A method for producing a low alloy structural thick steel plate

Abstract

The present application belongs to the technical field of plate production, and relates to a production method of low-alloy structural thick steel plate. The chemical composition of the steel plate is as follows: C: 0.09% to 0.12%, Si: 0.15% to 0.55%, Mn: 1.40% to 1.70%, P: ≤0.014%, S: ≤0.005%, Cr: ≤0.30%, Ni: ≤0.25%, Nb: 0.025% to 0.035%, Ti: 0.008% to 0.020%, Al: 0.020% to 0.050%, and the rest is Fe and inevitable impurities. The key production steps include converter smelting, refining, continuous casting, heating, rolling and heat treatment, and the problem of poor uniformity of mechanical properties in the thickness direction of the plate is solved. The production method combines the advantages of hot rolling process and heat treatment process, and the produced thick steel plate has excellent low-temperature toughness and good welding performance, and the production of the plate with a wide chemical composition range is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of plate production, and particularly relates to a production method of low-alloy structural thick steel plate. BACKGROUND

[0002] The composition and process of the steel plate greatly vary with the thickness, which is caused by the difference in cooling rate at different thickness positions of the steel plate. For the steel plate with small thickness, the difference in cooling rate is small in the whole thickness, and the cooling rate is also large, so the conventional process can be used to realize excellent performance. With the increase of the thickness of the steel plate, especially for the plate with thickness > 50 mm, the insufficient cooling rate of the steel plate and the difference in cooling rate at different thickness positions of the steel plate will have adverse effects on the performance of the steel plate, such as the difference in strength and toughness at different thickness positions of the steel plate.

[0003] For the steel plate delivered in the hot-rolled state (including NR, CR, TMCP, etc.), the manufacturer can refine the grains through rough and fine rolling, increase the microstructure and distortion energy in the structure, and promote the phase change of the steel plate after rolling and cooling to improve the uniformity of the structure in the thickness range of the steel plate, and the core strength can reach more than 420 MPa. However, for the steel plate with large thickness, the chemical composition and rolling and cooling process of the steel plate need to be greatly adjusted to ensure the comprehensive performance of the steel plate. When the cooling rate inside the steel plate reaches the set target, the cooling rate on the surface of the steel plate is usually too fast to produce a structure with poor strength and toughness matching on the surface of the steel plate.

[0004] For the steel plate delivered after heat treatment, the austenite structure of the steel plate after heating is close to the equilibrium state (i.e. the structure refinement effect of the rolled steel plate has been eliminated), and the structure and performance thereof basically only depend on the cooling rate and the hardenability of the material itself. The core cooling rate of the thick steel plate is difficult to further improve due to the limitation of the material itself and the existing heat treatment process, thereby causing a large difference in performance at the inner position of the thick plate. SUMMARY

[0005] The technical problem to be solved by the present application is to improve the uniformity of the mechanical properties of the plate in the thickness direction, and a production method of low-alloy structural thick steel plate is provided. The new process of controlled rolling and controlled cooling + heat treatment is adopted to realize the uniformity of the structure and performance of the plate in the thickness direction. The produced thick steel plate has excellent low-temperature toughness and good welding performance.

[0006] In order to achieve the above purpose, the technical scheme of the present application is as follows:

[0007] A production method of low-alloy structural thick steel plate, the steel plate has the following chemical components in percentage by mass: C: 0.09% to 0.12%, Si: 0.15% to 0.55%, Mn: 1.40% to 1.70%, P: ≤0.014%, S: ≤0.005%, Cr: ≤0.30%, Ni: ≤0.25%, Nb: 0.025% to 0.035%, Ti: 0.008% to 0.020%, Al: 0.020% to 0.050%, and the rest is Fe and inevitable impurities.

[0008] Further, the steel plate has the following chemical components in percentage by mass: C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Cr: 0.04%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest is Fe and inevitable impurities.

[0009] The roles of the elements in the application are as follows:

[0010] Carbon: Carbon exists in the form of interstitial solid solution in steel, and can also form carbides with other elements; carbon can greatly improve the strength of steel; the change of the content of carbon in steel will change the content of ferrite and pearlite in steel, and affect the toughness and welding process performance of the steel.

[0011] Silicon: Silicon is one of the deoxidizing elements in the steelmaking process, and exists in steel as a solid solution element, which can significantly improve the strength of the steel; silicon also has various influences on the carbides in steel, which can change the type and morphology of the carbides, promote the decomposition and refinement of the carbides, and also affect the transformation and distribution of the carbides to some extent.

[0012] Manganese: Manganese is one of the main alloying elements in steel, which can improve the strength through solid solution strengthening; manganese can reduce the critical transformation temperature to refine pearlite and indirectly improve the strength of steel; manganese can eliminate or weaken the hot brittleness of steel caused by sulfur; manganese can improve the low-temperature impact toughness of steel; and manganese is a carbide-forming element that affects the microstructure and performance of steel.

[0013] Chromium: Chromium itself has high hardness, and can significantly increase the surface hardness and wear resistance of steel when added to alloy steel; chromium can combine with iron in steel to form a layer of Cr2O3 oxide film that can protect steel from corrosion; chromium can significantly improve the corrosion resistance of steel, making it perform more excellently in humid or corrosive environments.

[0014] Nickel: Nickel can be infinitely solid solution with iron, expand the austenite phase region, conducive to the formation and stability of austenite; nickel can strengthen ferrite by solid solution strengthening and refine and increase pearlite; nickel can improve the low temperature toughness of steel; nickel can improve the weldability of steel, reduce the precipitation of intermetallic compounds, prevent and reduce their harmful effects; nickel can increase the stacking fault energy of steel, increase the mobile dislocations during deformation, thereby improving the plasticity and toughness of steel.

[0015] Niobium: As a micro-alloying element, a small amount of niobium can have a great impact on the performance of steel. Niobium can effectively refine the austenite grains during heating and rolling, inhibit austenite recrystallization, and improve the mechanical properties of steel. Niobium can reduce the interlamellar spacing of pearlite, reduce polygonal ferrite, refine the structure and improve impact toughness; niobium has a significant effect on the yield strength of steel.

[0016] Titanium: Titanium is an alloying element. Titanium has strong affinity with nitrogen and carbon, which is beneficial to controlling the surface quality of slab. Titanium can inhibit the growth of austenite grains during heating. Titanium is beneficial to refining austenite grains during heating and rolling. Titanium precipitates can improve the mechanical properties of steel. Titanium can also improve the toughness of steel at low temperature.

[0017] Aluminum: Aluminum is a strong deoxidizer and is added to molten steel as a deoxidizing alloy. At the same time, Al has strong affinity with N, which can inhibit the harm of nitrogen in steel. Al can refine the structure of steel, thereby improving the strength and toughness of steel.

[0018] Phosphorus: Phosphorus is a harmful element in steel, which can reduce the plasticity and toughness of steel, especially at low temperature. Phosphorus also deteriorates the welding performance of steel.

[0019] Sulfur: Sulfur is a harmful element in steel, which can cause thermal embrittlement at high temperature, reduce the plasticity and toughness of steel, and cause internal cracks in steel. Sulfur and manganese produce manganese sulfide, which affects the uniformity of steel and has adverse effects on performance.

[0020] Further, a production method of a low-alloy structural thick steel plate includes the following main steps:

[0021] (1) Converter smelting: top and bottom combined blowing converter, control the tapping temperature of 1560-1640℃, the tapping P of converter is less than or equal to 0.008%, and deoxidizing agent and alloy are added during tapping for deoxidization and alloying;

[0022] (2) Refining: After the molten steel in the ladle is heated by the LF furnace, the chemical composition is accurately adjusted, calcium treatment is carried out in the later stage of refining, and then the molten steel is treated in the RH furnace or VD furnace, and the molten steel is static after vacuum treatment;

[0023] (3) continuous casting: the overheat degree of the tundish is 8-30℃, the dynamic soft reduction or heavy reduction technology is used to improve the internal quality of the continuous casting billet, and the thickness size of the continuous casting billet is 300-450mm;

[0024] (4) heating: the slab is heated to 1100-1200℃ in the step continuous heating furnace and then discharged;

[0025] (5) rolling: two-stage controlled rolling is adopted, the rough rolling temperature is ≥950℃, the thickness of the intermediate billet is ≥ the target steel plate thickness+40mm, the intermediate billet is water-cooled, the target finish rolling temperature is 800-840℃, and the post-rolling is directly water-cooled to accelerate cooling, and the re-red temperature is 420-460℃;

[0026] (6) heat treatment: the furnace temperature is 890-930℃, and the steel plate is discharged and air-cooled and then water-cooled.

[0027] Preferably, the deoxidizer is an aluminum block or a silicon-manganese deoxidizer, and the alloy is a low-P alloy to avoid P rising in the molten steel.

[0028] Further, the chemical composition accurate adjustment in the refining stage refers to adding ferro-niobium, ferro-nickel and ferro-titanium for chemical composition accurate adjustment after the LF furnace is powered and heated.

[0029] Further, the molten steel is statically placed for 10-20min after the vacuum treatment in the refining stage.

[0030] Further, the water cooling time in the rolling stage is 50-120s.

[0031] Further, the furnace time in the heat treatment stage is calculated according to the following formula:

[0032] T=(0.3-0.5×H)min / mm

[0033] Wherein, T is the furnace time, and H is the thickness of the low-alloy structural steel plate.

[0034] Further, the steel plate is discharged, air-cooled to 700-750℃ and then water-cooled in the heat treatment stage.

[0035] Further, the mechanical properties of the steel plate at the near-surface, quarter and half positions in the thickness direction are as follows: yield strength ≥420MPa, tensile strength ≥500MPa, and elongation ≥25%.

[0036] Invention principle: the low alloy structural thick steel plate of the present application adopts low carbon composition system, adds appropriate Si, Mn and small amount of Ni, adds small amount of Nb, Ti, Al to refine the organization, strictly limits harmful elements such as P, S and the like in the steelmaking process, further improves the cleanliness of molten steel by adopting LF, vacuum treatment; adopt large thickness continuous casting billet, refine austenite grain through heating temperature control and rough rolling process, promote the fine grain structure after austenite transformation in the thick steel plate to improve the strength and toughness of the steel plate, the surface of the steel plate will produce bainite and ferrite-pearlite mixed structure after rolling, through the austenitization of the surface of the steel plate and the appropriate cooling process, the surface structure of the steel plate is transformed into fine ferrite-pearlite structure to improve the strength and toughness of the surface. At the same time, the in-furnace time of the steel plate is strictly limited to avoid the austenitization of the thick steel plate core to retain the excellent fine structure of the steel plate core; by controlling the heating process and cooling process of heat treatment, the cooling rate of the critical phase change zone of the thick position of the steel plate can be controlled to avoid the deterioration of the performance of the transition zone of the steel plate.

[0037] The low alloy structural thick steel plate of the present application has good mechanical properties and weldability. The production method adopted combines the advantages of hot rolling process and heat treatment process, realizes the production of medium plate in a wide chemical composition range, solves the problem of poor consistency of the structure and performance in the full thickness range of the thick steel plate, and effectively improves the toughness and weldability of the steel plate. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is the near-surface section metallographic structure diagram of example 1 of the present application;

[0039] Figure 2 is the 1 / 4 section metallographic structure diagram of example 1 of the present application;

[0040] Figure 3 is the 1 / 2 section metallographic structure diagram of example 1 of the present application. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0042] Example 1:

[0043] The low-alloy structural thick steel plate of the embodiment has the following chemical composition in mass percentage: C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Cr: 0.04%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest is Fe and inevitable impurities.

[0044] The specific production method steps are as follows:

[0045] (1) Converter smelting: top and bottom combined blowing converter, converter tapping temperature 1626℃, converter tapping measurement P=0.008%, aluminum block, silicon manganese deoxidizer is added during tapping process, silicon iron, silicon manganese, low carbon manganese iron is added for alloying.

[0046] (2) Refining: after the ladle steel is sent to the LF furnace for electric heating, niobium iron, nickel iron, titanium iron is added for chemical composition accurate adjustment, calcium treatment is carried out in the later stage of refining; after the LF refining is completed, the RH furnace is entered for vacuum treatment, and the molten steel is static for 15 min after the vacuum treatment is completed.

[0047] (3) Continuous casting: the superheat degree of the tundish is 9-15℃, dynamic soft reduction is adopted to improve the internal density of the slab and reduce internal segregation, and the thickness size of the continuous casting slab is 300mm.

[0048] (4) Heating: the slab is heated in the step continuous heating furnace, and the discharge temperature is 1189℃.

[0049] (5) Rolling: two-stage controlled rolling is adopted, and the finished product specification is 75x3100mm. The rough rolling temperature is 1100-1170℃, the intermediate slab thickness is 150mm, the intermediate slab is water cooled for 70s, the rough rolling temperature is 820℃, the finish rolling temperature is 805℃, and the red temperature is 430℃.

[0050] (6) Heat treatment: the heat treatment furnace temperature is 900℃, the furnace time is 25min, and the steel plate is discharged and air cooled to 730℃ and then water cooled.

[0051] The near-surface section metallographic structure of the low-alloy structural thick steel plate produced by the above method is shown in Figure 1 , the 1 / 4 section metallographic structure is shown in Figure 2 , and the 1 / 2 section metallographic structure is shown in Figure 3 .

[0052] Example 2:

[0053] The low yield ratio carbon-manganese low temperature steel plate described in this embodiment has the following chemical composition in mass percent: C: 0.12%, Si: 0.32%, Mn: 1.47%, P: 0.009%, S: 0.0015%, Cr: 0.17%, Ni: 0.02%, Nb: 0.031%, Ti: 0.012%, Al: 0.036%, and the balance of Fe and inevitable impurities.

[0054] The specific production method steps are as follows:

[0055] (1) Converter smelting: top and bottom combined blowing converter, converter tapping temperature 1612℃, converter tapping measurement P=0.007%, aluminum block, silicon-manganese deoxidizer is added during tapping process, silicon iron, silicon-manganese, low-carbon manganese iron is added for alloying.

[0056] (2) Refining: After the ladle steel is sent to the LF furnace for electric heating, niobium iron, chromium iron, and titanium iron are added for chemical composition accurate adjustment, calcium treatment is carried out in the later stage of refining; after LF refining is completed, the RH furnace is entered for vacuum treatment, and the molten steel is left standing for 15 min after vacuum treatment.

[0057] (3) Continuous casting: the superheat degree of the tundish is 9-15℃, dynamic soft reduction is used to improve the internal density of the slab and reduce internal segregation, and the thickness size of the continuous casting slab is 350mm.

[0058] (4) Heating: the slab is heated in a step continuous heating furnace, and the discharge temperature is 1147℃.

[0059] (5) Rolling: two-stage controlled rolling is used, and the finished product specification is 90x2600mm. The rough rolling temperature is 1070-1130℃, the intermediate slab thickness is 180mm, the intermediate slab is water cooled for 90s, the precision rolling opening temperature is 840℃, the precision rolling final rolling temperature is 795℃, and the red temperature is 420℃.

[0060] (6) Heat treatment: the heat treatment furnace temperature is 910℃, the furnace time is 30min, and the steel plate is discharged to air cooling to 730℃ and then water cooled.

[0061] Example 3:

[0062] The low yield ratio carbon-manganese low temperature steel plate described in this embodiment has the following chemical composition in mass percent: C: 0.12%, Si: 0.32%, Mn: 1.47%, P: 0.009%, S: 0.0015%, Cr: 0.17%, Ni: 0.02%, Nb: 0.031%, Ti: 0.012%, Al: 0.036%, and the balance of Fe and inevitable impurities.

[0063] The specific production method steps are as follows:

[0064] (1) Converter smelting: top and bottom combined blowing converter, converter tapping temperature 1606℃, converter tapping measurement P=0.006%, adding aluminum block, silicon manganese deoxidizer in tapping process, adding silicon iron, silicon manganese, low carbon manganese iron for alloying.

[0065] (2) Refining: after the ladle molten steel is heated by the LF furnace, niobium iron, chromium iron, nickel iron, titanium iron are added for accurate adjustment of chemical composition, calcium treatment is carried out in the later stage of refining; after the LF refining is completed, the RH furnace is entered for vacuum treatment, and the molten steel is static for 18 min after the vacuum treatment is completed.

[0066] (3) Continuous casting: the superheat of the tundish is 8-15℃, dynamic soft reduction is adopted to improve the internal density of the slab and reduce internal segregation, and the thickness size of the continuous casting slab is 450 mm.

[0067] (4) Heating: the slab is heated in a step continuous heating furnace, and the discharge temperature is 1137℃.

[0068] (5) Rolling: two-stage controlled rolling is adopted, and the finished product specification is 120x2200 mm. The rolling temperature is 1060-1130℃, the intermediate slab thickness is 200 mm, the intermediate slab is water cooled for 90 s, the rough rolling temperature is 840℃, the finish rolling temperature is 800℃, and the red temperature is 430℃.

[0069] (6) Heat treatment: the heat treatment furnace temperature is 900℃, the in-furnace time is 40 min, and the steel plate is discharged to air cooling to 740℃ and then water cooled.

[0070] Comparative Example 1:

[0071] The low alloy structural thick steel plate described in the comparative example has the following chemical composition in mass percent: C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest is Fe and unavoidable impurities.

[0072] The specific production method steps are as follows:

[0073] (1) Converter smelting: top and bottom combined blowing converter, converter tapping temperature 1626℃, converter tapping measurement P=0.008%, adding aluminum block, silicon manganese deoxidizer in tapping process, adding silicon iron, silicon manganese, low carbon manganese iron for alloying.

[0074] (2) Refining: after the ladle molten steel is heated by the LF furnace, niobium iron, nickel iron, titanium iron are added for accurate adjustment of chemical composition, calcium treatment is carried out in the later stage of refining; after the LF refining is completed, the RH furnace is entered for vacuum treatment, and the molten steel is static for 15 min after the vacuum treatment is completed.

[0075] (3) continuous casting: the overheat of tundish is 9-15 ℃, the internal density of slab is increased and the internal segregation is reduced by dynamic soft reduction, and the thickness size of continuous casting slab is 300 mm.

[0076] (4) heating: the slab is heated in a step continuous heating furnace, and the discharge temperature is 1170 ℃.

[0077] (5) rolling: two-stage controlled rolling is adopted, and the finished product specification is 75x3100 mm. The rough rolling temperature is 1100-1160 ℃, the intermediate slab thickness is 150 mm, the rough rolling temperature of finish rolling is 820 ℃, the finish rolling final rolling temperature is 794 ℃, and the red temperature is 440 ℃.

[0078] (6) heat treatment: the hearth temperature of heat treatment furnace is 900 ℃, the furnace time is 130 min, and the steel plate is discharged and air cooled.

[0079] Table 1 mechanical properties at different positions of examples and comparative examples

[0080]

[0081]

[0082] From Table 1, it can be seen that by using the method of the present application, the mechanical properties of low alloy structural thick steel plate at near surface, one quarter and one half can be ensured to be uniform and meet the requirements.

[0083] The above only describes the preferred embodiments of the present application, and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for producing a low alloy structural thick steel plate, characterized in that: The main steps include: (1) Converter smelting: top and bottom double-blown converter, control the tapping temperature at 1560-1640℃, converter tapping P≤0.008%, add deoxidizer and alloy for deoxidation and alloying during the tapping process; (2) Refining: After the molten steel in the ladle is heated by power in the LF furnace, the chemical composition is precisely adjusted. Calcium treatment is performed in the later stage of refining, and then the ladle enters the RH furnace or VD furnace for vacuum treatment. After vacuum treatment, the molten steel is allowed to stand; (3) Continuous casting: The tundish superheat temperature is 8-30°C. Dynamic light reduction or heavy reduction technology is used to improve the internal quality of the continuous casting billet. The thickness of the continuous casting billet is 300-450mm. (4) Heating: The slab is heated to 1100-1200°C in a step-beam continuous heating furnace and then taken out of the furnace; (5) Rolling: adopt two-stage controlled rolling, in which the rough rolling temperature is ≥950℃, the intermediate billet thickness is ≥the target steel plate thickness + 40mm, the intermediate billet is water-cooled for 50-120s, the target final rolling temperature of the finishing rolling is 800-840℃, and the steel is directly put into water for accelerated cooling after rolling, and the red-return temperature is 420-460℃; (6) Heat treatment: heating furnace temperature 890 ~ ​​930 ℃, furnace time T = (0.3 ~ 0.5 × H) min / mm, H is the thickness of the low alloy structural steel plate, the steel plate is air-cooled to 700 ~ 750 ℃ ​​after being taken out of the furnace and then water-cooled; The chemical composition of the steel plate is as follows by mass: C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Cr: 0.04%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the remainder is Fe and unavoidable impurities; The mechanical properties of the steel plate near the surface, at one-quarter and at one-half in the thickness direction are yield strength ≥420 MPa, tensile strength ≥500 MPa, and elongation ≥25%.

2. The method for producing a low alloy structural thick steel plate according to claim 1, characterized in that: The deoxidizer is aluminum block or silicon manganese deoxidizer, and the alloy is a low-P alloy to avoid P increase in molten steel.

3. The method for producing a low alloy structural thick steel plate according to claim 1, characterized in that: The precise adjustment of chemical composition in the refining stage refers to adding ferroniobium, ferronickel and ferrotitanium to precisely adjust the chemical composition after the LF furnace is powered on and heated.

4. The method for producing a low alloy structural thick steel plate according to claim 1, wherein: After vacuum treatment in the refining stage, the molten steel is left to stand for 10 to 20 minutes.

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