JPH0813028A - Production of precipitation hardening steel material having high tensile strength and high toughness - Google Patents

Abstract

PURPOSE:To provide high strength and high toughness by means of precipitation hardening treatment, while obviating the necessity of refining treatment of hardening and tempering after hot working, by specifying a composition. CONSTITUTION:A steel, having a composition containing, by weight ratio, 0.3-0.8% C, 0.1-1.5% Si, 0.2-2.0% Mn, <=1.5% Cr, <=0.1% Al, 30-100ppm N, and 0.08-0.5% V and/or 0.01-0.1% Nb, is used. This steel is rolled or hot-forged, reheated to a temp. not lower than the austenitizing temp., and cooled down to 500 deg.C at a rate of <=100 deg.C/min. By this method, a tensile strength of >=750N/mm<2> and a toughness equal to or higher than that of heat treated steel can be obtained. Further, the metallic structure of this steel is composed of ferrite and pearlite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to machine structural steel,
In particular, the present invention relates to a method for producing a precipitation hardening type high strength, high toughness structural steel having a ferrite / pearlite structure.

[0002]

2. Description of the Related Art Conventionally, tempered and tempered tempered steel is widely used for machine structural steels which are required to have high strength and high toughness. However, this tempering process
Since the heat treatment is required twice or more, the cost is high, and energy saving has been demanded in recent years, and it is desired to omit the heat treatment.

From such a flow, a precipitation hardening type high strength steel having a ferrite-pearlite structure added with V and Nb has been developed and used as a non-heat treated steel which does not require heat treatment. For example, Japanese Patent Publication No. 61-2728 discloses C: 0.
10-0.22wt%, Si: 0.02-0.80wt%, Mn: 0.6-2.00wt
%, Al: 0.005 to 0.1 wt% of steel containing Mo, Nb and V added together, after hot working such as rolling or forging, it is heated again to austenite temperature and then 800 ° C.
Steel material with a ferrite / pearlite structure with a tensile strength of 57 kgf / mm ² or more, which is subjected to a normalizing process of cooling from 5 to 500 ° C at an average cooling rate of 5 ° C / min or more and 30 ° C / min or less, and then tempering Has been proposed. However, this technique requires a tempering process after normalizing.

In Japanese Patent Publication No. Sho 63-23261, C: 0.3
~ 0.8 wt%, Si: 0.15 to 1.5 wt%, Mn: 0.5 to 2 wt%,
Ti: 0.01 to 0.05 wt%, N: 0.29 Ti to 0.025 wt% and V: 0.
A non-tempered tough steel is proposed, in which steel containing at least one of 01 to 0.1 wt% and Nb: 0.01 to 0.1 wt% is hot-rolled or hot-forged, and in some cases only normalizing. ing.

However, since these non-heat treated steels are inferior in toughness to the heat treated steels, they may not be used as a substitute for the heat treated steels. Further, since non-heat treated steel is not heat-treated, its mechanical properties are greatly influenced by rolling conditions. Regarding toughness, it is generally known that the finer the crystal grains, the higher the toughness. A material having a relatively small size can have a large reduction ratio and the structure is made finer by rolling. However, as the product size becomes larger, the reduction ratio is decreased and the structure is not made finer, so that the structure tends to have low toughness. Further, since the cooling rate after rolling becomes slower as the size becomes larger, there is a problem that crystal grains become coarse and precipitates easily become coarse, resulting in a decrease in strength and toughness.

[0006]

The heat-treated tempered steel used for mechanical structures, which is subjected to quenching and tempering, requires heat treatment twice or more, which causes a problem of high cost. Further, the non-heat treated steel has a lower toughness than the heat treated steel, and there is a problem that the toughness is significantly deteriorated especially when the product size is large.

An object of the present invention is to solve the above-mentioned problems and provide a technique for producing a high tensile strength and high toughness steel in which heat treatment is performed only once.

[0008]

The present invention provides a weight ratio of C:
0.3-0.8%, Si: 0.1-1.5%, Mn: 0.2-2.0%,
Cr: 1.5% or less, Al: 0.1% or less, N: 30 to 100 ppm and V: 0.08 to 0.5%, Nb: Steel containing one or two of 0.01% to 0.1% is rolled or hot forged. After that, heat again above the austenitizing temperature and keep the temperature up to 500 ° C for 100
Characterized by cooling at a cooling rate of ℃ / min or less,
A method for producing a precipitation hardening type high tensile strength and high toughness steel material having a ferrite / pearlite structure having a tensile strength of 750 N / mm ² or more.

[0009]

The present invention will be described in detail below. First, the reason for limiting the components will be described. C is 0.3 to obtain strength
% Or more, but if it exceeds 0.8%, proeutectoid cementite occurs and the toughness is impaired, so the range was made 0.3 to 0.8%.

Si is required to be 0.1% or more as a deoxidizing agent during steel making, but if it exceeds 1.5%, toughness is impaired, so 0.1 to 10% is required.
The range was 1.5%. Mn is 0.2-2.0 for the same reason as Si.
% Content is required. Cr improves strength, but 1.
If it exceeds 5%, the toughness and machinability will deteriorate, so it was made 1.5% or less.

V and Nb are elements that have a large effect on precipitation hardening, and it is necessary to contain one or two. However, even if added in a large amount, it does not contribute to the strength increase,
V was 0.08 to 0.5% and Nb was 0.01 to 0.1%. N
Is an element that contributes to precipitation hardening by combining with V and Nb, but if contained in a large amount, the toughness decreases, so 30 to 100 p
The range was pm.

Further, Al is necessary as a deoxidizing agent,
Since addition of a large amount has no effect, it was limited to 0.1% or less. In addition to these elements, one or more elements such as Ti, Ni and Mo may be added depending on the required material properties such as strength, toughness and machinability. Ti is N in steel
Although it binds to and contributes to precipitation hardening, addition of a large amount has no effect, so the content was limited to 0.1% or less.

Ni and Mo improve the strength, and when added in a small amount, they improve toughness, but when added in a large amount, they reduce toughness and machinability, so Ni is 0.2% or less and Mo is 0.3%.
The following additions are desirable. Next, regarding normalizing processing,
In order to improve the rough structure after hot working and to recover elongation and toughness, it is necessary to heat above the austenitizing temperature, to refine the structure by recrystallization and to solidify the precipitation hardening element. Note that the higher the heating temperature, the more the amount of solid solution of the additive component increases, the more precipitation hardening during cooling increases, and the higher the strength becomes, but the toughness decreases due to the coarsening of the austenite grains. From the viewpoint of improving the toughness, the heating temperature is preferably 1000 ° C or lower. After austenitization, 100 ℃ / min (at the surface cooling rate,
5) at a cooling rate above the average value between the start of cooling and 500 ° C)
When cooled below 00 ° C, bainite or martensite structure is formed, and in the case of non-tempered steel that is not tempered, the toughness becomes poor.
It was set to 00 ° C / min or less. At a cooling rate of 100 ° C / min or less
Since the ferrite / pearlite transformation is completed when cooled to 500 ° C, it may be cooled rapidly afterwards with water, etc., and the cooling rate is about 1 ° C / min (corresponding to the center part when air-cooling a steel bar of about φ200 mm). However, sufficient strength and toughness can be obtained by selecting the components within the specified range. Also,
The CCT diagram for steel No. 1 in Table 1 is shown in Fig. 1. If the cooling rate is increased to more than 100 ℃ / min before reaching 500 ℃, the ferrite-pearlite transformation will not be completed and the martensite Since the structure becomes mixed with iron and bainite, and the toughness is impaired, 100 ℃ /
It is necessary to keep the cooling rate below min.

[0014]

EXAMPLES Table 1 shows the main chemical components of the test materials used in Examples and Comparative Examples. The steel shown in Table 1 was melted in a converter to manufacture a steel ingot. The test materials shown in Table 2 were hot-worked steel ingots with a wrought forming ratio of 2.36,
It is manufactured into a round bar. For the tensile test, a JIS No. 4 tensile test piece sampled from 1/4 of the diameter was used. For the impact test, a JIS No. 3 impact test piece was used to measure the impact value at 20 ° C. Test No. 7 is as-rolled material from Steel No. 1 steel and Test No. 8 from Steel No. 2 steel.

Looking at the results of tensile strength in Table 2 by steel number, as can be seen by comparing test numbers 1 and 10,
For both Nb and V, the steel No. 3 which is within the specified range of the present invention does not reach a tensile strength of 750 N / mm ² even if the same treatment is applied.
When compared within the same steel number, the as-rolled material has a higher tensile strength, and the normalized material tends to have a higher tensile strength as the heat treatment temperature increases.

Looking at the impact value within the same steel number, the as-rolled material is low, and the normalized material tends to have a higher impact value as the heat treatment temperature is lowered. Test No. 9 in which the cooling rate after austenitizing was increased to 100 ° C./min or more had high strength but markedly low toughness. From the above results,
It is understood that the toughness of the steel materials having the components within the scope of the present invention can be improved by performing a proper normalizing treatment, and the tensile strength and the impact value can be controlled by the heat treatment temperature.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

INDUSTRIAL APPLICABILITY According to the present invention, precipitation hardening type high strength steel having a ferrite / pearlite structure can be made tougher by normalizing. In particular, it is effective for products with a large product size where the reduction rate cannot be increased. Also,
There is also an effect that the tensile strength and impact value can be adjusted by the heat treatment temperature.

[Brief description of drawings]

FIG. 1 is a CCT diagram of a steel No. 1 sample.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inori, Tabata Sokyu Kawashima, Kurashiki, Okayama Prefecture 1-chome (No address) Inside Mizushima Works, Kawasaki Steel Co., Ltd. (72) Inventor, Kazuo Asao Mizushima Kawasaki, Kurashiki, Okayama 1 chome (without street number) Kawasaki Steel Works Mizushima Steel Works

Claims (1)

[Claims] 1. A weight ratio of C: 0.3-0.8%, Si: 0.1-
1.5%, Mn: 0.2 to 2.0%, Cr: 1.5% or less, Al: 0.1
% Or less, N: 30 to 100 ppm and V: 0.08 to 0.5%, Nb:
Steel containing one or two of 0.01-0.1%,
After rolling or hot forging, it is heated again to the austenitizing temperature or higher and cooled to 500 ° C at a cooling rate of 100 ° C / min or less, and a ferrite / pearlite structure having a tensile strength of 750 N / mm ² or more is formed. A method for producing a precipitation hardening type high tensile strength and high toughness steel material having the same.