WO2023246904A1

WO2023246904A1 - Ultrahigh reaming steel and manufacturing method therefor

Info

Publication number: WO2023246904A1
Application number: PCT/CN2023/101852
Authority: WO
Inventors: 王焕荣; 张晨; 杨阿娜; 庞厚君; 范佳杰
Original assignee: 宝山钢铁股份有限公司
Priority date: 2022-06-22
Filing date: 2023-06-21
Publication date: 2023-12-28
Also published as: CN117305693A

Abstract

The present invention provides ultrahigh reaming steel and a manufacturing method therefor. The steel comprises the following components in percentage by weight: C: 0.03-0.09%; Si: ≤0.2%; Mn: 0.5-2.0%; P: ≤0.02%; S: ≤0.003%; Al: 0.2-1.2%; N: ≤0.004%; Ti: 0.05-0.20%; Mo: 0.05-0.5%; Mg: ≤0.005%; O: ≤0.003%; B: ≤0.001%; and the balance being Fe and other inevitable impurities. C, Mn, Mo and B in the steel satisfy the following: 0.25≤2×C+Mn/3+Mo+150×B≤1.5; during calculation, the numerical value before the percent sign for the mass percentage of the corresponding element is substituted. The steel in the present invention has excellent matching of strength, plasticity and reaming performance, and can be applied in passenger vehicle chassis parts needing high strength and thinning, such as a control arm and a subframe.

Description

Ultra-high hole expansion steel and manufacturing method thereof

Technical field

The invention belongs to the field of steel and its manufacturing method, and particularly relates to an ultra-high hole expansion steel and its manufacturing method.

Background technique

Pickling products are commonly used for many parts in passenger cars, especially chassis and body parts. Lightweighting of passenger cars is the development trend of the automotive industry. High-strength and weight reduction is an inevitable requirement for subsequent new models. This will inevitably lead to a higher grade of steel and will also bring about changes in the chassis structure: if the parts are more complex, the materials will be required. Performance, surface and forming technologies such as hydroforming, hot stamping, laser welding, etc. have put forward higher requirements, which in turn have higher requirements for material strength, stamping, flanging, springback and fatigue properties.

At present, the high-expansion steel commonly used by domestic auto parts companies is high-strength steel with a tensile strength of less than 600MPa and less than 400MPa. High-expansion steel with a tensile strength of 780MPa is gradually being used in batches in China. Due to the increase in steel strength, higher requirements have been placed on the elongation and hole expansion rate of steel, two important indicators in the forming process. In order to further reduce process costs, some passenger car companies have further improved the performance requirements of materials. For example, when producing structural parts such as automobile chassis control arms, in order to reduce the stamping process and reduce costs, it is required to have high strength and high plasticity while further improving the hole expansion rate index. For example, it is required to further increase the hole expansion rate index from the current ≥50% to ≥70% based on the existing 780MPa grade high hole expansion steel. At present, 780MPa high-pore expansion steel mostly adopts the design idea of high silicon composition system. The structure is mainly bainite. At the same time, some trace elements are added to the steel to provide a certain precipitation strengthening effect. The surface of the pickled steel strip not only has obvious red iron scale, but also the hole expansion rate is basically between 50-65%, and the elongation rate of the bainite structure is low. None of these can meet the higher hole expansion rate proposed by the user. performance requirements.

There are already some solutions for 780MPa grade pickling highly expanded steel in the prior art. like:

Chinese patent CN103602895A provides a low-carbon Nb-Ti micro-alloyed high hole expansion steel. Its composition design features low carbon and high silicon combined with Nb-Ti micro-alloying. The hole expansion rate of the steel is ≥50%. but The design of high silicon content usually results in red iron scale on the surface of the steel plate. In addition, the coiling temperature range required to form bainite is around 500°C. It is difficult to control the temperature of the entire length of the steel coil, which can easily cause large fluctuations in the performance of the entire length.

Chinese patent CN105821301A provides an 800MPa grade hot-rolled high-strength and high-pore-expanding steel. Its composition design feature is also low carbon and high silicon combined with Nb-Ti microalloying. The Ti content in the steel is very high, ranging from 0.15 to 0.18%. Therefore, in the actual production process, the surface of strip steel with this composition will have defects such as red iron scale, and ultra-high Ti content will easily form coarse TiN in the steel, which is very detrimental to the stability of the hole expansion rate.

Chinese patent CN108570604A provides a 780MPa grade hot-rolled, pickled, high-pore-expanding steel whose composition design features are low carbon, high aluminum and high chromium. The process design adopts a three-stage cooling process. However, this three-stage cooling process is very difficult to control, and the actual hole expansion rate is not high.

Chinese patent CN114107792A provides a 780MPa grade hot-rolled, pickled, high-pore-expanding steel. Its composition is characterized by low carbon and high titanium with an appropriate amount of molybdenum added. Since the phase transformation process of molybdenum-containing steel is relatively slow, the phase transformation process mainly occurs after coiling. Therefore, in the actual production process of this steel, there are problems such as low strength of the inner and outer rings of the steel coil.

Contents of the invention

The object of the present invention is to provide an ultra-high hole expansion steel and a manufacturing method thereof. The steel of the present invention has good surface quality, excellent and stable mechanical properties, and can be used in passenger car chassis parts such as control arms and subframes that require high strength and thinning.

As we all know, under normal circumstances, the elongation of a material is inversely proportional to the hole expansion rate, that is, the higher the elongation, the lower the hole expansion rate; conversely, the lower the elongation, the higher the hole expansion rate. Therefore, it is very difficult to achieve high elongation and ultra-high hole expansion rate at the same time, and it is even more difficult to ensure uniform performance across the entire length of the strip. Under the same or similar strengthening mechanism, the higher the strength of the material, the lower the hole expansion rate. In order to obtain steel with good plasticity and expansion and flanging properties, a better balance between the two is needed. In order to obtain a good match of strength, plasticity and hole expansion, the addition of more silicon elements seems to be indispensable for high strength, high plasticity and high hole expansion steel. However, the high silicon composition design brings poor steel plate surface, that is, when hot The red iron scale defects formed in the rolling process are difficult to completely remove in the subsequent pickling process, causing striped red iron scale to appear on the surface of the pickled high-strength steel, seriously affecting the surface quality.

In order to satisfy users' demands for higher surface quality, better performance stability, good plasticity and ultra The existing pickling 780MPa grade high-expansion steel needs to be improved to meet the needs of high hole expansion.

The invention adopts the composition design of low silicon (or even no Si), low carbon and high aluminum to avoid the appearance of red iron scale on the surface of strip steel, thereby improving the surface quality of pickling high-strength steel.

Specifically, the first aspect of the invention provides a steel whose composition in weight percentage is: C 0.03～0.09%, Si≤0.2%, Mn 0.5～2.0%, P≤0.02%, S≤0.003%, Al 0.2～1.2%, N≤0.004%, Ti 0.05～0.20%, Mo 0.05～0.5%, Mg≤0.005%, O≤0.003%, B≤0.001%, the balance is Fe and other inevitable impurities, so The relationship between C, Mn, Mo and B in the above steel satisfies:
0.25≤2×C+Mn/3+Mo+150×B≤1.5,

The value before the percent sign is entered into the weight percentage of the corresponding element when calculating. For example, when the C content in steel is 0.05%, substitute the value 0.05 for calculation.

Preferably, 0.5≤2×C+Mn/3+Mo+150×B≤1.2.

Unless otherwise specified, the content of chemical elements in steel refers to the weight fraction of the element.

Preferably, the steel of the present invention also contains one or more selected from the group consisting of Nb, V, Cu, Ni and Cr, wherein, in terms of weight percentage, Nb≤0.06%, V≤0.10%, preferably ≤0.05wt%, Cu ≤0.5%, preferably ≤0.3%, Ni≤0.5%, preferably ≤0.3%, Cr≤0.5%, preferably ≤0.3%.

Preferably, the composition of the above-mentioned steel also satisfies at least one of the following: Si≤0.15wt%, Mn 1.0～1.6wt%, S≤0.0015wt%, Al 0.5～1.0wt%, N≤0.003wt%, Ti 0.07～ 0.11wt%, Mo 0.15～0.45wt%, Ni≤0.03wt%, B≤0.0005wt%.

The design ideas of each element in the ultra-high expansion steel of the present invention are as follows:

Carbon is a basic element in steel and one of the important elements in the present invention. Carbon can expand the austenite phase area and stabilize austenite. As an interstitial atom in steel, carbon plays a very important role in improving the strength of steel, among which it has the greatest impact on the yield strength and tensile strength of steel. In the present invention, since the structure to be obtained is close to full ferrite, in order to obtain high-strength steel with a tensile strength of 780 MPa, the carbon content must be ensured to be above 0.03%. When the carbon content is below 0.03%, the tensile strength of the ferrite structure is difficult to reach 780MPa; but the carbon content cannot be higher than 0.09%. If the carbon content is too high, pearlite structure is easily formed during the phase change process, which is detrimental to the hole expansion performance. Therefore, the carbon content should be controlled between 0.03-0.09%.

Silicon is a basic element in steel, but it is an impurity element in the present invention. As mentioned before, in order to meet the user's requirements for high strength, high plasticity and ultra-high hole expansion rate, it is usually added during component design. Add more silicon. However, the high-silicon composition design brings about a reduction in the surface quality of the steel plate and more red iron sheet defects. In the present invention, in order to ensure good surface quality, the silicon content in the steel is strictly controlled. According to a large amount of statistical data in actual production, when the silicon content is below 0.2%, surface red iron scale defects can be avoided during the hot rolling process. Generally, when the silicon content is below 0.15%, it is guaranteed that no red scale will appear. Therefore, the silicon content in steel is within 0.2%, preferably within 0.15%.

Manganese is the most basic element in steel and one of the most important elements in the present invention. Mn is an important element in expanding the austenite phase area. It can stabilize austenite, refine grains and delay the transformation of austenite to pearlite. In the present invention, in order to ensure the strength and grain refinement effect of the steel plate, the Mn content is usually above 0.5%. At the same time, the Mn content generally should not exceed 2.0%, otherwise Mn segregation will easily occur during steelmaking and hot cracking will easily occur during slab continuous casting. Therefore, the Mn content in steel is 0.5-2.0%, preferably 1.0-1.6%.

Phosphorus is an impurity element in steel. P is easily segregated to the grain boundaries. When the content of P in the steel is high (≥0.1%), Fe ₂ P is formed and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the content, the better. Generally, when the P content is within 0.02%, the performance of the steel is better and the cost of steelmaking will not be increased.

Sulfur is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. Especially when the contents of S and Mn are both high, more MnS will be formed in the steel. MnS itself has a certain degree of plasticity. During the subsequent rolling process, MnS deforms along the rolling direction, which not only reduces the transverse plasticity of the steel, but also increases the anisotropy of the structure, which is detrimental to the hole expansion performance. Therefore, the lower the S content in steel, the better. In order to reduce the MnS content, the S content needs to be strictly controlled. In the present invention, the content of S is within 0.003%, preferably below 0.0015%.

Aluminum is one of the most important elements in this invention. In addition to the conventional deoxidation and nitrogen fixation effect, the addition of aluminum into steel also plays an important role in the present invention, which is to accelerate the phase change process, so that the phase change of the strip steel is completed on the laminar cooling roller table and before coiling, so as to avoid strip coiling. After removal, due to the different cooling rates of the inner and outer rings of the steel coil, uneven precipitation of nanoscale carbides results in large fluctuations in the performance of the entire length of the strip. The amount of aluminum added to steel is closely related to the austenite stabilizing elements carbon and manganese, as well as the key elements molybdenum and boron that inhibit ferrite phase transformation. Generally speaking, the higher the content of carbon, manganese, molybdenum, and boron, the higher the content of aluminum. Therefore, depending on the carbon, manganese, molybdenum and boron content in the steel, the aluminum content is usually between 0.1-1.5%, preferably between 0.5-1.0%.

Nitrogen is an impurity element in the present invention, and the lower its content, the better. But nitrogen is an inevitable element in the steelmaking process. Although its content is small, it is closely related to strong carbide-forming elements such as Ti The TiN particles formed by these combinations have a very negative impact on the properties of steel, especially on the hole expansion performance. Due to the square shape of TiN, there is a large stress concentration between its sharp corners and the matrix. During the process of hole expansion deformation, the stress concentration between TiN and the matrix can easily form a crack source, thus greatly reducing the hole expansion performance of the material. Since the present invention adopts a high titanium design in the component system, in order to minimize the adverse impact of TiN on hole expansion. Therefore, the nitrogen content in the present invention is 0.004% or less, preferably 0.003% or less.

Titanium is one of the important elements in the present invention. Ti mainly plays two roles in the present invention: first, it combines with the impurity element N in the steel to form TiN, which plays a part of the role of "nitrogen fixation"; second, it forms a uniform dispersion from the ferrite during the coiling phase transformation process. Fine nanoscale carbides improve strength, plasticity and hole expansion rate. When the titanium content is less than 0.05%, there is no obvious precipitation strengthening effect; when the titanium content is higher than 0.20%, coarse TiN can easily lead to poor impact toughness of the steel plate. Therefore, the titanium content in the steel of the present invention is between 0.05-0.20%, preferably between 0.07-0.11%.

Molybdenum is one of the important elements in the present invention. The addition of molybdenum to steel can greatly delay the phase transformation of ferrite and pearlite, which is beneficial to obtaining an irregular ferrite structure. Molybdenum and titanium are added to the steel at the same time, and the nanoscale titanium carbide molybdenum precipitate formed has the effect of resisting high-temperature coarsening, which can ensure that no coarsening occurs for a long time after coiling and the strength is reduced. At the same time, molybdenum has strong resistance to welding softening. Since the main purpose of the present invention is to obtain ferrite plus nano-precipitation structure, adding a certain amount of molybdenum can effectively reduce the degree of welding softening. Therefore, the content of molybdenum in the present invention is between 0.1-0.5%, preferably between 0.15-0.45%.

Magnesium is one of the important elements in the present invention. Magnesium added to steel can preferentially form dispersed fine MgO during the steelmaking stage. These fine MgO can serve as nucleation points for TiN. In the subsequent continuous casting process, it can effectively increase the nucleation points of TiN and reduce the size of TiN. Since TiN has a great influence on the hole expansion rate of the final steel plate, it can easily cause the hole expansion rate to be unstable. Therefore, the Mg content in the steel of the present invention is within 0.005%.

Oxygen is an unavoidable element in the steelmaking process. For the present invention, the oxygen content in the steel can generally reach less than 30 ppm after deoxidation, which will not cause obvious adverse effects on the performance of the steel plate. Therefore, the O content in the steel of the present invention is within 30 ppm.

Niobium is one of the additive elements in the present invention. Niobium is similar to titanium and is a strong carbide element in steel. Adding niobium to steel can greatly increase the non-recrystallization temperature of the steel. During the finishing rolling stage, deformed austenite with a higher dislocation density can be obtained. During subsequent phase transformation, The final tissue can be refined. However, the amount of niobium added cannot be Too much. On the one hand, if the amount of niobium added exceeds 0.06%, it is easy to form relatively coarse niobium carbonitrides in the structure, consuming part of the carbon atoms, thereby reducing the precipitation strengthening effect of carbides. At the same time, the high content of niobium can easily cause anisotropy of the hot-rolled austenite structure, which is inherited to the final structure during the subsequent cooling phase transformation process, which is detrimental to the hole expansion performance. Therefore, the niobium content in steel is usually ≤0.06%, preferably ≤0.03%.

Vanadium is an additive element in the present invention. Similar to titanium and niobium, vanadium is also a strong carbide-forming element. However, the solid solution or precipitation temperature of vanadium carbide is low, and it is usually completely dissolved in austenite during the finishing rolling stage. Only when the temperature is lowered and phase transformation begins, vanadium begins to form in the ferrite. Since the solid solubility of vanadium carbide in ferrite is greater than the solid solubility of niobium and titanium, the size of vanadium carbide formed in ferrite is larger, which is not conducive to precipitation strengthening and contributes far to the strength of steel. Less than titanium carbide or titanium molybdenum carbide. However, the formation of vanadium carbide also consumes a certain amount of carbon atoms, which is detrimental to the improvement of the strength of steel. Therefore, the added amount of vanadium in steel is usually ≤0.10%, preferably ≤0.05%.

Copper is an additive element in the present invention. Adding copper to steel can improve the corrosion resistance of steel. When it is added together with the P element, the corrosion resistance effect is better; when the amount of Cu added exceeds 1%, under certain conditions, an ε-Cu precipitation phase can be formed, causing A strong precipitation strengthening effect is achieved. However, the addition of Cu can easily cause "Cu embrittlement" during the rolling process. In order to make full use of Cu's corrosion resistance improvement effect in certain applications without causing significant "Cu embrittlement", Cu is usually added. The content of elements is controlled within 0.5%, preferably within 0.3%.

Nickel is an additive element in the present invention. Nickel added to steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper. Nickel added to steel has little effect on the tensile properties of the steel, but it can refine the structure and precipitated phases of the steel, greatly improving the low-temperature toughness of the steel. ;At the same time, in steel with added copper element, adding a small amount of nickel can inhibit the occurrence of "Cu embrittlement". Adding higher nickel has no significant adverse effect on the properties of the steel itself. If copper and nickel are added at the same time, it can not only improve the corrosion resistance, but also refine the structure and precipitated phases of the steel, thereby greatly improving the low-temperature toughness of the steel. But both copper and nickel are relatively expensive alloy elements. Therefore, in order to reduce the cost of alloy design as much as possible, the addition amount of nickel is usually ≤0.5%, preferably ≤0.3%.

Chromium is an additive element in the present invention. Chromium is added to steel mainly to improve the strength of steel through solid solution strengthening or structure refinement. Since the structure in the present invention is fine bainitic ferrite plus nano-precipitated carbides, and after the high-temperature hooding process, the movable dislocations in the structure are reduced, making the yield strength and tensile strength of the steel better. The ratio of strength to strength is relatively high, usually reaching above 0.90. add Adding a small amount of chromium element can appropriately reduce the yield strength of steel, thereby reducing the yield-strength ratio. In addition, the addition of a small amount of chromium can also play a role in improving corrosion resistance. Usually the addition amount of chromium is ≤0.5%, preferably ≤0.3%.

Boron is an impurity element in the present invention. Since boron can quickly segregate to the austenite grain boundary during the finishing rolling stage, it strongly inhibits the ferrite phase transformation. Considering the present invention, it is expected to obtain the entire ferrite structure as ferrite before hot rolling and coiling. Therefore, the boron element content must be strictly limited. The amount of boron added to steel is usually ≤0.001%, preferably ≤0.0005%.

Preferably, the steel of the present invention has a yield strength of ≥700MPa, a tensile strength of ≥780MPa, an elongation in the transverse direction A50 of ≥17%, and a hole expansion rate of ≥80%.

Considering the manufacturing cost of steel, preferably, the yield strength of the steel is below 850MPa, the tensile strength is below 900MPa, the transverse elongation A50 is below 25%, and the hole expansion rate is below 115%.

Preferably, the structure of the steel of the present invention is more than 95% by volume, preferably more than 97% by volume, of ferrite and less than 5% by volume, preferably less than 3% by volume of pearlite, and the ferrite contains dispersed nanoscale carbonization. things.

Most of the existing 780MPa grade high-expansion steel adopts high Ti design in composition design, and alloy elements such as Nb, Mo, and Cr are added at the same time. The structural transformation process mainly occurs after coiling. Considering that the cooling speed of the inner, middle and outer rings of the steel coil is different after coiling, the strength of the steel coil at different locations fluctuates greatly. Especially within a certain length range of the inner and outer rings of the steel coil, there is a large difference in performance from the middle ring. , resulting in obvious differences in the hole expansion performance of the strip.

In order to improve the uniformity of the performance of the steel coil throughout the length, the present invention adds more Al in the composition design, and at the same time controls the contents of C, Mn, Mo, and B elements that have an important impact on the ferrite phase transformation, so that the strip steel can The air cooling stage of the laminar cooling roller table before coiling completes the phase change process, thereby obtaining a strip with good uniformity of structure and precipitation, and improving the performance stability of the entire length of the strip.

Another aspect of the present invention provides a method for manufacturing the above-mentioned steel, comprising the following steps:

1) Smelting and casting

According to the above composition, the molten steel is smelted in a converter or electric furnace, then refined in a vacuum furnace, and then cast into a slab or ingot;

2) Reheating of billet or ingot

Heating temperature ≥1200℃, holding time: 1 to 2 hours;

3) Hot rolling and cooling of billet or ingot

Opening rolling temperature: 1050~1150℃, 3~5 passes of rough rolling at high pressure above 1050℃ and cumulative deformation ≥50% to obtain an intermediate billet, and then air-cooling or water-cooling the intermediate billet to 950~1000℃. Then perform 5 to 7 passes of finishing rolling with the cumulative deformation ≥ 70%, and the final rolling temperature is 850 to 950°C to obtain a steel strip;

Laminar flow cooling is used for cooling. After final rolling, the above steel strip is water-cooled to 550~650°C at a cooling rate of ≥10°C/s for coiling. After coiling, it is cooled to room temperature at a cooling rate of ≤50°C/h to obtain heat. Rolled strip steel.

Preferably, the above method also includes step 4) pickling, wherein the pickling speed of the hot-rolled strip is 30-140m/min, the pickling temperature is controlled at 75-85°C, and the tensile straightening rate is controlled at ≤3%. , rinse at a temperature range of 35 to 50°C, dry and oil the surface at a temperature of 120 to 140°C.

The beneficial effects of the method for manufacturing the above steel in the present invention are as follows:

The present invention adopts specially controlled low-carbon and high-aluminum composition design, and can obtain a high-surface ultra-high hole expansion steel with excellent performance stability through high-temperature coiling process on the hot continuous rolling production line. Due to the innovative design of the component system, the strip steel can complete the phase transformation before coiling, avoiding the structural uniformity problems caused by different cooling rates in the inner, middle and outer rings of the steel coil after coiling, making the steel coil performance uniform Greatly improve.

In the steelmaking process, the present invention uses Mg deoxidation to preferentially form dispersed and fine MgO in the molten steel, creating more nucleation points for the formation of TiN in the subsequent continuous casting process, which can effectively refine TiN particles and improve Hole expansion rate stability.

The rolling opening temperature of the present invention is 1050-1150°C, and 3-5 passes of rough rolling are performed under high pressure and the cumulative deformation is ≥50% above 1050°C. The main purpose is to refine the austenite grains while retaining more More solid solution titanium. The intermediate billet is then air-cooled or water-cooled to 950-1000°C, and then subjected to 5 to 7 passes of finish rolling with a cumulative deformation of ≥70%. Then after the final rolling between 850-950°C, the steel plate is water-cooled to 550-650°C at a cooling rate of ≥10°C/s, and then slowly cooled to room temperature after coiling. The specific manufacturing process is shown in Figure 2.

In the rough rolling and finishing rolling stages, the rolling rhythm should be completed as quickly as possible to ensure that more titanium is solidly dissolved in austenite. After the final rolling, the strip is cooled online to 550-650°C at a cooling rate of ≥10°C/s to obtain ferrite and nano-precipitation structures. According to actual production experience, depending on the strip thickness and composition, The strip steel completes the entire phase transformation process within 5-20 seconds on the layer-cooled roller table, thereby obtaining a more uniform structure and precipitation.

In the subsequent pickling process, the thermal stress unevenness generated inside the high-temperature coiled steel coil will be reduced and homogenized during pickling and straightening, which further improves the structural uniformity of the steel and is conducive to obtaining high-quality Pickled ultra-high hole expansion steel with surface, high plasticity, ultra-high hole expansion rate and good performance stability.

Compared with the existing technology, the solution of the present invention has the following advantages:

Chinese patents CN103602895A and CN105821301A adopt high-silicon composition designs, but the present invention adopts a unique composition design of low silicon or even no silicon, low carbon and high aluminum, which can avoid the appearance of red iron scale on the surface of the strip and improve the pickling strength. Surface quality of steel.

The steel in Chinese patent CN108570604A adopts a low silicon composition design, with a silicon content of 0.05 to 0.5%. However, there is still no guarantee that the red iron sheet defects will be completely eliminated on the strip surface. Moreover, the three-stage cooling process is difficult to control, and performance stability is difficult to guarantee.

Chinese patents CN105154769A and CN114107792A. Since the composition of steel contains molybdenum and other elements that inhibit ferrite phase transformation, the phase transformation process occurs after coiling. In actual production, there are problems such as large fluctuations in the performance of the inner and outer rings of steel coils.

The present invention adopts a new low-carbon and high-aluminum composition design idea. By accurately controlling the content of carbon, manganese, molybdenum and boron, high strength, high plasticity and ultra-high hole expansion can be obtained by using a simple rolling process. Hot-rolled steel coils with good matching rate and full-length performance stability.

After the pickling process, the internal stress in the ferrite structure is reduced and homogenized. The uniformly fine and dispersed nano-scale carbides in the ferrite give the steel plate high strength and high plasticity on the one hand, and at the same time, the good structure and uniform distribution of internal stress give the steel plate an ultra-high hole expansion rate.

The method of the present invention can be used to produce ultra-high hole expansion steel with yield strength ≥700MPa and tensile strength ≥780MPa, but with good elongation (transverse A50≥17%) and high hole expansion performance (hole expansion rate ≥80% ), shows good performance stability, can achieve excellent surface, strength, plasticity and hole expansion performance matching, and is suitable for the manufacturing of complex parts such as automobile chassis and subframes that require high-strength thinning and hole expansion flanging.

Description of the drawings

Figure 1 is a schematic diagram of the rolling and cooling process of steel according to the present invention;

Figure 2 is a typical metallographic photograph of the steel in Example 2 of the present invention;

Figure 3 is a typical metallographic photograph of the steel in Example 4 of the present invention;

Figure 4 is a typical metallographic photograph of the steel in Example 6 of the present invention.

Detailed ways

The present invention will be further described below in conjunction with the embodiments and drawings.

The components of the steels in the examples and comparative examples of the present invention are shown in Table 1. The balance of the components in Table 1 is Fe and inevitable impurities.

The process path for manufacturing steel in the embodiment of the present invention is:

1) Smelting and casting

According to the composition shown in Table 1, it is smelted in a converter or electric furnace, then secondary refined in a vacuum furnace, and then cast into a slab or ingot.

2) Reheating of billet or ingot

Heating temperature ≥1200℃, holding time: 1 to 2 hours.

3) Hot rolling and cooling of billet or ingot

Opening rolling temperature: 1050~1150℃, 3~5 passes of rough rolling at high pressure above 1050℃ and cumulative deformation ≥50% to obtain an intermediate billet, and then air-cooling or water-cooling the intermediate billet to 950~1000℃. Then perform 5 to 7 passes of finishing rolling with the cumulative deformation ≥ 70%, and the final rolling temperature is 850 to 950°C to obtain strip steel;

Laminar flow cooling is used for cooling. After final rolling, the strip is water-cooled to 550-650°C at a cooling rate of ≥10°C/s for coiling. After coiling, it is cooled to room temperature at a cooling rate of ≤50°C/h.

The specific process is shown in Figure 1.

Table 2 shows the production process parameters of steel according to the embodiment of the present invention. Table 3 shows the performance parameters of the steels in the examples and comparative examples of the present invention.

The steel in Comparative Examples 1-3 is selected from CN103602895A, and the steel in Comparative Example 4 is selected from CN114107792A.

Table 1 gives the ingredient differences between Examples and Comparative Examples. It can be seen from Table 1 that the composition designs of the comparative examples are all low-aluminum designs, and the composition designs of comparative examples 1-3 also include high-silicon designs, while the composition designs of the present invention are silicon-free and high-aluminum. The two are completely different in composition design. different.

As can be seen from Table 3, the yield strength of the steel coil obtained according to the composition and process of the present invention ≥700MPa, tensile strength ≥780MPa, elongation transverse A50≥17%, hole expansion rate ≥80%.

It can also be seen from Table 3 that although Comparative Examples 1 to 3 are similar to the present invention in terms of yield strength, tensile strength and elongation, the hole expansion rate index of Comparative Examples 1 to 3 is significantly lower than that of the embodiments of the present invention.

The yield strength, tensile strength and elongation of the steel in Table 3 were tested in accordance with GB/T 228.1-2021 "Tensile Test of Metal Materials Part 1: Room Temperature Test Method".

The hole expansion rate of steel was tested in accordance with GB/T 24524-2021 "Experimental Method for Hole Expansion of Metal Material Thin Plate and Thin Strip".

Figures 2 to 4 respectively show typical metallographic photos of the steels in Examples 2, 4 and 6 of the present invention.

It can be clearly seen from the figure that by using the composition and process path designed in the present invention, a structure mainly composed of ferrite and containing a very small amount of pearlite can be obtained. Specifically, the ferrite content in the steel is more than 97% by volume, the pearlite content is less than 3% by volume, and the ferrite contains dispersed nanoscale carbides.

The steel in the embodiment of the present invention shows a good match of high strength, high plasticity and ultra-high hole expansion rate, and has excellent comprehensive performance.

It can be seen from the above examples and comparative examples that the 780MPa high-strength steel involved in the present invention has high strength, high plasticity and ultra-high hole expansion rate, and is particularly suitable for automobile chassis structures that require high strength thinning and hole expansion flanging forming. It can also be used for the manufacture of parts such as control arms, etc., and can also be used for complex parts such as wheels that require drilling, and has broad application prospects.

Claims

A kind of steel whose composition in weight percentage is: C 0.03～0.09%, Si≤0.2%, Mn 0.5～2.0%, P≤0.02%, S≤0.003%, Al 0.2～1.2%, N≤0.004%, Ti 0.05～0.20%, Mo 0.05～0.5%, Mg≤0.005%, O≤0.003%, B≤0.001%, the balance is Fe and other inevitable impurities, among C, Mn, Mo and B in the steel The time satisfies the following:
0.25≤2×C+Mn/3+Mo+150×B≤1.5,

The value before the percent sign is entered into the weight percentage of the corresponding element when calculating.
The steel according to claim 1, characterized in that the steel further contains one or more selected from the group consisting of Nb, V, Cu, Ni and Cr, wherein, in terms of weight percentage, Nb≤0.06% and V≤0.10 %, preferably ≤0.05%, Cu≤0.5%, preferably ≤0.3wt%, Ni≤0.5%, preferably ≤0.3%, Cr≤0.5%, preferably ≤0.3%.
The steel according to claim 1, characterized in that the composition of the steel also satisfies at least one of the following: Si≤0.15wt%, Mn 1.0～1.6wt%, S≤0.0015wt%, Al 0.5～1.0wt %, N≤0.003wt%, Ti 0.07～0.11wt%, Mo 0.15～0.45wt%, Ni≤0.03wt%, B≤0.0005wt%.
The steel according to any one of claims 1 to 3, characterized in that the steel has a yield strength ≥700MPa, a tensile strength ≥780MPa, a transverse elongation A50≥17%, and a hole expansion rate ≥80%.
The steel according to any one of claims 1 to 4, characterized in that the structure of the steel is 95 volume % or more, preferably 97 volume % or more ferrite, and 5 volume % or less, preferably 3 volume % or less pearlescent. The ferrite contains dispersed nanoscale carbides.
A method for manufacturing the steel according to any one of claims 1 to 5, characterized in that the method includes the following steps:

1) Smelting and casting

The molten steel is smelted in a converter or an electric furnace according to the composition according to any one of claims 1 to 5, and then is refined in a vacuum furnace and then cast into a slab or ingot;

2) Reheating of billet or ingot

Heating temperature ≥1200℃, holding time: 1 to 2 hours;

3) Hot rolling and cooling of billet or ingot

Opening rolling temperature: 1050~1150℃, 3~5 passes of rough rolling under high pressure above 1050℃ And the cumulative deformation is ≥50%, and an intermediate billet is obtained. The intermediate billet is then air-cooled or water-cooled to 950-1000°C, and then 5-7 passes of finishing rolling are performed and the cumulative deformation is ≥70%. The final rolling temperature is between 850 and 850°C. 950℃ to obtain steel strip;

Laminar flow cooling is used for cooling. After final rolling, the steel strip is water-cooled to 550-650°C at a cooling rate of ≥10°C/s for coiling. After coiling, it is cooled to room temperature at a cooling rate of ≤50°C/h to obtain Hot rolled strip.
The method according to claim 6, characterized in that the method further includes step 4) pickling, wherein the pickling operating speed of the hot-rolled strip is 30-140m/min, and the pickling temperature is controlled at 75 ~85℃, control the tensile straightening rate at ≤3%, rinse in the temperature range of 35~50℃, and dry and oil the surface between 120~140℃.