JPS63130748A - Non-heattreated tough and hard steel having high strength - Google Patents

Abstract

PURPOSE:To improve toughness by specifying respective quantities of C, Cr, (Mn+Cr), and Nb and also by specifying the structure after forging and cooling in a V-added non-heattreated steel. CONSTITUTION:This non-heattreated steel has a composition consisting of, by weight, 0.05-0.25% C, <=1.0% Si, 0.5-3.0% Mn, 0.5-3.0% Cr, 0.01-0.10% Nb, 0.01-0.20% V, and the balance Fe and satisfying (Mn+Cr)=1.5-4.5 and also has a structure, after forging (representing forging, rolling, etc.) and cooling, composed of single phase of bainite or (bainite + <=25vol% ferrite). Moreover, either or both of <=0.7% Mo and <=2% Ni and/or one or more kinds among <=0.15% S, <=0.3% Pb, 0.005% Ca, <=0.3% Bi, and <=0.3% Te may further be incorporated to the above composition, if necessary.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is suitably used as a material for parts that are required to have excellent toughness, such as automobile suspension parts. This relates to high-strength non-thermal toughened steel.

(Prior art) Conventionally, mechanical structural steel used as a material for various mechanical structural parts is processed into the shape of the part and then subjected to heat treatment, that is, quenching and tempering. is normal. In this heat treatment, the steps typically include (1) heating to form austenite, (2) cooling from austenite, and (2) tempering after quenching.

However, when performing such heat treatment (tempering treatment), a heat treatment furnace to reheat the parts, a quenching tank to cool the parts, and a tempering furnace to temper the parts are required. , the number of man-hours and equipment costs increase.

Therefore, in order to obtain a certain degree of strength and toughness without the above-mentioned heat treatment, it has been attempted to use non-thermal steel with V (vanadium) added to the material. This non-tempered steel utilizes the precipitation hardening described in (1), and has a ferrite-pearlite structure.

(Problems to be Solved by the Invention) However, as mentioned above, the non-tempered steel added with V still has insufficient toughness, and is used especially in parts that require excellent toughness, such as automobiles. It is not suitable for application to suspension parts.

(Object of the Invention) The present invention has been made by focusing on such conventional problems, and provides a high-strength, non-heat-treated tough steel that can obtain high toughness without heat treatment and is also excellent in strength. It is intended to.

[Structure of the Invention] (Means for Solving the Problems) The high-strength non-thermal toughened steel according to the present invention has C:0 in weight%.
．． 05 to 0.25%, Si: 1.0% or less, Mn: 0.
5-3.0%, Cr: 0.5-3.0%, Mn+Cr
: 1, 5-4, 5%, Nb: 0.01-0.10
%, V: 0.01 to 0.20%, and if necessary, one or two selected from Mo: 0.7% or less, Ni: 2% or less, and also if necessary S20 accordingly.
15% or less, Pb: 0.3% or less, Ca: 0.005%
Hereinafter, it contains one or more selected from Bi: 0.3% or less, Te: 0.3% or less, the remainder substantially consists of Fe and impurities, and the forging (herein, forging, rolling, etc.) (Representative of plastic working.) φ The structure after cooling is characterized by a single phase of bainite or bainite + 25% by volume or less of ferrite.

In other words, the high-strength, non-tempered tough steel of the present invention uses a steel with a relatively low C content to obtain a structure consisting of low-C bainite or bainite + ferrite with excellent toughness, and has ferrite in the structure. If V exists, the fatigue strength decreases, so V is added to precipitate V carbonitrides in the ferrite to strengthen it, and as mentioned above, the C content is lowered to improve toughness. , M n
It is characterized in that the strength is increased by increasing the amount of +Cr to some extent.

Next, the reason for limiting the component range (wt%) of the high-strength, non-tempered tough steel according to the present invention will be explained.

C: 0.05 to 0.25%, C is an element added to ensure the strength of mechanical structural parts, and for this purpose it was contained in an amount of 0.05% or more. However, if it is too large, the toughness will decrease, so it is set at 0.125% or less.

Si: 1.0% or less St is an element that has a deoxidizing effect during steel melting, but if it is too large, it reduces toughness, so it was set to 1.0% or less.

Mn: 0.5-3.0% Mn has a deoxidizing and desulfurizing effect during steel melting, controls the form of inclusions, improves hardenability, and increases the strength of mechanical structural parts. It is an effective element for increasing the carbon content, and in order to obtain such an effect, it is contained in an amount of 0.5% or more. However, if the content is too large, machinability and ductility are reduced, so the content is set at 3.0% or less.

Cr: 0.5-3.0% Cr is an effective element for improving hardenability and increasing the strength of mechanical structural parts.
．． The content was 5% or more. However, if the content is too high, the strength will become too high and the toughness will decrease, so the content was set at 3.0% or less.

Mn+Cr: l, 5-4.5 Mn and Cr are effective elements for increasing strength as mentioned above, and Mn is added to compensate for the decrease in strength due to low C and to ensure good toughness. 1 for 100 r
．． It was set at 5% or more. However, if the amount of Mn + Cr is too large, the strength becomes too high and the toughness is reduced, so the Mn + Cr content is regulated to 4.5% or less.

Nb: 0.01 to 0.10% Nb is an effective element for facilitating the precipitation of bainite and increasing strength, so the content was set to 0.01% or more in order to obtain this effect. However, if the content is too large, the toughness will be reduced, so the content is set to 0.10% or less.

V: 0.01-0.20% ■ is an effective element for precipitating V carbon/nitride in ferrite and increasing the strength of mechanical structural parts, so in order to obtain this effect, The content was set to 0.01% or more. However, even if a large amount is added, the improvement in the effect is small and it is economically disadvantageous, so it is set at 0.20% or less.

One or two types selected from Mo: 0, 7% or less, Ni: 2% or less. Both Mo and Ni are effective elements for increasing strength, so they can be used in machines that require even higher strength. In the case of structural parts, one or two selected from Mo and Ni may be contained, preferably at least 0.01% for MO, and at least 0.1% for Ni, if necessary. .

However, even if a large amount of MO is added, the effect of addition is not improved much, and instead it reduces the toughness and becomes economically disadvantageous.
There is a need for i to be 2% or less.

S:0. i5% or less, Pb: 0.3% or less, Ca: 0.
0.005% or less, Bi: 0.3% or less, Te2: 0.3% or less - S, Pb, Ca, Bi, and Te are all elements that improve machinability. Therefore, when good machinability is required for mechanical structural parts, it is advisable to add an appropriate amount of one or more selected from these. However, if the amount added is too large, it will reduce hot workability and deteriorate toughness, so even if it is added,
S is 0.15% or less, Pb is 0.3% or less, and Ca is 0.
0.005% or less, Bi 0.3% or less, and Te 0.3% or less.

In the high-strength, non-tempered tough steel of the present invention having such a chemical composition, the structure after forging (representing plastic working such as forging, rolling, and rolling) and cooling is a single-phase bainite structure. Or bainite + ferrite is regulated to be 25% by volume or less. In other words, good strength and toughness can be obtained by making the base structure a single phase of bainite or bainite + 25% by volume or less of ferrite, and when martensite and pearlite coexist, good toughness can be obtained. This is because it is not possible to obtain Since fatigue strength decreases if the amount of ferrite is too large, it is limited to 25% by volume or less.

(Example 1) After steel having the components shown in Table 1 was melted, it was formed into an ingot, and subjected to blooming rolling and product rolling to obtain a rolled material having a diameter of 50 mm. Next, each rolled material was hot forged at 12,500C to obtain a forged material with a diameter of 25 mm, and the cooling rate after hot forging was controlled so that the structure became bainite + lO ~ 15% by volume ferrite. It was used as a material.

Next, the Vickers hardness and Charpy impact value at -50°C (the test piece was JIS No. 3) of each of the above-mentioned test materials were investigated, and Table 1 also shows p.

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Comparative steel No. 1, which has too little Cr content, has high hardness but a fairly low impact value and poor toughness, while comparative steel No. 2, and 3, which do not contain Nb and V, have low strength. Comparative steel N with too much C content
Although steel No. 024 had a high hardness, its impact value was quite low and its toughness was poor, and comparative steels No. 5 and No. 5, which contained too little Cr, were found to have low strength.

On the other hand, inventive steels Nos. 6 to 14, which had appropriate chemical component ratios, all had high impact values and were found to have excellent toughness. and,
When an appropriate amount of one or more of M o and Ni is added (No, 9.10, 13°14), it is possible to further increase the strength, and S, Pb, Ca, Bi, T
ecy) When an appropriate amount of one or more of these is added (No,
11, 12, 13.

14) was confirmed to have good machinability.

(Example 2) A rolled material of N087 (diameter 50 mm) shown in Table 1 adopted in Example 1 was selected, and 1250 mm was applied to this rolled material.
Hot forging was carried out at 0.degree. C. to obtain a forged material with a diameter of 25 mm, and the cooling rate after hot forging was controlled to obtain the microstructure shown in Table 2, and these were used as test materials.

Next, the Vickers hardness of each sample material and -50°C
When we investigated the Charpy impact value (the test piece was JIS No. 3), we found that the cooling rate after hot forging was too high and martensite was present in the microstructure, as shown in Table 2. If it is (symbol D), the hardness is high but the impact value is low and the toughness is poor.
On the other hand, if the cooling rate after hot forging is too low and the amount of ferrite in the microstructure is too large (code C) or if pearlite is present (code E), the hardness is small and the impact value is low. It was recognized that.

On the other hand, if the cooling rate after hot forging is appropriate,
When the microstructure is a single bainite phase (symbol A
) and when the amount of ferrite is 10% by volume (symbol B
) was found to have appropriate hardness, high impact value, and excellent toughness.

Furthermore, in the above sample steel No. 7, the amount of ferrite relative to bainite in the microstructure was changed by controlling the cooling rate after hot forging in the same manner as in the above case, and the relationship between this amount of ferrite and the fatigue limit was The results shown in Figure 1 were obtained.

As is clear from the results shown in Figure 1, in order to obtain a good fatigue limit, it is necessary to
It was confirmed that in order to make the fatigue limit ratio to G) 0.8 or more, the amount of ferrite to bainite needs to be 25% by volume or less.

[Effects of the Invention] As explained above, the high-strength non-thermal toughened steel according to the present invention has C: 0.05 to 0.25%, Si:
1.0% or less, Mn: 0, 5-3.0%, Cr
:0.5~3.0%, Mn+Cr:1.5~4.5
%, Nb: 0.01-0.10%, V: 0.01-0.
20%, and optionally one or two selected from Mo: 0, 7% or less, Ni: 2% or less, and also optionally S: 0.15% or less, Pb
: 0.3% or less, Ca: 0.005% or less, Bi
0.3% or less, Te: 0.3% or less, and the remainder substantially consists of Fe and impurities, and the structure after forging and cooling is a single bainite phase. Or, since it is bainite + 25% by volume or less of ferrite, high toughness can be obtained without heat refining, and it also has excellent strength.
There is no need for thermal treatment after hot plastic working, and therefore, it is possible to eliminate the need for quenching equipment or tempering equipment, and it is possible to obtain high-quality mechanical structural parts with stable characteristics at a low cost. It brings about a very good effect of being able to do it.

[Brief explanation of the drawing]

Figure 1 is a graph showing an example of the results of investigating the relationship between the amount of ferrite to bainite in the microstructure and the fatigue limit ratio for tempered steel of the same hardness for test steel No. 7 in Table 1. .

Claims (4)

[Claims] (1) In weight%, C: 0.05-0.25%, Si: 1.0% or less, Mn: 0.5-3.0%, Cr: 0.5-3.0%, Mn+Cr: 1.5 to 4.5%, Nb: 0.01 to 0.10%, V: 0.01 to 0.20%, and the remainder substantially consists of Fe and impurities, and the structure after forging and cooling is A high-strength non-thermal tough steel characterized by being a single phase of bainite or bainite + 25% by volume or less of ferrite. (2) In weight%, C: 0.05-0.25%, Si: 1.0% or less, Mn: 0.5-3.0%, Cr: 0.5-3.0%, Mn+Cr: 1.5-4.5%, Nb: 0.01-0.10%, V: 0.01-0.20%, Mo: 0.7% or less, Ni: 2% or less A high-strength, non-tempered tough steel containing one or two types, with the remainder substantially consisting of Fe and impurities, and whose structure after forging and cooling is a single phase of bainite or bainite + 25% by volume or less of ferrite. . (3) In weight%, C: 0.05-0.25%, Si: 1.0% or less, Mn: 0.5-3.0%, Cr: 0.5-3.0%, Mn+Cr: 1.5-4.5%, Nb: 0.01-0.10%, V: 0.01-0.20%, and S: 0.15% or less, Pb: 0.3% or less, Ca
: 0.005% or less, Bi: 0.3% or less, Te: 0.
Contains one or more selected from 3% or less, the remainder substantially consists of Fe and impurities, and the structure after forging and cooling is bainite single phase or bainite + ferrite with 25% or less by volume. High-strength, non-tempered tough steel. (4) In weight%, C: 0.05-0.25%, Si: 1.0% or less, Mn: 0.5-3.0%, Cr: 0.5-3.0%, Mn+Cr: 1.5-4.5%, Nb: 0.01-0.10%, V: 0.01-0.20%, Mo: 0.7% or less, Ni: 2% or less 1 type or 2 types, S: 0.15% or less, Pb: 0.3% or less, Ca
: 0.005% or less, Bi: 0.3% or less, Te: 0.
Contains one or more selected from 3% or less, the remainder substantially consists of Fe and impurities, and the structure after forging and cooling is bainite single phase or bainite + ferrite with 25% or less by volume. High-strength, non-tempered tough steel.