A microalloyed low cost martensitic stainless steel and a method for manufacturing the same

Abstract

The present application relates to a kind of microalloy low-cost martensitic stainless steel and its manufacturing method, based on the corrosion-resistant element Ni, Cr rich in laterite nickel ore, with laterite nickel ore smelting molten iron as the basis, while using ultra-low P industrial pure iron and high-carbon chromium iron for batching, to design the chemical composition of microalloy low-cost martensitic stainless steel, it includes the following chemical composition by mass percentage: C:0.26~0.29%, Si:0.19~0.49%, Mn:0.19~0.49%, S≤0.0015%, P≤0.015%, Ni:1.1~1.9%, Cr:12.1~13.9%, Ti:0.0050~0.0060%, 90×10 ‑4 %≤Zr+Mg≤130×10 ‑4 %, and the content of Zr, Mg satisfies Zr / Mg=2~4, the balance is Fe and inevitable impurities.The present application reduces production cost, improves the performance of local corrosion resistance, especially the performance of seawater corrosion resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metal materials, and particularly relates to a micro-alloy low-cost martensitic stainless steel and a manufacturing method thereof. BACKGROUND

[0002] With the consumption of conventional iron ore resources, laterite nickel ore as a surface layer mineral resource rich in micro-alloying elements and rare earth elements is increasingly concerned, and a large number of researchers have studied the smelting and application of laterite nickel ore. As a commonly used stainless steel, martensitic stainless steel is used in daily production and life, and whether the smelting and application of the marine corrosion-resistant martensitic stainless steel can be carried out by using laterite nickel ore has not been deeply studied by relevant researchers.

[0003] Martensitic stainless steel has excellent corrosion resistance and is mainly used as a raw material of tool steel, trimming die, cold cutting scissors, other ultra-high strength stainless steel, etc. In the process of daily life, Cl -The coastal, marine and other use environments cause serious damage to the passive film on the surface of stainless steel, and easily induce local corrosion at the defects of micro-nano particles such as corrosion active inclusions in steel, which seriously damages the continuity and uniformity of stainless steel and seriously affects the performance of steel materials. As the most common inclusion in stainless steel, MnS inclusion has a lower electrode potential than the passive film on the surface of the surrounding matrix, and often acts as an anode to dissolve preferentially. The active dissolution of sulfide generates aggressive ions that further promote the dissolution of inclusions and the surrounding matrix of stainless steel [Krawiec H, Vignal V, Heintz O, et al. Influence of the dissolution of MnS inclusions under free corrosion and potentiostatic conditions on the composition of passive films and the electrochemical behaviour of stainless steels. [J]. Electrochim. Acta., 2006, 51(16): 3235-3243; Krawiec H, Vignal V, Oltra R. Use of the electrochemical microcell technique and the SVET for monitoring pitting corrosion at MnS inclusions [J]. Electrochem. Commun., 2004, 6(7): 655-660]. In addition, Williams et al. found that there is a 100 nm FeS-rich halo around the MnS inclusions in stainless steel, and believed that the dissolution of FeS is the main reason for the initiation of pitting corrosion in stainless steel [Williams D E, Kilburn M R, Cliff J, et al. Composition changes around sulphide inclusions in stainless steels, and implications for the initiation of pitting corrosion [J]. Corros. Sci., 2010, 52(11): 3702-3716].Wranglen et al. pointed out that inclusions in steel are divided into active inclusions and non-active inclusions, and the sulfur contamination zone around sulfide is more likely to dissolve relative to the matrix, and such inclusions are active inclusions; and the matrix around non-active inclusions will not dissolve [Wranglen G. Pitting and sulphide inclusions in steel[J]. Corros. Sci, 1974, 14: 331-349].

[0004] By adopting different deoxidation methods to modify the inclusions in steel, the type of inclusions generated in steel can be controlled, thereby affecting the effect on the local corrosion process. It is reported that TiN and ZrO2 inclusions have better pitting corrosion resistance than MnS inclusions. Liu Chao et al. proved by CSAFM technology that TiN and ZrO2 inclusions do not have electrical conductivity, so they cannot form galvanic corrosion with the iron matrix. Wei Wentui and Yin Zhaochao et al. calculated the work function of different types of inclusions in steel by first principles, and the results showed that TiN and ZrO2 inclusions have higher work function than sulfide inclusions and Fe matrix, indicating that TiN and ZrO2 inclusions are more stable. In view of this, the present application provides a micro-alloyed low-cost martensitic stainless steel and a manufacturing method thereof. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a micro-alloyed low-cost martensitic stainless steel and a manufacturing method thereof. The purpose is to reduce the production cost of martensitic stainless steel and improve the local corrosion resistance, especially the seawater corrosion resistance.

[0006] In order to solve the above technical problems, the first object of the present application is to provide a micro-alloyed low-cost martensitic stainless steel, which comprises the following chemical components in mass percentage: C: 0.26-0.29%, Si: 0.19-0.49%, Mn: 0.19-0.49%, S≤0.0015%, P≤0.015%, Ni: 1.1-1.9%, Cr: 12.1-13.9%, Ti: 0.0050-0.0060%, 90×10 -4 %≤Zr+Mg≤130×10 -4 %, and the content of Zr and Mg satisfies Zr / Mg=2-4, and the balance is Fe and inevitable impurities.

[0007] The present application is based on the fact that laterite nickel ore is rich in corrosion-resistant elements Ni and Cr, and the chemical composition of the martensitic stainless steel is designed based on the laterite nickel ore smelting molten iron, and the ultra-low P industrial pure iron and high-carbon chromium iron are used for batching, so as to improve the corrosion resistance of the matrix. More importantly, in view of the local corrosion induced by large sulfides and oxides in the conventional martensitic stainless steel, the present application uses Zr and Mg (90x10 -4 %≤Zr+Mg≤130x10 -4 % and the content of Zr and Mg satisfies Zr / Mg=2-4, to replace the conventional Si and Al deoxidization, so as to form fine and dispersed composite sulfides and oxides, and the intergranular corrosion sensitivity is obviously reduced. Through the comparative evaluation of the local corrosion resistance of the micro-alloy low-cost martensitic stainless steel of the present application and the conventional martensitic stainless steel in 3.5% sodium chloride solution (simulated seawater), it is shown that the corrosion resistance of the micro-alloy martensitic stainless steel of the present application, especially the seawater corrosion resistance, is significantly improved.

[0008] The micro-alloy low-cost martensitic stainless steel of the present application is mainly used for making corrosion-resistant medium, parts and structural parts subjected to high impact load, such as blades used in steam turbines, valves used in hydraulic pressure machines and important structural parts of hydraulic pressure machines; at the same time, the micro-alloy low-cost martensitic stainless steel of the present application can also be used for making parts with high hardness and high wear resistance, such as cold stamping die steel, plastic die steel and shafts, antibacterial tools used in medical surgical procedures; in addition, the micro-alloy low-cost martensitic stainless steel of the present application can be used for making parts and large equipment used in strong acid and alkali and gas corrosion environment, and stainless steel cutters used for scissors and other continuously used cutters.

[0009] In the micro-alloy low-cost martensitic stainless steel of the present application, each element plays the following role:

[0010] C: the content is generally controlled to be less than or equal to 0.03%, and the low carbon content not only ensures the good strength and toughness of the material, but also greatly improves the weldability, cold workability and corrosion resistance of the steel. However, with the decrease of the carbon content, the strength of the steel will be reduced, and Ti, Nb and other elements are added in the martensitic age-hardening stainless steel to make up for the strength of the steel, and a dispersion strengthening phase is formed during the heat treatment process. C and Cr can form stable carbides such as Cr 23 C6 precipitated at the grain boundary, which not only reduces the effective Cr content in the steel matrix but also deteriorates the corrosion resistance of the material, therefore, the mass content of C in the present application is 0.26-0.29%, which is beneficial to the improvement of corrosion resistance.

[0011] Cr: Cr plays a key role in the effect of corrosion resistance of martensitic stainless steel, the Cr content of conventional martensitic stainless steel is generally 10.5% to 18%, in the material resistance to environmental corrosion, Cr can effectively improve the pitting potential of the material, thereby reducing the sensitivity of pitting corrosion, Cr can form a dense Cr2O3 protective film on the surface of the steel, thereby achieving good corrosion resistance. In the aging heat treatment, the combination of chromium and other alloying elements leads to the reduction of the effective Cr content of the matrix, ensuring that the basic requirement of the corrosion resistance of martensitic stainless steel must contain 10.5% of chromium content. At the same time, when the Cr content is too high, some ferrite will appear in the matrix after solid solution treatment, which will reduce the amount of martensite in the matrix, and the ferrite in the matrix has a certain influence on the hot plasticity of the material, which will reduce the strength, corrosion resistance and transverse toughness of the steel. In order to obtain good performance, the weight percentage of Cr added in the application is 12.1% to 13.9%.

[0012] Ni: The nickel content of conventional martensitic stainless steel is in the range of 6% to 11%, Ni performs excellently in expanding the austenite phase region and improving the stability of austenite, it is a typical element of forming austenite, nickel element will reduce the initial transformation temperature Ms of martensite, which is more intense than the effect of chromium. In order to ensure that the martensitic stainless steel is completely transformed into low-carbon lath martensite during the cooling process from high temperature to room temperature during solid solution treatment, the nickel content in the material cannot be too high. Nickel is infinitely soluble in the iron matrix, which makes it difficult to decompose the screw dislocation in the material, and makes cross slip occur, which can reduce the interaction between dislocations and interstitial atoms. In order to make the martensitic stainless steel have good toughness, Ni can form intermetallic compounds with Ti, Al and other elements in the aging heat treatment. The precipitation strengthening effect of these intermetallic compounds is the key to improve the strength of high-strength martensitic stainless steel, so the mass content of Ni added in the application is 1.1% to 1.9%.

[0013] Mn: Mn has a small effect on expanding the austenite region, the stability of Mn to the austenite phase region is slightly lower than that of Ni, and Mn has a good promoting effect on the hardenability of steel. The Mn content in martensitic stainless steel is generally ≤1%, manganese element can replace nickel element in martensitic stainless steel, which can reduce the cost of martensitic stainless steel and save nickel resources. It should be noted that this is only to replace nickel to obtain austenite in organization, but it cannot replace nickel in corrosion resistance. The addition of Mn will deteriorate the corrosion resistance of low Cr content stainless steel. As a deoxidizer in martensitic stainless steel, manganese can form MnS with sulfur element to inhibit the adverse effect of sulfur on the matrix, so as to improve the hot plasticity of stainless steel. Therefore, the mass content of Mn added in the application is 0.19% to 0.49%.

[0014] Si: Silicon is a strong delta ferrite stabilizing element, which can increase the temper brittleness temperature of the martensitic stainless steel, and inhibit the decomposition of residual austenite. Silicon can form silicon oxide with oxygen atoms on the surface of the stainless steel, so as to improve the high temperature resistance and sulfuric acid corrosion resistance of the stainless steel. Silicon can also improve the oxidation resistance of the steel. In the corrosion environment containing Cl - , silicon can inhibit the further formation of pitting corrosion, and has a good improvement effect on the pitting corrosion of the martensitic age-hardened stainless steel. However, the content of silicon should not be too high, otherwise the plasticity and toughness of the stainless steel will be seriously affected, and the weldability will also be reduced. Therefore, the content of silicon added in the present application is 0.19-0.49%.

[0015] S: Sulfur is considered as a harmful impurity in austenitic stainless steel. The harmful effects of sulfur mainly include: reducing the hot plasticity of the austenitic stainless steel, and affecting the hot workability of the steel, which is related to the precipitation of MnS or (Fe, Mn) S along the grain boundary at high temperature; sulfur also reduces the corrosion resistance of the austenitic stainless steel, and MnS is easily dissolved in acidic chloride solution, which often becomes a corrosion source to cause a significant reduction in the pitting corrosion resistance and crevice corrosion resistance.

[0016] P: Phosphorus is generally considered as a harmful impurity in austenitic stainless steel. The harmful effects of phosphorus mainly include: significantly reducing the corrosion resistance of chromium-nickel austenitic stainless steel in solid solution state and sensitized state in nitric acid of various concentrations; significantly enhancing the intergranular corrosion sensitivity of chromium-nickel austenitic stainless steel in solid solution state in concentrated nitric acid and nitric acid containing Cr 5+ , and reducing the corrosion resistance under these use conditions.

[0017] Zr: Zirconium is a strong carbide forming element, a strong deoxidizing element and a complex oxysulfide forming element. A small amount of zirconium can have the effects of degassing, purification and grain refinement, which is beneficial to improve the low temperature performance and stamping performance of the stainless steel, and significantly improve the hardenability of the steel when dissolved in austenite. In austenitic steel, it can prevent intergranular corrosion of the steel in oxidizing medium. Due to the fixed carbon and precipitation hardening effect, the high temperature performance of the high strength steel can be improved, such as creep strength.

[0018] Mg: Magnesium is a strong deoxidizing element and a complex oxysulfide forming element. Magnesium can reduce the number, size and distribution of inclusions in the steel, and improve the morphology of inclusions. Trace amounts of magnesium can improve the size and distribution of carbides in the stainless steel. The MgO inclusions formed have the effect of pinning the austenite grain boundary, and have a good control effect on the grain size.

[0019] In summary, the present application uses the molten iron rich in chromium and nickel from laterite nickel ore smelting as smelting raw material, and uses ultra-low P industrial pure iron and high-carbon chromium iron for batching, saves the precious metal Ni, and the prepared low-cost martensitic stainless steel does not contain high-cost alloy elements such as Mo and Nb, and good mechanical properties are obtained by using low-cost silicon, manganese, chromium and other elements; meanwhile, zirconium and magnesium elements are added, the welding performance is improved, and good corrosion resistance is obtained.

[0020] The present application has the following advantages:

[0021] (1) The present application uses the molten iron rich in chromium and nickel from laterite nickel ore smelting as smelting raw material, and uses ultra-low P industrial pure iron and high-carbon chromium iron for batching, saves the precious metal Ni, and good mechanical properties are obtained by using low-cost silicon, manganese, chromium and other elements; the present application adds zirconium and magnesium elements, the plasticity and toughness are improved, and the welding performance is improved.

[0022] (2) Compared with conventional martensitic stainless steel, the constant electrode potential saturation current density of the micro-alloyed low-cost martensitic stainless steel of the present application is obviously reduced, and the seawater corrosion resistance is more excellent than that of traditional martensitic stainless steel; on the one hand, the composite deoxidizing elements Zr and Mg significantly modify the corrosion active inclusions, so that the microstructure and morphology are obviously changed, the corrosion active MnS is wrapped by TiN, the density of corrosion active inclusions is greatly reduced, and the seawater local corrosion resistance is obviously improved; on the other hand, the Ni and Cr elements brought by laterite nickel ore improve the corrosion resistance of the matrix, and to some extent, help to improve the seawater corrosion resistance.

[0023] On the basis of the above technical scheme, the present application can be further improved as follows.

[0024] Further, it comprises the following chemical components in mass percentage: C: 0.26-0.27%, Si: 0.19-0.29%, Mn: 0.19-0.29%, S≤0.0013%, P≤0.012%, Ni: 1.1-1.4%, Cr: 12.1-13%, Ti: 0.0050-0.0060%, 90×10 -4 %≤Zr+Mg≤130×10 -4 %, and the content of Zr and Mg satisfies Zr / Mg=2-4, and the balance is Fe and inevitable impurities.

[0025] Further, it comprises the following chemical components in percentage by mass: C: 0.27-0.29%, Si: 0.29-0.49%, Mn: 0.29-0.49%, S: ≤0.0013%, P: ≤0.012%, Ni: 1.4-1.9%, Cr: 13-13.9%, Ti: 0.0050-0.0060%, 90×10 -4 %≤Zr+Mg≤130×10 -4 %, and the content of Zr and Mg satisfies Zr / Mg=2-4, and the balance is Fe and inevitable impurities.

[0026] Further, it comprises the following chemical components in percentage by mass: C: 0.27%, Si: 0.29%, Mn: 0.29%, S: 0.0013%, P: 0.012%, Ni: 1.4%, Cr: 13%, Ti: 0.0060%, Zr: 0.0040%, Mg: 0.0020%, and the balance is Fe and inevitable impurities.

[0027] The second object of the present application is to provide a preparation method of the micro-alloy low-cost martensitic stainless steel.

[0028] Step 1, the following raw materials are taken according to the weight ratio: ultra-low P industrial pure iron 30-50%, high-carbon chromium iron 10-20%, and red soil nickel ore smelted molten iron 40-60%; the ultra-low P industrial pure iron and the high-carbon chromium iron are first added into a medium-frequency furnace for melting, and then the red soil nickel ore smelted molten iron is added into the medium-frequency furnace, to obtain low-phosphorus raw material for smelting martensitic stainless steel, with P≤0.015wt%;

[0029] Step 2: the low-phosphorus raw material for smelting martensitic stainless steel obtained in step 1 is sequentially subjected to refining, continuous casting, rolling and heat treatment, to obtain the micro-alloy low-cost martensitic stainless steel.

[0030] The beneficial effects of the above scheme are: the present application is based on the fact that red soil nickel ore is rich in corrosion-resistant elements Ni and Cr, and smelted molten iron is used as the basis, and ultra-low P industrial pure iron and high-carbon chromium iron are used for batching, to design the chemical composition of the martensitic stainless steel, so as to improve the corrosion resistance of the matrix and reduce the cost.

[0031] Further, in step 1, the following raw materials are taken according to the weight ratio: ultra-low P industrial pure iron 35-40%, high-carbon chromium iron 13-10%, and red soil nickel ore smelted molten iron 50-52%.

[0032] Further, in step 1, the weight content of chemical components P and C in the ultra-low P industrial pure iron is respectively: P≤0.005%, and C≤0.01%.

[0033] Further, in step 1, the high-carbon ferrochrome has the following weight content of chemical components Cr, C and P: Cr≥60%, C≤9.5%, and P≤0.03%.

[0034] Further, in step 1, the molten iron smelted from the laterite nickel ore has the following weight content of chemical components Cr and P: Cr≥4%, and P≤0.03%.

[0035] Further, in step 2, the low-phosphorus raw material of the smelted martensitic stainless steel obtained in step 1 is sequentially subjected to the following specific steps of refining, continuous casting, rolling and heat treatment:

[0036] Step 2-1, refining: the low-phosphorus raw material of the smelted martensitic stainless steel is subjected to secondary refining in a LF furnace, and according to the chemical components in the micro-alloyed low-cost martensitic stainless steel, Si, Mn, Ni, Cr alloy fine-tuning and Ti, Zr, Mg micro-alloying treatment are performed to obtain a micro-alloyed stainless steel.

[0037] Step 2-2, continuous casting: the micro-alloyed stainless steel is subjected to continuous casting under the conditions of a tundish temperature of 1465-1480℃ and a working drawing speed of 1.05-1.20m / min to obtain a continuous casting billet.

[0038] Step 2-3, rolling: the continuous casting billet is subjected to rolling at a temperature of 1150-1250℃ to obtain a hot-rolled black skin coil.

[0039] Step 2-4, heat treatment: the hot-rolled black skin coil is heated to a temperature of 950-1050℃, held for 60-120min, then water quenched, and then subjected to medium-temperature tempering at 450-650℃, and cooled to room temperature in air after tempering to obtain the micro-alloyed low-cost martensitic stainless steel. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 Figure is an inclusion characterization diagram of a conventional martensitic stainless steel;

[0041] Figure 2 Figure is an inclusion characterization diagram of the micro-alloyed low-cost martensitic stainless steel of the present application;

[0042] Figure 3 Figure is an optical micrograph of corrosion active inclusions of the present application; wherein (a) is a conventional martensitic stainless steel; (b) is the micro-alloyed low-cost martensitic stainless steel of the present application;

[0043] Figure 4 Figure is a potentiostatic polarization curve comparison diagram of the present application in a 3.5wt.% NaCl solution;

[0044] Figure 5Potentiodynamic polarization curves of the present application in 3.5 wt.% NaCl solution. DETAILED DESCRIPTION

[0045] The principles and features of the present application are described below, and the examples are only used to explain the present application, and are not intended to limit the scope of the present application.

[0046] Example 1

[0047] The present embodiment relates to a kind of microalloyed nickel-saving martensitic stainless steel, it includes the following chemical components by mass percentage:0.26wt% C, 0.19wt% Si, 0.19wt% Mn, 0.0010wt% S, 0.010wt% P, 1.1wt% Ni, 12.1wt% Cr, 0.0040wt% Zr, 0.0010wt% Mg, 0.0050wt% Ti, 86.139wt% Fe.

[0048] The present embodiment relates to a kind of microalloyed low-cost martensitic stainless steel preparation method, including the following steps:

[0049] Step 1: using ultra-low P industrial pure iron as smelting base material, mass ratio is 40%; High-carbon chromium iron is added, mass ratio is 10%; Then add the molten iron using laterite nickel ore smelting, mass ratio is 50%.Among them, P≤0.0035wt% in industrial pure iron, C≤0.005wt%.Cr≥60wt% in high-carbon chromium iron, C≤6.5wt%.Cr≥60wt% in molten iron using laterite nickel ore smelting, P≤0.01wt%.Ultra-low P industrial pure iron (mass ratio 40%) and high-carbon chromium iron (mass ratio 10%) are melted by using intermediate frequency furnace, then molten iron (mass ratio 50%) using laterite nickel ore smelting is added, low-phosphorus raw material for smelting stainless steel can be obtained, P≤0.014wt% in low-phosphorus raw material for smelting stainless steel.

[0050] Step 2: refining, continuous casting, rolling and solid solution treatment are carried out according to the following steps:

[0051] Step 2-1: LF furnace refining: slagging and skimming before tapping, try to clean the slag; white slag tapping, a small amount of lime and fluorite can be used to adjust the slag reducibility and fluidity; tapping temperature 1530℃; first pass electricity to melt the top slag, then fine-tune Si, Mn, Ni, Cr alloy and Ti, Zr, Mg micro-alloying treatment, to obtain the chemical composition by weight percentage: 0.26wt% C, 0.19wt% Si, 0.19wt% Mn, 0.0010wt% S, 0.010wt% P, 1.1wt% Ni, 12.1wt% Cr, 0.0040wt% Zr, 0.0010wt% Mg, 0.0050wt% Ti, 86.139wt% Fe; the LF refining furnace smelting time is 45min.

[0052] Step 2-2: continuous casting: tundish temperature 1465℃; working speed 1.05m / min; wide face water volume of crystallizer is 175m 3 / h, narrow face water volume is 20m 3 / h, the temperature difference between inlet and outlet water is 5℃; the crystallizer uses non-sine vibration, vibration frequency is 140CPM, amplitude is 3.0mm; the crystallizer protective slag is high-nitrogen protective slag.

[0053] Step 2-3: rolling: the continuous casting billet is heated first, then hot-rolled to get hot-rolled black skin coil; the hot-rolling temperature is 1150℃.

[0054] Step 2-4: heat treatment: heated to temperature 950℃, held for 60min, then water quenched, then medium-temperature tempered at 450℃, cooled to room temperature in air, to get micro-alloyed low-cost martensitic stainless steel.

[0055] Example 2

[0056] This example relates to a kind of micro-alloyed nickel-saving martensitic stainless steel, which includes the following chemical composition by mass percentage: 0.27wt% C, 0.29wt% Si, 0.29wt% Mn, 0.0013wt% S, 0.012wt% P, 1.4wt% Ni, 13.0wt% Cr, 0.0040wt% Zr, 0.0020wt% Mg, 0.0060wt% Ti, 84.7247wt% Fe.

[0057] The preparation method of a kind of micro-alloyed low-cost martensitic stainless steel related by this example, including the following steps:

[0058] Step 1: using ultra-low P industrial pure iron as smelting base, mass ratio 35%; adding high-carbon chromium iron, mass ratio 13%; then adding molten iron smelted from laterite nickel ore, mass ratio 52%. The P in the industrial pure iron is ≤0.0050wt%, and the C is ≤0.01wt%. The Cr in the high-carbon chromium iron is ≥60wt%, and the C is ≤9.5wt%. The Cr in the molten iron smelted from laterite nickel ore is ≥60wt%, and the P is ≤0.03wt%. The low-phosphorus raw material for smelting stainless steel, P ≤0.015wt%, can be obtained by melting the ultra-low P industrial pure iron (mass ratio 35%) and the high-carbon chromium iron (mass ratio 13%) in a medium-frequency furnace, and then adding the molten iron smelted from laterite nickel ore (mass ratio 52%).

[0059] Step 2: refining, continuous casting, rolling and solid solution treatment are carried out as follows:

[0060] Step 2-1: LF furnace external refining: slagging and slagging before tapping, and trying to clean the slag; white slag tapping, a small amount of lime and fluorite can be used to adjust the slag reducibility and fluidity; the tapping temperature is 1630℃; the top slag is first melted by electricity, and then Si, Mn, Ni, Cr alloy fine adjustment and Ti, Zr, Mg micro-alloying treatment are carried out, so that the chemical composition by weight percentage includes: 0.27wt% C, 0.29wt% Si, 0.29wt% Mn, 0.0013wt% S, 0.012wt% P, 1.4wt% Ni, 13.0wt% Cr, 0.0040wt% Zr, 0.0020wt% Mg, 0.0060wt% Ti, 84.7247wt% Fe; the smelting time of the LF refining furnace is 60min.

[0061] Step 2-2: continuous casting: the tundish temperature is 1470℃; the working pulling speed is 1.10m / min; the wide surface water quantity of the crystallizer is 185m 3 / h, the narrow surface water quantity is 22m 3 / h, and the temperature difference between the inlet and outlet water is 6℃; the crystallizer adopts non-sine vibration, the vibration frequency is 140CPM, and the amplitude is 3.0mm; the crystallizer protective slag is high-nitrogen protective slag.

[0062] Step 2-3: rolling: the continuous casting blank is first heated, and then hot-rolled to obtain a hot-rolled black skin coil; the temperature of the hot-rolling is 1200℃.

[0063] Step 2-4: heat treatment: heating to a temperature of 1000℃ for 80min, water quenching, then medium-temperature tempering at 500℃, and cooling to room temperature in air, to obtain a micro-alloyed low-cost martensitic stainless steel.

[0064] Example 3

[0065] The embodiment relates to a kind of microalloyed nickel-saving martensitic stainless steel, which includes the following chemical components by mass percentage: 0.29wt% C, 0.49wt% Si, 0.49wt% Mn, 0.0015wt% S, 0.015wt% P, 1.9wt% Ni, 13.9wt% Cr, 0.0040wt% Zr, 0.0020wt% Mg, 0.0060wt% Ti, 82.9015wt% Fe.

[0066] The embodiment relates to a kind of microalloyed low-cost martensitic stainless steel preparation method, including the following steps:

[0067] Use ultra-low P industrial pure iron as smelting base material, mass ratio is 30%; Add high-carbon chromium iron, mass ratio is 20%; Then add molten iron smelted by laterite nickel ore, mass ratio is 60%. Among them, P≤0.0050wt% in industrial pure iron, C≤0.01wt%. Cr≥60wt% in high-carbon chromium iron, C≤9.5wt%. Cr≥60wt% in molten iron smelted by laterite nickel ore, P≤0.03wt%. Ultra-low P industrial pure iron (mass ratio 30%) and high-carbon chromium iron (mass ratio 20%) are melted by medium frequency furnace, then molten iron smelted by laterite nickel ore (mass ratio 60%) is added, and low-phosphorus raw material for smelting stainless steel can be obtained, P≤0.012wt%.

[0068] Step 2: refining, continuous casting, rolling and solid solution treatment are carried out according to the following steps:

[0069] Step 2-1: LF furnace refining: slagging and slagging before tapping, try to clean the slag; white slag tapping, a small amount of lime and fluorite can be used to adjust the slag reducibility and fluidity; the tapping temperature is 1830℃; first, the top slag is melted, then the Si, Mn, Ni, Cr alloy is finely adjusted, and the Ti, Zr, Mg microalloying treatment is carried out, to obtain the chemical composition by weight percentage: 0.29wt% C, 0.49wt% Si, 0.49wt% Mn, 0.0015wt% S, 0.015wt% P, 1.9wt% Ni, 13.9wt% Cr, 0.0040wt% Zr, 0.0020wt% Mg, 0.0060wt% Ti, 82.9015wt% Fe; the smelting time of the LF refining furnace is 90min.

[0070] Step 2-2: continuous casting: tundish temperature 1480℃; working pulling speed 1.20m / min; wide surface water amount of crystallizer is 195m 3 / h, narrow surface water amount is 25m 3 / h, the temperature difference between inlet and outlet water is 7℃; the crystallizer adopts non-sine vibration, vibration frequency is 140CPM, amplitude is 3.0mm; the crystallizer protection slag is high-nitrogen protection slag.

[0071] Step 2-3: Rolling: the continuous casting billet is heated and then hot-rolled to obtain a hot-rolled black skin coil; the temperature of the hot-rolling is 1250℃.

[0072] Step 2-4: Heat treatment: heated to a temperature of 1050℃, held for 120 min, water quenched, then tempered at a medium temperature of 650℃, and cooled to room temperature in air to obtain the micro-alloyed martensitic stainless steel.

[0073] The material prepared in Example 1 is selected for various tests, and the results are as follows:

[0074] (1) Inclusion characterization

[0075] The inclusion morphology in the conventional martensitic stainless steel and the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application is observed by SEM, and the inclusion composition is analyzed by EDS, and the results are as follows: Figure 1 and 2 It can be seen from Figure 1 that the inclusions in the conventional martensitic stainless steel are in the form of long strips, and the EDS spectrum confirms that Ti and N are distributed consistently, Al and O are distributed consistently, and Mn and S are distributed consistently; therefore, it is inferred that the inclusions in the conventional martensitic stainless steel are a composite inclusion composed of Al2O3+MnS+TiN, and the size of the inclusions is about 3-5μm. It can be seen from Figure 2 that the inclusions in the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application are in a complex form, and there is an ellipsoidal compound inside. The EDS spectrum confirms that Zr and O are distributed consistently, and Mn and S are distributed consistently; therefore, it is inferred that the inclusions in the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application are a multi-component composite inclusion, and the size of the inclusions is about 4μm.

[0076] According to the basic principles of metallurgical thermodynamics, Zr and Mg are both strong oxide-forming elements, and the use of Zr and Mg for composite deoxidization is beneficial to the removal of free oxygen content in the molten steel. The density of ZrO2 is 5.68g / cm 3 , which is greater than the density of Al2O3 (3.97g / cm 3 ), and especially the density of ZrO2 and molten steel (7.15g / cm 3The ZrO2 particles are more closely related to the Al2O3 particles, so once stable oxides are formed at high temperatures, ZrO2 floats uniformly in the molten steel, while Al2O3 collides and aggregates on the surface, becoming part of the slag. The portion of Al2O3 that fails to float remains in the steel as large, clustered inclusions. The electrical conductivity of the oxides is a key factor in their movement in the molten steel. According to existing research, the driving force for the movement of Al2O3 in molten steel is greater than that for ZrO2. During electrorefining, ZrO2 particles tend to repel each other and are difficult to aggregate, while Al2O3 easily collides and forms large particles that float to the surface of the molten steel and are absorbed by the surface coating agent. Therefore, compared with conventional Al and Si deoxidation, Zr and Mg composite deoxidation can form fine, dispersed composite oxides, in which… Figure 2 The experimental results prove this point.

[0077] (2) Microscopic photographs of microstructures

[0078] Corrosion-active inclusions were examined in conventional martensitic stainless steel and the microalloyed low-cost martensitic stainless steel prepared in Example 2 of this invention. Microscopic images of the examined microstructures are shown below. Figure 3 As shown in the figure. Statistical analysis was performed on the density of corrosion-active inclusions in conventional martensitic stainless steel and the microalloyed low-cost martensitic stainless steel prepared in Example 2 of this invention. Black dots appeared on the surface of conventional martensitic stainless steel, which were pitting corrosion caused by the dissolution of corrosion-active inclusions. In contrast, the surface of the microalloyed low-cost martensitic stainless steel prepared in Example 2 of this invention showed virtually no corrosion-active inclusions. The density of corrosion-active inclusions on the sample surface was statistically analyzed. The density of corrosion-active inclusions in conventional martensitic stainless steel was 0.515 inclusions / mm². 2 The density of corrosion-active inclusions in the microalloyed low-cost martensitic stainless steel prepared in Example 2 of this invention is 0.047 inclusions / mm². 2 Statistical analysis shows that the low-cost microalloyed martensitic stainless steel prepared in Example 2 of this invention, using laterite nickel ore for smelting, exhibits a significantly lower density of corrosion-active inclusions compared to conventional martensitic stainless steel after treatment with elements such as Zr. According to literature reports, MnS inclusions, either directly or in their surrounding area, readily dissolve in sodium chloride solution, inducing pitting corrosion and exhibiting high corrosion activity. After Zr-based composite deoxidation, the originally highly corrosive MnS inclusions in the steel are modified into composite inclusions, such as ZrO2 and MgO, which possess higher stability and greater corrosion resistance in sodium chloride solution. Therefore, Zr and Mg composite deoxidation reduces the density of corrosion-active inclusions in the steel.

[0079] (3) Corrosion test

[0080] The micro-alloyed low-cost martensitic stainless steel prepared in Example 2 was subjected to electrochemical test by using a Zahner Zennium electrochemical workstation, and a standard three-electrode system was selected, the research and development steel was used as a working electrode, a platinum sheet was used as an auxiliary electrode, and a saturated calomel electrode (SCE) was used as a reference electrode. The static potential method was used to identify the stability of the overall corrosion resistance of the maximum saturation current density under the static potential condition in the simulated environment. At the same time, the research and development steel was subjected to constant potential polarization test, the potential of the constant potential polarization curve test was -300 mV, the test time was 3600 s, and the saturation current density I max was recorded after the test was completed. The potentiodynamic polarization curve test was performed at a scanning rate of 0.5 mV / s, the scanning range was ± 300 mV relative to the open circuit potential (OCP), and the test solution selected was a 3.5 wt.% NaCl solution.

[0081] The constant potential polarization corrosion test of the conventional martensitic stainless steel and the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application was performed by referring to the self-designed corrosion evaluation method, and the saturation current density results of the conventional martensitic stainless steel and the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application are shown in Table 1. Figure 4 Under the same corrosion time, the smaller the saturation current density value, the better the corrosion resistance of the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application. The saturation current density value of the conventional martensitic stainless steel is 15.26 mA·cm 2 , and the saturation current density of the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application is 1.44 mA·cm 2 . The saturation current density of the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application is 13.82 mA·cm 2 lower than that of the conventional martensitic stainless steel. Therefore, based on the development of the micro-alloyed low-cost martensitic stainless steel prepared in Example 2 of the present application from laterite nickel ore, the types and forms of inclusions are effectively changed by multi-element composite deoxidization of Zr, Mg and the like, the density of corrosion active inclusions is significantly reduced, the constant potential polarization saturation current density value is significantly reduced, and the local corrosion resistance is obviously better than that of the conventional martensitic stainless steel.

[0082] Figure 5Potentiodynamic polarization curves of the conventional martensitic stainless steel and the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application in 3.5wt.% NaCl solution. The self-corrosion potential and the corrosion current density of the test sample can be obtained by the potentiodynamic polarization curve test and Tafel fitting, which are used to judge the change trend of the corrosion rate of the sample and the corrosion reaction mechanism. The corresponding corrosion potential and corrosion current density values of the conventional martensitic stainless steel and the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application are shown in Table 1. The self-corrosion potential values of the conventional martensitic stainless steel and the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application are -0.212 V and -0.385 V, respectively. The corrosion current density values of the conventional martensitic stainless steel and the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application are 4.749 x 10 -6 A·cm 2 and 3.926 x 10 -6 A·cm 2 , respectively. As can be seen from Table 1, the self-corrosion potential of the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application is higher than that of the conventional martensitic stainless steel, and the corrosion current density value of the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application is smaller than that of the conventional martensitic stainless steel. The more positive the self-corrosion potential is, the better the thermodynamic stability of the metal is, and the less the tendency of corrosion is. The smaller the corrosion current density value is, the slower the corrosion rate of the metal is. The microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application has a smaller corrosion current density and a more positive self-corrosion potential, so the corrosion resistance of the microalloyed low-cost martensitic stainless steel prepared in Example 2 of the present application is better than that of the conventional martensitic stainless steel.

[0083] Table 1 Fitting results of the potentiodynamic polarization of the two martensitic stainless steels

[0084]

[0085] In conclusion, the present application uses the hot metal rich in chromium and nickel smelted from laterite nickel ore as smelting raw material, saves the precious metal Ni, and obtains good mechanical properties by using low-cost silicon, manganese, chromium and other elements; the present application adds zirconium, magnesium and other elements, improves the plasticity and toughness, and improves the welding performance; compared with conventional martensitic stainless steel, the constant electrode potential saturation current density of the micro-alloy low-cost martensitic stainless steel of the present application is obviously reduced, and the seawater corrosion resistance is more excellent than that of traditional martensitic stainless steel; on the one hand, the composite deoxidizing elements Zr and Mg significantly modify the corrosion active inclusions, so that the microstructure and morphology are obviously changed, the corrosion active MnS is wrapped by TiN, the density of corrosion active inclusions is greatly reduced, the seawater local corrosion resistance is significantly improved, on the other hand, the Ni and Cr elements brought by laterite nickel ore improve the corrosion resistance of the matrix, which helps to improve the seawater corrosion resistance to a certain extent.

[0086] In the description of the present specification, the description referring to the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, different embodiments or examples described in the present specification and the features of different embodiments or examples can be combined and modified by those skilled in the art without contradiction, within the scope of the present application.

[0087] Although the embodiments of the present application have been shown and described above, it should be understood that the above-described embodiments are exemplary, and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A microalloyed, low-cost martensitic stainless steel, characterized in that, It includes the following chemical composition by mass percentage: C: 0.27%, Si: 0.29%, Mn: 0.29%, S: 0.0013%, P: 0.012%, Ni: 1.4%, Cr: 13%, Ti: 0.0060%, Zr: 0.0040%, Mg: 0.0020%, with the balance being Fe and unavoidable impurities; The preparation method of this microalloyed low-cost martensitic stainless steel includes the following steps: Step 1: Take the following raw materials according to the following weight ratio: 30-50% ultra-low phosphorus industrial pure iron, 10-20% high-carbon ferrochrome, and 40-60% molten iron from laterite nickel ore smelting; first, add the ultra-low phosphorus industrial pure iron and the high-carbon ferrochrome to the medium-frequency furnace for melting, and then add the molten iron from laterite nickel ore smelting to the medium-frequency furnace to obtain low-phosphorus raw materials for smelting martensitic stainless steel. Step 2: The low-phosphorus raw material for smelting martensitic stainless steel obtained in Step 1 is sequentially refined, continuously cast, rolled and heat-treated to obtain microalloyed low-cost martensitic stainless steel.

2. The microalloyed low-cost martensitic stainless steel according to claim 1, characterized in that, In step 1, the following raw materials are taken according to the following weight ratio: 35-40% ultra-low P industrial pure iron, 13-10% high carbon ferrochrome, and 50-52% molten iron smelted from laterite nickel ore.

3. The microalloyed low-cost martensitic stainless steel according to claim 1, characterized in that, In step 1, the weight contents of chemical components P and C in the ultra-low P industrial pure iron are P≤0.005% and C≤0.01%, respectively.

4. The microalloyed low-cost martensitic stainless steel according to claim 1, characterized in that, In step 1, the weight contents of the chemical components Cr, C and P in the high carbon ferrochrome are as follows: Cr≥60%, C≤9.5%, P≤0.03%.

5. The microalloyed low-cost martensitic stainless steel according to claim 1, characterized in that, In step 1, the weight contents of the chemical components Cr and P in the molten iron produced by smelting laterite nickel ore are Cr≥4% and P≤0.03%, respectively.

6. The microalloyed low-cost martensitic stainless steel according to claim 1, characterized in that, In step 2, the specific steps for refining, continuous casting, rolling, and heat treatment of the low-phosphorus raw material for smelting martensitic stainless steel obtained in step 1 are as follows: Step 2-1, Refining: The low-phosphorus raw material for smelting martensitic stainless steel is refined in an LF furnace, and alloy fine-tuning and micro-alloying treatment are carried out according to the chemical composition of the micro-alloyed low-cost martensitic stainless steel to obtain micro-alloyed stainless steel. Step 2-2, continuous casting: The microalloyed stainless steel is continuously cast at a tundish temperature of 1465~1480℃ and a working casting speed of 1.05~1.20 m / min to obtain a continuously cast billet; Steps 2-3, rolling: The continuously cast billet is rolled at a temperature of 1150~1250℃ to obtain hot-rolled black coil; Steps 2-4, heat treatment: The hot-rolled black steel coil is heated to 950~1050℃ and held for 60~120 min, then water quenched, and then tempered at a medium temperature of 450~650℃. After tempering, it is cooled to room temperature in air to obtain microalloyed low-cost martensitic stainless steel.

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