CN100447285C - Soft magnetic structural steel plate excellent in weldability and manufacturing method thereof - Google Patents

Abstract

Soft magnetic structural steel plate with excellent welding processability, using ultra-low C, 1.45%~1.65% Si content, medium and low Mn content, Ni and Cr alloying, adding 0.008%~0.016% Ti, and controlling N content below 0.0055% , and use Ca or rare earth element REM treatment and other technical means to optimize the recrystallization controlled rolling and accelerated cooling process and subsequent slow cooling process, so that the grain size of the finished soft magnetic structural steel plate is 10-30 μm, and excellent mechanical properties and electromagnetic properties are obtained. performance and weldability, especially improving the crack-like grain boundary cracks in the steel plate manufacturing and welding process, that is, the so-called copper brittleness, which further improves the electromagnetic properties of the steel plate; at the same time, this soft magnetic structural steel plate also has excellent low temperature impact toughness, To ensure the safety of the soft magnetic structural steel plate used in severe cold regions (below -30°C), the production process is simplified, the production cost is reduced, and the composition design system of the soft magnetic structural steel is further expanded.

Description

Soft magnetic structural steel plate excellent in weldability and manufacturing method thereof

technical field

The invention relates to a kind of maglev track beam special, which has high strength, excellent electromagnetic properties, low temperature toughness, good weldability, high magnetic flux density, high resistivity and excellent anti-magnetic timeliness, Atmospheric corrosion resistance, especially soft magnetic structural steel plates with excellent on-site welding processability and high electromagnetic properties, are especially suitable for use as side guide plates on maglev train track beams and other atmospheric corrosion-resistant engineering structural parts with electromagnetic performance requirements.

Background technique

Ordinary soft magnetic structural steel plates for magnetic levitation first appeared in Germany. German Thyssen Steel AG applied for patents US 4350525 "Magneticsuspension railroad parts" and US Patent 6287395 "High-energy weldable soft magnetic parts" in the United States in 1982 and 1999, respectively. Steel and its application on magnetic levitation rail components”, reveals the invention process of soft magnetic structural steel plate, US 4350525 and US Patent 6287395 expound the role of each alloy element in soft magnetic structural steel. However, the soft magnetic structural steel plate produced according to this patented technology has low physical quality and low impact toughness, especially low temperature impact toughness below 0°C, and the transverse impact energy (Akv) at 0°C is between 12J and 57J. , which may bring hidden dangers to train operation; at the same time, the magnetic induction is not high, B40 is only about 1.60T, especially under low magnetic field, B3 is only 0.60T, the electromagnetic conversion efficiency is low, the ineffective loss is large, and the power loss is large. Adapt to increasingly stringent environmental protection requirements.

Baosteel applied for the Chinese patent ZL01126937.5 "High-performance soft magnetic steel for maglev trains" in 2001. The performance of the soft magnetic structural steel plate used for maglev track beams has been greatly improved compared with the German invention, but its magnetic flux density and resistivity And low temperature impact toughness is still not high enough (B40 is about 1.60T~1.61T, B3 is about 0.9T~1.00T, resistivity ρ is about 0.39~0.40μΩm, 0℃ Akv≥140J), especially when the impact toughness is lower than -10℃ , cannot guarantee ≥27J, and the weldability is also poor. The welding line energy can only be controlled at ≤12kJ/cm to ensure the heat-affected zone (HAZ) 0℃ Akv≥27J. Therefore, this kind of soft magnetic structural steel plate is used in the colder north There are still major security risks.

The Chinese patent ZL02136192.4 applied by Baosteel in 2002, the performance of the soft magnetic structural steel plate used for the maglev track beam is greatly improved compared with the Chinese patent ZL01126937.5 applied by Baosteel in 2001, and its magnetic flux density and resistivity have a large The range is improved (B40 is about 1.65T~1.68T, B3 is about 1.15T~1.30T, and the resistivity ρ is about 0.42~0.45μΩm). Although the low temperature impact toughness has been greatly improved, Akv (-20°C) > 150J, the weldability is also good. Greatly improved, larger heat input welding can be used, simulated welding heat-affected zone (HAZ) -20°C Akv>27J (simulation parameters: peak temperature is 1350°C, t _8/5 is 50 seconds, single cycle), but- The impact toughness Akv of the base metal and the welded joint below 20°C fluctuates greatly, especially the local brittle zone (LBZ) in the welding heat-affected zone, and the Akv below -20°C cannot meet ≥27J. At the same time, the process is relatively complicated and the production cost is also high. Higher, off-line normalization heat treatment is required.

The Chinese patent ZL03116097.2 applied by Baosteel in 2003 not only has a simple manufacturing process, but also lower production costs, without the need for off-line normalization heat treatment, and the performance of the steel plate is also significantly improved compared with the Chinese patent ZL02136192.4 applied by Baosteel in 2002. Improvement, especially low-temperature impact toughness has been greatly improved, Akv (-30°C) ≥ 100J, weldability has also been greatly improved, and larger heat input welding can be used to simulate welding heat-affected zone (HAZ) -30°C Akv ≥ 21J ( Simulation parameters: peak temperature is 1350°C, t _8/5 is 50 seconds, single cycle), but the impact toughness Akv of the base metal and welded joint below -30°C fluctuates greatly, especially the local brittle zone in the welding heat-affected zone (LBZ), the Akv below -30°C cannot always meet ≥27J, especially the study found that the alloy system with high Si content determines that the low temperature toughness of the soft magnetic structural steel plate cannot stably meet the use safety in cold regions below -30°C.

The Chinese patent ZL200410017999.7 applied by Baosteel in 2004 uses extremely low C, medium Si content, high Als, high Mn, medium Cr content, ferromagnetic element Ni alloying, Ca or rare earth element REM treatment, and uses controlled Ti /N between 2.5 and 3.2 and other technical means, optimize the recrystallization controlled rolling and accelerated cooling process〖RCR+ACC or RCR+IDQ(InterruptedDirect Quenching)〗and the subsequent slow cooling process, so that the grain size of the finished soft magnetic structural steel plate At 10-30 μm, excellent mechanical properties, electromagnetic properties and weldability are obtained. Akv≥150J at -40°C, larger heat input welding can be used to simulate welding heat-affected zone (HAZ) -40°C Akv≥50J (simulation parameters: peak temperature is 1350°C, t _8/5 is 50 seconds, single cycle ), which meets the safety of use in cold regions below -30°C.

However, although the aforementioned Baosteel soft magnetic structural steel has high strength, low temperature toughness, magnetic flux density, resistivity and excellent mechanical properties of welded joints, that is, high joint strength, low temperature toughness (weld metal, fusion line and heat-affected zone). However, the on-site welding process is not stable. The main reason is that the steel plate contains a high amount of Cu (~0.35%). When the on-site welding operation is improper, it is easy to form cracks in the grain boundary near the fusion line, resulting in component repair. Even scrapping will not only affect the project progress and cost, but more importantly, the crack-like grain boundary cracks near the fusion line will affect the safety of the engineering structure; During the heating process of the slab, the cracked grain boundary cracks on the surface are called copper brittleness, which affects the progress of the steel plate production and manufacturing process. Cycle and delivery schedule, which in turn affects the progress of engineering construction; thirdly, when there is a high Cu content in the steel, the magnetic flux density of the steel plate is reduced, the coercive force is increased, and the electromagnetic properties of the steel plate are reduced.

Contents of the invention

The purpose of the present invention is to design a brand-new composition system, give full play to the potential of the composition design itself, and realize the organic unity of high strength, high toughness, good weldability and excellent electromagnetic properties of the soft magnetic structural steel plate; more importantly, Completely eliminate crack-like grain boundary cracks that may appear during the production and welding of soft magnetic structural steel plates, improve the on-site welding process and process adaptability of steel plates, and further improve the electromagnetic properties of steel plates. That is, adopt ultra-low C, 1.45%-1.65% Si content, medium-low Mn content, Ni and Cr alloying, add 0.008%-0.016% Ti, control N content below 0.0055%, and use Ca or rare earth element REM treatment And other technical means, optimize the recrystallization controlled rolling and accelerated cooling process〖RCR+ACC or RCR+IDQ(Interrupted DirectQuenching)〗and the subsequent slow cooling process, so that the grain size of the finished soft magnetic structural steel plate is 10-30μm, and excellent Mechanical properties, electromagnetic properties and weldability, especially improve the crack-like grain boundary cracks in the steel plate manufacturing and welding process, which is the so-called copper brittleness, and further improve the electromagnetic properties of the steel plate.

The technical solution of the present invention is that the soft magnetic structural steel plate with excellent welding processability has the composition weight percentage as follows:

C: 0.030% to 0.060%

Si: 1.45% to 1.65%

Mn: 0.40% to 0.80%

P: ≤0.015%

S: ≤0.004%

Als: 0.040% to 0.060%

Cr: 0.40% to 0.60%

Ti: 0.008% to 0.016%

N: ≤0.0055%

Ni: 0.70～1.00%

Ca: 0.0010%～0.0060% or REM0.010%～0.040%

The remainder is iron and unavoidable inclusions;

And, Pcm≤0.20%, Pcm=wt%C+wt%Si/30+(wt%Mn+wt%Cu+wt%Cr)/20+wt%Ni/60+wt%Mo/15+wt%V /10+5wt%B.

Further, the composition weight percentage of the soft magnetic structural steel plate with excellent welding processability of the present invention can also be:

C: 0.040% to 0.050%

Si: 1.50% to 1.60%

Mn: 0.50% to 0.70%

P: ≤0.013%

S: ≤0.003%

Als: 0.040% to 0.060%

Cr: 0.45% to 0.55%

Ti: 0.010% to 0.014%

N: ≤0.0045%

Ni: 0.80-0.90%

Ca: 0.0020%～0.0050% or REM0.02%～0.03%

The remainder is iron and unavoidable inclusions;

And, Pcm≤0.18%.

The structure of the above-mentioned soft magnetic structural steel plate is an equiaxed ferrite grain of 10 μm to 30 μm.

The manufacturing method of the soft magnetic structural steel plate with excellent welding manufacturability of the present invention comprises the following steps:

a) Casting, low temperature casting, casting temperature ≤ 1560°C;

b) The heating of the slab adopts ultra-low temperature, and the heating temperature of the slab is controlled between 1000°C and 1150°C;

c) Rolling, within the complete recrystallization temperature range, continuous rolling is carried out with a large rolling pass reduction rate to ensure complete recrystallization of the deformed metal. The rolling pass reduction rate is ≥ 12%, and the total rolling reduction rate ≥70%, control the finish rolling temperature at 780~880℃;

d) The transfer time from the end of rolling to the start of accelerated cooling should be controlled as short as possible, try to control it within 30 seconds, and at the beginning of accelerated cooling, the temperature of the steel plate must be above Ar ₃ point, with a rate of ≥10℃/ s Cooling rate to cool below 550°C;

e) Subsequently, the steel plates are stacked and slowly cooled to room temperature or slowly cooled to room temperature; or directly quenched to the quenching stop temperature after rolling. Slow cooling in stacks or slow cooling pits to below 200°C, then naturally air-cooled to room temperature; the slow cooling process of steel plates is that the slow cooling time above 350°C should not be less than 24 hours.

Wherein, the casting process in step a) adopts a continuous casting process; the casting temperature is between 1525°C and 1550°C.

Step b) heating the slab The heating temperature is controlled between 1050°C and 1100°C.

Step c) continuous rolling, the reduction rate of each rolling pass is ≥ 15%, and the total reduction rate of hot rolling is ≥ 80%.

Step d) The transfer time from the end of rolling to the start of accelerated cooling is controlled within 15 seconds; and when the accelerated cooling starts, the temperature of the steel plate must be above the Ar ₃ point and cooled to 450°C at a cooling rate of ≥15°C/s ~550°C.

Step e) After rolling, it is directly quenched to the quenching stop temperature, and the QST is also controlled between 450°C and 550°C.

The technical problem to be solved by the present invention is to completely eliminate the crack-like grain boundary cracks that may appear in the production and welding process of the soft magnetic structural steel plate, improve the on-site welding process and process adaptability of the steel plate, and further improve the electromagnetic properties of the steel plate ; At the same time, the newly developed soft magnetic structural steel plate should have higher strength, high low temperature impact toughness, excellent electromagnetic properties and weldability, to ensure the safety of soft magnetic structural steel plate used in severe cold regions (below -30°C). At the same time, the production process is simplified, the production cost is reduced, and the composition design system of soft magnetic structural steel is further expanded to maintain Baosteel's international leading position in the manufacturing technology of soft magnetic structural steel plate.

Carbon: It is well known that carbon has a great influence on the electromagnetic properties, low-temperature impact toughness and weldability of soft magnetic structural steel. From the perspective of improving the electromagnetic properties, low-temperature impact toughness and weldability of steel, it is better to hope that the C content in steel is relatively low; but from The strength of soft magnetic structural steel, more importantly, from the perspective of microstructure control in the hot rolling process and normalizing process, the C content should not be too low, because the Si content in soft magnetic structural steel is relatively high, and the C content is too low (<0.03%) Ac ₁ , Ac ₃ , Ar ₁ , and Ar ₃ are high, which brings big problems to the uniform and refined structure of hot rolling and normalizing, and it is easy to form mixed crystal structure, resulting in low low temperature impact toughness and welding heat of soft magnetic structural steel. Low-temperature impact toughness in the affected area deteriorates; secondly, too low C will lead to faster grain growth, resulting in coarsening of the structure and deteriorating low-temperature impact toughness; at the same time, too low C content is not good for atmospheric corrosion resistance, especially salt spray corrosion. At the same time, because the Si content in the soft magnetic structural steel is very high, it strongly inhibits the precipitation of carbides and promotes the formation of MA, so the C content in the steel cannot be higher than 0.06%. When the C content in the steel is higher than 0.06%, the welding heat-affected zone A large number of (HAZ) MA islands are formed, seriously deteriorating the impact toughness of HAZ, and when the C content in the steel is high, the electromagnetic properties of the steel plate will also be greatly degraded. Based on the above factors and considering that the maximum solid solubility of C in ferrite is about 0.02%, the content of C is controlled between 0.030% and 0.060%.

Silicon: Si in steel can increase the resistivity and magnetic permeability of steel, reduce magnetostriction, eddy current loss and hysteresis loss, adding a certain amount of Si to soft magnetic structural steel can greatly improve the electromagnetic properties and resistance of steel rate, and too low silicon (<1.40%) is not good for reducing magnetostriction, eddy current loss and hysteresis loss; but because Si is a strong ferrite stabilizing element, adding too much Si (>1.65%) will not only The magnetic flux density is reduced, resulting in high Ac ₁ , Ac ₃ , Ar ₁ , and Ar ₃ , which brings big problems to the uniform and refined structure of hot rolling and normalizing, and it is easy to form mixed crystal structure, resulting in low temperature of soft magnetic structural steel. Low impact toughness and low-temperature impact toughness deterioration in the welding heat-affected zone; secondly, Si is an brittle element in steel, excessive alloying not only causes great brittleness to the steel itself, but also seriously damages the weldability of the steel. In addition, Si can inhibit C precipitates from austenite and ferrite, improves the hardenability of steel, and promotes the formation of A/M islands. Based on the above factors, the Si content in the steel is controlled between 1.45% and 1.65%.

Manganese: As an alloying element in soft magnetic structural steel, in addition to increasing its resistivity, strength and toughness, Mn also has the ability to expand the austenite phase region, reduce the temperature of Ac ₁ , Ac ₃ , Ar ₁ , and Ar ₃ points, and refine Ferrite grain effect; but under the condition of higher Ni content (~1.00% Ni), adding too much Mn (>0.80%) will reduce the magnetic flux density of soft magnetic structural steel and improve the hardenability of soft magnetic structural steel , affect the weldability of soft magnetic structural steel, especially when welding with small input energy, it is easy to form brittle and hard structures such as martensite. Considering the influence of the above factors, the Mn content is controlled between 0.40% and 0.80%.

Phosphorus: P, as a harmful inclusion in steel, has a huge detrimental effect on the electromagnetic properties and mechanical properties of soft magnetic structural steel, especially low-temperature impact toughness and weldability. Theoretically, the lower the requirement, the better. The cost and the logistics of the steelmaking plant are smooth, and the P content is required to be controlled at ≤0.015%.

Sulfur: S, as a harmful inclusion in steel, has a great damage effect on the electromagnetic properties of soft magnetic structural steel. More importantly, S combines with Mn in steel to form MnS inclusions. In the hot rolling process, the plasticity of MnS makes MnS extends along the rolling direction and forms a MnS inclusion band along the rolling direction, which seriously damages the transverse impact toughness, Z-direction performance and weldability of the steel plate. At the same time, S is also the main element that produces hot brittleness during hot rolling. Theoretically, the lower the requirement, the better, but considering the steelmaking conditions, steelmaking cost and the principle of smooth logistics of the steelmaking plant, the S content is required to be controlled at ≤0.004%.

Chromium: Cr plays the same role as Cu. Adding a certain amount of Cr element to soft magnetic structural steel can also improve the atmospheric corrosion resistance of soft magnetic structural steel. Adding too little Cr is not enough to improve the atmospheric corrosion resistance of soft magnetic structural steel. If too much Cr (>0.60%) is added, it will seriously damage the electromagnetic properties and low temperature toughness of the steel. More importantly, the Cr content in the steel is too high When , it promotes the growth of upper bainite Bu and the formation of M-A islands in the welded HAZ, and reduces the low-temperature impact toughness of the HAZ. In addition, Cr is a relatively expensive element. Therefore, starting from the comprehensive consideration of the composition system of soft magnetic structural steel, the optimal Cr content is controlled at 0.40% to 0.60%.

Nickel: Ni is the only element that can improve the strength, low temperature toughness and weldability of steel at the same time; secondly, Ni is also a ferromagnetic element, and Fe-Ni alloy is a soft magnetic material with excellent performance. Adding Ni not only will not reduce the electromagnetic properties of the material, but will further improve the electromagnetic properties; again, when more than 0.70% Ni element is added to the steel, excellent atmospheric corrosion resistance can be obtained; therefore, theoretically speaking, the Ni content in the steel Within a certain range (≤1.20%), the higher the better, but Ni is a very precious element. Considering the performance/price ratio, the appropriate addition amount is 0.70%-1.00%.

Titanium: When a small amount of Ti is added to the steel, Ti combines with N in the steel to form TiN particles with high stability, which can inhibit the growth of austenite grains in the welding heat-affected zone (HAZ) and change the secondary phase transformation products, and improve the softness. Weldability of magnetic structural steels. If the Ti content is too small (<0.008%), the number of TiN particles formed is insufficient, which is not enough to inhibit the austenite grain growth of the HAZ and change the secondary phase transformation products to improve the low temperature toughness of the HAZ. When the Ti content is too much (>0.016%), large-sized TiN particles are precipitated during the solidification process of molten steel. Such large-sized TiN particles not only cannot inhibit the growth of austenite grains in the HAZ, but become the source of crack initiation. Nucleation point; In addition, too much TiN will lead to a decrease in the electromagnetic properties and an increase in the coercive force of the soft magnetic structural steel. Therefore, from the perspective of improving the weldability of the soft magnetic structural steel plate without damaging the electromagnetic properties of the soft magnetic structural steel, the Ti content control range is 0.008% to 0.016%.

Nitrogen: The control range of N corresponds to the control range of Ti, and its fundamental starting point is to inhibit the precipitation of TiN particles from molten steel during the solidification process of molten steel or molten steel (TiN particles precipitated from molten steel are not only rare in number, but more importantly The size is very coarse, such coarse TiN particles not only cannot inhibit the growth of austenite grains in the HAZ, but become the starting point of crack initiation); but should be controlled to precipitate from the solid phase after the molten steel is completely solidified. In this way, the precipitated TiN particles are not only fine but also dispersed, which can effectively refine the grains and improve the weldability of the soft magnetic structural steel. According to the above analysis, the precipitation start temperature of TiN particles should be lower than 1450°C, and according to log[%Ti][%N]=4.72-16192/T, the N content should be controlled below 0.0060%. However, when the N content is high, the free [N] content in the steel increases, and the soft magnetic structural steel may have severe magnetic aging during later use. It will damage the low-temperature impact toughness of HAZ and deteriorate the weldability of soft magnetic structural steel. Therefore, the N content is controlled at ≤0.0055%.

Ca or REM: Ca or REM treatment of steel, on the one hand, can further purify the molten steel, on the other hand, denature the sulfide in the steel to make it into non-deformable, stable and fine spherical sulfide, and improve the soft magnetic field. Improve the electromagnetic properties of steel, inhibit the hot brittleness of S, improve the impact toughness and Z-direction performance of soft magnetic steel, and improve the anisotropy of impact toughness of soft magnetic structural steel. The amount of Ca or REM added depends on the S content in the steel. If the amount of Ca or REM added is too low, the treatment effect will not be great; if the amount of Ca or REM added is too high, the size of Ca or REM (O, S) will be too large. , the brittleness also increases, which can become the starting point of fracture cracks, which reduces the low-temperature toughness of steel, and at the same time reduces the purity of steel and pollutes molten steel. Generally control the content of Ca or REM according to ESSP=(wt%Ca)[1-124(wt%O)]/1.25(wt%S), where ESSP is the sulfide inclusion shape control index, and the value range is between 0.5 and 5 It is appropriate, so the control range of Ca or REM content is 0.0010%≤Ca≤0.0060% (0.010%≤REM≤0.040%).

Acid-soluble aluminum Als: Als in soft magnetic structural steel can fix the free [N] in the steel, prevent the formation of iron nitrides from damaging the magnetism and prevent the free [N] from producing magnetic aging under the action of temperature and stress, and has the ability to improve the softness. The electrical resistivity of the magnetic steel plate, while reducing the free [N] of the welding heat-affected zone (HAZ), improves the low-temperature impact toughness of the HAZ; but adding excessive Als to the steel will not only reduce the magnetic flux density of the steel, but also form in the steel A large number of dispersed needle-like Al ₂ O ₃ inclusions damage the electromagnetic properties, low-temperature impact toughness and weldability of the steel. More importantly, Al is a strong deoxidizer. Under the condition that Als can fix the free N in the steel, that is, Al ≥10 (Ntotal-0.292Ti), the Als content should be reduced as much as possible. According to the composition system of soft magnetic structural steel, the optimal Als content is controlled between 0.040% and 0.060%.

Pcm≤0.20%, preferably Pcm≤0.18%, wherein Pcm=wt%C+wt%Si/30+(wt%Mn+wt%Cu+wt%Cr)/20+wt%Ni/60+wt%Mo/ 15+wt%V/10+5wt%B.

The present invention also has a related innovation point:

Copper, as we all know, soft magnetic structural steel is used as the side guide plate of the maglev track beam, so it is required to have excellent atmospheric corrosion resistance. Cu is a very effective weather-resistant element. Adding a certain amount of Cu to the soft magnetic structural steel can be very Greatly improve its atmospheric corrosion resistance; but the addition of Cu will bring the following problems:

First of all, Cu tends to form crack-like grain boundary cracks near the welding fusion line, causing components to be repaired or even scrapped, which not only affects the progress and cost of the project, but more importantly, the crack-like grain boundary cracks near the welding fusion line affect the engineering structure. safety;

Secondly, when there is a high Cu content in the steel (~0.35%), it is easy to cause surface cracks in the process of slab casting, slab heating, and steel plate rolling, which is the so-called copper brittleness, which affects the production of steel plates. The progress of the manufacturing process will cause the slab or steel plate to be scrapped in severe cases, which not only increases the manufacturing cost of the steel plate, but more importantly affects the steel plate manufacturing cycle and delivery schedule, thereby affecting the construction progress of the project;

Thirdly, when there is a high Cu content in the steel, the magnetic flux density of the steel plate is reduced, the coercive force is increased and the electromagnetic properties of the steel plate are reduced;

In addition, the Cu content is high, and the oxide cortex is dense when heated, which is difficult to remove. It is more difficult to remove it by high-pressure water, and if it cannot be removed, it will even cause surface defects on the steel plate.

Therefore, the steel of the present invention does not add Cu element.

Microstructure type of soft magnetic structural steel (base steel plate): uniform and fine equiaxed ferrite grains. The soft magnetic structural steel plates used on the track beams of high-speed maglev trains require not only excellent mechanical properties, especially low-temperature impact toughness, but also good electromagnetic properties. Therefore, the microstructure of the soft magnetic structural steel sheet is different from the usual welded structural steel sheet, and also different from the ordinary non-oriented electrical steel sheet. For ordinary welded structural steel plates, in order to improve the low-temperature impact toughness of the base metal and the welding heat-affected zone, it is beneficial to form extremely fine ferrite grains (<10 μm) or fine low-carbon bainite structures; and for For ordinary non-oriented electrical steel sheets, in order to improve their electromagnetic properties, it is advantageous to form uniform and coarse equiaxed ferrite (>100 μm). For soft magnetic structural steel plate, if its microstructure is extremely fine ferrite grains (<10μm) or fine low carbon bainite structure, although its mechanical properties, especially low temperature impact toughness can be improved, the electromagnetic properties Serious deterioration, especially the magnetic flux density under low magnetic field is greatly reduced, because the ferrite grain size is too small (<10μm), the grain boundary area increases, and the role of grain boundary pinning magnetic domain wall movement is strengthened. For small low carbon In the bainite structure, in addition to irregular grain boundaries pinning the magnetic domain wall movement, more importantly, a large number of crystal defects in the ferrite lath, such as dislocations and subgrain boundaries, also seriously hinder the magnetic domain wall movement, seriously damage the electromagnetic properties of the soft magnetic structural key; if the grain size of the soft magnetic structural steel plate is too coarse (> 30 μm), although the electromagnetic properties of the soft magnetic structural steel plate, especially the magnetic flux density under low magnetic field, can be improved, its low temperature impact toughness, In particular, the low temperature impact toughness of the welded heat-affected zone deteriorates sharply. Therefore, the microstructure of the soft magnetic structural steel plate should be uniform and fine equiaxed ferrite grains, and the ferrite grain size is between 10 μm and 30 μm.

According to the present invention, the molybdenum structure of the soft magnetic structure with excellent low-temperature impact toughness (below -30°C) is uniform and fine (10 μm-30 μm) equiaxed ferrite grains.

The casting process of the present invention is recommended to adopt the continuous casting process. The continuous casting process focuses on controlling the casting temperature and the solidification speed of the steel liquid. The casting temperature is ≤ 1560 ° C, preferably between 1525 ° C and 1550 ° C, and the low temperature casting method is better. The solid velocity of the molten steel from the liquidus line to the solidus line is one of the key processes of the present invention, which must be strictly controlled. Under the condition that no cracks occur in the slab, the faster the solid velocity, the better.

Ultra-low temperature slab heating is adopted, and the slab heating temperature is controlled between 1000°C and 1150°C, preferably between 1050°C and 1100°C, to ensure that the austenite grain size of the original slab is uniform and fine.

In the complete recrystallization temperature range, rapid continuous rolling is carried out with a large rolling pass reduction rate to ensure complete recrystallization of the deformed metal. For this reason, the rolling pass reduction rate is ≥ 12%, preferably ≥ 15%. The total rolling reduction rate is ≥70%, preferably ≥80%; the final rolling temperature is controlled above Ar ₃ +30°C, where Ar ₃ (°C)=910-310[%C]-80[%Mn]-20 [%Cu]-15[%Cr]-55[%Ni]-80[%Mo]-0.35 (t-8), where t is the plate thickness (mm).

The transfer time from the end of rolling to the start of accelerated cooling should be controlled as short as possible, try to control it within 30 seconds, preferably within 15 seconds, and it is particularly important that the steel plate temperature must be at the beginning of accelerated cooling. Above the Ar ₃ point, cool at a cooling rate of ≥10°C/s, preferably ≥15°C/s to below 550°C, preferably between 450°C and 550°C, and then slowly cool by stacking steel plates or slowly cooling in a slow cooling pit to room temperature; or carry out direct quenching after rolling (DQ-Direct Quenching) to the quenching stop temperature (QST-Quenching Stop Temperature), QST is also controlled below 550°C, preferably between 450°C and 550°C, and then the steel plates are stacked After slow cooling or slow cooling in slow cooling pits to below 200°C, naturally air cool to room temperature; the slow cooling process of steel plates is that the slow cooling time above 350°C shall not be less than 24 hours; slow cooling by stacking or slow cooling pits The purpose is to eliminate the internal stress formed inside the steel plate by accelerated cooling, improve the electromagnetic properties and remove H from the steel plate.

Beneficial effects of the present invention

By designing a brand-new composition system, the present invention gives full play to the potential of the composition design itself, and realizes the organic unity of high strength, high toughness, good weldability and excellent electromagnetic properties of the soft magnetic structural steel plate, and more importantly, completely eliminates the soft Crack-like grain boundary cracks that may appear during the production and welding of magnetic structural steel plates can improve the on-site welding process and process adaptability of steel plates, and further improve the electromagnetic properties of steel plates. That is, adopt ultra-low C, 1.45%-1.65% Si content, medium-low Mn content, Ni and Cr alloying, add 0.008%-0.016% Ti, control N content below 0.0055%, and use Ca or rare earth element REM treatment and other technical means, optimize the recrystallization controlled rolling and accelerated cooling process [RCR+ACC or RCR+IDQ (Interrupted Direct Quenching)] and the subsequent slow cooling process, so that the grain size of the finished soft magnetic structural steel plate is 10-30 μm, and excellent The mechanical properties, electromagnetic properties and weldability of the steel plate are especially improved, so-called copper brittleness, which is the so-called copper brittleness, in the process of steel plate manufacturing and welding process, which further improves the electromagnetic properties of the steel plate; at the same time, the soft magnetic structural steel plate also has excellent Excellent low-temperature impact toughness ensures the safety of soft magnetic structural steel plates used in severe cold regions (below -30°C), simplifies the production process, reduces production costs, further expands the composition design system of soft magnetic structural steels, and maintains Baosteel’s The international leading position in the manufacturing technology of soft magnetic structural steel plate.

Description of drawings

Fig. 1 is a schematic diagram of the microstructure of Steel Example 3 of the present invention.

Detailed ways

Embodiment: The manufacturing process of the soft magnetic structural steel plate of the present invention is as follows: smelting in a vacuum induction furnace in a laboratory, and casting starts immediately after the composition of the molten steel reaches the target composition, and the casting temperature of the molten steel is 1525°C to 1550°C. The soaking time of the slab in the heating furnace = plate thickness (mm) × (0.5 ~ 1.0) min/mm, the soaking temperature is 1050°C ~ 1100°C, the hot rolling adopts 9-pass rolling process, and the pass reduction rate Between 13% and 18%, the cumulative reduction rate is 70% to 90%, the final rolling temperature is 800°C to 850°C, the transfer time between the end of final rolling and the start of accelerated cooling is 15s to 30s, and the accelerated cooling is started When the surface temperature of the steel ingot is above 800°C, the accelerated cooling rate is 10°C/s-20°C/s, the cooling stop temperature is 500°C-550°C, and then slowly cooled to below 200°C (slow cooling rate ≤ 1°C/min ), then let it cool to room temperature.

See Table 1-Table 3 for other embodiments.

Table 3 Corrosion weight loss rate in an atmosphere containing a salt content of 6.0 mg/m ² d (mg/m ² × day)

steel sample Corrosion weight loss rate g/(m²hr) Relative corrosion rate (%) Example 1 1.07 60.8 Example 2 1.08 61.3 Example 3 1.09 61.9 Example 4 1.05 59.7 Example 5 1.06 60.2 Example 6 1.03 59.0 Q235 1.76 100 Germany 1.28 73 Baosteel 2001 1.10 62 Baosteel 2002 1.11 62 Baosteel 2003 1.06 60 Baosteel 2004 1.08 61

The maglev express train system is a fast, safe, efficient and environment-friendly means of transportation. At present, the fastest maglev train in the world has reached a speed of over 560 km/h. With the 9.11 terrorist incident in the United States and a series of air crashes around the world recently, the aviation industry has been hit unprecedentedly. It will set off a climax of the construction of the maglev express train system; as the key component of the track beam of the maglev express train system - the side-guided soft magnetic structural steel plate, especially the high-performance soft magnetic structural steel plate used in coastal areas (mostly in coastal areas of my country and other countries in the world) In developed regions, the possibility of building maglev trains is the greatest), which will surely have broad market prospects.

With the development of my country's national economy and society, the development of the maglev express train system has been placed on the daily agenda. At the same time, the development of the maglev train system can improve the technical level of my country's overall machinery manufacturing and automatic control, and drive the development of related industries. The maglev demonstration line of Pudong International Airport, a national key project, has been fully completed. Its completion will surely drive the rapid development of my country's maglev train transportation system and related industries, and will bring unlimited business opportunities. Magnetic levitation side-guided soft magnetic structural steel plate is the key component of the maglev guide rail. It can not only absorb the bearing capacity, guiding force and driving force in the maglev track, but also has high magnetic flux density, high resistivity, and good aging resistance. Sex, anti-magnetic timeliness and good weldability, so it has become a high value-added product with high efficiency, energy saving and environmental protection. Because the maglev train system is highly monopolistic (there are only two in the world: Japan and Germany, and only the German maglev express train system is practical), the production technology of the maglev side-guided soft magnetic structural steel plate is also highly monopolistic and confidential . Soft magnetic structural steel plate belongs to a new type of steel for our country, which has never been researched and produced by other domestic iron and steel enterprises except Baosteel. Because of its large consumption (~300 tons/km) and high price, it will become the research and development object of many large iron and steel enterprises.

Claims (10)

1. the good soft magnetic structural steel plate of weldprocedure, its composition weight percent is:

C：0.030％～0.060％

Si：1.45％～1.65％

Mn：0.40％～0.80％

P：≤0.015％

S：≤0.004％

Als：0.040％～0.060％

Cr：0.40％～0.60％

Ti：0.008％～0.016％

N：≤0.0055％

Ni：0.70～1.00％

Ca:0.0010%～0.0060% or REM0.010%～0.040%

All the other are iron and inevitably are mingled with;

And, Pcm≤0.20%, Pcm=wt%C+wt%Si/30+ (wt%Mn+wt%Cu+wt%Cr)/20+wt%Ni/60+wt%Mo/15+wt%V/10+5wt%B.

2. the good soft magnetic structural steel plate of weldprocedure as claimed in claim 1 is characterized in that,

C：0.040％～0.050％

Si：1.50％～1.60％

Mn：0.50％～0.70％

P：≤0.013％

S：≤0.003％

Als：0.040％～0.060％

Cr：0.45％～0.55％

Ti：0.010％～0.014％

N：≤0.0045％

Ni：0.80～0.90％

Ca:0.0020%～0.0050% or REM0.02%～0.03%

All the other are iron and inevitably are mingled with;

And, Pcm≤0.18%.

3. the good soft magnetic structural steel plate of weldprocedure as claimed in claim 1 or 2 is characterized in that it is organized as the equiaxed ferritic grain of 10 μ m～30 μ m.

4. the soft magnetic structural steel board fabrication method good as the weldprocedure of claim 1 or 2, it comprises the steps:

A) casting, low temperature casting, pouring temperature≤1560 ℃;

B) the very low temperature heating is adopted in slab heating, and slab heating temperature is controlled between 1000 ℃～1150 ℃;

C) rolling, in the perfect recrystallization temperature range, big rolling pass draft carries out continuous rolling, guarantees deformable metal generation perfect recrystallization, and rolling pass draft 〉=12% rolls total draft 〉=70%, and the control finishing temperature is at 780～880 ℃;

D) be controlled in 30 seconds to the biography time of putting beginning to quicken to cool off from rolling the end, and when quickening the cooling beginning, steel billet temperature must be at Ar ₃More than the point, be cooled to below 550 ℃ with 〉=10 ℃/s speed of cooling;

E) steel plate stacking slow cooling or burial pit slow cooling subsequently is to room temperature; Perhaps roll back direct quenching and stop temperature to quenching, quenching stops temperature and also is controlled at below 550 ℃, below steel plate stacking slow cooling or the burial pit slow cooling to 200 subsequently ℃ after, natural air cooling is to room temperature; The retarded cooling process of steel plate is for must not be lower than 24 hours in the slow cooling time more than 350 ℃.

5. the soft magnetic structural steel board fabrication method that weldprocedure as claimed in claim 4 is good is characterized in that, the step a) casting technique adopts continuous casting process.

6. the soft magnetic structural steel board fabrication method that weldprocedure as claimed in claim 4 is good is characterized in that, the step a) pouring temperature is at 1525 ℃～1550 ℃.

7. the soft magnetic structural steel board fabrication method that weldprocedure as claimed in claim 4 is good is characterized in that, the step b) slab heating temperature is controlled between 1050 ℃～1100 ℃.

8. the soft magnetic structural steel board fabrication method that weldprocedure as claimed in claim 4 is good is characterized in that, step c) continuous rolling, rolling pass draft 〉=15%, the total draft of hot rolling 〉=80%.

9. the soft magnetic structural steel board fabrication method that weldprocedure as claimed in claim 4 is good is characterized in that, step d) is controlled in 15 seconds to the biography time of putting beginning to quicken to cool off from rolling the end; And when quickening the cooling beginning, steel billet temperature must be at Ar ₃More than the point, be cooled between 450 ℃～550 ℃ with 〉=15 ℃/s speed of cooling.

10. the soft magnetic structural steel board fabrication method that weldprocedure as claimed in claim 4 is good is characterized in that, step e) is rolled back direct quenching and stopped temperature to quenching, and quenching stops temperature and also is controlled between 450 ℃～550 ℃.

Images