ULTRA-PURE FERRITIC STAINLESS STEEL AND ITS MANUFACTURING METHOD AND USE

Abstract

An ultra-pure ferritic stainless steel, a manufacturing method, and use thereof. The ultra-pure ferritic stainless steel comprises in percentage by weight, C0.025%, N0.025%, Si1.00%, Mn1.20%, Cr: 18.00%-24.00%, Nb: 0.40%-0.75%, Mo: 1.75%-2.50%, W: 0.80%-1.20%, Cu: 0.30%-0.60%, Al0.015%, Ti0.01%, P0.03%, S0.01%, and satisfies 3.4%2Nb+Mo+W5.2% and 32%Cr+4.7Mo+2.4W+11.5Cu45%, with the balance being Fe and inevitable impurities. With the composite strengthening of Nb, W and Mo, and the reasonable matching of Cu, Al and Ti, the obtained ultra-pure ferritic stainless steel can meet the requirements of working and serving at higher temperatures, has good high-temperature strength, high-temperature fatigue life and high-temperature oxidation resistance, has good cold working performance and good brazing performance.

Claims

1. An ultra-pure ferritic stainless steel comprising in percentage by weight: C0.025%, N0.025%, Si1.00%, Mn1.20%, Cr: 18.00%-24.00%, Nb: 0.40%-0.75%, Mo: 1.75%-2.50%, W: 0.80%-1.20%, Cu: 0.30%-0.60%, Al0.015%, Ti0.01%, P0.03%, S0.01%, and satisfying 3.4%2Nb+Mo+W5.2% and 32%Cr+4.7Mo+2.4W+11.5Cu45%, with the balance being Fe and inevitable impurities.

2. The ultra-pure ferritic stainless steel according to claim 1 comprising in percentage by weight: C0.015%, N0.015%, Si0.80%, Mn1.00%, Cr: 18.00%-21.00%, Nb: 0.40%-0.65%, Mo: 1.90%-2.40%, W: 0.90%-1.20%, Cu: 0.3%-0.5%, Al0.013%, Ti0.01%, P0.03%, S0.005%, and satisfying 3.6%2Nb+Mo+W4.9% and 33%Cr+4.7Mo+2.4W+11.5Cu41%, with the balance being Fe and inevitable impurities.

3. The ultra-pure ferritic stainless steel according to claim 1 comprising in percentage by weight: C0.01%, N0.01%, Si: 0.30%-0.80%, Mn: 0.40%-1.00%, Cr: 18.00%-20.00%, Nb: 0.50%-0.60%, Mo: 2.00%-2.30%, W: 1.00%-1.20%, Cu: 0.35%-0.45%, Al0.01%, Ti0.008%, P0.02%, S0.003%, and satisfying 4.0%2Nb+Mo+W4.8% and 34%Cr+4.7Mo+2.4W+11.5Cu39%, with the balance being Fe and inevitable impurities.

4. A manufacturing method of an ultra-pure ferritic stainless steel, comprising a smelting process, a casting process, a hot rolling process, a continuous annealing process after hot rolling, a cold rolling process, a continuous annealing process after cold rolling, wherein the composition of molten steel prepared in the smelting process is the same as that of the ultra-pure ferritic stainless steel according to claim 1.

5. The manufacturing method of an ultra-pure ferritic stainless steel according to claim 4, wherein in the hot rolling process, a heating temperature of a cast billet before hot rolling is 1200-1260 C., and a coiling temperature after hot rolling is 620-700 C.

6. The manufacturing method of an ultra-pure ferritic stainless steel according to claim 4, wherein in the continuous annealing process after hot rolling, an annealing temperature is 1000-1060 C. and a holding time is controlled at 1.5-2.0 min/mm.

7. The manufacturing method of an ultra-pure ferritic stainless steel according to claim 4, wherein a total deformation rate in the cold rolling process is 55%.

8. The manufacturing method of an ultra-pure ferritic stainless steel according to claim 4, wherein in the continuous annealing process after cold rolling, an annealing temperature is 1000-1060 C. and a holding time is controlled at 0.9-1.2 min/mm.

9. Use of the ultra-pure ferritic stainless steel according to claim 1 in the preparation of an automobile exhaust system.

10. An automobile exhaust system, wherein at least a portion of the automobile exhaust system comprises the ultra-pure ferritic stainless steel according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0021] Various other advantages and benefits will become clear to those skilled in the art by reading the following detailed description of the preferred embodiments. The drawings are only for the purpose of illustrating the preferred embodiment, and are not construed as limiting the present invention.

[0022] FIG. 1 shows the metallographic structure of a finished product of the ultra-pure ferritic stainless steel prepared in Example 1 of the present invention;

[0023] FIG. 2 is a comparison chart of high-temperature strengths of stainless steels prepared in Examples 1-3 of the present invention and Comparative Examples 4-7;

[0024] FIG. 3 is a comparison chart of high-temperature oxidation weight gains of stainless steels prepared in Example 1 of the present invention and Comparative Examples 4-7;

[0025] FIG. 4 is shows the brazing material spreadability test result of a brazing wettability test of the ultra-pure ferritic stainless steel prepared in Example 1 of the present invention.

[0026] FIG. 5 shows the brazing material spreadability test result of a brazing wettability test of the stainless steel 444 prepared in Comparative Example 5.

DESCRIPTION OF EMBODIMENTS

[0027] In order to fully understand the purpose, characteristics and efficacy of the present invention, the present invention will be described in detail reference to the following specific embodiments. Except for the following contents, the process of the present invention adopts conventional methods or devices in the field. Unless otherwise specified, the following terms have the meanings commonly appreciated by those skilled in the art.

[0028] When a numerical range is disclosed herein, the range is regarded as continuous and includes the minimum and maximum values of the range, and every value between such minimum and maximum values. Further, when a range refers to integers, it includes every integer between the minimum value and the maximum value of the range. Further, when multiple ranges are provided to describe features or features, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein should be construed as including any and all subranges subsumed therein.

[0029] Specifically, in a first aspect, the present invention provides an ultra-pure ferritic stainless steel, which comprises in percentage by weight: C0.025%, N0.025%, Si1.00%, Mn1.20%, Cr: 18.00%-24.00%, Nb: 0.40%-0.75%, Mo: 1.75%-2.50%, W: 0.80%-1.20%, Cu: 0.30%-0.60%, Al0.015%, Ti0.01%, P0.03%, S0.01%, and satisfies 3.4%2Nb+Mo+W5.2% and 32%Cr+4.7Mo+2.4W+11.5Cu45%, with the balance being Fe and inevitable impurities.

[0030] According to the present invention, the high-temperature strength of the material is enhanced by composite strengthening of Nb, Mo and W, and the corrosion resistance and cold working formability of the material are collaboratively enhanced by adding Cu. By controlling the content of Al and Ti, the brazing wettability of the material can be improved. The obtained ultra-pure ferritic stainless steel material has good high-temperature strength, high-temperature fatigue life and high-temperature oxidation resistance, and has good cold working performance and brazing processability, and can realize industrial production.

[0031] The function of each element in the ultra-pure ferritic stainless steel of the present invention will be described in detail as follows:

[0032] C, N: In ferritic stainless steel, the solubility of C in a-phase matrix is low, and it will decrease rapidly with the decrease of temperature, therefore it is likely to combine with Cr to form Cr.sub.23C.sub.6 and aggregate at the grain boundary, which is the source of high-temperature brittleness and intergranular corrosion of ferrite. The solubility of N in ferritic stainless steel is extremely low, and chromium nitride will also be formed. C and N have a strong affinity with Nb and Ti, therefore Nb and Ti are commonly used as stabilizing elements of C and N. Ti (C, N) and Nb(C, N) are uniformly distributed in the grain, and do not aggregate at the grain boundary like Cr.sub.23C.sub.6, which can improve the intergranular corrosion resistance and mechanical properties of ferritic stainless steel. Therefore, C and N should be controlled as low as possible, and in the present invention the control ranges of the two are both 0.025%, preferably 0.015%, and most preferably 0.010%.

[0033] Si and Mn: Si is a ferrite-forming element, and Mn is a weak austenite forming element and is the main deoxidizer in the smelting process. Si inhibits the oxidation of metal elements on the surface of the material in ferritic stainless steel and increases the oxidation resistance of the steel to some extent. In the present invention, Si content is controlled to be 1.00% and Mn content is controlled to be 1.20%; preferably the Si content is 0.80% and the Mn content is 1.00%; and most preferably the Si content is 0.30%-0.80% and the Mn content is 0.40%-1.00%.

[0034] Cr: Cr is the most important alloying element in stainless steel, and it is one of the main alloying elements in stainless steel for oxidation resistance and corrosion resistance. Cr forms a dense oxide film of Cr.sub.2O.sub.3 on the surface of the material, which hinders the diffusion of oxygen and metal ions, thus improving the oxidation resistance and strength of steel. The Cr content in the present invention is 18.00%-24.00%, preferably 18.00%-21.00%, and most preferably 18.00%-20.00%.

[0035] Nb, W, Mo: Nb, W, Mo not only can improve the corrosion resistance of stainless steel, but also can improve high-temperature performance of stainless steel. Mo can significantly promote the enrichment of chromium in the passivation film, thus enhancing the stability of the passivation film and strengthening the corrosion resistance of chromium in steel. Mo improves the strength of stainless steel, including high-temperature strength, so that the high-temperature durability and creep properties are obviously improved, and the thermal stability is improved. The function of W is similar to that of Mo. On the one hand, Nb is added as a stabilizing element for C and N, on the other hand, it can improve the high-temperature strength of ferritic stainless steel through solution strengthening or precipitation phase strengthening, especially its solution strengthening effect is the most prominent. The present invention controls the Nb content to be 0.40%-0.75%, the Mo content to be 1.75%-2.50%, and the W content to be 0.80%-1.20%; preferably, the content of Nb is 0.40%-0.65%, the content of Mo is 1.90%-2.40%, and the content of W is 0.90%-1.20%; most preferably, the content of Nb is 0.50%-0.60%, the content of Mo is 2.00%-2.30%, and the content of W is 1.00%-1.20%.

[0036] Cu: Cu can increase the Cr content in the passivation film, thus improving the corrosion resistance of the passivation film, and to a certain extent, it can increase the pitting potential of the ultra-pure ferritic stainless steel and improve the crevice corrosion resistance. The precipitation of Cu particles can improve the high-temperature strength and fatigue strength of steel. Cu can also improve the elongation at break of the ultra-pure ferritic stainless steel in the tensile test, improve its cold formability, and make up for the decrease of cold formability caused by increasing strengthening alloying elements. The content of Cu in the present invention is controlled to be 0.30%-0.60%, preferably 0.30%-0.50%, and most preferably 0.35%-0.45%.

[0037] The composition control of Nb, W, Mo, Cr and Cu should also satisfy 3.4%2Nb+Mo+W5.2% and 32%Cr+4.7Mo+2.4W+11.5Cu45%.

[0038] Al, Ti: The research shows that for ferritic stainless steel, the lower Al and Ti content can improve the wettability of stainless steel to the brazing material during brazing. It is reasonable that the Al content is 0.015% and the Ti content is 0.01%.

[0039] P, S: As harmful elements, P and S should be controlled as low as possible. In the present invention, P content is 0.03% and S content is 0.01%.

[0040] In another aspect, the present invention further provides a manufacturing method of an ultra-pure ferritic stainless steel, which comprises a smelting process, a casting process, a hot rolling process, a continuous annealing process after hot rolling, a cold rolling process, and a continuous annealing process after cold rolling.

[0041] In the smelting process, molten iron may be used as a raw material, which is subjected to molten iron pretreatment, converter smelting, VOD furnace smelting and LF furnace smelting before continuous casting, or may be carried out through three-step removal for molten iron (desilication, desulfurization and dephosphorization of molten iron), AOD furnace smelting, VOD furnace smelting and LF furnace smelting before continuous casting. The smelting process can also use scrap steel as a raw material, which is subjected to electric furnace smelting, AOD furnace smelting, VOD furnace smelting and LF furnace smelting before continuous casting. According to the actual demand, die casting can also be carried out. The chemical composition after smelting should meet the above design composition.

[0042] Heat treatment is needed both after hot rolling and cold rolling to restore the structure and improve the processability of the material. The main process parameters are as follows.

[0043] Preferably, the cast billet is heated before hot rolling, and then subjected to hot rolling which comprises rough rolling, finish rolling and coiling to obtain a hot-rolled steel coil. The heating temperature of the cast billet is 1200-1260 C., which ensures that the cast billet is burnt through and reduces the load of the rolling mill. The coiling temperature is 620-700 C. to prevent high-temperature brittleness and second phase precipitation, and to recover and recrystallize the hot rolled structure.

[0044] Heat treatment is respectively carried out both after the hot rolling process and the cold rolling process to improve the processability of the material. Continuous annealing can be used for the heat treatment.

[0045] Preferably, the hot-rolled steel coil is continuously annealed at the annealing temperature of 1000-1060 C. with a holding time being controlled at 1.5-2.0 min/mm, thereby softening and improving the mechanical properties of the hot rolled material and improving the toughness of the material, so as to facilitate the subsequent cold rolling processing.

[0046] Preferably, after annealing, the stainless steel is subjected to acid pickling to remove the surface scale.

[0047] Preferably, the total deformation rate in the cold rolling process should be 55% (preferably 65%), thereby refining the grain size of finished products, improving the elongation of materials and improving the formability of materials.

[0048] The rolling passes, rolling pressure and pass deformation of cold rolling can be carried out according to existing technology, and will not be specifically limited in the present invention.

[0049] Preferably, the cold-rolled steel coil is continuously annealed at the annealing temperature of 1000-1060 C. with a holding time being controlled at 0.9-1.2 min/mm, thereby softening and improving the mechanical properties of the cold-rolled material and improving the toughness of the material, so as to facilitate the subsequent processing of finished materials into parts with certain shapes.

[0050] Preferably, after annealing, the stainless steel is subjected to acid pickling to remove the surface scale.

[0051] According to the manufacturing method of an ultra-pure ferritic stainless steel, after smelting, continuous casting, hot rolling and cold rolling, the obtained ferritic stainless steel cold plate has a grain size of grade 6-8, and a tensile strength of 530-580 MPa. The high-temperature tensile strength at 900 C. is greater than 44 MPa, and the high-temperature tensile strength at 1000 C. is greater than 24 MPa. A high cycle fatigue test is carried out at 950 C. and 15 MPa according to the stress ratio R=0.1, and the service life is more than 10 times. In the high-temperature oxidation resistance test at 900 C., the oxidation weight gain is 1.4-1.5 mg/cm.sup.2 after holding for 1000 hours.

EXAMPLES

[0052] The present invention will be further illustrated by way of examples, but it is not limited to the scope of the examples. The experimental methods without specific conditions in the following examples follow the conventional methods and conditions.

Example 1

[0053] Molten iron was used as a raw material and was subjected to molten iron pretreatment, converter smelting, VOD furnace smelting and LF furnace smelting, then was continuously casted into a continuous casting billet, the chemical composition of the casting billet is shown in Table 1.

[0054] Hot rolling, first continuous annealing, cold rolling, and second continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters are shown in Table 2.

[0055] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

Example 2

[0056] Three-step removal for molten iron, AOD furnace smelting, VOD furnace smelting and LF furnace smelting was carried out and then the molten metal was continuously casted into a continuous casting billet. The chemical composition of the casting billet is shown in Table 1.

[0057] Hot rolling, first continuous annealing, cold rolling, and second continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters are shown in Table 2.

[0058] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

Example 3

[0059] Scrap steel was used as a raw material for smelting and was subjected to electric furnace smelting, AOD furnace smelting, VOD furnace smelting and LF furnace smelting, then was continuously casted into a continuous casting billet. The chemical composition of the casting billet is shown in Table 1.

[0060] Hot rolling, first continuous annealing, cold rolling, and second continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters are shown in Table 2.

[0061] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

Comparative Example 4: Stainless Steel 441

[0062] Three-step removal for molten iron, AOD furnace smelting, VOD furnace smelting and LF furnace smelting was carried out and then the molten metal was continuously casted into a continuous casting billet. The chemical composition of the casting billet is shown in Table 1.

[0063] Hot rolling, continuous annealing, cold rolling and continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters were set according to the conventional preparation process of stainless steel 441.

[0064] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

Comparative Example 5: Stainless Steel 444

[0065] Molten iron was used as a raw material and was subjected to molten iron pretreatment, converter smelting, VOD furnace smelting and LF furnace smelting, then was continuously casted into a continuous casting billet, the chemical composition of the casting billet is shown in Table 1.

[0066] Hot rolling, continuous annealing, cold rolling and continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters were set according to the conventional preparation process of stainless steel 444.

[0067] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

Comparative Example 6: Stainless Steel 444Nb

[0068] Scrap steel was used as a raw material for smelting and was subjected to electric furnace smelting, AOD furnace smelting, VOD furnace smelting and LF furnace smelting, then was continuously casted into a continuous casting billet. The chemical composition of the casting billet is shown in Table 1.

[0069] Hot rolling, first continuous annealing, cold rolling and second continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters were set according to the conventional preparation process of stainless steel 444Nb.

[0070] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

Comparative Example 7: Stainless Steel 445J2

[0071] Molten iron was used as a raw material and was subjected to molten iron pretreatment, converter smelting, VOD furnace smelting and LF furnace smelting, then was continuously casted into a continuous casting billet, the chemical composition of the casting billet is shown in Table 1.

[0072] Hot rolling, continuous annealing, cold rolling and continuous annealing were sequentially carried out on the continuous casting billet to obtain ultra-pure ferritic stainless steel. The process parameters were set according to the conventional preparation process of stainless steel 445J2.

[0073] A cold-rolled product of the ultra-pure ferritic stainless steel was tested for grain size, normal-temperature tensile and high-temperature tensile. The test results are shown in Table 3.

TABLE-US-00001 TABLE 1 Composition Summary of Ferritic Stainless Steels Prepared in Examples 1-3 and Comparative Examples 4-7 (wt. %) C Si Mn P S Cr Ni Mo Al Cu Nb Ti W N Example 1 0.0098 0.37 0.82 0.024 0.0028 18.22 0.093 2.23 0.010 0.41 0.52 0.006 1.09 0.0088 Example 2 0.0088 0.42 0.68 0.18 0.0015 18.35 0.19 2.16 0.009 0.43 0.55 0.005 1.04 0.0092 Example 3 0.0091 0.45 0.76 0.21 0.0012 18.19 0.23 2.32 0.009 0.46 0.53 0.007 1.12 0.0076 Comparative 0.0098 0.18 0.10 0.020 0.001 17.98 0.079 0.081 0.02 0.40 0.171 0.086 Example 4 Comparative 0.0071 0.19 0.22 0.021 0.001 17.95 0.077 1.88 0.077 0.10 0.16 0.20 0.096 Example 5 Comparative 0.0102 0.43 0.79 0.025 0.003 18.31 0.092 1.91 0.011 0.04 0.61 0.009 0.0089 Example 6 Comparative 0.0117 0.18 0.19 0.021 0.001 22.21 0.121 1.86 0.110 0.11 0.29 0.21 0.0122 Example 7

TABLE-US-00002 TABLE 2 Main Process Parameters of Examples 1-3 First continuous Second continuous Hot rolling annealing Cold rolling annealing Billet Holding Total Holding heating Coiling Thickness Annealing time deformation Thickness Annealing time Example temperature C. temperature C. mm temperature C. min/mm rate % mm temperature C. min/mm 1 1235-1245 660~680 3.0 1030-1045 1.7 67 1.0 1030-1045 1 2 1235-1245 660~680 5.0 1030-1045 1.6 60 2.0 1040-1055 0.9 3 1235-1245 660~680 4.0 1030-1045 1.6 70 1.2 1025-1040 1.1

TABLE-US-00003 TABLE 3 Summary of Properties of Ferritic Stainless Steels in Examples 1-3 and Comparative Examples 4-7 High-temperature Elongation at break Tensile strength, MPa oxidation weight gain, Grain size in normal temperature Normal mg/cm.sup.2 Item grade % temperature 800 C. 900 C. 1000 C. 900 C., 1000 h Examples 1 8 30 552 86 47 27 1.45 2 7 30 561 85 48 27 1.44 3 7 30.5 556 84 46 26 1.42 Comparative 4 7 35 455 61 29 15 1.78 examples 5 7 32 507 68 34 16 1.71 6 7 29 541 70 40 19 1.64 7 7 30 543 84 44 21 1.59

[0074] As can be seen from Table 3, the grain size of the cold-rolled products of Examples 1-3 of the present invention is grade 7-8, and the metallographic structure of the ultra-pure ferritic stainless steel of Example 1 is shown in FIG. 1. In Examples 1-3 of the present invention, at normal temperature, the elongation at break is 30-30.5% and the tensile strength is 552-561 Mpa.

[0075] According to the high-temperature tensile test, the high-temperature strengths of the stainless steels in Examples 1-3 of the present invention and Comparative Examples 4-7 are shown in FIG. 2. The high-temperature tensile strength of the material of the present invention is greater than 44 MPa at 900 C. and is greater than 24 MPa at 1000 C., which are higher than those of the materials of Comparative Examples 4-7.

[0076] The high cycle fatigue test was carried out according to the stress ratio R=0.1 at 950 C. and 15 MPa, and the stainless steels in Examples 1-3 all had a service life of more than 10.sup.7 times.

[0077] At the high temperature of 900 C., the stainless steels of Example 1 and Comparative Examples 4-7 were subjected to continuous oxidation test for 1000 hours. As shown in FIG. 3, the oxidation weight gain of the material of the present invention was 1.44 mg/cm.sup.2, which was lower than those of the materials in Comparative examples 4-7.

[0078] Brazing wettability test was carried out on the cold-rolled product of Example 1. Brazing material was coated on the surface of the sample and heated at 1100 C. in reducing atmosphere for 30 minutes, and the spreadability of the brazing material was observed. The brazing material spreading area of the material of the present invention was obviously larger than that of the stainless steel prepared in Comparative Example 5, as shown in FIG. 4. It proves that the material of the present invention has good brazing material wettability and good brazing performance.

[0079] It should be noted that the above embodiments are only used to illustrate, rather than to limit the technical solution of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, it should be appreciated by those skilled in the art that the technical solution described in the foregoing embodiments can still be modified, or some technical features thereof can be equivalently replaced. However, these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the scope of the present invention.