Economical non-magnetic high-strength high-corrosion-resistance stainless steel and its preparation process

Abstract

The application discloses an economical non-magnetic high-strength high-corrosion-resistance stainless steel, and chemical components of the steel are as follows in percentage by weight: C: 0.1-0.2%; Si: 0.4-1.0%; Mn: 14.0-20.0%; Cr: 14.0-20.0%; Ni: 1.0-2.0%; Cu: 0.5-1.0%; N: 0.25-0.40%; Mo: 0.5-1.5%; P: ≤0.030%; S: ≤0.003%; and the rest is Fe and inevitable impurity elements; meanwhile, P+S<0.03%, Md 30 ≤-60 DEG C, and a high-temperature solidification mode factor K is less than 1.4. By controlling the high-temperature solidification mode factor K and Md 30 , deformation martensite formed after quenching and cold rolling can be avoided, and the steel is non-magnetic and high-strength. And by improving the process, surface defects caused by solidification cracks of the non-magnetic high-strength stainless steel are solved.

Description

Technical Field

[0001] This invention relates to steel smelting processes, and more particularly to an economical non-magnetic, high-strength, and highly corrosion-resistant stainless steel and its preparation process. Background Technology

[0002] When selecting stainless steel materials for automobiles, the choice must be based on the required usage conditions and functional requirements. Generally, the following factors need to be considered:

[0003] 1. Strength: Stainless steel materials must have sufficient strength and rigidity to ensure the normal use of automotive parts.

[0004] 2. Corrosion resistance: Since automobiles are exposed to various harsh climates and environments during operation, stainless steel materials need to have excellent corrosion resistance to ensure the lifespan of the vehicle.

[0005] 3. Processing performance: Stainless steel materials need to be easy to process and form in order to meet the design requirements of different automotive parts.

[0006] Yield strength R p0.2 High-strength stainless steels reaching 1000 MPa can be categorized into three types: austenitic stainless steel (strength increased by work hardening to generate deformed martensite), precipitation-hardening or age-hardening stainless steel (strength increased by aging treatment to precipitate fine precipitates), and high-carbon martensitic stainless steel. Work-hardening austenitic stainless steel, which increases strength by deformed martensite, requires a certain amount of cold rolling deformation. The abundant deformed martensite and internal stress in its microstructure make it prone to aging cracking and stress corrosion cracking during use. Precipitation-hardening or age-hardening stainless steel and high-carbon martensitic stainless steel can achieve very high strength, but their elongation is generally less than 20%, resulting in poor formability. Furthermore, all three types of high-strength stainless steel contain a certain amount of martensite, exhibiting strong magnetism. Ferrite analysis shows that the magnetic phase content is generally greater than 15%, making them unsuitable for automotive applications requiring non-magnetic properties.

[0007] Patent application CN116219286A discloses a nickel-saving, high-strength, ductile-strength bimodal austenitic stainless steel and its preparation method. The method discloses a method for preparing a yield strength of not less than 650 MPa, tensile strength of not less than 1150 MPa, uniform elongation of not less than 45%, total elongation of not less than 55%, and strength-ductility product of not less than 65 GPa%. Through a combination of chemical composition and production process, an austenitic structure with two grain sizes is prepared. Although the magnetic properties of the steel in this composition system are relatively weak, the properties of the material prepared by this method exhibit significant anisotropy and poor performance stability.

[0008] The patent application file with the patent publication number CN113817969A discloses a high-strength super-corrosion-resistant non-magnetic stainless steel and a preparation method thereof. The chemical composition of the product is: Cr: 17% to 23%, Mn: 17% to 23%, Co: 17% to 23%, Si: 0.5% to 3%. Since the composition contains a high amount of scarce element Co, it is not suitable for the new energy vehicle industry.

[0009] Therefore, there is an urgent need to develop an economic non-magnetic high-strength high-corrosion-resistant stainless steel that can be applied to structural parts of new energy vehicles. SUMMARY

[0010] One of the purposes of the present application is to provide an economic non-magnetic high-strength high-corrosion-resistant stainless steel.

[0011] The technical solution for achieving the purpose of the present application is: an economic non-magnetic high-strength high-corrosion-resistant stainless steel,

[0012] The weight percentage of the chemical composition is as follows:

[0013] C: 0.1 to 0.2%;

[0014] Si: 0.4 to 1.0%;

[0015] Mn: 14.0 to 20.0%;

[0016] Cr: 14.0 to 20.0%;

[0017] Ni: 1.0 to 2.0%;

[0018] Cu: 0.5 to 1.0%;

[0019] N: 0.25 to 0.40%;

[0020] Mo: 0.5 to 1.5%;

[0021] P: ≤0.030%;

[0022] S: ≤0.003%;

[0023] The rest is Fe and inevitable impurity elements;

[0024] Meanwhile, P+S <0.03%, Md 30 ≤-60℃, and the high-temperature solidification mode factor K is less than 1.4, wherein the value of Md 30 The calculation formula of K is as follows:

[0025] Md 30= 551 - 462*(%C+%N) - 9.2*%Si - 8.1*%Mn - 13.7*%Cr - 18.5*%Mo - 29*(%Ni+%Cu) - 68*%Nb

[0026] K = (%Cr+%Mo+1.5*%Si) / (%Ni+35*%C+20*%N+0.25*%Cu)

[0027] In the component design of the present application:

[0028] C is a strong austenite forming element, which helps to stabilize the austenite structure at room temperature and during cold deformation, and solid solution in the matrix can improve the strength and hardness.

[0029] Si is an important deoxidizing element in the steelmaking process, and is also a ferrite forming element. With the increase of silicon content, the high temperature ferrite content increases. In order to ensure non-magnetic, more austenite forming elements need to be added to balance the formation of ferrite, so as to avoid the formation of high temperature ferrite remaining at room temperature. But too low silicon content is difficult to realize under the current smelting conditions.

[0030] Mn is a weak austenite forming element, but it is a strong austenite stabilizing element. In order to avoid the transformation of austenite to deformation martensite during cold deformation, a high manganese content is required. At the same time, manganese can also dissolve N to increase the N content in the matrix.

[0031] Cr is a strong ferrite forming and stabilizing element, which narrows the austenite region and is the main element to improve corrosion resistance, especially the resistance to pitting corrosion. The equivalent index PREN of pitting corrosion is closely related to the Cr content. But when the Cr content is too high, high temperature ferrite is prone to precipitate during solidification, which remains at room temperature and makes the steel have weak magnetism, which does not meet the use requirements.

[0032] Ni is an austenite forming and stabilizing element, which is beneficial to improve the forming performance of austenitic stainless steel and effectively reduce the aging cracking after stamping. But Ni is a precious metal element, which has a greater impact on cost.

[0033] Cu is an austenite forming element, which is beneficial to improve the forming and stamping performance at room temperature, but too high Cu will increase the cost and deteriorate the hot working performance.

[0034] N is a strong austenite forming element, and the solid solution of N can improve the stress corrosion and pitting corrosion resistance of austenitic stainless steel. But the precipitates of N can reduce the hot plasticity and corrosion resistance of the steel,

[0035] Mo is a ferrite forming element, which can significantly improve the stress corrosion and pitting corrosion resistance of austenitic stainless steel. Especially the pitting corrosion resistance is equivalent to 5 times of chromium.

[0036] P and S are both inevitable impurity elements, but have adverse effects on performance.

[0037] Md in the present application 30 is the temperature for generating 50% deformation martensite after cold deformation with a true strain of 30%, and is an empirical formula, indicating that the higher the content of alloying elements in austenite, the lower the Md 30 value, and the less likely the occurrence of martensite transformation. The requirement of Md 30 less than -50℃ is to avoid the transformation of deformation martensite during cold deformation.

[0038] The high-temperature solidification mode factor K is the ratio of high-temperature ferrite-forming elements to high-temperature austenite-forming elements, and is a coefficient created by the inventor through numerous experiments to find the correlation between the parameters of each component. In the calculation formula, the denominator is the ferrite-forming element, the numerator is the austenite-forming element, and the fraction represents the tendency of ferrite formation during high-temperature solidification. If K is greater than 1.4, ferrite phase will be preferentially precipitated during solidification, and part of the ferrite phase and high-temperature molten steel will gradually transform into austenite phase as the solidification proceeds. Due to the presence of more ferrite-forming elements, the unspent ferrite phase will remain during subsequent processing, and the high-temperature ferrite will remain to room temperature, resulting in magnetism. If K is less than 1.4, due to the presence of more austenite-forming elements, austenite phase will be preferentially precipitated during solidification. Although the austenite phase and high-temperature molten steel will transform into ferrite phase, as the solidification process progresses, the ferrite phase and high-temperature molten steel will also transform into austenite phase. Due to the presence of less ferrite phase, the ferrite phase and liquid will completely transform into austenite phase during the subsequent solidification process, and high-temperature ferrite will not remain to room temperature, thereby avoiding the magnetism caused by high-temperature ferrite.

[0039] Further, to ensure a certain pitting corrosion resistance, the pitting resistance equivalent PREN value should be greater than or equal to 21, wherein PREN=%Cr+3.3*%Mo+16*%N. The pitting resistance equivalent PREN value is to ensure corrosion resistance, especially pitting corrosion resistance.

[0040] The second object of the present application is to provide a preparation process of an economical non-magnetic high-strength corrosion-resistant stainless steel.

[0041] The technical solution for achieving the second object of the present application is: a preparation process of an economical non-magnetic high-strength corrosion-resistant stainless steel, which comprises the following steps:

[0042] (1) converter smelting according to the chemical composition of the first object of the application;

[0043] (2) refining; in the refining process, the composition is adjusted, and the Md 30 , K value and PREN value meet the range of values obtained by the calculation formula in the first object of the application;

[0044] (3) Casting;

[0045] (4) Hot rolling: the slab temperature sent to the hot rolling heating furnace is 600-900℃, the total furnace time is 0.9-1.0 times of the slab thickness, the heating temperature is 1200-1220℃, and the heated slab is rolled to the target thickness through a hot rolling continuous rolling mill or a furnace rolling mill;

[0046] The solidification mode with K less than 1.4 is a full austenite solidification mode or an austenite ferrite solidification mode, and the sensitivity of crack generation in the solidification process is high, and the subsequent manufacturing process needs to avoid the defects possibly generated by the solidification cracks. The slab temperature sent to the hot rolling heating furnace should be greater than 600℃ to ensure the generation of the slab cooling cracks in the full austenite solidification mode or the austenite ferrite solidification mode;

[0047] (5) Black skin coil obtained by coiling: the coiling temperature is 780-810℃, and rapid cooling is performed after coiling to reduce the precipitation of precipitates;

[0048] (6) Black skin coil rolling: after the black skin coil is rolled by 30-40% reduction, annealing and pickling are performed, the annealing temperature is 1080-1120℃, and the pickling temperature is 50-70℃;

[0049] (7) Quenching and cold rolling: after pickling, quenching and cold rolling by 18-20% reduction are performed.

[0050] Compared with the published patent technology, the present application has the following advantages:

[0051] ① Through the design and optimization of alloy components, the content of valuable metal elements is replaced by economic Mn and N, and the alloy cost is reduced.

[0052] ② By controlling the high-temperature solidification mode factor K and Md 30 , the high-temperature ferrite with magnetism is avoided to be formed in the high-temperature solidification process, and a certain amount of twin crystals is formed after quenching and cold rolling, the generation of the twin crystals is equivalent to the refinement of the grains, the strength can be improved, the deformation martensite is avoided to be formed after quenching and cold rolling, the non-magnetic and high-strength steel is realized.

[0053] ③ Compared with the conventional austenitic stainless steel cold-rolled product, the slab at temperature is directly sent to the heating furnace to reduce the energy consumption, the crack formation opportunity of the full austenite solidification mode or the austenite ferrite solidification mode with high crack sensitivity is avoided, and one cold rolling pass is reduced, and the process cost is saved.

[0054] ④ The present application solves the surface defects of the non-magnetic high-strength stainless steel due to the solidification cracks, reduces the quenching and cold rolling reduction, and further improves the corrosion resistance.

[0055] The stainless steel obtained by the application is especially suitable for manufacturing structural parts of light-weight new energy vehicles, such as B columns and cross beams. DETAILED DESCRIPTION

[0056] The following is a detailed description of the specific embodiments of the process for smelting 200 series stainless steel from high-carbon ferromanganese according to the application: Example 1

[0057] A preparation process of an economical non-magnetic high-strength corrosion-resistant stainless steel, comprising the following steps:

[0058] (1) Converter smelting according to the component design of the product; the chemical components are as follows in terms of weight percentage:

[0059] C: 0.12%;

[0060] Si: 0.6%;

[0061] Mn: 18.0%;

[0062] Cr: 14.0%;

[0063] Ni: 1.0%;

[0064] Cu: 0.5%

[0065] N: 0.35%;

[0066] Mo: 0.5%;

[0067] P: ≤0.030%;

[0068] S: ≤0.003%;

[0069] The rest is Fe and inevitable impurity elements;

[0070] Meanwhile, P+S < 0.03%, Md 30 : -62.01℃, K: 1.25, PREN: 21.25

[0071] (2) Refining: adjust the components in the refining process, and make the Md 30 , K value and PREN value meet the requirements of the application;

[0072] (3) Casting: after the molten steel is settled for 15 minutes, casting is performed, and a slab with a thickness of 200mm is cast;

[0073] (4) Hot rolling: the slab sent to the hot rolling heating furnace has a temperature of 650℃, the heating temperature is 1200-1220℃, the discharge temperature is 1210℃, the total furnace time is 180min, and the heated slab is rolled to the target thickness of 3mm through a hot rolling continuous rolling mill or a furnace rolling mill;

[0074] (5) The black skin coil is obtained by coiling at 780℃, and then rapidly cooling to reduce the precipitation of precipitates;

[0075] (6) The black skin coil is rolled by 30% reduction, and then annealed and pickled, the annealing temperature is 1080℃, and the pickling temperature is 50℃;

[0076] (7) The black skin coil is quenched and tempered by 20% reduction, and finally degreased.

[0077] The chemical compositions of Examples 1-5 and Comparative Examples 1 and 2 are shown in Table 1.

[0078] Table 1: Weight percentage of each chemical component (unit: %)

[0079]

[0080] The process parameters of Examples 1-5 and Comparative Examples 1 and 2 are shown in Table 2.

[0081] Table 2:

[0082]

[0083] The product properties obtained by Examples 1-5 and Comparative Examples 1 and 2 are shown in Table 3.

[0084] Table 3

[0085]

[0086] As shown in Table 3, compared with Comparative Examples 1 and 2, the yield strength of the product obtained by the preparation process of Examples 1-5 is greater than 1000 MPa, the tensile strength is greater than 1250 MPa, the elongation is greater than 20%, and the pitting potential is greater than 250 mV, which are all obviously improved compared with the comparative examples, realizing high strength and high corrosion resistance while having the characteristics of non-magnetic, and solving the surface quality problem caused by unavoidable crack defects during the solidification process of non-magnetic steel.

[0087] The above is only an embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent process transformation or direct or indirect application in other related technical fields using the content of the present application is also included in the patent protection scope of the present application.

Claims

1. An economical, non-magnetic, high-strength, and highly corrosion-resistant stainless steel, characterized in that: Its chemical composition by weight percentage is as follows: C：0.1～0.2％； Si: 0.4–1.0%; Mn: 14.0~20.0%; Cr：14.0～20.0％； Ni: 1.0–2.0%; Cu: 0.5–1.0%; N：0.25～0.40％； Mo: 0.5–1.5%; P：≤0.030％； S：≤0.003％； The remainder consists of Fe and unavoidable impurity elements; At the same time, P+S < 0.03%, Md 30 ≤-60℃, high-temperature solidification mode factor K is less than 1.4, where Md 30 The numerical values ​​of and the formula for calculating K are as follows: Md 30 =551-462*(%C+%N)-9.2*%Si-8.1*%Mn-13.7*%Cr-18.5*%Mo-29*(%Ni+%Cu)-68*%Nb; K=(%Cr+%Mo+1.5*%Si) / (%Ni+35*%C+20*%N+0.25*%Cu).

2. A preparation process for the economical non-magnetic high-strength and high-corrosion-resistant stainless steel according to claim 1, characterized in that: It includes the following steps: (1) Smelting in a converter according to the chemical composition described in claim 1; (2) Refining; the composition is finely adjusted in the refining process, and Md is made more refined. 30 The K value and the PREN value meet the range requirements of the values ​​obtained by the calculation formula in claim 1; (3) Casting; (4) Hot rolling: The temperature of the slab sent to the hot rolling furnace is 600℃~900℃, the total time in the furnace is 0.9~1.0 times the thickness of the slab, the heating temperature is 1200℃~1220℃, and the heated slab is rolled to the target thickness through a hot rolling continuous rolling mill or a furnace coil mill. (5) Rolling up to obtain black leather rolls: The rolling temperature is 780℃~810℃, and the rolls are cooled quickly after rolling to reduce the precipitation of precipitates; (6) Rolling of black leather rolls: After rolling with a reduction of 30% to 40%, the black leather rolls are annealed and pickled. The annealing temperature is 1080℃ to 1120℃, and the pickling temperature is 50℃ to 70℃. (7) Tempering and cold rolling: After pickling, tempering and cold rolling with a reduction of 18% to 20% is carried out.

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