JPS6075517A - Manufacture of nonrefined forged steel article - Google Patents

Abstract

PURPOSE:To improve the strength, toughness and ductility by subjecting a steel contg. prescribed percentages of C, Si, Mn, V, etc. to hot forging and heat treatment each at a prescribed temp. CONSTITUTION:A steel consisting of, by weight, 0.2-0.6% C, 0.15-1% Si, 0.6- 2% Mn, 0.03-0.3% V, <=0.12% S, 0.015-0.06% Al, 0.005-0.02% N and the balance Fe is manufactured by melting. The steel is forged at 750-1,100 deg.C starting temp. and 750-1,050 deg.C finishing temp., and it is cooled to a temp. between the temp. at which ferrite-pearlite transformation is finished and 100 deg.C in boiling water.

Description

[Detailed description of the invention] The present invention relates to a method for manufacturing non-thermal forged steel products.

Conventionally, mechanical structural parts used in automobiles and construction machinery have been produced by hot forging carbon steel or alloy steel for mechanical structural use, then reheating, and subjecting it to heat treatment such as quenching and tempering. They are manufactured and put into use with strength characteristics depending on the purpose and use. However, the above-mentioned heat treatment requires a large amount of thermal energy cost, and the manufacturing cost inevitably increases due to an increase in the number of processing steps, an increase in the number of products in progress, and the like.

Therefore, in recent years, in the manufacture of forged steel products, in order to simplify the manufacturing process, and in particular to omit the quenching and tempering process, so-called precipitation hardening type carbon steels and alloy steels containing trace amounts of V, Nb, T+, etc. The material is so-called hot-forged non-tempered steel with added alloying elements, and by using the heating during hot forging, the forging and the subsequent cooling process, it is possible to obtain the required properties while leaving it to cool after forging. Heat-treated forged steel products are attracting attention, and some have already been put into practical use.

Such non-thermal forged steel products are usually manufactured as follows. That is, forging of the steel material is started at a temperature of 1150 to 1300°C, and after the forging is finished at a temperature of 1100 to 1300°C, it is left to cool in the atmosphere or is blast cooled. However, according to such conventional methods, although it is possible to obtain the required strength by adjusting the chemical components of the steel material, such as the amounts of added C, Mn, V, etc., and a decrease in ductility is unavoidable.

As a result of intensive research to solve the above-mentioned problems in manufacturing non-thermal forged steel products, the inventors of the present invention have determined that the forging start temperature and end temperature are By setting the temperature to a lower temperature than conventional methods and by using a method of cooling down to 100°C in boiling water after forging, it is possible to form a fine pearlite/ferrite structure in the steel. Thus, we have discovered that it is possible to obtain a non-thermal forged steel product that has high strength, excellent toughness and ductility, and is also improved in terms of machinability compared to conventional heat-treated steel forgings, leading to the present invention. be.

The method for producing a non-thermal forged steel product according to the present invention includes, in weight percent, CO, 20-0.60%, Sto, 15-1.0%, Mn 0.60-2.0%, V 0.03-0.30. % SO, 12% or less, 1 O, 015 to 0.060%, NO, 005 to 0.020%, and the balance substantially consisting of iron and unavoidable impurities as the steel material, 750 to 1100' Forging is started at a temperature of c, and after forging is finished at a temperature of 750 to 1050°C, cooling is performed in boiling water to a temperature range from the temperature at which ferrite-pearlite transformation ends to 100°C.

First, the reasons for limiting the composition of the hot forged non-tempered steel used as the raw material steel in the method of the present invention will be explained.

C is added as an essential element to ensure the strength of forged steel products, and also to form carbides in V, Cr, etc., and exhibit its precipitation strengthening effect.
．． If it is less than 20%, such reinforcing effect is poor; on the other hand, if it is 0.
When it exceeds 60%, carbides are excessively produced, resulting in unnecessarily high hardness and decreased toughness. Therefore, the C content range is 0.20 to 0.60%.

In addition to deoxidizing St, it is an effective element for strengthening the ferrite structure after cooling for forging, but if it is less than 0.15%, the strength will be insufficient, and if it exceeds 1.0%, it will deteriorate the toughness and machinability. deteriorate. Therefore, the content range of St is 0.15 to 1.
Set to 0%.

Mn is an essential element for increasing the strength of steel forgings, and is also an element for improving hardenability, but 0.
When it is less than 60%, the effect of increasing strength is poor, and 2
．． If the content exceeds 0%, the ductility deteriorates significantly and the hardness increases, which has a detrimental effect on machinability, so the content should be in the range of 0.60 to 2.0%. .

(2) In the method of the present invention, carbonitrides are formed with C and N during cooling of the raw steel after forging to strengthen the steel,
It is also an important element for improving its toughness, but if its content is less than 0.03%, this effect will be poor, while if it is contained in an amount greater than 0.30%, it will not increase the strength. At the same time, the toughness is reduced. Therefore, the content range of (1) is set to 0.03 to 0.30%.

S is added to improve the machinability of steel forgings. However, if the content exceeds 0.12%, the toughness value decreases, so the upper limit is set to 0.12%.

Aρ is effective for deoxidizing effect and refining grain size, but in order to effectively exhibit this effect, it must be at least 0.01
It is necessary to contain 5%. However, even if it is contained in a large amount exceeding 0.060%, the effect will only increase slightly and it will also have a detrimental effect on the stiffness, so the content range is 0.015 to 0.060%. do.

As mentioned above, N is an element useful for strengthening steel by combining with V and Aβ to form carbonitrides and making crystal grains finer. To effectively exhibit this effect, it is necessary to add at least 0.005%;
．． Even if it is contained in a large amount exceeding 0.020%, no increase in the effect can be expected, and it also causes deterioration of toughness, so the content is set at 0.005 to 0.020%.

Furthermore, the raw material steel used in the present invention may be impregnated in an amount of 1.0% or less, as necessary, in order to improve various mechanical properties, such as increasing hardenability when surface hardening a forged steel product. Cr can be added. When the Cr content exceeds .theta.%, the hardenability increases due to the relatively large amount of Mn in the steel material, resulting in an appropriately cooled structure such as martensite, which is not preferable.

In the method of the present invention, steel having the above-mentioned composition is used as a raw material steel, and after being heated to a temperature at which the V contained therein is completely austenitized or higher, a temperature of 750 to 11
Forging starts at a temperature of 00°C and ends at a temperature of 750 to 1050°C.

When this forging start temperature is lower than 750°C, forging itself is already difficult and sufficient formability cannot be obtained. On the other hand, when the forging start temperature exceeds 1100°C and therefore the end temperature exceeds 1050°C, the austenite crystal grains in the raw steel become coarse and hardenability increases, and proper cooling structures such as bainite occur during cooling after forging. As a result, toughness and ductility decrease.

In the method of the present invention, after forging is started at a predetermined temperature and finished at a predetermined temperature,
By cooling the steel to a temperature ranging from the temperature at which ferrite/pearlite transformation ends to 100°C, for example, 100°C in boiling water, a fine ferrite/pearlite structure is generated in the steel, resulting in high strength and high strength for steel forgings. It can impart toughness and ductility. Furthermore, by using boiling water as a cooling medium, it is possible to achieve more uniform cooling compared to air or blast cooling, and the cooling capacity is greater than that of air or blast, so it is more efficient than conventional methods. The time required for ferrite-pearlite transformation can be shortened.

Furthermore, since most of the scale of the forged steel product is peeled off during cooling, the shot blasting step that was conventionally necessary can be omitted in some cases.

As described above, according to the method of the present invention, the composition of the steel material is set within a predetermined range, and when the steel material is hot forged and cooled, the starting temperature, ending temperature, and cooling method are selected. In addition to the high strength required for the first time,
A non-thermal forged steel product having toughness and ductility can be obtained.

Example weight percentage: G O, 44%, Si 0.25%, Mn 0.85%, P O, 022%, S O, 024%, N O, 010%1. 67! Hot forged non-thermal steel 545CV steel with a composition of 0.025%, VO, 098%, and 0.11% Cr, and CO, 45%, SiO, 24%, Mn 0.83%, PO, Cr-free steels having the following compositions: 0.020%, S-0.022%, NO.010%, Aβ 0.024%, and VO.096% were heated to a temperature at which ■ becomes austenitic, and then subjected to various treatments. By starting forging at a temperature of , the forging end temperature was varied, a hot forged product having a shaft portion was produced in this way, and then this was cooled with boiling water or left to cool in the atmosphere. Tensile test pieces and JIS No. S test pieces were taken from the longitudinal direction of the shaft portion of the obtained steel forgings, and their mechanical properties were investigated. The results are shown in the drawing.

The forging start temperature is 1100°C, the finishing temperature is 1075°C, and the forged steel product is then cooled with boiling water. It has a ferrite-pearlite structure with some bainite mixed in, and therefore has a higher tensile strength than when cooled in the air. It has great strength, but poor aperture and impact value.

However, the forging starting temperature is 1000℃ and the finishing temperature is 102℃.
5°C, and then cooled with boiling water.The 0.2% yield strength of the forged steel 1 is still about 10 kgf/- higher, and the reduction of area and impact value are equal to or higher than those of the case where the steel is cooled in the atmosphere.Thus, the present invention According to the method described above, it is possible to obtain a non-thermal forged steel product having high strength and excellent toughness and ductility.

[Brief explanation of the drawing]

The drawing shows a method for obtaining a forged steel product by hot forging 545CV steel and Cr-free steel, then cooling with boiling water or cooling in the atmosphere.
1 is a graph showing the relationship between forging start and end temperatures and the mechanical and impact properties of the resulting forged steel product. Patent applicant Kobe Steel, Ltd. Representative Patent attorney Ittsu Makino Department 2

Claims (1)

[Claims] (]) In weight% CO, 20-0.60%, SiO, 15-1.0% Mn 0.60-2.0% VO, 03-0.30% SO, 1.2% or less A7! Forging is started at a temperature of 750 to 100"C using a steel containing 0.015 to 0.060% N O, 005 to 0.020%, and the balance substantially iron and unavoidable impurities. , at a temperature of 750 to 1050'C. After completing forging, the product is cooled in boiling water to a temperature range from the temperature at which ferrite-pearlite transformation ends to 100'C. Method. [2) In weight% CO, 20-0.60%, Si 0.15-1.0% Mn 0.60-2.0% VO, 03-0.30% SO, 12% or less Aβ 0. A temperature of 750 to 1100°C using steel as a raw material containing 015 to 0.060% N O, 005 to 0.020%, 1.0% or less of Cr, and the balance substantially consisting of iron and unavoidable impurities. Start forging with
After completing forging at a temperature of 750 to 1050°C, the temperature at which ferrite-pearlite transformation ends in boiling water is 1
A method for producing a non-thermal forged steel product characterized by cooling to a temperature range of up to 00°C.