Application of the hottest NB microalloying techno

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Application of niobium microalloying technology in thin slab continuous casting and rolling production line

Abstract: in recent years, China has put into operation and is building as many as 10 thin slab continuous casting and rolling production lines (CSP). How to produce high-quality steel plates on CSP, especially HSLA steel plates, is a common concern. However, due to the differences in production processes, the traditional composition design and hot working process cannot be simply applied to CSP. Based on the basic principles of physical metallurgy of niobium in the traditional process and the characteristics of CSP process, this paper will find out the relationship between composition rolling process microstructure properties suitable for CSP process, and produce high-quality and high-performance steel plates on CSP

1. Preface

thin slab continuous casting and rolling technology is the latest achievement of the world's steel industry since the 20th century. It is a cutting-edge and revolutionary technology in the contemporary metallurgical field. It is the third technological revolution in steel production after the invention and application of oxygen converter and continuous casting technology. CSP has become a hot spot in the world's steel industry. Since the CSP of Nucor company was put into operation in 1987, more than 32 thin slab continuous casting and rolling production lines have been put into operation or under construction all over the world, forming an annual capacity of 46.3 million tons

at present, China has put into production, under construction and continued construction of 24 sets of strip mills, with a total design capacity of 42.95 million tons. Among them, 14 sets of continuous and semi continuous mills and Steckel mills; There are 10 CSP lines for thin slab continuous casting and rolling mills, with a design capacity of 29.2 million tons, of which Zhujiang steel, Anshan steel, Handan steel, Baotou Steel, Tangshan Steel and Maanshan Steel have been completed and put into operation

experience at home and abroad has proved that microalloying technology can provide necessary preconditions for accurately setting the required microstructure for CSP production line by thermomechanical rolling, and can stably produce high-quality niobium containing microalloyed steel strip. Today, there is no difference between the strip steel quality produced by CSP and that produced by traditional hot strip mills. In addition to the inability to produce O5 and AA plates with extremely high surface quality requirements, the coverage of steel varieties produced by CSP process has expanded and is close to more than 70% of strip varieties. With the development of the second generation CSP process technology, the production varieties will be further expanded, such as dual phase steel, TRIP steel, microalloyed fine grain steel, pipeline steel, etc

NB microalloying technology is widely used in the production of traditional hot strip mills; For the new CSP production process, after more than ten years of development and research, we have also gained some experience. Based on the physical and metallurgical basis of high strength low alloy (HSLA) steel, this paper expounds the basic principles of microalloying and thermomechanical treatment of steel, and combined with the characteristics of CSP process, preliminarily gives the suitable chemical composition design and process conditions for the production of niobium containing steel

2. Physical metallurgical basis of steel

2.1 strengthening mechanism of HSLA steel

from the analysis of hall Petch formula, we can see that there are several different methods to improve the strength:

σ = σ 0+ σ s+ σ t+ σ p+ σ D+kyd1/2


σ: Yield strength

σ 0: intergranular friction

σ s: Solid solution strengthening

σ t: Texture strengthening

σ p: Precipitation strengthening

σ d: Dislocation strengthening

kyd1/2: fine grain strengthening

recently, different interpretations have been put forward on the universal practicality of this formula from several aspects. Different ferrite states have different strengthening mechanisms. When the microstructure we obtained is polygonal ferrite, the strengthening effect is mainly fine grain strengthening, dislocation strengthening and precipitation strengthening. With the decrease of phase transformation temperature, the ferrite grain size decreases, the type and grain boundary also change slightly, KY decreases, and the grain refinement and strengthening effect decreases. When the microstructure is non polygonal ferrite, it is mainly the contribution of dislocation strengthening and solid solution strengthening to the strength; For acicular ferrite, the strengthening effect of fine grain is very small, mainly dislocation strengthening and solid solution strengthening

among these items contributing to strength, several other strengthening mechanisms will reduce the toughness of steel, and only grain refinement can improve both strength and toughness. Among them, niobium is the most effective grain refinement element

2.2 action mechanism of microalloyed elements

nb, V and Ti, as microalloyed elements in steel, significantly affect the microstructure of steel through their solute drag effect and the formation of carbides and nitrides. Each element has its own characteristics

nb is the most effective microalloying element for grain refinement. The main ways for NB to refine grains in steel are as follows: (1) the drag effect of solid solution Nb atoms can delay recrystallization and delay γ → α The diffusion transformation reduces the transformation temperature and refines the grains; (2) The formation of carbonitrides can prevent grain growth, recrystallization and precipitation strengthening. The soaking insoluble carbonitrides can obviously prevent the coarsening process of the soaking austenite grains by pinning the grain boundaries, and the precipitated particles in the final rolling passes and even the subsequent cooling process can effectively prevent the growth of recrystallized austenite; The strain induced carbonitrides precipitated during rolling effectively prevent austenite recrystallization by pinning grain boundaries and sub grain boundaries; Dispersed fine carbonitrides play a role in precipitation strengthening. Ti forms nitrides that are stable at high temperature, eliminating free nitrogen in steel, which is beneficial to the toughness of steel and indirectly improves the effect of Nb. V hardly forms precipitates in austenite, and γ/α During or after the phase transformation, a large amount of precipitation can be produced to strengthen precipitation

therefore, composite microalloying is usually one of the best alloy design methods for producing HSLA steel

2.3 austenite adjustment

austenite adjustment means that the austenite after hot rolling has the appropriate structure and composition, so that the desired ferrite structure can be obtained under the appropriate cooling conditions. One of the main purposes of austenite adjustment is to obtain the maximum grain boundary effective area (SV)

two different processes can be used for austenite adjustment during hot rolling, namely, traditional controlled rolling (CCR) and recrystallization controlled rolling (RCR)

for the CCR process, the deformation is carried out in the range below the recrystallization stop temperature T5. The higher the temperature of T5, the more rolling passes can be arranged, the larger the SV and the more effective the process. Among the three Microalloying Elements Nb, V and Ti, the addition of Nb has the greatest effect on improving T5, followed by Ti and V. With the increase of Nb content, the recrystallization stop temperature rises sharply, because in all possible precipitation systems, only NBC has high supersaturation in most typical hot rolling temperature ranges

for RCR process, deformation is carried out in the temperature range higher than T95. It is required that the steel controlled rolling by this process must have a low recrystallization stop temperature and an appropriate mechanism to inhibit the coarsening of fine grains within the pass transit time, so as to maintain the fine grain size. Therefore, the grain coarsening inhibition force must be greater than the grain coarsening driving force FGC

in terms of alloy design, it must be considered that the effect of solid solution Nb and precipitated Nb on inhibiting recrystallization is different. Solid solution NB can inhibit recrystallization in a short time, while precipitated NB can play a role in a long time

3. Application characteristics of Nb containing steel on CSP

3.1 comparison between traditional process and CSP process

due to different processes, CSP line is obviously different from traditional process:

(1) traditional rolling process is long process, while CSP is short process. The general process of traditional rolling is:

slab thickness 250mm → cutting → stacking → slow cooling to room temperature → cold charging after about 2 weeks → heating → heat preservation → cooling → rough rolling → finish rolling → water cooling → coiling

and CSP general process Jinan new era Gold Testing Instrument Co., Ltd. has an excellent technical team as follows:

slab → cutting → slow cooling (or fast cooling) → finish rolling → water cooling → coiling

(2) the total compression ratio is different because the slab thickness is different. For the traditional process, the slab thickness is generally about 250mm. After rough rolling and finish rolling, the final product is obtained, and its total compression ratio is large; For CSP line, the continuous casting billet is generally 70mm and directly enters the finishing mill, with a small total compression ratio

(3) the solid solution and precipitation states of microalloyed elements are different. For HSLA steel, Nb, V and Ti are generally used as microalloying elements. During the cooling process of the continuous casting billet after smelting and casting, when it is cooled below about 1080 ℃, it forms a relatively coarse star like precipitate as the most important automotive lightweight material, which is a microalloyed element carbon/nitride or carbon nitride. This kind of star precipitates will remain in the steel as the slab cools down. They have no effect on inhibiting the growth of austenite grains and will not produce precipitation strengthening effect. This star precipitate exists not only in Nb steel, but also in v-n-ti steel. For the traditional hot rolling, the slab will be sent back to the heating furnace for heating and insulation, with the heating temperature as high as 1200 ℃ - 1250 ℃, and the star precipitates will dissolve into austenite, which will precipitate in the subsequent rolling process, which can not only refine the grains after phase transformation, but also play the role of precipitation strengthening. For CSP line, considering the problem of steel leakage during continuous casting, when the temperature of molten steel drops below 1080 ℃ after passing through the crystallizer, secondary cooling section and straightening section, star shaped precipitates are generated in the continuous casting slab, and then enter the heating furnace. The heating temperature is generally at 115, and Nanshan Aviation Materials Industrial Park is about 0 ℃ for aircraft. Therefore, the coarse star precipitates formed in the slab will not be eliminated in the subsequent process, and they will remain in the strip after rolling and crimping as the continuous casting slab enters the finishing mill. In CSP line, due to the production of star precipitates, microalloyed elements are wasted. Calculation and test show that 0.02% of Nb added to steel is generally retained in star precipitates. Therefore, in order to make NB play its beneficial role fully, the amount of Nb added is higher than that of conventional components, that is, the total amount of Nb = NB effective +nb star, and the total amount of Nb added to steel is the sum of the effective amount of Nb and the amount of Nb in the star precipitate. Therefore, the strength of the strip produced by CSP line is lower than that of the conventional process under the condition of the same microalloying element content

(4) the austenite state of billets before finishing rolling is different, because the thermal history of billets in different processes is different. For HSLA steel, in the traditional production process, the slab must first be cooled to room temperature after continuous casting, which occurs in this process γ → α Phase transformation, through a phase transformation, the grain is refined, and the microalloyed carbon/nitride precipitates; After a period of time, the cold billets are sent to the heating furnace for reheating, and experience α → γ Phase transformation, grain coarsening will occur during heating, but due to the pinning effect of microalloyed element precipitates on austenite grain boundaries, austenite grain coarsening is not obvious, and the grain size is generally 300 μ m; Then austenite recrystallizes repeatedly during rough rolling, which refines the grain size to μ M then enter finishing rolling. In CSP process, there is one less thermal cycle, that is, the slab is cooled to more than 950 ℃ after continuous casting, and then enters the heating furnace, which does not happen in this process γ → α Phase transformation, as cast austenite grain size is generally 700 μ M is even larger, 20 times larger than the austenite grain in the traditional process; Then it goes directly to finish rolling F1, and the temperature at F1 inlet is also very high

3.2 key points of CSP production of Nb containing HSLA steel

with the gradual increase of CSP production lines put into production in China, production experience is gradually accumulated and enriched, and the thinking and understanding of CSP production process and performance are gradually deepened. According to the standard provided by the equipment manufacturer, e) the oil gauge in the oil pump is used to show the amount of oil in the oil pump. The process of producing plain carbon steel and C-Mn steel with small thickness reflects few problems, and the production is very smooth; But when producing HSLA steel with high strength

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