PROCESS AND DEVICE FOR PRODUCING HOT-ROLLED STRIP FROM SILICON STEEL

Abstract

A method and a combined casting/rolling installation for production/processing of high-quality hot-rolled strip into grain-oriented electrical steel strip at low cost, provides for: a) melting a steel having a chemical composition in wt % of Si 2 to 7%, C 0.01 to 0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%, P<0.1%, remainder Fe and impurities; b) casting a strand having a thickness of 25 to 150 mm on a continuous casting installation; c) rolling into a strip in up to 4 rolling passes directly after casting the strand, wherein at least in one rolling pass a true strain is >30% or the total true strain of all passes is >50%; d) heating the strip to a final temperature of 1050 to 1250° C., preferably 1100 to 1180° C.; e) finish rolling the strip in a second rolling train, then f) cooling and winding the strip.

Description

TECHNICAL FIELD

The present invention relates to a process and a device for producing hot-rolled strip from silicon-alloyed steels for further processing to form grain-oriented magnetic steel strip. The further processing of the hot strip is not the subject of this application; it takes place by heat treatment and cold-rolling.

BACKGROUND

Grain-oriented magnetic steel strip, for example for subsequent processing to form laminated magnetic steel sheet for transformers or electrical machines, is distinguished by low specific remagnetization losses and a high magnetic permeability. As the consumption of electrical energy increases and ever higher demands are placed on the efficiency of electrical machines, there is a high demand for high-quality and inexpensive magnetic steel sheets.

The production of magnetic steel strip can be subdivided into the following process steps: steel production, hot-strip production and cold-strip production, heat treatment and strip coating (see Fact Sheet 401, “Elektroband und -blech” [Magnetic steel strip and sheet], Stahl-Informations-Zentrum, Dusseldorf, Edition 2005).

Combined casting/rolling installations are known to a person skilled in the art for particularly economical production of high-quality hot strips, for example for subsequent processing to form automotive steel sheets (see e.g. EP 1662011 A1).

WO 98/46802 A1 discloses a process for producing grain-oriented magnetic steel sheets, wherein either a) a specific steel alloy is melted and a thin strand is cast therefrom in a continuous casting installation, then the strand is separated, the slabs are annealed, finish-rolled and cooled, and the hot strip is wound up; or b) a specific steel alloy is melted and a thin strand is cast therefrom in a continuous casting installation, then the strand is finish-rolled and cooled, and the hot strip is wound up.

After the working steps as per a) or b), the hot strip is substantially annealed, rolled to the final thickness in a cold-rolling mill train, decarburized and subjected to targeted secondary recrystallization. The molten steel alloy contains what are known as growth inhibitors, specifically sulfides, carbides or nitrides of the elements Mn, Cu and Al, which prevent the grain growth of the microstructure present after the finish-rolling. Depending on the temperature, these precipitations also affect the recrystallization as early as during the deformation, and immediately thereafter, in a manner such that a microstructure can be produced which, in further consequence, is suitable for producing a material with the desired grain properties.

The process according to the prior art for the production of hot-rolled strip either consumes a large amount of energy, or results in losses in quality of the grain-oriented magnetic steel sheets which are further processed. The equalizing furnaces used for annealing the slabs are additionally not very compact, which in turn increases the capital costs for the overall installation.

SUMMARY

According to various embodiments a process and a combined casting/rolling installation of the type mentioned in the introduction can be provided, with which high-quality hot-rolled strip for further processing to form grain-oriented magnetic steel strip with outstanding magnetic, electrical and geometrical properties can be produced at low cost.

According to an embodiment, a process for producing hot-rolled strip from silicon-alloyed steels on a combined casting/rolling installation for further processing to form grain-oriented magnetic steel strip, may comprise the following process steps in the sequence given: a) a steel having a chemical composition (in % by weight) of Si 2 to 7%, C 0.01 to 0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%, P<0.1%, remainder Fe and impurities is melted; b) a strand having a thickness of 25 to 150 mm is cast on a continuous casting installation; c) a strip is rolled in up to 4 roll passes immediately after the strand has been cast, wherein at least in one roll pass a degree of deformation is >30% or the total degree of deformation of all the passes is >50%; d) the strip is heated to a final temperature of 1050 to 1250° C., preferably 1100 to 1180° C.; e) the strip is finish-rolled in a second rolling mill train, and then f) the strip is cooled and coiled.

According to a further embodiment, the final temperature after the strip has been heated may be maintained for a duration t, where t>15 s, preferably >60 s. According to a further embodiment, the final temperature of the strip can be maintained in a continuous furnace. According to a further embodiment, the final temperature of the strip can be maintained during winding-up and subsequent unwinding in a coiling furnace. According to a further embodiment, the strip can be finish-rolled in 2 to 6, preferably 3 to 5, roll passes in the second rolling mill train. According to a further embodiment, the strip may have a final rolling temperature of 900 to 1050° C. after the finish-rolling. According to a further embodiment, the strip can be cooled to a coiling temperature of 300 to 600° C. by means of an intensive cooling step within 10 s, preferably within 6 s, after the finish-rolling. According to a further embodiment, at the start of the intensive cooling step, the strip can be cooled at a cooling rate which is twice as high, preferably three times as high, as the cooling rate at the end of the cooling step. According to a further embodiment, the sum of the alloying elements Cu+Mn in the steel melt can be >0.35% by weight, preferably >0.55% by weight. According to a further embodiment, the sum of the alloying elements S+N in the steel melt can be >100 ppm, preferably >200 ppm. According to a further embodiment, the quotient of the alloying elements Cu/Mn in the steel melt can be >2.5, preferably >3.5.

According to another embodiment, a combined casting/rolling installation for producing hot-rolled strip from silicon-alloyed steels for further processing to form grain-oriented magnetic steel strip, may comprise a continuous casting installation, a first rolling mill train, a heating device, a second rolling mill train, a cooling section and a winding-up device, wherein the first rolling mill train is arranged directly downstream of the continuous casting installation, and a coiling furnace or a continuous furnace for introducing heat and/or maintaining the temperature of the hot strip is located between the heating device and the second rolling mill train.

According to a further embodiment of the installation, the continuous casting installation may be in the form of a thin-slab continuous casting installation. According to a further embodiment of the installation, the first rolling mill train may comprise up to four rolling stands. According to a further embodiment of the installation, the second rolling mill train may comprise 2 to 6, preferably 3 to 5, rolling stands.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features will become apparent from the following description of non-limiting exemplary embodiments, where reference is made to the following figures:

FIG. 1 is a schematic illustration of a combined casting/rolling installation for the discontinuous production of hot-rolled strip for further processing to form grain-oriented steel sheets,

FIG. 2 is a schematic illustration of a combined casting/rolling installation for the continuous production of hot-rolled strip for further processing to form grain-oriented steel sheets.

DETAILED DESCRIPTION

A high-quality hot-rolled strip of this type is understood to mean a hot strip in which the growth inhibitors are distributed in the hot strip in finely dispersed form and homogeneously. This object is achieved by a process in which the following process steps are carried out in the sequence given on a combined casting/rolling installation:

a) a steel having a chemical composition (in % by weight) of S±2 to 7%, C 0.01 to 0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%, P<0.1%, remainder Fe and impurities is melted;

b) a strand having a thickness of 25 to 150 mm is cast on a continuous casting installation;

c) a strip is formed by rolling in up to 4 roll passes immediately after the strand has been cast, wherein at least in one roll pass a degree of deformation is >30% or the total degree of deformation of all the passes is >50%;

d) the strip is heated to a final temperature of 1050 to 1250° C., preferably 1100 to 1180° C.;

e) the strip is finish-rolled in a second rolling mill train, and then

f) the strip is cooled and coiled.

In this production process, the formation of homogeneously distributed growth inhibitors present in finely dispersed form, specifically sulfides, nitrides and carbides of the elements Mn, Cu, Al but also Cr, is promoted by the melting of a specific steel alloy (step a) and the rolling of a strip, which follows immediately after a thin strand has been cast (step b), with high degrees of deformation (step c) on a first rolling mill train. The degree of deformation φ is defined as φ=h₀−h₁/h₀, where h₀ is the thickness before the deformation and h₁ is the thickness of the strip or strand after one or more deformation steps; in this application, the degree of deformation is given in %. The heating of the strip (step d) has the effect that the further precipitation of growth inhibitors is stopped, and precipitations which have already formed with given kinetics are dissolved again. When the temperature is reduced again during finish-rolling on a second rolling mill train (step e) and the subsequent cooling of the strip (step f), further homogeneously distributed growth inhibitors present in finely dispersed form are formed. The production process can either be carried out continuously, i.e. on the basis of a strand or an unseparated strip, or in discontinuous batch operation, i.e. on the basis of slabs.

In one embodiment of the production process, the final temperature after the strip has been heated is maintained for a duration t, where t>15 s, preferably >60 s. Owing to this measure, a relatively high proportion of precipitations which may already be present in coarse clusters in the strip is dissolved. It is not expedient to maintain the temperature for a time t where t>90 s, since all the precipitations are already present in dissolved form after this time.

In continuous operation, the final temperature of the strip is advantageously maintained in a continuous furnace, which, by way of example, is in the form of a gas-fired furnace or of an induction furnace. This makes it possible to maintain the temperature of the strip in a particularly compact manner in continuous operation.

In discontinuous batch operation, the final temperature of the strip is advantageously maintained by winding-up and unwinding in a coiling furnace. This makes it possible to maintain the temperature of the strip in a particularly compact manner in discontinuous operation.

In one embodiment of the process, the strip is finish-rolled in 2 to 6, preferably in 3 to 5, roll passes on a second rolling mill train. This makes it possible to produce common strip thicknesses in a particularly economical manner.

In the case of finish-rolling, it is expedient if the strip has a final rolling temperature of 900 to 1050° C. after the finish-rolling. This ensures that the strip is finish-rolled in a favorable temperature range.

A further embodiment consists in the fact that the strip is cooled to a coiling temperature of 300 to 600° C. by means of an intensive cooling step within max. 10 s, preferably within max. 6 s, after the finish-rolling.

A further embodiment variant of the process consists in the fact that, at the start of the intensive cooling step, the strip is cooled at a cooling rate which is twice as high, preferably three times as high, as the cooling rate at the end of the cooling step. This temperature regime ensures that the microstructure present after the finish-rolling is “frozen” as quickly as possible for the subsequent steps.

In terms of the formation of growth inhibitors, it is advantageous for the sum of the alloying elements Cu+Mn in the steel melt to be >0.35% by weight, preferably >0.55% by weight. For the formation of a sufficiently high number of growth inhibitors, it is advantageous for the sum of the alloying elements S+N in the steel melt to be >100 ppm, preferably >200 ppm. An adequate quantity of Cu, Mn, S and N in the steel melt is advantageous in order to make it possible for a sufficient quantity of growth inhibitors to be precipitated into the hot strip.

The quotient of the alloying elements Cu/Mn in the steel melt is advantageously >2.5, preferably >3.5. Since Cu sulfides have a smaller size and a lower precipitation temperature than Mn sulfides, and are therefore to be preferred, it is advantageous if the steel melt contains more Cu than Mn. Since, however, Mn has more affinity to S than Cu, a “surplus” of Cu has to be present, in order to make it possible to form a higher quantity of Cu sulfides than Mn sulfides.

According to a further embodiment, for continuous operation the first rolling mill train is arranged directly downstream of the continuous casting installation, and a continuous furnace for introducing heat and/or maintaining the temperature of the hot strip is located between the heating device and the second rolling mill train. This configuration of the installation makes it possible to carry out the process according to various embodiments in a particularly economical manner with a high product quality, i.e. high production performance (continuous operation), low energy costs (amount of energy used to heat the hot strip is minimized) and low capital costs (compact installation).

One embodiment of the combined casting/rolling installation consists in the fact that the continuous casting installation is in the form of a thin-slab continuous casting installation. A further embodiment consists in the fact that the first rolling mill train comprises up to four rolling stands. A further embodiment consists in the fact that the second rolling mill train comprises 2 to 6, preferably 3 to 5, rolling stands. As a result of these measures, the capital costs for the first rolling mill train and the second rolling mill train are kept low (common hot-strip thicknesses can be produced on a few rolling stands).

Exemplary Embodiment 1

FIG. 1 shows a combined casting/rolling installation 1 for producing hot-rolled strip from silicon-alloyed steels; the installation parts for the further processing of the hot strip to form a grain-oriented magnetic steel strip are not shown. The states, i.e. the temperatures and thicknesses, of the strand or strip in the individual process steps are shown in table I; the states are denoted as P1 to P15. In a continuous casting installation 2 for producing thin slabs, a strand 3 having a thickness of 90 mm is cast from a specific steel alloy, consisting (in % by weight) of Si 3.2%, C 0.08%, Mn 0.1%, Cu 0.3%, Sn 0.08%, S 0.01%, Al 0.03%, Cr 0.1%, N 0.012%, P 0.05%, remainder Fe and impurities. Immediately after full solidification (strand temperature 1174° C., state P1), the strand 3 is subjected to a first rolling step, consisting of 2 roll passes, on a first rolling mill train 5. Here, the individual degrees of deformation are in each case 53% and 52%, i.e. first a strip having a thickness of 42 mm (state P2) and then a strip having a thickness of 20 mm (state P3) are rolled. The temperature of the strip after the first pass is 1171° C., and after the second pass is 1086° C. This first rolling step promotes the formation of homogeneously distributed clusters of growth inhibitors present in finely dispersed form, specifically sulfides, nitrides and carbides of the elements Cu, Al, Mn and Cr, in the strip, as a result of which further grain growth is inhibited. After the first rolling step, a roller table is used to convey the strip 4 to a heating device 6, in the form of an induction furnace, in which the incoming strip cooled to 944° C. (state P4) is heated to a final temperature of 1150° C. (state P5). Then, the temperature of the strip is maintained in a coiling furnace 7 (temperature on entry into the coiling furnace is 1134° C., state P6) for at least 30 s. The residence time of a strip region, the so-called local residence time, is different depending on the strip position. Owing to the winding-up and unwinding of the strip, it is the case, for example, that the head of the strip—present before the winding—remains in the coiling furnace for a longer time than the end of the strip; in this sense, the head of the strip present before the winding becomes the end of the strip, and vice versa. The heating of the strip 4 prevents precipitation of growth inhibitors until the strip is finish-rolled in a second rolling mill train 8; by maintaining the temperature for a time t, coarse clusters of growth inhibitors are dissolved, which are formed again in finely distributed form when the temperature is again reduced during the finish-rolling. After the near-net strip has been wound up and unwound in the coiling furnace 7, scale is removed from the strip by means of a descaling installation 12, as a result of which the temperature of the strip drops from 1101° C. to 1070° C. (temperatures before and after the descaling, states P7 and P8). Then, the strip is finish-rolled to a hot-strip final thickness of 2.6 mm on a second rolling mill train 8 in four roll passes (individual degrees of deformation 55, 53, 28 and 16%, i.e. strip thicknesses of 9.1, 4.3, 3.1 and 2.6 mm, states P9 to P12). During these roll passes, the strip cools down from 1043° C., 1012° C. and 984° C. to a final rolling temperature of 955° C. after the last roll pass. After the finish-rolling, the strip is cooled on a cooling section 9 within 3 s after the last pass in the second rolling mill train 8 from 932° C. (cooling section entry, state P13) to a temperature of 560° C. at the exit from the cooling section (state P14). During the finish-rolling and cooling of the strip, the clusters of growth inhibitors present in the strand are precipitated in finely dispersed form, i.e. with a typical cluster size <60 nm. After the hot strip has been cut by means of shears 10, the strip is wound up in a winding-up device 11; the winding temperature is 540° C. here (state P15). In subsequent production steps (not shown in more detail), the present hot strip is annealed, rolled to the final thickness in a cold-rolling mill train, decarburized and subjected to targeted secondary recrystallization.

Exemplary Embodiment 2

FIG. 2 shows a further combined casting/rolling installation for the continuous production of hot-rolled strip from silicon-alloyed steels; the installation parts for the further processing of the hot strip to form a grain-oriented magnetic steel strip are again not shown. The states P1 to P5 and P7 to P15 of the strand or strip in the individual process steps can be gathered from table I. Here, in turn, a specific steel alloy (see exemplary embodiment 1 for the chemical composition) is melted and a strand 3 is cast therefrom in a continuous casting installation 2 (state P1). Immediately after full solidification, the strand is subjected to a first rolling step, consisting of 2 roll passes, on a first rolling mill train 5 (states P2 and P3). Then, the strip 4 is heated in a heating device 6, in the form of an induction furnace (states P4 and P5). The significant difference with respect to exemplary embodiment 1 is that the temperature of the strip 4 is then maintained for at least 15 s after heating in a continuous furnace 13, in the form of a gas-fired furnace; the local residence time in the continuous furnace is constant for all strip regions (head of the strip, foot of the strip). The further process steps (descaling P7 to P8, finish-rolling P9 to P12, cooling P13 to P14 and coiling P15) can be gathered from exemplary embodiment 1.

	TABLE I
		Thickness	Temp.
	Location	[mm]	[° C.]
P1	End of the combined casting/rolling	90	1174
	installation
P2	After the first pass in the first	42	1171
	rolling mill train
P3	After the second pass in the first	20	1086
	rolling mill train
P4	Entry to the heating device	20	944
P5	Exit from the heating device	20	1150
P6	Entry to the coiling furnace	20	1134
P7	Entry to the descaling installation	20	1101
P8	Exit from the descaling installation	20	1070
P9	After the first pass in the second	9.1	1043
	rolling mill train
P10	After the second pass in the second	4.3	1012
	rolling mill train
P11	After the third pass in the second	3.1	984
	rolling mill train
P12	After the fourth pass in the second	2.6	955
	rolling mill train
P13	Entry to the cooling section	2.6	932
P14	Exit from the cooling section	2.6	560
P15	In the winding-up device	2.6	540

LIST OF REFERENCE NUMERALS

• 1 Combined casting/rolling installation
• 2 Continuous casting installation
• 3 Strand
• 4 Strip
• 5 First rolling mill train
• 6 Heating device
• 7 Coiling furnace
• 8 Second rolling mill train 9 Cooling section
• 10 Shears
• 11 Winding-up device
• 12 Descaling installation
• 13 Continuous furnace

Claims

1. A process for producing hot-rolled strip from silicon-alloyed steels on a combined casting/rolling installation for further processing to form grain-oriented magnetic steel strip, comprising the following process steps in the sequence given: a) melting a steel having a chemical composition (in % by weight) of Si 2 to 7%, C 0.01 to 0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%, P<0.1%, remainder Fe and impurities;b) casting a strand having a thickness of 25 to 150 mm on a continuous casting installation;c) rolling a strip in up to 4 roll passes immediately after the strand has been cast, wherein at least in one roll pass a degree of deformation is >30% or the total degree of deformation of all the passes is >50%;d) heating the strip to a final temperature of 1050 to 1250° C. or 1100 to 1180° C.;e) finish-rolling the strip in a second rolling mill train, and thenf) cooling and coiling the strip.

2. The process according to claim 1, wherein the final temperature after the strip has been heated is maintained for a duration t, where t>15 s or >60 s.

3. The process according to claim 2, wherein the final temperature of the strip is maintained in a continuous furnace.

4. The process according to claim 1, wherein the final temperature of the strip is maintained during winding-up and subsequent unwinding in a coiling furnace.

5. The process according to claim 1, wherein the strip is finish-rolled in 2 to 6 or 3 to 5 roll passes in the second rolling mill train.

6. The process according to claim 1, wherein the strip has a final rolling temperature of 900 to 1050° C. after the finish-rolling.

7. The process according to claim 1, wherein the strip is cooled to a coiling temperature of 300 to 600° C. by means of an intensive cooling step within 10 s or within 6 s after the finish-rolling.

8. The process according to claim 7, wherein, at the start of the intensive cooling step, the strip is cooled at a cooling rate which is twice as high or three times as high as the cooling rate at the end of the cooling step.

9. The process according to claim 1, wherein the sum of the alloying elements Cu+Mn in the steel melt is >0.35% by weight or >0.55% by weight.

10. The process according to claim 1, wherein the sum of the alloying elements S+N in the steel melt is >100 ppm or >200 ppm.

11. The process according to claim 1, wherein the quotient of the alloying elements Cu/Mn in the steel melt is >2.5 or >3.5.

12. A combined casting/rolling installation for producing hot-rolled strip from silicon-alloyed steels for further processing to form grain-oriented magnetic steel strip, comprising a continuous casting installation, a first rolling mill train, a heating device, a second rolling mill train, a cooling section and a winding up device, wherein the first rolling mill train is arranged directly downstream of the continuous casting installation, and a coiling furnace or a continuous furnace for at least one of introducing heat and maintaining the temperature of the hot strip is located between the heating device and the second rolling mill train.

13. The installation according to claim 12, wherein the continuous casting installation is in the form of a thin-slab continuous casting installation.

14. The installation according to claim 12, wherein the first rolling mill train comprises up to four rolling stands.

15. The installation according to claim 12, wherein the second rolling mill train comprises 2 to 6 or 3 to 5 rolling stands.

16. A system for producing hot-rolled strip from silicon-alloyed steels and further processing to form grain-oriented magnetic steel strip, comprising: a) means for melting a steel having a chemical composition (in % by weight) of S±2 to 7%, C 0.01 to 0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%, P<0.1%, remainder Fe and impurities;b) means for casting a strand having a thickness of 25 to 150 mm;c) means for rolling a strip in up to 4 roll passes immediately after the strand has been cast, wherein at least in one roll pass a degree of deformation is >30% or the total degree of deformation of all the passes is >50%;d) means for heating the strip to a final temperature of 1050 to 1250° C. or 1100 to 1180° C.;e) means for finish-rolling the strip, andf) means for cooling and coiling the strip.

17. The system according to claim 16, wherein the system is configured to maintain the final temperature after the strip has been heated for a duration t, where t>15 s or >60 s.

18. The system according to claim 17, wherein the system is configured to maintain the final temperature of the strip in a continuous furnace.

19. The system according to claim 16, wherein the system is configured to maintain the final temperature of the strip during winding-up and subsequent unwinding in a coiling furnace.

20. The system according to claim 16, wherein the system is configured to finish-roll the strip in 2 to 6 or 3 to 5 roll passes in the second rolling mill train.