Martensitic stainless steel having excellent intergranular corrosion resistance and method for manufacturing the same

Abstract

This invention discloses a martensitic stainless steel with excellent resistance to intergranular corrosion and its manufacturing method. The chemical composition of the steel, by weight percentage, includes: C: 0.12%–0.24%; Si: 0.6%–1.2%; Mn: 0.5%–0.8%; P ≤ 0.030%; S ≤ 0.005%; Cr: 9.0%–11.5%; Y: 0.01%–0.02%; Mg: 0.005%–0.008%, with the balance being Fe and unavoidable impurities. The production process includes smelting, heating, rolling, and heat treatment. By adding the rare earth element Y, the diffusion rate of Cr during the sensitization process of martensitic stainless steel is reduced, thus slowing down the formation of carbides (Mn). 23 The precipitation of C6, along with the enrichment of Y at the grain boundaries, increases the grain boundary potential, strengthens the grain boundaries, inhibits the segregation of Cr at the grain boundaries, thereby reducing the formation of Cr-depleted areas and enhancing the resistance to intergranular corrosion.

Description

Technical Field

[0001] This invention belongs to the field of ferrous metal materials technology, and specifically relates to martensitic stainless steel with excellent resistance to intergranular corrosion and its manufacturing method. Background Technology

[0002] Intergranular corrosion is generally considered the main form of corrosion in austenitic stainless steel and has received considerable attention. Martensitic stainless steel, due to its low cost and good overall performance, has seen increasingly wider applications in recent years, and its corrosion resistance has gradually become a focus of attention. In particular, it exhibits significant intergranular corrosion in highly corrosive environments containing chlorides and sulfides. Therefore, improving the intergranular corrosion resistance of martensitic stainless steel is of significant practical importance for further expanding its application areas.

[0003] There are currently no published patents related to solving the problem of intergranular corrosion in martensitic stainless steel. Summary of the Invention

[0004] The purpose of this invention is to provide a martensitic stainless steel with excellent resistance to intergranular corrosion and its manufacturing method. By optimizing the steel composition and process design, the sensitivity of martensitic stainless steel to intergranular corrosion is reduced.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] One of the technical solutions of this invention is to provide a martensitic stainless steel with excellent resistance to intergranular corrosion. The chemical composition of the steel, by weight percentage, includes C: 0.12% to 0.24%; Si: 0.6% to 1.2%; Mn: 0.5% to 0.8%; P≤0.030%; S≤0.005%; Cr: 9.0% to 11.5%; Y: 0.01% to 0.02%; Mg: 0.005% to 0.008%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, Y+Mg: 0.015%~0.023%, intergranular carbide size ≤40nm, the microstructure of stainless steel is tempered sorbite.

[0008] Furthermore, the reactivation rate Ra = Ir / Ia × 100% determined by the dual-ring electrochemical potentiodynamic reactivation method is below 55%, where Ir represents the reactivation current and Ia represents the activation current.

[0009] Furthermore, the stainless steel plate thickness is 25–100 mm, and the yield strength R p0.2 Its tensile strength is 780-840 MPa, and its tensile strength R is... m It ranges from 900 to 960 MPa.

[0010] Furthermore, the elongation A of the stainless steel is 17.5% to 19.0%, the reduction of area Z is 66% to 69%, and the average impact energy at 20°C is 45 to 60 J.

[0011] The second technical solution of this invention provides a method for manufacturing martensitic stainless steel with excellent resistance to intergranular corrosion. The production process includes: smelting, heating, rolling, and heat treatment, wherein:

[0012] The target heating temperature for the billet is 1150–1200℃, and the soaking time is 2–3 hours. After the billet is taken out of the furnace, the iron oxide scale is removed. The initial rolling temperature of the billet is not less than 1100℃. The first two passes are rolled with a large reduction of not less than 30 mm per pass. The final rolling temperature is controlled at 890–910℃. After rolling, the steel plate is air-cooled on a cooling bed until the surface temperature is ≤100℃ to obtain an air-quenched martensite structure. Then, it is subjected to high-temperature tempering treatment to obtain a tempered sorbite structure. The tempering temperature is 570–640℃, and the holding time is 4–5 min / mm.

[0013] Furthermore, the steel billet is a continuously cast slab, and the iron oxide scale is removed by high-pressure water at 18 MPa or above during the rolling process. The thickness of the steel plate after final rolling is 25-100 mm.

[0014] Furthermore, the smelting process includes: preparing scrap steel according to the alloy element ratio, adding carbon powder and smelting it through EAF, then refining it through AOD and LF, adding magnesium slag during the smelting process, adding Y as Fe-10% Y master alloy, and finally continuously casting it into a slab.

[0015] Furthermore, the continuous casting process adopts constant casting speed, with the casting speed controlled at 0.8 to 1.0 m / min, the superheat of the tundish controlled at 20 to 30°C, and the secondary cooling water ratio controlled at 0.18 to 0.22 L / kg, in order to increase the equiaxed crystal ratio in the center of the billet. At the same time, electromagnetic stirring is put in, with a current intensity of 600 to 800 A.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) By adding rare earth element Y, the diffusion rate of Cr during the sensitization process of martensitic stainless steel was reduced, and the carbide M was slowed down. 23 The precipitation of C6, coupled with the enrichment of Y at grain boundaries, increases the grain boundary potential, strengthens the grain boundaries, and inhibits Cr segregation at grain boundaries. This reduces the formation of Cr-depleted zones and enhances resistance to intergranular corrosion. The introduction of trace amounts of Mg interacts with Y to improve the steel's crystal structure, disrupt the network structure of intergranular carbides, reduce the size of intergranular carbides to ≤40 nm, and promote their uniform dispersion, thereby reducing the tendency for intergranular corrosion.

[0018] (2) By reasonably matching the continuous casting speed, superheat and secondary cooling water strength, combined with electromagnetic stirring, high temperature and large reduction rolling and other process methods, the component segregation is improved and the uniformity of the structure is improved, thereby avoiding the formation of corrosion micro-cells between the precipitates and the matrix structure due to different chemical sites, and the occurrence of electrochemical corrosion.

[0019] (3) The yield strength R of the steel grade of this invention p0.2 ≥780MPa, tensile strength R m ≥900MPa, stainless steel elongation A≥17.5%, reduction of area Z≥66%, average impact energy R at 20℃. KV ≥45J, reactivation rate Ra≤55%. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0021] The martensitic stainless steel with excellent resistance to intergranular corrosion provided by this invention has the following chemical composition by weight percentage: C: 0.12%–0.24%; Si: 0.6%–1.2%; Mn: 0.5%–0.8%; P≤0.030%; S≤0.005%; Cr: 9.0%–11.5%; Y: 0.01%–0.02%; Mg: 0.005%–0.008%, Y+Mg: 0.015%–0.025%, with the balance being Fe and unavoidable impurities.

[0022] The reasons for using the above-mentioned components are as follows:

[0023] (1) C: C is an important element for the austenitization of martensitic stainless steel, but C will promote the segregation of carbides at grain boundaries, resulting in Cr-depleted regions near the grain boundaries and causing intergranular corrosion. Therefore, the present invention controls the C content in the steel to be in the range of 0.12% to 0.24%.

[0024] (2) Si: Si has a good deoxidizing effect on molten steel, which is beneficial to improving the intergranular corrosion resistance. However, excessive Si content will affect the hot working performance of steel and easily form defects such as porosity and looseness. In this invention, the Si content is controlled at 0.6% to 1.2%.

[0025] (3) Mn: Mn can reduce the M at grain boundaries through kinetic mechanisms. 23The precipitation of C6 reduces the susceptibility to intergranular corrosion, while increased Mn content increases the temper brittleness of the steel. In this invention, the Mn content is controlled at 0.5–0.8%.

[0026] (4) P: P is a harmful element in steel, which can easily cause cold brittleness. However, strict control of P content will increase production costs. Therefore, this invention controls the P content in steel to ≤0.030%.

[0027] (5) S: S is also a harmful element in steel, which can seriously impair the corrosion resistance of steel. Therefore, this invention requires that the S content in the steel be ≤0.005%.

[0028] (6) Cr: Cr can strongly improve the passivation ability of martensitic stainless steel. At the same time, Cr will close the austenite phase region and promote ferrite formation, resulting in a decrease in the strength and impact toughness of the steel. Therefore, the Cr content in the steel is controlled at 9.0% to 11.5% in this invention.

[0029] (7) Y: The diffusion coefficient of rare earth element Y is higher than that of Cr, and it is easier to accumulate in the grain boundary, which can strengthen the grain boundary and thus inhibit the growth of Cr carbides in the grain boundary. However, excessive Y will lead to the formation of a large number of oxide inclusions in the steel. Therefore, the Y content is controlled to be 0.01% to 0.02% in this invention.

[0030] (8) Mg: Mg is a strong deoxidizing element that can purify molten steel. However, excessive addition will affect the hot working performance of steel. Therefore, the Mg content in this invention is controlled at 0.005% to 0.008%.

[0031] (9) Y+Mg: The addition of Y and Mg together produces a composite effect, which can improve the crystal structure of steel, destroy the network structure of intergranular carbides, thereby reducing the size of intergranular carbides and promoting their uniform dispersion, thereby reducing the tendency of intergranular corrosion. However, excessive amounts can easily cause composite inclusions to agglomerate and grow. Therefore, in this invention, the Y+Mg content is controlled at 0.015% to 0.025%, preferably 0.015% to 0.023%.

[0032] The present invention provides a method for manufacturing martensitic stainless steel with excellent resistance to intergranular corrosion. The method involves feeding scrap steel according to the alloy element ratios described above, adding carbon powder, smelting via EAF, and then refining via AOD and LF. Magnesium slag is added during the smelting process, and Y is added as a Fe-10% Y master alloy. Finally, the steel is continuously cast into slabs. The continuous casting process employs constant casting speed, controlled at 0.8–1.0 m / min, to avoid longitudinal cracks on the slab surface. The tundish superheat is controlled at 20–30°C, and the secondary cooling water ratio is controlled at 0.18–0.22 L / kg to increase the equiaxed grain ratio at the slab center. Simultaneously, electromagnetic stirring is employed to reduce center segregation, with a current intensity of 600–800 A.

[0033] To avoid the formation of high-temperature ferrite, the target heating temperature of the slab is 1150–1200℃, with a soaking time of 2–3 hours to promote the decomposition of intergranular carbides. After exiting the furnace, the slab is quickly sent to a descaling machine to remove iron oxide scale. To avoid increased deformation resistance during rolling leading to plate shape defects, the initial rolling temperature of the slab is not less than 1100℃. The first two rolling passes are rolled with a high temperature and large reduction of not less than 30mm / pass to fully break down the liquid carbides in the steel. The final rolling temperature is controlled at 890–910℃ to improve the uniformity of the grain structure in the thickness direction of the steel plate. The steel plate thickness is 25–100mm. After rolling, the steel plate is air-cooled on a cooling bed until the surface temperature is ≤100℃ to obtain an air-quenched martensite structure. Then, it undergoes high-temperature tempering treatment to obtain a tempered sorbite structure with good comprehensive properties. The tempering temperature is 570–640℃, and the holding time is 4–5min / mm.

[0034] The specific embodiments of the martensitic stainless steel with excellent resistance to intergranular corrosion and its manufacturing method of the present invention are as follows:

[0035] Each embodiment was carried out EAF melting and AOD+LF refining according to the alloy element ratio in Table 1. The continuous casting process parameters are shown in Table 2, and the slab heating, rolling and heat treatment process parameters are shown in Table 3.

[0036] Table 1. Chemical composition (%) of steel smelting in each embodiment

[0037]

[0038] Table 2. Steel continuous casting process parameters for each embodiment.

[0039] Example Pulling speed / m / min Superheat / °C Secondary cooling water volume (L / kg) Current intensity / A 1 0.9 25 0.20 700 2 1.0 20 0.18 600 3 0.8 30 0.22 800 4 1.0 20 0.18 800 5 0.8 24 0.19 600 6 1.0 25 0.20 600 Comparative Example 1 1.2 32 0.40 0 Comparative Example 2 1.4 17 0.35 1000 Comparative Example 3 1.2 34 0.30 500

[0040] Table 3 Steelmaking process parameters for each embodiment

[0041]

[0042] The reactivation rate of the examples and comparative samples was determined using the dual-ring electrochemical potentiodynamic reactivation method (DL-EPR), and the mechanical properties at room temperature were also tested. The results are shown in Table 4. As can be seen from Table 4, while maintaining the mechanical properties, the reactivation rate (below 55%) of each example of the present invention is lower than that of the comparative example, indicating that the steel of the present invention has less sensitivity to intergranular corrosion, that is, better resistance to intergranular corrosion.

[0043] Table 4. Test results of intergranular carbide size, mechanical properties, and reactivation rate.

[0044]

[0045] Note: Reactivation rate Ra = (Ir / Ia × 100%), where Ir represents the reactivation current and Ia represents the activation current.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A martensitic stainless steel with excellent resistance to intergranular corrosion, characterized in that, The chemical composition of the steel, by weight percentage, includes: C: 0.12%–0.24%; Si: 0.6%–1.2%; Mn: 0.5%–0.8%; P ≤ 0.030%; S ≤ 0.005%; Cr: 9.0%–11.5%. Y：0.01％～0.02％； Mg: 0.005%~0.008%, balance is Fe and unavoidable impurities; production process includes: smelting, heating, rolling, heat treatment, target heating temperature of billet is 1150~1200℃, soaking time is 2~3h, and iron oxide scale is removed after billet is taken out of the furnace. The initial rolling temperature of the billet is not less than 1100℃. The first two passes are rolled with a large reduction of not less than 30mm per pass. The final rolling temperature is controlled at 890~910℃. After rolling, the steel plate is air-cooled to a surface temperature of ≤100℃ to obtain an air-quenched martensite structure. Then, it is subjected to high-temperature tempering treatment to obtain a tempered sorbite structure. The tempering temperature is 570~640℃ and the holding time is 4~5min / mm.

2. The martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 1, characterized in that, Y+Mg: 0.015%~0.023%, intergranular carbide size ≤40nm.

3. The martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 1, characterized in that, The reactivation rate Ra = Ir / Ia × 100%, determined by the bicyclic electrochemical potentiodynamic reactivation method, was below 55%. Ir represents the reactivation current, and Ia represents the activation current.

4. The martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 1, characterized in that, The stainless steel plate has a thickness of 25-100mm, a yield strength Rp0.2 of 780-840MPa, and a tensile strength Rm of 900-960MPa.

5. The martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 1, characterized in that, The elongation (A) of stainless steel is 17.5%–19.0%, the reduction of area (Z) is 66%–69%, and the average impact energy at 20℃ is 45–60 J.

6. A method for manufacturing a martensitic stainless steel with excellent resistance to intergranular corrosion as described in any one of claims 1 to 5, the production process comprising: Smelting, heating, rolling, and heat treatment, characterized in that, The target heating temperature for the billet is 1150–1200℃, and the soaking time is 2–3 hours. After the billet is taken out of the furnace, the iron oxide scale is removed. The initial rolling temperature of the billet is not less than 1100℃. The first two passes are rolled with a large reduction of not less than 30 mm per pass. The final rolling temperature is controlled at 890–910℃. After rolling, the steel plate is air-cooled to a surface temperature of ≤100℃ to obtain an air-quenched martensite structure. Then, it is subjected to high-temperature tempering treatment to obtain a tempered sorbite structure. The tempering temperature is 570–640℃, and the holding time is 4–5 min / mm.

7. The method for manufacturing martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 6, characterized in that, The steel billet is a continuously cast slab. During the rolling process, high-pressure water of 18MPa or higher is used to remove iron oxide scale. The thickness of the steel plate after final rolling is 25-100mm.

8. The method for manufacturing martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 6, characterized in that, The smelting process includes: preparing scrap steel according to the proportion of alloying elements, adding carbon powder, smelting with EAF, refining with AOD and LF, adding magnesium slag during the smelting process, and finally continuously casting into slabs.

9. The method for manufacturing martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 8, characterized in that, The continuous casting process adopts constant casting speed, with the casting speed controlled at 0.8 to 1.0 m / min, the superheat of the tundish controlled at 20 to 30°C, and the secondary cooling water ratio controlled at 0.18 to 0.22 L / kg, in order to increase the equiaxed crystal ratio in the center of the billet. At the same time, electromagnetic stirring is put in, with a current intensity of 600 to 800 A.

10. The method for manufacturing martensitic stainless steel with excellent resistance to intergranular corrosion according to claim 6, characterized in that, The rolled steel plate is air-cooled on a cooling bed.

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