WO2002048410A1 - Method for the production of hot strip or sheet from a micro-alloyed steel - Google Patents

Abstract

The invention relates to a method for the production of hot strip or sheet, whereby a micro-alloyed steel, containing, in addition to the microalloying elements ( in wt. %) C: 0.05 0.12 %, Si: 0.2 0.5 %, Mn: 1.5 2.2 %, Al: 0.02 0.05 %, P: ≤ 0.025 %, S: ≤ 0.01 %, the remainder as iron and unavoidable impurities, which is cast to give an input stock in the form of slabs, pre-blocks or thin slabs. The input stock is heated to a temperature of 1300 - 1350 DEG C, pre-rolled with a deformation value of 36 % to 43 %, the pre-rolled input stock is thermomechanically hot-rolled at a rolling end temperature above the Ac3 temperature, the hot strip is cooled at a rate of at least 15 DEG C/s to a winding temperature of at least 590 DEG C and at most 630 DEG C and the hot strip finally wound. Hot strips can thus be produced economically, which even at high thickness can have a particularly high strength, by the above method.

Description

 Process for producing warπ-band or sheet from a micro-alloyed steel

The invention relates to a method for producing hot strip or sheet from ikrolegierteir. Steel.

From the Patent Abstract of Japan for JP 61-281814 A, a method for producing high-strength hot strips is known. The hot strips consist of a steel which in weight% is at most 0.3% C, at most 0.5% Si, 0.3 - 2.0% Mn, <0.05% Al, 0.01 - 0.02 % Nb, 0.01 - 0.2% V and possibly 0.01 - 0.2% Mo and 0.01 - 0.2% Ti, balance iron and unavoidable impurities. A raw material produced from this steel is finish-rolled into hot strip at a final roll temperature of at least 750 ° C., which is then immediately cooled to a coiling temperature of at most 650 ° C. The hot strip is then subjected to a final heat treatment.

Practical tests have shown that the strength of the hot strips produced by the known method at greater thicknesses of 8 mm and more does not meet the requirements which arise, for example, when constructing load-bearing structural elements of motor vehicle chassis.

The object of the invention is, starting from the prior art explained above, an inexpensive method for producing hot strips specify that have a high strength even with larger thicknesses.

This object is achieved according to the invention by a method for producing hot strips or sheets having a minimum yield strength of 700 N / mm ² with a thickness of 8 mm, in which a micro-alloyed steel, which in addition to micro-alloying elements (in% by weight) 0.05 - 0.12% C, 0.2-0.5% Si, 1.5-2.2% Mn, <0.025% P, <0.01% S, balance contains iron and unavoidable impurities, to a starting material such as slabs , Thin slabs or blooms, is cast, in which the primary material is heated to a temperature of 1300 - 1350 ° C, in which the heated primary material is pre-rolled with a degree of deformation of 36% to 43%, in which the pre-rolled primary material is thermo-mechanical at one Above the Ac ₃ temperature lying roll end temperature is hot rolled to a hot strip, in which the hot strip with a

Cooling rate of at least 15 ° C / s is cooled to a coiling temperature of at least 590 ° C and at most 630 ° C, with which the cooled hot strip is finally coiled.

Surprisingly, it has been shown that the inventive design of the contents of the individual alloy elements and the targeted coordination of the final roll temperature, the cooling rate and the reel temperature can produce hot strips which have the highest strengths even with thicknesses of more than 8 mm. Thus, in the case of a hot strip assembled and produced according to the invention with a thickness of 16 mm Yield point still at least 700 N / mm ² . A yield strength of at least 760 N / mm ² can be achieved for thicknesses of 13 mm. Additional heat treatment after the cooling in the coil is not required.

The high strength of steel produced according to the invention is achieved by the combined use of precipitation, fine grain and mixed crystal strengthening. The selected contents of the alloying elements C and Mn lead to the desired solid solution strengthening. By bringing the primary material to a temperature of 1300 ° C to 1350 ° C before the preliminary rolling, the precipitates of the alloy elements Ti, V and Nb which may be present are completely dissolved. The degrees of deformation set during the subsequent pre-deformation lead to a fine, evenly distributed and recrystallized austenite grain.

The excellent strength values make hot strip produced according to the invention particularly suitable for the production of load-bearing components on motor vehicles, such as, for example, heavily loaded side members of trucks.

The steel preferably contains at least one of the microalloying elements V, Mo, Ti, Nb in the following contents (in% by weight) 0.08-0.12% V, 0.1-0.2% Mo, 0.08-0, 11% Ti and 0.05-0.06% Nb. The Al content should be between 0.02 - 0.05% by weight. The micro alloying element niobium hinders austenite grain growth. It also hinders them Recrystallization of the austenite grains during hot rolling.

The recrystallization during hot rolling is also avoided by hot-rolling according to the invention at temperatures above the Ac ₃ temperature. The austenite / ferrite conversion takes place during the strip cooling behind the last stand of the hot rolling finishing train. In this way, a recrystallized, fine-grain structure with low pearlite contents is obtained. The ferrite grains have a size of 11 to 14 ASTM.

The cooling rate of at least 15 ° C./s is selected in order to cool the hot strip sufficiently quickly from the hot rolling end temperature, which is preferably at least 840 ° C., to the coiling temperature. When cooling in the coil from this reel temperature, the precipitation hardness maximum is reached by the micro-alloying elements Ti and V and Nb. This effect can be achieved particularly reliably if the reel temperature is in the range from 600 ° C. to 620 ° C.

Hot strips, the yield strength of which is at least 700 N / mm ² even with a thickness of 16 mm, can be produced particularly reliably in the procedure according to the invention if the steel (in% by weight) 0.06-0.08% C, 0, Contains 2-0.3% Si, 1.95-2.1% Mn, <0.02% P and <0.005% S. The Ti content, if present, should not be more than 0.1% by weight. According to another variant of the invention, hot strips, the yield strength of which is at least 760 N / mm ² at a thickness of 13 mm, can be produced reliably by the steel (in% by weight) 0.10-0.12% C, 0 , 4 - 0.5% Si, 1.95 - 2.1% Mn, <0.02% P, <0.005% S contains. The steel preferably has at least one of the microalloying elements V, Mo, Ti, Nb in the following contents (in% by weight): 0.08-0.10% V, 0.1-0.2% Mo, 0.09 - 0.11% Ti, 0.05 - 0.06% Nb.

The steel used according to the invention can contain one or more of the elements N, Cu, Ni, Sn, B or As as a result of production or to express certain properties, the sum of the contents of these elements being not more than 0.1% by weight.

If slabs are used as the raw material, the holding time should be at least 135 minutes during the heating of the slabs to ensure that the raw material is safely warmed up.

The invention is explained below using exemplary embodiments:

example 1

A steel melt with (in% by weight)

C: 0.075%

Si: 0.254%

Mn: 2.011%

P: 0.015% S: 0.003%

AI: 0.02%

N: 0.007%

Cu: 0.017%

Cr: 0.039%

Ni: 0.021%

Sn: 0.004%

V: 0.098%

Mon: 0.121%

Ti: 0.080%

Nb: 0.060%

B: 0.0003%

As: 0.002%

Rest of iron and unavoidable impurities,

was spilled into slabs. The slabs were then preheated at a temperature of 1350 ° C. for 135 minutes and then rolled to a thickness of 55 mm at a degree of deformation between 36% and 43%. The pre-rolled slabs were thermo-mechanically finished rolled in a finishing mill to a thickness of 16 mm. In the course of this hot rolling, the temperature control and the degree of deformation were coordinated with one another in such a way that the hot-rolled hot strip A had an optimal microstructure for the intended use. The final roll temperature was 840 ° C.

The hot strip A emerging from the finishing rolling mill was immediately water-cooled to a reel temperature of 610 ° C. and coiled. In the course of water cooling, cooling speeds of at least 15 ° C / s were achieved. Tensile tests were carried out on samples AI-A8 obtained from the hot strip A thus produced, the results of which are given in Table 1:

Table 1

Notched bar impact tests were also carried out on samples A9 - AI2 of the hot strip. The results are shown in Table 2:

Table 2

Example 2:

One off (in% by weight)

C: 0.115

Si: 0.449 Ό

Mn: 1.988 "S P: 0.019%

S: 0.003%

AI: 0.03%

N: 0.007%

Cu: 0.017%

Cr: 0.037%

Ni: 0.02%

Sn: 0.003%

V: 0.092%

Mon: 0.108%

Ti: 0.101%

Nb: 0.055%

B: 0.0003%

As: 0.003%

Rest of iron and unavoidable impurities,

composite steel melt was cast into slabs. As in the example of hot strip A, the slabs were preheated at a temperature of 1350 ° C. for 135 minutes, then rolled to a thickness of 55 mm and hot-rolled in a finishing mill at a final roll temperature of 840 ° C. The thickness of the hot rolled strip was 13 mm.

The hot strip B emerging from the finishing rolling mill was, like the hot strip A, immediately water-cooled and coiled to a coiling temperature of 610 ° C. In the course of water cooling, cooling rates of 15 ° C / s were again achieved.

Tensile tests were carried out on samples B1-B8 obtained from the hot strip B produced in this way, the results of which are given in Table 3:

Table 3

The results of the notched bar impact tests also carried out on samples B9 - B12 of hot strip B are given in Table 4:

Table 4

The tests carried out clearly confirmed the excellent mechanical properties of the hot strips A, B produced according to the invention.

Claims

PAT E N TAN S P RÜ C H E 1. Process for the production of hot strip or sheet, in which a micro-alloyed steel, which in addition to micro-alloying elements (in% by weight) C: 0.05 - 0.12%, Si: 0.2-0.5%, Mn: 1.5 - 2.2%, AI: 0.02 - 0.05%, P: <0.025%, S: <0.01%, Remainder contains iron and unavoidable impurities, is cast into a raw material such as slabs, blooms or thin slabs, at which the primary material is heated to a temperature of 1300 - 1350 ° C, in which the heated primary material is pre-rolled with a degree of deformation of 36% to 43%, in which the pre-rolled raw material is hot-rolled to a hot strip at a final roll temperature above the Ac ₃ temperature, where the hot strip with a Cooling rate of at least 15 ° C / s is cooled to a reel temperature of at least 590 ° C and at most 630 ° C, with which the cooled hot strip is finally coiled. Method according to claim 1, d a d u r c h g e k e n n z e i c h n e t, that the steel contains at least one of the microalloying elements V, Mo, Ti, Nb in the following contents (in% by weight): V: 0.08 - 0.12%, Mo: 0.1 - 0.2%, Ti: 0.08 - 0.11%, Nb: 0.05-0.06%. 3. The method according to any one of the preceding claims, characterized in that ß the hot strip or sheet has a minimum yield strength of 700 N / mm ² at a thickness of more than 8 mm. 4. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, d a ß the steel (in% by weight) C: 0.06 - 0.08%, Si: 0.2 - 0.3%, Mn: 1.95 - 2.1%, AI: 0.02 - 0.05%, P: <0.02%, S: contains <0.005%. 5. The method of claim 4, d a d u r c h g e k e n n z e i c h n e t, d a ß the Ti content max. Is 0.1% by weight. 6. The method according to any one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t, d a ß the steel (in% by weight) C: 0.10 - 0.12%, Si: 0.4 - 0.5%, Mn: 1.95 - 2.1%, ^' AI: 0.02 - 0.05%, P: <0.02%, S: <0.005% contains. Method according to Claim 6, d a d u r c h g e k e n n z e i c h n e t, that the steel contains at least one of the microalloying elements V, Mo, Ti, Nb in the following contents (in% by weight): V: 0.08 - 0.10%, Mo: 0.1 - 0.2%, Ti: 0.09 - 0.11%, Nb: 0.05-0.06%. 8. The method according to claim 6 or 7, characterized in that ß the hot strip or sheet with a thickness of 13 mm has a minimum yield strength of 760 N / mm ² . 9. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, d a ß slabs are processed as a raw material and the holding time during heating of the slabs is at least 135 minutes. 10. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the final roll temperature is at least 840 ° C. 11. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the reel temperature is at least 600 ° C and at most 620 ° C. 12. The method according to any one of the preceding claims, characterized in that the steel optionally contains one or more of the elements N, Cu, Ni, Sn, B or As, the sum of the contents of these elements not more than 0.1 wt. -%.