Marine work steel and preparation method thereof

Abstract

The invention discloses maritime work steel which comprises the following chemical components in percentage by mass: 0.060 to 0.08 weight percent of C, 0.15 to 0.25 weight percent of Si, 1.35 to 1.50 weight percent of Mn, less than or equal to 0.0080 percent of P, less than or equal to 0.0050 weight percent of S, 0.060 to 0.080 weight percent of Mo, 0.035 to 0.045 weight percent of Nb, 0.035 to 0.045 weight percent of V, less than or equal to 0.050 weight percent of N and Allt. The alloy comprises the following components in percentage by weight: 0.010% of magnesium, 0.0020-0.0030% of magnesium, 0.015-0.025% of Ti, 0.0045-0.0060% of O and the balance of iron and inevitable impurity elements. The maritime work steel has good high heat input welding performance and excellent HAZ low-temperature toughness.

Description

TECHNICAL FIELD

[0001] The present application relates to a marine steel material for a ship, in particular to a marine steel and a preparation method thereof, and particularly relates to a marine steel material with improved low-temperature toughness of a base body and HAZ and a production method thereof. BACKGROUND

[0002] With the rapid development of the shipbuilding industry and the wide application of large heat input welding, the low-temperature toughness of marine steel for ship structures is increasingly required. Ship body steel is joined by welding, but in the welded joint, especially the HAZ, there is a phenomenon of grain coarsening and deterioration of toughness due to the welding heat effect, which affects the stability and safety of the ship body. This key problem is more pronounced with the increase of welding line energy. The reason is that the cooling speed of HAZ slows down and the quenching property decreases with the increase of welding line energy, and then coarse island-shaped martensite is generated. In order to improve the toughness of HAZ, the method of oxide metallurgy has been widely concerned, including the first generation of oxide metallurgy, TiN particles pinning austenite grain boundaries, the second generation of oxide metallurgy using TiO x particles to promote intracrystalline acicular ferrite (hereinafter referred to as IAF) nucleation to improve HAZ performance, and the third generation of oxide metallurgy using Mg / Ca oxide particles to inhibit the growth of austenite grains. Among them, adjusting the steel alloy composition and then controlling the precipitation of second phase particles and the generation of inclusions as a key link has an important influence on improving the toughness of the heat-affected zone of large heat input welding.

[0003] Currently, there are still few researches and reports on using steel composition design to improve the toughness of HAZ by different mechanisms during rolling and large heat input welding.

[0004] The invention with the publication number CN113637917A "690MPa grade ultra-high strength special thick ship plate steel with excellent low-temperature impact performance and production method thereof" can ensure strength and low-temperature toughness, but requires the composite addition of high content of Ni (0.90-2.0wt%), Cr (0.40-0.60wt%), and Cu (0.15-0.25wt%), which has a high production cost. High content of Cr element can increase the strength and hardness of the steel in the rolled state, but at the same time, it reduces the plasticity and toughness. The two-stage controlled rolling + quenching heat treatment + tempering heat treatment process is adopted, and the manufacturing process and procedure are complex, and the delivery cycle of the steel plate is long.

[0005] The application patent with the publication number CN113322408A "A large line energy welding EH550 MPa grade quenched and tempered offshore steel plate and a manufacturing method thereof" adjusts the ratio of deoxidizing alloy Al / Ti to induce acicular ferrite in the HAZ during solidification and phase transformation, thereby increasing the low-temperature toughness of the HAZ. However, the Ti content is less than 0.012wt%, and the Al and Ti combined inclusions are easy to agglomerate, which reduces the effective inclusion amount and affects the IAF induction effect, and the low-temperature toughness of the HAZ after large line energy welding is significantly reduced. SUMMARY

[0006] The present application aims to provide a 420MPa grade offshore steel plate with excellent low-temperature performance of the base body and HAZ, and a manufacturing method thereof. The steel plate has the characteristics of ultrahigh strength and uniform microstructure and performance. The yield strength of the rolled steel plate is ≥420MPa, the tensile strength is 580-630MPa, and the elongation after fracture is ≥19%. The low-temperature toughness is excellent, the impact energy at -20℃ is ≥237J, and the impact energy at -40℃ is ≥197J. After welding with a large line energy input of 50-200KJ / cm, the low-temperature toughness of the HAZ is excellent, the impact energy at 20℃ is ≥193J, and the impact energy at 40℃ is ≥182J.

[0007] The first aspect of the present application provides an offshore steel plate. The chemical composition of the offshore steel plate includes, by mass percentage: C: 0.060-0.080wt%, Si: 0.15-0.25wt%, Mn: 1.35-1.50wt%, P: ≤0.0080wt%, S: ≤0.0050wt%, Mo: 0.060-0.080wt%, Nb: 0.035-0.045wt%, V: 0.035-0.045wt%, N: ≤0.050wt%, Al: <0.010wt%, Mg: 0.0020-0.0030wt%, Ti: 0.015-0.025wt%, O: 0.0045-0.0060wt%, and the balance being iron and unavoidable impurities.

[0008] The offshore steel plate according to the first aspect has a grain size of 65-120μm, and the average size of the second phase particles in the steel is <90nm; and / or the number of second phase particles is 1.5-3.5 / μm. 2 .

[0009] The offshore steel plate according to the first aspect has a yield strength of ≥420MPa, a tensile strength of 580-630MPa, an elongation after fracture of ≥19%, an impact energy at -20℃ of ≥237J, and an impact energy at -40℃ of ≥197J.

[0010] The offshore steel plate according to the first aspect has a volume fraction of IAF in the HAZ microstructure of ≥37% under the condition of welding with a line energy of 50-200kJ / cm.

[0011] Preferably, the HAZ organization has a grain size of 80-150 μm, an impact energy at -20℃ ≥193 J, and an impact energy at -40℃ ≥182 J.

[0012] The second aspect of the present application provides a method for preparing the offshore steel of the first aspect, the method comprising:

[0013] (1) Pretreatment: desulfurization by mechanical stirring method;

[0014] (2) Smelting: smelting by converter, dephosphorization and desulfurization during the process;

[0015] (3) Refining: refining the converter molten steel by LF refining furnace;

[0016] (4) Continuous casting: after the refining is completed, the temperature is adjusted and the molten steel is cast without oxidation to obtain a steel slab;

[0017] (5) Controlled rolling and controlled cooling: after the steel slab is homogenized, controlled rolling and controlled cooling are performed to obtain a rolled steel plate.

[0018] According to the method of the second aspect, in step (1), the pretreatment endpoint S≤0.004wt%; and / or

[0019] After the pretreatment, the slag is cleaned, and the bright surface of the molten iron is ≥90%.

[0020] According to the method of the second aspect, in step (2), the end control conditions of the smelting step are:

[0021] The smelting endpoint C≤0.06wt%, and the smelting endpoint P≤0.005wt%;

[0022] Secondary slag blocking is used, and the slag is strictly controlled at the furnace mouth and the tapping hole during tapping;

[0023] The tapping temperature is greater than 1558℃;

[0024] The ladle is clean without cold steel, and the red ladle is tapped; and / or

[0025] The converter tapping uses a slide plate to block the slag, and the amount of P returned is strictly controlled to be ≤0.002wt%.

[0026] According to the method of the second aspect, in step (3), the refining step comprises:

[0027] In the LF refining furnace, the converter molten steel is heated to 1590-1610℃ under argon protection, a top slag is covered, and after the top slag is melted, aluminum particles are used for deoxidation, and then ferrosilicon and ferromanganese are added in sequence, and after the alloy is melted and cleaned, refining is performed;

[0028] Before tapping, niobium iron, molybdenum iron, vanadium iron, and titanium iron alloy are added;

[0029] The ladle is moved to the wire feeding position, and the MgO-CaO cored wire is added and then soft blown.

[0030] According to the method of the second aspect, the top slag composition is CaO:SiO2:MgO:CaF2=53:27:10:10;

[0031] The oxygen content in the aluminum particle deoxidization step is controlled at 45-60 ppm;

[0032] The refining time after the alloy is melted and cleaned is 8-10 min;

[0033] The MgO-CaO cored wire is added at a speed of 3-4 m / min, preferably 3.5 m / min;

[0034] The soft blowing temperature is 1570-1595℃, preferably 1585℃; and / or

[0035] The soft blowing time is 10-15 min, preferably 12 min.

[0036] According to the method of the second aspect, in step (4), the pouring temperature is 1535-1545℃, the pouring speed is ≤1 m / min, and the pouring speed is preferably 0.7-1 m / min.

[0037] According to the method of the second aspect, in step (5), the homogenization temperature is 1150-1200℃, and the homogenization time is 9-13 min / cm.

[0038] The open rolling temperature is 1000-1050℃, the single pass reduction rate is ≥10%, the final rolling temperature of the rolling is ≥820℃, and the cumulative reduction rate is ≥35%; and / or

[0039] The open cooling temperature is 800-820℃, the final cooling temperature is 300-400℃, and the average cooling speed is 7-10℃ / s.

[0040] According to the method of the second aspect, the thickness of the rolled steel plate is 10-30 mm.

[0041] The offshore steel material has the following advantages and beneficial effects, but is not limited to:

[0042] The base body and the offshore steel material with excellent HAZ low-temperature toughness of the application adopt suitable component design, and through reasonable control of each alloying element in the steel, TiN, Nb(C / N) and other second phase particles are precipitated in the rolling process, the grains are refined, the low-temperature toughness of the rolled plate is improved, the rolled material with impact energy ≥237J at-20℃ and impact energy ≥197J at-40℃. The second phase particles such as TiN and Nb(C / N) precipitated in the rolling process of the offshore steel with excellent HAZ low-temperature toughness are dissolved back during welding, the grain boundary is released, the grain is expanded, the inclusion-induced IAF nucleation is promoted, the grain is refined, and the low-temperature toughness of the HAZ of the rolled material is improved, under the condition that the welding input line energy is 50-200KJ / cm, the impact energy at-20℃ is ≥193J, and the impact energy at-40℃ is ≥182J. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The evolution of the inclusion type with the T.O. content (total oxygen content) is shown, where Figure 1 (a) shows the case of T.O. = 0.0110wt%, Figure 1 (b) shows the cases of T.O. = 0.0061wt%, T.O. = 0.0037wt%.

[0044] Figure 2 The evolution of the inclusion number with the T.O. content is shown. DETAILED DESCRIPTION

[0045] The application will be further described in detail below by means of the accompanying drawings and examples. Through these descriptions, the features and advantages of the application will become more apparent.

[0046] The term "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any implementation described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other implementations. Unless specifically stated otherwise, the drawings are not drawn to scale and are merely intended to conceptually illustrate aspects of the embodiments.

[0047] In addition, the technical features involved in different embodiments of the application described below can be combined with each other as long as there is no conflict.

[0048] Before the technical solutions of the application are described, the terms used herein are defined as follows:

[0049] The term "offshore steel" refers to: structural steel for ships and offshore engineering.

[0050] The term "second phase particle" refers to: alloy compound particles formed by alloying elements of steel during heat treatment.

[0051] The term "IAF" refers to: Intragranular Acicular Ferrite.

[0052] The term "HAZ" refers to: Heat-affected-zone, welding heat-affected zone.

[0053] The term "LF furnace" refers to: Ladle Furnace, steel ladle furnace.

[0054] The present application provides a kind of offshore steel, according to mass percentage, the chemical composition of the offshore steel includes: C:0.060~0.080wt%, Si:0.15~.25wt%, Mn:1.35~1.50wt%, P≤0.0080wt%, S:≤0.0050wt%, Mo:0.060~0.080wt%, Nb:0.035~0.045wt%, V:0.035~0.045wt%, N≤0.050wt%, Al<0.010wt%, Mg:0.0020~0.0030wt%, Ti:0.015~0.025wt%, O:0.0045~0.0060wt%, the balance is iron and inevitable impurity elements.

[0055] The mechanism of action of each alloying component in the steel of the present application is as follows:

[0056] C: C is a necessary element for strengthening steel, which has a significant effect on improving the hardenability of steel through solid solution and precipitation strengthening, but increasing C content will seriously affect the welding performance and HAZ low temperature toughness of steel, so reducing C content is the basis for designing steel alloy components for large heat input welding, considering product performance, the steel material described in the present application preferably has a C content of 0.060~0.080wt%.

[0057] Si: Si is a solid solution strengthening element and also a common deoxidizer, which can dissolve in ferrite and austenite, is beneficial to the improvement of steel plate strength, hardness and high temperature oxidation resistance, and makes up for the reduction of steel strength due to the reduction of C content, but too high Si content will inhibit the formation of acicular ferrite and promote packet size coarsening, which will seriously reduce the plasticity, low temperature toughness and weldability of ultra-high strength steel, considering the economy and feasibility of refining ultra-high strength steel, the steel material described in the present application preferably has a Si content of 0.15~0.25wt%.

[0058] Mn: Mn is a good deoxidizer and desulfurizer, can form high sulfur content inclusions with S, and can promote IAF nucleation by using the mechanism of manganese-lean zone. Mn element can improve the strength and hardness of ferrite and austenite in steel, also has the effect of expanding the austenite phase region, reducing the Ar3 point temperature, refining the ferrite grain and improving the low temperature toughness of the steel plate. With the increase of the strength of the ultra-high strength ship plate steel, the Mn content in the steel gradually increases, therefore, the Mn content in the steel material described in the application is preferably in the range of 1.35-1.50wt%.

[0059] P: As a harmful element in steel, P is easy to segregate seriously, segregate in the center of the steel plate and gather at the grain boundary, etc., which can have serious adverse effects on the plasticity and low temperature toughness of the steel plate, and also can reduce the welding performance of the steel plate. The P content in the steel material of the application is controlled to be lower than 0.008wt%.

[0060] S: S is also a well-known harmful element of steel material, which can cause hot brittleness of steel, and sulfide inclusions can also become a crack source to cause cracks in steel. But S is easy to combine with Mn to form MnS, and the manganese-lean zone produced under certain conditions has a significant effect on promoting IAF induction, which can significantly refine the grain and improve the toughness of the steel material. The S content in the steel material of the application is controlled to be ≤0.0050wt%.

[0061] Mo: Mo is a necessary alloying element for improving the hardenability of thick plate steel. Mo can narrow the austenite phase region, inhibit the decomposition of austenite and the formation of grain boundary ferrite, and promote the formation of IAF and bainite microstructure. Mo can also significantly improve the strength of the steel material through solid solution strengthening. However, increasing Mo will increase the cost. In order to avoid the presence of too much M-A component, the Mo content in the steel needs to be controlled at a relatively low level. Therefore, the Mo content in the steel material described in the application is controlled in the range of 0.060-0.080wt%.

[0062] Nb: Nb element can effectively refine the grain of the rolled material by increasing the austenite recrystallization temperature, thereby improving the toughness. It also can play a role in precipitating strengthening the steel material by precipitating Nb(C, N). However, during welding, when the heat input energy is large (>60KJ / cm), Nb will deteriorate the toughness due to the promotion of granular bainite formation, and the increase of Nb content will also reduce the low temperature toughness of the HAZ of the steel material. The Nb content in the steel material described in the application can be controlled in the range of 0.035-0.045wt%.

[0063] V: V has the effects of precipitation strengthening and grain refinement, which can improve the strength of the steel material without reducing the toughness. With the increase of the grade of ultra-high strength steel material, the V content tends to increase, and the increase of V content will not have a significant effect on the low temperature toughness of the HAZ of the steel material. The addition of V also has the trend of promoting IAF transformation. The V content in the steel material described in the application can be controlled in the range of 0.035-0.045wt%.

[0064] N: TiN precipitated by Ti and N combination pins austenite grain boundary movement at high temperature, and then refines the grain, which is an important method for the first generation of oxide metallurgy to improve the toughness of HAZ. When the atomic ratio of Ti and N in the steel is 1:1, the TiN particles are fine and dispersed, and the pinning effect on the high temperature austenite grain boundary is significant, so that the steel plate is easy to obtain excellent toughness. In order to ensure the content of TiN in the steel plate, considering the generation of Ti oxide, according to the addition amount of Ti and the oxygen content in the steel and other nitrogen-fixing elements (such as Nb), the N content is controlled to be ≤0.050wt%, and preferably the N content is controlled to be ≤0.0050wt%.

[0065] Al: Al is a deoxidizing and grain refining element that must be added in the present application, and requires a certain addition amount. However, due to the combined addition of other fine-grain elements, and excessive Al elements will lead to the easy production of casting slab hot cracks in steel, thereby reducing the toughness of the steel, and affecting the Ti content in the inclusions. Therefore, the Al content in the steel material described in the present application is controlled to be below 0.01wt%.

[0066] Mg: Mg oxide and sulfide inclusions have the characteristics of dispersed distribution in the steel and stability at high welding temperature, which can effectively pin the austenite grain boundary, inhibit the growth of austenite grain, and then refine the grain. However, excessive Mg content will increase the area of austenite grain boundary, promote the transformation of grain boundary and side plate ferrite, and reduce the IAF nucleation ability. Therefore, the Mg content in the steel material described in the present application is controlled to be 0.0020-0.0030wt%.

[0067] Ti: Ti can fix N and C elements, and the precipitated TiN can effectively pin the austenite grain boundary, refine the grain, and then improve the toughness of the rolled material. TiO x is a very effective IAF nucleation inclusion in steel, which can significantly promote IAF nucleation and refine the grain. However, excessive Ti content will form coarse titanium carbonitride, which becomes a crack source, and then leads to reduced toughness. Therefore, the Ti content in the steel material described in the present application is preferably controlled to be 0.015-0.025wt%.

[0068] In one embodiment, the average size of the second phase particles in the offshore steel is <90nm; and / or the number of second phase particles is 1.5-3.5 / μm 2 .

[0069] In one embodiment, the grain size of the offshore steel is 65-120μm, the yield strength is ≥420MPa, the tensile strength is 580-630MPa, the elongation after fracture is ≥19%, the impact energy at -20℃ is ≥237J, and / or the impact energy at -40℃ is ≥197J.

[0070] In one embodiment, the offshore steel has a volume fraction of IAF in the HAZ structure ≥ 37% under the welding condition of 50-200 kJ / cm linear energy.

[0071] Preferably, the grain size in the HAZ structure is 80-150 μm, the impact energy at -20 ℃ is ≥ 193 J, and the impact energy at -40 ℃ is ≥ 182 J.

[0072] The HAZ inclusions of the rolled plate of the present application are MgO+Mg-Ti-O type composite inclusions, and under the welding condition of 50-200 KJ / cm linear energy, the HAZ structure is mainly inclusion-induced IAF and lath bainite structure.

[0073] Figure 1 The evolution of inclusion type with T.O. content (total oxygen content) is shown, wherein Figure 1 (a) the case of T.O. = 0.0110 wt% is shown, Figure 1 (b) the cases of T.O. = 0.0061 wt%, T.O. = 0.0037 wt% are shown. As shown in the figure, Figure 1 under the same smelting condition, with the decrease of oxygen content, the inclusions in the steel change to MnO-MgO-Ti3O5 system → high MgO+MgO-Ti3O5-Al2O3 system → high MgO system, the inclusions in the steel with too high oxygen content are MnO-MgO, which has no effect on inducing acicular ferrite and affects the cleanliness of the steel, and with too low oxygen content, the inclusions change to MgO, the fine and dispersed inclusions can pin the austenite grain boundary, but affect the growth space of acicular ferrite, when the oxygen content is 0.0061 wt%, the high MgO+MgO-Ti3O5-Al2O3 system inclusions can be obtained, which makes the inclusions fine and dispersed and at the same time induces acicular ferrite with high Ti content. Figure 2 The evolution of inclusion number with T.O. content is shown. As shown in the figure, Figure 2 when the oxygen content is too high (T.O. = 0.0110 wt%), there are a large number of large-size inclusions, which are not conducive to inducing acicular ferrite nucleation, and when the oxygen content is too low, the number of inclusions is significantly reduced, which cannot provide enough nucleation sites for acicular ferrite.

[0074] The present application also provides a preparation method of the offshore steel, comprising:

[0075] (1) pretreatment: desulfurization by mechanical stirring method;

[0076] (2) smelting: smelting by converter, and dephosphorization and desulfurization during the process;

[0077] (3) refining: refining the converter molten steel by LF refining furnace;

[0078] (4) continuous casting: after the refining is completed, the temperature is adjusted and the molten steel is cast without oxidation to obtain a steel plate blank;

[0079] (5) controlled rolling and controlled cooling: the steel plate blank is homogenized, and then controlled rolling and controlled cooling are performed to obtain a rolled steel plate.

[0080] The present application is based on steel composition design, and accurately controls the precipitation amount, precipitation temperature and melting temperature of Nb, Mo and V, so that they are rapidly precipitated at low temperature during rolling, and the composition, alloying sequence, adding speed of Al, Mg and Ti and the superheat and speed during continuous casting are controlled to generate a large amount of Mg-Ti-O beneficial complex inclusions, and a rolled steel plate for large heat input welding is obtained by rolling the steel material with a specific alloy composition, and the thickness specification of the finished steel plate is 10-30 mm.

[0081] The production process of the offshore steel in the present application is divided into smelting period, continuous casting period and controlled rolling and controlled cooling period.

[0082] Smelting period: based on the molten industrial process, the oxide metallurgical micro-alloying elements are added in a certain order and time, and the steel liquid composition qualified offshore steel is smelted by strictly controlling the adding amount, adding method and adding opportunity.

[0083] Continuous casting period: the steel is cast without oxidation protection, the pouring temperature is controlled at 1535-1545℃, and the pouring speed is ≤0.7-1 m / min. The steel plate blank is obtained.

[0084] Controlled rolling and controlled cooling period: based on the steel plate obtained by continuous casting, the steel plate is homogenized and then subjected to controlled rolling and controlled cooling treatment, and the average size of the second phase particles such as TiN and Nb(C / N) in the finally obtained offshore steel is <90 nm, and the particle number is 1.5 / μm 2 .

[0085] The present application is based on alloy composition design, and TiN, Nb(C / N) and other second phase particles are precipitated in the process of controlled rolling and controlled cooling after smelting and continuous casting, and a 420MPa offshore steel with excellent low temperature performance and a maximum thickness of 30mm is obtained; during the large heat input welding process, the TiN, Nb(C / N) and other second phase particles are dissolved back, the grain boundaries are released, the Mg element is used to make the effective inclusions dispersedly distributed, part of the MgO particles can also play the role of pinning austenite grain boundaries at high temperature when the TiN, Nb(C / N) and other second phase particles are dissolved back, and by adding other fine-grained elements to reduce the Al content, the TiO x content in the inclusions is increased, and the inclusion-induced IAF nucleation rate is improved, the inclusion-induced IAF nucleation is promoted, and the offshore steel with excellent low temperature toughness in the HAZ is obtained.

[0086] In one embodiment, in step (1), the pretreatment endpoint S is ≤0.004wt%, the slag is cleaned, and the bright surface of the molten iron is ≥90%.

[0087] In order to ensure that the content of S in the final product marine steel is ≤0.0050wt%, the end point S is ≤0.004% in the pretreatment desulfurization process.

[0088] In one embodiment, in step (2), the end control condition of the smelting step is:

[0089] The smelting end C is ≤0.06wt%, and the smelting end P is ≤0.005wt%;

[0090] Secondary slag stopping is adopted, and the slag is strictly prevented from falling at the furnace mouth and the tapping hole during tapping;

[0091] The tapping temperature is greater than 1558℃;

[0092] The ladle is clean without cold steel, and the red ladle is used for tapping; and / or

[0093] The sliding plate slag stopping is adopted for the converter tapping, and the amount of P returned is ≤0.002wt%.

[0094] The red ladle tapping refers to the operation of preheating the ladle lining to 800-1000℃ before tapping, so as to reduce the temperature drop during tapping, thereby reducing the tapping temperature, increasing the amount of scrap steel, and improving the furnace life.

[0095] In one embodiment, in step (3), the refining step comprises:

[0096] In the LF refining furnace, the converter molten steel is heated to 1590-1610℃ under the protection of argon, and the top slag is covered. After the top slag is melted, the aluminum particles are deoxidized, and then the ferrosilicon and ferromanganese are added in sequence. After the alloy is melted and cleaned, the refining is carried out.

[0097] The ferro-niobium, ferro-molybdenum, ferro-vanadium and ferro-titanium alloys are added before tapping.

[0098] The ladle is moved to the wire feeding position, and the magnesia calcium cored wire is added for soft blowing.

[0099] The top slag composition is CaO:SiO2:MgO:CaF2=53:27:10:10;

[0100] The oxygen position control in the aluminum wire deoxidization step is 45-60ppm;

[0101] The refining time after the alloy is melted and cleaned is 8-10min;

[0102] The addition speed of the magnesia calcium cored wire is 3-4m / min, preferably 3.5m / min;

[0103] The soft blowing temperature is 1570-1595℃, preferably 1585℃; and / or

[0104] The soft blowing time is 10-15 minutes, preferably 12 minutes.

[0105] The soft blowing ensures sufficient mixing of oxygen and blowing of the metal liquid to promote the reduction reaction and mixing process, thereby improving the reaction rate and reaction effect in the furnace.

[0106] In one embodiment, in step (2), the pouring temperature is 1535-1545°C, and the pouring speed is ≤1 m / min, preferably 0.7-1 m / min.

[0107] In one embodiment, in step (3), the homogenization temperature is 1150-1200°C, and the homogenization time is 9-13 min / cm.

[0108] The rolling open rolling temperature is 1000-1050°C, the single pass reduction rate is ≥10%, the final rolling temperature of the rolling is 850-900°C, and the cumulative reduction rate is ≥35%.

[0109] The open cooling temperature is 800-820°C, the final cooling temperature is 300-400°C, and the average cooling speed is 7-10°C / s.

[0110] In one embodiment, the thickness of the rolled steel plate is 10-30 mm.

[0111] In one specific embodiment, the preparation method of the present application comprises the following steps:

[0112] The preparation method comprises the following steps:

[0113] 1) Hot metal pre-desulfurization, S ≤ 0.004wt%, clean slag, and hot metal bright surface ≥ 90%;

[0114] 2) Converter smelting: end point C ≤ 0.06wt%, end point P ≤ 0.005wt%, using secondary slag stopping, and tapping strictly prohibited with slag at the furnace mouth and tapping hole;

[0115] The tapping temperature is required to be greater than 1558°C;

[0116] The ladle is required to be clean without cold steel, and the red ladle is tapped;

[0117] The converter tapping uses a sliding plate to stop slag, and the amount of P returned is ≤ 0.002wt%.

[0118] 3) Using an LF refining furnace, the raw material is converter molten steel, and the temperature is raised under the protection of argon blowing;

[0119] 4) When the temperature is raised to 1590-1610°C, immediately cover the top slag, the composition of the top slag is CaO:SiO2:MgO:CaF2=53:27:10:10, and after the top slag is melted, the aluminum particles are deoxidized;

[0120] After deoxidation, silicon iron and manganese iron are added in sequence, and after the alloy is melted clean, power stirring is performed for 8-10 min;

[0121] Before tapping, high-purity iron alloy micro-alloying such as niobium iron, molybdenum iron, vanadium iron and titanium iron is added in sequence;

[0122] 5) The ladle is moved to a wire feeding position to add magnesium calcium oxide slow-release cored wire for oxide metallurgical micro-alloying, the magnesium calcium oxide slow-release cored wire is added at a speed of 3.5 m / min, and after the magnesium calcium oxide cored wire is added, soft blowing is performed, and the soft blowing time is 10-15 min;

[0123] 6) After refining is completed, the temperature is adjusted, and tapping is performed under non-oxidizing protection, and the tapping temperature is controlled at 1535-1545℃, and the tapping speed is ≤0.7-1 m / min. The steel plate blank is obtained;

[0124] 7) The steel plate blank is kept at 1150-1200℃, the holding time is 9-13 min / cm, and then controlled rolling and controlled cooling are performed;

[0125] 8) The opening rolling temperature of rolling is 1000-1050℃, the single pass reduction rate is ≥10%, and the final rolling temperature of rolling is ≥820℃, and the cumulative reduction rate is ≥35%;

[0126] 9) The opening cooling temperature is 800-820℃, the final cooling temperature is 300-400℃, the average cooling speed is 7-10℃ / s, and then air cooling is performed to room temperature;

[0127] 10) The thickness of the finished rolled steel plate is 10-30 mm.

[0128] The present application does not have special restrictions on the source of all raw materials, and is a conventional product that can be obtained by purchase unless otherwise specified.

[0129] Examples 1-4:

[0130] The offshore steel with excellent low-temperature toughness of the base and HAZ described in the present application is prepared by the following steps:

[0131] 1) The molten iron is pre-desulfurized by mechanical stirring method, S≤0.004wt%, the slag is cleaned, and the bright surface of the molten iron is ≥90%;

[0132] 2) Converter smelting: the smelting endpoint C≤0.06wt%, the smelting endpoint P≤0.005wt%, secondary slag stopping is adopted, and slag is strictly prohibited at the furnace mouth and tapping hole during tapping;

[0133] The tapping temperature is required to be greater than 1558℃;

[0134] The ladle is required to be clean without cold steel, and the red ladle is tapped;

[0135] The converter tapping adopts slide plate slag stopping, and the amount of P returned is controlled to be less than or equal to 0.002wt%.

[0136] 3) The LF refining furnace is adopted, the raw material is converter molten steel, and the temperature is raised under the protection of argon gas;

[0137] 4) When the temperature is raised to 1590-1610℃, the top slag is immediately covered, the composition of the top slag is CaO:SiO2:MgO:CaF2=53:27:10:10, and the aluminum particles are deoxidized after the top slag is melted

[0138] After deoxidation, ferrosilicon and ferromanganese are sequentially added, and after the alloy is melted and cleaned, electric stirring is performed for 8-10min;

[0139] Before tapping, high-purity ferroalloy microalloying such as ferro-niobium, ferro-molybdenum, ferro-vanadium, and ferro-titanium is sequentially added;

[0140] 5) The ladle is moved to the wire feeding position to add magnesium oxide calcium slow-release cored wire for oxide metallurgical microalloying, the addition speed of the magnesium oxide calcium slow-release cored wire is 3.5m / min; after the magnesium oxide calcium cored wire is added, soft blowing is performed, the soft blowing temperature is 1585℃, and the soft blowing time is 10-15min;

[0141] 6) After refining is completed, the temperature is adjusted, and tapping is performed under non-oxidizing protection, the pouring temperature is controlled to be 1535-1545℃, the pouring speed is 0.7-1m / min, and a slab is obtained;

[0142] 7) The slab is kept at 1150-1200℃, the holding time is 9-13min / cm, and then controlled rolling and controlled cooling are performed;

[0143] 8) The open rolling temperature is 1000-1050℃, the single pass reduction rate is greater than or equal to 10%, the final rolling temperature of the rolling is greater than or equal to 820℃, and the cumulative reduction rate is greater than or equal to 35%;

[0144] 9) The open cooling temperature is 800-820℃, the final cooling temperature is 300-400℃, the average cooling speed is 7-10℃ / s, and then air cooling is performed to room temperature;

[0145] 10) The thickness of the finished rolled steel plate is 10-30mm.

[0146] Two pieces of the same size of the rolled steel plate are welded as a base material, the joint form is butt welding, the groove is a 45° V-shaped groove, the welding method is submerged arc automatic welding, the welding method is single-layer single-pass welding, the selected flux is alkaline flux, and the input line energy is 50KJ / cm, 100KJ / cm, and 200KJ / cm.

[0147] Table 1 lists the percentage content of each alloying component (the balance being Fe and unavoidable impurities) of the base and HAZ low-temperature toughness excellent offshore steel steel materials of Examples 1-4. Table 2 lists the preparation parameters of the offshore steel steel materials of Examples 1-4. Table 3 lists the rolled materials and different input line energy HAZ low-temperature toughness of Examples 1-4. Table 4 lists the base and HAZ microstructure characteristics of the steel materials of Examples 1-4.

[0148] Table 1 Chemical composition of offshore steel steel materials of Examples 1-4

[0149]

[0150]

[0151] Table 2 Preparation parameters of offshore steel steel materials of Examples 1-4

[0152]

[0153] Table 3 Offshore steel rolled materials and different input line energy HAZ low-temperature toughness of Examples

[0154]

[0155]

[0156] *The Charpy impact performance test of the embodiments of the present application was carried out according to GB / T 229 Metal Materials Charpy Pendulum Impact Test Method.

[0157] Table 4 Base and HAZ microstructure characteristics of steel materials of Examples 1-4

[0158]

[0159] The above describes the present application in combination with preferred embodiments, but these embodiments are only exemplary and serve only to illustrate. On this basis, various substitutions and improvements can be made to the present application, and these all fall within the protection scope of the present application.

Claims

1. A steel for marine structures, characterized in that, The chemical composition of the offshore steel by mass percentage comprises: C: 0.060-0.080wt%, Si: 0.15-0.25wt%, Mn: 1.35-1.50wt%, P: ≤0.0080wt%, S: ≤0.0050wt%, Mo: 0.060-0.080wt%, Nb: 0.035-0.045wt%, V: 0.035-0.045wt%, N ≤0.050wt%, Al <0.010wt%, Mg: 0.0020-0.0030wt%, Ti: 0.015-0.025wt%, O: 0.0045-0.0060wt%, and the balance is iron and inevitable impurity elements.

2. The offshore steel according to claim 1, characterized in that, The sea work steel has a grain size of 65-120 μm, the average size of the second phase particles in the steel is < 90 nm, and / or the number of the second phase particles is 1.5-3.5 per μm 2 .

3. The offshore steel according to claim 1, characterized in that, The offshore steel has a yield strength ≥420 MPa, a tensile strength of 580-630 MPa, an elongation after fracture ≥19%, an impact energy at -20°C ≥237 J, and an impact energy at -40°C ≥197 J.

4. The offshore steel according to claim 1, characterized in that, The offshore steel has a volume fraction of IAF in the HAZ structure ≥37% under the welding condition of a linear energy of 50-200 kJ / cm. Preferably, the grain size in the HAZ structure is 80-150 μm, the impact energy at -20°C ≥193 J, and the impact energy at -40°C ≥182 J.

5. The method of producing a marine steel according to any one of claims 1 to 4, characterized in that, The method comprises: (1) Pretreatment: desulfurization by mechanical stirring method; (2) Smelting: smelting by converter, and dephosphorization and desulfurization during the process; (3) Refining: refining the converter molten steel by LF refining furnace; (4) Continuous casting: after the refining is completed, the temperature is adjusted to non-oxidizing casting to obtain a steel slab; (5) Controlled rolling and controlled cooling: after the steel slab is homogenized, controlled rolling and controlled cooling are performed to obtain a rolled steel plate.

6. The method of claim 5, wherein, In step (1), the pretreatment endpoint S ≤0.004wt%; and / or After the pretreatment, the slag is cleaned, and the bright surface of the molten iron is ≥90%.

7. The method of claim 5, wherein, In step (2), the smelting step endpoint control conditions are: The smelting endpoint C ≤0.06wt%, and the smelting endpoint P ≤0.005wt%; Secondary slag blocking is used, and the slag is strictly controlled to be prevented from falling at the furnace mouth and the tapping hole during tapping; The tapping temperature is greater than 1558℃; The ladle is clean without cold steel, and the red ladle is tapped; and / or The converter tapping uses a slide plate to block the slag, the slag is strictly controlled to be prevented from falling, and the P return amount is ≤0.002wt%.

8. The method of claim 5, wherein, In step (3), the refining step comprises: In the LF refining furnace, the converter molten steel is heated to 1590-1610℃ under argon protection, a top slag is covered, the top slag is melted, and then aluminum particles are used for deoxidation, after the deoxidation, ferrosilicon and ferromanganese are sequentially added, and after the alloy is melted and refined, the refining is performed; Before tapping, ferro-niobium, ferromolybdenum, ferrovanadium, and ferrotitanium alloys are added; The ladle is moved to the wire feeding position, and after a magnesium calcium oxide cored wire is added, soft blowing is performed.

9. The method of claim 6, wherein, The top slag composition is CaO:SiO2:MgO:CaF2=53:27:10:10; The oxygen position in the aluminum particle deoxidation step is controlled to be 45-60 ppm; The refining time after the alloy is melted and refined is 8-10 min; The magnesium calcium oxide cored wire adding speed is 3-4 m / min, and preferably 3.5 m / min; The soft blowing temperature is 1570-1595℃, and preferably 1585℃; and / or The soft blowing time is 10-15 min, and preferably 12 min.

10. The method of claim 5, wherein, In step (4), the pouring temperature is 1535-1545 °C, and the pouring speed is ≤ 1 m / min, preferably 0.7-1 m / min.

11. The method of claim 5, wherein, In step (5), the homogenization temperature is 1150-1200 °C, and the homogenization time is 9-13 min / cm. The rolling start temperature is 1000-1050 °C, the single pass reduction is ≥ 10%, the rolling finish temperature is ≥ 820 °C, and the cumulative reduction is ≥ 35%; and / or The start cooling temperature is 800-820 °C, the finish cooling temperature is 300-400 °C, and the average cooling speed is 7-10 °C / s.

12. The method of claim 5, wherein, The thickness of the rolled steel plate is 10-30 mm.

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