RU2696920C1 - Method of production of rolled stock for pipes of main pipelines with simultaneous provision of uniform elongation and cold resistance - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: after melting steel is continuously cast into slabs, heated to rolling temperature, controlled hot rolling is carried out with accelerated step-by-step cooling and final slow cooling of sheets in stack. At that, hot rolling is finished in austenite area at temperature T_kp±50 °C, determined depending on chemical composition of steel by formula: T_kp=880-400[C]-70[Mn]+25[Si]-35[Ni]-25[Cr]-20[Cu]±30 °C, accelerated cooling of sheets with water is performed from temperature T_kp±50 °C austenite region at rate of 10÷50 deg/s to temperature Bs±50 °C, determined by formula Bs=695-320[C]-15[Cr+Cu+Ni]-25[Mn], after stopping water supply to roll, cooling is performed in air at rate of 0.1÷3 °C/s for extraction α-phase and enrichment with carbon of unconverted austenite over time t±10 s, determined proceeding from equation t=94[Nb]+22[V]+40[Mo]+10[Cr]+5[Mn]-18[Si]-6[Ni]-3[Cu]-30[C], and final slow cooling of sheets in piles is carried out at rate of 0.01÷0.001 deg/s in air up to 100 °C.

EFFECT: method is proposed for production of rolled sheet with thickness of 12–48 mm for production of pipes of main pipelines with diameter of up to 1,420 mm with provision of a portion of viscous components making fractures in testing by falling load of not less than 85 % at test temperature of -20 °C, impact viscosity (KCV) at test temperature of -40 °C is not less than 250 J/cm², high values of uniform elongation at achievement of strength properties in pipes from this rolled stock at the level of K60-K80 (X70-X100).

1 cl, 3 tbl

Description

The invention relates to the field of metallurgy, in particular to the production of sheet metal with a thickness of 12-48 mm at a reversible plate mill for the manufacture of pipes of main pipelines with a diameter of up to 1420 mm.

A known method for the production of rolled products with ferrito-martensito / bainitic structure, described in patent RU 2151214. According to this invention, a steel billet with the following ratio of chemical elements, wt. %: carbon - 0.05 ÷ 0.12; silicon - 0.01 ÷ 0.50; Manganese - 0.4 ÷ 2.0; niobium - 0.03 ÷ 0.12; vanadium - 0.05 ÷ 0.15; molybdenum - 0.2 ÷ 0.8; titanium - 0.015 ÷ 0.03; aluminum - 0.01 ÷ 0.03; iron - the rest; may additionally contain chromium - 0.3 ÷ 1; before hot rolling, it is heated to a temperature preferably in the range of 1150 ÷ 1250 ° C sufficient to dissolve essentially all vanadium and niobium carbonitrides. Then, in one or several passes, the billet is hot rolled: the first compression with a total deformation of 30–70% in the temperature range in which austenite recrystallizes; the second reduction of 40 ÷ 70% in a lower temperature range in which austenite does not recrystallize, but above the Ar ₃ point; the third reduction of 15 ÷ 20% after cooling the peals in air to a temperature in the range between the conversion points of Ar ₃ and Ar ₁ . After rolling, the roll is cooled accelerated at a rate of at least ~ 25 ° C / s, preferably ~ 35 ° C / s to a temperature of no higher than 400 ° C, at which further conversion to ferrite is excluded, and, if desired, a rolled, hardened, high-strength sheet steel suitable for the production of pipes for piping is cooled by air to room temperature.

The main disadvantage of the known production method is that the parameters of the technology of hot rolling and accelerated cooling are not related to the chemical composition of the steel (except for critical points), and therefore are not optimal for steels of various compositions with the content of chemical elements within the stated limits. In addition, deformation in the austenitic-ferritic region reduces ductility and increases the tendency to brittle fracture in the z-direction of the sheet metal, primarily as a result of hardening released in this temperature range of ferrite. Another disadvantage of this production method is the mandatory need for alloying steel with molybdenum in an amount of at least 0.2%, which has significant limitations for the production of rolled products of strength class K60. In addition, the addition of molybdenum leads to a rise in price of steel, so the use of this chemical element is not always advisable from an economic point of view, especially in the production of sheet metal of the above strength class.

The closest technology for the production of plate low-alloy strip is the method described in patent RU No. 2393236 (prototype), including steel smelting, casting, heating and hot rolling of the billet, accelerated cooling of the finished steel, characterized in that the steel of the following chemical composition is melted, wt. %: carbon 0.03-0.20, manganese 0.50-2.10, silicon 0.10-0.50, niobium 0.01-0.15, aluminum 0.01-0.10, titanium 0.005- 0.05, nitrogen 0.002-0.012, sulfur 0.0005-0.010, phosphorus 0.003-0.050, iron - the rest, hot rolling is completed in the temperature range from (Arz + 30 ° C) to (Arz-30 ° C), the subsequent accelerated cooling is carried out in two stages: at the first stage at a speed of 10-30 deg / s to a temperature of 650-550 ° C, then after a pause of 3-10 s at a second stage at a speed of 5-20 deg / s to a temperature of 550-450 ° C and subsequent cooling in air to 100 ° C is carried out slowly at a speed of 0.1-0.01 deg / s.

The disadvantage of this method for the production of plate is that it does not provide at the same time high strength, ductility and uniform elongation. In addition, the parameters of the hot rolling technology are not related to the chemical composition of the steel, therefore they are not optimal for steels of various compositions with the content of chemical elements within the stated limits.

The technical result of the invention consists in the simultaneous provision of the share of the viscous component in the fracture of the samples when tested with a falling load of at least 85% at a test temperature of -20 ° C, impact strength (KCV) at a test temperature of -40 ° C of at least 250 J / cm ² high values of uniform elongation when strength properties are achieved in pipes from this product at the level of K60-K80 (X70-X100).

The technical result is achieved in that in the proposed method for the production of rolled products for pipes of main pipelines while ensuring uniform elongation and cold resistance, including steel smelting, continuous casting of slabs, heating and hot rolling of slabs, followed by stepwise accelerated cooling and final delayed cooling of the sheets in the stack, in difference from the prototype:

- hot rolling is completed in the lower temperature range of the austenitic region at a temperature determined depending on the chemical composition of the steel by the formula: T _kn = 880-400C-70Mn + 2SSi-35Ni-25Cr-20Cu;

- accelerated cooling of peals with water is carried out from the austenitic region at a speed of 10 ÷ 50 deg / s to a temperature of Bs ± 50 ° C, determined by the formula: Bs = 695-320 [C] -15 [Cr + Cu + Ni] -25 [Mn ];

- after the cessation of water supply to the roll, cooling is performed at a rate of 0.1 ÷ 3 ° C / s for 15-30 seconds to isolate the α phase and enrich the carbon with unconverted austenite. The pause time t ± 10 s, is selected based on the equation: t = 94 [Nb] +22 [V] +40 [Mo] +10 [Cr] +5 [Mn] -18 [Si] -6 [Ni] -3 [Cu] -30 [C];

- then carry out accelerated cooling of peals at a speed of 10 ÷ 50 deg / s to a temperature determined by the formula:

- the final cooling of the peals is carried out slowly at a speed of 0.01 ÷ 0.001 deg / s in air to 100 ° C after stacking the sheets in the feet.

The invention consists in the following.

First, a continuously cast billet is made of steel with a given chemical composition. Further, from continuously cast slabs in the conditions of a reversible rolling mill equipped with an accelerated cooling installation, which allows for regulated accelerated cooling of peals, sheet metal sheets of specified sizes are manufactured using thermomechanical rolling technology (TMP).

Hot rolling of the strip, according to the proposed method, is carried out according to regulated temperature and deformation conditions in order to form a fine-grained structure in the finished product with an ordered distribution of defects in the crystal lattice, providing an increase in the yield strength, impact strength, the share of the viscous component in the fracture (ICE) and lowering the viscous temperature -fragile transition. In order to form the finest possible grain structure in rolled products, but not to allow ferrite to be released during deformation, it is necessary to complete the deformation at a temperature determined depending on the chemical composition of the steel according to the formula: Т _кп = 880-400С-70Mn + 255i -3SNi-2SCr-20Cu ± 20 ° C. If this temperature is exceeded, the effect of hardening in the final stage of rolling will not be maximum, which will cause a decrease in strength and viscosity properties. Upon completion of the deformation below this temperature, deformation of the precipitated ferrite will occur, which will lead to a decrease in toughness and uniform elongation.

After the hot rolling is completed, the peals are subjected to stepwise regulated cooling in several stages. Initial accelerated cooling of the peals with water is carried out from the austenitic region at a rate of 10 ÷ 50 deg / s to the temperature of the beginning of the bainitic transformation Bs ± 50 ° C, determined by the formula: Bs = 695-320 [C] -15 [Cr + Cu + Ni] - 25 [Mn]. The accelerated cooling in the first stage to the temperature of the onset of bainitic transformation allows to obtain fine-grained ferrite without the presence of bainite. If the completion temperature of the first cooling stage is higher than Bs, then the ferrite formed will be larger, as will the second structural component formed in the second stage of accelerated cooling. This will lead to a decrease in viscosity properties. If the completion temperature of the first cooling stage is lower than Bs, then upper bainite will be formed, the presence of which will adversely affect the toughness and uniform elongation.

After the water supply to the peat is cut off, slow cooling in air is carried out at a rate of 0.1 ÷ 3 ° C / s to isolate the α phase and enrich the unconverted austenite with carbon. for the time determined from the equation: t = 94 [Nb] +22 [V] +40 [Mo] +10 [Cr] +5 [Mn] -18 [Si] -6 [Ni] -3 [Cu] - 30 [C] ± 10 s. If the cooling time in air is shorter, the enrichment of the residual austenite with carbon will not take place completely and upon subsequent cooling it will turn into bainite with the presence of carbides, which will negatively affect the toughness and uniform elongation. If the cooling time in air is longer, then austenite will begin to decompose into ferrite and carbides, due to saturation with carbon, which will also negatively affect the toughness and uniform elongation.

Then carry out accelerated cooling of peals with a speed of 10 ÷ 50 deg / s to a temperature determined by the formula

необходимо для стабилизации обогащенных углеродом не превращенных порций аустенита. При превышении данной температуры возможен распад аустенита на карбиды и феррит при дальнейшем замедленном охлаждении, что негативно отразится на ударной вязкости и равномерном удлинении. Если температура завершения второго этапа ускоренного охлаждения будет ниже

, то возможна недостаточная стабилизация аустенита и превращение его в мартенсит. Это может вызвать значительное упрочнение металла и снижение равномерного удлинения.Accelerated cooling of peals to temperature

necessary for stabilization of carbon-enriched unconverted portions of austenite. If this temperature is exceeded, austenite can decompose into carbides and ferrite with further delayed cooling, which will negatively affect the toughness and uniform elongation. If the completion temperature of the second stage of accelerated cooling is lower

, then insufficient stabilization of austenite and its transformation into martensite is possible. This can cause significant hardening of the metal and a decrease in uniform elongation.

The final cooling of the peals is carried out slowly at a speed of 0.01 ÷ 0.001 deg / s in air to 100 ° C after stacking the sheets in the feet. The absence of this stage can cause cracks of hydrogen origin in the metal.

Due to the application of the proposed method of hot rolling with subsequent multi-stage cooling of the rolled products, both high strength, toughness, cold resistance, ductility, uniform elongation and the proportion of the viscous component during the falling load test (ICE at IPG) are simultaneously ensured.

Examples

Slabs of 4 swimming trunks were produced. The chemical composition of the melts and the chemical composition of the steel according to the prototype method are presented in Table 2. The metal was smelted by the converter method, subjected to out-of-furnace treatment and cast on a curved continuous casting machine. To compare the influence of the production methods according to the invention and the prototype on the structure and mechanical properties, slabs were rolled on a 5000 single-strand reversing mill on sheets of various thicknesses, followed by regulated cooling, including using UCO. Technological parameters of hot rolling and subsequent MA of the compared options for the manufacture of sheets are given in table. 2, - 3. Modes 1-1; 1-2; 1-3; 1-4; 2-1; 2-2; 2-3; 2-4; 2-5; 3-1; 3-2; 3-3; 3-4 and 4-1 are made according to the invention; 2-6; 2-7; 2-8 - outside the claimed range of technological parameters of the invention; 5, 6, 7 - by the prototype method.

According to the proposed method, after heating to temperatures of 11554-1170 ° C, the slabs were rolled in two stages. During the rough rolling phase, as a result of repeatedly alternating acts of deformation and static recrystallization, a substantial grinding of austenitic grain occurred. The final stage of hot rolling was the deformation of the metal in the region of suppression of recrystallization processes. After rolling, the sheets of the invention were subjected to multi-stage cooling at various speeds at each stage. During the first cooling stage, the peals were cooled at different speeds to the temperature of completion of the first cooling stage. Next, a pause in cooling was carried out for a regulated time. At this stage of sheet cooling, the polymorphic γ → α transformation took place according to the normal mechanism with the formation of polygon morphology ferrite and carbon enrichment of unconverted portions of austenite. Further, in order to transform, by the shear or intermediate mechanism, the remaining carbon-rich austenite into martensite-bainitic structural constituent sheets, they were cooled at various rates to the temperature of completion of the second cooling stage. The final cooling of the sheets to ambient temperature was carried out slowly after storage in stacks to prevent the formation of cracks of hydrogen origin.

During hot rolling of the sheet in regimes outside the declared range, the temperature of completion of the first stage of cooling of the sheet rolled in mode 2-6 was higher than the required range; when rolling sheet 2-7, the pause time between the first and second cooling stages was above the required range; when rolling the sheet in mode 2-8, the temperature of the completion of the second cooling stage was below the required range; when rolling sheet 2-9, the temperature of the end of rolling (T _CP ) was below the permissible temperature range.

Mechanical properties were determined on transverse samples. Tests on static tension was performed on fivefold polnotolschinnyh samples in accordance with the GOST 1497 and ASTM A370, with determination of ultimate tensile strength (σ _c) the yield strength (σ _t), ratio of yield strength to ultimate strength (σ _r / σ _c), the relative elongation (δ ₅ and δ _2ʹʹ ) and uniform elongation (δ _p ). Dynamic tests for shock bending of samples with a sharp stress concentrator at negative temperatures -20; -40; ° С were carried out according to GOST 9454 with the determination of impact strength (KCV), IPG samples with the assessment of ICE on the fracture surface were made in accordance with GOST 30456-97.

When assessing the mechanical properties, it is seen that the sheets produced according to the invention have better cold resistance, uniform elongation and impact strength compared to sheets produced according to the prototype and sheets produced outside the claimed range.

* - prototype method, ** - outside the declared range

* - prototype method, ** - outside the declared range

Claims (9)

Method for the production of rolled products for pipes of main pipelines, including steelmaking, continuous casting of slabs, heating and hot rolling of slabs, followed by stepwise accelerated cooling and final delayed cooling of the sheets in the stack, characterized in that the hot rolling is completed in the lower temperature range of the austenitic region at a temperature T _kp ± 30 ° C, determined depending on the chemical composition of the steel according to the formula T _kn = 880-400 [C] -70 [Mn] +25 [Si] -35 [Ni] -25 [Cr] -20 [Cu], accelerated cooling of peals with water is carried out from the temperature of the austenitic region T _kp ± 30 ° C at a speed of 10 ÷ 50 degrees / s to a temperature Bs ± 50 ° C, determined by the formula Bs = 695-320 [C] -15 [Cr + Cu + Ni] -25 [Mn], after stopping the supply of water to the roll, it is cooled in air at a rate of 0.1 ÷ 3 ° C / s for a time t ± 10 s, determined by the equation t = 94 [Nb] +22 [V] +40 [Mo] +10 [Cr] +5 [Mn] -18 [Si] -6 [Ni] -3 [Cu] -30 [C], then carry out accelerated cooling of peals with a speed of 10 ÷ 50 deg./s to a temperature of Tzo ₂ ± 50 ° C, determined by the formula

the final delayed cooling of the sheets in the foot is carried out at a rate of 0.01 ÷ 0.001 degrees / s in air to 100 ° C.