EP0939140B1 - Steel for use at high temperatures - Google Patents

Abstract

A chromium-molybdenum-vanadium steel has specified impurity content and accompanying elements for producing tools with isotropic properties. A ferrous metal alloy, of composition (by wt.) 0.25-0.79% C, 1.10-7.95% Cr, 0.56-3.49% Mo, 0.26-1.48% V and balance Fe, contains impurity and accompanying elements in the following quantities: (solid solution elements) 0.098-0.29% Mn and NOTGREATER 0.35 Ni+Co+Cu+W; (impurity elements) NOTGREATER 0.007% S+P and NOTGREATER 0.0014% O; (oxide-forming elements) 0.081-0.25% Si+Al and 0.00014-0.0009% Ca+Mg; (monocarbide-forming elements) NOTGREATER 0.008% Ti+Nb+Zr+Hf+Ta; (grain boundary active impurity elements) NOTGREATER 0.009% As+Bi+Sb+Sn+Zn+B; and (gases) NOTGREATER 0.008% N+H.

Description

Kohlenstoff

0,25 bis 0,79

Chrom

1.10 bis 7,95

Molybdän

0,56 bis 3,49

Vanadin

0,26 bis 1,48

Rest Eisen sowie die Verunreinigungs- und Begleitelemente unter anderen Mangan, Nickel, Kupfer, Wolfram, Schwefel, Phosphor, Sauerstoff, Silizium, Aluminium, Kalzium, Magnesium, Zinn, Antimon, Arsen sowie Stickstoff und Wasserstoff.The invention relates to a steel for use at elevated temperatures, for example hot working steel, containing in% by weight.

carbon

0.25 to 0.79

chrome

1.10 to 7.95

molybdenum

0.56 to 3.49

vanadium

0.26 to 1.48

Remainder iron as well as the impurities and accompanying elements including manganese, nickel, copper, tungsten, sulfur, phosphorus, oxygen, silicon, aluminum, calcium, magnesium, tin, antimony, arsenic as well as nitrogen and hydrogen.

Iron-based alloys, which are thermally quenched and tempered up to one Temperature in the range of 550 ° C and higher can be used, the so-called Hot work tool steels, mainly carbon, - chrome, - vanadium, molybdenum- and / or tungsten alloyed steels. After hardening and if necessary, repeated tempering of such materials are included Temperatures of 500 to 550 ° C hardness values of 51 to 55 HRC achievable, so that from this advantageously highly stressed hot work tools such as block sensors, Matrices and press domes for extrusion presses, die casting tools, Warm shear blades and the like can be produced. Such a steel is used in Breitler R., "Recent developments in the steels for pressure die casting ", North American Die Casting Association, Illinois, USA, 1991, pages 231 → 235.

Hot work tools are usually one in addition to a thermal one exposed to effective high mechanical stress at the same time, resulting in results in a particularly critical form of material stress. That often through elaborate machining tools should therefore be high Heat resistance and the like toughness and tempering resistance low Hot wear behavior and low sensitivity to hot cracks, high Resistance to temperature changes and the like thermal shock resistance have a high level of economy in its use or in the To achieve product manufacturing. Because of the mostly triaxial Stress is also an isotropy of the properties of the deformed Required.

It is known to improve the isotropy of mechanical properties the starting material, in particular the casting blocks of the alloy, one Submit diffusion annealing to increase the intensity of the micro segregations out. Remelting processes are also widely used for this purpose due to the solidification, particularly good prerequisites for one provide low micro-segregation block structure.

In order to further improve the material properties, attempts have also already been made Alloys with far lower than technically relevant impurities and To produce accompanying elements. However, this is a very effective measure expensive because the steel is made from the purest materials in particular delivered metallurgical vessels must be melted. For example discloses Bodnar, R., "Effects of Manganese, Silicon and Purity on the design of 3.5 NiCrMoV, ICrMoV, and 2.25 Cr-IMo Baintic alloy steels ", Met. Trans. A (1989), 20A (8), pages 1445-1460, a Cr-Mo-V Steel with low levels of impurities.

The invention is based on the object for an alloy of the beginning mentioned type the border areas of the pollution and accompanying elements specify within which a largely at elevated temperatures deformed material with isotropic and improved property profile can be created.

This object is achieved by a steel according to claim 1.

The advantages achieved by the invention essentially result from that even with possibly increased levels of individual impurities or Accompanying elements reduce them within a total concentration the content of other elements can be restricted by kinetics and therefore the material properties at high temperature are not disadvantageous influence, but improve overall, the range of the Material properties and their deformation-related differences in longitudinal and Transverse direction are significantly reduced. Thus, when using the alloy achieved economic advantages and in particular the Usage properties of the tool can be improved such that a Many times the service life is achieved even under rough operating conditions.

Extensive studies have shown that the material parameters and their unwanted differences in the workpiece or in the Direction of exposure of the same not exclusively from the content of the respective pollution and accompanying elements are dependent, but that Individual values and total values of the equally effective element groups have a decisive influence on the properties of the material. Within a Sum range or below one as maximum with regard to reaching sum limit recognized by desired material properties for several The respective concentration of the individual components in the elements can be advantageous Element group can be set. It is therefore no longer necessary, as in the Expert opinion was required, all unwanted or disadvantageous elements to have the lowest levels in steel, but rather what is possible is economically advantageous, sum values and / or sum maximum values for these to be observed in order to achieve the highest material quality.

Regarding the alloy according to the invention, it should be noted: Manganese is an element that can be stored in the mixed crystal and increases in this function the strength of the matrix or the material. Manganese also binds with others Elements to the sulfur of the steel for its mechanical properties less harmful sulfides. However, it was recognized that only in the area from 0.098 to 0.29% by weight of manganese in the material is advantageously effective. lower Levels of 0.098 do not ensure sulfur binding and soften if necessary, the mixed crystal of the alloy suddenly, but cause Manganese levels above 0.29% also increased signs of septicemia on the Grain boundaries and thus a loss in toughness of the material.

Ni and in particular Co, like Mn, solidify the mixed crystal and effect how the person skilled in the art is aware of an increased heat resistance of the material. Likewise, Cu and W in low contents act in the presence of stronger and / or more concentrated carbide formers. The However, studies have shown that the maximum total value of Ni + Co + Cu + W 0.28% by weight should not be exceeded because despite higher contents improved material strength markedly loss of toughness of the material Temperatures above 480 ° C and longer dwell times or with frequent heating enter.

The impurity elements P and S should naturally be low and one Do not exceed a total limit of 0.007% by weight in order to be high to achieve mechanical material properties. It was surprising that the Investigations of an interchangeability of these elements with regard to the Characteristics of the alloy showed, although only sulfur Manganese and, where appropriate, elements involved in inclusion such as Ca in particular are bound or participate in the sulfide formation.

Oxygen significantly reduces the mechanical properties of the material elevated temperature if its individual content is higher than 0.0014% by weight. It was found to be evident with respect to the standard enthalpies of formation of Oxides in combination with the one defined within the limits of the invention Composition of substances a maximum limit for an advantageously finely dispersed and coagulated excretion form of the non-metallic inclusions is present and that at higher oxygen values, limit enrichments as well Configuration phenomena occur that have an adverse effect.

In connection with the binding of oxygen, but also with the Inclusion morphology, which has a significant influence on the mechanical Has material values, the contents of oxide-forming elements can be seen, where the type of melting and solidification of the material or block are taken into account. To even when using vacuum melting technology dense, low-segregation blocks as the starting material for tool production it is necessary to obtain minimum Si + Al contents equal to 0.081% by weight to be observed because lower concentrations cause a carbon-oxygen reaction or can cause unfavorable boundary oxide formation. Although the Alloy metals Si and Al in higher proportions, as the expert knows, the Oxidation resistance and the hot crack resistance positively influence and cause high 0.2 proof stress values, it was surprisingly found that about a total limit of 0.25% by weight, the heat toughness in particular Al levels above 0.015 wt% deteriorate, with higher Mn levels are reinforcing.

Ca and Mg are extremely strong oxide and oxysulfide formers, but should not be used essential dimensions as intercalation elements in the mixed crystal due to their different atomic radii and reaction affinities exist, whereby as Total maximum content 0.0009% by weight was determined. On the other hand, however, is a Presence of these elements in the material for an oxygen and Sulfur setting important, so that their lower limit of 0.00014 wt .-% is set according to the invention because lower values mean a steep drop in some Material properties.

In the general view of the person skilled in the art, strong carbide formers, in particular the monocarbide-forming elements Ti, Nb, Zr, Hf, Ta in concentrations up to 0.1% with carbon contents up to 0.5% extremely effective grain refining agents, which, due to their finely dispersed carbide deposits, the isotropy of Promote materials in tempered condition. In the generic alloy is achieved by the monocarbide formers a refining effect that sharp-edged excretions, however, lead to tensile loads and elevated temperatures lead to crack initiation, which is striking at levels greater than than 0.008 wt% of Ti + Nb + Zr + Hf + Ta, making this value the represents the maximum limit according to the invention.

W, which can also form monocarbide in high concentrations, works however, also in small proportions in the mixed crystal should, as mentioned earlier, with the other storage elements Do not exceed the total limit of 0.35% by weight.

The cause of the property deterioration of the grain boundary Contamination metals are not yet scientifically researched for hot-work steel sufficiently clarified. However, it was found that a total As + Bi + Sb + Sn + Zn + B of greater than 0.009% by weight a disproportionate drop the mechanical values, in particular transverse to the direction of deformation, with temperature increases above 400 ° C, so that this total maximum value especially not with regard to the isotropy of the material according to the invention should be exceeded.

Although by vacuum treatment of the liquid steel its gas content As has been found, degassing technology must be used which reduce the concentration of N + H to values below 0.008% by weight reduced to an unfavorable influence of these gas fractions on the Material properties, especially at elevated temperatures, as well as a to avoid deteriorated hot formability of the material.

Ni+Co+Cu+W

= MAX 0,24

S+P

= MAX 0,004

O

= MAX 0,0008

Si+Al

= MAX 0,19

Ca+Mg

= MAX 0,0006

Ti+Nb+Zr+HF+Ta

= MAX 0,006

As+Bi+Sb+Sn+Zn+B

= MAX 0,005

N+H

= MAX 0,006

aufweist.The material properties, in particular their isotropy in the tool, can be further increased if the alloy values or total values for the following elements or element groups in% by weight

Ni + Co + Cu + W

= MAX 0.24

S + P

= MAX 0.004

O

= MAX 0.0008

Si + Al

= MAX 0.19

Ca + Mg

= MAX 0.0006

Ti + Nb + Zr + Ta + HF

= MAX 0.006

As + Bi + Sb + Sn + Zn + B

= MAX 0.005

N + H

= MAX 0.006

having.

Mn

= 0,125

Ni+Co+Cu+W

= 0,0029

S+P

= 0,00011

O

= 0,00004

Si+Al

= 0,019

Ca+Mg

= 0,000021

N+H

= 0,00011

aufweist, sind optimale mechanische Eigenschaftsmerkmale des Werkstoffes mit hoher Treffsicherheit erreichbar.If further, as can be advantageously provided, the alloy sum total minimum values for the following element groups in wt .-%

Mn

= 0.125

Ni + Co + Cu + W

= 0.0029

S + P

= 0.00011

O

= 0.00004

Si + Al

= 0.019

Ca + Mg

= 0.000021

N + H

= 0.00011

optimal mechanical properties of the material can be achieved with high accuracy.

It is particularly advantageous if the alloy is using ladle metallurgical Process and is made using the VAR process because of it an efficient control and limitation of the total maximum values as well as a Setting in total ranges of contamination and accompanying elements can be done.

In the following, the invention is compared using only one each Execution examples showing hot work steel alloys closer explained.

To the influence of the impurities and accompanying elements on the mechanical To clearly extract the values of hot-work steel at elevated temperatures, are only those materials of the extensive tests in Tables 1 to 5 cited, the approximately equal contents of basic elements, namely C, Cr, V and have extension Mo. However, this is not meant to be a limitation with regard to the chemical composition of the generic Represent iron-based alloy.

Table 1 shows the chemical composition of steels A to N with the contents of the basic elements as well as the individual and total values for the respective concentration of the contamination and accompanying elements.
Table 2 to Table 4 show the individual contents of impurities and accompanying elements found for steels A to N. Table 5 shows the measured mechanical values of the material at 5.43 times its deformation with absolute values and those in relation to steel A.

A high-strength steel was used as comparative alloy A in the tables according to DIN material number 1.2343 (property specifications in DIN 17350) selected and its determined material values for the heat toughness, the Heat wear resistance and machinability set at 100%.

The test melts were carried out using ladle metallurgical methods produced, deoxidized with different agents and cast into electrodes, after which by melting them, partly in a vacuum - Arc furnace, block production took place. These blocks were replaced by one Hot forging with a degree of deformation of 5.43 times to bar steel formed on which after hardening followed by two temperings chemical and mechanical material testing was carried out, the Results are summarized in the tables.

Alloys A to G, J to L and N were produced using a special technology, whereby a slag treatment with high Ca activity was carried out in the pan. Grain refining agents were also used for steels A to G and K to N. Impure scrap was, as it turned out disadvantageously in the following, used especially for alloys A, B and M. Steels H and I were manufactured with particular care with regard to the ingredients and alloys as well as the block melting. These materials according to the invention, according to H and I in Tables 1 to 4, listed in Table 5, provide significantly improved mechanical characteristics and thus an outstanding creep behavior of a tool made from them.

Claims (4)

1. A steel, in particular a hot-work steel, with a degree of deformation of more than 3.5 times for the manufacture of tools with largely isotropic properties, in particular largely isotropic mechanical material properties at temperatures up to 550 °C and an improved properties profile, containing in % by weight

   Carbon

   0.25 to 0.79

   Chromium

   1.10 to 7.95

   Molybdenum

   0.56 to 3.49

   Vanadium

   0.26 to 1.48

   and
   Elements which may be incorporated in the mixed crystal:

   Mn

   = MIN 0.098   MAX 0.29

   and

   Ni+Co+Cu+W

   = MAX 0.35

   Contaminating elements:

   S + P

   = MAX 0.007

   Oxygen:

   O

   = MAX 0.0014

   Oxide-forming elements:

   Si + Al

   = MIN 0.081   MAX 0.25

   and

   Ca+Mg

   = MIN 0.00014   MAX 0.0009

   Monocarbide-forming elements:

   Ti+Nb+Zr+Hf+Ta

   = MAX 0.008

   Grain-boundary-effective contaminating elements:

   As+Bi+Sb+Sn+Zn+B

   = MAX 0.009

   Gases:

   N+H

   = MAX 0.008

   Remainder iron.
2. A steel according to Claim 1, characterised in that the alloy exhibits total values for the following element groups in % by weight

   Mn

   = MAX 0.21

   Ni+Co+Cu+W

   = MAX 0.24

   S + P

   = MAX 0.004

   O

   = MAX 0.0008

   Si+Al

   = MAX 0.19

   Ca + Mg

   = MAX 0.0006

   Ti+Nb+Zr+Hf+Ta

   = MAX 0.006

   As+Bi+Sb+Sn+Zn+B

   = MAX 0.005

   N+ H

   = MAX 0.006

3. A steel according to Claim 1 or 2, characterised in that total minimum values for the following element groups are present in the alloy in % by weight

   Mn

   = 0.125

   Ni+Co+Cu+W

   = 0.0029

   S + P

   = 0.00011

   O

   = 0.00004

   Si+Al

   = 0.019

   Ca+Mg

   = 0.000021

   N+H

   = 0.00011

4. A steel according to one of Claims 1 to 3, characterised in that the alloy is produced by means of crucible-metallurgical methods and with the use of the VAR process.