CN1470653A

CN1470653A - Converter steelmaking process

Info

Publication number: CN1470653A
Application number: CNA031462065A
Authority: CN
Inventors: 浏刘; 刘浏; 佟溥翘; 金振坚; 郑丛杰; 曾加庆; 高建军; 布焕存; 胡砚斌; 米谷明
Original assignee: Central Iron and Steel Research Institute
Current assignee: Central Iron and Steel Research Institute
Priority date: 2003-07-04
Filing date: 2003-07-04
Publication date: 2004-01-28
Anticipated expiration: 2023-07-04
Also published as: CN1189575C

Abstract

The convertor steel-smelting method adopts the top and bottom combined converting process, uses molten iron and solid iron material as raw material, their weight percentage are, molten iron 85%-95% and solid iron material 5-15%. The pretreatment of desilication, dephosphorization and decarbonization of molten iron and decarbonizing refining process are implemented in same convertor, i.e. front stage is pretreatment of molten iron, and late stage is decarbonizing refining stage, in the pretreatment stage the dephosphorization agent can be added, the top-bottom combined converting process can be adopted, top converting adopts flow rate changed oxygen supply, and its medium and late stage adopts weak oxygen supply, and the decarbonizing refining stage adds lime, makes secondary slag formation and adopts large quantity of oxygen to make top and bottom combined converting process.

Description

Converter steelmaking method

Technical Field

The invention belongs to the field of ferrous metallurgy, and relates to a converter steelmaking production process. The method is suitable for three-removal pretreatment and decarburization refining of desiliconizing, dephosphorizing and decarburizing of molten iron in various converters.

Background

Phosphorus is a harmful impurity in steel, is easy to segregate in grain boundaries to cause cold brittleness of the steel, and obviously reduces the low-temperature impact toughness of the steel. This detrimental effect of phosphorus generally increases with increasing carbon, nitrogen and oxygen content in the steel. In addition, the segregation degree of phosphorus in the casting blank is high, and the harmful effect of phosphorus is more prominent. With the improvement of the quality requirement of users on steel, the requirement of the phosphorus content in steel is more and more strict, and the phosphorus content is required to be reduced as much as possible. Therefore, the reduction of the phosphorus content in steel becomes an important measure for improving the quality of steel.

Dephosphorization in steel making usually adopts an oxidation process. The converter has high end point temperature, which is not beneficial to dephosphorization; rephosphorization is often caused during tapping. Therefore, it is difficult to stably produce low-phosphorus steel with [ P]less than or equal to 0.015% by using the traditional converter steelmaking process.

The molten iron pretreatment process using the torpedo car and the hot metal ladle for conveying molten iron as refining containers lays a foundation for mass production of high-purity steel. However, the operation is limited due to problems such as a long time from tapping of the blast furnace to tapping of the converter, a large temperature drop, generation of a large amount of bubbles in the pretreatment of molten iron, and the like. From the heat balance condition of converter smelting, the heat of converter smelting comes from sensible heat and chemical heat brought by molten iron. When the desiliconization and dephosphorization process of the molten iron is adopted, the temperature of the molten iron added into the converter is lower because of the chemical heat of the molten iron and the reduction of the temperature of the molten iron due to the burning loss of chemical elements and the addition of a large amount of dephosphorization agent. Taking the Jun Lai factory as an example, the temperature drop before and after dephosphorization of molten iron is generally more than 100 ℃. The temperature of the molten iron added into the converter is only 1240 ℃ because the temperature of the molten iron added into the converter is reduced by about 30 ℃ after the molten iron is dephosphorized, thereby bringing a plurality of difficulties to the converter for blowing. Because of insufficient heat, the scrap steel ratio in the converter charging is greatly reduced, and simultaneously, slag iron in the blowing is often adhered to an oxygen lance, especially when high-carbon steel is smelted, the heat is insufficient, even an external heat source is sometimes needed, for example, coke is added during blowing, so that the problem of insufficient blowing heat of the converter is solved.

The traditional 'three-removal' pretreatment process method for desiliconizing, dephosphorizing and desulfurizing molten iron is to respectively carry out 'three-removal' pretreatment and converter refining on the molten iron in two converters. For example, the NKK Fushan factory in Japan adopts the desulfurized molten iron to carry out dephosphorization in a special dephosphorization converter mode, and then adds another converter again to carry out decarburization refining, thereby increasing the scheduling difficulty and also causing the temperature loss. Resulting in low steel-making efficiency, high energy consumption and high cost (Shenhu Steel works: 38 (1): 9; iron と steel, 1990 (11): 1817).

Disclosure of Invention

The invention aims to provide a converter steelmaking method which has high efficiency, low energy consumption and low cost, and can carry out three-way decarburization pretreatment and decarburization refining on molten iron in the same converter.

The converter steelmaking method adopts a top-bottom combined blowing process, takes molten iron and solid iron materials as raw materials, and the weight percentage of the molten iron and the solid iron materials is as follows: 85-95% of molten iron and 5-15% of solid iron material. The three-step desiliconization, dephosphorization and desulfurization pretreatment and decarburization refining of molten iron are carried out in the same converter in stages, namely the molten iron pretreatment stage is at the early stage, and the decarburization refining stage is at the later stage; and when the pretreatment is finished, the furnace is turned over and the slag is poured, the initial stage of 1/2-2/3 is poured, and then the decarburization refining stage is carried out.

The process of the pretreatment period of the three-dehydration of the molten iron comprises the following steps:

(1) adding dephosphorizing agent at the time of blowing, wherein the adding amount is 10-20kg/t_{Molten iron}Chemical of dephosphorizing agentThe components by weight percentage are as follows: CaO 40-70%, iron oxide 15-45%, CaF₂15-20%, and the iron oxide can be steel sheet or converter smoke.

(2) The pretreatment stage adopts top-bottom combined blowing, the top blowing adopts variable flow oxygen supply, and the initial top blowing oxygen supply intensity is 2.8-3.5Nm³At the middle and later stages, weak oxygen supply is adopted, and the oxygen supply intensity is controlled to be 2.8Nm³Min gradually decreased to 0.8Nm³/t.min；

(3) Strong nitrogen stirring is adopted in bottom blowing, and the oxygen supply intensity of the initial bottom blowing is 0.07-0.1Nm³T.min, gradually increasing to 0.15-0.30Nm in the middle and later stages³/t.min，

(4) At the end of the pretreatment, the molten iron contains 3.0-4.0% of [ C], 0.02-0.03% of [ S]and [ P], and the [ Si]is the trace.

The later stage decarburization refining process comprises the following steps:

(1) and (5) entering a decarburization refining period at the same time of finishing the pretreatment of the molten iron. After the pretreatment is finished, pouring 1/2-2/3 of initial period, then adding lime, slagging again, the adding amount of lime is 10-15kg/t_{Molten iron}Adopting a less slag smelting process;

(2) the top-bottom combined blowing with large oxygen supply is adopted until the end point of the steel making, and the top-blowing oxygen supply strength is 3.5-4.5nNm³At t.min, the bottom-blown stirring strength is 0.06-0.15Nm³And t.min, wherein the bottom blowing gas is nitrogen or argon. And timely switching is performed according to the quality requirement of the steel grade.

For the steel-making process, the dephosphorization reaction is as follows:

△G₁₅°＝-204，450+83.55T

the influence of the change of the components of the steelmaking slag on the dephosphorization capability of the slag is researched by G.W.Healy. An equation for estimating the phosphorus distribution between slag and steel is proposed:

\lg \frac{(% P)}{[% P]} = 22350 / T - 16.0 + 0.08 \times (% CaO) + 2.5 \times \lg (% TFe)

according to the analysis of the above formula, the distribution ratio of phosphorus between slag and steel can be increased by lowering the treatment temperature, increasing the basicity and the oxidizability of the slag. According to the principle, the converter top-bottom combined blowing process is adopted, the solid iron material is added according to the proportion of 5-15%, 10-20kg/t of dephosphorization agent is added during blowing, the molten pool is promoted to form liquid slag as soon as possible, and the initial slag is formed within 2.5-3 min. In the pretreatment period, top blowing adopts a variable flow oxygen supply process, so that molten iron desiliconization, manganese and phosphorus can be rapidly carried out, and the temperature of a molten pool is controlled at 1320-1380 ℃. The oxygen supply mode from the top of the furnace to the top of the furnace is large and small, and the main purpose is to meet the requirements of early-stage rapid desiliconization and slag melting by temperature rise. The oxygen supply is reduced in the middle and later stages, and the excessive de-C and temperature rise of the molten pool are mainly inhibited, so that the temperature of the molten pool is kept in a low-temperature interval suitable for de-P; the continuous stirring with large air quantity at the bottom in the middle and later periods aims to create good dynamic conditions and promote the full reaction and mixing among the steel slag so as to ensure that the P removal reaction is closer to the balance.

By adopting the method, on the premise of not prolonging the operation period of the converter, the dephosphorization rate of the converter reaches 90 percent, the desulfurization rate reaches 60 percent, the desiliconization effect can meet the requirements of various steel types, the oxygen supply intensity reaches 3.5-4.5 nm/tmin,

compared with the prior art, the invention has the following advantages:

1) because the three-dehydration pretreatment and the decarburization refining of the molten iron are completed in the same converter, the molten iron after the pretreatment of the molten iron is cancelled to be added into the converter again, the operation time is shortened, and the use efficiency of the converter is improved.

2) The temperature drop of molten iron generated by adding iron again is reduced, energy is saved, and the cost is reduced.

3) Obviously improves the dephosphorization rate and the desulfurization rate, and is beneficial to improving the quality of steel grade.

Detailed Description

According to the converter steelmaking method, 3 furnaces of steel are smelted on a 90-ton converter, and the raw materials of the steel are blast furnace molten iron and scrap steel. The ratio of molten iron to scrap, and the molten iron composition and temperature are shown in Table 1. After the molten iron is added into the converter, firstly, the three-step blowing pretreatment is carried out, the top and bottom combined blowing is started, and simultaneously, the dephosphorization agent is added, wherein the adding amount of the dephosphorization agent and the chemical components of the dephosphorization agent are shown in table 2. After the pretreatment is finished, the furnace is turned over to discharge slag, 1/2-2/3 primary slag is poured, and then lime is added to carry out decarburization refining. The pretreatment period, the top-bottom combined blowing system for decarburization refining, and the amount of lime added are shown in Table 3. After the decarburization refining was completed, slag was removed and steel was tapped, and the chemical composition and tapping temperature of the steel grade obtained were as shown in Table 4.

Table 1 examples raw material ratios and molten iron parameters

Furnace number	Raw material ratio (wt%)		Temperature of molten iron Degree (. degree. C.)	Molten iron ingredient (wt%)
	Raw material ratio (wt%)			Molten iron ingredient (wt%)						Molten iron	Solid iron material	C	Si	Mn	P	S	Fe
	1	93		Scrap steel 7	1239	4.3	0.346	0.136	0.073	Molten iron	Solid iron material	C	Si	Mn	P	S	Fe	0.014	Surplus
2	1	93	90	Scrap steel 7	1239	4.3	0.346	0.136	0.073	Scrap Steel 10	1288	4.28	0.395	0.158	0.08	0.016	Surplus	0.014	Surplus
2	3	92	90	Scrap Steel 8	1265	4.41	0.24	0.75	0.082	Scrap Steel 10	1288	4.28	0.395	0.158	0.08	0.016	Surplus	0.023	Surplus

TABLE 2 example dephosphorization agent addition amount and dephosphorization agent chemical composition in pretreatment period

Furnace number	Amount of dephosphorizing agent added kg/t_{Molten iron}	Dephosphorization agent component (wt%)
		Dephosphorization agent component (wt%)			CaO	Iron oxide	CaF₂
		1	11.1	51	CaO	Iron oxide	CaF₂	Iron scale 32	17
2	20.0	1	11.1	51	42	Iron scale 39	19	Iron scale 32	17
2	20.0	3	17.7	68	42	Iron scale 39	19	Converter flue dust 16	16

TABLE 3 examples top and bottom blowing oxygen supply system

Furnace with a heat exchanger Number (C)	Oxygen supply system in pretreatment period Strength (Nm)³T.min). times. (Sec.)		Oxygen supply system in dephosphorization refining period Strength (Nm)³T.min). times. (Sec.)		Decarburization refining period Amount of lime added Molten iron of kg/t
						Top blowing	Bottom blowing	Top blowing	Bottom blowing
	1	2.0×430	0.06×430	3.0×581		Top blowing	Bottom blowing	Top blowing	Bottom blowing	0.06×581	12.0
2	1	2.0×430	0.06×430	3.0×581	3.0×150 1.88×278	0.065×428	3.0×631	0.06×631	11.6	0.06×581	12.0
2	3	3.05×180 1.22×530	0.24×710	3.0×760	3.0×150 1.88×278	0.065×428	3.0×631	0.06×631	11.6	0.05×760	12.5

TABLE 4 examples of the chemical composition and tapping temperature of the steel grade at the end of the smelting

Furnace with a heat exchanger Number (C)	Chemical composition (wt%) of steel						Tapping temperature Degree of rotation ℃
	Chemical composition (wt%) of steel							C	Si	Mn	P	S	Fe
	1	0.049	0.007	0.0031	0.005	0.016		C	Si	Mn	P	S	Fe	Surplus	1658
2	1	0.049	0.007	0.0031	0.005	0.016	0.052	0.008	0.0029	0.004	0.014	Surplus	1671	Surplus	1658
2	3	0.1	0.01	0.005	0.006	0.020	0.052	0.008	0.0029	0.004	0.014	Surplus	1671	Surplus	1667

Claims

A converter steelmaking method adopts top-bottom combined blowing, takes molten iron and solid iron materials as raw materials, and is characterized in that the molten iron and the solid iron materials have the following weight percentages: 85-95% of molten iron and 5-15% of solid iron material; three-decarburization pretreatment and decarburization refining of molten iron desiliconization, dephosphorization and decarburization are carried out in the same converter in stages, namely the molten iron pretreatment stage is at the early stage, and the decarburization refining stage is at the later stage; and when the pretreatment is finished, the converter is turned over to discharge slag, the initial stage of 1/2-2/3 is turned over, and then the decarburization refining stage is carried out.
The method according to claim 1, wherein the process of the pretreatment stage of the molten iron triple dehydration comprises the following steps:

(1) adding dephosphorizing agent at the time of blowing, wherein the adding amount is 10-20kg/t_{Molten iron}；

(2) The pretreatment stage adopts top-bottom combined blowing, the top blowing adopts variable flow oxygen supply, and the initial top blowing oxygen supply intensity is 2.8-3.5Nm³At the middle and later stages, weak oxygen supply is adopted, and the oxygen supply intensity is controlled to be 2.8Nm³Min gradually decreased to 0.8Nm³/t.min；

(3) Strong nitrogen stirring is adoptedin bottom blowing, and the oxygen supply intensity of the initial bottom blowing is 0.07-0.1Nm³T.min, gradually increasing to 0.15-0.30Nm in the middle and later stages³/t.min；

(4) When the pretreatment is finished, the [ C]in the molten iron is 3.0-4.0%, the [ S]and the [ P]are 0.02-0.03%, and the [ Si]is a trace;
3. the method according to claim 1, characterized in that the post decarburization refining process is:

(1) at the end of the pretreatmentPouring out 1/2-2/3 initial slag, adding lime immediately, and slagging again, wherein the adding amount of lime is 10-15kg/t_{Molten iron}；

(2) The top and bottom composite blowing with large oxygen supply is adopted until the end point of steel making, and the oxygen supply intensity of top blowing is strongIs 3.5-4.5Nm³At t.min, the bottom-blown stirring strength is 0.06-0.15Nm³And t.min, wherein the bottom blowing gas is nitrogen or argon.
4. The method according to claims 1 and 2, characterized in that the dephosphorizing agent comprises the following chemical components in percentage by weight: CaO 40-70%, iron oxide 15-45%, CaF₂15-20%, and the iron oxide can be steel sheet or converter smoke.
5. The method according to claim 1 and 2, characterized in that medium-alkalinity slag is adopted in the pretreatment period, the slag alkalinity R is 2.0-3.0, and the final slag alkalinity in the decarburization refining period is 3.2-3.7.
6. The method as claimed in claims 1 and 2, characterized in that the bath temperature at the end of the pretreatment period is 1320-1380 ℃.
7. The method according to claim 2 or 3, wherein after the completion of the smelting and the tapping of the slag, the final slag is recovered and can be used for the return of the molten iron for the pretreatment.