Patent Description:
In the traditional process for steel production, tin (Sn) and copper (Cu) are typical residual elements or harmful elements in steel. It is very difficult and expensive to remove Sn and Cu fully during the steelmaking process. Generally, once the steel contains Sn and Cu, they cannot be eliminated thoroughly. Instead, the contents of Sn and Cu can only be reduced by diluting molten steel, which leads to an increased smelting cost for steel products.

In recent years, due to the repeated recycling of steel scrap, more and more steel scrap resources, and a continually decreased electricity price, short-flow steelmaking with an electric furnace based on steel scrap has risen and has been popularized domestically. As a result, the contents of Sn, Cu and other residual elements in the steel get higher and higher. Sn and Cu in steel are elements prone to segregation, and they may be enriched easily at grain boundaries to cause defects such as cracks. Therefore, the contents of Sn and Cu elements are controlled strictly in the traditional process. In common structural steel, definite requirements are imposed on the contents of both Sn and Cu: Sn (wt%) ≤<NUM>%; Cu (wt%) ≤<NUM>%.

Therefore, if the residual elements such as Sn and Cu in steel (especially steel scrap) can be utilized reasonably so as to "turn harm into benefit", it will have a positive influence on the entire metallurgical industry. Particularly, effective utilization of the existing steel scrap, or low quality or poor quality mineral resources (high tin ore, high copper ore) can be achieved; the recycling of steel can be promoted; the production cost can be reduced; and the sustainable development of the steel industry can be realized.

Traditional thin strip steel is mostly produced by multi-pass continuous rolling of a cast slab having a thickness of <NUM>-<NUM>. The traditional hot rolling process is: continuous casting + cast slab reheating and heat preservation + rough rolling + finish rolling + cooling + coiling. Particularly, a cast slab having a thickness of about <NUM> is firstly obtained by continuous casting; the cast slab is reheated and held; then, rough rolling and finish rolling are performed to obtain a steel strip having a thickness generally greater than <NUM>; and finally, laminar cooling and coiling are performed on the steel strip to complete the entire hot rolling production process. If a steel strip having a thickness of less than or equal to <NUM> is to be produced, it is relatively difficult, because subsequent cold rolling and annealing of the hot-rolled steel strip are generally necessary. In addition, the long process flow, the high energy consumption, the large number of unit devices, and the high capital construction cost result in high production cost.

The thin slab continuous casting and rolling process flow is: continuous casting + heat preservation and soaking of the cast slab + hot continuous rolling + cooling + coiling. The main differences between this process and the traditional process are as follows: the thickness of the cast slab in the thin slab process is greatly reduced to <NUM>-<NUM>. Because the cast slab is thin, the cast slab only needs to undergo <NUM>-<NUM> passes of rough rolling (when the thickness of the cast slab is <NUM>-<NUM>), or does not need to undergo rough rolling (when the thickness of the slab is <NUM>). In contrast, the continuous casting slab in the traditional process needs to be rolled repeatedly for multiple passes before it can be thinned to the required gauge before finish rolling. In addition, the cast slab in the thin slab process does not undergo cooling, but enters a soaking furnace directly for soaking and heat preservation, or a small amount of heat is supplemented. Hence, the thin slab process greatly shortens the process flow, reduces energy consumption, reduces investment, and thus reduces production cost. However, due to the fast cooling rate, the thin slab continuous casting and rolling process increases the steel strength and yield ratio, thereby increasing the rolling load, so that the thickness gauge of the hot-rolled products that can be economically produced cannot be too thin, generally ≥ <NUM>. See Chinese patents <CIT> (<CIT>), <CIT> (<CIT>) and <CIT> (<CIT>). Moreover, Sn and Cu elements are not involved in these patent applications.

The endless thin slab continuous casting and rolling process (ESP in short) rising in recent years is an improved process developed on the basis of the above semi-endless thin slab continuous casting and rolling process. The ESP realizes endless rolling for continuous casting of a slab, and eliminates the flame cutting of the slab and the heating furnace that is used for heat preservation, soaking and transition of slabs. The length of the entire production line is greatly shortened to about <NUM> meters. The slab produced by continuous casting with a continuous casting machine has a thickness of <NUM>-<NUM> and a width of <NUM>-<NUM>. The slab produced by continuous casting passes through an induction heating roll table to effect heat preservation and soaking on the slab. Then, the slab enters the rough rolling, finish rolling, laminar cooling, and coiling processes to obtain a hot-rolled plate. Since this process realizes endless rolling, a hot-rolled plate having a minimum thickness of <NUM> can be obtained, which expands the range of the gauge of hot-rolled plates. In addition, the output of a single production line can reach <NUM> million t/year. At present, this process has been developed and promoted rapidly, and there are a plurality of ESP production lines in operation around the world.

The thin strip continuous casting and rolling process has a shorter process flow than the thin slab continuous casting and rolling. The thin strip continuous casting technology is a cutting-edge technology in the research field of metallurgy and materials. Its appearance brings about a revolution to the steel industry. It changes the production process of steel strip in the traditional metallurgical industry by integrating continuous casting, rolling, and even heat treatment, so that the thin strip blank produced can be formed into a thin steel strip at one time after one pass of online hot rolling. Thus, the production process is simplified greatly, the production cycle is shortened, and the length of the process line is only about <NUM>. The equipment investment is also reduced accordingly, and the product cost is significantly reduced. It is a low-carbon, environmentally friendly process for producing a hot-rolled thin strip. The twin-roll thin strip continuous casting process is the main form of the thin strip continuous casting process, and it is also the only thin strip continuous casting process that has been industrialized in the world.

A typical process flow of twin-roll thin strip continuous casting is shown by <FIG>. The molten steel in a ladle <NUM> passes through a ladle shroud <NUM>, a tundish <NUM>, a submerged nozzle <NUM> and a distributor <NUM>, and is then directly poured into a molten pool <NUM> formed with side sealing devices 6a, 6b and two counter-rotating crystallization rolls 8a, 8b capable of rapid cooling. The molten steel solidifies on the circumferential surfaces of the rotating crystallization rolls 8a, 8b to form a solidified shell which gradually grows, and then forms a <NUM>-<NUM> thick cast strip <NUM> at the minimum gap (nip point) between the two crystallization rolls. The cast strip is guided by a guide plate <NUM> to pinch rolls <NUM> and sent to a rolling mill <NUM> to be rolled into a thin strip of <NUM>-<NUM>, and then cooled by a cooling device <NUM>. After its head is cut off by a flying shear <NUM>, it is finally sent to a coiler <NUM> to be coiled into a coil.

Martensite in carbon steel is formed by rapidly cooling or quenching austenite. Austenite has a special FCC crystal structure. Under natural cooling, austenite transforms into ferrite and cementite. However, under rapid cooling or quenching conditions, the austenite with the FCC crystal structure transforms into ferrite having a highly strained BCT crystal structure which is a supersaturated solid solution of carbon. The large number of dislocations caused by shear strain is the initial strengthening mechanism of this steel. The martensitic effect begins when the austenite reaches a temperature at which the martensitic transformation begins in the cooling process and the parent austenite becomes thermodynamically unstable. When the sample is quenched, the proportion of the austenite that transforms to martensite increases continuously until the transformation is completed at a lower transformation temperature.

Martensitic steel is used more and more often in certain fields that require high strength, such as automotive steel. A typical tensile strength of martensitic steel is generally in the range of <NUM>-<NUM> MPa. Martensitic steel is mainly used for safety components for body collision protection such as bumpers. In recent years, high-strength steel accounts for a yearly increasing fraction of the steel used in a vehicle. In the automotive industry, the use of thin-gauge, high-strength martensitic steel products provide broad room for weight reduction, energy saving and fuel economy improvement.

A thin strip continuous casting process is used to produce martensitic steel. Due to the thin thickness, the thin strip continuous casting process has strong manufacturing and cost advantages for a thin-gauge hot-rolled high-strength product having a thickness of less than or equal to <NUM>. The characteristic thicknesses of martensitic steel strip products directly supplied in hot-rolled state are <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, etc. Due to the small product thickness, many manufacturers are limited by the capacity of the traditional hot continuous rolling line in producing traditional thin-gauge martensitic steel. Generally, a hot continuous rolling process is used first for the production, followed by cold rolling, then entering a continuous annealing line for heating to an austenite phase region, and finally quenching to form martensitic steel. This production process increases the production cost for producing the thin-gauge martensitic high-strength steel.

When hot-rolled strip steel is used as a thin-gauge hot-rolled plate or a hot-rolled product in place of a cold-rolled product, high surface quality of the strip steel is required. It is generally required that the thickness of the oxide scale on the surface of strip steel should be as thin as possible. This requires control of the formation of the oxide scale on the cast strip in the subsequent stages. For example, in a typical twin-roll continuous casting process for thin strip steel, a closed chamber device is used from the crystallization rolls to the inlet of the rolling mill to prevent oxidation of the cast strip. Addition of hydrogen to the closed chamber device as disclosed in <CIT> (<CIT>) and control of the oxygen content to be less than <NUM>% in the closed chamber device as disclosed in <CIT> can both help to control the thickness of the oxide scale on the cast strip surface. However, there are few patents relating to how to control the thickness of the oxide scale in the conveying process from the rolling mill to the coiler, especially in the process of cooling the strip steel by laminar cooling or spray cooling. When the high-temperature strip steel is in contact with the cooling water, the thickness of the oxide scale on the surface of the cast strip grows rapidly. At the same time, the contact of the high-temperature strip steel with the cooling water may also cause many problems: first, water spots (rust spots) may be formed on the surface of the strip steel, which will affect the surface quality; second, cooling water for laminar cooling or spray cooling tends to cause local uneven cooling on the surface of the strip steel, resulting in a nonuniform microstructure inside the strip steel, so that the properties of the strip steel are not uniform and the product quality is affected; third, the local uneven cooling on the surface of the strip steel may cause deterioration of the strip shape, which affects the shape quality.

Moreover, the <CIT>, <CIT> and <CIT> disclose steel sheets/thin steel strips and their manufacturing methods according to prior art.

However, because the thin strip continuous casting process itself is characterized by rapid solidification, the steel produced by this process generally has problems such as nonuniform structure, low elongation, high yield ratio and poor formability. At the same time, the austenite grains in the cast strip are obviously not uniform, such that the structure of the final product obtained after austenite transformation is not uniform, either. Hence, the properties of the product are not stable. Therefore, it is difficult and challenging to use a thin strip continuous casting production line to produce high-strength products needed in the automobile industry and petrochemical industry. Therefore, when high-strength martensitic steel is to be produced by thin strip continuous casting, the production is impossible if the traditional composition and process are copied. Breakthrough in composition and process is necessary.

One object of the present invention is to provide a martensitic steel strip and a manufacturing method thereof, wherein a thin strip continuous casting process is employed to produce a hot-rolled thin-gauge martensitic steel strip, so that a good number of complicated intermediate steps in the traditional production of a martensitic steel strip may be obviated. A desired product thickness can be achieved by single-pass online hot rolling, and the product may be marketed directly for use without cold rolling, so as to achieve the purpose of "replacing cold rolling with hot rolling". According to the present invention, full use of Cu and Sn elements in steel scrap can be made to "turn harm into benefit" in terms of the Cu and Sn elements in the steel scrap, thereby promoting recycling of steel scrap resources, reducing production cost effectively, and realizing sustainable development of the steel industry. Compared with a traditional cold-rolled martensitic steel strip, the present invention achieves great reduction of energy consumption and CO<NUM> emission in production, and provides a green product.

To achieve the above object, the technical solution of the present invention is defined in claim <NUM>. Further improvements are subject to the dependent claims.

According to the present invention, residual elements such as Sn and Cu in steel scrap are utilized in smelting to produce molten steel, and micro-alloy elements such as Nb/Mo and the like as well as optional B are selectively added to the steel. In the smelting process, the basicity for slagging, the type and melting point of the inclusions in the steel, the free oxygen content in the molten steel, and the content of acid-soluble aluminum Als are controlled. Then, twin-roll thin strip continuous casting is performed to cast a cast strip having a thickness of <NUM>-<NUM>. After the cast strip exits crystallization rolls, it directly enters a lower closed chamber having a non-oxidizing atmosphere, and enters an on-line rolling mill for hot rolling under closed conditions. The rolled strip steel is rapidly quenched by gas atomization and cooled to <NUM> or less. The steel coil produced finally can be subjected to isothermal tempering treatment, or enters a continuous annealing production line for aging treatment. The gas atomization cooling can effectively reduce the thickness of the oxide scale on the surface of the strip steel, increase the temperature uniformity of the strip steel, and improve the surface quality of the strip steel.

In the chemical composition design of the martensitic steel strip according to the present invention:.

B is an active element that is prone to segregation, and it tends to segregate at the grain boundary. When B-containing steel is produced by the traditional process, the B content is generally controlled very strictly, usually around <NUM>-<NUM>%. In the thin strip continuous casting process, the solidification and cooling rate is fast. Hence, the segregation of B can be inhibited effectively, and more B can be solid dissolved. Therefore, the limitation to the B content can be relaxed appropriately. Coarse BN particles can also be produced by controlling the process appropriately to inhibit precipitation of fine AIN. In this way, B plays a role in nitrogen fixation. As shown by other studies, when B is added in combination with Nb and Mo, better effects can be achieved. Particularly, the possibility of segregation of C atoms may be decreased, and the precipitation of Fe<NUM>(C,B)<NUM> at grain boundaries may be avoided. Hence, it is possible to add more B. Therefore, when B is added, a higher B content is used in the present invention than in the traditional process, and the range is <NUM>-<NUM>%.

A manufacturing method for the martensitic steel strip according to the present invention comprises the following steps:.

Further, the manufacturing method also comprises step <NUM>): follow-up heat treatment, wherein the steel coil produced is subjected to isothermal tempering treatment at an isothermal tempering temperature of <NUM>-<NUM> for an isothermal tempering time of <NUM>-<NUM>; or the steel coil produced enters a continuous annealing production line for aging treatment at an aging temperature of <NUM>-<NUM> for an aging time of <NUM>-<NUM> in the continuous annealing.

Preferably, in step <NUM>), <NUM>% steel scrap may be selected as the raw material for the smelting without pre-screening, and an electric furnace is used for the smelting to produce molten steel. Alternatively, a converter is used for the smelting, wherein steel scrap is added to the converter in an amount of at least <NUM>% of the raw material for the smelting without pre-screening. Then, the molten steel is delivered to an LF furnace, VD/VOD furnace or RH furnace for refining.

Preferably, in step <NUM>), the non-oxidizing gas includes an inert gas, N<NUM>, or a mixed gas of CO<NUM> gas produced by sublimation of dry ice, N<NUM> and H<NUM>.

Preferably, in step <NUM>), the gas atomization cooling utilizes a gas-water ratio of <NUM>:<NUM>-<NUM>:<NUM>, a gas pressure of <NUM>-<NUM> MPa, and a water pressure of <NUM>-<NUM> MPa. As used herein, the gas-water ratio refers to the flow ratio of compressed air to water, and the unit of the flow is m<NUM>/h.

Preferably, in step <NUM>), the coiling utilizes double-coiler coiling or Carrousel coiling.

In the manufacturing method of the martensitic steel strip according to the present invention:
In order to improve the castability of the molten steel for thin strip continuous casting, the basicity a=CaO/SiO<NUM> for slagging in the steelmaking process is controlled at a<<NUM>, preferably a<<NUM>, or a=<NUM>-<NUM>.

In order to improve the castability of the molten steel for thin strip continuous casting, it is necessary to obtain a low-melting-point MnO-SiO<NUM>-Al<NUM>O<NUM> ternary inclusion, as shown in the shaded area in <FIG>. The MnO/SiO<NUM> in the MnO-SiO<NUM>-Al<NUM>O<NUM> ternary inclusion is controlled at <NUM>-<NUM>, preferably <NUM>-<NUM>.

In order to improve the castability of the molten steel for thin strip continuous casting, O is an essential element to form an oxide inclusion in the steel. Since it's necessary to form the low-melting-point MnO-SiO<NUM>-Al<NUM>O<NUM> ternary inclusion according to the present invention, the free oxygen content [O]Free in the molten steel is required to be in the range of <NUM>-<NUM>%.

In order to improve the castability of the molten steel for thin strip continuous casting, among the above components, Mn and S must be controlled to satisfy the following relationship: Mn/S><NUM>.

After the cast strip exits the crystallization rolls, the cast strip has a temperature of <NUM>-<NUM>, and it enters a lower closed chamber directly, wherein a non-oxidizing gas is supplied to the lower closed chamber, wherein an oxygen concentration in the lower closed chamber is controlled at <<NUM>%; wherein the anti-oxidation protection provided by the lower closed chamber to the cast strip extends to the inlet of the rolling mill; and wherein the cast strip has a temperature of <NUM>-<NUM> at an outlet of the lower closed chamber.

When B is present, the theoretical basis for precipitation of the BN phase in the cast strip occurring in the lower closed process is as follows:
The thermodynamic equations between boron and nitrogen, and between aluminum and nitrogen in γ-Fe in steel are as follows: <MAT> <MAT>.

As shown by <FIG>, the temperature at which BN begins to precipitate in the steel is around <NUM>, and the precipitation of BN levels off at <NUM>, while the precipitation of AIN has just begun (the temperature at which AIN begins to precipitate is around <NUM>). The precipitation of BN precedes AIN thermodynamically. Therefore, with the use of reasonable process control measures according to the present invention, the combination of B and N is completed in a lower enclosed chamber to generate coarse BN particles, thereby homogenizing the structure of austenite grains. This inhibits precipitation of fine AIN, and thus weakens the pinning effect of fine AIN on the grain boundary, so that the growth ability of grains is improved, and austenite grains are coarsened. As a result, subsequent martensite transformation is favored. In addition, the combination of B and N can effectively prevent appearance of the low-melting-point phase B<NUM>O<NUM> at the grain boundary.

Post-rolling cooling is performed on the on-line hot-rolled steel strip. Particularly, gas atomization rapid quenching is performed for the cooling to cool the strip steel to <NUM> or less. The gas atomization cooling process can effectively reduce the thickness of the oxide scale on the strip steel surface, improve the temperature uniformity of the strip steel, and promote the surface quality of the strip steel. The gas atomization cooling utilizes a gas-water ratio of <NUM>:<NUM>-<NUM>:<NUM>, a gas pressure of <NUM>-<NUM> MPa, and a water pressure of <NUM>-<NUM> MPa. After gas atomization, a high-pressure water mist is formed and sprayed on the surface of the steel strip. On the one hand, it plays a role in reducing the temperature of the steel strip. On the other hand, the water mist forms a dense gas film which covers the surface of the strip steel to protect the strip steel from oxidation, thereby effectively suppressing the growth of the oxide scale on the surface of the hot-rolled strip steel. With the use of this cooling process, the problems caused by traditional spraying or laminar cooling can be avoided, and the surface temperature of the strip steel can drop uniformly, so as to increase the temperature uniformity of the strip steel, and achieve the effect of homogenizing the internal microstructure. At the same time, the cooling is uniform, and the shape quality and performance stability of the strip steel can be improved. In addition, the thickness of the oxide scale on the surface of the strip steel can be reduced effectively. The cooling rate of the gas atomization rapid cooling is in the range of ≥<NUM>/s. The strip steel is cooled to <NUM> or less rapidly. After the cooling, the microstructure of the steel strip is martensite (M), or martensite (M) + bainite (B), wherein when the microstructure is martensite (M) + bainite (B), the volume fraction of martensite (M) is at least <NUM>%. In some embodiments, the cooling rate is in the range of <NUM>-<NUM>/s.

After the poor-quality head portion of the hot-rolled and cooled strip steel is cut off with a head shear, the strip steel is directly coiled into a coil. The coiling utilizes double-coiler coiling or Carrousel coiling to ensure continuous production of strip steel. The coiling temperature may be in the range of <NUM>-<NUM>.

Optionally, the steel coil produced may be subjected to isothermal tempering treatment, wherein the isothermal tempering temperature is <NUM>-<NUM>, and the isothermal tempering time is <NUM>-<NUM>; or the steel coil produced may enter a continuous annealing production line for aging treatment, wherein the aging temperature is <NUM>-<NUM>, and the aging time is <NUM>-<NUM> in the continuous annealing.

After the above manufacturing process, the final martensitic steel strip has a yield strength of <NUM>-<NUM> MPa, a tensile strength of <NUM>-<NUM> MPa, and an elongation of <NUM>-<NUM>%, for example <NUM>-<NUM>%. In some embodiments, the martensitic steel strip according to the present invention has a yield strength of <NUM>-<NUM> MPa, a tensile strength of <NUM>-<NUM> MPa, and an elongation of <NUM>-<NUM>%.

Compared with the prior art, the present invention has the following differences and improvements:
The most significant features which distinguish the present invention from the existing thin strip continuous casting technology include the roll diameter of the crystallization roll and the corresponding molten steel distribution mode. The technical feature of the EUROSTRIP technology is the crystallization rolls having a large diameter of Φ1500mm. Due to the large crystallization rolls together with the large capacity of the molten pool, it's easy to distribute the molten steel, but the cost for manufacturing the crystallization rolls and the cost for operation and maintenance are high. The technical feature of the CASTRIP technology is the crystallization rolls having a small diameter of Φ500mm. Due to the small crystallization rolls together with the small capacity of the molten pool, it's difficult to distribute the molten steel, but the cost for manufacturing the casting machine and the cost for operation and maintenance are low. In order to address the challenge of uniform distribution of molten steel in the small molten pool, CASTRIP adopts a three-stage system for dispensing and distributing molten steel (a tundish + a transition piece + a distributor). The use of a three-stage distribution system for molten steel leads to a direct increase in the cost of refractory materials. More importantly, the three-stage distribution system for molten steel extends the flow path of the molten steel, and the temperature drop of the molten steel is also larger. In order to achieve the required temperature of the molten steel in the molten pool, the tapping temperature needs to be increased greatly. The increased tapping temperature will lead to problems such as increased steelmaking cost, increased energy consumption and shortened life of refractory materials.

The crystallization rolls according to the present invention have a diameter of <NUM>-<NUM>, with crystallization rolls having a roll diameter of <NUM> being preferred. A two-stage system for dispensing and distributing molten steel (a tundish + a distributor) is adopted. The molten steel flowing out of the distributor forms different distribution patterns along the roll surfaces and the two side surfaces, and flows in two paths without interfering with each other. Due to the use of a two-stage distribution system, in contrast to a three-stage distribution system, the cost of refractory materials is reduced greatly; and the flow path of the molten steel is shortened, so that the temperature drop of the molten steel is reduced, and the tapping temperature can be lowered. Compared with the three-stage distribution system, the tapping temperature can be lowered by <NUM>-<NUM>. The decreased tapping temperature can effectively reduce the cost of steelmaking, save energy and prolong the life of refractory materials. The combined use of crystallization rolls having a preferred roll diameter of Φ800mm and a two-stage system for dispensing and distributing molten steel according to the present invention not only meets the requirement of stable distribution of molten steel, but also achieves the goals of simple structure, convenient operation and low processing cost.

The prior art production of thin strip products by thin strip continuous casting and the corresponding processes are reported in many patents. However, the production of B-containing martensitic steel by thin strip continuous casting has not been reported yet. Nevertheless, it is considered that the following patents are comparable to the present invention in terms of process control and equipment. The details are as follows:
<CIT> discloses a martensitic steel and a manufacturing method thereof. The following composition by weight percentage are used in this patent publication: C=<NUM>%-<NUM>%, Cr<<NUM>%, Mn=<NUM>%-<NUM>%, Si=<NUM>%-<NUM>%, Cu=<NUM>%-<NUM>%, Nb<<NUM> %, Mo<<NUM>%, Al<<NUM>%, and a balance of Fe and unavoidable impurities caused by smelting. Cu is mentioned among the chemical elements in this patent publication, but Sn and B are not mentioned. In the claims in this patent publication, the molten steel solidifies under a heat flow of higher than <NUM> MW/m<NUM> to form a steel strip having a thickness of <<NUM>, and the reduction rate of the on-line hot rolling is <NUM>-<NUM>%. Rapid cooling is performed after the rolling, so that the strip steel has a microstructure of martensite or martensite+bainite with a volume fraction of martensite being at least <NUM>%. It only mentions post-rolling rapid cooling, and it is silent on any way to achieve rapid cooling.

Chinese Patent Publication <CIT> discloses a method for preparing a martensitic steel having high strength and toughness by a thin strip casting, rolling and aging process. The following composition by weight percentage are used in this patent publication: C=<NUM>%-<NUM>%, Mn=<NUM>%-<NUM>%, Si=<NUM>%-<NUM>%, Mo=<NUM>-<NUM>%, V=<NUM>-<NUM>%, Nb= <NUM>-<NUM>%, Cr=<NUM>-<NUM>%, P<<NUM>%, S≤<NUM>%, and a balance of Fe and unavoidable impurities. The chemical elements in this patent publication do not include Cu, Sn, B, etc. An important feature of the patent publication is that the steel strip must be aged to improve the properties of the steel.

<CIT>, <CIT>, <CIT>, and Chinese Patent Applications <CIT> (<CIT>), <CIT> (<CIT>), <CIT> (<CIT>) disclose a method of producing a micro-alloyed thin steel strip having a thickness of <NUM>-<NUM> by using a thin strip continuous casting and rolling process. The chemical composition used in this method are C: ≤ <NUM>%, Mn: <NUM>-<NUM>%, Si: <NUM>-<NUM>%, Al: ≤ <NUM>%, and also at least one of Nb: <NUM>-<NUM>%, V: <NUM>-<NUM>%, and Mo: <NUM>-<NUM>%. Under the conditions of hot rolling reduction rate of <NUM>-<NUM>% and coiling temperature of ≤<NUM>, the microstructure of the hot-rolled strip is bainite + acicular ferrite. According to the above patent applications, the alloy elements mainly existing in a solid solution state in the cast strip inhibit recrystallization of austenite after hot rolling. Even if the reduction rate reaches <NUM>%, the recrystallization of austenite is also very limited. Since the hot rolling reduction rate of <NUM>-<NUM>% does not cause recrystallization of austenite, the hardenability of coarse austenite remains after hot rolling, so that the structure of bainite + acicular ferrite is obtained at room temperature. No temperature range used for hot rolling is disclosed by the above patent applications. Nevertheless, an article (C. Killmore, etc. Development of Ultra-Thin Cast Strip Products by the CASTRIP® Process. AIS Tech, Indianapolis, Indiana, USA, May <NUM>-<NUM>, <NUM>) related to these patent applications reports that the temperature used for hot rolling is <NUM>. Within the above composition system, the thin strip continuously cast low carbon micro-alloyed steel products produced by this method have a yield strength that can reach <NUM> MPa, a tensile strength that can reach <NUM> MPa, and an elongation of ≤<NUM>% or <NUM>%. The thin strip continuous casting process is generally followed by only <NUM>-<NUM> rolling mills, and the hot rolling reduction rate is usually difficult to exceed <NUM>%. Hence, deformation has little effect in refining grains. If austenite grains are not refined by recrystallization, the nonuniform austenite structure is difficult to be effectively improved after hot rolling, and the bainite + acicular ferrite structure produced by transformation of unevenly sized austenite is also very uneven, so the elongation is not high.

<CIT> (<CIT>) proposes another method for producing a micro-alloyed thin steel strip having a thickness of <NUM>-<NUM> using the thin strip continuous casting and rolling process. The micro-alloyed steel composition system used in this method comprises C: <NUM>-<NUM>%, Mn: <NUM>-<NUM>%, Si: <NUM>-<NUM>%, Al: ≤ <NUM>%, S: ≤ <NUM>%, P: ≤ <NUM>%, Cr: <NUM>-<NUM>%, Ni: <NUM>-<NUM>%, Mo: ≤<NUM>%, N: <NUM>-<NUM>%, and a balance of Fe and unavoidable impurities. The hot rolling of the cast strip is performed in the range of <NUM>-(Ar1-<NUM>) °C, corresponding to hot rolling in the austenite region, the austenitic-ferrite two-phase region, or the ferrite region. The hot rolling reduction rate is <NUM>- <NUM>%. In this method, an on-line heating system is designed to be positioned after the thin strip continuous casting and rolling unit, and the heating temperature range is <NUM>-<NUM>. The purpose is that, after the steel strip is hot rolled in different phase regions, the temperature of the steel strip can be held for a period of time to allow for complete recrystallization, so that the steel strip can obtain better matched strength and plasticity. When this method is used for production, it is necessary to add an on-line heating system in the design of the production line. Because the length of the heating time depends on the strip speed and the length of the heating furnace, the heating furnace must be long enough to ensure heating uniformity. This not only increases the investment cost, but also significantly increases the footprint of the thin strip casting and rolling line, reducing the advantages of the line.

The main advantages of the present invention include:.

The present invention will be further described with reference to the following examples, but these examples by no means limit the present invention.

Referring to <FIG>, the molten steel that conforms to the chemical composition designed according to the present invention passes through a ladle <NUM>, a ladle shroud <NUM>, a tundish <NUM>, a submerged nozzle <NUM> and a distributor <NUM>, and is then directly poured into a molten pool <NUM> formed with side sealing devices 6a, 6b and two counter-rotating crystallization rolls 8a, 8b capable of rapid cooling. The molten steel solidifies on the circumferential surfaces of the rotating crystallization rolls 8a, 8b to form a solidified shell which gradually grows, and then forms a <NUM>-<NUM> thick cast strip <NUM> at the minimum gap (nip point) between the two crystallization rolls. After the cast strip <NUM> exits the crystallization rolls 8a and 8b, the temperature of the cast strip is <NUM>-<NUM>, and the cast strip enters a lower closed chamber <NUM> directly. The lower closed chamber <NUM> is supplied with an inert gas to protect the strip steel, i.e. protecting the strip steel from oxidation. The anti-oxidation protective atmosphere may be N<NUM>, or Ar, or other non-oxidizing gas, such as CO<NUM> gas obtained by sublimation of dry ice. The oxygen concentration in the lower closed chamber <NUM> is controlled to be <<NUM>%. The anti-oxidation protection provided by the lower closed chamber <NUM> to the cast strip <NUM> extends to the inlet of the rolling mill <NUM>. The temperature of the cast strip at the outlet of the lower closed chamber <NUM> is <NUM>-<NUM>. Then, the cast strip is delivered to the hot rolling mill <NUM> through a swinging guide plate <NUM>, pinch rolls <NUM> and a roll table <NUM>. After hot rolling, a hot rolled strip of <NUM>-<NUM> in thickness is formed. Post-rolling cooling is performed with the use of a gas atomization rapid cooling device <NUM> to cool the strip steel to <NUM> or less to improve the temperature uniformity of the strip steel. After the head portion of the strip steel is cut off by a flying shear <NUM>, the cut head portion falls into a flying shear pit <NUM> along a flying shear guide plate <NUM>, and the hot-rolled strip with the head portion cut off enters a coiler <NUM> for coiling. After the steel coil is taken off the coiler, it is cooled in air to room temperature.

In addition, the steel coil produced may also be subjected to isothermal tempering treatment, or enters a continuous annealing production line for aging treatment.

The chemical compositions of the Examples according to the present invention are shown in Table <NUM>, wherein the balance is Fe and other unavoidable impurities. The process parameters of the manufacturing method according to the present invention are shown in Table <NUM>, and the mechanical properties of the hot-rolled strips obtained finally are shown in Table <NUM>.

To sum up, the martensitic steel manufactured from the steel composition in the designed scope provided according to the present invention using the thin strip continuous casting technology has a yield strength of <NUM>-<NUM> MPa, a tensile strength of <NUM>-<NUM> MPa, and an elongation of <NUM>-<NUM>%, for example <NUM>-<NUM>%. The cold working bending performance of the martensitic steel is qualified. It can be widely used in the field of high-strength automotive steel, such as safety components for body collision protection, for example, bumpers, anti-collision beams, etc. In the automotive industry, it provides broad room for weight reduction, energy saving and fuel economy improvement.

The B-containing chemical compositions of the Examples according to the present invention are shown in Table <NUM>, wherein the balance is Fe and other unavoidable impurities. The process parameters of the manufacturing method according to the present invention are shown in Table <NUM>, and the mechanical properties of the hot-rolled strips obtained finally are shown in Table <NUM>.

Claim 1:
A martensitic steel strip comprising the following composition by weight percentage: C:
<NUM>-<NUM>%, Si: <NUM>-<NUM>%, Mn: <NUM>-<NUM>%, P≤<NUM>%, S≤<NUM>%, N: <NUM>-<NUM>%, Als: <<NUM>%, optional B: <NUM>-<NUM>%, total oxygen [O]T: <NUM>-<NUM>%; and a balance of Fe and other unavoidable impurities, and, at the same time, the following conditions are satisfied:
it comprises one or both of Cu: <NUM>-<NUM>% and Sn: <NUM>-<NUM>%;
it comprises one or both of Nb: <NUM>-<NUM>% and Mo: <NUM>-<NUM>%;
Mn/S><NUM>, wherein:
martensite has a volume fraction of at least <NUM>%,
the martensitic steel strip has a thickness of <NUM>-<NUM>, and
the martensitic steel strip has a yield strength of <NUM>-<NUM> MPa, a tensile strength of <NUM>-<NUM> MPa, and an elongation of <NUM>-<NUM>%.