strength and high toughness can be obtained and the machinability deteriorates. is there.

In other words, extremely low C having good machinability
The inadequate strength of the martensitic steel can be increased to at least the same level as that of the conventional tough steel by working in a quasi-austenite state and subsequently cooling and martensitizing.

In the present invention, one or more of S, Pb, Bi, Ca, and Te are used as free-cutting components, S: 0.03 to 0.20%, Pb: ≤ 0.30%,
Bi: ≦ 0.15%, Ca: ≦ 0.05%, Te: ≦
It can be contained at 0.1% (claim 2). Thereby, machinability can be effectively improved without particularly impairing the mechanical properties of steel.

In the present invention, as a processing condition for the raw material having the above composition, the raw material is once heated to 860 ° C. or more to austenitize, then cooled to a temperature range of 550 ° C. to 900 ° C., and plastic working is performed in that temperature range. After performing, 100 ° C /
It is possible to form martensite by cooling to an Mf point of 300 ° C. or less at an average cooling rate of at least minutes (claim 3).

As a method for producing a martensitic non-heat treated steel, the composition of the steel may be the composition of claim 1 and then simply quenched as described above. In this case as well, good machinability can be obtained as compared with the conventional tough steel. However, according to claim 3, the structure is austenitized and then cooled to a specific temperature, that is, 550 to 900 ° C. Therefore, a method of performing cooling and martensite after plastic working can be suitably used. Thus, in the latter case, the machinability can be made equal to or higher than that of the former, though the strength level is greatly increased as compared with the former.

In this case, a method such as air cooling or blast cooling can be used as a cooling method after austenitization, and a method such as water cooling or oil cooling as a cooling method for martensite after plastic working. Can be used.

In the present invention, a reheating treatment can be performed for 5 minutes or more at a temperature in the range of 200 to 600 ° C. after martensitization after the plastic working. Thereby, machinability can be more effectively improved.

Next, the reasons for limiting each chemical component in the present invention will be described in detail. C: 0.04 to 0.10% Martensite hardness is determined by the amount of C added. In the present invention, 0.04% or more must be added. C also has the effect of increasing the deformation resistance during metastable austenite.
0% or less to keep deformation resistance low. According to the present invention, by adjusting the amount of C added to 0.04 to 0.10%, 100
A strength of 0 MPa to 1400 MPa can be realized.

Si: 0.10 to 1.0% Si is an element which enhances the hardenability, but at the same time, the deformation resistance, especially the deformation resistance in the cold, increases. %.

Mn: 1.5-3.5% Mn is an inexpensive element and one of the elements that most enhances hardenability. In the present invention, 1.5% or more is added for the main purpose of securing hardenability. However, an excessive addition exceeding 3.5% increases the deformation resistance, so the upper limit is made 3.5%.

P: ≦ 0.04% P is an element causing grain boundary segregation, and its upper limit must be suppressed. Therefore, it is set to 0.04% or less.

Cr: 0.5 to 2.0% Cr is an element that enhances hardenability, and an element that enhances softening resistance during reheating treatment. Further, since there is an effect of increasing the hardness after plastic working such as forging due to work induced precipitation or the like, 0.5 to 2.0% is added.

Ni: ≦ 1.0% Cu: ≦ 0.5% Ni and Cu are not effective as much as Mn and Cr, but may be added to enhance the hardenability without increasing the deformation resistance. it can. However, the respective upper limits are 1.0% for Ni and 0.5% for Cu.
It is necessary to

Mo: ≦ 1.0% V: ≦ 0.3% These elements are elements that enhance hardenability and elements that increase softening resistance during reheating. The addition of Cr in combination increases the amount of work-induced precipitation of carbides, so that the hardness after plastic working such as forging increases, and a great effect is brought about. However, the upper limit of Mo is 1.0
% And V, it is necessary to set the upper limit to 0.3%. The upper limit of 0.3% of V is the solid solubility limit during austenitization.

B has the effect of significantly increasing the hardenability when added in a small amount. However, if the addition amount is too large, the hardenability is rather deteriorated.
It is desirable to set it in the range of 005 to 0.01%.

Al: ≦ 0.1% Al combines with N to form a stable nitride and prevents crystal grains from becoming coarse. However, when the addition amount increases, the forgeability deteriorates, so the upper limit of the addition amount is set to 0.1%.

Ti: 0.015 to 0.060% Ti also forms a stable nitride like Al, and prevents crystal grains from becoming coarse. It also has the effect of preventing BN when B is added. The upper limit is set at 0.06 for reasons of impaired forgeability.
0%. When B is added, it is desirable to add 2.5 times or more the amount of N.

N: ≦ 0.03% When the amount of N is large, the effective amount of B decreases.
3%.

S: 0.03 to 0.20% Pb: ≤ 0.30% Bi: ≤ 0.15% Ca: ≤ 0.05% Te: ≤ 0.1% These elements enhance machinability. Works, but forgeability,
It is also an element that inhibits mechanical properties. Therefore, the upper limit of the addition amount is set to the above value.

[0043]

Next, embodiments of the present invention will be described in detail. <Example 1> The raw materials of the chemical components shown in Tables 1 and 2 are shown in FIG.
According to the process shown, a blank which had been subjected to forward extrusion (area reduction rate 60%) or block forging (rolling rate 30%) was cut and finished to a predetermined size to obtain a test piece. Various tests were performed using the test pieces to evaluate the characteristics. In addition, JIS-S35VC, S45V
C, S55VC and SCR420 were used.

[0044]

[Table 1]

[0045]

[Table 2]

[1. Hardenability] First, FIG. 2 shows the relationship between hardenability on the horizontal axis and hardness on the vertical axis in the case of processing according to Process A and Process B shown in FIG.

In the processing (ausforming) according to the process B, since the diffusion transformation such as the ferrite transformation and the bainite transformation is remarkably promoted, the hardness is greatly reduced in a region where the index indicating the hardenability is low. However, when the index representing the hardenability is 3.5 or more, the hardness becomes higher than the normal hardness after quenching (process A), and the original effect of ausforming appears. In the process B, the forging temperature was set to 500 to 900 ° C.

Next, FIG. 3 shows that the index indicating hardenability is 3.
1 shows the relationship between forging temperature and hardness after quenching (after martensitic transformation) for 1 (steel type I), 4.3 (steel type C), and 5.1 (steel type M). For steel types M (5.1) and C (4.3) having sufficiently high indices indicating hardenability, M
550-9 above the s point and below the recrystallization temperature
It can be seen that in the range of 00 ° C., a hardness higher than that of the hot forged material at 1100 ° C. (forged and quenched material of simple quenching) can be obtained. Here, even in forging at 500 ° C., hardness equal to or higher than that of a simple quenched material (forged quenched material) is obtained, but other mechanical properties are reduced as can be seen from the results of a toughness test described below. Therefore, the forging temperature needs to be 550 ° C. or higher.

FIG. 8 shows the relationship between the added amount of the free-cutting component (S + Pb + 2 × Bi amount) and the hardness for the invention steels N, O, P, Q, R and S. Incidentally, the forging temperature in the process B is 700 ° C. As shown in this figure,
The decrease in hardness is small with an increase in the amount of the free-cutting component. Further, from this figure, it is understood that the hardness is higher than that of the simple hardened material by performing the processing according to the process B.

[2. FIG. 4 shows the forging temperature of the inventive steels I, C, and M and the impact value (JIS-
No. 3 test piece). Inventive steels C and M, for which sufficient hardenability has been secured, processed according to process B, exhibit higher toughness than the simple hardened material despite the increase in hardness in FIG. Where Ms
At 500 ° C. forging near the point, a significant decrease in toughness is observed. This indicates that the lower limit of the effective forging temperature in the process B is 550 ° C.

FIG. 9 shows the relationship between the added amount of free-cutting components (S + Pb + 2 × Bi amount) and the impact value for the invention steels N, O, P, Q, R and S (the forging temperature of process B is 7).
00 ° C). In this result, although the impact value is reduced as the amount of S + Pb + 2 × Bi increases, sufficient toughness is obtained despite the high hardness of 380 HV (the tensile strength is about 1200 MPa). Therefore, it can be seen that high strength and high toughness are maintained even when a free-cutting component is added in a fixed amount. It can be seen from FIG. 9 that the case processed according to the process B retains higher toughness than the simple quenched material.

[3. Machinability] Next, FIG. 5 shows steel types A, B,
It shows the relationship between the C content and the hardness after quenching for C, D, E, and F when treated according to process A and when treated according to process B. In such a low C amount range, the hardness increases significantly even with a slight increase in the C amount. From FIG. 5, it can be seen that the hardness of the process B material is higher than that of the simple hardened material.

On the other hand, FIG. 6 shows the relationship between the C content and the machinability when working is performed in accordance with the process A and the process B, that is, the drill feed speed when the drill reaches the end of its life when the cutting amount is 1000 mm. I have.

As shown in this figure, the machinability surely decreases with an increase in hardness (increase in the amount of C), but in FIG. 5, there is a difference in hardness level between process A and process B. Despite this, the machinability results in FIG. 6 show that the machinability is exactly the same for both process A and process B. That is, it can be seen that when processing is performed in accordance with the process B, hardness and strength can be effectively increased while maintaining machinability at or above the same level.

FIG. 7 shows the hardness and machinability (cutting amount 100
Drill feed rate when the life of the drill reaches 0 mm)
And the relationship. In FIG. 7, J is used as a comparative material.
The test results for IS-S35VC to S45VC and SCR420-HT are also shown.

In the case of the invention steels A, B, C, D and E, the balance between the strength and the machinability is clearly superior to that of the JIS steel type in both the case where the processing is performed according to the process A and the case where the processing is performed according to the process B. ing.

The process B material, which has been strengthened by ausforming, has a higher strength-machinability balance than the process A material.

FIG. 10 shows the above invention steels N, O, P, Q, R,
It shows the relationship between the amount of free-cutting component added (S + Pb + 2 × Bi amount) and machinability when S is processed according to processes A and B (forging temperature of process B is 70%).
0 ° C). As is apparent from this figure, it can be confirmed that the machinability is effectively enhanced by adding the free-cutting component.

[4. Characteristics after reheating] FIG. 11 shows invention steel C
2 shows the relationship between the reheating temperature and the hardness when the test piece obtained by processing according to Process A and Process B is reheated after martensitic transformation. In this figure, the curve of Process B is always located on the higher hardness side than that of Process A, indicating that the softening resistance during the reheating treatment is high.

FIG. 12 shows the relationship between the reheating temperature and the machinability. The method for evaluating the machinability is the same as described above. As apparent from this figure, the process B
The material shows substantially the same machinability, although the hardness level is clearly higher than the process A material in FIG. These values are sufficiently high compared to JIS-general steel.

<Embodiment 2> Next, an embodiment in which the present invention is applied to a connecting rod for an automobile engine will be described. At that time, the trial production conditions were as shown in Table 3, and the cutting test conditions were as shown in Table 4.

[0062]

[Table 3]

[0063]

[Table 4]

FIG. 13 shows a cutting position in the cutting test.
FIG. 14 shows the positions of the hardness measurement cross sections. The prototype and test are based on Invention Steel C and JIS-S45C.
(HT).

FIG. 15 shows the results of measuring the hardness of the cut portions (measurement portions) AA ', BB', and CC 'in FIG. As shown in this figure, the steel A subjected to the ausforming processing of the invention steel C is S45C
The hardness value is higher by about 100 HV than that of the HT material.

Next, FIG. 16 shows the results of a cutting life test. In the case of the invention steel C, although the hardness was higher than that of the S45C-HT material (see FIG. 15), It can be confirmed that the machinability is rather high.

Although the embodiment of the present invention has been described in detail, this is merely an example, and the present invention can be carried out in various modified forms without departing from the gist thereof.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a pattern of a heating and processing process employed in an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an index indicating hardenability and hardness obtained in an example of the present invention.

FIG. 3 is a diagram illustrating a relationship between forging temperature and hardness obtained in an example of the present invention.

FIG. 4 is a diagram illustrating a relationship between a forging temperature and an impact value obtained in an example of the present invention.

FIG. 5 is a diagram showing the relationship between the C content and hardness obtained in an example of the present invention.

FIG. 6 is a diagram showing the relationship between the C content and machinability obtained in an example of the present invention.

FIG. 7 is a diagram illustrating a relationship between hardness and machinability obtained in an example of the present invention.

FIG. 8 is a graph showing the relationship between the amount of free-cutting components added and the hardness obtained in the example of the present invention.

FIG. 9 is a graph showing the relationship between the added amount of free-cutting components and the impact value obtained in the example of the present invention.

FIG. 10 is a diagram showing the relationship between the amount of free-cutting components added and the machinability obtained in the example of the present invention.

FIG. 11 is a diagram showing a relationship between reheating temperature and hardness obtained in an example of the present invention.

FIG. 12 is a diagram showing a relationship between reheating temperature and machinability obtained in an example of the present invention.

FIG. 13 is a view showing a cutting portion of a connecting rod experimentally manufactured in an example of the present invention.

FIG. 14 is a diagram showing a hardness measurement site of a connecting rod experimentally manufactured in the example.

FIG. 15 is a diagram showing a relationship between a measurement site and hardness of the connecting rod experimentally manufactured in the example.

FIG. 16 is a diagram showing a relationship between a machined portion of a connecting rod experimentally manufactured in the same example and machinability.

Claims (4)

[Claims] C: 0.04 to 0.10% Si: 0.10 to 1.0% Mn: 1.5 to 3.5% P: .ltoreq.0.04% Cr: 0.5% by weight Al: ≦ 0.1% N: ≦ 0.03% Ti: 0.015 to 0.060% and Mn + Cr + Mo + V + Cu + Ni / 2 + XB
≧ 3.5 (however, Ni: ≦ 1.0%, Cu: ≦ 0.5%,
Mo: ≦ 1.0%, V: ≦ 0.3% and 0.0005 ≦
XB = 0.5 when B ≦ 0.01%, 2.5 × B ≦ Ti
A high-strength and high-toughness martensitic non-heat-treated steel excellent in machinability, characterized by satisfying the following conditions. 2. The method according to claim 1, further comprising one or more of S, Pb, Bi, Ca, and Te as free-cutting components: S: 0.03 to 0.20% Pb: ≦ 0.30% Bi : ≦ 0.15% Ca: ≦ 0.05% Te: ≦ 0.1% High strength with excellent machinability, characterized in that:
High toughness martensitic non-heat treated steel. 3. The method for producing a martensitic non-heat treated steel according to claim 1 or 2, wherein the material is once heated to 860 ° C. or higher to austenitize, and then cooled to 550 to 550 ° C.
After the temperature is lowered to 900 ° C., plastic working is performed in the temperature range, and thereafter, the material is cooled to an Mf point of 300 ° C. or less at an average cooling rate of 100 ° C./min or more to form martensite. Method for producing high-strength, high-toughness martensitic non-heat treated steel with excellent heat resistance. 4. The high machinability excellent in machinability according to claim 3, wherein after the martensitization after the plastic working, a reheating treatment is performed in a temperature range of 200 to 600 ° C. for 5 minutes or more. A method for producing a high-strength, high-toughness martensitic non-heat treated steel