EP1453984B1 - Method for the production of hot strip or sheet from a micro-alloyed steel - Google Patents

Abstract

The invention relates to a method for producing hot strip or hot plate, in which a microalloyed steel, which, apart from microalloying elements, comprises (in weight %) 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%; with the remainder being iron and unavoidable impurities, is cast to form a raw material such as slabs, blooms or thin slabs; in which the raw material is heated to a temperature of 1300-1350° C.; in which the heated raw material is rough rolled at a degree of deformation of 36% to 43%; in which the rough rolled raw material is thermomechanically hot rolled at a final rolling temperature which exceeds the Ac3 temperature so as to form a hot strip; in which the hot strip is cooled at a cooling rate of at least 15° C./s to a coiling temperature of at least 590° C. and at most 630° C. at which temperature the cooled hot strip is finally coiled. With such a method, hot strip which is of particularly high strength even at larger thicknesses can be produced economically.

Description

The invention relates to a method for producing Hot strip or sheet of micro-alloyed steel.

From the Patent Abstract of Japan to JP 61-281814 A is a method for producing high-strength hot strips known. The hot strips are made of a steel that is in % By weight not more than 0.3% C, not more than 0.5% Si, 0.3 2.0% Mn, ≤ 0.05% Al, 0.01-0.02% Nb, 0.01-0.2% V and optionally 0.01-0.2% Mo and 0.01-0.2% Ti, balance Contains iron and unavoidable impurities. On off This raw material produced in this steel is used in a Roll end temperature of at least 750 ° C to hot strip finish rolled, which immediately afterwards on a cooled to a maximum of 650 ° C reel temperature becomes. Subsequently, the hot strip is a final Subjected to heat treatment.

Practical experiments have shown that the strength of hot strips produced by the known method larger thicknesses of 8 mm and more does not meet the requirements met, for example, in the construction of load-bearing Design elements of motor vehicle chassis result.

The object of the invention is, starting from the above-explained prior art cost-effective method for producing hot strips indicate that even with larger thicknesses a high Possess strength.

This object is achieved according to the invention by a method according to claim 1. For producing hot strips or sheets possessing a minimum yield strength of 700 N / mm ² at a thickness of 8 mm, a microalloyed steel is produced 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 iron and unavoidable impurities, to a starting material like slabs, thin slabs or blooms, potted; the starting material is heated to a temperature of 1300-1350 ° C; the heated starting material is pre-rolled at a degree of deformation of 36% to 43%; the pre-rolled starting material is thermo-mechanically hot rolled to a hot strip at a rolling end temperature above the Ac ₃ temperature; the hot strip is cooled at a cooling rate of at least 15 ° C / s to a coiler temperature of at least 590 ° C and at most 630 ° C, and finally the cooled hot strip is coiled.

Surprisingly, it has been shown that can be produced by the inventive dimensioning of the contents of the individual alloying elements and the targeted tuning of the rolling end temperature, the cooling rate and the coiling temperature hot strips, which have the highest strengths even at thicknesses of more than 8 mm. Thus, in a hot strip composed and manufactured according to the invention with a thickness of 16 mm, the yield strength is still at least 700 N / mm ² . For thicknesses of 13 mm, a yield strength of at least 760 N / mm ² can be achieved. A supplementary heat treatment following the cooling in the coil is not required.

The high strength of steel produced according to the invention is due to the combined use of the elimination, Fine grain and solid solution hardening achieved. there lead the selected contents of the alloying elements C and Mn to the desired solid solution hardening. By doing the raw material before roughing to a temperature of 1300 ° C is brought to 1350 ° C, the Precipitates of micro-alloying elements Ti, V and Nb completely dissolved. The at the subsequent pre-deformation set Deformations lead to a fine, even dispersed and recrystallized austenite grain.

The excellent strength values make According to the invention produced hot strip in particular suitable for the production of load-bearing components Motor vehicles, such as heavily loaded Longitudinal beams of trucks.

The steel contains at least one of Micro-alloying elements V, Mo, Ti, Nb in the following Held (in 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 and 0.05% by weight. The The micro-alloying element niobium hinders this Austenite grain. In addition, it hinders the Recrystallization of the austenite grains during the Hot rolling.

Recrystallization during hot rolling is also avoided by hot rolling according to the invention at temperatures above the Ac ₃ temperature. The austenite / ferrite transformation occurs during strip cooling behind the last stand of the hot rolling mill. In this way, a recrystallized, fine-grained microstructure with low Perlitanteilen is obtained. The ferrite grains have a size of 11 to 14 ASTM.

The cooling rate of at least 15 ° C / s is chosen to the hot strip of the preferably at least 840 ° C hot rolling end temperature sufficient cool down to the reel temperature quickly. At the of this reel temperature outgoing cooling in the coil is the precipitation hardness maximum by the Microalloying Ti and V and Nb achieved. This effect can be achieved particularly reliably if the reel temperature in the range of 600 ° C to 620 ° C lies.

Hot strips, whose yield strength is at least 700 N / mm ² even with a thickness of 16 mm, can be produced particularly reliably with the method according to the invention if the steel (in weight%) is 0.06-0.08% C, 0, 2 - 0.3% Si, 1.95 - 2.1% Mn, ≤ 0.02% P and ≤ 0.005% S contains. In this case, if present, the Ti content should not be more than 0.1% by weight.

According to another variant of the invention, hot strips whose yield strength is at least 760 N / mm ² at a thickness of 13 mm can thereby be reliably produced by the steel (in weight%) being 0.10-0.12% C, 0 , 4 - 0.5% Si, 1.95 - 2.1% Mn, ≤ 0.02% P, ≤ 0.005% S. In this case, the steel preferably has at least one of the micro-alloying 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 in the invention can production-related or to the expression of certain Properties optionally one or more of the elements N, Cu, Ni, Sn, B or As, where the sum of the Content of these elements not more than 0.1 wt .-% is.

If slabs are processed as starting material, then the Holding time during heating of slabs at least 135 minutes to ensure a safe warmth of the To ensure starting material.

The invention is based on Embodiments explained:

example 1

C:

0,075 %

Si:

0,254 %

Mn:

2,011 %

P:

0,015 %

S:

0,003 %

Al:

0,02 %

N:

0,007 %

Cu:

0,017 %

Cr:

0,039 %

Ni:

0,021 %

Sn:

0,004 %

V:

0,098 %

Mo:

0,121 %

Ti:

0,080 %

Nb:

0,060 %

B:

0,0003 %

As:

0,002 %

Rest Eisen und unvermeidbare Verunreinigungen,
wurde zu Brammen vergossen. Die Brammen wurden anschließend bei einer Temperatur von 1350 °C für 135 Minuten vorgewärmt und dann bei einem zwischen 36 % und 43 % liegenden Verformungsgrad auf eine Dicke von 55 mm vorgewalzt. Die vorgewalzten Brammen wurden in einer Fertigwalzstaffel auf eine Dicke von 16 mm thermomechanisch fertig warmgewalzt. Im Zuge dieses Warmwalzens wurden die Temperaturführung und Umformgrade so aufeinander abgestimmt, daß das fertig warmgewalzte Warmband A eine für den Verwendungszweck optimale Gefügestruktur aufweist. Die Walzendtemperatur betrug dabei 840 °C.A molten steel with (in% by weight)

C:

0.075%

Si:

0.254%

Mn:

2.011%

P:

0.015%

S:

0.003%

al:

0.02%

N:

0.007%

Cu:

0.017%

Cr:

0.039%

Ni:

0.021%

Sn:

0.004%

V:

0.098%

Not a word:

0.121%

Ti:

0.080%

Nb:

0.060%

B:

0,0003%

As:

0.002%

Residual iron and unavoidable impurities,
was shed to slabs. The slabs were then preheated at a temperature of 1350 ° C for 135 minutes and then pre-rolled to a thickness of 55 mm with a degree of deformation between 36% and 43%. The pre-rolled slabs were hot rolled to a thickness of 16 mm thermomechanically finished in a finishing roll. In the course of this hot rolling, the temperature control and degrees of deformation were coordinated so that the finished hot-rolled hot strip A has an optimum for the intended use microstructure. The rolling end temperature was 840 ° C.

The hot strip A emerging from the finish rolling scale was immediately after a reel temperature of 610 ° C water cooled and reeled. In the course of Water cooling were cooling rates of at least 15 ° C / s achieved.

Tensile tests were carried out on samples A1-A8 obtained from the hot strip A produced in this way, the results of which are given in Table 1: R _eh [N / mm ² ] R _m [N / mm ² ] R _eh / R _m A ₅ [%] A ₈₀ [%] A1 725 830 87.3 19.5 19.6 A2 701 816 85.9 21.5 21.6 A3 712 824 86.4 20.2 20.2 A4 753 842 89.5 21.5 20.8 A5 763 841 90.7 19.9 19.1 A6 772 846 91.2 21.0 20.3 A7 761 840 90.6 20.2 19.4 A8 760 858 88.5 19.0 18.4

Impact impact tests were also carried out on samples A9 - A12 of the hot strip. Their results are given in Table 2: sample direction Prüftemp. [K] Kv [J] A9 Along 233 86.6 A10 crosswise 233 49.5 A11 Along 233 86.0 A12 crosswise 233 58.6

Example 2:

C:

0,115 %

Si:

0,449 %

Mn:

1,988 %

P:

0,019 %

S:

0,003 %

Al:

0,03 %

N:

0,007 %

Cu:

0,017 %

Cr:

0,037 %

Ni:

0, 02 %

Sn:

0,003 %

V:

0,092 %

Mo:

0,108 %

Ti:

0,101 %

Nb:

0,055 %

B:

0,0003 %

As:

0,003 %

Rest Eisen und unvermeidbare Verunreinigungen,
zusammengesetzte Stahlschmelze wurde zu Brammen vergossen. Die Brammen wurden wie beim Beispiel des Warmbands A bei einer Temperatur von 1350 °C für 135 Minuten vorgewärmt, anschließend auf eine Dicke von 55 mm vorgewalzt und in einer Fertigwalzstaffel bei einer Walzendtemperatur von 840 °C warmgewalzt. Die Dicke des fertig gewalzten Warmbandes betrug 13 mm.One out of (in% by weight)

C:

0,115%

Si:

0.449%

Mn:

1,988%

P:

0.019%

S:

0.003%

al:

0.03%

N:

0.007%

Cu:

0.017%

Cr:

0.037%

Ni:

0, 02%

Sn:

0.003%

V:

0.092%

Not a word:

0.108%

Ti:

0.101%

Nb:

0.055%

B:

0,0003%

As:

0.003%

Residual iron and unavoidable impurities,
composite molten steel was poured into slabs. The slabs were preheated as in the example of the hot strip A at a temperature of 1350 ° C for 135 minutes, then pre-rolled to a thickness of 55 mm and hot rolled in a finishing roll at a rolling finish temperature of 840 ° C. The thickness of the finished rolled hot strip was 13 mm.

The emerging from the finishing roll scale hot strip B was just like the hot strip A immediately after water cooled to a reel temperature of 610 ° C and coiled. In the course of the water cooling were again Cooling speeds of 15 ° C / s 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: R _eh [N / mm ² ] R _m [N / mm ² ] R _eh / R _m A ₅ [%] A ₈₀ [%] B1 767 899 85.3 15.5 15.4 B2 771 893 86.3 18.1 18.1 B3 783 916 85.5 15.8 15.8 B4 796 875 91.0 18.5 18.0 B5 784 872 89.9 19.0 18.4 B6 792 871 90.9 19.9 19.3 B7 794 890 89.1 19.9 19.2 B8 776 871 89.0 18.6 18.1

The results of the notched bar impact tests on samples B9 - B12 of hot strip B are also given in Table 4: sample direction Prüftemp. [K] Kv [J] B9 Along 233 55.5 B10 crosswise 233 36.2 B11 Along 233 78.4 B12 crosswise 233 37.4

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

Claims (10)

1. A method for producing hot strip or hot plate

   in which a microalloyed steel which, apart from at least one of the microalloying elements V, Mo, Ti, Nb in the following contents (in weight %) V: 0,08 - 0,12 %, Mo: 0,1 - 0,2 %, Ti: 0,08 - 0,11 %, Nb: 0,05 - 0,06 % comprises the following elements (in weight %)

   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 %

   as well as optionally one or several of the elements N, Cu, Ni, Sn, B and As, with the remainder being iron and unavoidable impurities, the total content of the optionally present elements being ≤ 0,1 weight %, is cast to form a raw material such as slabs, blooms, or thin slabs;

   in which the raw material is heated to a temperature of 1300 - 1350 °C;

   in which the heated raw material is rough rolled at a degree of deformation of 36 % to 43 %;

   in which the rough rolled raw material is thermomechanically hot rolled at a finish roll temperature which exceeds the Ac₃ temperature so as to form a hot strip;

   in which the hot strip is cooled at a cooling rate of at least 15 °C/s to a coiling temperature of at least 590 °C and at most 630 °C;

   at which temperature the cooled hot strip is finally coiled.
2. The method according to claim 1, characterised in that at a thickness exceeding 8 mm, the hot strip or hot plate has a minimum yield point of 700 N/mm².
3. The method according to one of claims 1 or 2, characterised in that the steel comprises (in weight %):

   C:

   0.06 - 0.08 %;

   Si:

   0.2 - 0.3 %;

   Mn:

   1.95 - 2.1 %;

   Al:

   0.02 - 0.05 %;

   P:

   ≤ 0.02 %;

   S:

   ≤ 0.005 %.

4. The method according to claim 3, characterised in that the Ti content does not exceed 0.1 weight %.
5. The method according to any one of claims 1 or 2, characterised in that the steel comprises (in weight %):

   C:

   0.10 - 0.12 %;

   Si:

   0.4 - 0.5 %;

   Mn:

   1.95 - 2.1 %;

   Al:

   0.02 - 0.05 %;

   S:

   ≤ 0.005 %.

6. The method according to claim 5, characterised in that the steel comprises at least one of the microalloying elements V, Mo, Ti, Nb in the following percentages (in weight %):

   V:

   0.08 - 0.10 %;

   Mo:

   0.1 - 0.2 %;

   Ti:

   0.09 - 0.11 %;

   Nb:

   0.05 - 0.06 %.

7. The method according to claim 5 or 6, characterised in that at a thickness of 13 mm, the hot strip or hot plate has a minimum yield point of 760 N/mm².
8. The method according to any one of the preceding claims, characterised in that slabs are used as raw material, with the holding time during heating of the slabs being at least 135 minutes.
9. The method according to any one of the preceding claims, characterised in that the final rolling temperature is at least 840 °C.
10. The method according to any one of the preceding claims, characterised in that the coiling temperature is at least 600 °C and at most 620 °C.