WO2021136336A1

WO2021136336A1 - Method for producing boron-added steel based on twin-roll strip casting

Info

Publication number: WO2021136336A1
Application number: PCT/CN2020/141215
Authority: WO
Inventors: Xinyuan Liu; Qingxiao FENG; Huayue SHI; Aihua Chen; Zhiqiao LIU; Hualong LI; Dongsheng Zhou; Yujun Liu; Yihai ZHU; Yiqing ZHAI; Yixin Shi; Yi Ma
Original assignee: Jiangsu Shagang Group Co., Ltd.; Institute Of Research Of Iron & Steel, Jiangsu Province/Sha-Steel, Co., Ltd.; Zhangjiagang Zhongmei Ucs Technology Co., Ltd.
Priority date: 2019-12-30
Filing date: 2020-12-30
Publication date: 2021-07-08
Also published as: CN111014603A; CN111014603B

Abstract

A method of twin-roll casting a boron-added steel having a composition of C: ≤ 0.03 wt. %, Si:≤ 0.3 wt. %, Mn: 0.3-0.8 wt. %, P: ≤0.02 wt. %, N:≤ 0.008 wt. %, S:≤ 0.003 wt. %, B: 0.0008-0.0015 wt. %, the balance being Fe and unavoidable impurities. The method includes the steps of: smelting the steel composition; strip casting the molten steel into a cast strip through a twin-roll caster; online one-pass hot rolling the cast strip into a hot-rolled strip with a certain thickness with a reduction of 25-60%; obtaining a steel strip with a polygonal ferrite microstructure after cooling and coiling. The boron-added steel has a tensile strength of at least 270 MPa and an elongation of at least 21%.

Description

METHOD FOR PRODUCING BORON-ADDED STEEL BASED ON TWIN-ROLL STRIP CASTING

Technical field

The present invention relates to iron and steel metallurgy, and particularly relates to a boron-added steel with excellent surface quality and mechanical properties that is produced by a continuous twin-roll casting apparatus and a production method thereof.

Background

Boron-added steel refers to steel with boron as a main alloy element. Boron can significantly improve steel hardenability and result in excellent mechanical properties.

At present, the main processes for producing boron-added steel include the following steps: converter or electric furnace → refining → continuous casting of thick slab or thin slab → heating in heating furnace → rolling → coiling. These processes have long production times and require large resource consumption and large waste water discharge.

Boron can improve a number of properties of steel.

However, boron also greatly increases crack sensitivity of steel and is a disadvantage of this basis.

Usually, to reduce crack sensitivity, Al or Ti is added to steel. These elements fix N in steel, so as to avoid the formation of BN and reduce the enrichment of BN at grain boundaries.

For example, Chinese patent application CN201710640124. X (CN107502826A) in the name of Rizhao Steel Holding Group Co Ltd discloses a method of producing boron-added steel of ASTM A572 steel, Gr50 grade based on thin slab continuous casting and rolling process, with the chemical composition of A572Gr50 containing Ti to reduce crack problems in thin slabs.

Chinese patent application CN201410311812.8 (CN105316558A) in the name of Benxi Steel Plate Co Ltd discloses a preparation method for boron-added steel for preventing corner cracking in cast billets, also discloses the use of Ti in the steel composition to fix N in steel to reduce cast billet cracking.

Although Ti can reduce the problem of slab cracking, it also increases production costs. The addition of Ti, TiO _x and TiN during continuous casting also increases the potential for blocking slab and billet casting nozzles, affecting the castability of molten steel, causing large fluctuations of liquid level of mould, and then affecting the slab/billet quality and product quality.

The above discussion of the prior art is not to be taken as an admission of the common general knowledge.

Summary

According to the present invention a twin-roll continuous casting production line is used in a method for continuous casting and hot rolling boron-added steel that does not have Ti in the steel composition, with the method producing steel strip products with excellent mechanical properties and surface quality.

The method of the present invention includes adjusting twin roll casting parameters such as casting pool atmosphere parameters, casting speed, hot box atmosphere parameters and other production parameters to suit the use of boron-added steel that does not have Ti in the steel composition.

The present invention discloses a method of producing a boron-added steel based on twin-roll strip casting, which includes the following steps:

1) smelting

smelting to obtain a molten steel having a composition of C: ≤ 0.03 wt. %, Si: ≤ 0.3 wt. %, Mn: 0.3-0.8 wt. %, P: ≤ 0.02 wt. %, N: ≤ 0.008 wt. %, S: ≤ 0.003 wt. %, B: 0.0008-0.0015 wt. %, the balance being Fe and unavoidable impurities;

2) strip casting

strip casting the molten steel into a cast strip with a thickness of 1.4-2.0 mm through a tundish, a transition piece, and a twin-roll caster;

3) online hot rolling

online hot rolling the cast strip into a hot-rolled strip with a thickness of 0.8-1.8 mm by one-pass hot rolling after passing cast strip from the twin-roll caster through a hot box protected by nitrogen; and

4) cooling and coiling

cooling the hot-rolled strip and coiling at a coiling temperature in a range of 600-720℃.

The unavoidable impurities may include impurities that are present as a result of the use of scrap steel being used in step 1) .

The method of producing a boron-added steel may include controlling a casting temperature of the molten steel to be in a range of 1600-1690℃.

The method of producing a boron-added steel may include controlling a casting speed in step 2) to be in a range of 40-80 m/min.

The method of producing a boron-added steel may include protecting a molten pool in step 2) by an inert gas such as nitrogen or argon.

The method of producing a boron-added steel may include protecting a molten pool in step 2) by controlling pressure in a space enclosing the molten pool to be in a range of 2-50 Pa higher than an outside atmospheric pressure.

The method of producing a boron-added steel may include protecting a molten pool in step 2) includes passing molten steel through a tundish and a transition piece to the twin-roll caster.

The method of producing a boron-added steel may include controlling a nitrogen flow rate to the hot box in step 3) to be in a range of 1000-3000 m ³/hr.

The method of producing a boron-added steel may include maintaining a temperature of the cast strip in step 3) in a range of 1200-1400℃.

The method of producing a boron-added steel may include controlling the hot rolling reduction of the cast strip in step 3) to be in a range of 25-60%.

The method of producing a boron-added steel may include coiling the hot rolled step in step 4) at a coiling temperature in a range of 620-700℃.

The present invention also discloses a boron-added steel strip produced by the above method of twin roll casting and hot rolling a boron-added steel.

The boron-added steel strip may have a microstructure that includes irregular polygonal ferrite.

The boron-added steel strip may have a tensile strength of at least 250 MPa, typically at least 270 MPa.

The boron-added steel strip may have an elongation rate of at least 20%, typically at least 21%.

Beneficial technical effects

Compared with the prior art described above, the characteristics and beneficial technical effects of the present invention at least include:

1) The production technology of twin-roll strip casting and rolling rather than the above-described main processes makes it possible to produce strip in a short production process and at a low production cost and alloy cost.

2) The boron-added steel does not need Ti and by controlling production parameters such as pool atmosphere, casting speed and hot box atmosphere, twin roll cast boron-added steel product can be produced with at least substantially no cracks, with excellent mechanical properties, including a tensile strength of ≥270MPa and an elongation rate is ≥ 21%, and excellent strip surface quality.

Brief description of the drawings

The present invention is described further by way of example with reference to the accompanying drawings, noting that the drawings and following description of the drawings are illustrative only of the embodiments of the present invention, rather than limitation to the present invention.

FIG. 1 is a diagrammatical side view of an embodiment of a twin roll caster that is suitable for use in the method of the invention;

FIG. 2 is an enlarged partial sectional view of a portion of the twin roll caster of FIG. 1;

FIG. 3 is a schematic view of a portion of twin roll caster of Fig. 2; and

FIG. 4 illustrates a metallographic structure of boron-added steel strip produced according to an embodiment of the present invention.

Detailed description

In order to make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, a clear and full description will be afforded below to the technical solutions of the embodiments of the present invention. The described embodiments are some of the embodiments of the invention, rather than all of the embodiments. All other embodiments obtained by the ordinary skilled in this art based on the described embodiments of invention without any creative endeavours fall into the protection scope of the invention.

Unless defined otherwise, the technical terms or scientific terminology as used in the present disclosure should take the meaning usually understood by the ordinary skilled in this art of invention.

Referring now to FIGS. 1, 2, and 3, a twin roll caster is illustrated that comprises a main machine frame 10 that stands up from the factory floor and supports a pair of counter-rotatable casting rolls 12 mounted in a module in a roll cassette 11. The casting rolls 12 are mounted in the roll cassette 11 for ease of operation and movement as described below. The roll cassette 11 facilitates rapid movement of the casting rolls 12 ready for casting from a setup position into an operative casting position as a unit in the caster, and ready removal of the casting rolls 12 from the casting position when the casting rolls 12 are to be replaced. There is no particular configuration of the roll cassette 11 that is desired, so long as it performs that function of facilitating movement and positioning of the casting rolls 12 as described herein.

The casting apparatus for continuously casting boron-added steel strip includes the pair of counter-rotatable casting rolls 12 having casting surfaces 13 laterally positioned to form a nip 18 there between. Molten steel is supplied from a ladle 13 through a metal delivery system to a metal delivery nozzle 17 (core nozzle) positioned between the casting rolls 12 above the nip 18. Molten steel thus delivered forms a casting pool 19 of molten metal above the nip 18 supported on the casting surfaces 13 of the casting rolls 12. This casting pool 19 is confined in the casting area at the ends of the casting rolls 12 by a pair of side closure plates, or side dams 20 (shown in dotted line in FIG. 3) . The upper surface of the casting pool 19 (generally referred to as the "meniscus" level) may rise above the lower end of the delivery nozzle 17 so that the lower end of the delivery nozzle 17 is immersed within the casting pool 19. The casting area includes the addition of a protective atmosphere above the casting pool 19 to inhibit oxidation of the molten metal in the casting area.

The ladle 13 typically is of a conventional construction supported on a rotating turret 40. For metal delivery, the ladle 13 is positioned over a movable tundish 14 in the casting position to fill the tundish 14 with molten metal. The movable tundish 14 may be positioned on a tundish car 66 capable of transferring the tundish 14 from a heating station (not shown) , where the tundish 14 is heated to near a casting temperature, to the casting position. A tundish guide, such as rails 39, may be positioned beneath the tundish car 66 to enable moving the movable tundish 14 from the heating station to the casting position.

The movable tundish 14 may be fitted with a slide gate 25, actuable by a servo mechanism, to allow molten steel to flow from the tundish 14 through the slide gate 25, and then through a refractory outlet shroud 15 to a transition piece or distributor 16 in the casting position. From the distributor 16, the molten steel flows to the delivery nozzle 17 positioned between the casting rolls 12 above the nip 18.

The side dams 20 may be made from a refractory material such as zirconia graphite, graphite alumina, boron nitride, boron nitride zirconia, or other suitable composites. The side dams 20 have a face surface capable of physical contact with the casting rolls 12 and molten metal in the casting pool 19. The side dams 20 are mounted in side dam holders (not shown) , which are movable by side dam actuators (not shown) , such as a hydraulic or pneumatic cylinder, servo mechanism, or other actuator to bring the side dams 20 into engagement with the ends of the casting rolls 12. Additionally, the side dam actuators are capable of positioning the side dams 20 during casting. The side dams 20 form end closures for the molten pool of steel on the casting rolls 12 during the casting operation.

FIG. 1 shows the twin roll caster producing the cast strip 21, which passes across a guide table 30 to a pinch roll stand 31, comprising pinch rolls 31A. Upon exiting the pinch roll stand 31, the thin cast strip 21 may pass through a hot rolling mill 32, comprising a pair of work rolls 32A, and backup rolls 32B, forming a gap capable of hot rolling the cast strip 21 delivered from the casting rolls 12, where the cast strip 21 is hot rolled to reduce the strip to a desired thickness, improve the strip surface, and improve the strip flatness. The work rolls 32A have work surfaces relating to the desired strip profile across the work rolls 32A. The hot rolled cast strip 21 then passes onto a run-out table 33, where it may be cooled by contact with a coolant, such as water, supplied via water jets 90 or other suitable means, and by convection and radiation. In any event, the hot rolled cast strip 21 may then pass through a second pinch roll stand 91 to provide tension of the cast strip 21, and then to a coiler 92. The cast strip 21 may be between about 1.3 and 2.0 millimeters in thickness before hot rolling.

At the start of the casting operation, a short length of imperfect strip is typically produced as casting conditions stabilize. After continuous casting is established, the casting rolls 12 are moved apart slightly and then brought together again to cause this leading end of the cast strip 21 to break away forming a clean head end of the following cast strip 21. The imperfect material drops into a scrap receptacle 26, which is movable on a scrap receptacle guide. The scrap receptacle 26 is located in a scrap receiving position beneath the caster and forms part of a sealed enclosure 27 as described below. The enclosure 27 is typically water cooled. At this time, a water-cooled apron 28 that normally hangs downwardly from a pivot 29 to one side in the enclosure 27 is swung into position to guide the clean end of the cast strip 21 onto the guide table 30 that feeds it to the pinch roll stand 31. The apron 28 is then retracted back to its hanging position to allow the cast strip 21 to hang in a loop beneath the casting rolls 12 in enclosure 27 before it passes to the guide table 30 where it engages a succession of guide rollers.

An overflow container 38 may be provided beneath the movable tundish 14 to receive molten material that may spill from the tundish 14. As shown in FIG. 1, the overflow container 38 may be movable on rails 39 or another guide such that the overflow container 38 may be placed beneath the movable tundish 14 as desired in casting locations. Additionally, an optional overflow container (not shown) may be provided for the distributor 16 adjacent the distributor 16.

The sealed enclosure 27 is formed by a number of separate wall sections that fit together at various seal connections to form a continuous enclosure wall that permits control of the atmosphere within the enclosure 27. Additionally, the scrap receptacle 26 may be capable of attaching with the enclosure 27 so that the enclosure 27 is capable of supporting a protective atmosphere immediately beneath the casting rolls 12 in the casting position. The enclosure 27 includes an opening in the lower portion of the enclosure 27, lower enclosure portion 44, providing an outlet for scrap to pass from the enclosure 27 into the scrap receptacle 26 in the scrap receiving position. The lower enclosure portion 44 may extend downwardly as a part of the enclosure 27, the opening being positioned above the scrap receptacle 26 in the scrap receiving position.

A rim portion 45 may surround the opening of the lower enclosure portion 44 and may be movably positioned above the scrap receptacle 26, capable of sealingly engaging and/or attaching to the scrap receptacle 26 in the scrap receiving position. The rim portion 45 may be movable between a sealing position in which the rim portion 45 engages the scrap receptacle 26, and a clearance position in which the rim portion 45 is disengaged from the scrap receptacle 26. Alternately, the caster or the scrap receptacle 26 may include a lifting mechanism to raise the scrap receptacle 26 into sealing engagement with the rim portion 45 of the enclosure 27, and then lower the scrap receptacle 26 into the clearance position. When sealed, the enclosure 27 and scrap receptacle 26 are filled with a desired gas, such as nitrogen, to reduce the amount of oxygen in the enclosure 27 and provide a protective atmosphere for the cast strip 21.

The enclosure 27 may include an upper collar portion 43 supporting a protective atmosphere immediately beneath the casting rolls 12 in the casting position. When the casting rolls 12 are in the casting position, the upper collar portion 43 is moved to the extended position closing the space between a housing portion 53 adjacent the casting rolls 12, as shown in FIG. 2, and the enclosure 27. The upper collar portion 43 may be provided within or adjacent the enclosure 27 and adjacent the casting rolls 12 and may be moved by a plurality of actuators (not shown) such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, and rotating actuators.

The casting rolls 12 are internally water cooled so that as the casting rolls 12 are counter-rotated, shells solidify on the casting surfaces 13, as the casting surfaces 13 move into contact with and through the casting pool 19 with each revolution of the casting rolls 12. The shells are brought close together at the nip 18 between the casting rolls 12 to produce a thin cast strip product 21 delivered downwardly from the nip 18. The thin cast strip product 21 is formed from the shells at the nip 18 between the casting rolls 12 and delivered downwardly and moved downstream as described above.

Embodiment 1

This embodiment is carried out according to the following method steps in accordance with one, but not the only embodiment of the invention:

1) smelting

a chemical composition of a molten steel is: C, 0.025 wt. %, Si, 0.16 wt. %, Mn, 0.35 wt. %, P, 0.012 wt. %, N, 0.005 wt. %, S, 0.0028 wt. %, B, 0.0009 wt. %, the balance is Fe and unavoidable impurities, wherein a casting temperature of the molten steel is controlled to be 1650℃.

2) strip casting

strip casting the molten steel into a cast strip with a thickness of 1.8 mm, wherein a casting speed is controlled to be 60 m/min, a molten pool atmosphere is protected by nitrogen, and a pressure is 10 Pa higher than the outside atmospheric pressure.

3) online hot rolling the cast strip

online hot rolling the cast strip into a hot-rolled strip with a thickness of 1.2 mm by one-pass hot rolling after passing through a hot box protected by nitrogen, wherein a nitrogen flow rate of the hot box is 1500 m ³/hr, and the temperature of the cast strip is maintained at 1300℃.

4) cooling and coiling

cooling the hot-rolled strip and coiling by a coiling temperature control mode, wherein the coiling temperature is controlled to be 700℃.

The mechanical properties of the boron-added steel obtained through the above steps were tested, and the tensile strength was found to be 340MPa and the elongation rate was found to be 32%. Surface inspection of the strip indicated that the surface quality of strip steel was good and there were no cracks in the surface.

Embodiment 2

1) smelting

a chemical composition of a molten steel is: C, 0.028 wt. %, Si, 0.16 wt. %, Mn, 0.8 wt. %, P, 0.015 wt. %, N, 0.004 wt. %, S, 0.0028 wt. %, B, 0.001 wt. %, the balance is Fe and unavoidable impurities, wherein a casting temperature of the molten steel is controlled to be 1630℃.

2) strip casting

strip casting the molten steel into a cast strip with a thickness of 1.4 mm, wherein a casting speed is controlled to be 80 m/min, a molten pool atmosphere is protected by argon, and a pressure is 5 Pa higher than the outside atmospheric pressure.

3) online hot rolling the cast strip

online hot rolling the cast strip into a hot-rolled strip with a thickness of 0.8 mm by one-pass hot rolling after passing through a hot box protected by nitrogen, wherein a nitrogen flow rate of the hot box is 2000 m ³/hr, and the temperature of the cast strip is maintained at 1350℃.

4) cooling and coiling

cooling the hot-rolled strip and coiling by a coiling temperature control mode, wherein the coiling temperature is controlled to be 620℃.

The mechanical properties of the boron-added steel obtained through the above steps were tested, and the tensile strength was found to be 430MPa and the elongation rate was found to be 25%. Surface inspection of the strip indicated that the surface quality of strip steel was good and there were no cracks in the surface.

The above description is merely an exemplary embodiment of the present invention, and it should be noted that those skilled in the art can also make several improvements and modifications, which should also be the scope of protection of the present invention, without departing from the principle of the present invention.

Claims

A method of producing a boron-added steel based on a twin-roll strip casting, the method comprising steps of:

1) smelting

smelting to obtain a molten steel having a composition of C: ≤0.03 wt.%, Si: ≤0.3 wt.%, Mn: 0.3-0.8 wt.%, P: ≤0.02 wt.%, N: ≤0.008 wt.%, S: ≤0.003 wt.%, B: 0.0008-0.0015 wt.%, the balance being Fe and unavoidable impurities;

2) strip casting

strip casting the molten steel into a cast strip with a thickness of 1.4-2.0 mm in a twin-roll caster;

3) online hot rolling

online hot rolling the cast strip into a hot-rolled strip with a thickness of 0.8-1.8 mm by one-pass hot rolling after passing cast strip from the twin-roll caster through a hot box protected by nitrogen;

4) cooling and coiling

cooling the hot-rolled strip and coiling at a coiling temperature control mode, wherein the coiling temperature is controlled to be in a range of 600-720℃.
The method according to claim 1, includes controlling a casting temperature of the molten steel in step 1) to be in a range of 1600-1690℃.
The method according to claim 1 or claim 2, includes controlling a casting speed in step 2) to be in a range of 40-80 m/min.
The method according to any one of the preceding claims, includes protecting a molten pool in step 2) by nitrogen or argon and controlling a pressure in a space enclosing the molten pool to be in a range of 2-50 Pa higher than an outside atmospheric pressure.
The method according to any one of the preceding claims, includes controlling a nitrogen flow rate to the hot box in step 3) to be in a range of 1000-3000 m ³/hr.
The method according to any one of the preceding claims, includes controlling the temperature of the cast strip in step 3) to be in a range of 1200-1400℃.
The method according to any one of the preceding claims, includes hot rolling the cast strip in step 3) to be in a reduction range of 25-60%.
A twin roll cast boron-added steel strip, produced by the method according to any one of the preceding claims.
The twin roll cast boron-added steel strip according to claim 8, having a microstructure that includes an irregular polygonal ferrite.
The twin roll cast boron-added steel strip according to claim 8 or claim 9, wherein the boron-added steel strip has a tensile strength of at least 270 MPa.
The twin roll cast boron-added steel strip according to claim 8 or claim 9, wherein the boron-added steel strip has an elongation rate of at least 21%.