ZA200506720B

ZA200506720B - A method for processing a steel product, and product produced using said method

Info

Publication number: ZA200506720B
Application number: ZA200506720A
Authority: ZA
Inventors: Der Winden Menno Rutger Van
Original assignee: Corus Technology Bv
Priority date: 2003-02-24
Filing date: 2004-02-13
Publication date: 2006-11-29
Also published as: US20070051439A1; JP2006520692A; AU2004213135A1; EP1599299B1; PT1599299E; ATE366149T1; DE602004007362D1; EP1599299A1; EP1449596A1; CN1767910A; WO2004073900A1; RU2005129721A; KR20050100701A; BRPI0407621A; MXPA05008979A; UA85550C2; BRPI0407621B1

Abstract

The invention relates to a method for processing a steel product, in which the steel product is passed between a set of rotating rolls of a rolling mill stand in order to roll the steel product. According to the invention, the rolls of the rolling mill stand have different peripheral velocities such that one roll is a faster moving roll and the other roll is a slower moving roll, and the peripheral velocity of the faster moving roll is at least 5% higher and at most 100% higher than that of the slower moving roll, and the thickness of the steel product is reduced by at most 15% per pass, and the rolling takes place at a maximum temperature of 1350 DEG C. <??>The invention also relates to a steel product produced using the method, and to the use of this steel product.

Description

A METHOD FOR PROCESSING A STEEL PRODUCT, AND

PRODUCT PRODUCED USING SAID METHOD

The invention relates to a method for processing a steel product, in which the steel product is passed between a * set of rotating rolls of a rolling mill stand. This rolling mill stand may be part of a rolling mill device consisting of one Or more rolling mill stands.

Rolling is a very standard operation for imparting desired dimensions and properties to metal in general and steel in particular. apart from obtaining the desired final geometry of the steel product, rolling also results in an improvement to the structure as a result of the metallurgical processes taking place during and after the rolling.

However, the conventional rolling, which for wide products is usually considered to be a plane strain compression process, results in a considerable change in thickness, which in some cases is undesirable or impossible. For example, in heavy construction it is necessary to have steel plate with 2a thickness of 60 to 150 mm for, inter alia, the production of off-shore platforms ox bridges.

Since cast steel slabs currently have a maximum thickness of less than 400 mm, the change in thickness caused by the rolling to 150 mm would only amount to approximately 60%.

Each pass through a conventional rolling mill stand usually results in a change in thickness of 10 to 30%.

The casting of slabs sometimes results in the formation of porosity in the slab, a characteristic which is inherent to the casting process. This porosity is closed up by the pressure applied as a result of the slabs being rolled a sufficient number of times. However, if it is necessary to form a plate with a very high thickness, the rolling only

CONFIRMATION COPY closes up the pores in the outermost layers of the slab, and not those in the core of the material. However. the oe pores in the core of the material are highly disadvantageous for the mechanical properties of the material, in particular for the toughness properties of the plate. Also, grain refinement only occurs in the outermost layers of the plate. To close up the pores by the application of pressure and to achieve grain refinement even in the core of the plate, the degree of rolling through the thick slab therefore has to be high, whereas the combination of starting thickness of the slab and final thickness of the steel product do often not allow a large thickness reduction.

It is possible to introduce a large equivalent strain into a product without imposing a large thickness reduction under laboratory conditions using small samples with the

Equal Channel Angular Extrusion (ECAE) method in which extreme shear strains are applied without changing the specimen’s dimension. In ECAE a billet is extruded through a die with two channels of equal cross-section that meet at an angle. Under ideal circumstances the billet is sheared on crossing the plane of intersection of the channels by an amount determined by the angle between the two channels. Since the cross section does not change during the process, it can be repeated thereby accumulating strain. However, this laboratory technique cannot be used for industrial production of steel products because of the very high process forces required, and the impossibility to up-scale this process for flat products of conventional dimensions.

It is an object of the invention to provide a method for introducing a large equivalent strain into the steel product without imposing an equivalent reduction in thickness of the product.

It is also an object of the invention to provide a method for processing a steel product which allows the properties of the product produced thereby to be improved.

Yet another object of the invention is to provide a method for processing a steel product which results in grain refinement in the product which is thereby produced.

Yet another object of the invention is to provide a method for processing continuously cast steel by means of which the properties of the slab or strip are improved.

It is another object of the invention to provide a method for processing a continuously cast steel slab or strip with which it is possible to close up pores in the cast material.

It is also an object of the invention to provide a steel product with improved mechanical properties which is produced with the aid of this method.

In the context of this invention, steel should be considered to comprise all ferrous alloys for example ultra-low carbon steels, low-carbon steels, medium to high carbon steels, electrical steels, and stainless steels. A steel product in the context of this invention comprises ingots, slabs, blooms, billets, bar, rod, strip and profiled sections.

One or more of these objects are addressed by a method for processing a continuously cast steel product, in which the steel is passed between a set of rotating rolls of a rolling mill stand in order to roll the steel product, wherein the rolls of the rolling mill stand have different peripheral velocities such that one roll is a faster moving roll and the other roll is a slower moving roll,

AMENDED SHEET DATED 24 OCTOBER 2006 wherein the peripheral velocity of the faster moving roll is at least 5% and at most 100% higher that that of the slower moving roll, wherein the thickness of the steel product is reduced by at most 15% for each pass. and wherein that the rolling takes place at a max imam temperature of 1350°C. as a result of the rolls being provided with a different peripheral velocity, shearing OCCUIS in the steel product and has been found to occur throughout the entire thickness of the product. It has been found that this requires a velocity difference of at least 5%. The shearing leads to pores in the continuously cast material being closed up to a considerable extent. This does not require a major change in thickness, but rather a change in thickness of at most 15% can suffice. preferably this thickness reduction is at most 8% and more preferable at most 5%. This is particularly advantageous in the ’ processing of those steel products where the dimensions of : the steel product at the start of the process do not allow a singificant reduction in the thickness direction, : pecause the thickness is substantially retained.

In addition, it is important that the rolling according to the invention can result in a grain refinement which occurs throughout the entire thickness of the rolled material, which is advantageous for the mechanical properties of the slab or strip. Inter alia, the strength of the material increases. The peneficial effects of smaller grain sizes are commonly known.

The rolling is preferably carried out at an elevated temperature. However, the maximum temperature is limited to 1350°C because the formation of low melting oxides on the surface of the steel product to be produced has to be avoided. The elevated temperature makes the rolling run more smoothly.

It is also expected that the processing according to the invention will result in a rolled sheet with less lateral spread.

The peripheral velocity of the faster moving roll is preferably at most 50% higher and more preferably at most 20% higher than that of the slower moving roll. If there is a high difference in velocity, there is a considerable risk of slipping between the rolls and the steel product, which would result in uneven shearing.

According to an advantageous embodiment, the rolling mill is designed in such a manner that the rolls have different diameters. This makes it possible to obtain the desired difference in peripheral velocity.

According to another advantageous embodiment, the rolls have a different rotational speed. This too makes it possible to obtain the desired difference in rotational speed.

It is also possible for these latter two measures to be combined, i.e. rolls with different diameters and different rotational speeds in order to obtain the desired difference in peripheral velocity of the rolls.

According to an advantageous embodiment of the method, the . 30 steel product is introduced between the rolls at an angle of between 5 and 45° with respect to the perpendicular to the plane through the center axes of the rolls.

Introducing the steel product between the rolls at an angle makes it easier for the rolls to grip the steel product, with the result that the change in thickness can be kept as low as possible. Experiments have also shown that after rolling the steel product has an improved straightness if it is introduced at an angle between the rolls. The steel product is preferably fed in at an angle of between 10 and 25°, and more preferably at angle of between 15 and 25°, since with such an angle the steel product comes out of the rolling mill with a good level of straightness. It should be noted that the latter effect is also dependent on the reduction in the size of the steel product, the type of steel product and the alloy and the temperature.

For this purpose, after the rolling has been carried out for the first time, the processing operating is preferably repeated one or more times. For example, sufficiently good grain refinement is obtained by carrying out the processing operating according to the invention three times. However, the number of times that the processing operation has to be carried out depends on the thickness of the steel product, the difference in peripheral velocity of the rolls and the desired grain refinement. It is desirable for the steel product to be introduced between the rolls at an angle of between 5 and 45°, preferably between 10 and 25° and more preferably between 15 and 25° during each processing operation. 25 .

If the processing operation according to the invention is repeated a number of times, according to an advantageous embodiment the steel product can be passed through the rolling mill stand in opposite directions for each pass. 10 The steel product then changes direction after each rolling operation and is always passed through the same rolling mill stand. In this case, the rolls have to rotate in opposite directions for each pass. In this case too, it is desirable for the steel product in each case to be introduced at an angle between the rolls.

according to another advantageous embodiment, the steel product is successively passed through two or more rolling mill stands. This method is suitable primarily for strip material, which in this way can undergo the desired processing operation very quickly.

According to a preferred embodiment of the invention the rolling is carried out on a steel product of which at least a skin layer has a substantially austenitic structure, and preferably on a steel product having a substantially austenitic structure throughout. Typical minimum temperatures range from 900 oc for an ultra low carbon steel to 800-870 °c for a low carbon steel (depending on the chemical composition of course) to about 723 °C for a steel with 0.8 %C. In all cases the maximum temperature is 1350 °C. In case of rolling an austenitic stainless steel, the rolling always takes place on an austenitic structure.

According to a second preferred embodiment the rolling is carried out on a steel product of which at least a skin layer has a substantially austenitic-ferritic two-phase structure, and preferably on a steel product having a substantially austenitic-ferritic two-phase structure throughout. Typical temperatures range for a low carbon steel from 723 °C ending at 800-870 °C. The temperature range decreases with increasing carbon contents to reduce to an eutectoid point of about 723 oc for a steel with 0.8 %C.

According to a third preferred embodiment the rolling is carried out on a steel product of which at least a skin layer has a substantially ferritic structure, and preferably on a steel product having a gubstantially ferritic structure throughout. For a low carbon steel with a carbon content higher than 0.02% the maximum : temnperature is about 723 °C, whereas for steels with lower carbon contents such as ultra low carbon steels the

Claims

1. A method for processing a steel product, in which the steel product is passed between a set of rotating rolls of a rolling mill stand in order to roll the steel product, characterized in that the rolls of the rolling mill stand have different peripheral velocities such that one roll is a faster moving roll and the other roll is a slower moving roll, in that the peripheral velocity of the faster moving roll is at least 5% higher and at most 100% higher than that of the slower moving roll, in that the thickness of the steel product is reduced by at most 15% per pass, and in that the rolling takes place at a maximum temperature of 1350°C.

2. The method as claimed in claim 1, in which the thickness of the steel product is reduced by at most 8% each pass.

3. The method as claimed in claim 2, in which the thickness of the steel product is reduced by at most 5% each pass.

4. The method as claimed in claim 1, 2 or 3, in which the peripheral velocity of the faster moving roll is at most 50% higher than that of the slower moving roll.

5. The method as claimed in claim 4, in which the peripheral velocity of the faster moving roll is at most 20% higher than that of the slower moving roll.

6. The method as claimed in one of the preceding claims, in which the rolling mill is designed in such a manner that the rolls have different diameters.

7. The method as claimed in one of the preceding claims, in which the rolls have different rotational speeds.

8. The method as claimed in one of the preceding claims, in which the steel product is introduced between the rolls at an angle of between 5 and 45° with respect to the AMENDED SHEET DATED 24 OCTOBER 2006 perpendicular to the plane through the center axes of the rolls.

9. The method as claimed in claim 8, in which the steel product is introduced between the rolls at an angle of between 10 and 25° with respect to the perpendicular to the plane through the center axes of the rolls.

10. The method as claimed in claim 8, in which the steel product is introduced between the rolls at an angle of between 15 and 25° with respect to the perpendicular to the plane through the center axes of the rolls.

11.The method as claimed in one of the preceding claims, in which the rolling operation is repeated one or more times after the rolling has been carried out for the first time.

12.The method as claimed in claim 11, in which the steel product is passed through the rolling mill stand in opposite directions for each pass.

13.The method as claimed in claim 11, in which the steel product is successively passed through two or more rolling mill stands.

14. The method as claimed in one of the preceding claims, in which the rolling operation as described in one of claims 1 - 13 is preceded or followed by a rolling operation which 1s carried out using a rolling mill in which the rolls have substantially identical peripheral velocities.

15. The method as claimed in any of the claims 1 to 14, in which the rolling is carried out on a steel product of which at least a skin layer has a substantially austenitic structure.

16.The method as claimed in claim 15, in which the rolling is carried out on a steel product having a substantially austenitic structure throughout. AMENDED SHEET DATED 24 OCTOBER 2006

17.The method as claimed in any of the claims 1 to 14, in which the rolling is carried out on a steel product of which at least a skin layer has a substantially austenitic-ferritic two-phase structure.

18. The method as claimed in claim 17, in which the rolling is carried out on a steel product having a substantially austenitic-ferritic two-phase structure throughout.

19.The method as claimed in any of the claims 1 to 14, in which the rolling is carried out on a steel product of which at least a skin layer has a substantially ferritic structure.

20.The method as claimed in claim 19, in which the rolling is carried out on a steel product having a substantially ferritic structure throughout.

21.The method as claimed in any of the claims 1 to 14, in which the rolling is carried out while the temperature of the steel product is higher than 0°C and lower than 720°C.

22.The method according to claim 21, in which the rolling is carried out on a steel product having a substantially martensitic structure.

23.A method for producing a steel product comprising continuous casting of a steel strand and one or more of the following process steps: e¢ heating and/or temperature homogenising the steel strand between a casting machine and a rolling device; e rolling the steel product in one or more rolling mill stands of the rolling device with rolls having substantially identical peripheral velocities; e accelerated cooling after the last rolling step: AMENDED SHEET DATED 24 OCTOBER 2006 e cutting the steel product into slabs or coils before or after rolling; e coiling the steel product; and e¢ cooling the steel product characterised in that between casting the strand and accelerated cooling or «coiling or cooling, or after cooling, the steel product is subjected to the method of any one of the claims 1 - 14.

24.The method for producing a steel product according to claim 23 characterised in that the thickness of the cast strand is below 150 mm.

25.The method for producing a steel product according to claim 24 characterised in that the thickness of the cast strand is below 100 mm.

26.The method for producing a steel product according to claim 25 characterised in that the thickness of the cast strand is below 80 mm.

27.The method for producing a steel product according to claim 23 characterised in that the thickness of the cast strand is below 20 mm.

28. The method for producing a steel product according to claim 27 characterised in that the thickness of the cast strand is below 10 mm.

29.The method for producing a steel product according to claim 28 characterised in that the thickness of the cast strand is below 5 mm.

30.The method according to any one of claims 23 to 29, wherein the steel product that is produced is a stainless steel product. AMENDED SHEET DATED 24 OCTOBER 2006

31.The method for producing a steel product according to any one of claims 23 - 30 characterised in that the rolling is carried out on a steel product having a substantially austenitic structure, in that the steel is acceleratedly cooled thereafter, in that the steel product essentially comprises ferrite, bainite and/or martensite, and in that the ferrite content after cooling is at least 60%.

32.The method for producing a steel product according to claim 31 in that the ferrite content after cooling is more than 70%.

33.The method for producing a steel product according to claim 32 in that the ferrite content after cooling is more than 80%.

34.The method for producing a steel product according to any one of claims 23 - 33 wherein the average grainsize of the steel product is smaller than 5 um.

35.The method for producing a steel product according to claim 34 wherein the average drainsize of the steel product is smaller than 2um.

36.The method for producing a steel product according to claim 35 wherein the average dgrainsize of the steel product is smaller than 1 um.

37.The method according to any of the claims 1 - 36 wherein the steel product is subjected to a heat treatment before or after the rolling step.

38.The method according to «claim 37 wherein the heat treatment is a normalising treatment, a full anneal, a stress relief anneal or a speroidisation annealing treatment.

39.The method as claimed in any of the claims 1 - 36 wherein a surface of the steel product which is to be rolled is covered by one or more layers prior to rolling. AMENDED SHEET DATED 24 OCTOBER 2006

40.The method as claimed in claim 39 wherein the covering layer is a metal.

41.The method as claimed in claim 40 wherein the metal is another steel.

42.The method as claimed in claim 41 wherein the steel is a steel with a different composition or a stainless steel, Titanium, Nickel, Copper, Aluminium or alloys thereof.

43.A steel product produced according to the method of any of the claims 1 - 42, in which the starting point is a steel ingot, in which steel product the pores in the core of the product have a maximum dimension of less than 200 um.

44.A steel product as claimed in claim 43 in which the pores in the core of the product have a maximum dimension of less than 100 um.

45.A steel product as claimed in claim 44 in which the pores in the core of the product have a maximum dimension of less than 20 um.

46.A steel product as claimed in claim 45 in which the pores in the core of the product have a maximum dimension of less than 10 pm.

47.A steel plate, strip or billet produced by continuous casting, using the method as claimed in one of claims 1 - 14, in which the pores in the core of the plate, strip or billet have a maximum dimension of less than 200 um.

48.A steel plate, strip or billet as claimed in claim 47 in which the pores in the core of the plate, strip of billet have a maximum dimension of less than 100 um.

49.A steel plate, strip or billet as claimed in claim 48 in which the pores in the core of the plate, strip of billet have a maximum dimension of less than 20 um. AMENDED SHEET DATED 24 OCTOBER 2006

50.A steel plate, strip or billet as claimed in claim 49 in which the pores in the core of the plate, strip of billet have a maximum dimension of less than 10 um.

S51.A steel strip produced according to any one of claims 23 to 29, 34, 35 or 36 wherein the steel is a HSLA-steel comprising at least one of the elements niobium, titanium, vanadium or boron, or wherein the steel is an ultra low carbon steel.

52.A steel strip according to claim 51 wherein the steel is at least partly stabilised.

53.A steel strip according to claim 52 wherein the steel is partly stabilised with at least one of the elements titanium, niobium or boron. AMENDED SHEET DATED 24 OCTOBER 2006