CN115652183A - Preparation method of alloy structural steel and alloy structural steel - Google Patents
Preparation method of alloy structural steel and alloy structural steel Download PDFInfo
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Abstract
The application discloses a preparation method of alloy structural steel and the alloy structural steel. The preparation method comprises the following steps: refining the primary molten steel in an LF furnace to obtain LF refined molten steel; refining the LF refined molten steel in an RH furnace, and removing vacuum in the RH furnace to obtain RH refined molten steel; and transferring the RH refined molten steel to a continuous casting platform, injecting the RH refined molten steel into a continuous casting tundish for continuous casting, and solidifying to obtain an alloy structural steel continuous casting slab. In the RH refining, a composition containing metallic calcium is added into the RH furnace 5-8min before the vacuum in the RH furnace is removed, so that the molten steel in the continuous casting tundish meets the condition that Ca/S is more than or equal to 0.5 and S is less than or equal to 10ppm, and the generation of MnS inclusions in a plate blank is eliminated or inhibited, thereby eliminating or reducing the adverse effect of long flaky MnS inclusions on the transverse impact toughness of a steel plate after hot rolling, ensuring the basic performance of the alloy structural steel and improving the transverse impact toughness of the alloy structural steel.
Description
Technical Field
The application belongs to the technical field of alloy structural steel, and particularly relates to a preparation method of alloy structural steel and the alloy structural steel.
Background
With the development of economy and society, the market needs a large amount of common steel, and is more prone to high efficiency, environmental protection and production and use of novel steel. The alloy steel structural steel has good comprehensive performance, good processing property, welding property, other special properties and the like, can meet the requirements of steel for various aspects such as chemical engineering petroleum, ships, boiler pressure vessels, vehicles, bridges, railway buildings, mines and the like, can meet the requirements of multiple layers in various fields, and can powerfully promote the development of modern construction.
The existing alloy steel structural steel plate, particularly the high-strength and ultrahigh-strength alloy structural steel plate, has lower toughness and is easy to cause cracks or fracture. Therefore, the toughness, particularly the transverse toughness, of the conventional alloy steel plate still needs to be improved.
Disclosure of Invention
In view of this, the present application provides a method of producing an alloy structural steel and an alloy structural steel, and aims to provide an alloy structural steel excellent in transverse toughness.
In a first aspect, an embodiment of the present application provides a method for preparing alloy structural steel, including the following steps:
KR desulfurization is carried out on blast furnace molten iron, and then converter smelting is carried out to obtain primary molten steel;
refining the primary molten steel in an LF furnace to obtain LF refined molten steel;
and performing RH furnace refining on the LF refined molten steel, and then removing the vacuum environment in the RH furnace to obtain RH refined molten steel, wherein in the RH refining, a composition containing metal calcium is added into the RH furnace 5-8min before the vacuum environment in the RH furnace is removed, so that the molten steel in a continuous casting tundish has uniform components and satisfies the following requirements: ca/S is more than or equal to 0.5;
transferring the RH refined molten steel to a continuous casting platform and injecting the RH refined molten steel into a continuous casting tundish for continuous casting, and solidifying to obtain an alloy structure steel slab;
and heating, rolling, cooling and coiling the alloy structural steel slab to obtain the alloy structural steel.
According to an embodiment of one aspect of the application, the composition containing the metallic calcium is added into the RH furnace 5-8min before the vacuum environment in the furnace is removed, so that the molten steel in the tundish satisfies the following conditions: ca/S is more than or equal to 1.
According to an embodiment of one aspect of the application, the composition containing the metal calcium is added into the RH furnace 5-8min before the vacuum environment in the furnace is removed, so that the molten steel in the tundish satisfies the following conditions: ca/S is more than or equal to 1.
According to an embodiment of one aspect of the application, after LF refining is carried out, S content of molten steel in the continuous casting tundish is less than or equal to 6ppm, so that S content of the molten steel in the continuous casting tundish is less than or equal to 10ppm.
According to an embodiment of one aspect of the present application, blast furnace molten iron is desulfurized by the KR so that the S content in the molten iron is controlled to be less than or equal to 10ppm at the end of the KR desulfurization.
According to an embodiment of one aspect of the application, KR desulfurization, converter smelting, LF furnace refining and RH furnace refining are controlled so that the S content of the molten steel in the continuous casting tundish is less than or equal to 5ppm.
According to an embodiment of one aspect of the application, scrap steel with the sulfur content of less than or equal to 0.025 percent is used in converter smelting, so that the S content in primary molten steel is less than or equal to 50ppm, and the ladle slag thickness of steel tapped from the converter is controlled to be less than or equal to 50mm, so that LF refining slagging deep desulfurization is facilitated, and the S content of LF outbound molten steel is less than or equal to 2ppm.
According to an embodiment of one aspect of the application, a deep desulfurization refining process is adopted in LF refining, the slag amount is more than or equal to 2.0kg/t, and the alkalinity of slag is 6-12, so that the S content of molten steel discharged from LF refining is less than or equal to 2ppm.
According to an embodiment of one aspect of the application, a composition containing metallic calcium is added into an RH refining furnace 5-8min before the vacuum environment in the furnace is removed, the vacuum is removed after circulation is carried out for 4-7 min under the condition that the vacuum degree in the RH furnace is 65-800 Pa, so that the molten steel in a continuous casting tundish is uniform in composition and Ca is more than or equal to 5ppm.
In a second aspect, the present application provides an alloy structural steel produced by the production method of the first aspect.
According to an embodiment of one aspect of the present application, the inclusions in the alloy structural steel satisfy: and grading the non-metallic inclusions of the product according to GB/T10561-2005, wherein the class B inclusions are less than or equal to 1.5 grade, the class D inclusions are 0-1.0 grade, and the class Ds inclusions are 0-1.0 grade.
According to an embodiment of one aspect of the application, the alloy structural steel comprises the following chemical components in percentage by mass: less than or equal to 0.35 percent of C, less than or equal to 0.70 percent of Si, less than or equal to 1.60 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.010 percent of S, less than or equal to 1.10 percent of Cr, less than or equal to 0.80 percent of Ni, less than or equal to 0.50 percent of Mo, less than or equal to 0.050 percent of Ti, 0.0005 to 0.006 percent of B, 0.015 to 0.1 percent of Als, and the balance of Fe and inevitable impurity elements.
Compared with the prior art, the application has at least the following beneficial effects:
according to the preparation method of the alloy structural steel, the LF refining molten steel is subjected to RH furnace refining, a composition containing metallic calcium is added into the RH furnace 5-8min before vacuum in the RH furnace is removed, molten steel components in a continuous casting tundish are uniform, the molten steel in the continuous casting tundish meets the condition that Ca/S is more than or equal to 0.5, and on the basis of ensuring the cleanliness of the molten steel, the generation of MnS inclusions in a plate blank is eliminated or inhibited, so that the adverse effect of long-strip flaky MnS inclusions formed after hot rolling on the transverse impact toughness of a steel plate, particularly a rolled plate is eliminated or reduced, the transverse impact toughness of the alloy structural steel is improved while the basic performance of the alloy structural steel is ensured, microcracks are prevented from being generated, and the phenomenon of cracks or fracture of the steel plate in the using process is avoided.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present application, and it is obvious for a person skilled in the art to obtain other drawings based on the drawings without any creative effort.
FIG. 1 shows MnS inclusions in NM450-SPT heat-treated steel sheets provided by examples of the present application;
FIG. 2 illustrates the coarsening and spheroidizing process of MnS inclusions in an ultra-low sulfur steel provided by the prior art during heating;
FIG. 3 shows MnS inclusions on fractures of samples for which conventional NM450 wear plate lateral impact power fails in the present application;
FIG. 4 shows the composition of CaS composite inclusions in a super tough wear plate NM450-SPT final plate provided by an embodiment of the present application.
Detailed Description
In order to make the purpose, technical solution and advantageous technical effects of the present application clearer, the present application is further described in detail with reference to the following embodiments. It should be understood that the embodiments described in this specification are only for the purpose of explaining the present application and are not intended to limit the present application.
For the sake of brevity, only some numerical ranges are explicitly disclosed herein. However, any lower limit may be combined with any upper limit to form ranges not explicitly recited; and any lower limit may be combined with any other lower limit to form a range not explicitly recited, and any upper limit may be combined with any other upper limit to form a range not explicitly recited. Also, although not explicitly recited, each point or individual numerical value between the endpoints of a range is encompassed within that range. Thus, each point or individual value can form a range not explicitly recited as its own lower or upper limit in combination with any other point or individual value or in combination with other lower or upper limits.
In the description of the present application, it is to be noted that, unless otherwise specified, "above" and "below" are inclusive of the present number, and "plural" of "one or more" means two or more.
The above summary of the present application is not intended to describe each disclosed embodiment or every implementation of the present application. The following description more particularly exemplifies illustrative embodiments. At various points throughout this application, guidance is provided through a list of embodiments that can be used in various combinations. In each instance, the list is merely a representative group and should not be construed as exhaustive.
The existing alloy steel structure steel plate has low toughness and is easy to crack or break in the using process. Therefore, there is a need to develop still higher toughness alloy structural steel plates to meet the demands of the market or users.
Mn element is the most obvious alloy element for improving the hardenability of steel, and Mn element in the steel with high heat treatment strength and ultrahigh strength steel structure is a commonly used alloy element. The addition of Mn element to steel can eliminate hot embrittlement caused by S element and improve strength and hardenability of steel, but the addition of Mn element inevitably forms MnS inclusions with S in steel.
MnS inclusions have excellent plasticity and extend into extremely thin band-like inclusions during hot rolling after heating of a slab to have a significant adverse effect on the transverse impact toughness of a steel sheet, for example, in a rolled coil, the band-like inclusions are mostly submicron-sized in thickness, and may have a length of up to hundreds of micrometers and a width of usually less than twenty micrometers
In the related art, the S content in the steel sheet of the alloy steel structural steel can be controlled to a level of not more than 5ppm. The inventors' studies have found that even in such ultra-low S content high strength and ultra-high strength alloy structural steels, particularly heat treated high strength steel sheets and ultra-high strength steel sheets for structural use, mnS inclusions constitute a hazard to the transverse impact toughness of the steel sheets if they are not subjected to a denaturing treatment such as a calcium treatment.
The high-strength and ultrahigh-strength steel plate produced by the plate rolling process usually adopts a rolling method, and the rolling is longitudinal rolling without transverse rolling. The research of the inventor finds that the higher the strength grade of the steel plate is, the more remarkable the influence of the inclusion on the toughness is, for example, mnS inclusion and B inclusion have remarkable adverse effect on the transverse impact toughness of the heat-treated ultrahigh-strength steel plate with the tensile strength requirement of NM450 and the like of more than or equal to 1250MPa respectively, and the improvement is urgently needed.
In view of this, the inventor provides a preparation method of an alloy structural steel, provides a preparation method of an alloy structural steel plate capable of obtaining higher transverse impact energy, and simultaneously provides a steel-making process capable of ensuring molten steel cleanliness. For example, the method can be used for preparing the NM450-SPT super-toughness wear-resistant steel plate with the transverse impact energy of more than or equal to 47J at the temperature of-40 ℃.
Preparation method of alloy structural steel
The embodiment of the application provides a preparation method of alloy structural steel on one hand, which comprises the following steps:
KR desulfurization is carried out on blast furnace molten iron, and then converter smelting is carried out to obtain primary molten steel;
refining the primary molten steel in an LF furnace to obtain LF refined molten steel;
and performing RH furnace refining on the LF refined molten steel, and then removing the vacuum environment in the RH furnace to obtain RH refined molten steel, wherein in the RH furnace refining, a composition containing metal calcium is added into the RH furnace 5-8min before the vacuum environment in the RH furnace is removed, the molten steel in a continuous casting tundish is made uniform, and the molten steel in the continuous casting tundish is made to meet the following conditions: ca/S is more than or equal to 0.5;
transferring the RH refined molten steel to a continuous casting platform and injecting the RH refined molten steel into a continuous casting tundish for continuous casting, and solidifying to obtain an alloy structure steel slab;
and heating, rolling, cooling and coiling the alloy structural steel plate blank to obtain the alloy structural steel plate.
In some embodiments, the molten steel in the continuous casting tundish satisfies: ca is more than or equal to 5ppm, and S is less than or equal to 10ppm.
In some embodiments, after the composition containing metallic calcium is added into the RH furnace, the molten steel is circulated for 4-7 min under the condition of 65-800 Pa of vacuum degree in the RH furnace to homogenize the components.
In the examples of the present application, after a composition containing metallic calcium was added to the RH furnace 5 to 8min before the vacuum atmosphere in the RH furnace was removed, the molten steel was circulated for 4 to 7min under a vacuum degree of 95 to 300Pa in the RH furnace, so that the molten steel components in the continuous casting tundish were uniform.
According to the embodiment of the application, the primary molten steel after the smelting of the converter is finished is poured into a steel ladle through a steel outlet. And transferring the ladle and the primary molten steel in the ladle to an LF furnace for LF refining. And after the LF furnace refining is finished, transferring the ladle and LF refining molten steel in the ladle to an RH furnace for RH refining. And after finishing the RH refining, transferring the ladle and the RH refined molten steel in the ladle to a continuous casting platform. And pouring RH refined molten steel in a ladle into a continuous casting tundish through a long nozzle on a continuous casting platform, and pouring the molten steel in the continuous casting tundish into a continuous casting crystallizer through a submerged nozzle (SEN) for solidification to prepare a plate blank.
According to the embodiment of the application, the composition containing metallic calcium generally contains silicon, iron, carbon and elemental metal calcium, and the mass fraction of the elemental metal calcium in the composition containing metallic calcium is 0.3% -2%. The above-described calcium metal-containing compositions typically comprise ferrosilicon, which is typically produced by carbothermic processes, containing greater than about 0.6% calcium metal.
In the prior art, ca treatment is usually carried out after LF furnace refining is finished, even if the Ca content in RH incoming molten steel reaches 25-45 ppm, the Ca content in the molten steel is almost completely volatilized in a vacuum environment after RH refining in a subsequent process, so that the Ca content in the molten steel in a continuous casting ladle is usually less than 5ppm, and the modification treatment of MnS inclusions cannot be realized, and long and extremely flaky MnS inclusions distributed along the rolling direction still exist in an alloy structure steel coil, so that the transverse impact toughness of a steel plate is remarkably reduced. After the refining in the LF furnace, ca treatment is performed to make solid Al in the molten steel 2 O 3 12CaO.7Al included in liquid state 2 O 3 With 3CaO. Al 2 O 3 The composite inclusion is not beneficial to floating removal in the RH refining process of the subsequent procedure, so that B-type inclusion, D-type inclusion and D-type inclusion in the alloy structural steel plate can be caused S The class value is higher than 1.5 grade, and the transverse and longitudinal impact toughness of the steel plate is reduced. The above factors result in the toughness, particularly the transverse impact toughness, of high-strength and ultrahigh-strength steel plates being at a low level.
In the prior art, calcium treatment is also carried out after the vacuum in the furnace is relieved after RH refining is finished, and calcium wires are fed into molten steel in a conventional operation. The following disadvantages of this method are: 1) After RH refining, because the slag surface of the steel ladle is easy to crust, when calcium wires are fed, the calcium wires are easy to penetrate through the slag layer and cannot be fed into molten steel; 2) The steel slag and the molten steel are easy to violently tumble, secondary pollution and secondary oxidation are caused, and the cleanliness of the molten steel is deteriorated. Because of the problems, the transverse impact energy of the heat treatment high-strength and ultrahigh-strength plates is generally improved by performing calcium treatment after the vacuum in the furnace is relieved after RH refining is finished, but the transverse impact energy is unstable, and the alloy structural steel which is produced in batches cannot be stably controlled to meet the requirement of transverse toughness.
The inventors have found that, when Ca/S of molten steel in a continuous casting tundish is less than 0.5 without or after Ca treatment, even if the S content is as low as 5ppm, long and extremely flaky MnS inclusions distributed in the rolling direction are present in the thickness center portion of the alloy structural steel.
Based on the above reasons, the inventors studied that the molten steel in the continuous casting tundish can satisfy Ca/S of 0.5 ppm or more and Ca of 5ppm or more by adding a composition containing metallic calcium into the RH furnace 5 to 8min before removing the vacuum in the RH furnace and circulating the molten steel for 4 to 7min under the condition of a vacuum degree of 65 to 800Pa during the RH refining process. At the moment, mnS inclusions in the plate blank can be eliminated or reduced, so that extremely thin strip-shaped MnS inclusions in a rolled plate rolled by the plate blank (if the MnS inclusions exist in the plate blank, the strip-shaped MnS inclusions extend to be extremely thin in the rolling direction in the rolling process) are eliminated or reduced, a large amount of strip-shaped extremely thin sheet-shaped MnS inclusions are distributed on the steel plate along the rolling direction to be controlled, the obvious adverse effect of the MnS inclusions on transverse punching power is eliminated or reduced, the cleanliness of molten steel can be ensured, the B type inclusions in the material sample are less than or equal to 1.5 grade, and the D type inclusions and Ds type inclusions in the material sample are less than or equal to 1.0 grade, so that the transverse impact power and the stability of the transverse impact power are obviously improved, the requirements of markets or users on the transverse toughness are better met, and the phenomenon of cracks or cracks in the use process is avoided.
The mechanism of formation of the extremely thin strip-like MnS inclusions is: in the cooling process after the continuous casting slab is solidified, the initial precipitation temperature of MnS inclusions can be calculated by a solid solubility product formula. The initial precipitation temperature in ferrite can be calculated from the formula lg [ Mn ] [ S ] =4.286-11906/T (1), and the initial precipitation temperature in austenite can be calculated from the formula lg [ Mn ] [ S ] =4.771-12073/T (2). T in the formulas (1) and (2) is absolute temperature, and after the absolute solid solution temperature is calculated, the absolute solid solution temperature is converted into the conventional solid solution temperature in centigrade used in actual production. For example, when an alloy structural steel continuous casting slab having a Mn content of 1.0% and an S content of 0.0015% is solidified and cooled to 1401.6 ℃, mnS inclusions begin to precipitate in ferrite and when the slab is cooled to 1316.6 ℃, mnS inclusions begin to precipitate in austenite. When the Mn content is constant, the initial precipitation temperature of MnS decreases with the decrease in the S content. For example, when the S content is reduced to 0.0005% and the Mn content is still 1.0%, the precipitation start temperatures of MnS in ferrite and austenite are 1296.3 ℃ and 1222.7 ℃, respectively.
The MnS inclusions in the conventional ultra-low sulfur steel slab coarsened (grown) and spheroidized to a micron-sized size in a heating process before hot rolling, as shown in fig. 2, and it is considered that the MnS inclusions have excellent plasticity. Through research of the inventor, mnS in the ultra-low sulfur alloy structural steel plate blank with the sulfur content less than or equal to 20ppm extends along the rolling direction in the hot rolling process to form long flaky inclusions in the plate, so that the thickness of the MnS in the rolled plate is generally in a submicron scale, the width of the MnS is generally not more than 20 micrometers, and the length of the MnS can reach hundreds of micrometers, as shown in figures 1 and 3. MnS inclusions on the fracture of the sample with unqualified lateral impact power of the conventional NM450 wear plate are shown in figure 3 below. It can be seen that in order to eliminate the adverse effect of MnS inclusions on the transverse impact energy, it is necessary to denature the MnS inclusions by adding metallic calcium even if the S content in the steel is as low as 5ppm level.
And (3) analyzing the components (mass percent) of MnS inclusions on the fracture of the sample with the unqualified transverse impact power of the conventional NM 450.
In the production process of the alloy structural steel plate, the improvement of the transverse impact energy of the steel plate is very necessary. For example, in the production process of the coiled plate, the thickness of the adopted plate blank is 230mm, the thickness of the hot rolled plate is usually less than or equal to 25.4mm, and the blank-to-plate compression ratio is usually more than or equal to 9, so that the length-width ratio of MnS inclusions in the plate is usually more than or equal to 9, and under the condition, the adverse effect of the MnS inclusions on the transverse impact energy of the steel plate is obviously more than the longitudinal impact energy.
In some embodiments, the composition containing metallic calcium is added 5-8min before the RH refined vacuum environment is removed, so that the molten steel in the tundish satisfies the following conditions: ca/S is more than or equal to 1.
In some embodiments, in the RH furnace refining process, after the composition containing the metallic calcium is added into the RH furnace 5-8min before the vacuum in the RH furnace is removed, the RH furnace is cycled for 4-7 min under the condition that the vacuum degree in the RH furnace is 65-800 Pa, so that the molten steel in the continuous casting tundish is uniform in composition and satisfies the following conditions: ca/S is more than or equal to 1.0, ca is more than or equal to 5ppm, and S is less than or equal to 5ppm.
In some embodiments, the S content of the molten iron is less than or equal to 10ppm by the KR desanding of the molten iron, the converter uses common scrap steel and controls the thickness of steel slag discharged by the converter to be less than or equal to 50mm, the LF furnace adopts a deep desulfurization refining process with large slag quantity and high alkalinity to ensure that the S content of the molten steel discharged from the LF furnace is less than or equal to 6ppm, and the RH furnace refining controls the sulfur return quantity to be less than or equal to 2ppm, so that the molten steel in the continuous casting tundish satisfies the following conditions: s content is more than 5ppm and less than or equal to 10ppm, and Ca/S content is more than or equal to 0.5 and less than 1.0.
At this time, the alloy structure steel plate has a partial modification effect on MnS inclusions, and the transverse impact of the alloy structure steel plate is improved. According to the embodiment of the application, the S content of the molten iron is less than or equal to 10ppm by removing S from the molten iron KR, the S content of primary molten steel is less than or equal to 50ppm by using clean steel scrap with the S content less than or equal to 0.025 percent in converter steelmaking, the S content of primary molten steel is less than or equal to 50ppm by controlling the thickness of steel slag discharged from the converter to be less than or equal to 50mm, and the LF furnace adopts a deep desulfurization refining process with large slag amount and high alkalinity to ensure that the S content of the molten steel discharged from the LF station is less than or equal to 2ppm, the refining sulfur return amount of the RH furnace is less than or equal to 1.5ppm, so that the molten steel in a continuous casting tundish meets the requirements of S content less than or equal to 5ppm and Ca/S content is more than or equal to 1, the MnS inclusion can be fully denatured, the adverse effect of the transverse impact energy of a steel plate caused by the MnS inclusion is eliminated, the transverse toughness of alloy structural steel is further improved, and the requirements of users or markets are met
In some embodiments, the LF furnace refining is performed such that the molten steel in the continuous casting tundish has an S content of 10ppm or less.
In some embodiments, after LF furnace refining, RH furnace refining is carried out, and S content of the molten steel in the continuous casting tundish is less than or equal to 8ppm by RH outbound molten steel, so that S content of the molten steel in the continuous casting tundish is less than or equal to 10ppm.
In some embodiments, the steel grade smelted in the previous smelting pass of the RH furnace is an ultra low sulfur steel with an S content of less than or equal to 10ppm, so that the RH off-station molten steel has an S content of less than or equal to 3.5ppm.
In some embodiments, the steel grade smelted in the previous smelting pass of the RH furnace has an S content of more than 10ppm, and the RH furnace is subjected to furnace washing treatment by using ultralow-sulfur molten steel with the S content of less than or equal to 10ppm so that the RH outbound molten steel has the S content of less than or equal to 3.5ppm.
In some embodiments, the molten steel in the continuous casting tundish is sufficient to: when the content of S is more than 5ppm and less than or equal to 10ppm and the content of Ca/S is more than or equal to 1, mnS inclusion can be fully denatured, the adverse effect of the MnS inclusion on the transverse impact energy of the heat-treated high-strength and ultrahigh-strength steel plate is eliminated, and the transverse impact energy is obviously improved.
In some embodiments, blast furnace molten iron is desulfurized by the KR such that the S content in the molten iron is controlled to be less than or equal to 10ppm at the end of the KR desulfurization.
According to the embodiment of the application, by controlling KR desulfurization, the S content of molten iron is less than or equal to 10ppm when KR desulfurization is finished, the positive effect of reducing the sulfur content of the molten steel in the converter is achieved, the desulfurization pressure of the LF furnace in the subsequent process is reduced, and the guarantee capacity that the S content of the molten steel in the continuous casting tundish is less than or equal to 5ppm is remarkably improved.
In some embodiments, clean scrap steel is used for KR desulfurization, converter steelmaking (which is beneficial to controlling the sulfur content of the molten steel discharged from the converter), LF furnace refining deep desulfurization and RH refining sulfur return amount are controlled, so that the S content of the molten steel in the continuous casting tundish is less than or equal to 5ppm.
In some embodiments, the RH furnace smelting continuous casting steel requires the S content of the extremely low S steel grade with less than or equal to 5ppm, and the former steel must be the steel grade with the S content of less than or equal to 10ppm in the continuous casting, so as to be beneficial to controlling the RH refining molten steel to return to the S content of less than or equal to 1.5ppm
In some embodiments, the S content of the RH furnace smelting continuous casting steel is required to be an extremely low S steel grade with the S content less than or equal to 5ppm, and if the S content of the former furnace steel is more than 10ppm, the RH furnace is washed by molten steel with the S content less than or equal to 10ppm.
According to the embodiment of the application, in the LF + RH duplex refining process, ca treatment is carried out after LF refining is finished, even if the Ca content in RH arrival molten steel reaches 25-45 ppm, the Ca in the molten steel is almost volatilized completely after RH treatment, the content of the Ca in a tundish is usually less than 5ppm, and the MnS inclusion cannot be fully denatured; ca treatment (Ca wire feeding) is carried out after RH refining is broken, which is easy to cause violent turnover of steel slag and molten steel, pollute the molten steel and cause secondary oxidation of refined molten steel, thus being not beneficial to the cleanliness of the molten steel.
In some embodiments, the waste steel with the sulfur content less than or equal to 0.025 percent is used in converter smelting, so that the S content in the primary molten steel is less than or equal to 50ppm, and the pressure of desulfurization in the LF furnace refining in the subsequent process is reduced; controlling the slag discharging amount of the converter tapping to ensure that the slag thickness in the steel ladle is less than or equal to 50mm so as to reduce the total content of FeO, mnO and P2O5 in the slag, facilitate the deep desulphurization of the LF slagging refining and reduce the return of molten steel to P
In some embodiments, a deep desulfurization refining process is adopted in LF furnace refining, the amount of slag is more than or equal to 2.0kg/t, and the alkalinity of the slag is 6-12, so that the S content of the molten steel discharged from the LF refining is less than or equal to 2ppm.
According to the embodiment of the application, for the ultra-low sulfur steel with the S content less than or equal to 10ppm, caS can be formed only in the solidification process of molten steel. The thermodynamic calculation results show that when the S content is less than or equal to 4.7ppm, caS cannot be formed at the temperature above the liquidus. However, the S element is the element having the highest segregation tendency among the elements in the steel, and has a segregation coefficient of (1-K) 0 ) Up to 0.95-0.98, [ E ]] δγ ＝0.02～0.05[E] Liquid for treating urinary tract infection . Because of serious segregation at the solidification front, even if the S content (average component) in the molten steel is reduced to 1-5 ppm, caS can still be formed in the process of casting blank solidification, [ ca ]]+[S]= CaS, caS with CaO, al 2 O 3 And MgO and other impurities are combined to form aCaS 2 O 3 、aCaS.bCaO.c Al 2 O 3 dMgO and the like. The plasticity of the composite inclusions is obviously weaker than that of MnS inclusions, and spindle-shaped inclusions are more difficult to deform in the hot rolling process to form in a plate instead of strip-shaped flaky inclusions, so that the area of the inclusions on the cross section of a transverse impact energy test sample is obviously reduced, and the transverse impact energy is obviously improved. For example, the NM450-SPT finished product treated by the effective calcium has no MnS inclusion, the sulfide inclusion is a composite inclusion of calcium sulfide, and the appearance and the composition are shown in figure 4 below. Therefore, it is necessary to perform calcium treatment on MnS material for an extremely low-sulfur alloy structural steel having an S content of 5ppm or less.
The compositional analysis in figure 4 is shown in the table below. (in mass percent)
In some embodiments, the ferrosilicon composition containing elemental calcium is added to the RH furnace 5-8min before the RH furnace removes the vacuum, and the vacuum is removed after 4-7 min of circulation to make the molten steel in the continuous casting tundish uniform and satisfy: s is less than or equal to 5ppm, ca is more than or equal to 5ppm, and Ca/S is more than or equal to 1. By adding the ferrosilicon composition containing the metal calcium at the final stage of RH refining, mnS inclusions in a plate blank can be effectively eliminated, extremely thin strip inclusions extending in the MnS rolling process are avoided, and the transverse impact toughness of a steel plate is improved.
According to an embodiment of the present application. By adopting the preparation method, the transverse impact toughness of the alloy structural steel is obviously improved, so that the alloy structural steel has stable mechanical properties and the inclusions are stable and controlled. In addition, in actual production, the molten steel must be homogenized by the number of molten steel cycles per minute after the ferrosilicon is charged into the RH furnace, and thus strict requirements are imposed on the grain size of the ferrosilicon.
Alloy structural steel
In a second aspect, the present application provides an alloy structural steel produced by the production method of the first aspect.
In some embodiments, the inclusions in the alloy structural steel satisfy: and grading the non-metallic inclusions of the product according to GB/T10561-2005, wherein the class B inclusions are less than or equal to 1.5 grade, the class D inclusions are 0-1.0 grade, and the class Ds inclusions are 0-1.0 grade. Therefore, the alloy structural steel plate meets the requirement of transverse toughness, and the phenomenon of cracks or fissures in the using process is avoided.
In some embodiments, the alloy structural steel comprises the following chemical components in mass percent: less than or equal to 0.35 percent of C, less than or equal to 0.70 percent of Si, less than or equal to 1.60 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.010 percent of S, less than or equal to 1.10 percent of Cr, less than or equal to 0.80 percent of Ni, less than or equal to 0.50 percent of Mo, less than or equal to 0.050 percent of Ti, 0.0005 to 0.006 percent of B, 0.015 to 0.1 percent of Als, and the balance of Fe and inevitable impurity elements.
In some embodiments, the mechanical properties of the alloy structural steel include: tensile strength not less than 1250MPa, surface Brinell hardness HBW 420-480 and elongation A 50 ≥7％。
The alloy structural steel has the chemical components and has the mechanical properties, and the transverse toughness meets the following requirements: the transverse impact energy at minus 40 ℃ is more than or equal to 47J.
According to the embodiment of the present application, the alloy structural steel may be a hot-rolled high-strength steel sheet and an ultra-high-strength steel sheet (e.g., an in-line quenched sheet), an off-line heat-treated high-strength steel sheet and an ultra-high-strength heat-treated steel sheet, and the steel sheet may include Si in an amount of 0.2% to 0.7%.
Examples
The present disclosure is more particularly described in the following examples that are intended as illustrative only, since various modifications and changes within the scope of the present disclosure will be apparent to those skilled in the art. All ingredients, percentages, and ratios reported in the following examples are on a weight basis, all articles used in the examples are commercially available or synthesized according to conventional methods and can be used without further treatment, and the equipment used in the examples is commercially available, unless otherwise specified.
Examples
The embodiment of the application provides a preparation method of alloy structural steel, which comprises the following steps:
KR desulfurization is carried out on blast furnace molten iron, and then converter smelting is carried out to obtain primary molten steel;
refining the primary molten steel in an LF furnace to obtain LF refined molten steel;
the method specifically comprises the following steps: when KR is desulfurized, the desulfurizing slag scraping cleanliness is more than or equal to 98 percent; the S is less than or equal to 10ppm after treatment. When smelting in a converter, clean scrap steel with the S content less than or equal to 0.025 percent is used; the end point S is less than or equal to 50ppm; the slag discharge amount of tapping is strictly controlled, and the thickness of ladle slag is less than or equal to 50mm. During LF refining, a deep S-removing refining process is adopted, the total slag amount is more than or equal to 2.0kg/t, the slag alkalinity range is 6-12, and the outbound S is less than or equal to 2ppm; the content of the silicon discharged from the converter is determined according to the content of the finished product silicon, for example, when the content of the finished product silicon is less than or equal to 0.30 percent, the converter and the LF process do not need to add ferrosilicon or ferrosilicon-manganese and other ferrosilicon-containing alloys; when the silicon content of the finished product is higher than 0.30 percent, the space for adding ferrosilicon and increasing silicon in the RH procedure is reserved and controlled according to 0.25 to 0.35 percent (target 0.30 percent), for example, when the silicon content of the finished product is 0.50 percent (range is 0.45 to 0.55 percent), the silicon at the station is controlled according to 0.15 to 0.25 percent (target 0.20 percent).
Carrying out RH furnace refining on the LF refined molten steel, and then removing vacuum in an RH furnace to obtain RH refined molten steel, wherein in the RH furnace refining, adding a silicon iron composition containing metal calcium into the RH furnace 5-8min before the vacuum in the RH furnace is removed, and circulating for 4-7 min under the condition that the vacuum degree in the RH furnace is 65-800 Pa so as to ensure that the molten steel in a continuous casting tundish has uniform components and the Ca/S is more than or equal to 1; the method specifically comprises the following steps of RH smelting: the front heat is the low-sulfur steel with S less than or equal to 10 ppm; adding 75 ferrosilicon containing about 0.6% of calcium into the final stage, wherein the grain size of the ferrosilicon is not less than 90% when the grain size is 10-30 mm, and the maximum grain size is not more than 50mm; the RH outbound sulfur is less than or equal to 3.5ppm, the Ca is more than or equal to 8ppm, and the sedation time is 20-40 min. The molten steel in the tundish meets the following requirements: s is less than or equal to 5ppm, and Ca is more than or equal to 5ppm.
Transferring the RH refined molten steel to a continuous casting platform and injecting the RH refined molten steel into a continuous casting tundish for continuous casting, and solidifying to obtain an alloy structure steel slab;
heating, rolling, cooling, coiling, flattening, quenching and low-temperature tempering are carried out on the alloy structural steel slab to obtain the alloy structural steel, namely the ultra-high toughness wear-resisting plate NM450-SPT.
Comparative example
The mechanical properties of the conventional NM450 steel sheet are shown in table 2 below.
Performance detection
Inclusion grade detection
Example 11 heats of steel were produced, for a total of 62 hot rolled coils, each coil of steel was tested for mechanical properties and inclusions using a sample of the finished product, for a total of 62 test specimens. And (3) carrying out detection grading on inclusions in the finished steel plate (sample) according to GB/T10561-2005, counting the proportion of the grades of each type of inclusions, and showing the detection result in table 1.
Table 1: grade ratio of various inclusions in ultra-high toughness NM450-SPT finished plate inclusion detection
Mechanical property detection
Detecting yield strength, tensile strength and elongation according to GB/T228.1-2021, detecting transverse impact energy at-40 ℃ or transverse impact energy at-20 ℃ according to GB/T229-2020, and detecting transverse impact energy at-40 ℃ according to GB/T
231.1-2018, and the surface Brinell hardness HBW is detected. Example 11 heats of steel were produced, and a total of 62 hot rolled coils were produced, and each coil of steel was tested for mechanical properties and inclusions using one sample plate, and a total of 62 test samples were obtained, and the test results are shown in table 2. To compare the effects of the examples, the mechanical properties of the conventional NM450 wear plates produced in the same period as the examples are also shown in table 2, and the number of samples is 50. It is noted that the example NM450-SPT detects-40 ℃ lateral impact power according to the user's request, while the conventional NM450 detects-20 ℃ lateral impact power.
Table 2: the mechanical properties of the ultra-high toughness NM450-SPT are compared with those of the conventional NM 450.
As can be seen from the above table, the adverse effect of MnS inclusion on the transverse impact energy is eliminated, the cleanliness of molten steel is ensured, B-type inclusion is less than or equal to 1.5 grade, D-type inclusion and Ds-type inclusion are less than or equal to 1.0 grade in a steel sample, and the transverse impact energy and the stability thereof are obviously improved.
The above description is only for the specific embodiments of the present application, but the scope of the present application is not limited thereto, and any person skilled in the art can easily think of various equivalent modifications or substitutions within the technical scope of the present application, and these modifications or substitutions should be covered within the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.
Claims (10)
1. A preparation method of alloy structural steel comprises the following steps:
KR desulfurization is carried out on blast furnace molten iron, and then converter smelting is carried out to obtain primary molten steel;
refining the primary molten steel in an LF furnace to obtain LF refined molten steel;
carrying out RH furnace refining on the LF refining molten steel, and then removing the vacuum environment in the RH furnace to obtain RH refining molten steel, wherein in the RH furnace refining process, a composition containing metal calcium is added into the furnace 5-8min before the vacuum environment in the RH furnace is removed so that the molten steel in a continuous casting tundish meets the condition that Ca/S is more than or equal to 0.5;
transferring the RH refined molten steel to a continuous casting platform and injecting the RH refined molten steel into a continuous casting tundish for continuous casting, and solidifying to obtain an alloy structure steel plate blank;
and heating, rolling, cooling and coiling the alloy structural steel slab to obtain the alloy structural steel.
2. The method according to claim 1, wherein the composition containing metallic calcium is added into the RH furnace 5 to 8min before the vacuum atmosphere in the furnace is removed so that the molten steel in the tundish satisfies the following conditions: ca/S is more than or equal to 1.
3. The production method according to claim 1, wherein LF refining is performed so that the S content of the molten steel in the continuous casting tundish is 10ppm or less.
4. The method according to claim 3, wherein the blast furnace molten iron is desulfurized by the KR so that the S content in the molten iron is controlled to be 10ppm or less at the end of the KR desulfurization.
5. The method according to claim 3, wherein the KR desulfurization, converter smelting, LF furnace refining and RH furnace refining are controlled so that the S content of the molten steel in the continuous casting tundish is less than or equal to 5ppm.
6. The preparation method of claim 5, wherein the scrap steel with the sulfur content of less than or equal to 0.025 percent is used in converter smelting, so that the S content in the primary molten steel is less than or equal to 50ppm, the ladle slag thickness is controlled to be less than or equal to 50mm during converter tapping, LF refining slagging deep desulfurization is facilitated, and the S content of the LF outlet molten steel is less than or equal to 2ppm; and/or the presence of a gas in the gas,
in the LF furnace refining, a deep desulfurization refining process is adopted, the slag amount is more than or equal to 2.0kg/t, and the alkalinity of slag is 6-12, so that the molten steel S of the LF outlet station is less than or equal to 2ppm.
7. The preparation method according to claim 1 or 2, characterized in that the composition containing metallic calcium is added into the RH refining furnace 5-8min before the vacuum environment in the furnace is removed, and the vacuum is removed after the RH refining furnace is circulated for 4-7 min under the condition that the vacuum degree in the RH furnace is 65-800 Pa, so that the molten steel in the continuous casting tundish has uniform components and the Ca content is more than or equal to 5ppm.
8. An alloy structural steel produced by the production method according to any one of claims 1 to 7.
9. The structural alloy steel according to claim 8, wherein the inclusions in the structural alloy steel satisfy: and grading the non-metallic inclusions of the product according to GB/T10561-2005, wherein the class B inclusions are less than or equal to 1.5 grade, the class D inclusions are 0-1.0 grade, and the class Ds inclusions are 0-1.0 grade.
10. The structural alloy steel as set forth in claim 9, characterized in that it comprises the following chemical components in mass percent: less than or equal to 0.35 percent of C, less than or equal to 0.70 percent of Si, less than or equal to 1.60 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.010 percent of S, less than or equal to 1.10 percent of Cr, less than or equal to 0.80 percent of Ni, less than or equal to 0.50 percent of Mo, less than or equal to 0.050 percent of Ti, 0.0005 to 0.006 percent of B, 0.015 to 0.10 percent of Als, and the balance of Fe and inevitable impurity elements.
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