CN108467985B

CN108467985B - Method for controlling inclusions in austenitic stainless steel for pressure container

Info

Publication number: CN108467985B
Application number: CN201810326005.1A
Authority: CN
Inventors: 李建民; 翟俊; 郎炜昀; 任选
Original assignee: Shanxi Taigang Stainless Steel Co Ltd
Current assignee: Shanxi Taigang Stainless Steel Co Ltd
Priority date: 2018-04-12
Filing date: 2018-04-12
Publication date: 2020-08-25
Anticipated expiration: 2038-04-12
Also published as: CN108467985A

Abstract

The invention relates to the field of stainless steel smelting, in particular to the field of austenitic stainless steel smelting for pressure containers. A method for controlling inclusions in austenitic stainless steel for a pressure vessel is characterized in that the austenitic stainless steel for the pressure vessel is produced by adopting a K-OBM-S furnace → VOD furnace → LF furnace flow, and tapping is carried out to the VOD furnace when the slag thickness in the K-OBM-S furnace is less than or equal to 50 mm; keeping the vacuum degree of the VOD furnace to be less than or equal to 2mbar in a boiling period, adding 15-18 kg of lime per ton steel, 3-6 kg of fluorite per ton steel and 4-5 kg of aluminum shot per ton steel in a reduction period, reducing for 15-25 min, and then tapping steel to an LF furnace; after slag melting is carried out in an LF furnace, 0.625 kg of aluminum powder per ton of steel is added to the slag surface, 1-2 m of pure calcium line per ton of steel is fed for micro-calcium treatment, 3.0-4.0L/min argon gas is blown down and stirred for 5min, 1.0-1.5L/min argon gas is blown down and stirred for 15-25 min, and the steel is taken out of the furnace, and continuous casting and calming are carried out for 30-35 min.

Description

Method for controlling inclusions in austenitic stainless steel for pressure container

Technical Field

The invention relates to the field of stainless steel smelting, in particular to the field of austenitic stainless steel smelting for pressure containers.

Background

The pressure vessel is an important part of some thermal power stations and nuclear reactors, and because it bears the key functions of equipment operation, energy conversion and the like, the pressure vessel is required to be capable of normally operating under severe environments such as high temperature, high pressure, fluid scouring and corrosion, and the pressure vessel in some special fields is required to have a design life of not less than 40 years and be not replaceable. Stainless steel is one of the important materials for constructing pressure vessels, which puts higher demands on the stainless steel material.

Compared with common stainless steel, the stainless steel for the pressure container has more strict requirements on the purity of the steel, including harmful elements, flaw detection, inclusion rating and total oxygen content, so that the conventional stainless steel refining process cannot meet the ultra-purity standard of the stainless steel for the pressure container.

Regarding austenitic stainless steel for pressure vessels, the smelting process of EAF → AOD → LF is mostly adopted at present, and the Si deoxidation or Si-Al composite deoxidation mode is mainly adopted, but five harmful elements are inevitably brought in due to the adoption of electric smelting scrap steel smelting, and in addition, the impurity removal effect is poor due to the short refining process, the B-type or C-type impurities exceed the standard, and the oxygen content exceeds 30 ppm. Patent application numbers 201410582328.9 and 201410582316.X and 201410582327.4 both disclose a preparation method of an austenitic stainless steel pressure vessel for nuclear power, which improves the comprehensive performance of austenitic stainless steel, but the aspects of deoxidation and inclusion removal are not elaborated, and the austenitic stainless steel is directly cast after AOD tapping, so that a necessary refining means is lacked, and the inclusion removal capability is insufficient.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the problem of insufficient impurity removal capability in the smelting process of austenitic stainless steel for the pressure container is solved.

The technical scheme adopted by the invention is as follows: a method for controlling inclusions in austenitic stainless steel for a pressure vessel is characterized in that a K-OBM-S furnace → a VOD furnace → an LF furnace is adopted for producing the austenitic stainless steel for the pressure vessel, pretreated molten iron subjected to dephosphorization and desulfurization is added into the K-OBM-S furnace to be prepared into required components, and the molten iron is discharged into the VOD furnace when the slag thickness is less than or equal to 50 mm; keeping the vacuum degree of the VOD furnace to be less than or equal to 2mbar in a boiling period, adding 15-18 kg of lime per ton steel, 3-6 kg of fluorite per ton steel and 4-5 kg of aluminum shot per ton steel in a reduction period, reducing for 15-25 min, and then tapping steel to an LF furnace; after slag melting is carried out in an LF furnace, 0.625 kg of aluminum powder per ton of steel is added to the slag surface, 1-2 m of pure calcium line per ton of steel is fed for micro-calcium treatment, 3.0-4.0L/min argon gas is blown down and stirred for 5min, 1.0-1.5L/min argon gas is blown down and stirred for 15-25 min, and the steel is taken out of the furnace, and continuous casting and calming are carried out for 30-35 min.

As a preferred mode: the refining slag system adopted in the LF furnace comprises the following components in percentage by mass: 55-60%; SiO2₂：5~10% ；Al₂O₃：15~20%；MgO：5~10%；CaF₂10-15%, and the others are impurities.

The invention has the beneficial effects that: the method comprises the steps of (1) adopting an all-aluminum deoxidation method, controlling the aluminum content to be 0.02-0.03%, carrying out micro-calcium treatment, wherein as-cast inclusions mainly comprise calcium aluminate coated magnesium aluminate spinel, the maximum size of the inclusions is 10 mu m, and the inclusions are not easy to deform and mainly comprise D-type inclusions in the rolling process; and the balanced oxygen content of the slag system is low and is lower than 15 ppm; the time for removing the inclusions by floating is prolonged, the number of the inclusions is small, and flaw detection incompatibility caused by the aggregation of the inclusions is avoided.

Detailed Description

Example 1

Steel grade 316H

Adding the pretreated molten iron into the K-OBM-S, wherein the thickness of tapping slag is 30mm, and the molten steel amount is 81.4 t;

vacuum degree of 1mbar in VOD boiling period, adding 1400kg of lime, 400kg of fluorite and 300kg of aluminum pills in reduction period, and reducing for 20 min;

adding 50kg of aluminum powder on the slag surface after LF enters the station for slagging, feeding a pure calcium line of 160m, stirring for 5min at the argon bottom blowing flow rate of 2 multiplied by 150L/min, then stirring for 20min at the argon bottom blowing flow rate of 2 multiplied by 50L/min, and calming for 30 min;

in this example, the lime contains 95% CaO and the fluorite contains CaF₂95 percent of Al, 99.6 percent of Al in the aluminum pill, 96.0 percent of Al in the aluminum powder and 0.22kg/m of metal-containing calcium in the pure calcium wire;

in the embodiment, the refining slag system comprises (by mass percent) CaO: 57.5 percent; SiO 2: 6.3 percent; al (Al)₂O₃：16.2%；MgO：6.9%；CaF₂: 12.4%, others are impurities.

The result of the analysis of the total oxygen content by sampling in the as-cast state of this example was 12ppm, and the maximum size of inclusions was 8.6 μm;

the 316H rolled stock obtained in the embodiment is subjected to gold sampling according to the national standard of GBT 10561-,

the rolled stock of this example was subjected to flaw detection analysis, and the flaw detection yield was 95%.

Example 2

Steel grade 316H

Adding the K-OBM-S into the pretreated molten iron, wherein the thickness of tapping slag is 30mm, and the molten steel amount is 83.5 t;

the vacuum degree in the VOD boiling period is 1mbar, 1500kg of lime, 500kg of fluorite and 400kg of aluminum pills are added in the reduction period, and the reduction time is 20 min;

adding 50kg of aluminum powder on the slag surface after LF enters the station for slagging, feeding 167m of pure calcium wire, stirring for 5min at the argon bottom blowing flow rate of 2X 140L/min, then stirring for 25min at the argon bottom blowing flow rate of 2X 45L/min, and keeping the temperature for 35 min;

in this example, the CaO content of lime is 95%, and CaF in fluorite₂95 percent of Al, 99.6 percent of Al in the aluminum pill, 96.0 percent of Al in the aluminum powder and 0.22kg/m of metal-containing calcium in the pure calcium wire;

in the embodiment, the refining slag system comprises CaO: 55.6 percent; SiO2₂：5.6% ；Al₂O₃：16.9%；MgO：8.1%；CaF₂: 12.6%, and the others are impurities.

The total oxygen content of the cast sample of this example was 9ppm, and the maximum size of the inclusions was 7.6 μm;

the rolled stock of this example was subjected to flaw detection analysis, and the flaw detection yield was 97%.

Comparative example

Steel grade 316H

Adding the K-OBM-S into the pretreated molten iron, wherein the thickness of tapping slag is 30mm, and the molten steel amount is 80.1 t;

in the VOD boiling period, the vacuum degree is 1mbar, in the reduction period, 550kg of ferrosilicon alloy, 1200kg of lime and 300kg of fluorite are added, and the reduction time is 10 min;

after the LF is in the station, slagging is carried out, stirring is carried out for 5min under the argon bottom blowing flow of 2 multiplied by 140L/min, then stirring is carried out for 15min under the argon bottom blowing flow of 2 multiplied by 45L/min, and the sedation time is 25 min;

in the comparative example, the Si content in the Si-Fe alloy is 75%, the CaO content in the lime is 95%, and the CaF content in the fluorite is 95%₂The content is 95 percent;

the result of the total oxygen content of the cast sample analysis of the comparative example is 30ppm, and the maximum size of the inclusion is 18 mu m;

gold phase samples are taken from the 316H rolled stock obtained in the comparative example according to the national standard of GBT 10561-,

the rolled stock of this example was subjected to flaw detection analysis, and the flaw detection yield was 80%.

Claims

1. A method for controlling inclusions in austenitic stainless steel for pressure vessels, characterized by comprising: the production of austenitic stainless steel for the pressure vessel adopts the flow of a K-OBM-S furnace → a VOD furnace → an LF furnace, molten iron which is subjected to dephosphorization and desulfurization through pretreatment is added into the K-OBM-S furnace to be prepared into required components, and the molten iron is tapped into the VOD furnace when the slag thickness is less than or equal to 50 mm; keeping the vacuum degree of the VOD furnace to be less than or equal to 2mbar in a boiling period, adding 15-18 kg of lime per ton steel, 3-6 kg of fluorite per ton steel and 4-5 kg of aluminum shot per ton steel in a reduction period, reducing for 15-25 min, and then tapping steel to an LF furnace; adopting an all-aluminum deoxidation method, controlling the aluminum content to be 0.02-0.03%, carrying out micro-calcium treatment, adding 0.625 kg of aluminum powder per ton of steel on the slag surface after slagging in an LF furnace, feeding 1-2 m of pure calcium wire per ton of steel for micro-calcium treatment, stirring for 5min by blowing argon gas at a rate of 3.0-4.0L/min per ton of steel, then stirring for 15-25 min by blowing argon gas at a rate of 1.0-1.5L/min per ton of steel, discharging, and carrying out continuous casting and calming for 30-35 min, wherein refined slag components adopted in the LF furnace are CaO in percentage by mass: 55-60%; SiO2₂：5~10% ；Al₂O₃：15~20%；MgO：5~10%；CaF₂10-15 percent of the slag system, the other impurities, as-cast inclusions mainly comprise calcium aluminate coated magnesium aluminate spinel, the maximum size of the inclusions is 10 mu m, the slag system is not easy to deform in the rolling process and mainly comprises D-type inclusions, the balanced oxygen content of the slag system is low and is lower than 15ppm, the floating removal time of the inclusions is prolonged, the number of the inclusions is small, and unqualified flaw detection caused by the aggregation of the inclusions is avoided.