Patent Application
20240287635
The present disclosure pertains to the field of metallurgical steelmaking technology, and in particular relates to a method for preparing a titanium-containing ultra-low-carbon steel.
In the production of ultra-low-carbon steel, aluminum is generally used to deoxygenate molten steel at the end. The product of deoxidization exists in the steel as an α-Al2O3 phase. The hardness of the α-Al2O3 inclusion is much higher than that of the steel. During subsequent cold rolling or other processing, the Al2O3 inclusion damages the steel matrix. The damage becomes an origin or inducement of cracking which damages the steel quality. It is proposed in Chinese Patent CN109402321B and Publication WO2021036974A1 that addition of rare earth to steel can effectively reduce defects caused by Al2O3 inclusion.
When manufacturing titanium-containing ultra-low-carbon steel, titanium in the steel often causes nozzle blocking, such that the liquid level fluctuation in the continuous casting mold increases, and thus the risk of entrapping mold fluxes into the molten steel to form defects increases. Even worse, the steel may congeal on the inner wall of the nozzle, leading to a decreased casting speed of the continuous casting machine, or even stop of pouring. It is generally believed that reducing the content of Al2O3, a deoxidization product, in steel is an effective way to solve the problem of nozzle blocking for titanium-containing ultra-low-carbon steel.
Addition of rare earth to titanium-containing ultra-low-carbon steel greatly increases the probability of nozzle blocking during continuous casting. In this case, even if the total oxygen in the steel can be controlled at less than 18 ppm or even lower such that the total amount of inclusions in the steel has reached an extremely low level accordingly, nozzle blocking still occurs frequently.
Therefore, for titanium-containing ultra-low-carbon steel, in order to reduce the harm of the Al deoxidization product (Al2O3) to cold-rolled steel, it is necessary to control the characteristics of oxide inclusions in the steel and ensure the stability of the pouring process during smelting.
Chinese Patent CN1678761B highlights addition of a rare earth metal (REM) to Al-deoxidized steel at a mass ratio REM/T.O=0.05-0.5 (the proportion of rare earth oxides in the final oxides is 0.5-15%), thereby reducing the amount of FeO or FeO· Al2O3 between adjacent Al2O3 particles in the steel, inhibiting agglomeration of Al2O3 particles, and ultimately improving the quality of the finished product. This technology is based on the following theory: FeO or FeO· Al2O3 is present between adjacent Al2O3 particles in steel (proposed by the inventors), and both of FeO and FeO· Al2O3 are present in a liquid state in the molten steel, such that Al2O3 inclusions in the steel agglomerate into large-sized particles. These large-sized inclusion particles are an important factor for deterioration of the quality of the finished product obtained subsequently.
Publication No. CN1218839A highlights that after decarburization of molten steel is completed, deoxidization with Ti, alloying, and addition of CaSi alloy or CaSi-REM alloy are performed in sequence to control the final oxide inclusion composition to be Ti2O3—CaO or REM oxide-Al2O3 composite inclusions containing a small amount of SiO2 or MnO, wherein the mass percentage of CaO+REM oxide is in the range of [5, 50], thereby obtaining a steel plate with an improved surface corrosion rate. This technology is based on the following theory: the residual oxide inclusions having a composition in a specific range in the steel do not clog the nozzle, and the inclusions can be finely dispersed (proposed by the inventors), so a steel plate with good surface properties can be obtained. As emphasized by this patent publication, the process effect thereof is achieved by controlling the Ti content (Ti/Al ratio) added for deoxidization, and the amount of Ca or REM added, such that the composition of the final inclusions is an oxide containing Ti, Ca/REM and Al.
The literature “Investigating the influence of Ti and P on the clogging of ULC steels in the continuous casting process” (C. Bernhard et al. InSteelCon 2011 Proceedings) reveals that under strictly controlled secondary oxidation conditions, titanium added to steel can improve wettability between alumina and molten steel, as well as wettability between an Al2O3-based refractory material and the molten steel. This will reduce the interface thermal resistance. Due to the increased speed of heat transfer between the molten steel and the immersed nozzle, nozzle blocking will be resulted due to low temperature.
In Publication No. WO2021036974A1, the inventors have proposed that in the manufacturing of titanium-containing ultra-low-carbon steel, after vacuum (RH, VD or VOD) decarburization is completed, Ti and Al are added one after another to molten steel for deoxidization, and then rare earth is added to the molten steel. This can effectively solve the smooth working problem of continuous casting process encountered by treatment of titanium-containing ultra-low-carbon steel with rare earth. In actual production, the reaction product of titanium added first and free oxygen in the molten steel floats to the top slag in the steel ladle and is absorbed by it, such that the titanium consumption is increased by 0.5 kg/t steel. In addition, first addition of Ti for pre-deoxidization will extend the vacuum treatment time by 5 minutes or longer. In other words, the operation of adding titanium first for pre-deoxidization in the treatment of titanium-containing ultra-low-carbon steel with rare earth increases the raw material cost of the product, prolongs the refining cycle, and increases the heat load in the smelting process.
In view of the above situation, there is an urgent need in the industry to develop a new method for preparing titanium-containing ultra-low-carbon steel, which method can effectively improve the characteristics of deoxidization inclusions in the steel, solve the smooth working problem of molten steel in casting process, reduce the rate of cold rolling defects caused by Al2O3, and improve the product quality of the titanium-containing ultra-low-carbon steel.
In view of the above-mentioned deficiencies existing in the prior art, one object of the present disclosure is to provide a method for preparing a titanium-containing ultra-low-carbon steel, wherein rare earth is used to modify the aluminum deoxidization product Al2O3 in the steel, so as to suppress its harmfulness. At the same time, the oxygen content in the molten steel and the rare earth metal purity are controlled in the refining process to eliminate the impact of the addition of the rare earth on smooth working of continuous casting process, thereby allowing for stable casting of the titanium-containing ultra-low-carbon steel treated with the rare earth. As a result, the characteristics of deoxidization inclusions in the steel are improved effectively; the smooth working problem of molten steel in continuous casting process is solved; the rate of cold rolling defects caused by Al2O3 is reduced; and the product quality of the titanium-containing ultra-low-carbon steel is improved.
In order to achieve the above object, the present disclosure adopts the following technical solution:
The present disclosure provides a method for preparing a titanium-containing ultra-low-carbon steel, comprising hot metal pretreatment, primary converter refining, vacuum refining, continuous casting, hot rolling, pickling and cold rolling;
Preferably, during the vacuum refining process:
Preferably, the oxide of Re2O3·Al2O3 in the molten steel is Ce2O3·Al2O3 or La2O3·Al2O3.
In one or more embodiments, after Al is added for deoxidization treatment, the circulation time of the molten steel is 3 to 10 minutes.
In one or more embodiments, after the additional alloying and rare earth components are added to the molten steel, the circulation time of the molten steel is 2 to 10 minutes.
Preferably, a vacuum refining device used in the vacuum refining process is an RH vacuum circulation degassing refining furnace (RH furnace) or a vacuum decarburization furnace (VD furnace) or a vacuum oxygen decarburization furnace (VOD furnace).
Preferably, in the hot metal pretreatment:
Preferably, during the primary converter refining process:
In the present disclosure, the ultra-low-carbon steel refers to steel with a mass percentage of carbon≤0.005% in the finished product.
Preferably, the titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C≤0.005%, Si≤0.05%, Mn: 0.05 to 0.3%, Al: 0.04 to 0.15%, Ti: 0.04 to 0.1%, P≤0.05%, S≤0.02%, N≤0.003%, and a balance of Fe and unavoidable impurities, wherein the Al content is greater than the Ti content.
In one or more embodiments, the titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C≤0.0018%, Si≤0.03%, Mn: 0.07 to 0.15%, Al: 0.04 to 0.07%, Ti: 0.04 to 0.06%, P<0.015%, S≤0.005%, N≤0.003%, and a balance of Fe and unavoidable impurities, wherein the Al content is greater than the Ti content.
It has been found according to the present disclosure that in the later stage of refining after the deoxidization treatment, the rare earth element (Ce or La) added to the molten steel reacts with the deoxidization product Al2O3 not removed from the molten steel as follows:
The possible values of n in the above formula are 11, 1, and 0. Accordingly, as the amount of the rare earth component added increases, the reaction products generated are Re2O3·11 Al2O3 (also known as β Al2O3), Re2O3·Al2O3(Ce2O3·Al2O3 or La2O3·Al2O3) and Re2O3, wherein Ce2O3·Al2O3 in the product Re2O3·Al2O3 is a liquid phase at 1600° C., the temperature of the molten steel. In a solid phase, its edges are smooth without obvious sharp angles, and the hardness is close to that of the steel matrix. In contrast, the Al2O3 crystal generated by conventional aluminum deoxidization in the steel belongs to an α crystal form which has a hexagonal crystal structure. It is a solid phase at the temperature of the molten steel, has sharp edges, and has a Mohs hardness of 9, which is much larger than other common materials. As compared with the original single-component Al2O3 inclusions, the probability of mechanical damage to the steel plate matrix caused by the Re2O3·Al2O3 inclusions in the titanium-containing ultra-low-carbon steel of the present disclosure is greatly reduced during cold rolling and subsequent cold processing, thereby reducing the degree of damage to the steel plate matrix, and improving the surface quality of the finished product. Typical inclusions (with the main component confirmed to be Re2O3·Al2O3) in the cold-rolled finished product produced by the process of the present disclosure are shown in
According to the present disclosure, it's believed that the reasons for titanium-containing ultra-low-carbon steel to cause easy nozzle blocking and difficult pouring are as follows: firstly, Ti in the molten steel improves the wettability of the interface between the Al2O3 skin layer and the molten steel, thereby reducing the size of the Al2O3 inclusions. The smaller the alumina inclusion particles are, the easier it is to cause nozzle blocking. Secondly, the better wettability provides more effective heat transfer between the clogs and the refractory material, thus causing formation of cold steel at the blocking locations, thereby further exasperating blocking.
The test results obtained by the inventors of the present disclosure show that: in the manufacturing of titanium-containing ultra-low-carbon steel, when a rare earth component is added to aluminum deoxidized steel, nozzle blocking tends to be aggravated, and the fluctuation of the liquid level in the mold increases, which seriously affects the smooth working of continuous casting process, reduces the proportion of qualified slabs, and worsens the quality of finished products.
Through multiple tests according to the present disclosure, it has been found that during vacuum refining of titanium-containing ultra-low-carbon steel, controlling the oxygen content of the molten steel at the end of decarburization and ensuring the purity of the added rare earth component, especially the oxygen content, can effectively suppress the impact of Ti in the molten steel on the surface wettability of Al2O3 in the molten steel, and in turn alleviate nozzle blocking during the continuous casting process, thereby ensuring a stable liquid level in the mold and a smooth working of continuous casting process.
In the present disclosure, before the decarburization treatment in the vacuum refining process, the free oxygen content in the molten steel is controlled so that the mass ratio of O to C in the molten steel satisfies O/C=1.25 to 2.15, preferably O/C=1.27 to 2.1, for example O/C=1.3 to 2.0; wherein the oxygen-to-carbon mass ratio is greater than 1.25, preferably greater than 1.27, for example greater than 1.3, so as to ensure the minimum amount of oxygen needed to remove carbon from the molten steel. Traditionally, it is believed that a sufficient excess of oxygen (O/C mass ratio ≥2.0) must be ensured in molten steel to maintain a high vacuum decarburization rate. The research according to the present disclosure has revealed that in actual production, when the initial oxygen-to-carbon mass ratio for vacuum decarburization is greater than 1.25, preferably greater than 1.27, for example greater than 1.3, the carbon in the molten steel can be reduced to 10 ppm or less within 17 minutes. The oxygen-to-carbon mass ratio is less than 2.15, preferably less than 2.1, for example less than 2.0, so as to ensure that the oxygen content in the molten steel at the end of decarburization is less than 350 ppm.
The decarburization treatment in the vacuum refining process according to the present disclosure enables the carbon content in the molten steel to be below the value required by the finished product. When the vacuum decarburization treatment is completed, the free oxygen O in the molten steel is in the range of 100 to 350 ppm. If the free oxygen is less than 100 ppm at the end of the decarburization, the decarburization time will be extended. The less the free oxygen, the longer the decarburization time is extended. If the free oxygen is more than 350 ppm at the end of the decarburization, there will be more deoxidization products in the molten steel. The Al2O3 content in the steel ladle slag will be higher, and the fluctuation of the liquid level in the mold will be increased significantly.
After the decarburization treatment in the vacuum refining process according to the present disclosure, the net circulation time of the molten steel after deoxidization by adding aluminum is required to be ≥3 minutes to ensure that the deoxidization product Al2O3 in the steel fully floats to the top slag in the steel ladle, so that most of the inclusions generated float to the top slag in the steel ladle.
In the later stage of the vacuum refining process according to the present disclosure (after deoxidization), additional alloying and rare earth (especially Ce or La) components are added, and then the composition of the molten steel is adjusted to the target range to control the composition of oxide inclusions in the steel. After the rare earth component is added, the circulation time of the molten steel is ≥2 minutes, so that the number of inclusions remaining in the molten steel is as small as possible.
The requirements for the rare earth component in the present disclosure include: 1) a total oxygen content T.O <100 ppm, because oxygen is a harmful component and will pollute the molten steel. The oxygen content should be as low as possible to ensure continuous casting smooth working of the molten steel; 2) a N content≤30 ppm, for controlling the titanium nitride content in the finished product to a low level; 3) a content of impurities other than the rare earth element(s) in the rare earth component <0.1 wt %; so as to achieve the object of making casting stable, improving the characteristics of the oxide inclusions, and reducing the cold rolled steel defects of the titanium-containing ultra-low-carbon steel.
Regarding the amount of the rare earth added according to the present disclosure, its upper limit is determined in terms of the ratio between the mass of the rare earth added (kg) and the total oxygen T.O (ppm) in the molten steel, i.e. REM/T.O=3.0. If the amount of the rare earth added exceeds a certain value, Al2O3 in the molten steel will be reduced completely, and all oxygen in the molten steel exists in the form of Re2O3. This will lead to two possible adverse consequences: 1) formation of a single rare earth oxide Re2O3 which has a large specific gravity and is not easy to float; 2) sharp rise of the free Re content in the steel, wherein the free Re will react with the refractory material, leading to contamination of the molten steel, melting or damaging the plug or nozzle in severe cases, such that the casting is rendered abnormal or interrupted. The lower limit of the amount of the rare earth added is determined according to REM/T.O=0.70. If the amount of the rare earth added is too low, there will be unstable Re2O3·11Al2O3 (β Al2O3) in the steel, or even sole Al2O3. As the temperature decreases, unstable β Al2O3 will decompose at medium and low temperatures, and eutectoid reaction takes place:
The method for preparing a titanium-containing ultra-low-carbon steel provided by the present disclosure can effectively improve the characteristics of the deoxidization inclusions in the steel, solve the casting process smooth working problem of molten steel, reduce the rate of cold rolling defects caused by Al2O3, and improve the product quality of the titanium-containing ultra-low-carbon steel. Specifically, the method includes the following beneficial effects:
Other features, objects and advantages of the present disclosure will become more apparent by reading the detailed description of the non-limiting embodiments with reference to the following drawings:
In order to better understand the above technical solutions of the present disclosure, the technical solutions of the present disclosure will be further described below with reference to the Examples.
As shown in
In the hot metal pretreatment, the hot metal is desulfurized by the KR process. After desulfurization, 3/4 of the top slag in the hot metal ladle is removed, wherein the S content in the hot metal after desulfurization is ≤20 ppm.
During the primary converter refining process, a top-bottom combined blowing process is used for the converter to ensure the strength of the bottom blowing. When the blowing is stopped, the free oxygen content in the molten steel is ≤600 ppm. During tapping from the converter, when the tapping amount reaches 1/5, lime is added to the steel ladle in an amount of 1.6 to 3 kg/t steel. When the tapping amount reaches 9/10, aluminum slag is added to the steel ladle in an amount of 1.0 to 1.4 kg/t steel. In some embodiments, after the tapping in the primary converter refining process is completed, the top slag in the steel ladle is modified, and the composition of the top slag in the steel ladle is adjusted to: CaO=40 to 50 wt %, FeO+MnO≤7.0 wt %. After the modification to the top slag in the steel ladle is completed, vacuum refining is performed.
During the vacuum refining process, in the early stage of the vacuum refining process, the free oxygen content in the molten steel is adjusted to satisfy the mass ratio O/C=1.25 to 2.15, preferably O/C=1.27 to 2.1, in some embodiments, O/C=1.3 to 2.0. Then, when the decarburization treatment is completed, the free oxygen O in the molten steel is between 100 and 350 ppm. In some embodiments, the free oxygen O is between 100 and 300 ppm. After Al is added for deoxidization treatment, the molten steel continues to circulate for a time ≥3 min. In the later stage of the vacuum refining process, additional alloying element(s) and rare earth(s) (including rare earth element Ce or La) are added. The composition and temperature of the molten steel are adjusted to the specified ranges, and the circulation time of the molten steel is ≥2 min. Finally, oxide Re2O3·Al2O3 (such as Ce2O3·Al2O3 or La2O3·Al2O3) is generated in the molten steel. The vacuum refining is completed. The amount of the rare earth added is based on the mass ratio REM/T.O=0.7 to 3.0, wherein REM represents the mass of the rare earth in kg, and T.O represents the total oxygen in the steel in ppm. In the rare earth added, the content of the impurities other than the rare earth element(s) is <0.1 wt %, wherein the total oxygen T.O is <100 ppm, and the N content is ≤30 ppm.
The steel types suitable for the above method for preparing a titanium-containing ultra-low-carbon steel are titanium-containing ultra-low-carbon steel products. Such titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C≤0.005%, Si≤0.05%, Mn: 0.05 to 0.3%, Al: 0.04 to 0.15%, Ti: 0.04 to 0.1%, P≤0.05%, S≤0.02%, N≤0.003%, and a balance of Fe and unavoidable impurities, wherein the Al content is greater than the Ti content to ensure that the final deoxidization of the molten steel before the rare earth is added is controlled by Al in the molten steel. During the casting process, the pass rate of the mold liquid level fluctuation within ±5 mm is >92%; and the pass rate of the mold liquid level fluctuation within ±3 mm is >32%.
The method for preparing the titanium-containing ultra-low-carbon steel of the present disclosure is further demonstrated below with reference to the specific Examples. In the Examples, the titanium-containing ultra-low-carbon steel comprises the following components in mass percentage: C≤0.0018%, Si≤0.03%, Mn: 0.07 to 0.15%, Al: 0.04 to 0.07%, Ti: 0.04 to 0.06%, P≤0.015%, S≤0.005%, N≤0.003%, and a balance of Fe and unavoidable impurities, wherein the Al content is greater than the Ti content.
The process path used in this Example was hot metal pretreatment (hot metal desulfurization and dephosphorization)→primary converter refining (top-bottom combined converter blowing, tapping)→modification to the top slag in the steel ladle→vacuum refining (decarburization, deoxidization, alloying and rare earth treatment)→continuous casting→hot rolling→pickling→cold rolling.
This Example demonstrates a typical smelting scheme according to the present disclosure. The KR process was used for desulfurization. After the desulfurization, 3/4 of the top slag of the hot metal ladle was removed. The S content in the desulfurized hot metal was 15 ppm. During the primary converter refining process, top-bottom combined blowing was employed. When the converter blowing was completed, C=220 ppm, O=580 ppm in the molten steel. Slag cutoff tapping was performed. In the early stage of tapping (when the tapping amount reached 1/5), lime was added in an amount of 2.2 kg/t steel. In the final stage (when the tapping amount reached 9/10), aluminum slag was added in an amount of 1.1 kg/t steel. Before the vacuum refining treatment, the composition of the top slag in the steel ladle was FeO+MnO=6.50 wt %, CaO: 42 wt %, and the slag thickness was 110 mm. In the early stage of the vacuum refining treatment (before the decarburization treatment), the free oxygen content in the molten steel was adjusted, so that the mass ratio O/C in the molten steel=1.27. When the decarburization in the vacuum refining process was completed, the free oxygen O in the molten steel: 320 ppm. Then, after Al was added for decarburization, the molten steel continued to circulate for 4.5 min. In the later stage of the vacuum refining process, additional alloying and rare earth components were added. The rare earth component was CeLa alloy (Ce:La mass ratio: 2:1). The content of the impurities other than the rare earth elements in the rare earth component was <0.1 wt %, wherein the total oxygen T.O was <100 ppm, and the N content was ≤30 ppm. The composition of the molten steel was adjusted to the specified range. After adding the rare earth, the molten steel was circulated for 5 minutes. After the refining was completed, continuous casting was performed, followed by hot rolling, pickling and cold rolling, where REM/T.O=1.2.
Process effects: during the continuous casting process in this Example, the pass rate of the mold liquid level fluctuation within ±5 mm was 94.2%, and the pass rate of the mold liquid level fluctuation within ±5 mm was 36%. For the cold-rolled steel in this Example, the undergrade ratio of steel slab was 40%, and the steel defect rate caused by Al2O3 was 0.02%.
Tables 1 and 2 show some other embodiments of the technical solution of the present disclosure used in actual production, as well as Comparative Group I using titanium pre-deoxidization and rare earth treatment, and Comparative Group II using a conventional process without rare earth treatment, for comparison. The process for Comparative Group I (Comparative Examples 1 to 6): hot metal pretreatment (desulfurization, dephosphorization)→primary refining (top-bottom combined converter blowing, tapping)→modification to the top slag in the steel ladle→vacuum refining (decarburization, titanium pre-deoxidization, Al deoxidization, alloying and rare earth treatment)→continuous casting→hot rolling→pickling→cold rolling. The process for Comparative Group II (Comparative Examples 7 to 12): hot metal pretreatment (desulfurization, dephosphorization)→primary refining (top-bottom combined converter blowing, tapping)→modification to the top slag in the steel ladle→vacuum refining (decarburization, Al deoxidization, alloying)→continuous casting→hot rolling→pickling→cold rolling. The differences of Examples 2 to 6, Comparative Group I and Comparative Group II from Example 1 in process parameters are shown in Table 1.
As shown in Tables 1 and 2, as compared with the process including titanium pre-deoxidization and rare earth treatment and the process including the conventional process without rare earth treatment, during the continuous casting process of the method for preparing the titanium-containing ultra-low-carbon steel of the present disclosure, the pass rates of the mold liquid level fluctuation within +5 mm and +3 mm are >92% and >32% respectively, better than the conventional process without rare earth treatment. The composition of the oxide inclusions in the titanium-containing ultra-low-carbon steel of the present disclosure changes from sole Al2O3 to Re2O3·Al2O3. The undergrade ratio of steel slab is about 35% for the titanium-containing ultra-low-carbon steel of the present disclosure, better than the conventional process without rare earth treatment (about 37% on average). The vacuum refining time is less than 27 minutes, comparable to the conventional process without rare earth treatment. The titanium consumption is comparable to the conventional process without rare earth treatment, and about 0.5 kg/t steel less than the process including first addition of titanium and rare earth treatment. With the use of the method for manufacturing a titanium-containing ultra-low-carbon steel according to the present disclosure, the vacuum refining time and the titanium consumption are comparable to the conventional process without rare earth treatment, and can ensure smooth working of continuous casting process, greatly reduce the cold rolling defect rate caused by Al2O3 (>90% lower), and significantly improve the product quality of the titanium-containing ultra-low-carbon steel.
Therefore, the method for preparing a titanium-containing ultra-low-carbon steel according to the present disclosure can effectively improve the characteristics of the deoxidization inclusions in the steel, solve the smooth working problem of the molten steel in continuous casting process, and reduce the rate of cold rolling defects caused by Al2O3 in the cold-rolled finished steel. It's suitable for improving the product quality of the titanium-containing ultra-low-carbon steel product, and valuable for promotion and application in steel mills.
One of ordinary skill in the art should recognize that the above Examples are only used to illustrate the present disclosure and are not used to limit the present disclosure. All changes and modifications made to the above Examples fall within the scope defined by the claims of the present disclosure without departing from the spirit and scope of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202110725238.0 | Jun 2021 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2022/101860 | 6/28/2022 | WO |
