EP2617855B1 - Low alloyed steel and components produced therefrom - Google Patents

Description

The invention relates to a low alloy steel with excellent processability, scale resistance and components made therefrom.

In particular, it relates to steel for forming parts, which provides forming parts with good scale resistance, and components made therefrom.

Low alloy refers to steels in which no alloying element exceeds an average content of 5 percent by mass.

• An erster Stelle wird der Kohlenstoffgehalt in Massenprozent mal 100 angegeben, gefolgt von den chemischen Elementsymbolen der Legierungselemente in der Reihenfolge sinkender Massenanteile, und am Ende in der gleichen Reihenfolge, getrennt durch Bindestriche, die Massenanteil der zuvor aufgeführten Legierungselemente, die mit folgenden Faktoren multipliziert werden, um auf größere ganze Zahlen zu kommen:
  • × 1000: B
  • × 100: C, N, P, S, Ce
  • × 10: Al, Cu, Mo, Ti, V, Be, Ta, Zr, Nb, Pb
  • × 4: Cr, Co, Mn, Ni, Si, W
  Stähle mit besonders geringem Kohlenstoffgehalt wurden in jüngerer Zeit für Umformteile, insbesondere für Fahrzeuge, Maschinenbau, Großmotorenbau etc. usw. breit verwendet, da die Stähle eine exzellente Verarbeitbarkeit aufweisen. Rohlinge für das Schmieden werden gewöhnlich durch Dekarbonisieren von geschmolzenem Stahl, welcher durch einen Konverter usw. hergestellt wurde, erhalten, wobei bspw. ein Vakuumentgasungsverfahren, wie das RH-Verfahren, verwendet wird, um die Kohlenstoffkonzentration auf eine besonders geringe Kohlenstoffkonzentration herabzusetzen. Danach findet meist kontinuierliches Gießen statt. Für Umform-Anwendungen wurde häufig als niedrig legierter Stahl 42CrMo4 IM Stahl bzw. 43CrMo4 verwendet.

The designation of the steel alloys is based on the following rule:

• In the first place, the carbon content is given in mass percentage times 100, followed by the chemical element symbols of the alloying elements in the order of decreasing mass fractions, and at the end in the same order, separated by hyphens, the mass fraction of the alloying elements listed above multiplied by the following factors to get bigger integers:
  • × 1000: B
  • × 100: C, N, P, S, Ce
  • × 10: Al, Cu, Mo, Ti, V, Be, Ta, Zr, Nb, Pb
  • × 4: Cr, Co, Mn, Ni, Si, W
  Particularly low carbon steels have recently been widely used for forming parts, especially for vehicles, engineering, large engine construction, etc., because the steels have excellent processability. Blanks for forging are usually obtained by decarburizing molten steel produced by a converter, etc., using, for example, a vacuum degassing method such as the RH method to lower the carbon concentration to a particularly low carbon concentration. Thereafter, mostly continuous casting takes place. For forming applications, 42CrMo4 IM steel or 43CrMo4 steel was often used as the low alloy steel.

Silicon steel of various composition is from the GB 2173216A known. The EP1961832A1 and the US 5846344A reveal highly elastic steels with one Carbon content up to 0.6%; and Si up to 2.5% and possibly some B - but they use expensive special elements such as Mn, Nb and V to obtain the desired elasticity.

The well-known 42CrMoS4 IM steel has a composition of Chemical composition (% by weight) min Max C 0.38 0.45 Si 0.40 Mn 0.70 0.90 P 0,035 S 0,035 Cr 0.90 1.20 Not a word 0.15 0.30

• Einschlüsse sind weniger abrasiv, sie wirken wie ein Schmiermittel und Barriere an Werkzeug/Werkstück-Kontaktstellen. Verglichen mit der Standard-Klasse der IM-Stähle ergibt sich bereits eine
  • Verbesserte Zerspanbarkeit mit reduzierten Bearbeitungskosten
  • Bis zu 30% längere Standzeiten für eine bestimmte Schnittgeschwindigkeit
  • Bis zu 20% höhere Schnittgeschwindigkeiten für eine bestimmte Standzeit Die in der bekannten Legierung eingesetzten Legierungsbestandteile des Stahls haben u.a. folgende Wirkungen

42CrMo4 In steel, when hardened and tempered, it has a tensile strength of 900 to approx. 1200 MPa, a yield strength Re MPa of at least 650 MPa. Advantages of this steel are:

• Inclusions are less abrasive, they act as a lubricant and barrier to tool / workpiece contact points. Compared to the standard class of IM steels, there is already one
  • Improved machinability with reduced machining costs
  • Up to 30% longer life for a specific cutting speed
  • Up to 20% higher cutting speeds for a certain service life The alloy constituents of the steel used in the known alloy have, inter alia, the following effects

carbon

Carbon lowers the melting point and increases the hardness and tensile strength through the formation of Fe3C. In larger quantities it increased the brittleness and lowered the forging ability, weldability, elongation at break and notched impact strength. In particular, moldability is lowered when it is added in a large amount. Here, therefore, the addition must be low.

chrome

Chromium lowers the critical cooling rate, increases wear resistance, heat resistance, scale resistance. The tensile strength is increased because chromium acts as a carbide former. From 12.2% by weight it increases the corrosion resistance (stainless steel), acts as a ferrite stabilizer. Unfortunately, it reduces impact energy and weldability, lowers heat and electrical conductivity. With chromium additives the best results of hardening or hardening are achieved.

molybdenum

It improves hardenability, tensile strength and weldability. Unfortunately, it reduces ductility and forgeability. Molybdenum also increases hardenability and advantageously complements chromium. In addition, Mo improves the heat resistance as well as the tempering resistance, a property that is particularly important in tempering.

sulfur

Sulfur increases the machinability, but reduces the ductility and thus forgeability of the iron alloy.

The well-known steel 42CrMo4 is extensively used. With the properties described, the materials are suitable for high and highest dynamic and static stability. Their application results from the required strength and toughness values, but always the dimensioning of the components must be taken into account. The mechanical workability of these steels, especially in hot and / or cold forming processes is excellent and they are therefore used extensively in vehicle construction, mechanical engineering, Oroßmotorenbau etc. For certain applications, however, they are not sufficiently resistant to scaling (thermally highly stressed parts) and do not have sufficient strength for lightweight steel components

Due to stricter environmental legislation, especially in the US, the pressures and thus also the temperatures in the combustion chamber of the diesel engines had to be increased in order to reduce pollutants in the exhaust gas.

Under the newer, tightened conditions for Ferrothermkolben should be in the combustion chamber, the temperatures up to 500 ° C and on the inside of the piston rather slightly lower.

The required load increases the aluminum alloy used until then for cars is less and less grown. As a solution offered in this case a two-part solution consisting of a heavy-duty piston upper part and the piston skirt. As a standard material for the upper piston part, the material 42CrMo4 in tempered design is also frequently chosen. The strength of these components is between 870 and 1080 MPa. Also, the heat resistance, alternating load resistance, thermal shock resistance and oxidation resistance of this tempering steel are just sufficient for the present conditions.

Because of the scale strength which can be improved for the new applications and the high prices for these conventional steels, which are in particular due to the Mo addition, an attempt is made to create a steel with better mechanical properties.

• bis 400 °C: Einsatz unlegierter und Mangan-legierter Stähle möglich bis 550 °C: Einsatz Mo(-V) legierter Stähle
• bis 600 °C: Einsatz mit Cr hochlegierter, zunderbeständiger Stähle > 600 °C: Einsatz hochlegierter, austenitischer Cr-Ni-Stähle - hochlegierte Stähle sind allerdings teuer.

So far it was assumed that:

• up to 400 ° C: Use of unalloyed and manganese-alloyed steels possible up to 550 ° C: Use of Mo (-V) alloyed steels
• up to 600 ° C: Use with Cr high-alloyed, scale-resistant steels> 600 ° C: Use of high-alloy, austenitic Cr-Ni steels - however, high-alloyed steels are expensive.

As a tinder-resistant and high-temperature-resistant steel, therefore, only high-alloyed steels were used with correspondingly high costs for the alloying elements.

Accordingly, it is an object of the present invention to improve the scale resistance of low alloy steels for high thermal strength steel parts.

The object is achieved by steel with the features of claim 1 and components made therefrom according to claim 2 and 3

The invention accordingly relates to low-alloy steel with the composition of claim 1.

The steels according to the invention contain at least 92.00% by weight of iron, preferably at least 96.00% by weight of iron. It is favorable if impurities and unavoidable elements are present in concentrations of less than 0.10% by weight, preferably less than 0.05% by weight.

A typical application is for components, in particular machine components, with a tensile strength of> 950-1250 [MPa], a yield strength of> 700 to about 770 [MPa]; an elongation at break> 10% and a scale resistance of about 600 ° C to about 650 ° C and more. Typical such components are engine components selected from the group consisting of pistons, also for internal combustion engines, crankshafts, connecting rods, steering parts, valve parts, conveyor belt parts, in particular for hot parts; but also power plant components; Fasteners for heat resistant areas, steam turbine parts, combustor parts for gas or oil burners; Exhaust systems and their parts. The properties of steels of the invention over known steels are 42CrMo4 42TBSi Tensile strength (Mpa) > 900 - 1100 1000-1200 Yield strength (Mpa) > 650 > 750 elongation > 12% > 10% scaling resistance up to 550 ° C up to approx. 650 ° C heat treatment QT QT workability: Good Good Reibschweißverhalten: Good analysis DIN EN 10083 DIN EN 10083 plus

Due to the addition of Si, the costs for the steel according to the invention are approximately equal to those of 42CrMo4, but at the same time a considerable increase in the scale strength by 100-150 ° C. and more and yield strength occurs. The yield strength increases by about 100 MPa for the steels according to the invention, accompanied by a slight reduction in the elongation at break. The processing does not change and can be carried out with the usual tools and procedures.

A typical steel according to the invention has a composition of: Chemical composition (% by weight) min Max C 0.38 0.45 Cr 0.9 1.2 Not a word 0 0.3 Mn 0.6 0.9 Ti 0.04 0.04 Si 4 4 B 0.005 0.005

Typical representatives of this group are steels such as 42TBSi The newly introduced alloy components have the following effects:

silicon

It increases the scale resistance, is a mixed crystal hardener and hinders the carbide formation. It makes the melt thinner in steel production and also acts as a reducing agent. Finally, it increases the tensile strength, the yield strength and the scale resistance and has a ferrite-stabilizing effect. Too much addition reduces the formability of the alloy.

titanium

Titanium prevents intergranular corrosion due to TiC formation in iron alloys. The strong nitride former (titanium) serves u.a. to protect boron by reaction with nitrogen. For example, when nitrogen is fixed with titanium, satisfactory hardenability occurs in the temperature range up to 1000 ° C when the steel contains about 5-20 ppm of boron. Ti is used to deoxidize the steel and to fix C and N as TiC and TiN, respectively. The Ti content must therefore be at least 0.02%. However, since the effect of adding Ti is saturated when the Ti content is over 0.08%, the upper limit of the Ti content is defined as 0.08%.

boron

Boron increases the yield strength and strength of the steel, even when added in the smallest amounts. It also acts as a neutron absorber and makes the steel suitable for nuclear power plant and similar applications. The addition of boron in an amount of up to 0.01% on austenitic steels also improves their high temperature resistance. Boron steels are high quality cold formed steels. The basic effect of boron in steel is shown in the improvement of hardenability, which is already evident at a very low concentration of 0.0010% boron. Even in the small amount up to 100 ppm, boron increases the hardenability more than other, more expensive elements that have to be used in much larger quantities

An outstanding feature of boron steels is the improvement of hardenability through the addition of even minute amounts of boron between 3 and 15 ppm.

The amount of boron is critical because an excessive amount of boron (> 30 ppm) can reduce toughness, lead to embrittlement and heat brittleness. The effect of boron on hardenability also depends on the amount of carbon in the steel, with the effect of boron increasing in inverse proportion to the percentage of carbon present.

Boron may also be ineffective if its condition is altered by improper heat treatment. For example, a high austenitizing temperature must be avoided as well as temperature ranges in which certain boron precipitates occur.

Generally, the hardenability of steel is largely due to the behavior of oxygen, carbon and nitrogen in steel. Boron reacts with oxygen to form bromide (B ₂ O ₃ ); with carbon to iron borcementite (Fe ₃ (GB)) and iron boron carbide (Fe ₂₃ (CB) ₆ ) and with nitrogen to boron nitride (BN). Boron loss can be by oxygen. The hardenability of boron steel is also closely related to austenitic conditions and usually falls by heating above 1000 ° C. Boron steels must also be tempered at a lower temperature than other alloyed steels of the same hardenability.

The steels according to the invention are used for many applications, such as wear-resistant material and as high-strength steel. Examples include punching tools, spades, knives, saw blades, safety supports in vehicles, etc.

Boron steels are indicated when the base material meets the mechanical requirements (toughness, wear resistance, etc.), but the hardenability is insufficient for the intended section size. Instead of requiring a higher alloyed and thus expensive steel, a user may employ appropriate amounts of boron, thereby providing suitable hardenability.

• Fig. 1: einen Schliff zweier Proben, die in einem Ofen jeweils 5 h 700°C mit einer geregelten Sauerstoffatmosphäre geglüht worden waren;
• Fig. 2: einen Schliff zweier Stahlproben, die in einem Ofen jeweils 5 h 750°C mit einer geregelten Sauerstoffatmosphäre geglüht worden waren; und
• Fig. 3 eine Auftragung der Kerbschlagzähigkeit, Zugfestigkeit, Einschnürung von Stahlproben gegen den Siliciumgehalt von verschiedenen 42CrMo4-Legierungen, die bei verschiedenen Temperaturen getempert worden waren.

Particular advantages of the inventive steels are good cold formability, extended tool life for tools made therefrom, improved weldability due to low carbon equivalents, lower tempering temperatures. This results in energy savings and good case hardening. The invention will be explained in more detail in detail with reference to the drawing and exemplary embodiments, to which, however, it should by no means be restricted. Showing:

• Fig. 1 : a finish of two samples, each annealed in an oven for 5 h at 700 ° C with a controlled oxygen atmosphere;
• Fig. 2 : a finish of two steel samples annealed in an oven for 5 hours at 750 ° C with a controlled oxygen atmosphere; and
• Fig. 3 a plot of notch impact strength, tensile strength, necking of steel specimens against the silicon content of various 42CrMo4 alloys annealed at different temperatures.

Embodiment 2

A 42TBSi cast steel billet is forged to a piston at 1150 ° C in a forging process. This piston thus prepared is used in a conventional manner as a combustion chamber for a gas engine. After a burning period of several months, no scaling of the steel surface of the bulb was found in the firing range / ignition range. By contrast, an identical piston made of 42CrMoS4 already showed clear signs of scaling after 70% of this time.

Embodiment 3

A forged steel billet of conventional 42CrMo4 (Sample 4) and a steel billet of steel according to the invention (42CrMo4 + 4% Si + 0.04 wt% Ti and 0.005 wt% B) (Sample 6) were transferred to an electric air circulation furnace and at 700 ° C annealed in the oven for 5 h. The controlled circulating air atmosphere with normal air in the oven always ensured the same amount of oxygen. Two more samples of conventional 42CrMo4 (Sample 4) and the steel of the invention (Sample 6) were annealed in the same furnace under the same conditions but at 750 ° C for 5 hours. The measured steel billets each came from cast, forged blocks, forged down to 45 mm diameter. You can clearly see from the Fig. 1 , the upper one of the 42CrMo4 steel after annealing at 700 ° C and ground down by the same conditions Annealed steel alloy according to the invention shows that the scale layer in the Si steel according to the invention is considerably thinner (8 micrometers compared to 30 micrometers) than in the conventional 42CrMo4 steel without addition of silicon - the Si steel material according to the invention thus scales up much slower / less.

Fig. 2 shows the same steel billets in a 5h annealing in the same convection oven at 750 ° C, the upper sample is the 42 CrMo4 steel and a compared to the treatment at 700 ° C thickened scale layer of max. 44 micrometers has formed, while the steel according to the invention a thin scale layer of max. 5 micrometers shows.

It can be seen from this that the silicon steel according to the invention is attacked oxidatively much less at elevated temperatures of oxygen than conventional low-alloyed CrMo4 steel. The steels of the invention thus achieve a scale strength that could previously be achieved only with expensive additives.

In Fig. 3 is a compilation of the properties of 42CrMo4 steels with silicon alloy up to about 4% depending on the silicon content and the temperature of the thermal aging graphically. The abscissa indicates the Si content in% by weight of a basic alloy 42CrMo4, while the left-hand ordinate indicates the tensile strength UTS in MPa. On the right ordinate the notched impact strength (KU) is indicated. There are curves for the constriction RofA (%) for the inventive Staht - shown once at low and once at high Si content. It can be seen that the constriction and the notched impact strength decrease as the tensile strength values increase. The notched impact strength decreases sharply from Si contents of more than 2.5% by weight. The properties also depend on the tempering temperature (low tempering / high tempering). The high tempering temperature at about 0.5 wt.% Si was 680 ° C while the low tempering temperature was 630 ° C. With an addition of about 2.5 Si, the high tempering temperature was 730 ° C and the low tempering temperature was 680 ° C. It can be clearly seen that with increasing Si content - depending on the tempering temperature - the tensile strength increases, while constriction and impact strength decrease. A higher tempering temperature deteriorates the impact value and constriction RoFa; whereas the RoFa constriction at low silicon content is higher for tempered steel than at low tempered steel - this RoFa ratio of Steel tempered with low temperature compared to RoFa of steel tempered with higher temperature reverses with increasing Si content; while the impact strength at higher silicon content becomes almost independent of the tempering temperature. The tensile strength increases with increasing tempering temperature and increasing Si content.

The invention thus also relates to machine components or components with a tensile strength of 1000 [MPa] and more for varying mechanical loads up to a temperature of 630 ° C, formed from a thermally tempered steel alloy according to the invention. In particular, the invention also relates to engine and / or drive components of vehicles.

Other machine components with alternating, mechanical and thermal stress are in modern technology increasingly higher, to the limits of the respective material resistance charged. This is especially true for engines because the weight reductions achieved thereby are also useful for fuel savings and the like. Of the materials from which these components are formed, high values for the toughness, strength, and ductility properties are required in the thermally tempered state, because these property values are of crucial importance to dimensional design of the parts. As evidenced by failure of parts in long-term operation, the properties of the material fatigue have to be taken into consideration in order to achieve a high level of operational safety.

For parts with significant, mechanical alternating load in the railway, automotive and aerospace industry, the inventive low-alloy tempered steels are now used advantageously. Use of steel alloys having a composition equal to that of tempered steels of the aforesaid kind has proven useful for producing highly stressed machine components, with their fatigue and thermal resistance properties sufficient for mechanical cycling in the limit of the materials used.
The description of the invention is merely exemplary and variations familiar to those skilled in the art are also within the scope of the invention as defined by the claims.

Claims (3)

1. steel, with the following alloying components:

   0.38 - 0.45 wt.% C;

   0,6 - 0,9 wt.% Mn

   max 0,25 wt.% P

   max. 0,035 wt.% S

   0,9 - 1,2 wt.% Cr

   0,15 - 0,30 wt.% Mo

   4 wt.% Si

   0,04 wt.% Ti

   0,005 wt.% B

   a remainder of iron.
2. structural component, especially machine component, made of a steel alloy according to claim 1, having a tensile strength of greater than about 1000 to about 2000 MPa, a yield strength of greater than about 700 to approximately 950 MPa; a break elongation of greater than about 17% and a scaling resistance of greater than about 650° C.
3. structural component according to claim 2, selected from a group consisting of pistons, crank shafts, connecting rods, steering parts, valve parts, conveyor parts, power plant components, replacement parts for heat-resistant areas, steam turbine parts, combustion chamber parts for gas and oil burners, and exhaust systems and their components.

Images