KR100737510B1 - Method for fabricating vehicle components and new use of a precipitation hardenable martensitic stainless steel - Google Patents

Abstract

The present invention relates to a composition and a method for producing a precipitate hardenable martensitic stainless steel product, the composition comprising at least 0.5 wt.% Cr, at least 0.5 wt.% Mo, and a total of 50 wt. It is characterized by exceeding%. The method steps include smelting the material into a casting, cold deformation several times to obtain at least 50% martensite after hot extrusion, and finally aging at 425 to 525 ° C. to obtain precipitation of semicrystalline particles. . The material can be used in vehicle parts where the demand for corrosion resistance, high strength and good toughness must be met.

Description

METHOD FOR FABRICATING VEHICLE COMPONENTS AND NEW USE OF A PRECIPITATION HARDENABLE MARTENSITIC STAINLESS STEEL}

The present invention relates to precipitate hardenable martensitic stainless steel, hereinafter referred to as stainless maraging steel. More specifically, the present invention relates to maraging steels for certain applications where various advantages can be obtained with regard to product properties and manufacturing processes, such as in the vehicle industry (eg, cars, trucks, motorcycles). will be.

In general, automobiles use carbon steel tubes as shock absorbers. These tubes are cured and surface treated in different ways, depending on the type of product. The manufacturing method involves many steps and curing processes and can be rejected because of the very high demands on the dimensional tolerances of such tubes.

It should be noted that the combination of martensite conversion and precipitation hardening of its own is known from Metall. Mater. Trans. A., 25A, 2225-2233, 1994. Precipitation in the martensite structure of an intermetallic compound of a semi-crystalline structure based on iron, molybdenum, chromium and silicon is disclosed in this document. Martensite in this alloy can be formed isothermally by modification as described in the above document or as described in Scripta Metallurgica et Materialia, 1995, Vol. 33, No. 9, pp. 1367-1373. have. It is known that these new forms of steel alloys exhibit a combination of good strength, corrosion resistance and ductility. In fact, tensile strengths of 2500-3000 MPa have been obtained for wire products in low temperature operation and aging conditions which make materials well suited for medical and dental instruments. However, this document does not disclose a manufacturing method that allows deformation to form steel products of the desired form by achieving optimal conditions between ductility, strength, formability and corrosion resistance and homogeneity of martensite distribution.

Steel alloys treated according to the present invention can be processed in the form of wires, tubes, rods and strips for further use in a variety of vehicles and automotive components. It is an object of the present invention to provide a highly efficient method for producing easily formable steel products having an even distribution of martensite and deposits which make them suitable for use in parts in the vehicle or automobile industry.

By using the stainless steel maraging steel according to the invention, the manufacturing process for obtaining the final product can be much shorter. Curing by precipitation of intermetallic compounds gives the product a very high level of strength. It is known that materials for application shock absorbers require very high demands on mechanical properties.

In contrast to conventional high strength steels, maraging steels have certain unique properties such as being hard to warp during hardening, good weldability and good combination of strength and toughness making them usable for many applications. Compared with conventional stainless steels, the physical properties of stainless steel maraging steels are similar to those of carbon steels used today.

According to a first aspect, the present invention comprises at least 0.5% by weight of chromium, at least 0.5% by weight of molybdenum and precipitates into the matrix of martensite with a composition in which the total amount of Cr, Ni, Fe is greater than 50% by weight. It provides a part made from a stainless steel maraging steel material having a microstructure including intermetallic particles, and having corrosion resistance, high strength and toughness.

According to another aspect, the present invention relates to a method for preparing an alloy comprising: smelting an alloy comprising a composition comprising at least 0.5 wt% chromium, at least 0.5 wt% molybdenum, and wherein the total amount of Cr, Ni, Fe is greater than 50 wt%; Casting an alloy; Hot extruding the casting followed by a number of cold deformation steps to obtain at least 50% martensite throughout its microstructure; And aging the alloy at 425 to 525 ° C. sufficient to obtain precipitation of semi-crystalline particles in the martensitic microstructures.

Martensitic stainless steel alloys according to the invention, more particularly suitably optimized components, contain at least 0.5% Cr and 0.5% Mo, and settling hardening in which the total amount of Cr, Ni, Fe exceeds 50% It has been found that possible stainless steel alloys are well suited for use in environments where the demand for excellent corrosion resistance with high strength and toughness must be met. One such field of application is in vehicles and parts of motor vehicles. More specifically, such alloys are fabricated such that precipitation of intermetallic semicrystalline particles is obtained at the martensite matrix.

One embodiment of the steel alloy according to the present invention

0.1 wt% carbon

Nitrogen up to 0.1 wt%

0.5 to 4 wt% copper

10-14 wt% chromium

Molybdenum 0.5-6 wt%

Nickel 7-11 wt%

Cobalt 0-9 wt%

Tantalum up to 0.1 wt%

Niobium up to 0.1 wt%

Vanadium up to 0.1 wt%

Tungsten up to 0.1 wt%

Aluminum 0.05 to 0.6 wt%

Titanium 0.4-1.4 wt%

Silicon up to 0.7 wt%

Manganese ≤ 1.0 wt%

Iron balance (excluding common impurities up to 0.5% by weight in total)

It is preferable that it consists of.

More preferably, the preparation of this alloy should be made in such a way that, after deformation, the precipitation appears as quasicrystalline particles in order to make the modified martensite. It has been found that enhanced mechanical properties can be obtained in this particular type of alloy if the total amount of deformation can take place without an intermediate annealing step between each deformation step and all deformation steps.

Fabrication of the material takes place by first smelting the iron-based alloy in an arc furnace under a protected atmosphere having the composition. The material is then poured to form a casting and hot extruded to obtain a hollow tube, which is then introduced into a pilgering mill with cold reduction, whereby the total degree of cold rolling is 50%. Above, preferably further deformation by cold drawing is sufficient to obtain a martensite level of at least 70%. The material is finally aged at 425-525 [deg.] C., preferably at about 475 [deg.] C. for 4 hours and then ready for use in a form suitable for vehicle parts or similar applications.

As a preferred embodiment, it has been found that the material with the composition and processed in the manner described above is very suitable for making shock absorbers for automobiles, which are generally manufactured as tubular parts.

Mechanical properties are particularly important for materials that must be very suitable for use in vehicle components. At the same time, the material should be easily formed to be manufactured in the form of wires, tubes, rods and strips for further use in this kind of application.

FIG. 1 shows Critical Pitting Temperature for 1RK91, AISI304 and AISI316 at various concentrations of sodium chloride using electrochemical CPT test with electrostatic potential measurement at +300 mVSCE, pH = 0.6, ground test sample (600 μm). (CPT). All results are the average of six measurements.

In order to investigate the mechanical properties of the material according to the invention, the material was fatigue tested along with other existing optional conventional carbon steel materials.

The invention will be further described with reference to the following examples, which are illustrative rather than limiting.

The iron-based alloy according to the present invention having the analysis as shown in Table 1 was subjected to this fatigue test.

Chemical composition of 1RK91 (% by weight) steel C + N Cr Mn Fe Ni Mo Ti Al Si Cu Sandvik 1RK91 <0.05 12.0 0.3 Remaining amount 9.0 4.0 0.9 0.30 0.15 2.0

   For comparison, a hard chromed standard carbon steel tube was chosen. The results of the comparative fatigue test are shown in Table 2 below.

Steel alloy Fatigue strength 1RK91 hard chromed C-steel 300 MPa 195 MPa

As is apparent from Table 2, the alloy of the present invention, 1RK91, has a significantly higher fatigue strength than the steel currently used as shock absorbers. This is mainly the result of selecting materials with precipitated quasicrystalline particles and martensite that appear in the microstructure after being made according to the present invention. Other representative properties for describing the level of mechanical properties are the hardness level and the E-coefficient (Young's modulus), usually expressed in GPa.

Table 3 below shows these values for the 1RK91 tube material selected according to the present invention compared to standard C-steel tubes as mentioned in the previous table.

Mechanical properties alloy Hardness (H _v )  E (GPa) 1RK91, aging   565   201 C-steel (surface area)   518   218 C-steel (center wall area)   314

As shown in Table 3, the hardness level for the 1RK91 alloy of the present invention is clearly higher than for standard carbon steels, even if the latter surface area is hard chromized. It is also important that the E-factor value is approximately the same as for carbon steel. In general, this is a surprising result because the E-coefficient value for a stainless steel alloy can never reach its level for carbon steel. Other measured values important for quantifying the mechanical properties of the material are shown in Table 4 below.

Mechanical test results alloy R _p 0.05 (MPa) R _p 0.2 (MPa) Rm (MPa) Tensile Strength A% (elongation) 1RK91 1830 1850 1870 6.7 C-Steel Control 578 635 644 13.3

It is evident from Table 4 that the 1RK91 alloy of the present invention will outperform standard carbon steels in its mechanical properties.

The tendency of thermal expansion is another important characteristic to be considered in connection with vehicle components such as shock absorbers. Table 5 below shows the thermal expansion values for the 1RK91 material compared to both the standard carbon steel type 4L7 and the standard 18 / 10-stainless steel alloy.

Thermal expansion value (μm / (m x ℃)) Temperature 1RK91 C-Steel 4L7 Alloy 18/10 30 to 100 11.48 12.3 16.7 30 to 200 11.87 12.8 17.3 30 to 300 12.19 13.5 17.8 30 to 400 12.45 14.0 18.1

Thermal expansion values are important in the manufacture and use of automotive components where there is a need for any error tolerance to be kept within very limited limits. An important conclusion that can be drawn from this table is that it has been found that using the steel according to the invention it is possible to obtain thermal expansion values comparable to those that can be obtained with conventional carbon steels, while at the same time the conventional carbon in mechanical properties It is better than steel.

Corrosion properties are also important in the materials used in vehicle components. At the same time, the material should be easily formed to allow manufacture in the form of wires, tubes, rods and strips for its further use in this kind of application. The material was tested in comparison with other existing optional stainless materials such as Tp 316 and Tp 304 to investigate the corrosion properties of the material according to the invention.

Although the present invention has been described with reference to the above embodiments, any changes and modifications will be apparent to those skilled in the art. Accordingly, the invention is limited only by the spirit and scope of the appended claims.

Claims (9)

It is made of a stainless steel maraging steel material having a microstructure in which the total amount of Cr, Ni, Fe exceeds 50% by weight and includes intermetallic particles precipitated to the base of martensite, 0.1 wt% carbon Nitrogen up to 0.1 wt% 0.5 to 4 wt% copper 10-14 wt% chromium Molybdenum 0.5-6 wt% Nickel 7-11 wt% Up to 9 wt% cobalt Tantalum up to 0.1 wt% Niobium up to 0.1 wt% Vanadium up to 0.1 wt% Tungsten up to 0.1 wt% Aluminum 0.05 to 0.6 wt% Titanium 0.4-1.4 wt% Silicon up to 0.7 wt% Manganese up to 1.0 wt%; And Iron balance, except common impurities A vehicle component having high corrosion resistance, high strength and toughness, characterized in that the microstructure comprises quasi-crystalline particles in the matrix of martensite. delete delete It is made of a stainless steel maraging steel material having a microstructure in which the total amount of Cr, Ni, Fe exceeds 50% by weight and includes intermetallic particles precipitated to the base of martensite, 0.1 wt% carbon Nitrogen up to 0.1 wt% 0.5 to 4 wt% copper 10-14 wt% chromium Molybdenum 0.5-6 wt% Nickel 7-11 wt% Tantalum up to 0.1 wt% Niobium up to 0.1 wt% Vanadium up to 0.1 wt% Tungsten up to 0.1 wt% Aluminum 0.05 to 0.6 wt% Titanium 0.4-1.4 wt% Silicon up to 0.7 wt% Manganese up to 1.0 wt%; And Iron balance, except common impurities A vehicle component having high corrosion resistance, high strength and toughness, characterized in that the microstructure comprises quasi-crystalline particles in the matrix of martensite. The method according to claim 1 or 4, A vehicle component, characterized in that the vehicle component is tubular. The method according to claim 1 or 4, And said vehicle component is a shock absorber. delete delete delete

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