EP1358359B1 - Steel and method for producing an intermediate product - Google Patents

Abstract

The present invention relates to a steel, in particular for tools exposed to corrosion, of the following composition (in mass-%): C: min. 0.02 and max. 0.12%; Si: max. 1.5%; Mn: more than 1.0-2.50%; P: max. 0.035%; S: min. 0.04% and less than 0.15%; Cr: more than 8.0% and less than 12%; Mo: more than 0.0% and max. 0.20%; V: more than 0.0% and max. 0.25%; Nb: more than 0.1% and max. 0.5%; N: at least 0.02% and max. 0.12%; Ni: max. 0.5%; B: max. 0.005%; Cu: max. 0.3%; Al: max. 0.035%; Sn: max. 0.035%; As: max. 0.02%; at least one of the elements Ca, Mg or Ce, wherein the sum of the contents of these elements is at least 0.0002% and max. 0.015%; with the remainder being iron and unavoidable impurities.

Description

For the production of tools that are practical Use corrosive media and are exposed to the at the same time, high demands are placed on their hardness become martensitic, corrosion-resistant Tool steels used.

Machining processes are an essential one Part of industrial production technology and a major cost bearer in the manufacture of tools for plastics processing. The economic Usability of steels of the type mentioned at the beginning therefore depends essentially on their machinability and their Corrosion resistance, which in turn is crucial is influenced by the chromium content of the steels. Under The term "machinability" is used in this context understood the property of a material to have certain conditions machined.

Special requirements for corrosion resistance of tools made from steels of the above mentioned type are produced in the area the plastics processing industry. So lead Contacts with the cooling and Detergents, the surrounding atmosphere and with the processed plastics themselves in many cases a corrosive stress on the respective tool.

A martensitic stainless steel with good Machinability is known from EP 0 721 513 B1. The known steel contains 10 to 14% by mass of chromium. to He also shows improvement in machinability at least 0.15% sulfur and 1.0 to 3.5% copper. The addition of copper also has a positive effect Influence on the hardness of the alloy.

In addition to that mentioned in the European patent specification described steel is a variety of chrome alloyed, corrosion-resistant steels known, their chromium content is between 11.0 and 17.0 mass%. They are for example those with material numbers 1.2080, 1.2082, 1.2083, 1.2085, 1.2201, 1.2314, 1.2316, 1.2319, 1.2361, 1.2376, 1.2378, 1.2379, 1.2380, 1.2436, 1.2601 in of the steels designated in the StahlEisen list. Are regular these steels with carbon, silicon and manganese alloyed. They also optionally contain carbide formers such as molybdenum, vanadium or tungsten.

The known steels are processed in Dependence on the respective carbon and Carbide. So the steels are the ones in question Kind on the one hand remunerated by the tool manufacturer Condition used with a hardness of 285 to 325 HB. at This hardness is a machining of the Material still possible. On the other hand, the steels in the soft annealed condition processed, the hardness of the Then steels is a maximum of 250 HB. So less hard Steels are easier to process. It must however, heat treatment after processing be performed to the usually required Achieve hardness of 46 to 60 HRC. Subsequently finishing is required.

Machining can be done by the The high final hardness required by the known Do not carry out steel economically anymore. This The problem is caused by the processing in the soft annealed condition with downstream Solved heat treatment. A disadvantage of the final However, heat treatment exists in addition to that for this additional operation costs incurred in that it thereby to crack formation and distortion of the component as a result the warming can come.

Another disadvantage of the well-known, in the steel-iron list steels is their due to the Carbon content and the alloy composition deteriorated weldability. A good But weldability is just in the area of Plastic processing indispensable. It is often in Result of subsequent changes in the design and necessary due to repairs becoming necessary, Carry out welding work on the tools.

The determination of one is made even more difficult Requirements in practice, especially when it comes to problems in plastics processing righteous steel in that such a steel not only corrosion-resistant, easy to machine and good weldable, but also be sufficiently tough must to the forces occurring in practical operation to be able to record. Otherwise there is a risk that the occurring high bending, torsion, pressure and Tensile forces also cause cracks.

It has been shown that the known steels all at the same time do not meet these requirements. So show good due to a higher sulfur content machinable steels have too little toughness while at due to an increase in the carbon content harder steels reduce the corrosion resistance is. JP-A-01 215 489 discloses a steel consisting of 0.05-0.17% C, 0.01-1.5% Si, 0.01-2% Mr, ≤ 0.025% P, ≤ 0.015% S, 8-12% Cr, ≤ 0.8% Ni, 0.5-3% Mo, 0.5-3% W, 0.1-0.2% Nb, ≤ 0.04% Al, 0.03-0.05% N, ≤ 0.010 and 0.0005-0.01 Ca, rest iron.

The object of the invention is a especially for the manufacture of tools for the plastics processing industry suitable steel find that with high hardness and corrosion resistance one that meets the practical requirements Toughness, machinability and weldability. In addition, a method for the production of Intermediate products from such a steel will be specified. Under the term "intermediate product" in this Context also long products, flat products or others Objects understood, which then another Processing to be fed.

C:

mindestens 0,02 und höchstens 0,12 %,

Si:

höchstens 1,5 %,

Mn:

mehr als 1,0 - 2,50 %,

P:

höchstens 0,035 %,

S:

mindestens 0,04 % und weniger als 0,15 %,

Cr:

mehr als 8,0 % und weniger als 12 %,

Mo:

mehr als 0,0 % und höchstens 0,20 %,

V:

mehr als 0,0 % und höchstens 0,25 %,

Nb:

mehr als 0,1 % und höchstens 0,5 %,

N:

mindestens 0,02 und höchstens 0,12 %,

Ni:

höchstens 0,5 %

B:

höchstens 0,005 %,

Cu:

höchstens 0,3 %,

Al:

höchstens 0,035 %,

Sn:

höchstens 0,035 %,

As:

höchstens 0,02 %,

mindestens eines der Elemente Ca, Mg oder Ce, wobei die Summe der Gehalte an diesen Elementen mehr als 0,0002 % und höchstens 0,015 % beträgt,
Rest Eisen und unvermeidbare Verunreinigungen.With regard to the material, this task is solved by a steel for tools which are particularly exposed to corrosion and which has the following composition (in mass%):

C:

at least 0.02 and at most 0.12%,

Si:

at most 1.5%,

Mn:

more than 1.0 - 2.50%,

P:

at most 0.035%,

S:

at least 0.04% and less than 0.15%,

Cr:

more than 8.0% and less than 12%,

Mo:

more than 0.0% and at most 0.20%,

V:

more than 0.0% and at most 0.25%,

Nb:

more than 0.1% and at most 0.5%,

N:

at least 0.02 and at most 0.12%,

Ni:

at most 0.5%

B:

at most 0.005%,

Cu:

at most 0.3%,

al:

at most 0.035%,

Sn:

at most 0.035%,

As:

at most 0.02%,

at least one of the elements Ca, Mg or Ce, the sum of the contents of these elements being more than 0.0002% and at most 0.015%,
Balance iron and unavoidable impurities.

The nioballoyed tool steel according to the invention has an optimized combination of machinability, hardness, Corrosion resistance, weldability and toughness on. It reaches hardness layers that are between 300 and 450 HB lie. Despite the relatively high sulfur content, it shows good toughness for steels of the generic type on the requirements that arise in practice enough.

To improve machinability, the invention Steels sulfur-alloyed, the proportion of which is less than 0.15 mass%. The steel preferably has at least 0.04 mass .-%, whereby a good Machinability can be guaranteed safely. Yet better machinability can be taken into account other conditions imposed on the composition can be achieved when steel according to the invention contains at least 0.07% by mass of sulfur.

Despite such a proportion of sulfur has steel according to the invention has good toughness. This is achieved in that the steel together with the Sulfur at least one of the elements calcium, manganese or contains cerium in quantities the total of which is more than 0.0002 however at most 0.015 mass%. These elements enable the formation of sulfides in the matrix of the Steel and thus improve its toughness. This can be achieved unerringly, for example, if the steel according to the invention 0.001 - 0.009 mass% Contains calcium.

By using low carbon levels of 0.12 mass% maximum and low nitrogen contents of at most 0.12% by mass and a niobium content of 0.11 up to 0.5% by mass in the steel according to the invention Hard phases are formed which lead to the hardness of 300 contribute up to 450 HB. At the same time, the hard phases in particularly fine and evenly distributed what a has a positive influence on the toughness properties.

These advantageous properties of alloying with niobium are particularly noticeable if the niobium content is set so that the hardness factor H _f for steel according to the invention fulfills the following condition: 0.047 <H f ≤ 0.095, where the hardness factor H _f according to the formula H f = 0.11% Nb / 7.14 calculated and the respective Nb content of the steel is designated with% Nb. In such a measurement of the niobium content, the carbon and nitrogen present are largely bound to hard phases by the niobium element, so that the chromium contained in the steel according to the invention with a content of less than 12% is fully available for the formation of corrosion-inhibiting passive layers. In this way, steel according to the invention has excellent corrosion resistance despite the relatively low chromium contents and high hardness.

In the case of steel according to the invention, the contents of such elements which could lead to crack formation in the weld seam are also reduced to a minimum. Optimal weldability of steel according to the invention can be ensured by the fact that according to the formula S f =% C + 5 ×% B + 2 ×% Cu + (% P +% S) / 2 + (% Mo +% Cr) / 4 +% Mn / 10 calculating welding factor S _f for steel according to the invention fulfills the following condition: S f <3.99, where with% C,% B,% Cu,% P,% S,% Mo,% Cr,% Mn the respective C-, B-, Cu-, P-, S-, Mo-, Cr-, Mn- Contents of the steel are labeled.

The toughness of the known tool steels mentioned at the beginning is determined by the carbon and carbide content as well as by the Amount of sulfur, distribution and morphology the sulfides negatively affected. Steel according to the invention contains a maximum of 0.12% carbon. That way its carbide content is also limited. In addition, with one steel according to the invention in that the contents in it grain-limiting elements is reduced to a minimum, toughness compared to other sulfur alloyed steels elevated.

It has been found that the elements with a grain boundary effect in steels of the type in question segregate at the grain boundaries during the solidification process and during hot forming and / or during heat treatment at certain temperatures. These segregations lead to a reduction in cohesion and thus preferentially form the source of cracks. By fulfilling the following condition for a steel according to the invention, the embrittlement factor KG _f , the negative influence of the elements which act on the grain boundaries and the associated risk of cracks can be minimized in a targeted manner: KG f <1.07, where the embrittlement factor KG _f according to the formula KG f = 2.97 ×% Cu + 3.2 × (% Sn +% As) + 0.55 ×% Al + 5.42 ×% P + 0.98% N calculated and designated% Cu,% Sn,% As,% Al,% P and% N the respective Cu, Sn, As, Al, P and N contents of the steel.

• Erschmelzen eines erfindungsgemäßen Stahls,
• Vergießen des Stahls zu einem Vormaterial, wie Blöcken, Brammen, Stranggußriegeln, Dünnbrammen oder gegossenem Band,
• Diffusionsglühen des Vormaterials bei einer 1200 - 1280 °C betragenden Temperatur,
• Warmumformen des geglühten Vormaterials zu dem Bauelement.

With regard to the method for producing an intermediate product for the production of components, in particular for the production of a tool subject to corrosion, from steel composed according to the invention, the above-mentioned object is achieved by performing at least the following production steps:

• Melting of a steel according to the invention,
• Casting the steel into a raw material, such as blocks, slabs, continuous cast bars, thin slabs or cast strip,
• Diffusion annealing of the primary material at a temperature of 1200 - 1280 ° C,
• Hot forming the annealed starting material into the component.

Due to the temperature range chosen according to the invention performed diffusion annealing of the primary material is a Compensation of the segregation-related segregations, so that an even distribution of the contained Alloying elements is achieved. In the subsequent The hot forming of the primary material to the intermediate product is the Structural structure and the material isotropy influenced. A improved structure of the structure and a higher isotropy of the Material can be achieved in that the Hot forming using a degree of deformation ϕ of is carried out at least 1.5.

In the context of the method according to the invention, the Hot forming as forging or, to manufacture larger ones Dimensions to be carried out as hot rolling. The Hot forming is preferred at temperatures of 850 ° C - 1100 ° C. In this temperature range the material used according to the invention has a low one Yield stress and high toughness, so that a optimal formability is given. The hot forming leaves thus quickly, inexpensively and with high output carry out.

The workpiece produced according to the invention is after Hot forming from the forming heat, preferably in air stored. When stored in air, the material becomes slow and completely from austenitic to martensitic Condition transferred. Such slow cooling will on the one hand, the desired hardness of the material of up to 450 HB set. On the other hand, heat and Conversion voltages largely avoided, so that none Delays or stress cracks in the finished intermediate product occur.

By an additional one, if necessary Heat treatment at temperatures from 850 ° C - 1050 ° C with subsequent controlled cooling on one Cooling medium such as air, oil, water or a polymer preferably tempering at temperatures between 400 ° C and 650 ° C follows, a hardness of the produced Intermediate are manufactured, which differ from the after Storage in air from the hardness present in the forming heat different. In particular, you can use this Heat treatment also lower hardness values up to one Achieve a lower limit of 300 HB.

Diag. 1

den Schneidenverschleiß im Bohrversuch aufgetragen über den Bohrweg,

Diag. 2

die für verschiedene Stähle ermittelte Schlagbiegearbeit aufgetragen über den Versprödungsfaktor KG_f.

The invention is explained in more detail below on the basis of exemplary embodiments. Show it:

Diag. 1

the cutting edge wear in the drilling test plotted over the drilling path,

Diag. 2

the impact bending work determined for various steels plotted against the embrittlement factor KG _f .

Table 1 compares the alloys of steels A, B, C according to the invention with the compositions of four comparative steels D, E, F, G lying outside the invention. Table 2 also shows the Brinell hardness values belonging to steels A to G as well as the hardness (H _f ), sweat (S _f ) and embrittlement factors (KG _f ).

To check the machinability of steels A - G were made on components made from these steels Drilling tests with uncoated twist drills from the High-speed steel with the material number 1.3343 carried out. For this purpose 24 mm deep holes were made into those with a hardness of 300 to 400 HB Steels drilled. The cutting speed was in each case 12 m / min and the feed 0.12 mm / rev.

After a total drilling distance of 200, 1200 and 2400 mm the wear on the twist drills Cutting edges measured. It turned out that the Steels A, B and C according to the invention despite their higher Hardness less wear on the cutting edges of the drill generate (Diag. 1). Your machinability is thus clearly compared to that of the conventional, outside the Invention steels D, E, F and G improved.

To determine the toughness of tool steels the impact bending test according to steel-iron test sheet 1314 carried out. This trial is used as a measure of that Toughness of a material used to smash unscarred samples necessary impact bending work determined. The samples used with the dimension 7 x 10 x 55 mm were from the direction of deformation of the checked steels A - G taken with a hardness from 300 to 400 HB.

The test was carried out at room temperature. Like the one in Diag. 2 summarized values for the impact bending work (mean values from 3 tested individual samples) show, with increasing embrittlement factor KG _f a significant decrease in the measured impact bending work can be determined. The steels A, B and C according to the invention have the desired high level of toughness with values well above 200 J, while for the steels D, E, F and G listed for comparison only values between 50 and 150 J could be measured with increasing embrittlement factor, their toughness was therefore significantly lower.

To determine the corrosion resistance of Table 1 to check the listed steels Immersion tests in a 0.5% aqueous Sodium chloride solution performed. After a dive From 1 hour, the samples were each half an hour air dried and then immersed again. To that was a total of nine diving and drying cycles Appearance of the formerly finely ground samples assessed.

After the experiments were over, the Steels A to C according to the invention practically none Rust attack on the surface of the samples, indicating adequate corrosion resistance suggesting. Steels D, E and G showed a strong attack by the Test solution so that most of the surface after the test cycles carried out had already been corroded. Only the comparative steel F was due to its high Chromium content and due to the lack of sulfur corrosion resistant. Because of the lack of sulfur in the composition, however, this steel F had the worst machinability of all examined Steels on.

The examples explained demonstrate that steel according to the invention on the one hand safely achieves the desired hardness of 300 HB to 450 HB and on the other hand is easy to machine. In contrast, this combination of properties is not achieved in the case of steels lying outside the invention, which do not meet the conditions to be observed for the hardness factor H _f according to the invention.

Comparable has been found in connection with the value to be observed for the weldability factor S _{f of} steels according to the invention. The comparative steels, the welding factor S _{f of which} are above the limit value provided according to the invention, have a significantly poorer welding behavior than the steels according to the invention. This is particularly evident in the occurrence of welding cracks, which can be avoided in the case of the steels not according to the invention, which require extensive preheating and post-treatment.

Erfindung Stahl Härte
[HB] S_f KG_f H_f A 395 3,28 0,76 0,0889 B 380 3,04 0,42 0,0646 C 370 3,54 0,69 0,0552 Vergleich Stahl Härte
[HB] S_f KG_f H_f D 330 5,12 2,95 0,1099 E 300 5,10 1,09 0,1099 F 315 4,99 0,98 0,1099 G 325 6,48 4,06 0,0005 Finally, the examples show that the inventive limitation of the contents of elements with grain boundary effects, such as Cu, Sn, As, Al, P and N, in the case of steels A, B, C keeps the respective embrittlement factor KG _f low and, as a result, one for steels good toughness has been achieved.

invention steel hardness
[HB] S _f KG _f H _f A 395 3.28 0.76 0.0889 B 380 3.04 0.42 0.0646 C 370 3.54 0.69 0.0552 comparison steel hardness
[HB] S _f KG _f H _f D 330 5.12 2.95 .1099 e 300 5.10 1.09 .1099 F 315 4.99 0.98 .1099 G 325 6.48 4.06 0.0005

Claims (15)

1. A steel, in particular for tools exposed to corrosion, of the following composition (in mass-%):

   C:

   min. 0.02 and max. 0.12 %;

   Si:

   max. 1.5 %;

   Mn:

   more than 1.0 - 2.50 %;

   P:

   max. 0.035 %;

   S:

   min. 0.04 % and less than 0.15 %;

   Cr:

   more than 8.0 % and less than 12 %;

   Mo:

   more than 0.0 % and max. 0.20 %;

   V:

   more than 0.0 % and max. 0.25 %;

   Nb:

   more than 0.1 % and max. 0.5 %;

   N:

   at least 0.02 % and max. 0.12 %;

   Ni:

   max. 0.5 %;

   B:

   max. 0.005 %;

   Cu:

   max. 0.3 %;

   Al:

   max. 0.035 %;

   Sn:

   max. 0.035 %;

   As:

   max. 0.02 %;

   at least one of the element Ca, Mg or Ce, wherein the sum of the contents of these elements is more than 0.0002 % and max. 0.015%;
   with the remainder being iron and unavoidable impurities.
2. The steel according to claim 1, characterised in that it contains 0.001 - 0.009 mass-% of Ca.
3. The steel according to one of claims 1 or 2, characterised in that its hardness factor Hf meets the following condition: 0.047 < Hf ≤ 0.095, wherein Hf = 0.11 - %Nb / 7.14 with %Nb designating the respective Nb content of the steel.
4. The steel according to one of the preceding claims, characterised in that its weld factor Sf meets the following condition: Sf < 3.99, wherein Sf = %C + 5x%B + 2x%Cu + (%P+%S)/2 + (%Mo+%Cr)/4 + %Mn/10 and wherein %C, %B, %Cu, %P, %S, %Mo, %Cr, %Mn designate the respective contents of C, B, Cu, P, S, Mo, Cr and Mn of the steel.
5. The steel according to one of the preceding claims, characterised in that its embrittlement factor KGf meets the following condition: KGf < 1.07;    wherein KGf = 2.97x%Cu + 3.2x(%Sn+%As) + 0.55x%Al + 5.42x%P + 0.98%N    with %Cu, %Sn, %As, %Al, %P and %N designating the respective contents of Cu, Sn, As, Al, P and N of the steel.
6. The steel according to any one of the preceding claims, characterised in that it comprises at least 0.05 mass-% of sulphur.
7. The steel according to any one of the preceding claims, characterised in that it comprises at least 0.07 mass-% of sulphur.
8. A method for producing an intermediate product for the production of components, in particular for the production of tools exposed to corrosion, made from a steel with a composition according to any one of claims 1 to 7, comprising the following steps:

   melting the steel;

   casting the steel to form a raw material such as ingots, slabs, continuous-cast bars, thin slabs or cast strip;

   diffusion annealing of the raw material at a temperature between 1200 and 1280 °C; and

   hot forming the annealed raw material to form the intermediate product.
9. The method according to claim 8, characterised in that hot forming is carried out by way of forging.
10. The method according to claim 8, characterised in that hot forming is carried out by way of hot rolling.
11. The method according to any one of claims 7 to 10, characterised in that following hot forming, the intermediate product is held where it is exposed to air.
12. The method according to any one of claims 7 to 11, characterised in that hot forming takes place at temperatures between 850 °C and 1150 °C.
13. The method according to any one of claims 7 to 12, characterised in that following hot forming, the intermediate product is heat treated at temperatures between 850 °C and 1050 °C and after heat treatment is subjected to controlled cooling with the use of a cooling medium such as air, oil, water or a polymer.
14. The method according to claim 13, characterised in that after cooling, tempering at temperatures between 400 °C and 650 °C is carried out.
15. The use of a steel with a composition according to any one of claims 1 to 6 for the production of tools for plastics processing.

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