EP3347500B1 - Steel, hot-rolled flat steel produkt, and manufacturing method of the product - Google Patents

Description

The invention relates to a steel suitable for producing a graphite-free hot-rolled steel flat product, a hot-rolled steel flat product with a graphite-free structure in the uncured state, the hot-rolled steel flat product in the hardened state, and a method for producing such hot-rolled steel flat products.

Steels of this type and the resulting hot-rolled flat steel products are required for the manufacture of hardened machine components that are used, for example, in agriculture, forestry and timber, and which are required for their intended use to have excellent fatigue strength, optimized hardenability and a high level of resistance to crack corrosion.

When we speak of "flat steel products", we mean rolled products such as rolled strips and sheets as well as blanks and blanks obtained therefrom, the width and length of which are in each case substantially greater than their thickness.

If in the present text information is given in "%" or "ppm" in connection with alloy information, this information always refers to the weight, unless expressly stated otherwise.

In the agricultural, forestry and timber industries, the service life of the wear parts used increases as a result of ever increasing ones Performance and availability requirements are becoming increasingly important. Downtimes due to change and maintenance should be as short as possible to ensure maximum availability of the usually expensive machines.

A particular stress profile arises, for example, in agricultural or forestry knives during harvesting from the fact that the knives are basically used, for example, for cutting comparatively soft clippings such as grass, but also for relatively hard parts, such as stones, roots and the like, during the cutting process , can meet. If the knife material is hard and brittle, there is a risk of the knife breaking.

The same problems arise with knives that are used in cutters for cutting, chopping or chopping grass, straw, cereals, branches and other plants and parts of plants. These include, in particular, cutting knives for rotary mowers, rotor knives, fodder mixing wagon knives, straw chopper knives or underfloor chopper knives, as well as the parts of plows or harrows that penetrate into the ground during field work in order to shred or circulate the soil.

For example, chopper knives for straw are between 3 mm and 5 mm thick in practice. They can be either fully coated or only locally edge-zone coated. Other knives are at least 1.3 mm thick, other wear parts such as the components used in harrows reach thicknesses of up to 20 mm.

The high abrasive wear to which the components made from steels of the type in question are exposed mainly results from the shock and friction loads that occur during use. Hard objects that come into contact with the wear-resistant steel parts continuously remove surface segments.

The load spectrum described above requires a high hardness of the steel from which the components in question are made. However, sufficient toughness must often be set to withstand the dynamic loads that occur during operation. In addition, there are high demands on the service life, which can be checked by fatigue tests.

Because corrosive attacks of the material can occur during use - such as contact with acidic or basic media (floors or cut materials) - sensitivity to hydrogen-induced failure also plays a decisive role. The tendency towards hydrogen-induced crack formation can be assessed on the basis of the diffusion coefficient to be determined for the steel.

Finally, a material of the type in question here must be suitable for complex shaping. For example, the steel material must be separated from the respective hot-rolled flat steel product forming the respective starting product by different separation processes and be able to be brought into the required form in a shaping process and also be remuneration-able. It should also be suitable for other processes that protect the surface or further increase hardness.

In the prior art, a wide variety of suggestions have been made for steels and flat steel products which are intended to meet these requirements.

In the DE 10 2010 050 499 B3 ( WO 2012/062281 A1 ), for example, the use of a hot-formed and press-hardened, wear-resistant steel component with a hardness between 500 and 700 HB in construction machines, agricultural machines and mining machines has been presented. For the manufacture of such steel components, four steel alloys are proposed which, in addition to their respective main constituent iron, contain unavoidable impurities (in% by weight) steel 1: 0.2-0.4% C, 0.3-0.8% Si, 1 , 0 - 2.5% Mn, max. 0.02% P, max. 0.02% S, max. 0.05% AI, max. 2% Cu, 0.1 - 0.5% Cr, max. 2% Ni, 0.1 - 1% Mo, 0.001 - 0.01% B, 0.01 - 1% W, max. 0.05% N, steel 2: 0.35 - 0.55% C, 0.1 - 2.5% Si, 0.3 - 2.5% Mn, max. 0.05% P, max. 0.01% S, max. 0.08% AI, max. 0.5% Cu, 0.1 - 2.0% Cr, max. 3.0% Ni, max. 1.0% Mo, max. 2.0% Co, 0.001 - 0.005% B, 0.01 - 0.08% Nb, max. 0.4% V, max. 0.02% N, max. 0.2% Ti, steel 3: 0.40 - 0.44% C, 0.1 - 0.5% Si, 0.5 - 1.2% Mn, max. 0.02% P, max. 0.005% S, max. 0.05% AI, max. 0.2% Cu, 0.3 - 0.8% Cr, 1.0 - 2.5% Ni, 0.2 - 0.6% Mo, 0.5 - 2.0% Co, 0.0015 - 0.005% B, 0.02 - 0.05% Nb, max. 0.4% V, max. 0.015% N, 0.01-0.05% Ti and steel 4: 0.42-0.45% C, 0.30-0.40% Si, 0.80-0.90% Mn, max. 0.012% P, max. 0.001% S, 0.020 - 0.050% Al, max. 0.10% Cu, 0.50 - 0.60% Cr, 2.00 - 2.20% Ni, 0.45 - 0.59% Mo, 0.90 - 1.10% Co, 0.002 - 0.004% B, max. 0.008% N, 0.015 - 0.025% Ti, max. Contain 0.030% Sn. The selection of the four steels is specifically based on the fact that the steel components are manufactured with bending angles of more than 5 degrees. This should make it possible, in particular, to use complex geometries of wear-resistant steel components.

In addition to the prior art explained above, the JP 2000 328182 A a structural steel for machine parts is known which, in addition to iron and unavoidable impurities, contains the contents of the elements listed below (in% by weight): 0.10-0.65% C, 0.03-2.00% Si, 0.30 - 2.50% Mn, 0.015 - 0.35% S, 0.005 - 0.060% AI, 0.0005 - 0.0100% B and 0.005 - 0.020% N, with the proviso that 0.3 ≤ B / N ≤ 1.2, as well as 0.01 - 0.30 Pb and / or 0.01 - 0.30 Bi and optionally one or more of the following elements: Cr: 0.1 - 2.0%, Mo: 0.05 - 1.00 %, Ni: 0.1 - 3.5%, V: 0.01 - 0.50%, Ti: 0.01 - 0.40%, Nb: 0.01 - 0.30%, Ca: 0.001 - 0.01%. The steel is at 1200 ° C for 30 min. held, hot rolled or forged and cooled in air. Elements made of structural steel can be heated to 850 ° C, hardened and tempered at 600 ° C.

Against the background of the prior art, the task has arisen of a steel, a flat steel product and a method for the same To provide manufacturing that make it possible to manufacture products that have a combination of toughness and fatigue strength that is optimized for use in the fields of agriculture, forestry or comparable applications.

The invention has achieved this object by means of an alloyed steel according to claim 1, a hot-rolled flat steel product obtained according to claim 2 and the method designed according to claim 8.

Advantageous refinements of the invention are specified in the dependent claims and are explained in detail below, like the general inventive concept.

• mit %Ni: jeweiliger Ni-Gehalt des Stahls
• %Si: jeweiliger Si-Gehalt des Stahls
• %Mn: jeweiliger Mn-Gehalt des Stahls
• %S: jeweiliger S-Gehalt des Stahls
• %Al: jeweiliger Al-Gehalt des Stahls

N: 0,0015 - 0.010 % Ni: 0,04 - 2 % Ti: 0,005 - 0,1 % V: 0,0080 - 0,1 % B: 0,0005 - 0,004 % Ca: 0,0005 - 0,004 % P: ≤ 0,030 % S: ≤ 0,005 % Al: ≤ 0,05 % Cu: ≤ 0,1 % Nb: ≤ 0,004 % H: ≤ 0,0001 % O: ≤ 0,0050 % sowie optional Mo: 0,006 - 0,02 %,
und als Rest aus Eisen und unvermeidbaren Verunreinigungen.A steel according to the invention accordingly consists of (in% by weight C: 0.4 - 0.7%, Si: 0.15 - 0.5% Mn: 0.8 - 2.0% Cr: 0.3 - 1.0%, the Cr content% Cr fulfills the following condition in each case:

% Cr ≥ (% Ni +% Si +% Mn +% S +% AI) / 5

• with% Ni: the respective Ni content of the steel
• % Si: respective Si content of the steel
• % Mn: respective Mn content of the steel
• % S: respective S content of the steel
• % Al: respective Al content of the steel

N: 0.0015 - 0.010% Ni: 0.04 - 2% Ti: 0.005 - 0.1% V: 0.0080 - 0.1% B: 0.0005 - 0.004% Ca: 0.0005 - 0.004% P: ≤ 0.030% S: ≤ 0.005% Al: ≤ 0.05% Cu: ≤ 0.1% Nb: ≤ 0.004% H: ≤ 0.0001% O: ≤ 0.0050% and optionally Mo: 0.006 - 0.02%,
and the rest of iron and unavoidable impurities.

Carbon is present in the steel according to the invention in a content of 0.4-0.7% by weight and in this content acts to increase hardness and strength. The minimum content of 0.4% is required for a sufficient basic hardness. In order to avoid impairment of the formability due to an excessively high C content, the C content can be limited to 0.6% by weight, whereby an optimal rollability of the steel can be ensured if the C content is 0.55% by weight. -% does not exceed. The positive effect of the presence of carbon in the steel according to the invention can be used particularly safely if the minimum carbon content is 0.47% by weight.

Silicon increases the hardenability in the steel according to the invention and is therefore present in the steel according to the invention in contents of 0.15-0.5% by weight. At higher contents, scale that is difficult to remove can form on the hot strip. With increasing Si content, the hydrogen permeability is also reduced. If graphite is formed, silicon promotes graphite particle growth in particular. It shows a low solubility in cementite and destabilizes it. For this reason too high Si content above 0.5% should be avoided. However, a minimum content of 0.15% by weight should be added to the steel in order to be able to use the positive effect of Si in the steel according to the invention in a reliable manner. By the Si content is limited to a maximum of 0.3% by weight, a particularly simple removal of scale adhering to the hot strip can be accomplished by means of pickling which is customary for this purpose and carried out in a conventional manner.

Sulfur acts as an accompanying steel element in the steel according to the invention in several ways. The S content should therefore be kept as low as possible. A maximum of 0.005% by weight of S has proven to be favorable, the limitation to a maximum of 0.0005% by weight particularly reliably preventing the negative influences of the presence of S in the steel according to the invention. By limiting the S content to the limits specified according to the invention, a reduction in the notched impact strength which occurs as a result of the higher S contents can also be avoided and damage by hydrogen penetrating into the steel can be avoided. Furthermore, the sulfur content is also minimized in order to avoid the setting of the titanium content provided according to the invention into titanium carbosulfide.

In order to achieve the desired low S contents, a calcium treatment is also carried out in addition to a conventional desulfurization treatment when the steel according to the invention is produced. CaSi is added, as a result of which Ca contents of 0.0005-0.004% by weight of Ca are present in the steel according to the invention. The increase in Si content associated with the addition of CaSi must also be taken into account when measuring the Si content. Optimally, the Ca content is limited to 0.0015% by weight in order to particularly reliably avoid the formation of inclusions which could have a negative effect on toughness.

Small additions of phosphorus of up to 0.030% by weight can have a favorable effect on the corrosion and strength properties, but at higher P contents there is a risk of embrittlement. To do this To avoid safely, the P content of a steel according to the invention can be limited to at most 0.025% by weight. However, in order to be able to use the favorable influence of P, it may be expedient to provide a minimum P content of 0.001% by weight in the steel according to the invention.

Chromium increases the hardenability and strength in the steel according to the invention and is a carbide former. With increasing Cr content, however, the hydrogen permeability is reduced. Below 0.3% by weight of Cr, however, the increase in hardenability and strength would be too small, whereas if the Cr content was more than 1.0% by weight, the hydrogen permeability would be reduced too much. Cr also prevents the early appearance of graphite after long annealing.

The void holes caused by the hardening process due to graphite accumulation must be avoided in order to avoid hydrogen accumulation, which would ultimately lead to material failure. Such holes as traps for hydrogen reduce the permeability (diffusion) for hydrogen. In addition, the holes remaining in the structure after graphite accumulations have a negative effect on the fatigue strength. Chromium, on the other hand, suppresses the nucleation and growth of the graphite particles.

• mit %Ni: jeweiliger Ni-Gehalt des Stahls
• %Si: jeweiliger Si-Gehalt des Stahls
• %Mn: jeweiliger Mn-Gehalt des Stahls
• %S: jeweiliger S-Gehalt des Stahls
• %Al: jeweiliger Al-Gehalt des Stahls

Using this positive effect of Cr, a steel according to the invention is therefore designed in such a way that any formation of graphite in the structure is avoided. To achieve this, a Cr content of up to 1% by weight is possible in the steel according to the invention. However, it turns out that taking into account the positive influences of the other alloy elements provided according to the invention, Cr contents of up to 0.7% by weight are regularly sufficient for this purpose. An addition of chromium of at least 0.3% by weight can prevent undesirable graphite formation in steels with simultaneous contents of Ni, Si, Mn, S and Al. The Cr content must be set as an additional condition for its minimum value that it corresponds to at least one fifth of the sum of the contents of Ni, Si, Mn, S and Al, so that:

% Cr ≥ (% Ni +% Si +% Mn +% S +% Al) / 5

• with% Ni: the respective Ni content of the steel
• % Si: respective Si content of the steel
• % Mn: respective Mn content of the steel
• % S: respective S content of the steel
• % Al: respective Al content of the steel

This condition should be fulfilled because otherwise the graphite is not safely avoided and at the usual austenitizing temperatures of the hardening process it dissolves much more slowly than the cementite and thus much less carbon would be available to increase the martensite hardness.

Molybdenum can optionally be added to a steel according to the invention in order to improve its through-hardenability. If flat steel products with a thickness of up to 15 mm are produced from steel according to the invention, contents of at least 0.006% by weight are sufficient for this purpose. With larger thicknesses, however, no more than 0.02% by weight should also be added, since otherwise there is a risk of graphite formation in the structure.

In the steel according to the invention, nickel acts both to increase hardenability and, particularly at low temperatures, to improve toughness. A minimum content of 0.04% by weight is required for this. However, because the formation of graphite would be favored if the Ni contents were too high, an upper limit of the Ni content of 2% by weight must be observed. The positive effect of Ni can be used particularly safely while avoiding negative effects if the Ni content is limited to a maximum of 0.35% by weight.

The presence of manganese, in particular in combination with the presence of nickel, can widen the phase space of the austenite lead to low temperatures. This undesirable effect is avoided by limiting the Mn content to a maximum of 2% by weight, in particular a maximum of 1.8% by weight. A minimum content of 0.8% by weight, in particular 1.0% by weight, of Mn should, however, be observed in order to ensure reliable sulfur binding in the form of manganese sulfides and thus to avoid low-melting iron eutectics.

Boron already affects the conversion of the steel in contents of 0.0005 - 0.004% by weight. In order to be able to use this effect, the alloy should be adjusted in such a way that the boron does not bind to BN. In order for boron to fully develop its hardening effect, it must be dissolved in the steel, so it must not be bound with nitrogen.

In order to bind the nitrogen present in the steel according to the invention, the steel contains 0.005-0.1% by weight of titanium. Ti forms nitrides, carbonitrides and carbosulfides and thus binds the constituents nitrogen and sulfur, which are unfavorable with regard to the properties of the steel, and at the same time contributes to increasing the strength and hardness of the steel according to the invention. In addition, the presence of Ti makes it possible to control austenite grain growth. The Ti content should be a minimum of 0.005% by weight and a maximum of 0.1% by weight, with Ti contents of up to 0.04% by weight proving to be particularly effective in relation to the action of Ti used according to the invention to have. In order to be able to use the favorable influences of Ti particularly safely, a minimum content of 0.01% by weight can be provided. So that there is always a sufficient amount of Ti in the steel for setting the S content "% S" and N content "% N" in the steel, the respective Ti content "% Ti" can also be set according to the following condition (in% by weight):
% S * 1.5 +% N * 3.5 + 0.005% ≤% Ti ≤% S * 1.5 +% N * 3.5 + 0.010%

In the alloy according to the invention, niobium is provided only as an accompanying element which is to be attributed to the unavoidable impurities in amounts in which it has no effect. Therefore, the Nb content is limited to at most 0.004% by weight.

Like titanium, vanadium is a microalloying element that forms carbonitrides, nitrides and carbides, which increases strength and hardness and thus contributes to the wear resistance of steel. In order to use these effects, the minimum vanadium content in the steel according to the invention is 0.0080% by weight. With contents of more than 0.1% by weight, however, there is no further increase in the positive influence of V. Vanadium can be used particularly effectively in contents of up to 0.05% by weight in the steel according to the invention.

Nitrogen, especially in the unbound state, contributes to the fatigue strength of the steel according to the invention. For this purpose, a minimum content of 0.0015% by weight is provided in the steel according to the invention. However, too high a non-set nitrogen content would lead to premature aging and thus to less favorable processing behavior of the steel according to the invention. Therefore, the N content of the steel is limited to at most 0.010% by weight. Negative effects of the presence of nitrogen can be safely avoided by reducing the nitrogen content to a maximum of 0.0070% by weight, in particular a maximum of 0.0040% by weight. These amounts of nitrogen present in the steel according to the invention are sufficient to ensure, in the presence of V, the formation of an amount of nitrides, carbides and mixed forms of these precipitates which is sufficient to improve the wear resistance. Another positive effect of nitrogen in the steel according to the invention is that N stabilizes existing iron carbides and therefore hinders graphite formation in the structure. A minimum content of 15 ppm N in the steel according to the invention must also be maintained for this effect.

The hydrogen content in the hot strip must be limited to a maximum of 1 ppm to prevent breaks due to hydrogen. Non-metallic inclusions can become places for hydrogen accumulation, reducing hydrogen permeability. This is accompanied by the risk of hydrogen-induced cracks. To prevent this, the oxygen content of the steel according to the invention is limited to a maximum of 0.0050% by weight, in particular a maximum of 0.0030% by weight.

The presence of large amounts of aluminum would favor the occurrence of graphite when the steel according to the invention solidifies. Larger graphite accumulations hardly dissolve during hardening and, as mentioned, can even leave holes in the structure where hydrogen could in turn accumulate. In order to prevent these negative effects, the Al content of the steel according to the invention is limited to a maximum of 0.05% by weight.

Copper is also only present in the steel according to the invention as an accompanying element, which has no positive effect on the desired properties of the steel. Rather, excessive Cu contents should be avoided, since Cu forms low-melting compounds in connection with sulfur and iron, which can cause surface defects and edge cracks during hot rolling. The Cu content of the steel according to the invention is therefore limited to a maximum of 0.1% by weight. The risk of edge cracking in particular can be excluded with particular certainty if the Cu content of the steel according to the invention is limited to at most 0.012% by weight.

Compared to previously used low-alloy Cr steels, the alloy concept according to the invention based on the combination of Mn, Ti, B and V contents in combination with a higher C content and the additional addition of nickel has proven to be successful in the production of hot rolled flat steel products, in particular Hot wide strip. This not only achieved optimized mechanical properties that led to a significantly increased fatigue strength, but also minimized susceptibility to corrosion. Compared to the steel concepts based on high-alloyed stainless steels, the invention enables a much more economical production due to the resource-saving, precisely dimensioned alloy of nickel and chromium.

A flat steel product according to the invention is characterized in that it consists of a steel according to the invention and in the uncured state has a pearlitic, graphite-free structure of at least 80% by volume. The absence of graphite makes the steel particularly suitable for hardening and minimizes the risk of hydrogen being embedded in the structure of the steel, which would favor the formation of cracks and reduce the fatigue strength of the steel.

The steel flat product produced and assembled in accordance with the invention has mechanical properties and wear resistance, which, in combination with optimized elongation properties, make it particularly suitable for use with impact loads, such as occur wherever appropriate equipment is used to move earth or stone or to process vegetation be made. Examples of this are agriculture or forestry, but also mining, construction, especially civil engineering, and the like. For example, highly wear-resistant and durable components of plows, harrows and knives or cutting members and the like can be produced from flat steel products according to the invention, which are required in agricultural implements and machines for tillage or harvesting.

In the hot-rolled, ready-to-use, unhardened state, flat steel products according to the invention, measured transversely to the rolling direction, have a yield strength ReH of 450-650 MPa and a tensile strength Rm of 750-950 MPa and have a uniform elongation Ag of 5 - 15% and an elongation A80 of 15 - 30%.

The graphite-free structure of flat steel products according to the invention in the unhardened state, in addition to the at least 80% by volume pearlite, consists in total of at most 10% by volume bainite or martensite and the rest consists of ferrite. The proportions of bainite and martensite in the structure of the steel according to the invention are to be kept as low as possible. In this way, a temperature change which is narrow in terms of temperature is achieved during the subsequent hardening and thus a homogeneous hardening structure, so that the highest possible pearlite contents of preferably at least 90% by volume are achieved. This in turn causes a homogeneous initial state, which leads to a correspondingly homogeneous hardening and tempering structure.

After hardening, flat steel products according to the invention or components produced therefrom have a structure which consists of at least 99% by volume of martensite and the remainder of residual austenite.

Steel flat products according to the invention obtained in this way achieve a hardness of 540-600 HV1 in the hardened state.

The notched bar impact test according to DIN EN ISO 148-1, January 2011 edition, at 25 ° C on an ISO-V standard specimen, of notched steel products according to the invention or components made therefrom and subsequently hardened, measured in the hardened state, measured at right angles to the rolling direction at room temperature 8 J / cm ² . The special wear resistance of flat steel products according to the invention or components made from them is expressed in that their fatigue strength Pü50, determined according to DIN 50100, is 220-400 MPa after hardening.

It is characteristic of the low tendency of flat steel products according to the invention to absorb hydrogen that the hydrogen diffusion coefficient determined according to DIN ISO 17081, June 2014 edition, typically 1.5 * 10 ^-7 cm ² / s to 9 * 10 ^-7 cm ² / s, is in particular 4.2 * 10 ^-7 cm ² / s to 5.2 * 10 ^-7 cm ² / s. A certain combination of precipitations and martensite structure and the avoidance of significant holes in the structure are responsible for the susceptibility to corrosion of a steel flat product made of steel according to the invention or of a component produced therefrom in the hardened state.

In the case of extremely abrasive loading of flat steel products according to the invention or components produced therefrom, there is a risk in practical use that the flat steel product or the component in question heats up to such an extent that austenitization takes place. The cooling then takes place at such a high speed that particularly hard martensite is formed. With regard to a good service life of the flat steel products or steel components obtained according to the invention, it has proven particularly advantageous here if the hydrogen diffusion coefficient of the steel is 4.2 * 10 ^-7 to 5.2 * 10 ^-7 cm ² / s.

1. a) Herstellen einer erfindungsgemäß legierten Stahlschmelze, wobei die Stahlherstellung eine Entstickung der Schmelze auf Werte unterhalb 100 ppm N und eine sekundärmetallurgische Ca-Behandlung umfasst;
2. b) Vergießen der Stahlschmelze zu einem Vorprodukt, wobei es sich bei dem Vorprodukt um eine Bramme, eine Dünnbramme oder ein gegossenes Band handelt kann;
3. c) Erwärmen des Vorprodukts auf eine 1150 - 1300 °C betragende Vorwärmtemperatur, wobei diese Erwärmung auch in einem Halten auf der jeweiligen Vorwärmtemperatur bestehen kann, wenn das Vorprodukt in ausreichend warmen Zustand in diesen Arbeitsschritt gelangt;
4. d) Warmwalzen des Vorprodukts zu einem Warmband, wobei die Warmwalzanfangstemperatur 900 - 1150 °C und die Warmwalzendtemperatur 780 - 880 °C beträgt;
5. e) Abkühlen des erhaltenen Warmbands auf eine 550 - 680 °C betragende Haspeltemperatur mit einer Abkühlgeschwindigkeit von 2 - 50 °C/s;
6. f) Haspeln des Warmbands bei der Haspeltemperatur zu einem fest gewickelten Coil;
7. g) Abkühlen des Coils mit einer mittleren Abkühlgeschwindigkeit, die im Coilkern 4,5 - 14,5 °C/h beträgt auf eine höchstens 100 °C betragende Grenztemperatur;
   und
8. h) von der Grenztemperatur ausgehendes Abkühlen des Coils auf Raumtemperatur an ruhender Luft.

The method according to the invention for producing a flat steel product according to the invention comprises at least the following steps:

1. a) production of a steel melt alloyed according to the invention, the steel production comprising a denitrification of the melt to values below 100 ppm N and a secondary metallurgical Ca treatment;
2. b) pouring the molten steel into a preliminary product, which preliminary product can be a slab, a thin slab or a cast strip;
3. c) heating the pre-product to a preheating temperature of 1150-1300 ° C, which heating can also consist in holding at the respective preheating temperature, if that Pre-product reaches this step in a sufficiently warm condition;
4. d) hot rolling the preliminary product to a hot strip, the hot rolling starting temperature being 900-1150 ° C and the hot rolling end temperature 780-880 ° C;
5. e) cooling the hot strip obtained to a reel temperature of 550-680 ° C with a cooling rate of 2-50 ° C / s;
6. f) reeling the hot strip at the reel temperature into a tightly wound coil;
7. g) cooling the coil at an average cooling rate which is 4.5-14.5 ° C / h in the coil core to a maximum temperature of 100 ° C;
   and
8. h) cooling of the coil from the limit temperature to room temperature in still air.

The method according to the invention for producing a steel is designed in such a way that it reliably supplies flat steel products which are both in the hot-rolled uncured state, i.e. after cooling in the coil, as well as after the treatment steps completed during their processing, are free of graphite.

Steel production involves denitrification of the melt to values below 100 ppm N and a secondary metallurgical Ca treatment to achieve minimal S contents.

The casting of the molten steel according to the invention can take place in the strand for the production of slabs or as near-final-size strip casting with subsequent hot rolling.

The respective preliminary product is hot rolled to a thickness of typically at least 1.2 mm and a maximum of 20 mm, whereby a minimum thickness of 4 mm should be observed when processing cast strip in order to avoid the unfavorable effects of coarser inclusions that occur due to the system.

The preheating temperature, i.e. the temperature to which the slabs are heated before the rolling process should not fall below 1150 ° C. in order to achieve as complete a resolution as possible of the precipitates of the microalloying elements formed in the previous casting process. At the same time, the preheating temperature should not be higher than 1300 ° C to avoid the formation of coarse austenite grain at the beginning of the rolling process.

Basically, the phase change in the Fe-C system takes place according to a stable system or according to the so-called metastable system. The end products are the carbon compound Fe3C or the pure carbon in the form of graphite. The alloying elements have different effects on this phase change process in the manner explained above.

A particularly slow cooling, that is to say with respect to the stable state diagram, must be particularly beneficial for the formation of graphite. The invention takes this into account when designing the temperature profile during hot strip production. Long dwell times at higher temperatures can have a direct or indirect effect on the formation of graphite via other mechanisms, for example, on the one hand favoring C diffusion at higher temperatures and on the other hand the stronger scaling of the strip contributes to the cooling of the hot strip in the coil due to the mutual contact lying windings is slowed down due to lower heat energy radiation. The weakening of the emission by the scale layer and the low thermal conductivity by the air layer have an impact. On top of that break up particularly thick layers of scale easily and can lead to surface defects. As a result, the temperature control at higher temperatures is also decisive in the pre-rolling stage for the later graphite formation.

In order not to get too thick a scale layer susceptible to tearing on the one hand, but on the other hand to keep the resulting layer sufficiently uniformly thin and adherent, a hot rolling start temperature of 900-1150 ° C in combination with a hot rolling end temperature of 780-880 is required in the process according to the invention ° C provided.

In contrast to steels with lower C contents, the rolling forces required for forming increase significantly in the case of steels with relatively high C contents, as are provided according to the invention, at hot rolling end temperatures of less than 780 ° C. To avoid this, the hot rolling end temperature in hot rolling according to the invention is at least 780 ° C. Above 880 ° C, however, an austenite grain that is too coarse and therefore less prone to transformation occurs. At higher hot rolling temperatures, there is also a risk that austenite transformation will result in hard phases during hot strip cooling, which would impair the toughness of the steel. Therefore, the hot rolling end temperature according to the invention should not exceed 880 ° C.

After hot rolling, the hot strip obtained is cooled to a coiling temperature which is 550-580 ° C. This cooling can be carried out in a conventional manner using a suitable medium at a cooling rate of typically 2-50 ° C./s.

The reel temperature is at least 550 ° C to prevent significant amounts of bainite or martensite from forming in the hot strip instead of the desired pearlite. If the reel temperature is too high of more than At 680 ° C, however, further austenite grain growth would be possible, which could have an unfavorable effect on the toughness of the hot strip and could also trigger cracks or breaks when it is subsequently unwound in the cooled state. The lower limit of the reel temperature can be raised to 600 ° C in order to achieve increased security against the occurrence of hard phases, particularly in the presence of segregations. If optimized toughness properties are to be guaranteed, the maximum reel temperature can be limited to 650 ° C.

The hot strip, which is tempered in the manner explained above and wound into a fixed coil in which the turns of the hot strip lie close together, is cooled in the coil to a target temperature which is at most 100 ° C. in the region of the coil core. The average cooling rate of this cooling in the coil core is 4.5-14.5 ° C / h and can be achieved in a manner known per se by means of water or air cooling.

When cooling in air, which is preferred according to the invention, the target temperature in the core area of the coil should be reached within 40-120 hours, cooling times of 40-60 hours having proven particularly advantageous. In contrast, cooling in the area of the free edges and surfaces of the coil can take place more quickly. Avoid quenching with cooling media of any kind.

In order to be able to reliably adhere to the cooling times specified according to the invention, the coil weight should in practice be a maximum of 38 t when cooling in air. Otherwise there is a risk that graphite will form in the microstructure as a result of cooling that takes place too slowly.

Of the hot-rolled flat steel products obtained in this way, typically in the form of steel plate or steel strip, the carbides can be individually or in the hood furnace after an optional heat treatment Stacks at annealing times of 10-100 hours in the temperature range of 660-740 ° C, in particular at temperatures of more than 660 ° C to less than 740 ° C, can be divided by a suitable process, such as laser cutting or punching, blanks or blanks, from which the components to be manufactured are then hot or cold formed in a manner known per se.

A hot-rolled flat steel product according to the invention can be cold-rolled in a conventional manner without graphite being formed, so that cold-formed flat steel products or components can also be hardened from the flat steel product according to the invention, and in the hardened state they have optimum wear resistance and also for the one provided here Have optimal mechanical properties.

• i) auf eine 830 - 950 °C betragende Austenitisierungstemperatur erwärmt,
• j) über 3 - 60 min bei der Austenitisierungstemperatur gehalten und
• k) anschließend mit einer Abkühlgeschwindigkeit von 40 - 250 °C/s abgeschreckt werden.

To maximize their hardness and wear resistance, the steel flat products according to the invention or the components formed therefrom can be subjected to a hardening process in which they

• i) heated to an austenitizing temperature of 830-950 ° C,
• j) held at the austenitizing temperature for 3 - 60 min and
• k) then quenched at a cooling rate of 40-250 ° C / s.

The austenitizing temperature in step i) should not be less than 830 ° C and the holding time in step j) should not be less than 3 minutes in order to achieve a sufficiently coarse austenite grain. In this way, the transformation inertia of large austenite grains prevents the transformation of the austenite into softer phase components such as ferrite or bainite and promotes the transformation into the martensite, so that after hardening in the respective Flat steel product or component has a structure consisting of at least 99% martensite.

However, the austenitizing temperature should not exceed 950 ° C and the holding time 60 min in order to avoid irreversible structural damage from overheating.

After the austenitizing, the flat steel product or the component formed therefrom is quenched at a cooling rate of 40-250 ° C./s in a manner known per se, for example by means of water or oil cooling. The cooling rate is at least 40 ° C / s to ensure a complete transformation of the austenite into martensite. The cooling rate is also limited to 250 ° C / s to avoid hard cracks.

The hardening carried out in this way can be followed by a tempering treatment in which the flat steel product or the component produced therefrom is kept at a tempering temperature of 150-350 ° C., in particular 150-300 ° C., for a period of 0.2-2 hours. to improve toughness. The respective flat steel product or the component formed from it can then be cooled in air to room temperature.

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

The steel melts S1-S6 were melted with the compositions given in Table 1 and cast into slabs.

Slabs consisting of steels S1, S2, S3, S5 and S6 have been heated to a preheating temperature T1 and then, starting from a hot rolling start temperature T2 and a hot rolling end temperature T3, were conventionally hot-rolled into hot strips W1 - W8 with a thickness of 2-6 mm.

After hot rolling, the hot strips were cooled to a reel temperature T4 at a cooling rate CR1 and were wound at this reel temperature T4 to form a coil, in which the turns were close to one another.

The hot strips W1-W8 were then cooled to a target temperature of 100 ° C. over a cooling time t1 in the coil, the cooling taking place in the core area of the coil at a cooling rate CR2. After cooling in the coil, the mixture was cooled to room temperature in air.

The pearlite content P, the ferrite content F, the sum of the bainite and martensite contents B + M, the graphite content G and the residual austenite content RA have been determined for the structure of the hot strip samples W1-W5 thus obtained. The yield strength ReH, the tensile strength Rm, the uniform elongation Ag and the elongation A80 have also been determined for the hot strip samples W1 - W5.

The parameters "preheating temperature T1", "hot rolling start temperature T2", "hot rolling end temperature T3", "cooling speed CR1", "coiling temperature T4", "cooling time t1" and "cooling speed CR2" set in the production of hot strip samples W1-W8 are given in Table 2 . Likewise in Table 2 for the hot strip samples W1-W5 are the "pearlite fraction P", the "ferrite fraction F", the "sum of the bainite and martensite fractions B + M", the "graphite fraction G" and the "residual austenite fraction RA", the " Yield strength ReH ", the" tensile strength Rm ", the" uniform elongation Ag "and the" elongation A80 ".

The hot strip samples W1 - W5, W7, W8 each additionally went through a hardening process, in which they were first heated to an austenitizing temperature T6, held there for an austenitizing period t2 and quenched at the cooling rate CR3 after the end of the austenitizing period t2.

The quenched samples W1-W5, W7, W8 then underwent a tempering treatment in which they were heated to a tempering temperature T7 and held there for a tempering period t3. After the end of the tempering period t3, the samples W1-W5, W7, W8 were cooled in air to room temperature.

The martensite content M, the graphite content G and the residual austenite content RA have been determined for the structure of the samples H1 - H5, H7, H8 hardened and tempered in this way. Likewise, the notched impact strength KBZ was determined in an impact test according to DIN EN ISO 148-1, January 2011 edition, at 25 ° C on an ISO-V standard sample and, according to DIN ISO 17081, June 2014 edition, the hydrogen diffusion coefficient k.

The relevant parameters "austenitizing temperature T6", "austenitizing time t2", "cooling rate CR3", "tempering temperature T7" and "tempering time t3", the "martensite content M", the "graphite content G", the "residual austenite content RA", the "notched impact strength KBZ "and the" hydrogen diffusion coefficient k ", insofar as they have been determined, are listed in Table 3.

Although the three steels S1 to S3 were produced from the hot strip production with almost identical production parameters except for the coiling temperature, they still have different structures depending on their chemical composition and the resulting different austenite transformation.

The metallographic assessment of the phase fractions of pearlite, ferrite, graphite and residual austenite (RA) was carried out light-optically on cuts at 1000x magnification, and the residual austenite fraction was additionally checked by X-ray.

Due to the different phase proportions of ferrite due to different reel temperatures, the results were otherwise the same chemical composition with higher ferrite content lower strength values and therefore higher elongation values. The mechanical properties Rm, ReH, Ag and A80 were determined in the tensile test according to DIN EN ISO 6892-1 with sample form 1.

The hot strip sample W5 already contains the undesired phase graphite in the unhardened structure with a proportion of 1.6% by volume. After a further possible cold rolling along with previous optional and subsequent optional annealing in the coil, an increase in the amount of graphite must be expected here.

In contrast, in the hot strip samples W1 and W2 consisting of the steel S1 according to the invention, no graphite is present in the structure. The chemical composition and its overall effect on austenite conversion are largely responsible for this.

For example, in the case of the hot strip specimen W5 which does not consist of steel S3 and which is not according to the invention, the alloy is selected such that graphite is formed in the structure and accordingly there is a risk of holes forming in the steel. As a result, hardened components which are produced from the hot strip specimen W5 do not meet the requirements placed on a hardened flat steel product according to the invention or a component produced therefrom.

After hot rolling, the hot strip sample W4 has higher ferrite fractions than the sample W1. As a result, the hardened sample H4 produced from this hot strip sample W4 does not achieve the total hardness required according to the invention. The reason for the comparably high proportion of ferrite is the low Ni content.

Surprisingly, it was found that the hardened sample H1 made of steel S1 not only has a higher toughness, but also has a particularly favorable fatigue strength, which is a measure of the service life of components made from this steel, which are used, for example, in agricultural equipment.

For example, the hardened samples H7 and H3 not according to the invention achieve endurance strengths Pü determined according to DIN 50100 of only a maximum of 183 MPa, whereas the sample H1 according to the invention with 245 MPa has a significantly better endurance strength value Pü in combination with optimal toughness. In contrast, the sample H8 not according to the invention has a high hardness, but not a good notched impact strength.

The wear due to impact loads has been tested by the notched impact tests mentioned above at 25 ° C on an ISO-V standard sample. The result is more than twice the value of the higher carbon steel of sample H8 and the lower carbon steel of sample H7 in sample H1.

For the variant H1 according to the invention, a fatigue strength Pü50 of 245 MPa was determined in accordance with DIN 50100, while the sample H7 not according to the invention achieved only Pü values of less than 190 MPa.

Tabelle 2 Probe Nr. Stahl T1 T2 T3 CR1 T4 CR2 t1 P F B+M G RA Reh Rm Ag A80 °C °C/s °C °C/h H Vol.-% MPa % W1 S1 1200 1100 840 40 654 12 52 80 20 0 0 0 450 750 15 30 W2 S1 1200 1100 840 40 646 12 52 90 10 0 0 0 650 950 5 15 W3 S2 1200 1100 840 40 645 12 52 90 10 0 0 0 250 550 15 30 W4 S2 1200 1100 840 40 654 12 52 70 30 0 0 0 380 740 8 15 W5 S3 1200 1100 840 40 650 12 52 48 50 0 1,6 0 340 505 14 29 W6 S4 1200 1100 840 40 650 12 52 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. W7 S5 1200 1100 840 40 650 12 52 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. W8 S6 1200 1100 840 40 650 12 52 n.a. n.a. n.a. n.a. n.a. n.a. n_.a. n.a. n.a. "n.a." = Nicht ausgewertet Tabelle 3 Probe Warm band Stahl T6 t2 CR3 T7 t3 M G RA HV1 Pü50 KBZ K °C min. K/s °C h % HV1 MPa J/cm² cm²/s H1 W1 S1 860 30 42 230 1 100 0 0 575 245 12 4,2^∗10^-7 H2 W2 S1 860 30 42 230 1 100 0 0 n_.a. n.a. n.a. 5,19^∗10^-7 H3 W3 S2 860 30 42 230 1 100 0 0 403 137 n.a. n.a. H4 W4 S2 860 30 42 230 1 100 0 0 407 n.a. n.a. n.a. H5 W5 S3 860 30 42 230 1 98 1,5 0 n.a. n.a. n.a. n.a. H6 W6 S4 Keine Härtung 1,87^∗10^-6 H7 W7 S5 860 30 42 230 1 100 0 0 545 183 4 n.a. H8 W8 S6 860 30 42 230 1 100 0 0 560 n.a. 5 n.a. "n.a." = Nicht ausgewertet The sample H6 not according to the invention and consisting of the steel S4 not according to the invention was intended for case hardening and therefore did not go through the hardening process according to the invention. The hydrogen diffusion coefficient determined for this sample H6 for comparison was 1.87 * 10-6 cm ² / s.

Table 2 Sample No. stole T1 T2 T3 CR1 T4 CR2 t1 P F B + M G RA deer Rm Ag A80 ° C ° C / s ° C ° C / h H Vol .-% MPa % W1 S1 1200 1100 840 40 654 12 52 80 20th 0 0 0 450 750 15 30th W2 S1 1200 1100 840 40 646 12 52 90 10th 0 0 0 650 950 5 15 W3 S2 1200 1100 840 40 645 12 52 90 10th 0 0 0 250 550 15 30th W4 S2 1200 1100 840 40 654 12 52 70 30th 0 0 0 380 740 8th 15 W5 S3 1200 1100 840 40 650 12 52 48 50 0 1.6 0 340 505 14 29 D6 S4 1200 1100 840 40 650 12 52 n / A n / A n / A n / A n / A n / A n / A n / A n / A W7 S5 1200 1100 840 40 650 12 52 n / A n / A n / A n / A n / A n / A n / A n / A n / A W8 S6 1200 1100 840 40 650 12 52 n / A n / A n / A n / A n / A n / A n _. a. n / A n / A "na" = not evaluated sample Warm band stole T6 t2 CR3 T7 t3 M G RA HV1 Pü50 KBZ K ° C min. K / s ° C H % HV1 MPa J / cm ² cm ² / s H1 W1 S1 860 30th 42 230 1 100 0 0 575 245 12 4.2 ^∗ 10 ^-7 H2 W2 S1 860 30th 42 230 1 100 0 0 n _. a. n / A n / A 5.19 ^∗ 10 ^-7 H3 W3 S2 860 30th 42 230 1 100 0 0 403 137 n / A n / A H4 W4 S2 860 30th 42 230 1 100 0 0 407 n / A n / A n / A H5 W5 S3 860 30th 42 230 1 98 1.5 0 n / A n / A n / A n / A H6 D6 S4 No hardening 1.87 ^∗ 10 ^-6 H7 W7 S5 860 30th 42 230 1 100 0 0 545 183 4th n / A H8 W8 S6 860 30th 42 230 1 100 0 0 560 n / A 5 n / A "na" = not evaluated

Claims (6)

1. Steel, which consists of (in % by weight) C: 0.4 to 0.7%, Si: 0.15 to 0.5% Mn: 0.8 to 2.0% Cr: 0.3 to 1.0%, wherein the Cr content %Cr in each case meets the following condition:
   
           %Cr≥(%Ni + %Si + %Mn + %S + %Al)/5
   
   

   with %Ni: respective Ni content of the steel

   %Si: respective Si content of the steel

   %Mn: respective Mn content of the steel

   %S: respective S content of the steel

   %Al: respective Al content of the steel

   N: 0.0015 to 0.010% Ni: 0.04 to 2% Ti: 0.005 to 0.1% V: 0.0080 to 0.1% B: 0.0005 to 0.004% Ca: 0.0005 to 0.004% P: ≤ 0.030% S: ≤ 0.005% Al: ≤ 0.05% Cu: ≤ 0.1% Nb: ≤ 0.004% H: ≤ 0.0001% O: ≤ 0.0050% and optionally Mo: 0.006 to 0.02%,
   remainder iron and unavoidable impurities.
2. Hot-rolled flat steel product, manufactured from a steel according to claim 1 and with a structure which is graphite-free in the unhardened state, which consists of at least 80% by volume perlite, of in total at most 10% by volume bainite or martensite and as the remainder of ferrite, wherein the flat steel product has, in the ready-to-use, unhardened state measured transverse to the rolling direction, a yield strength ReH of 450 to 650 MPa and a tensile strength Rm of 750 to 950 MPa and wherein the flat steel product, in the unhardened state, has a uniform elongation Ag of 5 to 15% and an elongation A80 of 15 to 30%.
3. Hot-rolled flat steel product, manufactured from a hot-rolled flat steel product according to claim 2, with a structure, which is in the hardened state and consists of at least 99% by volume martensite and as the remainder of residual austenite, wherein the hardness of the flat steel product, in the hardened state, is 540 to 600 HV1, wherein the flat steel product, in the hardened state, has a fatigue strength Pü50 of 220 to 400 MPa, wherein the hydrogen diffusion coefficient of the flat steel product in the hardened state is 1.5*10^-7 cm²/s to 9*10^-7 cm²/s and wherein the notch impact strength of the flat steel product in the hardened state measured transverse to the rolling direction at room temperature is at least 8 J/cm², wherein these parameters are measured according to the standards indicated in the description.
4. Method of manufacturing a flat steel product formed according to claims 2 to 3, comprising at least the following work steps:

   a) manufacturing a steel melt alloyed according to claim 1, wherein the steel manufacture comprises denitrifying the melt to values below 100 ppm N and a secondary metallurgical Ca treatment;

   b) casting the steel melt into an intermediate product;

   c) heating the intermediate product to a pre-heating temperature of 1150 to 1300°C;

   d) hot rolling the intermediate product into a hot strip, wherein the hot rolling starting temperature is 900 to 1150°C and the hot rolling end temperature is 780 to 880°C;

   e) cooling the obtained hot strip to a coiling temperature of 550 to 680°C at a cooling speed of 2 to 50°C/s;

   f) coiling the hot strip at the coiling temperature into a firmly wound coil;

   g) cooling the coil at a medium cooling speed which is 4.5 to 14.5°C/h in the coil core to a boundary temperature of at most 100°C;

   h) cooling the coil proceeding from the boundary temperature to room temperature in stationary air in order to obtain a hot-rolled flat steel product in the unhardened state with graphite-free structure;

   i) optionally hardening the flat steel product or the component formed from the flat steel product by heating the flat steel product or the respective component to an austenitization temperature of 830 to 950°C, holding it for 3 to 60 mins at the austenitization temperature and then quenching it at a cooling speed of 40 to 250°C/s;

   j) optionally passing through an annealing treatment, in the case of which the flat steel product or the components formed therefrom are held at an annealing temperature of 150 to 350°C after hardening for a duration of 0.2 to 2 hours.
5. Method according to claim 4, characterised in that the coiling temperature is 600 to 650°C.
6. Method according to any one of claims 4 or 5, characterised in that the cooling duration in work step g) is 40 to 120 hours.