TECHNICAL FIELD The invention relates to the use of a tool steel having a specific chemical composition for motor-car body sheet pressing tools.

BACKGROUND OF THE INVENTION

For motor-car body sheet pressing tools there are today used a large variety of steel grades, including high alloyed steels, such as for example steels of grade SIS 2310 having the nominal composition 1.55 C, 0.30 Si, 0.25 Mn, 12.0 Cr, 0.80 Mo, 0.80 V, balance iron, as well as steels having a medium or comparatively low content of alloying elements. Among the latter ones one may in the first place mention steels of grade SIS 2260 having the nominal composition

1.00 C, 0.15 Si, 0.60 Mn, 5.3 Cr, 1.1 Mo, 0.20 V, balance iron, and the steel grade which is marketed under the trade name UHB FERM0 and which has the nominal composition 0.48 C, 0.40 Si, 0.90 Mn, 1.50 Cr, balance iron and impurities. Steels of type grade SIS 2260, which is the one which is most similar to the steel of the present invention, are used in the first place when the requirement on a high wear resistance is crucial for the choice of steel grade, while UHB FERM0 is used when good flame hardening features and a good toughness of the steel are crucial factors. It is a disadvantage that one and the same steel grade does not fulfill high demands in terms of wear resistance, flame hardening features as well as toughness, since the stock-keeping of a plurality of steel grades increases the over-all costs.

BRIEF DISCLOSURE OF THE INVENTION it is an object of the invention to offer a tool steel with features which better meets with the requirements for the use of the tool steel for motor-car body sheet pressing tools than known steel grades used today for the said purpose. Particularly it is an object to bring about a tool steel which is useful for large, casted tools for pressing body sheets in the motor-car industry. More particularly it

is an object to bring about a tool steel which shall be able to be used as a universal material in the said technical field, the concept "universal material" implying that the steel shall have a high wear resistance combined with a good toughness ^' , particularly in cutting edges, i.e. features which are characteristic features for those two tool steels which for the present are the dominating materials for motor-car body sheet pressing tools. The tools shall be possible to cast in large dimensions and shall be able to be used after suitable flame hardening treatment in the as cast condition. The flame harde- ning features shall be favourable, which implies a comparatively low flame hardening temperature and a broad flame hardening temperature range which with a high accuracy gives desired hardness to the surface layer of the tool.

Further, the steel shall have a good hardenability, i.e. may be through-hardened also in large dimensions, and the maximum hardening temperature should conveniently be lower than 1020°C. Besides being lame hardenable it is also desirable that the steel can be induction hardened and most conveniently also be CVD-coated and/or nitrogen case hardened. Good flame hardening features, however, are more important than the feasibility of CVD-coating the tool or of nitrogen case hardening it.

Still another object is that the steel and the tool made from the steel has good shape permanence to heat and that the dimension change which may occur during conventional hardening will not cause that the tool will shrink.

Still another object is that the steel has a good weldability, such that the tool made from the steel can be weld repaired without preheating.

Besides the above objects the steel i.a. for costs reasons also shall have only a moderate total content of alloying elements.

These and other objects and advantages of the invention can be achieved therein that the invention is characterized through what is stated in the appending claims and in the following description of a preferred embodiment and from the following report of achieved results.

DESCRIPTION OF PREFERRED EMBODIMENT AND REPORT OF ACHIEVED RESULTS The examined steels had the following nominal compositions (balance iron and impurities and accessory elements in normal amounts)

Ni

1.0

From steels Nos. 1-5 with the above nominal compositions there were produced 50 kg ingots and from these ingots there were made test specimens (test bars). The test bars were hardened through heating for 30 minutes at different temperatures between 900°C and 1050°C and subsequent cooling in air. The achieved hardnesses are illustrated graphically in the diagram in the drawing. The hardening procedure can be said to be a measurement of the flame hardening features due to the small dimensions of the test bars, which may correspond to the condi- tions at a flame hardening procedure. Best values in this respect were obtained with steel No. 2, which also satisfies other desired features of the steel of the invention. The composition of steel No. 2 there¬ fore corresponds to the nominal composition of the steel of the invention. In the diagram there have also been made corresponding graphs for the commercial reference steels Nos. 6 and 7.

It is a characteristic feature of the steel of the invention, as represented in the first place by steel No. 2, that there is achieved a high and substantially constant hardness within a broad temperature range. More particularly there is obtained a hardness between 61 and 63 HRC through hardening within the entire temperature range 950- 1050°C. Without binding the scope of the invention to any particular theory as far as the reason of the achieved results is concerned, there is a fair basis for the assumption that the results have been achieved through the fact that:

— One has achieved a comparatively low carbide stability, which allows a rapid dissolution of the carbides and a high hardness in the case of low austenitization temperatures;

- One has a low content of carbide forming elements and a carbon content adapted thereto, which should lie between 0.5 and 0.7 %, preferably between 0.55 and 0.65 %, so that a comparatively high carbon content will exist in solid solution after hardening; If the carbon content is too low, a sufficient hardness will not be achieved, and if the carbon content is too high, the steel will be too hard and consequently also brittle, at the same time as the hardness will strongly depend on the flame hardening temperature, which means that one will have a too narrow temperature range within which the desired hardness can be achieved;

- One has a comparately low content of strongly ferrite forming elements (Si and Mo) and a comparatively high content of the austenite stabilizing elements (Mn and/or Ni) , which together with in the first place carbon will press down the A.-line and there- through increase the austenite region as compared to the conven¬ tional steels Nos. 6 and 7 in Table 1. On the other hand the Si- content should be sufficiently high to give a good flowability during casting, however not higher than 1.0 %. The Mo content should not be higher than 0.8 % , while Ni + — should lie between 0.3 and 1.8 %. However, the steel,, as is conventional, should contain at

least 0, 5 % Mn. Also nitrogen is a strong austenite former but should not exist in amounts exceeding 0.1 % in order not to make the steel brittle.

Also steel No. 4 has good flame hardening features, but the hardness level is not as high as for steel No. 2. Steel No. 3 also has a good flame hardening characteristic so far that there is achieved a hard¬ ness exceeding 60 HRC already when hardening from a temperature between 950° a.1000°C. However, the hardness will be lower when hardening from higher temperatures, which is disadvantageous. As far as steel No. 3 is concerned, it is also a disadvantage that the through-hardening ability is impaired due in the first place to the lower amount of chromium. This to some extent also concerns steel No. 3. Steel No. 6, which corresponds to grade SIS 2260, is very hard but has poor flame hardening features, which is revealed by the graph in the drawing, and steel No. 7, which corresponds to the conventional steel grade UHB FERM0, has a low hardness but better flame hardening features, which is also apparent from the graph.