Ultra-high strength steel easy to weld and welding joint

Abstract

The invention relates to ultrahigh-strength steel easy to weld and a welding joint. The ultrahigh-strength steel easy to weld comprises, by mass, 0.10%-0.13% of C, 0.2%-0.5% of Si, 0.8%-1.2% of Mn, 0.03%-0.10% of Al, 0.3%-0.6% of Cr, 0.1%-0.3% of Mo, 0.07%-0.10% of Ti, 0.04%-0.08% of V, smaller than or equal to 0.02% of P, smaller than or equal to 0.005% of S and the balance Fe and other inevitable impurities. According to the invention, the components adopt the design of no Ni, low Mo, no Nb and high Ti, and meet the following relational expressions: Si / Al = 4-8, and C / Ti = 1.1-1.5, the alloying cost is low, and the strengthening and toughening effect is good; two-stage quenching and rapid tempering are adopted, so that the heating, heat preservation and cooling time is effectively shortened, and the production efficiency is high; welding wire components and welding parameters are optimized, the flow ratio of protective gas is controlled to be 1: (10-25), and an obtained welding joint is excellent in performance. The thickness of the steel plate is smaller than or equal to 5 mm, it is guaranteed that the yield strength is larger than or equal to 1150 MPa, the tensile strength is larger than or equal to 1250 MPa, and the ballistic work at the temperature of-40 DEG C is larger than or equal to 70 J, meanwhile, the tensile strength of a steel plate welding joint is larger than or equal to 1150 MPa, and the ballistic work

Description

Technical Field

[0001] The present invention relates to the field of ultra-high strength steel, and specifically provides an ultra-high strength steel and a welded joint that are easy to weld. Background Art

[0002] As equipment and devices develop towards large-scale and lightweight, the strength requirements for steel materials continue to increase. 1100MPa-grade ultra-high-strength steel is mainly used in large or ultra-large equipment. These application scenarios put forward more stringent technical requirements on the strength, toughness and weldability of steel plates. First of all, the quenching and tempering time of traditional ultra-high-strength steel is relatively long. Long-term heat treatment will lead to the growth of steel plate grains and the coarsening of carbides, which will reduce the production efficiency of steel plates while damaging their strength and toughness. Secondly, the high-temperature thermal cycle of steel plate welding is not conducive to the strengthening and toughening of the heat-affected zone. Generally, it is necessary to add high-temperature softening elements such as Cr and V and elements that are beneficial to toughness such as Ni and Mo to improve weldability, thereby pushing up alloying costs. Therefore, it is urgent to explore an innovative solution to reduce alloying costs, improve heat treatment efficiency and optimize welding performance without sacrificing the high strength and toughness of steel plates, so as to solve the bottleneck problem of industry development.

[0003] Patent CN 108315671 B discloses a low yield ratio ultra-high strength steel with a yield strength of 1000MPa and a preparation method thereof. The invention adopts controlled rolling and controlled cooling, offline quenching and low-temperature tempering to obtain a fine lath martensite structure with high dislocation density. However, the quenching and holding time is up to 90 minutes, which makes the grains and carbides in the steel plate structure coarsened, resulting in low strength and toughness. At the same time, the patent does not involve the weldability of the product.

[0004] Patent CN 114196879 B discloses a structural steel plate with a yield strength of 1000 MPa and a manufacturing method thereof. In order to ensure high toughness, the invention adds a large amount of precious metal alloy elements, such as 10-15% Ni and 0.8-1.5% Mo, to the steel plate, which greatly increases the alloying cost and is not conducive to industrial production. At the same time, the patent does not involve the weldability of the product.

[0005] Patent CN 117904408 A discloses a flash heat treatment process for ultra-high-strength steel plates. The invention obtains fine tempered martensite and nano-scale carbides by means of rapid heating, short-term heat preservation and rapid cooling during quenching, which enables the steel to have good strength and toughness without significantly increasing costs. However, the invention heats the steel plate to 600~650℃ and keeps it warm for 20~40min before flash heat treatment, which reduces the production efficiency of flash heat treatment and weakens its efficiency advantage in industrial production. At the same time, the patent does not involve the weldability of the product.

[0006] In the existing technology, there are few patents that adopt a low-cost alloying solution with no Ni, low Mo, no Nb, and high Ti, supplemented by innovative two-stage quenching and rapid tempering technology, to produce ultra-high strength steel and its welded joints with both ultra-high strength and good toughness with lower alloying cost, high heat treatment efficiency and optimized welding process. Summary of the invention

[0007] In view of the shortcomings of the prior art, the present invention provides a method for preparing an ultra-high strength steel and a welded joint that is easy to weld. By adding an appropriate amount of cheap Si, Al, Ti and a small amount of precious metal elements such as Cr and V, and combining an innovative two-stage quenching and rapid tempering technology, fine high-temperature tempered martensite and high-temperature stable and finely dispersed TiC and VC are obtained with low alloying cost and high heat treatment efficiency, thereby improving the strength and toughness of the steel; in conjunction with an optimized welding process, finely dispersed TiN and VN are further introduced into the welded joint structure, pinning the grain boundaries during the welding thermal cycle to inhibit the growth of grains, thereby improving the strength and toughness of the welded joint.

[0008] The present invention provides an ultra-high strength steel and a welded joint that are easy to weld, wherein the ultra-high strength steel has the following composition percentages by mass:

[0009] C: 0.10-0.13%, Si: 0.2-0.5%, preferably 0.25-0.4%, Mn: 0.8-1.2%, Al: 0.03-0.10%, preferably 0.5-0.10%, Cr: 0.3-0.6%, Mo: 0.1-0.3%, Ti: 0.07-0.10%, V: 0.04-0.08%,

[0010] P≤0.02%, S≤0.005%, the rest are Fe and other inevitable impurities.

[0011] Furthermore, a low-cost component design scheme of no Ni, low Cr, no Nb, and high Ti is adopted. Ni, Cr, and Nb are all precious metals, and reducing their content can effectively reduce the alloying cost; the cost of Ti is relatively low. Through a higher Ti addition amount, combined with a reasonable system design, uniformly dispersed Ti carbides are formed and the grains are refined, which can effectively improve the strength and toughness of the steel and compensate for the loss of strength and toughness caused by the no Ni, low Cr, and no Nb design. At the same time, Ti carbides inhibit the coarsening of grains during the welding thermal cycle and improve the welding performance of the steel.

[0012] The present invention controls the content of Si element within the range of 0.2-0.6wt%; when the Si content is greater than or equal to 0.2wt%, it can have a deoxidation effect and can produce solid solution strengthening, but when its content exceeds 0.5wt%, it will cause cold brittleness, which is not conducive to the welding performance of steel.

[0013] The present invention controls the content of Al element in the range of 0.03-0.10wt%; when the Al content is greater than or equal to 0.03wt%, it can have a strong deoxidation and nitrogen fixation effect, but when its content exceeds 0.10wt%, Al oxide inclusion defects are easily generated in the steel.

[0014] The present invention controls the content of the C element within the range of 0.10-0.13wt%; when the C content is greater than or equal to 0.10wt%, the strength of the steel can be effectively improved at a very low cost. However, when the C content exceeds 0.13wt%, the carbon equivalent of the steel will increase, which is not conducive to the welding performance of the steel.

[0015] The present invention controls the content of Ti element within the range of 0.07-0.10wt%; when the Ti content is greater than or equal to 0.07wt%, it tends to combine with C element to form a high-temperature stable fine TiC phase, which will not be significantly dissolved after welding thermal cycle, and can improve the strength of the welded joint. However, when the content exceeds 0.10wt%, it will cause the carbide of Ti to coarsen and reduce the toughness of the steel.

[0016] Preferably, the Si / Al mass ratio and C / Ti mass ratio of the obtained steel plate need to satisfy the following conditions at the same time:

[0017] The present invention accurately controls the Si / Al mass ratio within the range of 4 to 8, that is, Si / Al=4 to 8. When the Si / Al ratio is within the above range, the interface migration rate during the austenite phase transformation in the steel can be synergistically improved, the diffusion rate of the Ti element during the TiC phase precipitation process can be improved, and the precipitation amount and distribution uniformity of the TiC phase can be increased, and the size of the TiC phase can be refined. This is conducive to improving the strengthening effect of the TiC phase and improving the strength of the steel welded joint.

[0018] The present invention accurately controls the C / Ti mass ratio within the range of 1.1 to 1.5, that is, C / Ti=1.1 to 1.5. When the C / Ti ratio is within the above range, it is beneficial to the refinement and homogenization of Ti carbides; when C / Ti<1.1, the Ti content is too low relative to the C content, which is not conducive to the formation of TiC, thereby reducing the strength of the weld joint; when C / Ti>1.5, the Ti content is too high relative to the C content, resulting in coarsening of TiC, thereby reducing the toughness of the weld joint.

[0019] Furthermore, on the premise of meeting the above-mentioned content requirements, the Si / Al mass ratio and the C / Ti mass ratio are further optimized, Si / Al=4~8, preferably 5~6, C / Ti=1.1~1.5, preferably 1.1~1.3.

[0020] The present invention controls the content of V element in the range of 0.04-0.08wt%; when the V content is greater than or equal to 0.04wt%, it is beneficial to precipitate fine dispersed VC and refine the grains, resulting in second phase strengthening and fine grain strengthening. However, V is a precious metal, and when its content exceeds 0.08wt%, the production cost will be increased.

[0021] The present invention controls the content of the Mn element within the range of 0.8-1.2wt%; when the Mn content is greater than or equal to 0.8wt%, it is helpful to produce solid solution strengthening and improve the hardenability of the steel, but when its content exceeds 1.2wt%, it will lead to segregation and the formation of inclusions such as MnS, thereby reducing the toughness of the steel.

[0022] The present invention controls the content of Cr element within the range of 0.3-0.6wt%; when the Cr content is greater than or equal to 0.3wt%, it is beneficial to reduce the phase transition point of steel, produce fine grain strengthening, and improve the hardenability of steel. However, when its content exceeds 0.6wt%, it will lead to the precipitation of coarse grain boundary carbides, reducing the toughness and welding performance of the steel.

[0023] Under the process conditions defined in the present invention, the content of the Mo element is controlled within the range of 0.1-0.3wt%; when the Mo content is greater than or equal to 0.1wt%, it is beneficial to improve the austenite stability and hardenability of the steel, but when its content exceeds 0.6wt%, it will lead to an increase in the carbon equivalent and reduce the welding performance of the steel.

[0024] The present invention discloses an ultra-high strength steel and a welded joint that are easy to weld, which are prepared by the following scheme: a steel plate is smelted according to the designed composition, and the steel plate is rolled and heat-treated to obtain a finished product;

[0025] The rolling includes rough rolling and finish rolling, wherein:

[0026] The starting temperature of rough rolling is ≥1170℃, preferably 1170~1200℃, and the reduction rate of at least one pass is ≥20%; the starting temperature of finishing rolling is ≥1050℃, preferably 1050~1080℃, the final rolling temperature is ≥820℃, preferably 820~850℃, and the reduction rate of the last pass is ≥15%. The reduction rate of at least one pass of rough rolling is ≥20% and the reduction rate of the last pass of finishing rolling is ≥15%, which are both conducive to refining the grain size of the steel plate.

[0027] The steel plate after final rolling is cooled to 500-550℃ at a rate of 60-100℃ / s, coiled and air-cooled to room temperature. After rapid cooling, the steel is coiled at 500-550℃ to obtain a uniform and fine troostite structure with dispersed carbides, which provides a large number of nucleation sites for austenitization in the rapid heat treatment process and provides a microstructural basis for the realization of rapid heat treatment.

[0028] After rolling, the thickness of the steel plate is ≤5mm. The thin plate design enables the ultra-high strength steel plate to significantly reduce the time required to achieve uniform heating and sufficient hardening during heat treatment, making rapid heat treatment possible.

[0029] The rapid heat treatment includes two-stage quenching and rapid tempering.

[0030] The two-stage quenching process rapidly heats the steel plate to 900~960℃ at a rate of 80~120℃ / s, keeps it warm for 3~6min, rapidly cools it to 500~600℃ at a rate of ≥100℃ / s in the first stage, and cools it to room temperature at 10~30℃ / s in the second stage. Among them, rapid heating and short-term heat preservation are conducive to avoiding structural coarsening; in the first stage, the steel plate undergoes phase transformation through extremely fast cooling speed, improves the degree of lattice distortion and carbon supersaturation inside the structure, thereby obtaining a fine and uniform structure, and provides a driving force for the precipitation of nano-carbide phases during tempering, making rapid tempering possible. Reducing the cooling rate in the second stage is conducive to reducing the excessive quenching internal stress caused by ultra-fast cooling in the first stage, which is conducive to the development of the toughness of the steel plate. Rapid tempering is performed after rapid quenching.

[0031] The rapid tempering process heats the steel plate to 500-600°C at a speed of 80-120°C / s, keeps the temperature for 3-6 minutes, and then cools the steel plate to room temperature at a speed of 10-30°C / s. Through ultra-rapid heating, short-time heat preservation and rapid cooling, the tempering time is greatly shortened while ensuring the effective precipitation of the nano-carbide phase, thereby improving the production efficiency.

[0032] The present invention discloses an ultra-high strength steel and a welded joint that are easy to weld. After tempering, the microstructure of the steel is fine tempered martensite + V, Ti carbides + Cr, Mo carbides, the average size of the tempered martensite grains is ≤6μm, and the carbides are evenly distributed and have an average size of ≤10nm.

[0033] The invention discloses an ultra-high strength steel and a welded joint that are easy to weld. After tempering, the yield strength of the steel is ≥1150MPa, the tensile strength is ≥1250MPa, the impact energy at -40°C is ≥70J, and the crack sensitivity coefficient P is cm 0.17~0.25%;

[0034] Among them, P cm =C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0035] An easy-to-weld ultra-high strength steel and a welded joint, wherein the tempered steel is used as the welding object and the welding is performed by gas shielded welding. The welding wire composition used in the welding is, by mass percentage, C: 0.10-0.13%, Si: 0.2-0.5%, Mn: 0.8-1.2%, Al: 0.03-0.10%, Cr: 0.3-0.6%, Mo: 0.1-0.3%, Ti: 0.10-0.15%, V: 0.04-0.08%, and the rest is Fe and other inevitable impurities; the welding current used is 140-180A, the welding voltage is 15-22V, and the welding heat input is ≤6kJ / cm; the shielding gases used are nitrogen and argon, the total gas flow rate is 20-25L / min, and the flow rate ratio of nitrogen to argon is 1:(10-25). In the welding environment, the nitrogen in the protective gas ionizes active N atoms, which combine with the Ti and V atoms in the weld joint to form TiN, thereby improving its strength and toughness matching. When the flow ratio of nitrogen and argon is within the range of 1: (10~25), it is conducive to the formation of fine dispersed TiN and VN in the organization, refines the grains of the weld joint, and improves its strength and toughness matching; when the nitrogen content in the protective gas is too high, the content of ionized active N atoms is too high, causing TiN to coarsen and reducing its beneficial effect; when the nitrogen content in the protective gas is too low, the content of dispersed TiN formed is too small, and precipitation strengthening is weakened.

[0036] The present invention discloses an ultra-high strength steel and a welded joint that are easy to weld, wherein the tensile strength is ≥1150MPa and the impact energy of the heat-affected zone at -20°C is ≥90J. Compared with the prior art, the present invention has low alloying cost and high heat treatment efficiency, and the obtained steel plate has not only excellent strength and toughness, but also excellent welding performance, and has great potential for industrial production.

[0037] The technical solution of the present invention has the following beneficial effects:

[0038] 1. The present invention improves the strength of steel and weld joint strength by using low-cost nano-carbonitrides of Ti element, reduces the addition of precious metal elements Ni, Cr, and Nb, and reduces alloying costs; by precisely controlling the Si / Al ratio and the C / Ti ratio, and satisfying the relationship: Si / Al=4~8, preferably 5~6, C / Ti=1.1~1.5, preferably 1.1~1.3, the grains can be refined and the uniform dispersion distribution of high-temperature stable TiC can be promoted, thereby optimizing the strength and toughness matching of the steel plate.

[0039] 2. Based on the composition design, the present invention combines the appropriate coiling temperature and the innovative rapid heat treatment system to quickly austenitize the steel structure, prevent the growth of austenite grains and the coarsening of carbides. The average size of the tempered martensite grains is ≤6μm, and the average size of the carbides is ≤10nm, which ensures the excellent strength, toughness and welding performance of the steel.

[0040] 3. The present invention optimizes the flow ratio of nitrogen and argon in the welding shielding gas to 1: (10-25), promotes the precipitation of fine dispersed TiN and VN in the weld joint, pins the grain boundaries during the welding thermal cycle to inhibit the growth of grains, produces precipitation strengthening and fine grain strengthening, and improves the strength and toughness of the weld joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 OM photo of the steel plate obtained for Example 1 of the ultra-high strength steel of the present invention;

[0042] Figure 2 The OM photograph of the fusion zone of the steel plate weld joint was obtained for Example 1 of the ultra-high strength steel of the present invention. DETAILED DESCRIPTION

[0043] In order to further illustrate the technical problems, technical solutions and technical effects to be solved by the present invention, a specific description will be given below in conjunction with the accompanying drawings and embodiments. The embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0044] V: 0.04~0.08%, P≤0.02%, S≤0.005%, the rest is Fe and other inevitable impurities;

[0045] Table 1 lists the mass percentages of the main alloying elements in the ultra-high strength steel embodiments and comparative examples of the present technical solution. Elements not listed in Table 1 and their corresponding mass percentages are as follows: Mn is 1.20%, Mo is 0.20wt%, Cr is 0.50%, V is 0.06%, and the balance is Fe and unavoidable impurities; these elements and their contents are consistent in the embodiments of the present invention and the comparative examples.

[0046] The present invention aims at the existing problems and provides an ultra-high strength steel and a welded joint that is easy to weld. The ingot is initially smelted and refined according to the above composition, and then continuously cast to form a cast ingot. Subsequently, the ingot is heated to 1150-1270°C and kept warm for more than 1.5 hours to prepare for the rolling process.

[0047] In view of the existing problems, the present invention provides an ultra-high strength steel and welded joint that are easy to weld. Table 2 lists some parameters of the rapid heat treatment and welding scheme of ultra-high strength steel embodiments and comparative examples. Other parameters are consistent in the embodiments of the present invention and comparative examples. Among them, the starting temperature of rough rolling is about 1180°C, the starting temperature of finishing rolling is about 1080°C, the final rolling temperature of finishing rolling is about 830°C, and the reduction rate of the last pass of finishing rolling is 10~18% (see Table 2 for details); the cooling process is controlled to cool the steel plate to about 530°C at a rate of 80°C / s, coil it and air-cool it to room temperature; the rapid quenching process is to quickly heat the steel plate to 930°C at a rate of 100°C / s, keep it warm for 2~6 minutes (see Table 2 for details), and then start two-stage quenching, first quickly cool the steel plate to 520°C at a cooling rate of 80°C / s, and then The steel plate is cooled to room temperature at a cooling rate of 15°C / s; the steel plate is heated to about 530°C at a rate of 80°C / s in the rapid tempering process, and then cooled to room temperature at a rate of 15°C / s after being kept warm for 1~6min (see Table 2 for details); the welding current used for steel plate welding is about 160A, the welding voltage is about 18V, and the welding heat input is 4~10kJ / cm (see Table 2 for details); the shielding gas for steel plate welding is nitrogen and argon, and the total gas flow rate is about 25L / min, wherein the flow ratio of nitrogen and argon is 1:(8~25) (see Table 2 for details). Finally, the thickness of the finished steel plate is 4mm. The performance indicators of the steel plate measured by the present invention are shown in Table 3.

[0048]

[0049]

[0050]

[0051] It can be seen from Tables 1-3 that the present invention obtains an ultra-high strength steel and a welded joint that are easy to weld under the synergistic effect of components and process parameters.

Claims

1. An easily weldable ultra-high strength steel and a welded joint, characterized in that: The following components are included in mass percentage: C: 0.10~0.13%, Si: 0.2~0.5%, Mn: 0.8~1.2%, Al: 0.03~0.10%, Cr: 0.3~0.6%, Mo: 0.1~0.3%, Ti: 0.07~0.10%, V: 0.04~0.08%, P≤0.02%, S≤0.005%, and the rest are Fe and other inevitable impurities.

2. The easily weldable ultra-high strength steel and welded joint according to claim 1, characterized in that: The Si / Al mass ratio and C / Ti mass ratio of the obtained steel satisfy the following relationship at the same time: Si / Al=4~8, C / Ti=1.1~1.

5.

3. The easily weldable ultra-high strength steel and welded joint according to claim 2, characterized in that: The Si / Al mass ratio and C / Ti mass ratio of the obtained steel satisfy the following relationships at the same time: Si / Al=5~6, C / Ti=1.1~1.

3.

4. The easily weldable ultra-high strength steel and welded joint according to claim 1, characterized in that: The production of the ultra-high strength steel and its welded joints includes raw material proportioning, primary refining and refining according to the mass percentage of the ultra-high strength steel, rapid composition analysis and composition adjustment before the furnace, and then continuous casting, controlled rolling, controlled cooling, rapid heat treatment and welding; The controlled rolling includes rough rolling and finish rolling, wherein: The starting temperature of rough rolling is ≥1170℃, the reduction rate of at least one pass is ≥20%, the starting temperature of finishing rolling is ≥1050℃, the final rolling temperature is ≥820℃, and the reduction rate of the last pass is ≥15%. The steel plate after final rolling is cooled to 500-550°C at a rate of 60-100°C / s, coiled and air-cooled to room temperature. The rapid heat treatment includes two-stage quenching and rapid tempering, wherein: The heating rate of two-stage quenching is 80~120℃ / s, the heating temperature is 900~960℃, the holding time is 3~6min, the first stage is rapidly cooled to 500~600℃ at a rate of ≥100℃ / s, and the second stage is cooled to room temperature at a rate of 10~30℃ / s. The heating rate of rapid tempering is 80~120℃ / s, the heating temperature is 500~600℃, the holding time is 3~6min, and the cooling rate is 10~30℃ / s.

5. The easily weldable ultra-high strength steel and welded joint according to claims 1 to 4, characterized in that: The thickness of the steel material is ≤5mm.

6. The easily weldable ultra-high strength steel and welded joint according to claim 4, characterized in that: After tempering, the microstructure of the steel includes tempered martensite and carbides, wherein the carbides are evenly distributed and the average size is ≤10nm; the average grain size of the tempered martensite is ≤6μm.

7. The easily weldable ultra-high strength steel and welded joint according to claim 3, characterized in that: After tempering, the yield strength of the steel is ≥1150MPa, the tensile strength is ≥1250MPa, the impact energy at -40℃ is ≥70J, and the crack sensitivity coefficient P cm It is 0.17~0.25%.

8. An easily weldable ultra-high strength steel and welded joint according to claims 1-6, characterized in that: The tempered steel is taken as the welding object and the welding is carried out by gas shielded welding. The composition of the welding wire used in welding is as follows by mass percentage: C: 0.10~0.13%, Si: 0.2~0.5%, Mn: 0.8~1.2%, Al: 0.03~0.10%, Cr: 0.3~0.6%, Mo: 0.1~0.3%, Ti: 0.10~0.15%, V: 0.04~0.08%, and the rest is Fe and other inevitable impurities.

9. The easily weldable ultra-high strength steel and welded joint according to claim 8, characterized in that: The welding current used is 140~180A, the welding voltage is 15~22V, the welding heat input is ≤6kJ / cm, the shielding gas used is nitrogen and argon, the total gas flow rate is 20~25L / min, and the flow ratio of nitrogen to argon is 1:(10~25).

10. The easily weldable ultra-high strength steel and welded joint according to claim 8, characterized in that: The tensile strength of the welded joint is ≥1150MPa and the impact energy of the heat affected zone at -20℃ is ≥90J.

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