JPH032352A - Production of spring steel wire with high anti-fatigue strength and cold forming spring steel wire - Google Patents

Abstract

PURPOSE:To obtain a steel wire capable of producing a spring having high anti-fatigue strength required at the time of hardening in the air by specially distributing the chemical components of a steel wire and specifying the amount, size, and composition of nonmetallic inclusions in the steel. CONSTITUTION:This spring steel wire is a steel wire having a composition consisting of, by weight, 0.55-0.70% C, 1.00-2.50% Si, 0.50-1.50% Mn, two or more kinds among 1.00-4.00% Ni, 0.50-2.50% Cr, and 0.10-0.90% Mo, either or both of 0.05-0.50% V and 0.05-0.50% Nb, and the balance Fe and also having a structure in which, as to nonmetallic inclusions, the index of nonviscous inclusions having a ratio of length (l) to width (d), (l/d), of <=5 in the L-section of the wire is regulated to <=10 and also the size of the maximum inclusion among the above nonviscous inclusions is regulated to <=15mu. This steel wire is heated up to >=850 deg.C, cooled in the air at 0.5-40 deg.C/sec cooling rate, and then tempered at 300-600 deg.C, by which the high strength steel wire having required strength can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a spring steel wire with excellent fatigue strength used for valve springs of automobile engines, etc., and a method for manufacturing the steel wire for springs.

[Prior art] Valve springs conventionally used in automobile engines, etc.
IS G3561, JIS3565, JIS3566,
It is common to use so-called oil-tempered wire, which is defined by the above standards, and is cold-formed into springs.

By the way, in recent years, as automobile engines have increased in output and vehicle bodies have become lighter, there has been an extremely high demand for higher stress in engine valve springs.In order to meet this demand, there is a strong demand for springs with high fatigue strength. Existing specified materials are becoming increasingly difficult to meet this requirement.

In order to meet this demand, spring steels with increased amounts of alloying elements have been proposed (for example, JP-A-59-177352, JP-A-62-107044, JP-A-62-17).
Publication No. 7152).

Furthermore, in order to increase fatigue strength, it is becoming common to harden the surface by nitriding, shot peening, etc.

The oil-tempered wire used in this spring is a so-called OT wire, which is manufactured by heating a steel wire above its transformation point, quenching it in oil, and then tempering it in lead iron to adjust its strength.

[Problem to be solved by the invention] However, even with these oil-tempered wires made of steel,
It was not possible to meet the increasingly strict requirements for higher fatigue strength of springs.

Furthermore, this oil tempering process has problems in that it uses oil as a quenching medium, resulting in a poor working environment and high heat treatment costs.

[Means for Solving the Problems] In order to solve the above problems, the present inventors investigated ways to increase the fatigue strength of springs, appropriately distributed the chemical components of the spring steel wire, and He discovered and completed a new method for producing high-strength steel wire for cold forming from spring steel wire in which the amount, size, and composition of nonmetallic inclusions were controlled.

That is, in the present invention, C: 0.55 to 0.70% and Si: 1.00 to 1.00% by weight.
2.50%Mn: 0.50-1.50%, Ni: 1.00-4.00%, Cr:
0.50~2.50%Mo: 0.10~0.90
%, 2 fm to 3 types, V: 0.05 to 0.50%, Nb: 0.05
The spring steel wire contains one or two of ~0.50% of Fe, with the remainder essentially consisting of Fe, and the nonmetallic inclusions contained in the steel are: ■ In the cross section of the rolled wire rod, length ρ and width d
The ratio of (JZ/d) is 5 or less and the non-viscous inclusion index is 10
The size of the largest inclusion among the non-viscous inclusions is 15μ or less.■ Also, the non-viscous inclusions are 5i02: 30-70%, ^
1203: 20% or less, CaO2 30% or less, MgO:
It is a non-viscous inclusion having a composition of 30% or less, and is characterized by heating the spring steel wire to a temperature of 850°C or higher, heating it in air for 0.5 to 100°C, and then tempering it in a range of 300 to 600°C. This is a method of manufacturing steel wire for cold-open forming springs.

The present inventors investigated ways to improve the fatigue life of springs, conducted multifaceted research on the chemical composition of steel, the influence of inclusions that are the starting point of fatigue fracture, and the manufacturing method of steel wire, and discovered the above chemical composition system. At the same time, we conducted the following experiments.

FIG. 1 is a diagram showing the relationship between the 50 million times fatigue strength and the non-viscous inclusion index after Nakamura rotary bending fatigue tests were conducted on oil tempered wires having a tensile strength of 230 kg/+nm2. The inviscid inclusion index is determined when the ratio of length (fL) to width (d) is 1/d≦ in the cross section of the rolled wire rod.
Regarding the inclusions in No. 5, the Japan Spring Manufacturers Association "Microscopic test method for non-metallic inclusions in spring steel materials" (1986
20th of the month).

From this figure, it can be seen that fatigue strength is improved by setting the inviscid inclusion index to 10 or less.

Figure 2 shows the fracture origin of a test piece that was tested at a test stress near the 50 million-cycle fatigue life and observed the fracture origin, and if the fatigue origin was an inclusion, the fatigue life was determined by the size of the inclusion. It is a diagram shown in stratification. The size of the inclusions was measured according to the method for measuring the size of inclusions using the inclusion microscopy test method described above.

From Figure 2, it can be seen that by reducing the size of inclusions to 15μ or less, the fatigue life becomes extremely long.In other words, the maximum size of non-viscous inclusions that become the starting point of fatigue fracture is
It is important to do the following:

Further analysis of the composition of these nonmetallic inclusions revealed that the composition of the inclusions was 5i (h: 30-70%, AJ2203
: 20% or less, CaO: 30% or less, MgO: 3
0% or less, the non-viscous inclusion index is 10
It has been found that the size is 15μ or less.

Furthermore, when manufacturing steel wire for cold forming from spring steel having the above components and inclusions, it is cooled in air at a cooling rate of 0.5 to δOO without quenching in oil, and then 300 to δOO The present invention was completed by discovering that a high-strength steel wire having the necessary strength can be produced by tempering at a temperature of .degree.

[For production] The reasons for limiting the constituent elements of the present invention will be explained below.

C is an element used to obtain the strength necessary for springs through heat treatment, and if it is less than 0.55%, the strength cannot be obtained;
．． If it exceeds 70%, the toughness and ductility will drop significantly, and as a result, cold forming into a spring will become difficult, so it must be avoided.

St has a deoxidizing effect and is solid-solved in the ferrite base, increasing strength and ensuring resistance to sagging, so 1.00% or more is necessary; however, if it exceeds 2.50%, it causes a decrease in toughness and This must be avoided since decarburization during production becomes severe and it becomes impossible to produce good rolled wire rods.

Mn is an element necessary to improve deoxidation and hardenability and ensure strength and toughness, and for this reason, it is necessary to have a content of 0.50% or more.

If it exceeds 1.50%, the difficulty in manufacturing increases and the toughness of the steel wire is impaired, so it must be avoided.

Ni, Cr, and Mo are two or three elements that improve the strength and toughness of the spring by increasing hardenability, tempering softening resistance, or precipitating fine carbides.
It is effective to add seeds in combination.

Therefore, it is necessary to add 1.00 to 4.00% of Ni. If it is less than 1.00%, the effect will not appear, and if it is less than 4.0%, the effect will not appear.
Even if it is added in an amount exceeding 0%, no further effect can be obtained, so it is excluded from the scope of the claims.

Cr must be contained in an amount of 0.50% or more, and if it exceeds 2.50%, the settling property deteriorates, so it must be avoided.

Mo is an element necessary for increasing resistance to temper softening and imparting strength and toughness to the spring by precipitating fine carbides, and for this purpose, it is necessary in an amount of 0.1 θ% or more. 0
．． Even if it is added in an amount exceeding 90%, no further effect can be obtained, so it is excluded from the scope of the claims.

Nb and V are elements added to improve strength and set property by refining crystal grains and precipitation hardening.
0.05% or more is required. Moreover, even if it is added in an amount exceeding 0.50%, the effect is saturated, so it is excluded from the scope of the claims.

Next, the present invention specifies the form, amount, and composition of inclusions. This is a technology that improves fatigue properties by specifying the composition of steel as described above and reducing the hard inclusions that become the starting point of fatigue fracture when manufacturing springs using the method described later. This is one of the points of the invention.

Regarding inclusions with a ratio of length (fL) to width (d) of λ/d≦5 in the cross section of a rolled wire rod, the Japan Spring Manufacturers Association "Microscopic test method for nonmetallic inclusions in spring steel materials" (
(January 20, 1988) when the non-viscous inclusion index exceeds 10, and when the size of the largest inclusion among the non-viscous non-metallic inclusions exceeds 15μ, the fatigue properties deteriorate. Therefore, you must avoid it.

The composition of the inclusions needs to contain 30 to 70% 5i02. If it is less than 30%, oxides in the steel will increase, and if it exceeds 70%, there will be too much hard 5i02, resulting in a decrease in fatigue strength.

As for Al2O5, if it exceeds 20%, the amount of hard inclusions will increase, so it must be avoided.

If the CaO content exceeds 30%, hard inclusions will result, so the appropriate range is 30% or less.

Regarding MgO, if it exceeds 30%, hard MgO-based inclusions will result, so the range was set to 30% or less.

When manufacturing steel wire for cold open forming from copper for springs, instead of the conventional method of manufacturing so-called OT wire by quenching it in oil and then tempering it in a lead bath, it is possible to simply Hardening is performed by cooling in air.

In this case, the heating temperature needs to be 850°C or higher. If the heating temperature is lower than this, sufficient strength and toughness cannot be obtained, so it must be avoided.

Cooling is performed at 0.5-b. If the cooling rate is less than 0.5°C/sec, sufficient strength cannot be obtained after quenching;
If the temperature is exceeded, problems such as quench cracking may occur, so both must be avoided.

The tempering temperature ranges from 300 to 600°C.

If the temperature is less than 300°C, the strength will be too high and the toughness will be impaired, so it must be avoided. On the other hand, if the temperature exceeds 600°C, the strength decreases rapidly, so it is excluded from the scope of claims.

[Example] A more detailed explanation will be given below with reference to Examples.

Steel having the chemical composition shown in Table 1 is heated to 120% and converted into a converter.
Ladle scouring, melting by continuous casting method, diameter 8.35 after casting
It was rolled into a wire rod of 100 mn.

This wire rod was annealed, pickled, and lubricated, and then drawn. This drawn wire material was heated, cooled, and tempered in a continuous furnace.

The steel wire produced in this manner was cold-formed into springs, subjected to low-temperature annealing, nitriding, shot peening, setting, etc., and processed into valve springs for automobiles.

For this spring, average wide cuff 0 kg/mm. Table 1 shows the results of a fatigue test conducted at a stress amplitude of 50 kg/mu++2.

As shown in Table 1, the valve spring of the present invention did not break even after being used over 50 million times.

[Effects of the invention] As explained in detail above, the present invention has succeeded in improving the fatigue life of the spring.As a result of the improved fatigue strength, the design stress of the spring can be increased, and the spring's @quantization and volume can be increased. It is remarkable that the performance of automobiles is improved through reduction of fatigue life, etc. Furthermore, when manufacturing the steel wire for processing these high fatigue life springs, it is possible to quench it in air, which makes it possible to simultaneously achieve the simplification of the manufacturing process. It has a great effect.

[Brief explanation of the drawing]

Figure 1 shows the relationship between fatigue strength and non-viscous inclusion index for an oil tempered wire with a tensile strength of 230 kg/mm2. FIG. 3 is a diagram showing the relationship between the size of inclusions at the starting point of a fracture surface and fatigue life. Microscopic testing method for non-metallic inclusions in spring steel materials (Japan Spring Industry Association S63.1.203

Claims (1)

[Claims] 1. C: 0.55-0.70%, Si: 1.00-2.50 in weight%
%Mn: 0.50-1.50%, Ni: 1.00-4.00%, Cr: 0.50-2.5
0% Mo: 2 or 3 of 0.10-0.90%, V: 0.05-0.50%, Nb: 0.05-0.50
%, and the remainder is substantially Fe.
In the rolled wire rod, the non-metallic inclusions have a ratio of length l to width d (l/d) of 5 or less and a non-viscous inclusion index of 10 or less in the L cross section of the rolled wire rod, and the non-viscous inclusions A high fatigue strength spring steel wire characterized in that the largest inclusion size in the wire is 15μ or less. 2. Inviscid inclusions are SiO_2: 30-70%, Al
_2O_3: 20% or less, CaO: 30% or less, MgO
The high fatigue strength spring steel wire according to claim 1, wherein the spring steel wire has a composition of 30% or less of non-viscous inclusions. 3. After heating the spring steel wire according to claim 1 or 2 to a temperature of 850°C or higher, cooling it in air at a cooling rate of 0.5 to 40°C/sec, and then tempering in the range of 300 to 600°C. A method for producing steel wire for cold-formed springs, characterized by: