JPS63303036A - High-strength steel wire - Google Patents

Abstract

PURPOSE:To manufacture a heat-resisting and high-strength steel wire having high strength and also having, owing to nitriding treatment, sufficient fatigue resistance and settling resistance even under severe service conditions, by working a low-alloy steel billet containing specific amounts of Cr, V, and Mo into a wire by means of hot rolling and cold wiredrawing. CONSTITUTION:A billet of low-alloy steel containing, by weight, 0.60-0.80% C, 1.30-1.80% Si, 0.40-0.70% Mn, 0.40-1.00% Cr, 0.20-0.60% V, and 0.10-0.25% Mo is hot-rolled so as to be worked into a wire rod. After heat treatment, such as patenting treatment, surface grinding is applied to the above wire rod to remove the surface defects of the wire rod, and successively, cold wiredrawing is applied to form the above wire rod into a wire in the final straight-line state, followed by final annealing. By this method, the heat-resisting and high-strength steel wire suitable, as high-strength steel wire, for material for piston rings and various fasteners and sufficiently capable of withstanding use, after formed into a spring, etc., and subjected to nitriding, even under severe service conditions of high stress, high temp., etc., can be obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a high-strength steel wire used in the manufacture of springs, piston rings, various fasteners, etc., and springs, etc., which are subjected to nitriding treatment after forming to produce high-stress, high-temperature steel wires. The present invention also relates to a heat-resistant, high-strength steel wire that can have sufficient fatigue resistance and fatigue resistance even under severe usage conditions such as high repetition rates.

(Prior art) Conventionally, steel wires for valve springs used in automobile engines include piano wires for valve springs, carbon steel oil tempered wires for valve springs, chrome vanadium steel oil tempered wires for valve springs, and steel wires for valve springs. Silicon chrome steel oil tempered wire is commonly used.

Among them, silicon chrome steel oil tempered wire for valve springs has been used in a wide range of applications in recent years as a valve spring material with excellent durability and heat resistance. However, as the performance of engines increases, the temperature applied to the spring material tends to increase due to the higher temperature of the atmosphere in which the spring is used and the heat generated by the high-speed operation of the spring, so the performance of conventional steel wire for valve springs is not always sufficient. However, there is a growing need for more heat-resistant steel wire.

In addition, nitriding is one of the methods of increasing the fatigue strength of springs and the like, and the scope of its use has been expanding in recent years.

There are various methods for this nitriding treatment, such as a method in a gas atmosphere or a method in which treatment is carried out in a salt bath, but in any of these methods, more stable nitriding characteristics can be obtained by treating at a higher temperature. Conventionally, when applying nitriding treatment in the field of valve springs, the nitriding treatment is performed on silicon chrome steel oil tempered wire for valve springs, which is the most heat-resistant of the steel wires for valve springs, after forming the spring. In this case, in order to prevent the internal hardness of the wire from decreasing, nitriding is performed at a temperature of 380°C to 420'C, and especially when nitriding is performed at a temperature of 450''C or higher, the internal hardness of the wire decreases. However, since the spring's resistance to fatigue deteriorates, its performance as a spring deteriorates.

Therefore, there is a strong desire for a heat-resistant high-strength steel wire not only from the viewpoint of use as a spring but also from the viewpoint of spring manufacturing.

(Problems to be Solved by the Invention) The present invention does not cause a decrease in fatigue strength or increase in fatigue of springs, piston rings, etc. due to higher engine speeds and higher temperatures in the atmosphere in which the springs are used, and The purpose of the present invention is to provide a high-strength steel wire with excellent heat resistance that can be nitrided more stably at higher temperatures than conventional ones when nitriding is performed for the purpose of improving fatigue strength and wear resistance.

(Means for Solving the Problems) The above-mentioned object of the present invention is to provide carbon in a weight ratio of 0.60 to 0.8.
0%, 5iH1, 30-1.70%, Mn; 0.40-
0.70%, CrHO, 40-1.00%, ■; 0.2
This is achieved by a high-strength steel wire characterized by containing 0 to 0.60% Mo, 0.10 to 0.25% Mo, and the balance consisting of Fe and inevitable impurities.

The high strength steel wire according to the present invention has a C; of 0.60 to 0.
80%, Si; 1.30-1.70%, Mn; 0.40
~0.70%, Cr; 0.40~1. 0%,■；0
．． 20-0.60%, Mo; 0.10-0.25%, the balance consisting of Fe and unavoidable impurities. After cold working into a wire, A. heating to a temperature above the transformation point. After austenitization, martensite or a mixed composite fiber of martensite and residual austenite is produced by quenching, and then tempering is performed at an appropriate temperature to produce the material.

After the high-strength steel wire of the present invention is formed into springs, piston rings, etc., it is subjected to nitriding treatment to improve fatigue resistance and wear resistance.

In addition, when manufacturing springs, piston rings, etc., if it is difficult to form strong steel wires using quenched and tempered high-strength steel wire, forming may be performed on the cold-worked wire before quenching and tempering. Predetermined properties can also be imparted by applying and then quenching and tempering treatment and nitriding treatment.

The reason for limiting the composition of the steel wire of the present invention will be explained below.

CTC is an effective element for increasing the strength of steel, but if it is less than 0.60%, sufficient strength cannot be obtained in order to obtain the strength targeted by the present invention. Moreover, if it exceeds 0.80%, the contribution to strength decreases and becomes a factor inhibiting toughness, so C was set in the range of 0.60 to 0.80%.

Si: Si is an effective element for improving the fatigue resistance of springs, but if it is less than 1.30%, its effect is small;
．． If it exceeds 70%, it is not desirable because it deteriorates workability in cold working and promotes decarburization in hot rolling and heat treatment. From this point of view, in order to obtain the target strength level of the present invention, the range of Si is 1.3.
The content was set at 0 to 1.70%.

Mn; Mn is an effective element for improving the hardenability of steel, but if it is less than 0.40%, its effect is small;
Even if it is added in an amount exceeding 70%, a large amount of retained austenite will be produced, and the hardenability will not be improved much and the effect will be reduced, so the range is set to 0.40 to 0.70%.

Cr; Cr is an element that dissolves in steel to improve hardenability and generates nitrides. If it is less than 0.40%, the effect will be small, and if it exceeds 1.00%, solid solution of carbides will be suppressed and strength will decrease, so 0.40 to 1.0%.
The range was 0%.

v; ■ is an element that is effective in particularly improving the settling resistance and preventing decarburization together with Cr; it also refines crystal grains, imparts toughness, and improves delayed fracture resistance.

It also shows a precipitation effect at around 500°C. If it is less than 0.20%, the refinement effect will be small, and if it exceeds 0.60%, there will be a strong tendency to suppress solid solution of carbides like Cr.
It was made into the range of 20-0.60%.

Mo: Mo is an effective element for improving the flattening resistance of the spring, and also increases the tempering softening resistance and provides heat resistance. If it is less than 0.10%, the above effects of addition will be small.

Furthermore, if the amount added is large, there are restrictions on wire manufacturing. During the manufacturing process of the quenched and tempered steel wire of the present invention, in order to cold work the wire to a predetermined wire diameter, it is necessary to perform heat treatment to withstand the working.

This heat treatment is preferably a constant temperature transformation heat treatment usually called a patenting treatment. According to our experiments, the amount of Mo added that enables mass-producible patenting processing is 0.25%.
is the limit, and if Mo is added in excess of this, the transformation from austenite to pearlite will not be completed during the patenting process, and martensite will partially form, making post-processing difficult. For the above reasons, the Mo range of the present invention is 0.10 to 0.2
It was set at 5%.

For the above-mentioned reasons, steel with a limited range of components is made into a wire rod by hot rolling after melting.

In the wire manufacturing process, the wire is subjected to heat treatment such as patenting treatment, and then cold wire drawing processing is performed. If the final wire diameter is small, heat treatment and wire drawing are repeated.

(Example) C in electric furnace, 0.60-0.80%, St; 1.30
~1.70%, Mn; 0.40-0.70%, Cr; 0
．． 40-1.00%, V; 0.20-0.60%, Mo
; After melting a steel ingot with the chemical composition of A to H in Table 1, which contains 0.10 to 0.25% and the balance is Fe and unavoidable impurities, it is hot rolled into a wire rod with a diameter of 8.01. year,
After heat treatment, surface defects of the wire are removed by surface grinding7.
4 tm, the diameter was reduced to 4.0 tm by cold working, and finally quenching and tempering were performed to obtain the mechanical properties shown in Table 2. In addition, symbol 1 in Tables 1 and 2
-L is a steel wire for valve springs according to the JIS standard selected for comparison with the steel wire of the present invention, and I and J are all steel wires of the present invention except that melting was performed in a converter. made using the same manufacturing method. Since L is a piano wire, it is made from a wire with a diameter of 9.0 ms, and is not quenched and tempered in the final process.

Table 3 shows the results of the fatigue strength of the steel wire of the present invention and the comparative steel wire determined by a rotary bending fatigue test. The test was conducted using the staircase method using 30 test pieces that were heat treated at 420°C for 20 minutes on a 700m long specimen, then subjected to sit peening and aging treatment at 200°C for 20 minutes. The statistical fatigue strength was determined using

The steel wire of the present invention has better fatigue strength than any conventional steel wire for valve springs.

Table 4 shows invention steel wires A-H and comparative steel wires 1. J.K.

The amount of residual shear strain is shown as the amount of residual shear strain when a torsional stress of 70 kgf/- is applied to the spring at a temperature of 230°C for 96 hours. Has low shear strain and excellent heat resistance.

The results of comparing the nitriding properties of the steel wire of the present invention with comparative steel wire I are shown in FIG. Comparative steel wire I was nitrided at 420°C and 500°C, and the invention steel wire was nitrided at 500°C, and the nitriding properties were evaluated based on the hardness distribution of the main part of the wire surface. The surface hardness of the steel wire is high, and the hardening depth is deeper than that of the comparative steel wire. Furthermore, although the steel wire of the present invention was nitrided at a higher temperature than the comparative steel wire, its internal hardness was the same as that of the comparative steel wire, which indicates that the steel wire of the present invention has excellent heat resistance.

Table 3 Table 4 (Effects of the Invention) As detailed above, by using the high-strength steel wire of the present invention, it is possible to perform a stable nitriding treatment on the spring at high temperatures, resulting in heat-resistant fatigue. It became possible to obtain springs with excellent strength.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the results of comparing the nitriding properties of the steel wire of the present invention with those of comparative steel wire (3).

Claims (1)

[Claims] Weight ratio of C: 0.60 to 0.80%, Si: 1.30 to
1.70%, Mn; 0.40-0.70%, Cr; 0.
40-1.00%, V; 0.20-0.60%, Mo;
A high-strength steel wire characterized in that it contains 0.10 to 0.25%, with the remainder consisting of Fe and unavoidable impurities.