RU2792549C1 - Method for the production of cold-resistant rolled steel sheet - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: production of sheet metal in thicknesses up to 50 mm from cold-resistant steel for use in heavy engineering, in building structures at low temperatures up to -70°C. A method for producing cold-resistant steel sheet includes making a steel billet, austenizing it, deforming it by rough and finish rolling, and cooling. The billet is obtained from steel containing, wt.%: carbon 0.04-0.20, silicon 0.1-0.5, manganese 0.9-1.9, sulfur - NMT 0.009, phosphorus - NMT 0.015, chromium - NMT 0.5, nickel - NMT 0.4, copper - NMT 0.4, aluminum - 0.02-0.07, vanadium - 0.002-0.10, niobium - 0.01-0,10, titanium - 0.003-0.10, molybdenum - 0.05-0.5, nitrogen - NMT 0.010, calcium - NMT 0.005, boron - NMT 0.005, arsenic - NMT 0.08, zirconium - NMT 0.2, iron and inevitable impurities - the rest. Austenization by heating the billet for rolling is carried out to a temperature of 1150-1300°C, start finishing rolling at a temperature of 880-990°C, and finish at a temperature of 810-920°C, then the resulting steel sheet is cooled in air to ambient temperature, then it is heat treated, at which it is heated to a temperature of 900-950°C, accelerated cooling with water up to a temperature of NMT 350°C, reheating to 500-690°C and subsequent cooling in air.

EFFECT: high mechanical properties, including at -70°C.

5 cl, 3 tbl, 1 ex

Description

The invention relates to metallurgy, and in particular to the production of sheet metal in thicknesses up to 50 mm from cold-resistant steel for use in heavy engineering, in building structures at low temperatures down to -70°C.

A known method for the production of high-strength weldable cold-resistant steel and products from it, containing in wt.%: carbon 0.08 - 0.10, silicon 0.30 - 0.40, manganese 0.65 - 0.75, chromium 0.45 - 0.55, nickel 1.65 - 1.75, copper 0.50 - 0.60, molybdenum 0.30 - 0.35, niobium 0.02 - 0.04, zinc 0.0001 - 0.01, bismuth 0.0001 - 0.005, antimony 0.0001 - 0.005, calcium 0.0001 - 0.01, aluminum 0.02 - 0.05, nitrogen 0.001 - 0.008, sulfur not more than 0.005, phosphorus not more than 0.012, the rest - iron and inevitable impurities, while the value of the carbon equivalent does not exceed 0.53% [RU No. 2731223, IPC C22C38/60, C22C38/48, 2020].

The disadvantage of this method is the high content of Nickel in the steel, which significantly increases the cost of metal products.

The closest analogue, taken as a prototype, to the claimed technical solution is a method for the production of cold-resistant sheet metal, according to which the workpiece is obtained from steel containing in wt.%: C (0.04 - 0.10), Mn (1.00 - 1 .40), Si (0.15 - 0.35), Ni (0.10-0.80), Al (0.02 - 0.06), Mo (0.01 - 0.08), Nb ( 0.02 - 0.06), V (0.02 - 0.10), S (0.001 - 0.008), P (0.003 - 0.012), iron - the rest, it is heated to 1140 - 1170 ° C, preliminary deformation at 940 - 990°C, then the resulting workpiece is cooled by 70 - 100°C, the final deformation is carried out at 830 - 750°C and cooled first accelerated to 550 - 400°C, and then slowly to a temperature not exceeding 150°C, while the carbon coefficient is not more than 0.38 [Patent RU No. 2345149, IPC C21D 8/02, C22C 38/12, C21D 9/46, 2009].

The disadvantages of this method are:

- loss of productivity on the mill;

- the method applies only to technology with cooling from rolling heating, excluding the state of delivery "quenching + tempering"; this method does not allow obtaining satisfactory impact strength properties at temperatures down to minus 70°C at thicknesses up to 50 mm.

The technical result of the invention is the development of a method for the production of cold-resistant rolled products with the required level of mechanical properties, including at -70°C, and a reduced cost of its production compared to the prototype.

The required level of mechanical properties refers to the following properties:

yield strength not less than 690 MPa,

ultimate strength not less than 770 MPa,

relative elongation not less than 14%.

The technical result is achieved by the fact that in the method for the production of cold-resistant sheet steel, including obtaining a billet from steel, its austenitization, deformation by rough and finish rolling and cooling, according to the invention, the billet is obtained from steel containing, wt.%:

Carbon 0.04 - 0.20 Silicon 0.1 - 0.5 Manganese 0.9 - 1.9 Sulfur no more than 0.009 Phosphorus no more than 0.015 Chromium no more than 0.5 Nickel no more than 0.4 Copper no more than 0.4 Aluminum 0.02 - 0.07 Vanadium 0.002 - 0.10 Niobium 0.01 - 0.10 Titanium 0.003 - 0.10 Molybdenum 0.05 - 0.5 Nitrogen no more than 0.010 Calcium no more than 0.005 Bor no more than 0.005 Arsenic no more than 0.08 Zirconium no more than 0.2 Iron and inevitable impurities rest,

austenization by heating the billet for rolling is carried out to a temperature of 1150 - 1300°C, finishing rolling is started at a temperature of 880 - 990°C, and finished at a temperature of 810 - 920°C, then the resulting rolled steel sheet is cooled in air to ambient temperature, then its heat treatment is performed, during which heating is carried out to a temperature of 900 - 950°C, accelerated cooling with water to a temperature of not more than 350°C, reheating to a temperature of 500 - 690°C and subsequent cooling in air.

Rolled steel sheet is characterized by the following characteristics:

yield strength not less than 690 MPa,

ultimate strength not less than 770 MPa,

relative elongation not less than 14%.

The duration of heating for rolling is at least 2.5 hours.

The duration of reheating (for tempering) is 0.8 - 4.0 min/mm of rolled products.

The carbon equivalent of steel is 0.41 - 0.55.

The essence of the proposed technical solution is as follows.

The selected chemical composition of steel provides the necessary set of technological and mechanical characteristics, at a lower cost, relative to analogues.

To obtain the required strength, the carbon content must be at least 0.04% and not more than 0.20%. The carbon content in an amount of more than 0.20% leads to a deterioration in the plastic properties of the steel.

Silicon deoxidizes steel, increases its strength and elasticity, silicon content of 0.1 - 0.5% provides sufficient strength characteristics of steel.

Manganese increases the strength of steel and also binds sulfur. When the manganese content is less than 0.9%, the steel is not strong enough. With a manganese content of more than 1.9%, the ductility of steel and its resistance to shock loads decrease.

Sulfur, phosphorus, arsenic are harmful impurities, therefore, the indicated values of sulfur content not more than 0.009%, phosphorus not more than 0.015% and arsenic not more than 0.08% are necessary to obtain high values of impact strength at low temperatures. With a sulfur content of more than 0.009%, a large amount of sulfide inclusions is formed in the steel, which significantly reduces the impact strength and crack resistance. Phosphorus is one of the elements with the highest tendency to segregation and the formation of segregations along grain boundaries, and, as a result, negatively affecting the impact strength of steel and crack resistance, therefore, the upper limit of the phosphorus content is set to no more than 0.015%.

Chromium and nickel increase the strength of steel. Increasing the content of chromium and nickel more than 0.5% and 0.4%, respectively, is not economically feasible.

Copper increases the stability of austenite, which is especially important during the final heat treatment and increases the corrosion resistance of steel, but its significant content leads to a high cost of finished rolled products. For the steel of the claimed alloying composition, the copper content is limited to 0.4%, which provides the required properties of rolled products.

The addition of aluminum is necessary for the deoxidation of steel. An aluminum concentration of more than 0.07% leads to the formation of corundum inclusions, which are stress concentrators and adversely affect the continuous casting process. Reducing the aluminum content to less than 0.02% can lead to a deterioration in the ductility and toughness of the steel.

Microalloying steel with vanadium, niobium and titanium effectively inhibits recrystallization and grain growth during heating, which in turn allows you to maintain the required level of mechanical properties, however, when the content of vanadium, niobium and titanium is more than 0.10% each, there is a significant increase in the cost of the steel production process, and also increases the tendency of steel to embrittlement.

The addition of molybdenum increases the strength of the steel. Molybdenum in an amount of less than 0.05% does not have a significant effect on the properties, and its content of more than 0.5% already leads to a significant increase in the cost of steel production.

Calcium is introduced to modify non-metallic inclusions. The calcium content of more than 0.005% will lead to the formation of a large number of inclusions - calcium aluminates, which will adversely affect the cold resistance of steel. The calcium content within the stated limits ensures the production of globular sulfides, which contributes to an increase in the level of impact strength at low temperatures.

Boron, added up to 0.005%, significantly increases the hardenability of the steel, promoting the formation of potentially hardening components, bainite or martensite, while slowing down the formation of softer ferritic and pearlitic structural components during steel cooling. Boron in an amount of more than 0.005% can contribute to the formation of embrittling Fe ₂₃ (C,B) ₆ particles.

The content of arsenic in an amount of not more than 0.08% avoids temper brittleness, which in turn increases the cold resistance of steel.

Zirconium has a hardening effect on steel. The increase in strength with zirconium occurs due to the formation of dispersed zirconium carbides and carbonitrides, which inhibit the growth of austenite grains, which leads to the formation of a hereditary fine-grained steel structure. With the indicated amounts of zirconium (no more than 0.2%), hardening is achieved without reducing plasticity, which is caused by the effect of dissolving zirconium in steel and grinding its substructure, which provides a fine-grained steel structure. The introduction of zirconium additionally changes the morphology and phase composition of sulfides, and also eliminates the formation of chains of non-metallic inclusions that reduce plastic properties.

The carbon equivalent of steel can be in the intervals (depending on the formula by which it is calculated):

- 0.41-0.51% - Seq=C+Mn/6+Si/24+Cr/5+Ni/40+Cu/13+V/14+P/2,

- 0.42-0.55% - CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15,

- 0.25-0.34% - CET=C+(Mn+Mo)/10+Ni/40+(Cr+Cu)/20.

Higher values of the carbon equivalent lead to a deterioration in the weldability of the steel.

Carry out austenitization with heating for rolling up to temperatures of 1150 - 1300°C. Exceeding the upper limit of the interval stimulates abnormal growth of austenite grains, leading to a decrease in strength and viscosity properties. If the lower limit of this heating temperature range is not reached, carbonitrides are poorly soluble in austenite, which has a negative effect on the course of recrystallization processes, and also reduces strength and viscosity properties.

The duration of heating for rolling less than 2.5 hours leads to heterogeneity of the structure over the entire cross section, in particular, to the preservation of coarse segregation in the middle, which negatively affects, in the future, the required mechanical properties.

The start of finishing rolling is carried out at temperatures of 880 - 990°C, and finished at temperatures in the range of 810 - 920°C, then the rolled products are cooled in air. The temperature of the beginning of finishing rolling in this range is necessary for more intensive grinding of austenite grains. At temperatures of the beginning of finishing rolling over 990°C and the end of finishing rolling over 920°C, austenite grains grow, which reduces the complex of mechanical properties, especially impact strength. At temperatures of the beginning of finishing rolling and the end of rolling below 880°C and 810°C, respectively, the rolled product cools down, which leads to an uneven microstructure of the rolled product and a high anisotropy of properties.

Heat treatment of rolled products after heating to a temperature of 900-950°C, followed by accelerated cooling with water to a temperature of not more than 350°C, provides an optimal uniform hardening structure throughout the entire thickness of the rolled product, and, therefore, allows achieving high properties throughout the entire sheet section.

Reheating (tempering) of hardened sheets in the range of 500-690°С allows to provide the required characteristics in terms of elongation and impact strength while maintaining strength characteristics.

The duration of reheating (for tempering) below 0.8 min/mm of rolled products does not provide heating of the sheet over the entire section, which in turn leads to anisotropy of properties, a decrease in ductility and toughness of rolled metal. An increase in the specific heating time above 4.0 min/mm of rolled products reduces its strength properties.

An example of the implementation of the method

Steel was smelted in an oxygen converter, and after out-of-furnace treatment and vacuum treatment, continuous casting into slabs with a cross section of 250 * 1630 mm was carried out. Further, heating was carried out for rolling to temperatures of 1150 - 1300°C and the sheets were rolled to a final thickness of 40 and 50 mm on a two-stand reversing mill (rolling to other thicknesses is also possible). Deformation in the roughing stand was carried out in the temperature range of 980 - 1120°C, with a total reduction ratio of 62 - 68%. The rolling was cooled to a temperature of 880 - 990°C. The final deformation was carried out in the finishing stand with strictly regulated reductions of 10–20% per pass in the temperature range of 810–990°C with a total reduction ratio of 50–55%, after which the rolled product was cooled in air.

Then the rolled product was heated to temperatures of 900 - 950°C and then accelerated cooling was carried out to a temperature of not more than 350°C, after which it was reheated to a temperature of 500 - 690°C with a holding time of 0.8 - 4 min/mm of rolled product.

According to the claimed method, 5 experiments were carried out. The chemical composition is given in table 1, the technological parameters are given in table 2, the mechanical properties are given in table 3.

Tensile tests were performed on cylindrical specimens according to GOST 1497 with a calculated length L=5.65√F ₀ taken across the rolling direction and impact strength specimens according to GOST 9454 with a V-shaped concentrator taken along the rolling direction.

As can be seen from the results of the experiments, rolled products produced by the proposed technology are characterized by the required level of mechanical properties. Also, the experiments showed that the cost of steel production according to the declared technology is 2.0 - 3.0% lower compared to the prototype production technology.

Table 1
Chemical composition of rolled products experiment number C Mn Si S P Cr Ni Cu Al Nb Ti V Mo IN N ₂ 1 0.155 1.14 0.29 0.002 0.009 0.26 0.16 0.04 0.040 0.027 0.011 0.005 0.23 0.002 0.005 2 0.159 1.12 0.38 0.006 0.009 0.41 0.06 0.02 0.030 0.022 0.08 0.004 0.15 0.0013 0.006 3 0.181 1.24 0.29 0.002 0.008 0.15 0.26 0.04 0.040 0.054 0.011 0.005 0.28 0.001 0.004 4 0.161 1.78 0.16 0.005 0.010 0.11 0.33 0.16 0.021 0.049 0.08 0.004 0.15 0.0011 0.005 5 0.172 1.52 0.22 0.005 0.008 0.31 0.18 0.31 0.050 0.028 0.08 0.004 0.15 0.0013 0.005

* - in experiments, the content of arsenic was 0.00

table 2
Controlled technological parameters experiment number Heating temperature for rolling, °C Т start of finishing rolling, °С T end of finishing rolling, °С T heating for heat treatment, °С Cooling temperature, °С T vacation, ° С reheating t, min/mm rolled 1 1170 940 821 930 311 570 1.5 2 1280 955 834 945 280 624 1.8 3 1220 950 822 920 304 672 2.0 4 1153 890 810 910 322 540 2.9 5 1170 900 812 905 333 535 1.0

Table 3 experiment number Tensile strength, σv, N / mm ² Yield strength, σt, N / mm ² Relative elongation, δ5,% Impact strength KCV at minus 40°C, J/ ^cm2 Impact strength KCV at minus 70°C, J/ ^cm2 1 820 740 18.5 229 / 218 / 116 161/181/198 2 790 700 19.0 233 / 230 / 234 223 / 226 / 125 3 789 693 19.0 229 / 208 / 198 171/181/198 4 850 748 17.0 234 / 233 / 238 225 / 230 / 224 5 810 721 17.5 224 / 213 / 113 162 / 180 / 168

Claims (10)

1. A method for the production of cold-resistant steel sheet, including obtaining a billet from steel, its austenitization, deformation by rough and finish rolling and cooling, characterized in that the billet is obtained from steel containing, wt.%: Carbon 0.04 - 0.20 Silicon 0.1 - 0.5 Manganese 0.9 - 1.9 Sulfur no more than 0.009 Phosphorus no more than 0.015 Chromium no more than 0.5 Nickel no more than 0.4 Copper no more than 0.4 Aluminum 0.02 - 0.07 Vanadium 0.002 - 0.10 Niobium 0.01 - 0.10 Titanium 0.003 - 0.10 Molybdenum 0.05 - 0.5 Nitrogen no more than 0.010 Calcium no more than 0.005 Bor no more than 0.005 Arsenic no more than 0.08 Zirconium no more than 0.2 Iron and inevitable impurities rest, austenization by heating the billet for rolling is carried out to a temperature of 1150 - 1300°C, finishing rolling is started at a temperature of 880 - 990°C, and finished at a temperature of 810 - 920°C, then the resulting rolled steel sheet is cooled in air to ambient temperature, then its heat treatment is performed, during which heating is carried out to a temperature of 900 - 950°C, accelerated cooling with water to a temperature of not more than 350°C, reheating to a temperature of 500 - 690°C and subsequent cooling in air. 2. The method according to p. 1, characterized in that the rolled steel sheet is characterized by the following characteristics: yield strength not less than 690 MPa, ultimate strength not less than 770 MPa, relative elongation not less than 14%. 3. The method according to p. 1, characterized in that the duration of heating for rolling is at least 2.5 hours. 4. The method according to p. 1, characterized in that the duration of reheating is 0.8 - 4.0 min / mm rolled. 5. The method according to p. 1, characterized in that the carbon equivalent of steel is 0.41 - 0.55.