ULTRAHIGH-STRENGTH MARAGING STAINLESS STEEL WITH MULTIPHASE STRENGTHENING AND PREPARATION METHOD THEREOF

Abstract

Disclosed is an ultrahigh-strength maraging stainless steel with multiphase strengthening and a preparation method thereof. The stainless steel has a composition in mass percentage as follows: 1.0-5.0% of Co, 6.0-10.0% of Ni, 11.0-17.0% of Cr, 0.3-2.0% of Ti, 3.0-7.0% of Mo, 0.08-1.0% of Mn, 0.08-0.5% of Si, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance.

Claims

1. An ultrahigh-strength maraging stainless steel with multiphase strengthening, wherein the stainless steel has a composition as follows in mass percentage: 1.0-5.0% of Co, 6.0-10.0% of Ni, 11.0-17.0% of Cr, 0.3-2.0% of Ti, 3.0-7.0% of Mo, 0.08-1.0% of Mn, 0.08-0.5% of Si, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance; a method for preparing the stainless steel comprises: 1) proportioning alloying elements; 2) vacuum smelting for an electrode in a vacuum induction melting furnace; 3) vacuum arc remelting; 4) high-temperature homogenizing annealing; 5) forging or hot rolling for cogging; 6) cold rolling for deformation; and 7) heat treating.

2. A method for preparing the ultrahigh-strength maraging stainless steel with multiphase strengthening according to claim 1, comprising: 1) proportioning alloying elements; 2) vacuum smelting for an electrode in a vacuum induction melting furnace; 3) vacuum arc remelting; 4) high-temperature homogenizing annealing; 5) forging or hot rolling for cogging; 6) cold rolling deformation; and 7) heat treating.

3. The method according to claim 2, wherein in step 1), the proportioning of alloying elements comprises: taking metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium, iron-silicon, and pure iron and inevitable impurities as a balance, according to a mass percentage of each element in the stainless steel.

4. The method according to claim 2, wherein in step 2), the vacuum smelting for an electrode in a vacuum induction melting furnace is conducted by high-vacuum smelting throughout at a vacuum degree of 0.1 Pa or less; pure iron, metal nickel, metal molybdenum and metal cobalt are added with the furnace, metal chromium and metal titanium are added from an overhead bunker, and industrial silicon and metal manganese are added from an alloy bunker; after the materials added with the furnace are molten down, the metals from the overhead bunker are added, molten totally and subjected to deoxidation alloying, and the metals from the alloy bunker are finally added; during smelting, refining is conducted at a temperature within a range of 1,550-1,650° C. for not less than 60 min under stirring for not less than 10 min; smelting composition is sampled on site and analyzed, and then is adjusted to achieve a target composition; pouring is conducted at a temperature within a range of 1,530-1,550° C., and heat preservation is conducted normally on a riser.

5. The method according to claim 2, wherein in step 3), the vacuum arc remelting is conducted at a melting rate within a range of 100-260 Kg/h, and during the remelting, a vacuum degree is maintained at 10.sup.−2 Pa or less.

6. The method according to claim 2, wherein in step 4), the high-temperature homogenizing annealing comprises: heating with the furnace in air, vacuum or a protective atmosphere at a rate of 100-180° C./h to a temperature within a range of 600-900° C. and maintaining for 4-8 h, then heating to a temperature within a range of 1,100-1,300° C. and maintaining for 20-50 h, and then conducting furnace cooling, air cooling or oil cooling to room temperature.

7. The method according to claim 2, wherein in step 5), the forging or hot rolling produces a square ingot or round ingot; the forging or hot rolling for cogging is conducted under conditions as follows: a billet is heated to a temperature within a range of 1,100-1,300° C. and maintained for 10-24 h for discharging and rolling; the forging or hot rolling begins at a temperature of 1,100° C. or higher and ends at a temperature of 950° C. or higher; a hot rolled sheet stock has a total rolling reduction of not less than 50%, and a forged billet has a forging ratio of not less than 6; after the forging or hot rolling, a resultant is cooled in an ice-water mixture to room temperature.

8. The method according to claim 2, wherein in step 6), the cold rolling deformation is conducted as follows: a cold rolled sheet stock has a total rolling reduction of not less than 65%; a tube stock, a bar stock, a wire stock or a section material is cold deformed by reciprocating tube rolling, groove rolling, universal rolling or drawing to obtain a size and specification of a product as required.

9. The method according to claim 2, wherein in step 7), the heat treating comprises: high-temperature quenching treatment, cryogenic treatment and aging treatment.

10. The method according to claim 9, wherein the high-temperature quenching treatment is conducted by maintaining at a temperature within a range of 1,050-1,200° C. for 5-30 min, and then quenching in an ice-water mixture at 0° C. for cooling.

11. The method according to claim 9, wherein the cryogenic treatment is conducted with liquid nitrogen for 4-10 h, followed by returning to room temperature.

12. The method according to claim 9, wherein the aging treatment is conducted at a temperature within a range of 450-600° C. for 0.5-500 h, followed by air cooling or quenching to room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows a metallographic morphology image after aging treatment in Example 1;

[0045] FIG. 2 shows a transmission electron microscope (TEM) image after aging treatment in Example 1;

[0046] FIG. 3 shows a engineering stress-strain graph after aging treatment in Example 2, where the abscissa refers to engineering strain and the ordinate refers to engineering stress;

[0047] FIG. 4 shows an X-ray diffraction (XRD) graph after aging treatment in Example 2, in which the abscissa refers to scanning angle and the ordinate refers to diffraction intensity; and

[0048] FIG. 5 shows an image of an amorphous ring on a lath boundary after aging treatment in Example 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] Hereinafter, the ultrahigh-strength maraging stainless steel with multiphase strengthening and the preparation method thereof according to the present disclosure will be further explained and illustrated with reference to the drawings and examples, which should not be regarded as an improper limitation to the technical solutions of the present disclosure.

Example 1

[0050] Pure iron, metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium and iron-silicon were taken as raw materials according to a composition of stainless steel in mass percentage as follows: 3.0% of Co, 11.0% of Cr, 0.08% of Mn, 5.0% of Mo, 8.0% of Ni, 0.4% of Si, 2.0% of Ti, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance. C, P and S were inevitable impurities.

[0051] A billet was prepared by vacuum melting throughout.

[0052] The high-temperature homogenizing annealing was conducted as follows: the billet was heated with a furnace in air at a heating rate of 100° C./h to 700° C. and maintained for 4 h, then was heated to 1,100° C. and maintained for 20 h, and then was cooled with the furnace to room temperature.

[0053] The hot rolling for cogging was conducted under the following conditions: the billet was heated to 1,100° C. and maintained for 10 h, and then was discharged and rolled; the hot rolling began at a temperature of 1,100±20° C. and ended at a temperature of 950° C. or higher; the hot rolled sheet stock had a total rolling reduction of 50%, and was cooled in an ice-water mixture.

[0054] The sheet stock was cold rolled with a total rolling reduction of 65%.

[0055] The sheet stock above was subjected to a high-temperature quenching treatment by being maintained at 1,200° C. for 5 min and then cooled in an ice-water mixture at 0° C. for quenching; subsequently, the sheet stock was subjected to a cryogenic treatment with liquid nitrogen for 5 h and returned to room temperature; thereafter, the sheet stock was subjected to an aging treatment at 450° C. for 20 h, and then air-cooled to room temperature.

[0056] The mechanical properties of Example 1 are shown in Table 1. The stainless steel sample has an average hardness of 569.3 HV, yield strength of 2,260 MPa, a tensile strength of 2,690 MPa, an elongation of 8.2% and a pitting potential of 0.14 V.sub.SCE. FIG. 1 shows a metallographic morphology image after aging treatment of Example 1. FIG. 2 shows a TEM image after aging treatment of Example 1, where a large amount of dislocations are observed in martensitic laths.

Example 2

[0057] Pure iron, metal chromium, metal nickel, metal manganese, metal molybdenum, metal cobalt, metal titanium and iron-silicon were taken as raw materials according to a composition of stainless steel in mass percentage as follows: 1.0% of Co, 12.0% of Cr, 0.5% of Mn, 6.0% of Mo, 9.0% of Ni, 0.5% of Si, 1.5% of Ti, 0.02% or less of C, 0.003% or less of P, 0.003% or less of S, and Fe as a balance. C, P and S were inevitable impurities.

[0058] A billet was prepared by vacuum melting throughout.

[0059] The high-temperature homogenizing annealing was conducted as follows: the billet was heated with a furnace in air at a heating rate of at 100° C./h to 700° C. and maintained for 4 h, then was heated to 1,150° C. and maintained for 20 h, and then was cooled with the furnace to room temperature.

[0060] The hot rolling for cogging was conducted under the following conditions: the billet was heated to 1,200° C. and maintained for 30 h, and then was discharged and rolled; the hot rolling began at a temperature of 1,150±20° C. and ended at a temperature of 950° C. or higher; the hot rolled sheet stock had a total rolling reduction of 50%, and was cooled in an ice-water mixture.

[0061] The sheet stock was cold rolled with a total rolling reduction of 85%.

[0062] The sheet stock above was subjected to a high-temperature quenching treatment by being maintained at 1,200° C. for 5 min and then cooled in an ice-water mixture at 0° C. for quenching; subsequently, the sheet stock was subjected to a cryogenic treatment with liquid nitrogen for 7 h and returned to room temperature; thereafter, the sheet stock was subjected to an aging treatment at 520° C. for 30 h, and then air-cooled to room temperature.

[0063] The mechanical properties of Example 2 are shown in Table 1. The stainless steel sample has an average hardness of 578.1 HV, yield strength of 2,280 MPa, a tensile strength of 2,740 MPa, an elongation of 10.5% and a pitting potential of 0.17 V.sub.SCE. FIG. 3 shows a stress-strain graph after aging treatment of Example 2. FIG. 4 shows an XRD graph after aging treatment of Example 2, where a precipitation of reverted austenite is observed. FIG. 5 shows an image of an amorphous ring on a lath boundary after aging treatment of Example 2.

[0064] In the examples above, the testing methods for corrosion resistance, hardness and tensile properties of the ultrahigh-strength maraging stainless steel with multiphase strengthening are as follows.

[0065] 1) Hardness: a hardness test was conducted on an HVS-50 Vickers hardness tester with a load of 1 Kg; an average value was taken after 5 dotting, and the results were listed in Table 1.

[0066] 2) Tensile mechanical properties: a tensile test was conducted on an electronic universal testing machine; the samples were prepared as rectangular samples with a nominal size of (2-3) mm×4 mm×20.6 mm; for each of the tensile strength, yield strength and elongation, an average value was taken from three samples treated identically, and the results were listed in Table 1.

[0067] 3) Corrosion Resistance

[0068] A sample was processed into a specification of 10 mm×10 mm×2 mm, and was exposed to 1 cm.sup.2 for experiment after being encapsulated with epoxy resin. The surface of the sample was polished to 2000# with sandpaper, scrubbed with ethyl alcohol to remove oil stains, washed with deionized water and air dried for later use. An experiment was conducted with a solution of 0.1M Na.sub.2SO.sub.4+xNaCl (pH=3) at temperature of 25° C. The electrochemical experiment was conducted using a CHI660E electrochemical workstation. The electrochemical experiment was conducted with a common three-electrode system, where the ultrahigh-strength stainless steel sample was used as a working electrode, a Pt sheet was used as an auxiliary electrode and a saturated calomel electrode (SCE) was used as a reference electrode. Before the electrochemical experiment, the sample was applied with a potential of −1.2 V.sub.SEC and subjected to potentiostatic polarization for 5 min to remove the oxide membrane formed on surface of the sample in the air. The system was stabilized for 30 min and recording was started. Potentiodynamic polarization was conducted at a scanning rate of 0.5 mV/S within a scanning potential range of −0.3 V (vs. open circuit potential E.sub.OC) to 1.5 V (vs. reference electrode potential E.sub.R), and the experiment was stopped after current varied stably. An average value was taken from three measurements and the results were listed in Table 1.

TABLE-US-00002 TABLE 1 Composition, hardness, tensile properties and pitting potential of examples Average Yield Tensile Pitting Chemical composition/% Aging hardness/ strength/ strength/ potential/ Co Ni Cr Ti Mo Mn Si Fe process HV MPa MPa Elongation/% V.sub.SCE Example 1 3.0 8.0 11 2 5 0.08 0.4 Bal. 450° C. 569.3 2260 2690 8.2 0.14 20 h Example 2 4.0 9.0 12 1.5 6 0.5 0.5 Bal. 520° C. 578.1 2280 2740 10.5 0.17 30 h Note: The contents of components such as C, P and S in examples in Table 1 conform to the elemental composition of the stainless steel according to the present disclosure. The content of C is 0.02% or less, the content of P is 0.003% or less and the content of S is 0.003% or less, all of which are not listed in Table 1. The abbreviation Bal. represents balance.