CN110656280A - Low-oxygen calcium-magnesium-containing sulfur series free-cutting steel and preparation method thereof - Google Patents
Low-oxygen calcium-magnesium-containing sulfur series free-cutting steel and preparation method thereof Download PDFInfo
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Abstract
The invention provides a low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel and a preparation method thereof. The low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel comprises the following chemical components: the weight percentage is 0.30% -0.42% C, 0.15% -0.35% Si, 0.75% -1.05% Mn, 1.0% -1.5% Cr, 0.04% -0.07% Mo, 0.001% -0.020% Ca, 0.001% -0.02% Mg, 0.02% -0.05% S, 0.025% -0.035% Alt, less than or equal to 0.015% P, less than or equal to 0.001% O, 0.002% -0.005% N. The free-cutting steel provided by the invention has the advantages of simple and easily-controlled components, excellent mechanical property and cutting property, and high surface smoothness after cutting.
Description
Technical Field
The invention relates to the field of metal materials, in particular to low-oxygen calcium-magnesium-containing sulfur-series free-cutting steel and a preparation method thereof.
Background
The free-cutting steel is widely applied to the fields of automobile parts, household appliances, precision instruments, instruments and the like, and becomes one of the most popular steel types developed and researched in recent years along with the rapid development of related industries and the continuous improvement of the requirements of users on the surface quality, the mechanical property and the precision of the parts. Compared with common steel, the product processed by the method has the advantages of good surface finish, small tool wear, long service life and low energy consumption. According to statistics, the proportion of the machining cost to the part manufacturing cost can reach 40% -60%. Therefore, the cutting performance of the steel is improved, and the product cost and the energy consumption in the machining process can be greatly reduced.
The chalcogenide free-cutting steel is one of the free-cutting steels with the largest yield and the widest application, and accounts for more than 90 percent of the yield of the whole free-cutting steel. The sulfide in the sulfur series free-cutting steel mainly exists in a form of MnS, and the MnS plays a role in chip breaking and lubrication in the cutting process, so that the cutting performance of the steel is improved, and the abrasion of a cutter is reduced. However, during rolling, MnS is elongated and deformed in the rolling direction, and the deformed MnS seriously deteriorates the transverse impact toughness of the steel, resulting in anisotropy of mechanical properties of the material.
According to the form and distribution of MnS in the steel casting state, the MnS can be divided into three types: when the [ O ] in the steel is more than 0.02 percent, the steel is I-type MnS, is spherical and is distributed dispersedly; when the [ O ] in the steel is less than 0.01 percent, the steel is II-type MnS, is dendritic and is distributed along a crystal boundary; when the [ O ] content is further reduced, it is III-type MnS, and it is in a lump form and irregularly distributed. The influence of the I-type MnS on the mechanical property of the steel is small, and the influence of the II-type MnS and the III-type MnS on the mechanical property of the steel is large (mainly reducing the transverse impact toughness). In order to control the form of sulfides in steel and reduce the adverse effect on the mechanical properties of steel, the oxygen content in steel is usually controlled to be above 0.01% to obtain type I MnS, as described in Chinese patent (patent publication No. CN103966531A), and in order to control the form of sulfides in steel, the oxygen content in steel is controlled to be between 0.015% and 0.02%. However, high oxygen content inevitably affects the purity of the steel and reduces the fatigue properties of the steel. To obtain high purity and ensure high fatigue properties, [ O ] in the steel is usually controlled to a low level, and therefore MnS is mainly in group II and group III in low oxygen steel. In order to eliminate or reduce the adverse effect of these two types of MnS on mechanical properties in low-oxygen steel and to ensure machinability of the steel, it is necessary to control the MnS morphology in the steel.
At present, the calcium treatment is mostly adopted to denature MnS in the steel to form Mn-Ca-S complex sulfide, so that the deformation resistance of the steel at high temperature is improved, and the anisotropy of the steel is reduced. However, due to the low solubility of Ca in steel (0.0314% at 1873K), calcium treatment alone often results in incomplete inclusion denaturation. When the content of Ca is too high, CaS inclusions can be formed, and the nozzle is blocked.
Disclosure of Invention
The present invention is directed to solving at least one of the problems of the prior art.
Accordingly, an object of one aspect of the present invention is to provide a low-oxygen calcium-magnesium-containing sulfur-based free-cutting steel.
Another aspect of the present invention is to provide a method for preparing the low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel.
To achieve the above objects, an embodiment of an aspect of the present invention provides a low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel having a chemical composition comprising: the weight percentage is 0.30% -0.42% C, 0.15% -0.35% Si, 0.75% -1.05% Mn, 1.0% -1.5% Cr, 0.04% -0.07% Mo, 0.001% -0.020% Ca, 0.001% -0.02% Mg, 0.02% -0.05% S, 0.025% -0.035% Alt, less than or equal to 0.015% P, less than or equal to 0.001% O, 0.002% -0.005% N.
In the low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel provided by the above embodiment of the present invention, O can be bonded with deoxidizing elements in the steel to form inclusions, thereby reducing the fatigue life of the steel and the tool life, and at the same time, O reduces the effect of Ca-denatured MnS in the steel, so the upper limit of the O content is set to 0.001%, and Ca and Mg elements are added to the steel to control the form of sulfides. The addition of Mg can lead oxide inclusions in steel to become finer and dispersed, and provide more nucleation cores for the precipitation of sulfides in the solidification process of molten steel; ca element is easier to be dissolved in MnS to form Ca-Mn-S composite sulfide, thereby improving the high-temperature deformation resistance of the sulfide. On one hand, Ca and Mg elements are added into steel, so that dispersed complex sulfides with proper grain size and hard cores are finally formed in the steel, the anisotropy of the steel is improved while the cutting performance of the steel is ensured, and on the other hand, the respective addition amount of the Ca and Mg complex treatment is inevitably reduced, so that the production cost of the steel is not increased compared with the case of singly adopting calcium treatment or magnesium treatment.
Embodiments of another aspect of the present invention provide a method for preparing the low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel of the above embodiments, comprising: the molten steel is firstly treated by magnesium and then treated by calcium, or the molten steel is simultaneously treated by calcium and magnesium.
According to the preparation method provided by the embodiment of the second aspect of the invention, magnesium treatment and calcium treatment are sequentially carried out on molten steel, or calcium and magnesium treatment is simultaneously carried out, the form of sulfide in the steel is controlled through Ca and Mg composite treatment, the respective advantages of the two alloy elements in the sulfide modification process are fully exerted, and the sulfide modification is more effective and thorough.
It should be noted that if the molten steel is subjected to the calcium treatment and then the magnesium treatment, S is combined with Ca to form CaS, and thus the steel obtained by the calcium treatment and then the magnesium treatment has poor properties.
In the above technical solution, preferably, the magnesium treatment includes feeding a magnesium wire or a magnesium-aluminum wire into the molten steel, the calcium treatment includes feeding a calcium wire into the molten steel, and the calcium-magnesium treatment includes feeding a calcium-magnesium wire into the molten steel.
Of course, the magnesium treatment may be feeding other lines containing magnesium.
In the above technical solution, preferably, the preparation method further includes: smelting in a primary smelting furnace; after the primary smelting furnace taps steel, the ladle is sent into an LF furnace for refining, and the refining slag is controlled to comprise 55-60% of CaO and 4-10% of SiO in percentage by mass2、2％～8％MgO、10％～17％Al2O3、1％～5％CaF2Less than 0.6 percent of T.Fe, the refining time of the white slag is more than 20min, and the total refining time is 35-60 min; after refining, sending the molten steel into a vacuum refining furnace for vacuum treatment; after the vacuum treatment, the molten steel is firstly subjected to magnesium treatment and then calcium treatment, or the molten steel is simultaneously subjected to calcium and magnesium treatment.
In the above technical scheme, preferably, when tapping from the primary smelting furnace, the mass percent of carbon in steel is 0.08-0.12%, and the slag comprises 40-55% by mass of CaO and 10-30% by mass of SiO2、2％～10％Al2O3、1％～6％CaF2Less than or equal to 15 percent of FeO and 5 to 10 percent of MgO, wherein the mass fraction of P in the molten steel is less than or equal to 0.01 percent; and/or when the primary smelting furnace taps steel, the thickness of the slag entering the steel ladle from the primary smelting furnace is less than 50mm, and the tapping temperature is 1630-1650 ℃; and/or when the steel is tapped from the primary smelting furnace, at least one of Si-Fe, Mn-Fe, Cr-Fe, Ni-Fe and Mo-Fe is added into the molten steel for alloying.
In the above technical solution, preferably, the smelting in the primary smelting furnace specifically includes: when the primary smelting furnace is an electric arc furnace, waste steel or a mixture of the waste steel and molten iron is used as a raw material, the mass fraction of the waste steel in the mixture is not less than 40%, before the raw material is added into the primary smelting furnace, the mass fraction of C in the raw material is controlled to be more than 1.5%, and the mass fraction of P in the raw material is controlled to be less than 0.045%; controlling the temperature of the raw material to be more than or equal to 1550 ℃ in the initial oxidation stage, and controlling the continuous decarbonization amount to be more than or equal to 0.30%; the alkalinity of the furnace slag in the oxidation process is 2.5-3.5; adding lime according to the alkalinity of the slag after the oxygen blowing is finished, and controlling the mass fraction of P in the steel to be less than 0.01 percent; or the smelting of the primary smelting furnace specifically comprises the following steps: when the primary smelting furnace is a converter, the mass fraction of the converter end point C is more than or equal to 0.08%, and the slag alkalinity is more than or equal to 3.5; the final slag alkalinity after the smelting in the converter is more than or equal to 3.0, and the mass fraction of P is less than or equal to 0.01%.
In the above technical scheme, preferably, after the ladle is sent into the LF furnace, lime is gradually added for slagging, and aluminum is used for deoxidizing the slag; and/or the total slag amount is 14kg/t steel to 17kg/t steel in the process of feeding the steel ladle into the LF furnace for refining.
In the above technical solution, preferably, the sending the molten steel into the vacuum refining furnace after refining specifically includes: controlling the vacuum treatment time to be 25-35 min, and gradually increasing the vacuum degree to 100 Pa; and finely adjusting the components of the molten steel in the final stage of the vacuum treatment.
In the technical scheme, preferably, when the vacuum refining furnace is an RH refining furnace, the stirring gas amount is 10NL/min/t steel to 20NL/min/t steel; when the vacuum refining furnace is a VD refining furnace, the stirring gas amount is 2NL/min/t steel to 5NL/min/t steel.
In the above technical solution, preferably, before or after the magnesium treatment and then the calcium treatment are performed on the molten steel, the method further includes: performing soft blowing treatment, wherein the soft blowing treatment time is longer than or equal to 15min, and the argon flow is 30-50L/min; and/or, after the molten steel is firstly subjected to magnesium treatment and then calcium treatment, or the molten steel is simultaneously subjected to calcium and magnesium treatment, the method further comprises the following steps: continuous casting molding or die casting molding, wherein the superheat degree of molten steel in a tundish is 10-35 ℃, protective pouring is adopted in the whole continuous casting molding process, and a crystallizer electromagnetic stirring and solidification tail end electromagnetic stirring process is adopted; the die casting molding adopts argon blowing protection pouring and adopts liquid protection slag.
The main technical key point of the chalcogenide free-cutting steel is to control the form of sulfide in the steel, improve the high-temperature deformation resistance of the steel, reduce the deformation of the steel in the forging or rolling process, and improve the transverse impact toughness of the steel while ensuring the cutting performance of the steel. The key points of the invention are as follows: the components of the steel are strictly controlled in the production process of a primary smelting furnace, a ladle refining furnace, a vacuum refining furnace (RH refining furnace or VD refining furnace), continuous casting and mold injection, and the inclusion in the steel is removed to the maximum extent by adopting comprehensive technologies of controlling slag amount, white slag refining, molten steel stirring, vacuum treatment, tundish flow control, electromagnetic stirring and the like. And the inclusions are subjected to Ca and Mg composite treatment, and the deformation of sulfides in the rolling process is reduced by controlling the forms of oxides and sulfides in the steel, so that the aim of improving the anisotropy of the steel while ensuring the cutting performance of the steel is fulfilled.
The preparation method comprises the following steps:
1. the primary smelting furnace can adopt electric arc furnace smelting or converter smelting.
2. Controlling the carbon content (mass percent) in steel to be 0.08-0.12% when tapping from a primary smelting furnace; the slag comprises, by mass, 40-55% of CaO, 210-30% of SiO, 78-10% of Al2O 32, 21-6% of CaF, less than or equal to 15% of FeO, 5-10% of MgO, and less than or equal to 0.01% of P in molten steel, the thickness of the slag entering a ladle from a primary furnace is controlled to be less than 50mm by adopting a slag blocking ball, a slag blocking cone or a sliding plate for slag blocking, the tapping temperature is controlled to be 1630-1650 ℃, and Si-Fe, Mn-Fe, Cr-Fe, NI-Fe and Mo-Fe are added for alloying during tapping.
3. Putting the ladle into an LF station (ladle refining furnace), transmitting electricity to heat for more than 10min, gradually adding lime to make slag, and deoxidizing the slag by adopting aluminum particles; the refining slag components (by mass percent) are controlled to be 55-60 percent of CaO and SiO24％～10％、MgO 2％～8％、Al2O310％～17％、CaF21 to 5 percent, less than 0.5 percent of T.Fe and the total slag amount of about 14 to 17kg/t steel, so as toControlling the steel-making cost, wherein the white slag refining time is more than 20min, and the total refining time is 35-60 min.
4. Carrying out vacuum treatment on the molten steel by using a VD refining furnace or an RH refining furnace, controlling the vacuum treatment time to be 25-35 min, and gradually increasing the vacuum degree to 100 Pa; amount of stirring gas: the RH refining furnace is 10NL/min/t steel to 20NL/min/t steel, and the VD refining furnace is 2NL/min/t steel to 5NL/min/t steel.
5. Fine adjustment is carried out on the components of the molten steel in the last stage of vacuum treatment; after the vacuum treatment is finished, performing magnesium treatment on the molten steel, and then performing calcium treatment, or performing calcium and magnesium treatment at the same time; the soft blowing time is ensured to be more than or equal to 15min, and the argon flow is 30L/min to 50L/min.
6. The superheat degree of the molten steel of the tundish is controlled at 10-30 ℃, the continuous casting molding adopts totally-enclosed pouring, and the crystallizer and the solidification tail end of a casting blank are electromagnetically stirred; the die casting molding adopts argon blowing protection pouring.
Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
The above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
FIG. 1 is an SEM photograph of sulfide in a steel ingot (furnace No. 7) which has not undergone the Ca/Mg composite treatment;
FIG. 2 is an SEM photograph of sulfide in a steel ingot (furnace No. 1) subjected to Ca and Mg composite treatment;
FIG. 3 is an SEM photograph of sulfides precipitated with oxides as cores in a steel ingot (No. 1) subjected to Ca and Mg composite treatment;
FIG. 4 is an SEM photograph of sulfides in steel after forging without Ca and Mg composite treatment (furnace No. 7);
FIG. 5 is an SEM photograph of a complex sulfide containing an oxide core in a steel (furnace No. 1) subjected to Ca and Mg complex treatment after forging;
fig. 6 is a schematic flow chart of a method for manufacturing a low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel according to an embodiment of the present invention.
Detailed Description
In order that the above objects, features and advantages of the present invention can be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings. It should be noted that the embodiments and features of the embodiments of the present application may be combined with each other without conflict.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced otherwise than as specifically described herein, and thus the scope of the present invention is not limited by the specific embodiments disclosed below.
The low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel and the method for manufacturing the same according to some embodiments of the present invention will be described below with reference to the accompanying drawings.
According to some embodiments of the present invention, a low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel is provided, which comprises the following chemical components (by mass): 0.30-0.42% of C, 0.15-0.35% of Si, 0.75-1.05% of Mn, 1.0-1.5% of Cr1.04-0.07% of Mo, 0.001-0.02% of Ca, 0.001-0.02% of Mg, 0.02-0.05% of S, 0.025-0.035% of Alt, less than or equal to 0.015% of P, less than or equal to 0.001% of O, 0.002-0.005% of N, and the balance of Fe and other elements, wherein the other elements refer to inevitable impurity elements or other undetected or undetected elements.
C is one of the most effective elements for improving the strength of the steel, but the toughness and the plasticity of the steel are reduced due to the excessively high carbon content, and the hardness of the steel is increased along with the increase of the carbon content, so that the abrasion of a cutter is increased, and the service life of the cutter is reduced; the carbon content is too low, the steel is soft, and the steel chips are too long and difficult to break in the cutting process, so that the cutting performance is influenced. The preferred carbon content of the present invention ranges from 0.30% to 0.42%, and preferably, the mass percentage of C may be, but is not limited to, 0.30%, 0.34%, 0.38%, or 0.42%.
Si acts as a solid solution strengthening effect in steel, and improves the strength of steel. However, too high a Si content lowers the cold formability of the steel. Meanwhile, the hardness of inclusions produced by combining Si and O is higher, so that the abrasion of the cutter is increased. The Si content of the invention is controlled in the range of 0.15-0.35%, and preferably, the Si mass percentage can be but not limited to 0.15%, 0.20%, 0.25%, 0.30% or 0.35%.
The Mn content is controlled between 0.75 percent and 1.05 percent. Mn is combined with S in the steel to form MnS, plays roles of lubrication and cutting in the cutting process, and improves the cutting performance of the steel. Preferably, the mass percentage of Mn may be, but is not limited to, 0.75%, 0.85%, 0.95%, or 1.05%.
Cr can improve the strength of steel and increase the oxidation resistance and corrosion resistance of steel. The Cr content is 1.0-1.5%. Preferably, the mass percentage of Cr may be, but is not limited to, 1.0%, 1.25%, or 1.5%.
The content of Mo is 0.04-0.07%. Mo can strengthen ferrite and improve the strength of steel, and meanwhile, Mo can greatly improve the brittleness of steel and improve the toughness of steel. Too high a content of Mo increases the cost and at the same time increases the hardness of the steel and increases the tool wear. Preferably, the mass percentage of Mo may be, but is not limited to, 0.04%, 0.05%, 0.06%, or 0.07%.
The content of Ca is 0.001-0.02%. The Ca can modify MnS in the steel to form Mn-Ca-S composite sulfide, thereby improving the high-temperature deformation resistance of the sulfide and improving the transverse impact toughness of the steel. CaS inclusions can be formed in steel due to the fact that the content of Ca is too high, water gaps are blocked, and meanwhile, the hardness of the CaS inclusions is high, so that abrasion of cutting tools can be increased. Too low Ca content may result in incomplete MnS modification in the steel. The preferable range of Ca content in the invention is 0.001-0.02%. Preferably, the mass percentage of Ca may be, but is not limited to, 0.001%, 0.005%, 0.01%, 0.015%, or 0.02%.
The Mg content is 0.001-0.02%. Mg is combined with Al and O in steel to form Mg-Al-O inclusion, so that a nucleation core is provided for the precipitation of sulfide in the solidification process, and meanwhile, Mg can be slightly dissolved in MnS in a solid solution manner, so that the high-temperature deformation resistance of the sulfide is improved. Too low Mg content to form nucleation cores, too little for Al2O3The denaturation of the inclusion is insufficient, and the effect is limited. The Mg content is too high, MgS inclusion can be formed, and the service life of the cutter is reduced. The preferable Mg content range of the invention is 0.001-0.02%. Preferably, the mass percentage of Mg may be, but is not limited to, 0.001%0.005%, 0.01%, 0.015% or 0.02%.
The S content is 0.02-0.05%. S is combined with Mn in the steel to form MnS, plays roles of lubrication and cutting in the cutting process, and improves the cutting performance of the steel. The content of S is too low, and the effect of improving the cutting performance of the steel is insufficient; too high content can form a large amount of MnS inclusions in the steel, so that the modification is difficult, and the transverse impact toughness of the steel is reduced. The preferable S content range of the invention is 0.02-0.05%. Preferably, the mass percentage of S may be, but is not limited to, 0.02%, 0.03%, 0.04%, or 0.05%.
The Alt content is 0.025-0.035%. Al is a deoxidizer of steel, reduces the content of O in the steel, and forms Mg-Al-O inclusion together with Mg to provide a nucleation core for the precipitation of sulfide in the solidification process. Preferably, the mass percentage of Alt may be, but is not limited to, 0.025%, 0.030%, or 0.35%.
P can form micro segregation when molten steel is solidified, and is segregated on austenite grain boundaries, so that the grain boundaries are embrittled, and the brittleness of steel is increased, and therefore, the upper limit of the content of P is set to 0.015%. Preferably, the mass percentage of P may be, but is not limited to, 0.005%, 0.01%, or 0.015%.
O can form inclusions by the deoxidizing elements in the steel, and reduces the fatigue life of the steel and the tool life, and O reduces the effect of Ca-denatured MnS in the steel, so the upper limit of the O content is set to 0.001%. Preferably, the mass percentage of O may be, but is not limited to, 0.0005% or 0.001%.
N can form nitride or carbonitride with alloy elements in the steel, so that grains are refined, and the toughness of the steel is improved; however, too high N content may reduce the mechanical properties of the steel and form large-grained nitride inclusions in the steel, which may seriously reduce the fatigue life of the steel, so that the N content in the steel is controlled to be between 0.002% and 0.005%. Preferably, the mass percentage of N may be, but is not limited to, 0.002%, 0.003%, 0.004%, or 0.005%.
According to the range of the chemical components of the steel grade, 8 furnaces of steel are smelted by a 30kg vacuum induction furnace; the 6-furnace steel is prepared according to the preparation method of the invention, namely, the 6-furnace steel is sequentially subjected to magnesium treatment and calcium treatment or simultaneously subjected to calcium and magnesium treatment, the furnace number is 1-6, the two-furnace steel is not subjected to Mg and Ca composite treatment and is used as comparison steel, and the furnace numbers are 7 and 8. Cast into ingots under vacuum and then forged into rods of 20mm diameter. The forging technological parameters are as follows: heating to 1250 ℃ along with the furnace, preserving heat for 20min, wherein the initial forging temperature is more than or equal to 1150 ℃, and the final forging temperature is more than or equal to 850 ℃. Table 1 shows the main chemical components of the 8-furnace test steels. Sampling on a steel ingot, detecting and analyzing the form of sulfide in steel, wherein the sulfide in the steel which is not subjected to Ca and Mg composite treatment is mainly distributed along grain boundaries, as shown in figure 1, the sulfide in the steel which is subjected to Ca and Mg composite treatment is granular, and the amount of the sulfide separated out along the grain boundaries is obviously reduced, as shown in figure 2. This is mainly because many fine oxides of Mg-Al-O are formed in the steel after the Ca and Mg composite treatment, and the steel nucleates and precipitates with these fine oxides as a core during the solidification process, as shown in fig. 3.
Table 1 main chemical composition (mass percentage) of test steel
Note: the chemical components not listed in the table are: the mass percent of Alt is 0.025-0.035%, and the mass percent of P is less than or equal to 0.015%.
The mechanical properties of the 8-furnace test steels were measured at room temperature (as shown in table 2): the tensile test is tested according to the GB/T228-2002 standard, and the sample size is L0-5 d0, and d 0-10 mm; the impact test was carried out using a pendulum impact tester according to GB/T229-1994, with specimen dimensions of 10X 55mm and a U-shaped recess. As can be seen from Table 2, the transverse impact toughness of the steel treated by Ca and Mg combination is significantly improved, which is mainly related to the form of sulfide in the steel. The sulfide in the steel not subjected to the Ca and Mg composite treatment has poor deformation resistance, and after the steel is deformed (forged or rolled), the sulfide in the steel elongates in the deformation direction, as shown in fig. 4. As shown in FIG. 3, the sulfide in the steel subjected to the Ca and Mg composite treatment contains a nucleation core, the deformation of the sulfide is much smaller, and as shown in FIG. 5, the transverse impact toughness of the steel is improved.
TABLE 2 mechanical Properties of the test steels
The test steels were tested for machinability and the results are shown in table 3. The tool life was checked according to the International standards Institute (ISO) standards (the tool life was determined as the amount of wear VB behind the tool reaching 0.3 mm).
TABLE 3 machinability of test steels
As shown in fig. 6, the preparation method in the present application includes:
step S10, smelting in a primary smelting furnace: the primary smelting furnace adopts an electric arc furnace or a converter for smelting.
When the primary smelting furnace is an electric arc furnace:
a) the raw material adopts scrap steel or a mixture of the scrap steel and molten iron, wherein the proportion of the scrap steel is not less than 40 percent, C in the raw material is controlled to be more than 1.5 percent before the raw material is put into a furnace, and P is controlled to be less than 0.045 percent;
b) oxygen blowing fluxing (oxygen purity is more than 99.5%), controlling the temperature of the raw materials to be more than or equal to 1550 ℃ in the initial oxidation stage, and continuously decarbonizing the raw materials to be more than or equal to 0.30%, ensuring enough decarbonizing amount, and removing gas and impurities in the steel by boiling the molten steel;
c) forming foam slag with good fluidity as early as possible in the oxidation process, and controlling the alkalinity of the slag to be 2.5-3.5;
d) according to the slag alkalinity R (CaO/SiO) after the oxygen blowing is finished2) And replenishing lime, controlling the phosphorus content in the steel to be stabilized below 0.01 percent, tapping, and finishing electric furnace smelting.
When the primary smelting furnace is a converter:
a) the carbon content at the end point of the converter is not less than 0.08 percent, and the molten steel is prevented from being oxidized; controlling slag basicity R (CaO/SiO)2)≥3.5；
b) Controlling the final slag alkalinity R (CaO/SiO) after the smelting of the converter is finished2) Stopping slag and tapping more than or equal to 3.0 percent and P less than or equal to 0.01 percent, and finishing converter smelting.
When the steel is tapped from the primary smelting furnace, the carbon content in the steel is controlled to be 0.08-0.12 percentThe slag comprises (by mass) CaO 40-55% and SiO210～30％、Al2O32～10％、CaF21-6% of FeO, less than or equal to 15% of MgO, less than or equal to 0.01% of P in molten steel, controlling the thickness of slag entering a steel ladle from a primary furnace to be less than 50mm by adopting a slag blocking ball or a slag blocking cone or a sliding plate to block slag, controlling the tapping temperature to be 1630-1650 ℃, and adding Si-Fe, Mn-Fe, Cr-Fe, NI-Fe and Mo-Fe for alloying during tapping.
Step S20, refining in a ladle refining furnace: putting the ladle into an LF station, transmitting electricity to heat for more than 10min, gradually adding lime to slag, and deoxidizing slag by adopting aluminum particles; the total slag amount is 14-17 kg/t steel, the refining slag components (by mass percent) are controlled to be 55-60 percent of CaO and SiO24～10％、MgO 2～8％、Al2O310～17％、CaF21-5%, less than 0.6% of T.Fe, more than 20min of white slag refining time, and 35-60 min of total refining time.
Step S30, vacuum processing: carrying out vacuum treatment on the molten steel by using a VD refining furnace or an RH refining furnace, controlling the vacuum treatment time to be 25-35 min, and gradually increasing the vacuum degree to 100 Pa; amount of stirring gas (Ar): RH is 10NL/min/t steel-20 NL/min/t steel, VD is 2NL/min/t steel-5 NL/min/t steel; fine adjustment is carried out on the components of the molten steel in the last stage of vacuum treatment; after the vacuum treatment is finished, magnesium treatment is firstly carried out on the molten steel, then calcium treatment is carried out on the molten steel, or calcium and magnesium treatment is carried out simultaneously; the soft blowing time is guaranteed to be more than or equal to 15min, the argon flow is 30-50L/min, argon is blown in by soft blowing, the soft blowing can be carried out after vacuum treatment before magnesium treatment or calcium and magnesium treatment, and the soft blowing treatment can also be carried out after calcium treatment and before calcium and magnesium treatment; and all components in the steel at the end of refining meet the component requirements of the low-oxygen calcium-magnesium-containing sulfur-series free-cutting steel in the application.
Step S40, continuous casting/die casting: the superheat degree of the molten steel in the tundish is controlled to be 10-35 ℃, protective pouring is adopted in the whole continuous casting process, electromagnetic stirring is adopted at the solidification tail end of a crystallizer and a casting blank, the components and the temperature of the molten steel are uniform, and the solidification structure of the molten steel is improved; controlling reasonable pulling speed according to the section size of the casting blank; the die casting adopts argon blowing protection pouring and liquid protection slag.
The preparation method of the low-oxygen calcium-containing magnesium-sulfur free-cutting steel is illustrated by taking a primary refining furnace as a converter and a vacuum refining furnace as an RH refining furnace as an example. The metallurgical production method of the low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel comprises the following steps: the molten iron pretreatment-converter-ladle refining furnace (LF furnace) -vacuum refining furnace (RH refining furnace) -continuous casting, the molten steel amount is 120 tons.
Raw materials: molten iron and scrap steel (the mass ratio of the molten iron to the scrap steel is 8:2), and the raw materials need to be kept clean and dry before entering the primary smelting furnace.
The converter process comprises the following steps: the carbon content of the converter end point is controlled to be 0.10 percent, and the final slag alkalinity (CaO/SiO)2) 3.5, the FeO content is less than 10 percent, and the P content is 0.012 percent; the tapping temperature is 1645 ℃; slag is blocked by adopting a slag blocking ball, and the thickness of a slag layer entering a steel ladle from a converter is controlled to be less than 50 mm; during the tapping process, Si-Fe, Mn-Fe, Cr-Fe, Ni-Fe and Mo-Fe are added for alloying.
The process of the ladle refining furnace (LF furnace) comprises the following steps: active lime is gradually added for slagging, Al particles are added to the surface of the slag for deoxidation, and the sum of MnO and FeO content in the slag is reduced to be less than 1.0%. The refining slag comprises the following components in percentage by mass: CaO 55-60%, SiO24％～10％、MgO 2％～8％、Al2O310～17％、CaF21 to 5 percent, less than 0.5 percent of T.Fe (total iron) and 17kg/t steel of total slag amount. The refining time of the white slag is 23min, and the total refining time is 57 min.
Vacuum refining furnace (RH) process: and (3) performing final deoxidation by adopting Al, gradually increasing the RH vacuum degree to 65Pa, controlling the RH argon blowing flow to 1200NL/min, and finely adjusting the components of the molten steel at the final stage of vacuum treatment, wherein the refining time is 30 min. After the vacuum treatment, firstly feeding 150 meters of Mg-Al wire and then 150 meters of Ca wire into the steel, wherein the soft blowing time is 18min, and the argon flow is 35L/min. The tapping temperature is controlled to be about 1560 ℃.
The continuous casting process comprises the following steps: optimizing a tundish flow field, wherein the superheat degree of molten steel is 30 ℃; electromagnetic stirring is adopted for the crystallizer and the solidification tail end of the casting blank; the whole process of continuous casting protects the casting and prevents the molten steel from breathing in. The cross section size of the continuous casting billet is 150 multiplied by 150mm, and the drawing speed is 2.1 m/min.
In summary, the low-oxygen calcium-magnesium-containing chalcogenide free-cutting steel provided by the embodiment of the invention controls the Alt (Alt represents the total aluminum content in the steel, is related to the oxygen content and the inclusions in the steel and is one of the important indexes of steel components) and the O content in the steel, and adds a certain amount of Mg and Ca elements in the steel to form dispersed composite sulfides with proper grain size and hard cores in the steel, thereby ensuring the cutting performance of the steel, improving the anisotropy of the steel, having excellent mechanical performance and cutting performance, and having high surface finish after cutting. The smelting production method comprises the following steps: the process comprises primary smelting, ladle refining, vacuum treatment (VD refining furnace or RH refining furnace), continuous casting/die casting and forming, and can obviously reduce the number of inclusions in steel, control the size and the shape of the inclusions, particularly control the form of sulfides in the steel, and improve the fatigue property and the isotropy of the steel.
In the description of the present invention, the term "plurality" means two or more unless explicitly specified or limited otherwise; the terms "connected," "secured," and the like are to be construed broadly and unless otherwise stated or indicated, and for example, "connected" may be a fixed connection, a removable connection, an integral connection, or an electrical connection; may be directly connected or indirectly connected through an intermediate. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
In the description of the present specification, it is to be understood that the terms "upper", "lower", "front", "rear", "left", "right", and the like indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of describing the present invention and simplifying the description, but do not indicate or imply that the referred device or unit must have a specific direction, be configured and operated in a specific orientation, and thus, should not be construed as limiting the present invention.
In the description herein, the description of the terms "one embodiment," "some embodiments," "specific embodiments," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A low-oxygen calcium-magnesium-containing sulfur-series free-cutting steel is characterized by comprising the following chemical components: the weight percentage is 0.30% -0.42% C, 0.15% -0.35% Si, 0.75% -1.05% Mn, 1.0% -1.5% Cr, 0.04% -0.07% Mo, 0.001% -0.020% Ca, 0.001% -0.02% Mg, 0.02% -0.05% S, 0.025% -0.035% Alt, less than or equal to 0.015% P, less than or equal to 0.001% O, 0.002% -0.005% N.
2. A production method for producing the low-oxygen calcium-magnesium-containing sulfur-based free-cutting steel according to claim 1, comprising:
the molten steel is firstly treated by magnesium and then treated by calcium, or the molten steel is simultaneously treated by calcium and magnesium.
3. The production method according to claim 2,
the magnesium treatment comprises feeding a magnesium wire or a magnesium-aluminum wire into the molten steel, the calcium treatment comprises feeding a calcium wire into the molten steel, and the calcium and magnesium treatment comprises feeding the calcium-magnesium wire into the molten steel.
4. The production method according to claim 2 or 3, characterized by further comprising:
smelting in a primary smelting furnace;
after the primary smelting furnace taps steel, the ladle is sent into an LF furnace for refining, wherein the refining slag comprises 55-60% CaO and 4-10% SiO in percentage by mass2、2％～8％MgO、10％～17％Al2O3、1％～5％CaF2Less than 0.6 percent of T.Fe, the refining time of the white slag is more than 20min, and the total refining time is 35-60 min;
after refining, sending the molten steel into a vacuum refining furnace for vacuum treatment;
after the vacuum treatment, the molten steel is firstly subjected to magnesium treatment and then calcium treatment, or the molten steel is simultaneously subjected to calcium and magnesium treatment.
5. The production method according to claim 4,
when the steel is tapped from the primary smelting furnace, the mass percent of carbon in steel is 0.08-0.12%, and the slag comprises 40-55% of CaO and 10-30% of SiO2、2％～10％Al2O3、1％～6％CaF2Less than or equal to 15 percent of FeO and 5 to 10 percent of MgO, wherein the mass fraction of P in the molten steel is less than or equal to 0.01 percent; and/or the presence of a gas in the gas,
when the primary smelting furnace taps steel, the thickness of the slag entering the steel ladle from the primary smelting furnace is less than 50mm, and the tapping temperature is 1630-1650 ℃; and/or the presence of a gas in the gas,
when the steel is tapped from the primary smelting furnace, at least one of Si-Fe, Mn-Fe, Cr-Fe, NI-Fe and Mo-Fe is added into the molten steel for alloying.
6. The production method according to claim 4,
the smelting of the primary smelting furnace specifically comprises the following steps:
when the primary smelting furnace is an electric arc furnace, waste steel or a mixture of the waste steel and molten iron is used as a raw material, the mass fraction of the waste steel in the mixture is not less than 40%, before the raw material is added into the primary smelting furnace, the mass fraction of C in the raw material is controlled to be more than 1.5%, and the mass fraction of P in the raw material is controlled to be less than 0.045%;
controlling the temperature of the raw material to be more than or equal to 1550 ℃ in the initial oxidation stage, and controlling the continuous decarbonization amount to be more than or equal to 0.30%;
the alkalinity of the furnace slag in the oxidation process is 2.5-3.5;
adding lime according to the alkalinity of the slag after the oxygen blowing is finished, and controlling the mass fraction of P in the steel to be less than 0.01 percent; alternatively, the first and second electrodes may be,
the smelting of the primary smelting furnace specifically comprises the following steps:
when the primary smelting furnace is a converter, the mass fraction of the converter end point C is more than or equal to 0.08%, and the slag alkalinity is more than or equal to 3.5;
the final slag alkalinity after the smelting in the converter is more than or equal to 3.0, and the mass fraction of P is less than or equal to 0.01%.
7. The production method according to claim 4,
after the ladle is sent into the LF furnace, lime is gradually added for slagging, and aluminum is adopted to deoxidize the slag; and/or the presence of a gas in the gas,
and in the process of feeding the ladle into the LF furnace for refining, the total slag amount is 14kg/t steel to 17kg/t steel.
8. The production method according to claim 4,
the step of sending the molten steel into a vacuum refining furnace for vacuum treatment after refining specifically comprises the following steps:
controlling the vacuum treatment time to be 25-35 min, and gradually increasing the vacuum degree to 100 Pa;
and finely adjusting the components of the molten steel in the final stage of the vacuum treatment.
9. The method according to claim 8,
when the vacuum refining furnace is an RH refining furnace, the amount of stirring gas is 10NL/min/t steel to 20NL/min/t steel; when the vacuum refining furnace is a VD refining furnace, the stirring gas amount is 2NL/min/t steel to 5NL/min/t steel.
10. The production method according to claim 2 or 3,
the method comprises the following steps of firstly carrying out magnesium treatment on the molten steel, and then carrying out calcium treatment, or before or after simultaneously carrying out calcium and magnesium treatment on the molten steel, further comprising: performing soft blowing treatment, wherein the soft blowing treatment time is longer than or equal to 15min, and the argon flow is 30-50L/min; and/or the presence of a gas in the gas,
the magnesium treatment is carried out to the molten steel earlier, calcium treatment is carried out again, or, after carrying out calcium magnesium treatment to the molten steel simultaneously, still include: continuous casting molding or die casting molding, wherein the superheat degree of molten steel in a tundish is 10-35 ℃, protective pouring is adopted in the whole continuous casting molding process, and a crystallizer electromagnetic stirring and solidification tail end electromagnetic stirring process is adopted; the die casting molding adopts argon blowing protection pouring and adopts liquid protection slag.
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