EP0181583B1 - Method of making a dual-phase hot-rolled steel strip - Google Patents

Abstract

The invention relates to a method of producing hot-rolled strip having a dual-phase structure from a slab previously produced by ingot casting or continuous casting. The slab contains carbon, manganese, silicon and chromium as essential constituents in addition to iron. The slab is heated up to the rolling temperature, hot-rolled at a temperature above Ar3, rapidly cooled from the rolling temperature and coiled at a relative low temperature. The characterizing features of the invention are that the hot-rolled strip (a) is produced from a steel which, in addition to 0.05 to 0.16% of C, 0.5 to 1.0% of Si, 0.3 to 1.5% of Cr, </=0.025% of P, </=0.015% of S, 0.02 to 0.10% of Al and </=0.011% of N, contains 0.2 to 0.4% of Mn, the remainder being iron and usual impurities, (b) is rapidly cooled, immediately after finish-rolling, from final rolling temperature down to the coiling temperature at a mean rate in the range from 30 DEG to 70 DEG C./s and without interruptions, and (c) is then coiled at a temperature in the range from 350 DEG to 190 DEG C.

Description

The invention relates to a method for producing hot strip with a two-phase structure from a slab previously produced by block casting or continuous casting, which contains iron, carbon, manganese, silicon and chromium as essential constituents, by heating the slab to the rolling temperature Hot and finish rolling at a temperature> A, ₃ , by accelerated cooling from the rolling heat and by coiling at a relatively low temperature.

Such a process for the production of hot strip with a two-phase structure made of fine-grained ferrite (> 70%) and grains of martensite dispersed therein, in which the slab essentially containing carbon, manganese, silicon and chromium is first heated to hot rolling temperature, then warm above Ar3 - and finish-rolled, then accelerated and finally coiled at low temperature, is known from EP-B-19193 and from EP-A-72 867.

In the process known from EP-B-19193, the slab, which is essentially 0.05-0.20% C, 0.5-1.5% Mn and 0.5-2.0% Si and possibly Cr, V, Mo, Ti and Nb, the rest contains iron, hot-rolled in the austenitic state, then cooled to a temperature in the range of approx. 800―650 ° C, coiled and held at this temperature for at least one minute. Subsequently, the strip is unwound in a further process step, cooled to a temperature <450 ° C. at a speed> 10 ° C./s and finally rewound at this temperature.

In the process known from EP-A-72 867, the slab heated to the rolling temperature, which is essentially 0.02-0.20% C, 0.5-2.0% Mn, 0.05-2.0% Si and 0.3-1.5% Cr as well as ≤0.15% P and ≤0.1% Al, the rest contains iron, hot rolled with a finish rolling temperature> 780 ° C. After leaving the finishing line, the hot strip is cooled at a speed> 40 ° C / s to an intermediate temperature T _N in the order of approximately 750-650 ° C and kept at this temperature for at least 5 seconds. This is followed by a further accelerated cooling at a rate> 50 ° C / s to a temperature in the range of 500-200 ° C before the strip is finally coiled at this temperature.

With both previously known methods, a low yield ratio (<approx. 0.70) and good cold formability of the hot-rolled strip or the sheets produced therefrom are achieved. In order to set the corresponding two-phase structure, a controlled or stepped cooling process is provided in both cases after the hot or finish rolling of the hot strip. Phases of accelerated cooling and phases of keeping the hot strip at a certain temperature (cooling in still air) alternate with each other. In terms of device, both cooling processes mean complex cooling sections or, in the case of the process known from EP-B-19 193, a second unwinding and winding device for the finished rolled hot strip.

On the other hand, EP-A-68 598 discloses a method for producing hot strip with a two-phase structure, low yield ratio and good formability, in which, in contrast to the two aforementioned methods, the hot strip after finishing rolling to a low coiling temperature without additional effort is cooled. This is essentially achieved in that the slab in addition to 0.03-0.15% C, 0.6―1.8% Mn, ≤0.10% Al, <0.008% S and possibly 0.2-2.0 % Si alone or together with Cr contains an increased phosphorus content in the range of 0.04-0.20%, balance iron. In addition, the slab must be heated to a certain temperature in the specified range of 1,100-1,250 ° C before it is subsequently hot-rolled and finish-rolled at a temperature in the range of 900-780 ° C and after finish-rolling at a speed in the range of 10-200 ° C / s is cooled and finally can be coiled at a temperature ≤450 ° C. This previously known method has the advantage that the hot strip can be produced on conventional rolling mills with the associated cooling section without additional equipment. On the other hand, however, the increase in the phosphorus content means a deterioration in the weldability of the hot strip. Furthermore, the tendency of the hot strip to embrittlement increases with increasing or increased phosphorus content. This temper embrittlement is particularly noticeable when the sheet made from the hot strip with the increased phosphorus content has to be welded during further processing, for example. In addition, in this previously known method, the temperature of the furnace for heating and heating the slab must be accurate to the rolling temperature can be set, this temperature from the predetermined temperature range is also below the usual temperatures.

The object of the invention is to provide a method for producing hot strip with a two-phase structure which, with at least the same property profile, namely low yield ratio (below approx. 0.70), isotropic cold formability, good weldability, with simple means, especially on conventional rolling mills with associated cooling section, ie can be produced without additional equipment.

• a) aus einem Stahl erzeugt wird, der neben 0,05 bis 0,16% C, 0,5 bis 1,0% Si, 0,3 bis 1,5% Cr, ≤ 0,025% P, --<0,015% S, 0,02 bis 0,10% AI und ≤0,011 % N, 0,2 bis 0,4% Mn, Rest Eisen und die üblichen Verunreinigungen enthält,
• b) unmittelbar nach dem Fertigwalzen von der Endwalztemperatur bis auf die Haspeltemperatur mit einer mittleren Geschwindigkeit im Bereich von 30 bis 70°C/s und ohne Unterbrechungen beschleunigt abgekühlt wird und
• c) anschließend bei einer Temperatur im Breich von 350 bis 190°C gehaspelt wird.

According to the invention, this object is achieved by a method of the type mentioned in the introduction in that the hot strip

• a) is produced from a steel which, in addition to 0.05 to 0.16% C, 0.5 to 1.0% Si, 0.3 to 1.5% Cr, ≤ 0.025% P, - <0.015% S, 0.02 to 0.10% Al and ≤0.011% N, 0.2 to 0.4% Mn, balance contains iron and the usual impurities,
• b) immediately after the finish rolling from the finish rolling temperature to the coiling temperature with an average speed in the range of 30 to 70 ° C / s and accelerated without interruptions and
• c) is then coiled at a temperature in the range of 350 to 190 ° C.

In comparison with the previously known state of the art, it is essential to the invention that the manganese content of the steel from which the slab is first produced and then the hot strip is reduced and to a low value in the range from 0.20 to 0.40%. is set. At the same time, the carbon content of the steel is preferably set to a value in the range from 0.05 to 0.12%.

The slab produced from a steel of the claimed composition can then be heated to the usual rolling temperature and heated through using the method according to the invention. No special measures are required in this regard.

According to the invention, the hot and finished rolling of the heated slab into the hot strip takes place at a temperature above and as close as possible to A, ₃ . With regard to this deformation during hot rolling and in particular during finish rolling in the last two stands of the finishing train, no further special measures are necessary when using the method according to the invention. For example, the deformation in the last two stands of the prefabricated structure is a maximum of 25%, preferably about 15%.

Following this, the hot strip is, according to the invention, cooled immediately after finish rolling with a final rolling temperature above A _r3 at an average speed in the range from 30 to 70 ° C./s and without interruptions and then coiled at a temperature in the range from 350 to 190 ° C. .

According to the invention, the slab additionally contains 0.01 to 0.04% titanium in a stoichiometric ratio to nitrogen in order to achieve an improvement in the cold formability at the low coiling temperature according to the invention. At the same time, nitrogen aging of the finished rolled hot strip or of the sheets produced therefrom is avoided.

The main advantage of the method according to the invention is that hot strip with a two-phase structure made of fine-grained, globular ferrite (> 80%) and grains of martensite dispersed therein can be produced on conventional hot strip mills with the associated downstream cooling section. Furthermore, the method according to the invention enables known measures to accelerate ferrite formation and its negative effects, such as to dispense with a high final deformation during hot rolling and finish rolling in the two-phase area. A high final deformation means undesirable high rolling forces and a deterioration in strip flatness and strip geometry and finish rolling in the two-phase area also mean high rolling forces, a deterioration in cold formability and anisotropic mechanical properties of the hot rolled strip.

In comparison to the two processes known from EP-B-19 193 and EP-A-72 867, changes in apparatus and additions to the cooling section in terms of device are not required. In addition, compared to the method known from EP-A-68 598, it is also not necessary to control the temperature at which hot rolling is started and to set it precisely beforehand. Rather, hot rolling can also be started in the process according to the invention at a temperature> 1,250 ° C.

Despite the note in EP-A-68 598, page 7, that an Mn content of at least 0.6% is required to set the two-phase structure, it has surprisingly been found that this also has a reduced Mn content according to the invention in the range of 0.2-0.4% is possible. Another advantage of the method according to the invention is that the manufacturing costs are reduced due to the reduced Mn content. In addition, the low Mn content according to the invention advantageously has the effect that practically no stretched sulfides (MnS) form, which usually cause a deterioration in the cold formability, particularly in the transverse direction, of high-strength steels. For this reason, a lowering of the sulfur content in the production of the steel can be dispensed with, for example, in the method according to the invention, in order to control the formation of elongated sulfides at higher Mn contents. However, this means a reduction in the production costs of the hot strips produced by the method according to the invention.

In contrast to the process previously known from EP-A-68 598, the process according to the invention makes it possible to dispense with the increased addition of phosphorus, which leads to embrittlement. The preferred phosphorus content is limited to - <0.015%.

Overall, hot strip with a two-phase structure made of> 80% fine-grained, globular ferrite and martensite and with a yield ratio <0.07 can be produced with the aid of the method according to the invention, which can be welded without problems and in both the longitudinal and transverse directions has good, uniform cold mobility.

Another advantage of the method according to the invention is that, in the hot strip produced by the method, after deformation and subsequent tempering treatment, for example, by baking an applied layer of lacquer, an additional increase in the yield strength is achieved. Furthermore, the low alloy content enables the production of hot strip with a two-phase structure with tensile strengths of 500 to 600 N / mm ² , which are particularly suitable for the production of parts that require high cold formability.

The invention is explained in more detail using the following exemplary embodiments.

Continuous cast slabs with the chemical compositions A and B given in Table 1 were first heated to a temperature of approximately 1,250 ° C. and heated through. Subsequently, they were hot-rolled at a temperature above A _r3 and _finish- rolled to a final thickness d (see Table 2). The final rolling or finish rolling temperature was placed as close as possible to A, ₃ . The finished rolled strips were then cooled without interruption at a medium speed in the range of 30-70 ° C./s on a conventional cooling section downstream of the hot strip mill and wound up at the various reel temperatures HT given in Table 2. The mechanical properties of the finished rolled sheets, as summarized in Table 2, resulted.

The values given in Table 2 make it clear that a yield strength ratio of <0.70 in the longitudinal direction L and in the transverse direction 0 could be achieved in the strips or sheets produced by the process according to the invention. It can also be seen from Table 2 that, according to the invention, a reel temperature HT in the range from 350-190 ° C. must be maintained. In contrast, the desired ferritic / martensitic structure is not achieved at higher reel temperatures, as can be seen from the pronounced yield strength and the higher yield ratio of samples A1 and B1.

In addition, Table 2 shows that the reel temperature HT should preferably be set to a temperature above 200 ° C, because at a lower reel temperature, s. Samples A3 and B3, the yield strength ratio increased again and the elongation at break A ₅ decreased to even worse values. Both have an unfavorable effect on the cold formability of the hot strip or sheet.

Furthermore, an artificial aging treatment at 100 ° C and for about an hour showed no change in the yield strength. On the other hand, for example in the case of the two strips or sheets A2 and B2, an increase in the yield strength in the range of 4080 N / mm ^{2 was determined} after a tempering treatment at approx. 170 ° C. and 20 minutes after the strips or sheets had previously been subjected to a three percent pre-ver ^f styling aid were subjected.

Claims (4)

1. A process for the production of hot rolled strip having a dual phase structure with martensite distributed in ferrite from a slab previously produced by ingot casting or continuous casting, by heating the slab to rolling temperature, hot rolling and finish rolling at a temperature of >A_r3, accelerated cooling from the rolling heat and coiling, wherein the hot rolled strip

a) is produced from a steel which contains in addition to 0.05 to 0.16% C, 0.5 to 1.0% Si, 0.3 to 1.5% Cr, ≤0.025% P, ≤0.015% S, 0.02 to 0.10% AI and ≤0.011 % N, 0.2 to 0.4% Mn and if necessary 0.01 to 0.04% titanium in stoichiometric ratio to the nitrogen present, residue iron and the usual impurities,

b) is immediately after finish rolling cooled with acceleration from the final rolling temperature to the coiling temperature at a mean speed in the range between 30 and 70°C/sec and without interruptions, and

c) is then coiled at a temperature in the range of 350 to 190°C.

2. A process according to claim 1, characterized in that 0.06 to 0.12% C is added to the steel.

3. A process according to claims 1 or 2, characterized in that the phosphorus content is limited to ≤0.015%.

4. A process according to one of claims 1 to 3, characterized in that the hot rolled strip is finish rolled at a temperature in the range of 20 to 50°C above A_r3 and coiled after accelerated cooling from the rolling heat at a temperature in the range of 330 to 260°C.