FI73467C - FOERFARANDE ATT TILLVERKA ETT SJAELVANLOEPANDE STAOL. - Google Patents

Abstract

Steel and process for its manufacture which is suitable for the production of springs, especially leafsprings for vehicles. The steel which is vacuum-treated is an ordinary low-carbon steel with 0.20 - 0.40 % C. Its composition is incidentally characterised in that the oxygen content does not exceed 0.040 % and in that boron is added to an amount of 0.002 - 0.004 % and titanium to an amount of 0.015 - 0.050 %. The addition of boron causes the hardenability to increase greatly, thus making it possible to keep the carbon content low and, in consequence, reducing the danger of destructive decarbonisation. A characteristic of the process of production consists in the fact that the steel is produced in an oxygen converter in such a manner that the nitrogen content in the ladle does not exceed 20 ppm, in that it is subsequently subjected to vacuum treatment and in that Al is added so that the amount of uncombined oxygen drops to max. 40 ppm, whereafter titanium and boron are added prior to continuous casting and rolling.

Description

1 73467

Method for making self - permeable steel

The invention relates to a method of manufacturing steel, which steel is suitable for the production of springs, in particular leaf springs for vehicles. The method is known above all for being an easy-to-process low-carbon carbon steel to which boron has been added to improve the complete hardening of the steel and to make it self-permeable.

In vehicles, flexible devices such as coil spring a and leaf spring a are used to dampen mainly impact forces.

Coil springs are often made of a round material and the elastic property of the steel used is obtained by cold working, above all by drawing the material. For vehicles with low wheel strain, these coil springs are well suited. For heavier vehicles, springs with a larger cross-sectional area are used, such as leaf spring a of various shapes. Leaf spring 11a has, among many other advantages, the feature that the stability of the spring in directions other than the intended elastic direction is high.

Since the force moment of the leaf springs is greatest in their central part, the largest cross-sectional area is desired at that point. This can be solved in several ways. The spring leaf can be shaped so that it is widest at its center and tapers in the directions of the ends. This spring shape is bulky and causes a great waste of material in the manufacture. The spring leaf can be made thicker in the middle. These springs are often referred to as parabolic leaf springs and have played an increasingly important role in recent times. Parabolic leaf springs require a relatively small space, but are instead expensive to manufacture. The most common 1 time spring type is the so-called. a layered leaf spring, and above all the type made of a flat bar which is cut into pieces of different lengths and stacked on top of each other. The top leaf is the longest and has a fastening ring with 2 73467 ends at both ends. The cost of manufacturing these springs is the cheapest. Due to the friction between the leaves, hysteresis is also caused in the springs, which has a damping effect on the suspension. In the manufacture of springs, high reproducibility requirements are placed on the steel material used in order to avoid asymmetry in the springs. In addition to the small deviation of the dimensions, a small deviation of the properties in the steel is also required.

The essential properties of the steel used for the springs are a high elastic limit and a high fatigue limit.

The energy storage efficiency of the spring material is proportional to the square (R) of the elastic limit. The elastic limit is the maximum specific load that can be applied to the substance to return it to the initial position without deforming the substance. In many steels, the values of the elastic limit and the elongation limit are almost the same.

The fatigue limit follows in principle the elastic limit, but is also above all dependent on the homogeneity of the structure as well as on the quality of the surface, and has a favorable smooth surface which is V · ;. hi i 1 i roofing as well as defects such as slag-1 oi s ta and others.

Normally, quenching and discharge are used as a means of e.g. to increase the elastic limit, especially in coarse steel materials. In this case, a steel to be hardened must be used, i.e. a steel whose composition is adapted to the thickness of the substance so that, when cooled, martensite penetrates all the way to the center of the cross-section. After quenching, the material is released, usually 3 73467

O

at a temperature of about 400-500 ° C to achieve the specified toughness. Unfortunately, however, the elastic limit is lowered due to this emission.

To date, the relatively high carbon content of spring steel has been the main means of achieving a high elastic limit. Hardness also increases as the carbon content increases.

Of the steels currently used in springs, mention may be made of SS 2090 as well as SS 2230. The markings are taken from the Svensk Standard.

SS 2090 contains 0.52 to 0.60% C and 1.5 to 2.0% Si. Thanks to the high Si value, an increased hardenability is achieved. But Si 1 as an alloying agent is currently expensive.

SS 2230 contains 0.48 to 0.55% C, a slightly increased silicon content and 0.70 to 1.00 Mn, 0.90 to 1.20% Cr and 0.10 to 0.20% V. This steel is used for heavier for springs. The increased content of silicon, manganese and chromium increases the coarseness above all else, while vanadium is added instead due to the grinding of the crystal structure.

It is expensive to produce steel due to 1 ejection additives.

Moreover, it is relatively difficult to machine in cutting and punching drilling or stamping work, such as drilling holes through leaves to hold a spring package together. There is also a risk of carbon loss on the surface of both steels now mentioned, which e.g. lowers the fatigue limit.

It has now been shown that it is possible to produce both cheaply and efficiently a substance for springs by adding small amounts of boron and, above all, by ensuring that the favorable properties of boron come into their own in steel. One significant advantage of the addition of boron is that the steel can be made self-permeable, i.e. normally no separate discharge is required after the hardening process.

The invention will be described in more detail in the appended claims, which show the essential features of the invention, as well as in the following section.

It has proved advantageous to choose an unalloyed carbon steel as the starting material of the invention, the composition of which is as follows: C = 0.20 - 0.40%

Si = 0.20 - 0.35%

Mn = 1,0 - 1,3% S = maximum 0,040% P = maximum 0,040%

Cr = a maximum of 0,60% and which in addition contains the normally occurring and acceptable impurities. The steel should also contain small amounts of boron, as well as possibly titanium. The amounts will be specified in more detail below.

Small amounts of boron 11 ä have a very favorable effect, especially on the hardenability and thus naturally on the elastic limit.

When comparing the hardness of different steels, the following formula is used:

_ Λ Si Mn Cu Ni Cr Mo V

P = C + - + - + - + - + - + - + - + 5 x B

C 30 20 20 60 20 15 10 in which the various alloying agents are expressed, for example, in percentages by weight. The higher the P ^ the better the hardness. As it turns out, the carbon and especially boron content 1a has a significant effect. In addition, at a boron content of 0.002 to 0.004%, the effect has been found to be most advantageous. For example, a higher concentration may make the steel brittle in hardening. Good spring steel-

II

5 73467 requires that P does not vary much between different parts of the finished steel, nor between different blast furnace smeltings. It has been found that a suitable P 2 value for spring steel should remain in the furnace charges between 0.28 and 0.36 and between them at most 0.05.

As can be seen from the formula, by adding boron, the C content can also be kept lower than in conventional spring steel. This, of course, lowers the price of steel, while its surface is less susceptible to sand loss, which lowers the fatigue limit. In addition, this low-carbon boron-containing steel can be easily machined with cutting and stamping tools, even in hot-rolled and unheated states, whereas conventional spring steel, due to its high carbon content, must first be annealed or machined in a hot state. Softening annealing also increases the risk of carbon loss on the steel surface.

The addition of boron makes the steel self-permeable, i.e. the toughness required for the springs is achieved immediately after hardening. As an example, the measured toughness values (char-py W) at room temperature range between 25 and 35 J. In low temperature applications and where the requirements for uniform mechanical properties are very high,. o hard, the emission can be carried out at a maximum of 300 C, edu11 ίο t at a maximum of 230 C, thus increasing the impact strength or smoothing the properties of the substance. It has been found that if hydrogen and oxygen are reduced above all in the steel melt, then the effect of boron becomes more pronounced. Steel for said purpose is most preferably produced in an oxygen gas converter. However, the amount of blowing and the use of the blowing pipe must be adjusted so that the nitrogen content of the steel when calculating does not exceed 20 ppm. In fact, this nitrogen limit is too high to be acceptable, but for practical reasons not much lower readings can be reached. The oxygen content is also too high. These amounts can be reduced to acceptable levels, for example, by vacuum treatment, initial addition, and addition of titanium prior to the addition of boron.

The vacuum process makes it possible both to achieve a low oxygen content through degassing and slag separation and to implement a control technique that minimizes nitrogen uptake from the air. After vacuum deoxidation, the steel is further reduced in a conventional manner by adding aluminum so that the maximum value of free oxygen in the smelting batch is a maximum of 40 ppm, preferably 15-20 ppm. The oxygen content limit is determined in part by the fact that free oxygen adversely affects the quality of the steel, and in part by the fact that titanium has a greater tendency to combine with oxygen than the remaining nitrogen it is intended to bind.

Boron is very sensitive to nitrogen, and despite the process, some nitrogen is always left. By adding titanium, the best attempt is made to render the nitrogen in the smelter harmless by binding it to titanium nitrite. A suitable titanium content to be added has proven to be 0.015 to 0.050% by weight and preferably 0.020 to 0.045.

Experience has shown that for sheet steel according to the invention the product between the nitrogen content and the titanium content, expressed as a percentage by weight of the smelting batch - 4 (Ti x N), should preferably be below 4 x 10. It is safer to decide the dosage of the amount of titanium in the melt by taking this fact into account.

One drawback is that the titanium nitrites developed in the process form inclusions which may affect the properties of the steel e.g. 1to breakability and fatigue strength.

I) 7 73467

To avoid this side effect, the nitrogen content of the steel should be as low as possible.

After the vacuum treatment, the steel is cast in a continuous casting furnace equipped with a sputter protection system, i.e. where the flowing melt is surrounded by a ceramic protective tube or other cover so that the steel melt does not come into contact with air during passage. This avoids direct steel / i1ma contact, which can cause nitrogen and oxygen to enter the steel. If necessary, use a protective atmosphere.

The rolling of 1a, once the melt has been cast into a product to be quenched, also has a considerable effect, above all, on the fatigue limit of the sheet-steel. The cross-section reduction must be at least 25: 1 to obtain a well-formed steel.

The slag residues which nevertheless remain in the steel may have changed to such a shape after relief rolling that their effect on fatigue and strength is generally negative. In order to avoid this, the melt precursor can be melted with metallic calcium by blowing it as a powder into the melt, primarily the so-called in a form containing by weight 20% or more of metallic calcium. This is preferably done before vacuum treatment. The effect is, in part, that the amount of sulfide slag in the steel is minimized, in part, the remaining sulfide slags after rolling are deformed to less dangerous round shapes. The disadvantage is that the process may increase the nitrogen content. However, this can be strictly avoided by following the usual practice in the steelmaking process, for example by using argon as a carrier gas when blowing a second clay powder, and otherwise by ensuring that air does not come into contact with the melt throughout the process until the steel has solidified.

Claims (2)

8 73467 A method for producing self-permeable low carbon carbon steel for springs, in particular leaf springs, characterized in that the steel melt is produced in an oxygen gas converter by adjusting the garden blowing and blower pipe , 20 to 0,40% by weight Si = 0,20 to 0,35 Mn = 1,0 to 1,3 S = maximum 0,040 P = maximum 0,040 - "- Cr = maximum 0,60 -" - and other normally occurring impurities , which are then added to the vacuum treatment and aluminum to such an extent that unbound oxygen decreases to a maximum of 40 ppm, preferably 15 to 20 ppm and titanium to 0.015 to 0.050% by weight, preferably 0.020 to 0.045% by weight in the melt, and that the product of titanium and nitrogen content - -4% by weight does not exceed 4 x 10, and that boron is finally added to 0.002% to 0.004% by weight, after which the melt is subjected to continuous casting of the steel melt without contact with air, and the material is hot-rolled so that the reduction in total cross-section1 is not less than 25: 1. Process according to Claim 1, characterized in that calcium silicon containing at least 20% by weight of metallic calcium is blown into the melt before the vacuum treatment.