CN112941410A

CN112941410A - Method for controlling alpha phase content in austenitic stainless steel

Info

Publication number: CN112941410A
Application number: CN202110125892.8A
Authority: CN
Inventors: 邹琪; 李刚亮; 翁佳龙; 丁大伟
Original assignee: Wuxi Flang Forging Co ltd
Current assignee: Wuxi Flang Forging Co ltd
Priority date: 2021-01-29
Filing date: 2021-01-29
Publication date: 2021-06-11

Abstract

The invention discloses a method for controlling alpha phase content in austenitic stainless steel, which comprises the following steps: smelting in an electric furnace; refining outside the furnace: after refining, casting molten iron into a steel ingot, and slowly cooling the steel ingot; forging heat treatment; testing the performance; machining; nondestructive testing; and (5) finishing. Through the mode, the chemical element content is adjusted according to the principle of improving the Ni equivalent and reducing the Cr equivalent. The high equivalent value of Ni is easy to form austenite, the reduction of the equivalent of Cr is easy to inhibit the formation of ferrite, the content of ferrite in the material is reduced, and the content of the alpha phase is effectively controlled to be not higher than 0.5 percent; the heat-insulating layer is additionally arranged outside the steel ingot mould, and the dry sand is filled, so that the cooling speed of the steel ingot during pouring is controlled, and the ferrite is slowly cooled to realize full transformation; the forging heating temperature is not allowed to be higher than 1170 ℃, the solution treatment temperature is 1060 +/-10 ℃, the heat preservation time is not more than 4 hours, and ferrite is avoided.

Description

Method for controlling alpha phase content in austenitic stainless steel

Technical Field

The invention relates to the technical field of alpha phase content control, in particular to a method for controlling alpha phase content in austenitic stainless steel.

Background

The austenitic stainless steel forging is widely applied and has large consumption in the industries of machinery, petroleum, chemical industry, nuclear power and the like. Austenitic stainless steels ideally should have a single phase austenitic structure, i.e., a microstructure with only the austenitic phase, with no or few second terms present. However, in practical applications, austenitic stainless steels often contain a certain amount of other phase structure, such as ferrite phase (α -phase) and carbides.

Under the common conditions, in the process of purchasing and using austenitic stainless steel in various industries, the ferrite content in the austenitic stainless steel is not controlled, and the corrosion resistance is qualified, but with the continuous progress of the technology and the continuous improvement of the engineering quality requirement, in the technical documents of construction projects such as petrifaction and nuclear power, the ferrite content in the austenitic stainless steel is required to be controlled frequently, the ferrite content of a forge piece without welding requirement is controlled to be not higher than 0.5%, the ferrite content of a forge piece with welding requirement is controlled to be 5% -8%, and the traditional austenitic stainless steel forge piece can not meet the alpha phase content requirement.

Disclosure of Invention

The invention mainly solves the technical problem of providing a method for controlling the content of alpha phase in austenitic stainless steel, which can inhibit the formation of high-temperature ferrite, reduce the content of ferrite in the material and effectively control the content of the alpha phase to be less than or equal to 0.5 percent.

In order to achieve the purpose, the technical scheme adopted by the invention for solving the technical problems is as follows:

a method for controlling alpha phase content in austenitic stainless steel comprises the following steps:

step one, smelting in an electric furnace: smelting furnace burden into primary molten steel in an electric furnace;

step two, refining outside the furnace: further refining the molten iron obtained in the first step into stainless steel by using a refining furnace, casting the molten iron into a steel ingot after refining, and slowly cooling the steel ingot; the stainless steel comprises the following chemical components: 0.02-0.03% of C, 1.70-2.00% of Mn, 8.5-13% of Ni, 0.07-0.10% of N, less than or equal to 0.03% of P, less than or equal to 0.03% of S, 18-19% of Cr, less than or equal to 0.5% of Si, and the balance of iron;

step three, forging: forging the stainless steel ingot obtained in the second step into a product forging by using a hydraulic press or an air hammer; the stainless steel comprises the following chemical components: 0.02-0.03% of C, 1.70-2.00% of Mn, 8.5-13% of Ni, 0.07-0.10% of N, less than or equal to 0.03% of P, less than or equal to 0.03% of S, 18-19% of Cr, less than or equal to 0.5% of Si, and the balance of iron;

step four, heat treatment: placing the forge piece naturally cooled in the third step into a resistance furnace for solution treatment;

step five, performance test: after the heat treatment, cutting test samples from the forged piece subjected to the heat treatment in the fourth step, processing the test samples into tensile samples and impact samples, and performing mechanical property tests; processing the sample into a ferrite test sample, and detecting the ferrite content;

step six, machining: machining the forged piece qualified in the mechanical property test in the fifth step into a stainless steel forged piece workpiece;

step seven, nondestructive testing: carrying out flaw detection on the machined workpiece through an ultrasonic flaw detector and a penetration flaw detector;

step eight, finished product: and packaging and warehousing the workpieces subjected to flaw detection.

Preferably, in the second step, the steel ingot is slowly cooled by adding the heat-insulating layer outside the steel ingot mould.

Preferably, the heat preservation layer is formed by rolling a steel plate, and dry sand is filled in the heat preservation layer.

Preferably, the thickness of the heat-insulating layer is 70-120 mm.

Preferably, the forging heating temperature in the third step is less than or equal to 1170 ℃.

Preferably, the solution treatment temperature adopted in the heat treatment process in the fourth step is 1060 +/-10 ℃, the liquid medium is cooled, and the heat preservation time of the solution treatment is not more than 4 hours.

Due to the application of the technical scheme, compared with the prior art, the invention has the following beneficial effects:

1. the chemical element content is adjusted by improving the Ni equivalent and reducing the Cr equivalent. The high equivalent value of Ni is easy to form austenite, and the reduction of the equivalent of Cr is easy to inhibit the formation of ferrite; the increase of Ni, C, N and Cu elements is beneficial to the increase of nickel equivalent, and the reduction of the content of Cr and Si elements is beneficial to the reduction of chromium equivalent; inhibit the formation of high-temperature ferrite, reduce the content of ferrite in the material and effectively control the content of the alpha phase to ensure that the content is not higher than 0.5 percent.

2. And (3) additionally arranging a heat-insulating layer outside the ingot mould, and filling dry sand, so that the cooling speed of the ingot mould during pouring is controlled, and ferrite is fully transformed by slow cooling.

3. The forging heating temperature is not allowed to be higher than 1170 ℃, the solution treatment temperature is 1060 +/-10 ℃, the heat preservation time is not more than 4 hours, and ferrite is avoided.

Detailed Description

The following detailed description of the preferred embodiments of the present invention is provided to enable those skilled in the art to more readily understand the advantages and features of the present invention, and to clearly and unequivocally define the scope of the present invention.

and additionally arranging a heat-insulating sleeve made of rolled steel plates outside the ingot mould to form a heat-insulating layer, wherein the distance between the heat-insulating sleeve and the ingot mould is 70-120 mm, and dry sand is filled between the heat-insulating sleeve and the ingot mould. Namely, the thickness of the heat preservation layer is 70-120 mm, and dry sand is filled in the heat preservation layer. And reducing the cooling speed, so that the ferrite content in the steel ingot is not higher than 0.5%, and the ferrite content in the forged piece is not higher than the design standard.

According to the liquid state crystallization characteristic of austenitic stainless steel, a high-temperature ferrite phase is firstly separated out from the liquid, and then, the austenite is generated through nucleation. The cooling speed should be strictly controlled in the process of casting the steel ingot and avoiding the situation that high-temperature ferrite is not ready to be converted and remains to the room temperature. When the high-temperature ferrite remains in the steel ingot at room temperature, the high-temperature ferrite cannot be eliminated by forging heating and heat treatment heating in the later period, and the high-temperature ferrite can be always remained in the forged piece. In order to avoid the existence of excessive high-temperature ferrite in the forged piece, the forged piece is slowly cooled in the steel ingot casting cooling process, so that the ferrite is fully converted into an austenite structure.

Step three, forging: forging the stainless steel ingot obtained in the second step into a product forging by using a hydraulic press or an air hammer;

in the third step, the forging heating temperature is less than or equal to 1170 ℃, the forging heating temperature is not too high, high-temperature ferrite can be generated at higher heating temperature, and once the high-temperature ferrite is generated, the performance is not favorable and cannot be eliminated.

And (3) carrying out simulation analysis and test verification by using finite element analysis software Deform in the forging forming process, and determining forging process parameters required by the specified ferrite content.

in order to achieve a good solution treatment effect, the solution treatment temperature adopted in the heat treatment process is 1060 +/-10 ℃, the liquid medium is cooled, the heat preservation time of the solution treatment is not more than 4 hours, and the austenite grains are prevented from growing too fast during heat treatment and heating. Since high-temperature ferrite is generated even when the solution treatment temperature is too high, the solution treatment temperature is also lowered in this production method.

Finite element analysis software DANTE is designed in the heat treatment process for simulation and process verification, and heat treatment process parameters which are required to be adopted for specifying the ferrite content are determined.

Step five, performance test: after the heat treatment, test specimens were cut out of the forged part heat-treated in the fourth step, and the specimens were processed into tensile specimens and impact specimens to conduct mechanical property tests. And processing the sample into a ferrite test sample, and detecting the ferrite content, wherein the ferrite content is mainly determined during detection, and other tests only need to meet the material standard SA 182.

Step six, machining: machining the forged piece qualified by the mechanical property test and the ferrite content test in the fifth step to obtain a stainless steel forged piece;

step seven, nondestructive testing: flaw detection and inspection are carried out on the machined workpiece through an ultrasonic flaw detector and a penetration flaw detector;

step eight, finished product: and packaging and warehousing the flaw-detected and inspected workpieces.

According to the method for controlling the alpha phase content in the austenitic stainless steel, the alpha phase content is controlled by three key points, 1, slow cooling is carried out during steel ingot casting; 2. controlling the forging heating temperature; 3. and controlling the heat treatment temperature.

The invention is specifically illustrated below with reference to specific examples:

example 1: and manufacturing a phi 600 multiplied by phi 350 multiplied by 150mm austenitic stainless steel forging made of SA182F304L stainless steel.

And (4) acceptance requirements: rm is more than or equal to 485MPa, Rp0.2 is more than or equal to 170MPa, A is more than or equal to 30 percent, wherein A represents the growth rate, ultrasonic detection and penetration detection are carried out according to NB2540 of the third volume of ASME, and the defects are in accordance with the standard specification.

The specific process flow is as follows:

raw material smelting → blanking → forging → heat treatment → sampling → performance test → nondestructive testing → machining → finished product.

The raw material used was a 1-ton-weight ingot whose chemical composition is shown in table 1. The slow cooling measure of the invention is adopted for steel ingot pouring and cooling, namely, a heat insulation layer is formed by additionally arranging a heat insulation sleeve made by rolling a steel plate outside the steel ingot mould, the thickness of the heat insulation layer is 100mm, and dry sand is filled in the heat insulation layer. During forging, after the steel ingot is heated, chamfering and cutting off a riser and a nozzle are performed, wherein the end of the riser is cut off by 15%, and the end of the nozzle is cut off by 5%. And forging the steel ingot into a forging round blank with the diameter of 300mm, cooling, sawing and blanking, wherein one steel ingot can be used for manufacturing the forged pieces of 3 embodiments.

Table 1 chemical composition%

Material of	C	Si	Mn	P	S	Cr	Ni	N
									SA182 F304L	0.024	0.45	1.9	0.025	0.02	18.6	9.7	0.082

Sawing the round forging blank into blanks of phi 300 multiplied by 400, putting the blanks into a forging heating furnace for heating, heating the blanks along with the furnace to 850 ℃, preserving the heat for 0.5 hour, heating the blanks along with the furnace to 1180 ℃, preserving the heat for 1.5 hours, and then starting forging: upsetting to 200mm height, upsetting ratio to 2, drawing to 400mm height, drawing to 2, upsetting to 150mm height, forging ratio to 2.6, punching, reaming and shaping to specified size.

The initial forging temperature is 1180 ℃, the final forging temperature is 950 ℃, the forging is carried out by two times of fire, the total forging ratio is 6.6, the final fire deformation is 20 percent, and the air cooling is carried out after the forging.

And (3) loading the forged workpiece into a heat treatment furnace, keeping the temperature for 3.5 hours at the solution treatment temperature of 1060 ℃, and cooling by water.

After the heat treatment is finished, selecting a forging piece in each batch, intercepting test materials on the forging piece, namely cutting a sample ring on one end face, intercepting various samples on the sample ring, and making mechanical property and ferrite content test items. The mechanical properties are shown in Table 2, wherein A represents the growth rate and Z represents the reduction of area.

TABLE 2 mechanical Property testing

Material of	Rm(MPa)	Rp0.2(MPa)	A(％)	Z(％)	Akv2(J)
						SA182F304L	552	255	52	71	266

Through metallographic detection, the ferrite content of the forging is 0.16%.

And machining the forge piece after the mechanical property test, wherein the machining aims to prepare for subsequent nondestructive testing, and the nondestructive testing items are ultrasonic testing and penetration testing. No defect with equivalent weight larger than 2mm is found in ultrasonic detection, no circular defect exceeding 1mm is found in penetration detection, and no linear defect is found.

Comparative example 1: manufacturing a phi 600 multiplied by phi 350 multiplied by 150mm austenitic stainless steel forging made of SA182F304L stainless steel, wherein the chemical components of the stainless steel are traditional chemical components, and concretely, see Table 3, the process method of the embodiment 1 is adopted;

TABLE 3 chemical composition%

Material of	C	Si	Mn	P	S	Cr	Ni	N
									SA182F304L	0.18	0.76	1.22	0.042	0.021	18.12	8.06	0.021

After the heat treatment is finished, selecting a forging piece in each batch, intercepting test materials on the forging piece, namely cutting a sample ring on one end face, intercepting various samples on the sample ring, and making mechanical property and ferrite content test items. The mechanical properties are shown in Table 4, wherein A represents the growth rate and Z represents the reduction of area.

TABLE 4 mechanical Properties test

Material of	Rm(MPa)	Rp0.2(MPa)	A(％)	Z(％)	Akv2(J)
						SA182F304L	532	233	55	72	278

Metallographic detection shows that the ferrite content is 0.86%.

Comparative example 2: manufacturing a phi 600 multiplied by phi 350 multiplied by 150mm austenitic stainless steel forging made of SA182F304L stainless steel, wherein the chemical components of the stainless steel are the chemical components in the embodiment 1, and the details are shown in Table 5; the manufacturing method adopts the traditional manufacturing method;

TABLE 5 chemical composition%

Material of	C	Si	Mn	P	S	Cr	Ni	N
									SA182F304L	0.024	0.45	1.9	0.025	0.02	18.6	9.7	0.082

The traditional manufacturing method is used, and the content of the traditional manufacturing method is as follows:

the difference between the traditional process and the process of example 1 is as follows: the steel ingot used in the traditional process does not adopt slow cooling measures during pouring, and is normally air-cooled. The forging heating temperature of the traditional process is 1220 ℃, and a certain amount of ferrite is generated due to higher heating temperature. The heat treatment temperature of the traditional process is 1120 ℃, and a certain amount of ferrite is also generated.

After the heat treatment is finished, selecting a forging piece in each batch, intercepting test materials on the forging piece, namely cutting a sample ring on one end face, intercepting various samples on the sample ring, and making mechanical property and ferrite content test items. The mechanical properties are shown in Table 6, wherein A represents the growth rate and Z represents the reduction of area.

TABLE 6 mechanical Properties test

Material of	Rm(MPa)	Rp0.2(MPa)	A(％)	Z(％)	Akv2(J)
						SA182F304L	546	258	53	69	262

Through metallographic detection, the ferrite content is 2.25%.

And machining the forge piece after the mechanical property test, wherein the machining aims to prepare for subsequent nondestructive testing, and the nondestructive testing items are ultrasonic testing and penetration testing. The ultrasonic detection and the penetration detection are qualified.

Comparing example 1 of the present invention with comparative examples 1, 2, the following conclusions were made:

comparative example 1 used conventional chemistry and the patented manufacturing process. Because the traditional chemical components do not improve the nickel equivalent and reduce the chromium equivalent, the ferrite content in the forge piece is increased compared with the embodiment, which shows that the chemical component adjustment has a good effect on reducing the ferrite content in the forge piece. The indexes such as mechanical property and the like are not changed greatly compared with the embodiment because the manufacturing process except the chemical components is not changed.

Comparative example 2 the chemical composition of the example and the conventional manufacturing process were used. Because slow cooling measures are not taken in steel ingot casting in the traditional process, more ferrite is not transformed and is kept to the room temperature when metal is solidified, and in addition, the traditional forging heating temperature and the heat treatment temperature are higher, a part of ferrite is also generated, so that the content of the ferrite in a forge piece reaches about 2 percent, and the manufacturing process disclosed by the invention plays a good role in inhibiting the generation of the ferrite.

The detection data show that the content of ferrite can be stably controlled to be not more than 0.5% by adopting the patented chemical components and the production forge piece of the manufacturing process, and the special requirements of users are met. Indexes such as the mechanical property of the forged piece manufactured by adopting the chemical components and the manufacturing process of the patent also meet the requirement of the SA182 standard.

The above description is only an embodiment of the present invention, and not intended to limit the scope of the present invention, and all modifications of equivalent structures and equivalent processes, or direct or indirect applications in other related fields, which are made by the contents of the present specification, are included in the scope of the present invention.

Claims

1. A method for controlling alpha phase content in austenitic stainless steel is characterized by comprising the following steps:

2. The method for controlling the alpha phase content in austenitic stainless steel according to claim 1, wherein: and in the second step, the steel ingot is slowly cooled by additionally arranging the heat-insulating layer outside the steel ingot mould.

3. The method for controlling the content of alpha phase in austenitic stainless steel according to claim 2, wherein: the heat preservation layer is formed by rolling steel plates, and dry sand is filled in the heat preservation layer.

4. The method for controlling the content of alpha phase in austenitic stainless steel according to claim 2, wherein: the thickness of the heat preservation layer is 70-120 mm.

5. The method for controlling the alpha phase content in austenitic stainless steel according to claim 1, wherein: the heating temperature of forging in the third step is less than or equal to 1170 ℃.

6. The method for controlling the alpha phase content in austenitic stainless steel according to claim 1, wherein: in the fourth step, the solution treatment temperature adopted in the heat treatment process is 1060 +/-10 ℃, the liquid medium is cooled, and the heat preservation time of the solution treatment is not more than 4 hours.