Patent ID: 12258648

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, steel is thermomechanically rolled in order to increase the properties of toughness and isotropy as well as other properties.

According toFIG.1, conventionally hot rolled steels, steels in which the rolled product is first heated to the hot-forming temperature and then rolled, by means of which the non-deformed grain is deflected in the rolling direction; already during the rolling, a recrystallization takes place after each roll pass, at the end of which the respective austenite grain has a globular form.

By contrast with this, thermomechanically rolled steels contain higher concentrations of carbide-forming elements, which form precipitation already during the hot rolling. The precipitation and the dissolved micro-alloying elements delay or suppress the recrystallization after the roll passes. Correspondingly, a recrystallization and a corresponding grain growth do not occur so that according toFIG.2, a globular structure according toFIG.1is not formed and instead, the austenite is in an elongated form.

InFIG.3the different austenite embodiments are shown, on the one hand, the globular recrystallized austenite (top) and on the other, the elongated, non-recrystallized austenite (bottom).

The difference between the normalized rolled steels with the globular recrystallized austenite grain on the one hand and the thermomechanically rolled steels with the non-globular, elongated, and deformed austenite grain is that the austenite grain of the thermomechanically rolled steel exhibits a much finer structure after the transformation.

Correspondingly, the forming has significant effects on the structure and properties; the properties cannot be achieved by means of the heat treatment alone.

The thermomechanically rolled steels used are so-called micro-alloyed steels.

FIG.4schematically depicts how from the austenite range, by means of different cooling curves, it is also possible to achieve different structures or microstructures. It shows that by means of different cooling paths, martensitic steels, complex-phase steels, dual-phase steels, and ferritic-bainitic steels can be achieved.

Prior conventional heat treatment routes are shown inFIG.5, lines1and2. For example, the thermomechanical rolling and a conventional quenching and tempering step (a slab quenching and tempering), which is used for sheets, and the thermomechanical rolling, which can be combined with a direct quenching step (DQ) and an annealing step (A)).

The method according to the invention (FIG.5, last line) provides a thermomechanical rolling, an optional direct quenching (with an optional annealing step), and then at least one very short-term, for example inductive, hardening step or quenching and tempering step.

Before this short-term inductive hardening step or quenching and tempering step, the hot strip is allowed to cool or is cooled to room temperature (e.g. after the direct hardening). A further processing from the rolling heat does not take place.

The temperature/time curves according to the prior art are shown inFIGS.6aand6b.

The differences in the structures are clear when known structures shown inFIG.6aandFIG.6bare compared to the structure produced according to the invention shown inFIG.10a. The structure of the thermomechanically rolled and short-term heat-treated steel according to the invention differs significantly from the conventionally treated steels; the smaller size and more isotropic form of the grain structure are particularly conspicuous.

Basically, the quenching and tempering step should be explained once again; the conventional quenching and tempering step is shown inFIG.6a.

In the conventional quenching and tempering, a product is first heated in a reheating furnace and is then thermomechanically rolled and completely cooled.

After the quenching and tempering, it is heated again to approx. 900° C. and then a rapid cooling in water is performed, followed by an annealing step at approx. 600° C. with a subsequent cooling in air.

The conventional heat treatments that are not according to the invention are thus the conventional hardening (H) or slab hardening, the conventional quenching and tempering (H+A) or slab quenching and tempering, and the conventional annealing (A) in the form of slab annealing or bell annealing.

In the conventional hardening or quenching and tempering, it is only possible to treat piece goods, which is relatively costly. In conventional thermomechanical rolling, the elongation of the structure produces an anisotropy of the properties; a slab annealing can achieve very good strength/toughness ratios, but it is only possible to heat treat slabs and not strips.

The solution according to the invention provides a thermomechanically produced hot strip (TM+DQ), which results in an elongated austenite grain and a homogeneous carbon distribution in the microstructure.

By contrast with conventional methods, however, the subsequent heat treatments (HST, AST) are performed as short-term heat treatments.

By contrast with the prior art, in the heating according to the invention, as shown in the figures described above, a rapid short-term heating is performed; for example, the heat source can be an inductive heating, but does not have to be.

According to the invention, hardening can be performed at least once and annealing can optionally be performed once. This yields a globular, fine austenite grain with a maximized strength and a maximized toughness.

According to the invention, the hardening can be performed once or twice; at 100 to 1000° K/s, the heating rates can be very high; the maximum temperature is set to >Ac3. According to the invention, this temperature is 800° C. to 1000° C., in particular between 820 and 970° C. The holding time is extremely short compared to the prior art and can be from 0.5 to 60 seconds, in particular from 0.5 to 5 seconds.

According to the invention, however, the heating rate can also be lower and can, for example, be 5 K/s, 10 K/s, or 15 K/s.

Preferably, but necessarily, the low holding times can be from 0.5 to 60 seconds, more preferably 0.5 to 20 seconds, in particular 0.5 to 5 seconds.

The subsequent cooling rates are set anywhere from >10° K/s up to greater than 60° K/s.

The optional annealing is performed at a maximum temperature below Ac1, which is normally from 300° C. to 700° C. In order to avoid a softening zone in subsequent welding processes, an annealing temperature of between 500° C. and 700° C. can be advantageous, but in order to increase the yield strength, a lower annealing temperature of 300° C. to 450° C. can be particularly advantageous.

The short-term heat treatments according to the invention are thus one the one hand hardening treatments or quenching and tempering treatments.

FIG.7shows that a thermomechanically rolled, directly hardened and annealed steel has an elongated structure, whereas the steel produced according to the invention (TM+DQ+A+HST/HST+AST) exhibits an isotropic globular structure.

This structure consists of 90% martensite (non-annealed or annealed), with the remainder being composed of austenite and bainite. The former austenite grain is globular, with the grain size being less than 20 pm and in particular less than 10 pm.

FIGS.8and9show examples of the mechanical properties that can be achieved as a function of the heat treatment routes and parameters for two respective alloy compositions A and B described in Tables 1 and 2 below. Table 1 shows selected properties of a steel (Material A) that has been heat treated according to the invention in contrast to conventionally heat treated steels. Table 2 shows selected properties of a steel (Material B) that has been heat treated according to the invention in contrast to conventionally heat treated steels.

TABLE 1Alloy Composition, Material A, Represented by FIG. 8MaterialCSiMnPSAlCrNiMaterial A0.090.121.640.0080.0010.0510.920.47MaterialMoCuVNbTiBNMaterial A0.220.020.110.0020.0260.00230.0051Rp.02RmKV@−40°Rm · KV@−40°MaterialProduction process[MPa][MPa]C. [J]C. [MPa · J]Material APrior artTM + DQ + H90711742327,002H: 920° C., 10′, HJ = 23,380TM + DQ + H + A8799342321,482H: 920° C., 10′, A = 570° C., 35′TM + DQ + A98310135353,689InventionTM + DQ + (A) + HST8941181115135,815HST: 950° C., 1″, HJ = 21,882TM + DQ + (A) + HST+ AST9029259386,025HST: 950° C., 1″, AST: 650° C., 1″

TABLE 2Alloy Composition, Material B, Represented by FIG. 9MaterialCSiMnPSAlCrNiMaterial A0.180.291.440.0080.0010.0520.731.02MaterialMoCuVNbTiBNMaterial A0.390.460.040.0380.0200.00030.0051Rp.02RmKV@−40°Rm · KV@−40°MaterialProduction process[MPa][MPa]C. [J]C. [MPa · J]Material BPrior artTM + DQ + H101213715575,405H: 920° C., 10′, HJ = 23,380TM + DQ + H + A103910644446,816H: 920° C., 10′, A = 570° C., 35′TM + DQ + A116912134655,798InventionTM + DQ + (A) + HST1082147490132,660HST: 950° C., 1″, HJ = 21,882

If a steel with the chemical composition shown in Table 1 is conventionally hardened, i.e. austenitized, i.e. kept at 920° C. for 10 minutes, then this yields an HJ parameter of 23380. The mechanical properties are an Rp0.2of 907 MPa, Rmof 1174 MPa, and a notched bar impact work KV of 23 joule. The product of Rm and KV is 27,002 MPaJ. If the same steel grade is quenched and tempered (austenitized again at 920° C. for 10 minutes and additionally annealed at 570° C. for 35 minutes), then the Rp0.2is 879 MPa, the Rmis 934 MPa, and the notched bar impact work is 23 joule. The product of Rm and KV is 21,482 MPaJ.

In the thermomechanically rolled, directly hardened, and annealed production route, the mechanical characteristic values are 983 MPa for Rp0.2, 1013 MPa for Rm, and 53 joule for the notched bar impact bending work and the product Rm*KV=53,689 MPaJ.

By contrast, with the same material, but using the short-term heat treatment (HST) according to the invention with a holding step at 950° C. for 1 second and subsequent cooling to room temperature (HJ=21882), an Rp0.2value of 894 MPa with a tensile strength of 1181 MPa and a notched bar impact bending work of 115 joule are achieved. These extremely good mechanical properties yield a product of Rm*KV of 135,815 MPaJ and represent an approximately five-fold increase over the value according to the prior art.

Even with an additional short-term annealing (AST) after the hardening treatment and cooling to room temperature, for example at 650° C. for only 1 second, an Rp0.2value of 902 MPa is achieved with a tensile strength of 925 MPa and a notched bar impact bending work of 93 joule (product 86,025 MPaJ).

This means that it has been possible to achieve an extreme increase in the properties in all ranges.

InFIG.8, the product of tensile strength and notch bar impact work at −40° C. as a function of the HJ parameter is plotted for different hardening processes. The light point corresponds to the above-described example A according to the invention with an HJ of 21,879 and the dark point corresponds to the prior art. The HJ value should be between 18000 and 22000 and the maximum temperatures should be in the range of 800-1000° C. With an excessively low HJP and excessively low maximum temperatures, a complete austenitization does not occur and the material cannot be completely hardened. The HJP and the maximum temperature of the hardening process, however, must also not be selected too high and in particular the HJP must be below 23000 since otherwise, the mechanical properties (especially the product of Rm and KV) can decrease drastically.

It is clear from Table 2 and from the comparison of the achievable values, though, that with a different alloy situation, the short-term heat treatment results in optimal combinations of properties.

Here, too, there is a clear relationship between a lower HJ value and the good mechanical properties; in material B, this was demonstrated primarily in the increase in the achieved notched bar impact bending work.

FIG.9once again shows the HJ parameter for heat treatments according to the invention with the example from Table 2 of HJ=21882 compared to the prior art (HJ=23380).

FIG.10ashows the temperature/time curve according to a possible embodiment of the invention together with the structures that are established.

First of all, it is clear that with the thermomechanical rolling, an elongated austenite grain is achieved, which is transformed by the direct hardening into a martensitic grain; optionally, an annealing treatment is performed.

By means of the possible short-term heat treatments, this grain, which is elongated and enriched with dislocations because of the thermomechanical treatment and direct hardening, is transformed into a fine, globular grain.

With the thermomechanical rolling according to the invention, in which the subsequent heat treatments are performed as short-term heat treatments, it is advantageous that a structure with improved properties is achieved; the short-term heat treatments also permit these heat treatment methods to be performed inline.

By means of the processing step or production step of welding, the introduced energy (heat and/or pressure) causes a local change in the structure and the mechanical properties. Products therefore have nonhomogeneous properties in the region of the weld seam.

If after production, the short-term heat treatment according to the invention is used after a processing step of “welding,” then as shown inFIG.10bfor a fusion welding process, a homogenization of the microstructure occurs in the weld seam region. The microstructure of the weld seam region and also its mechanical properties are thus brought into line with those of the rest of the product.

This is true for both fusion-welded connections such as laser welds and for pressure-welded connections such as high-frequency welds.

The invention will be explained in greater detail based on an example:

The product according to the invention is produced in that first, a steel melt with the composition according to the invention, particularly the chemical composition indicated inFIG.8or9, is melted in the steel mill and after the secondary metallurgical treatment, is cast into a slab ingot in a continuous casting machine.

The slab ingot is then heated to a temperature in the range from 1100° C. to 1300° C., in particular 1200° C. to 1260° C., descaled, and then thermomechanically hot rolled into a steel strip; in the hot rolling of the slab ingot, the initial rolling temperature is in the range from 1000° C. to 1250° C. and the final rolling temperature is greater than 800° C. and in particular, is between 830° C. and 930° C. In this case, a significant part of the forming takes place below the recrystallization stop temperature as a result of which, the austenite is elongated, as shown inFIG.2. After the hot rolling, the steel strip is cooled from the final rolling temperature to the coiling temperature by means of water exposure and is coiled. In the present example, the coiling temperature is below the martensite start temperature, i.e. less than 500° C., in particular less than 250° C., and is achieved at a cooling rate of greater than 25° C./s, in particular between 40° C./s and 100° C./s.

The steel strip, with or without a preceding blank cutting (e.g. cross-cutting or longitudinal cutting), is optionally subjected to a heat treatment; the temperature assumes values below the Al temperature, in particular below 700° C. Blanks made of the steel strip produced according to the invention can optionally be connected by means of a welding process. In this case, these blanks can have different dimensions or chemical compositions.

The steel strip or blank is formed into a component (profile, tube, edge piece) in a forming process with an optionally integrated welding process (e.g. in a roll-profiling machine or bending machine).

According to the invention, the steel strip is subjected to a short-term heat treatment before the forming or the formed component (profile, tube, edge piece). In this case, the product is first heated at least once to a maximum temperature above Ac3; typically, this is 800° C. to 1000° C., in particular however 820° C. to 970° C., briefly kept at this temperature, and then rapidly cooled. The heating rates, depending on the cross-section of the product to be heated, are greater than 5 K/s, preferably greater than 10 K/s, particularly preferably greater than 50 K/s, in particular greater than 100 K/s. The holding time at the maximum temperature is 0.5 to 60 seconds, for example 1 to 10 s; then, a cooling is performed at cooling rates between 10 K/s and up to greater than 60 K/s.

After the hardening, the steel strip can be subjected to another annealing treatment before the forming or the formed component (profile, tube, edge piece) can be subjected to another annealing treatment. In the latter, the sheet is heated at a heating rate of up to 1000 K/s, in particular 400° C./s to 800° C./s, to a maximum temperature below Ac1, which usually means 300° C. to 700° C., for example 550° C. The holding time at the maximum temperature is 0.5 to 60 seconds, for example 1 to 10 s; then, a cooling is performed at cooling rates between 10 K/s and up to greater than 60 K/s.

The invention will be explained in greater detail based on a specific example:

The product according to the invention is produced in that first, a steel melt with the composition according to the invention, in particular the chemical composition indicated in Table 1 is melted in the steel mill and after secondary metallurgical treatment, is cast into a slab ingot in a continuous casting machine.

The slab ingot is then heated to a temperature of 1225° C., descaled, and then conventionally hot-rolled into a steel strip; in the hot rolling of the slab ingot, the initial rolling temperature is 1110° C. and the final rolling temperature is 890° C. In this case, a significant part of the forming takes place below the recrystallization stop temperature as a result of which, the austenite is elongated, as shown inFIG.2. After the hot rolling, the steel strip is cooled from the final rolling temperature to the coiling temperature by means of water exposure and is coiled. In the present example, the coiling temperature is 125° C. and is achieved at a cooling rate of 55° C./s.

The steel strip, which has been cut to length and has a thickness of 5 mm, is formed in a roll-profiling machine into a closed profile with a square cross-section and the outer dimensions of 50 mm×50 mm.

According to the invention, the profile is subjected to a short-term heat treatment.

In this case, the profile is initially heated to a maximum temperature above Ac3, to 950° C. in the present example, is briefly held at this temperature, and is then rapidly cooled. The heating rates are 15 K/s. The holding time at the maximum temperature is 1 second; finally, a cooling at cooling rates of 20 K/s is performed. The Hollomon-Jaffee parameter of the short-term hardening that is performed is 21882.