Preparation method of nb-based high-strength deep drawing steel

Abstract

The application discloses a preparation method of Nb high-strength deep-drawing steel, which comprises the following steps: a steelmaking process, a hot rolling process, a cold rolling process and a continuous annealing process; the chemical components of the high-strength deep-drawing steel comprise the following components in percentage by weight: C is less than or equal to 0.0030%; Si is less than or equal to 0.03%; Mn is 0.30-0.45%; P is 0.030-0.045%; S is less than or equal to 0.015%; Alt is 0.020-0.070%; Nb is 0.050-0.060%; N is less than or equal to 0.0040%; B is 0.0003-0.0010%, and the balance is Fe and inevitable impurities. The application aims to provide a preparation method of Nb high-strength deep-drawing steel, through reasonable regulation and control of the contents of Nb, Mn and P elements and optimization of a production process, the forming performance of the automobile steel is improved while the strength performance of the automobile steel is ensured.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy and metallic materials technology, and particularly relates to a method for preparing Nb-based high-strength deep-drawing steel. Background Technology

[0002] High-strength IF deep-drawing steel with a yield strength of 180MPa is widely used in various automotive components due to its excellent strength-to-weight ratio, including front and rear door panels, left and right front fenders, upper and lower trunk lid panels, and engine hood panels. The demand is high and the usage is substantial. Phosphorus strengthening, as an economical and effective strengthening method, enhances the strength of steel through solid solution strengthening. However, the addition of phosphorus in traditional methods often presents a series of process challenges. With the increasing demands for green manufacturing and cost control, developing an efficient and stable preparation method to achieve target performance has become a key direction for technological upgrading in the industry.

[0003] Currently, the preparation technology of phosphorus-strengthened steel largely relies on composite microalloying, such as the synergistic effect of elements like niobium, vanadium, and titanium, to control toughness while improving strength. However, the introduction of phosphorus easily leads to increased cold brittleness and decreased formability, limiting its application in demanding applications. Among existing methods, niobium microalloying can refine grains and improve toughness, but the preparation process of pure Nb-based steel is complex and costly, and it is difficult to accurately balance the strengthening effect of phosphorus and the microalloying effect of niobium. In addition, traditional processes are sensitive to composition control and heat treatment parameters, which can easily cause performance fluctuations, making it challenging to consistently achieve a yield strength of 180 MPa, thus requiring innovative solutions.

[0004] To address these shortcomings, this invention proposes a method for preparing pure Nb-based high-strength deep-drawing steel, aiming to overcome the deficiencies of existing technologies. This method optimizes the ratio of niobium to phosphorus and manganese, combined with a specific rolling process, to achieve a synergistic effect of grain refinement and solid solution strengthening. Utilizing the unique advantages of pure Nb-based steel and optimizing the production process, the negative effects of phosphorus are effectively suppressed, ensuring that the steel achieves a yield strength of 180 MPa while also possessing good formability. Simultaneously, it reduces performance fluctuations at the beginning and end of the coil, lowers the product's sensitivity to the process, and makes its performance more stable. This method is simple, highly controllable, and provides a new pathway for the industrialization of high-performance structural steel, possessing significant technical and economic value. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing Nb-based high-strength deep-drawing steel. By rationally controlling the content of Nb, Mn, and P elements and optimizing the production process, the formability of automotive steel can be improved while ensuring its strength performance.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a method for preparing Nb-based high-strength deep-drawing steel, comprising: steelmaking process - hot rolling process - cold rolling process - continuous annealing process; wherein: the steelmaking process includes: converter top and bottom combined blowing smelting - RH - slab continuous casting; the hot rolling process includes: billet heating - high-pressure water descaling - fixed width press - E1R1 roughing mill rolling - E2R2 roughing mill rolling - insulation cover - flying shear - high-pressure water descaling - F1-F7 finishing mill rolling - dense laminar flow cooling - coiling; the cold rolling process includes: pickling and uncoiling - welding - tension leveling - pickling - rinsing - drying - edge trimming - continuous rolling mill cold rolling - coiling; the continuous annealing process includes: continuous annealing and uncoiling - welding - cleaning - inlet looper - annealing furnace - outlet looper - leveling - edge trimming - surface inspection - oiling - sampling - coiling - weighing - packaging; characterized in that:

[0008] 1) Converter production

[0009] After KR desulfurization treatment, molten iron is smelted in a converter. The sulfur content of the molten iron entering the converter must be less than 0.005%, and the slag removal area must be greater than 95%. The tapping temperature of the converter must be ≥1660℃, and a turnover ladle must be used for tapping. The temperature drop at tapping must be less than 70℃. The RH molten steel supplied to the converter must have [C] ≤0.04%, [Si] ≤0.03%, [Mn] ≤0.45%, and [O] : 0.040-0.07%.

[0010] 2) RH furnace production

[0011] The RH furnace undergoes deep decarburization. After decarburization, aluminum granules are added for deoxidation based on the oxygen content. After circulating for at least 4 minutes, alloys such as ferromanganese, ferrosilicon, and ferroniobium are added to adjust the composition. After adjusting the composition, ensure vacuum circulation for at least 6 minutes.

[0012] 3) Continuous casting production

[0013] For slab continuous casting, the superheat of the tundish during the casting process is controlled within the range of 30-45℃, and the production speed is 1.0-1.8m / min;

[0014] 4) The heating temperature during billet heating is 1180-1230℃, the heating time is 70-130min, the soaking temperature is 1180-1230℃, the soaking time is 25-60min, and the furnace exit temperature is 1190-1240℃.

[0015] 5) The finishing rolling temperature is 890-940℃, and the coiling temperature is 560-600℃;

[0016] 6) In the continuous annealing process, the annealing heating and soaking temperature is 780-810℃, the slow cooling section outlet temperature is ≤670℃, the rapid cooling section outlet temperature is ≤430℃, the over-aging section temperature is ≤400℃, the final cooling section outlet temperature is ≤150℃, the leveling machine elongation is 0.7-0.9%, and the process section speed is 50-180m / min;

[0017] The chemical composition of the high-strength deep-drawing steel by weight percentage includes: C ≤ 0.0030%; Si ≤ 0.03%; Mn: 0.30-0.45%; P: 0.030-0.045%; S ≤ 0.015%; Alt: 0.020-0.070%; Nb: 0.050-0.060%; N ≤ 0.0040%; B: 0.0003-0.0010%, with the balance being Fe and unavoidable impurities.

[0018] Furthermore, the hot-rolled thickness is 3.0mm-5.5mm, and the cooling mode adopts front-end centralized cooling.

[0019] Furthermore, the cold-rolled thickness is 0.60-1.80 mm.

[0020] Furthermore, the metallographic structure is ferrite.

[0021] Furthermore, the metallographic grain size is 9.0-11.0 grade.

[0022] Furthermore, it achieves uniformity of mechanical properties for products of different thickness groups, with a steel strip surface quality FD and a surface roughness range of 0.6-1.9μm.

[0023] Furthermore, the mechanical properties of the deep-drawing steel meet the following requirements: yield strength 180-240 MPa, tensile strength ≥340 MPa, and elongation after fracture A. 80mm ≥34%, r 90 ≥1.7,n 90 ≥0.19.

[0024] The role and mechanism of each alloying element in this invention:

[0025] Carbon (C) in steel primarily exists as interstitial atoms dissolved in the steel or forming alloyed cementite with Fe or other metallic elements. However, interstitial C atoms can cause lattice distortion in the metal, reducing formability (such as deep drawing performance). Therefore, ultra-low C design is used to eliminate its negative impact on formability, hence the C content is set to ≤0.003%. Similarly, nitrogen (N) atoms also occupy interstitial spaces in the lattice, and their content must be strictly limited to improve the ductility of the steel; the N content is set to ≤0.0040%.

[0026] The main role of silicon (Si) in steel is to inhibit cementite formation and to act as a solid solution strengthening element, improving the strength and hardness of the ferrite matrix. In automotive steel, its main function is to assist in deoxidation; however, excessive Si content can deteriorate surface quality, so the Si content range is set at ≤0.03%.

[0027] As a solid solution strengthening element and carbide-forming element in steel, manganese (Mn) can simultaneously strengthen ferrite and cementite, improving the strength and hardness of steel. Furthermore, Mn can replace Fe and S in steel to form MnS, avoiding the formation of FeS (FeS readily forms low-melting-point compounds with Fe), which leads to hot brittleness in steel. It also acts as a deoxidizer. Mn can lower the pearlite transformation temperature, thereby reducing the interlamellar spacing of pearlite. However, increasing the Mn content significantly reduces the weldability of steel, increases grain size, and increases the susceptibility to white spots and billet segregation. Since cementite is absent in automotive steel, its strengthening effect on carbides is not considered. When the Mn content > 0.80%, excessive Mn leads to increased grain size, affecting the microstructure changes during production and reducing the steel's ductility and toughness. Considering all factors, the Mn content range is set at ≤0.30-0.45%.

[0028] In automotive steels, nitrogen (Nb) mainly exists in the form of carbides or nitrides. On the one hand, it can completely fix interstitial atoms, preventing them from interfering with the crystal lattice and significantly improving the deep-drawing performance and resistance to aging. On the other hand, it can also increase the recrystallization temperature, playing a role in precipitation strengthening and grain refinement. Furthermore, studies have found that adding Ti to Nb-containing steel causes some Nb to co-precipitate with Ti at high temperatures due to TiN-induced precipitation, resulting in a significant increase in precipitate size. Therefore, from the perspective of fully utilizing Nb precipitation strengthening and reducing process sensitivity, adding Nb alone is more reasonable. However, due to the high price of Nb, considering cost and yield factors, based on the calculation formula Nb addition ≥ (6.64 × N content + 7.74 × C content) / (yield 90%-98%), the Nb content range is set at 0.050-0.060%.

[0029] Both phosphorus (P) and sulfur (S) are impurity elements in general steel that cannot be completely removed. They significantly increase the steel's crack susceptibility, raise its low-temperature brittle transition temperature, and reduce its low-temperature impact resistance. Therefore, without affecting the steel's properties, it is generally desirable to keep the P and S content as low as possible. High-strength deep-drawing steel requires the addition or retention of a certain amount of phosphorus (P) to improve its strength through solid solution strengthening. However, to reduce the harmful effects of P and improve deep-drawing performance, the P content is set at 0.030-0.045%, thus improving strength while avoiding its cold brittleness effects. Sulfur (S) remains an impurity element in automotive steel, so its content must be controlled below 0.015%.

[0030] Al plays three key roles: First, as a strong deoxidizer, it effectively removes oxygen from molten steel during smelting, purifying the steel. Second, aluminum combines with nitrogen (N) to form fine AlN particles, which pin grain boundaries and inhibit austenite grain growth, thereby refining the final ferrite grains and significantly improving the strength and toughness of the steel. Finally, by refining grains and altering precipitates, aluminum indirectly improves the distribution of phosphorus (P) at grain boundaries, alleviating the tendency for cold brittleness caused by phosphorus segregation, which is crucial for maintaining the plasticity and stamping formability of materials with high phosphorus content. Excessive Al addition can form coarse Al₂O₃ or calcium aluminate inclusions, and can also affect the fluidity of molten steel and exacerbate surface defects in the billet; therefore, its content is controlled at 0.020-0.070%.

[0031] In high-phosphorus interstitial steel, boron (B) can alleviate the tendency of phosphorus (P) to segregate at grain boundaries, reducing cold brittleness. Simultaneously, B can hinder ferrite nucleation, delay the transformation of austenite to ferrite, and improve microstructure uniformity. Insufficient B content prevents effective grain boundary segregation, while excessive B content leads to the precipitation of coarse BN phases, reducing the steel's strength and toughness. Therefore, its content is controlled between 0.0003-0.0010%.

[0032] The reasons for selecting the above process parameters are as follows:

[0033] Setting a lower coiling temperature in the hot rolling process can prevent the growth of Nb-containing precipitates, enhance grain refinement and strengthening, thus ensuring product strength. Combined with adjustments to the continuous annealing process, this can reduce the amount of P and Mn added to the steel, increasing the r-value and improving formability. Furthermore, a lower coiling temperature reduces the temperature difference between the inner and outer coils, resulting in more uniform Nb precipitation, reduced performance fluctuations at the beginning and end of the coil, decreased product sensitivity to the process, and more stable product performance.

[0034] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0035] This invention provides a method for preparing Nb-based high-strength deep-drawing steel. The purpose of this invention is to provide a method for preparing Nb-based high-strength deep-drawing steel by rationally controlling the content of Nb, Mn, and P elements and optimizing the production process, thereby improving the formability of the automotive steel while ensuring its strength. Practical use has proven that the performance is suitable, and its mechanical properties meet the following requirements: yield strength 180-240 MPa, tensile strength ≥340 MPa, and elongation after fracture A. 80mm ≥34%, r 90 ≥1.7,n 90 ≥0.19. This method for producing automotive steel is simple, highly operable, and easy to promote and apply. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] Figure 1 The metallographic structure is that of the finished product. Detailed Implementation

[0038] A method for preparing low-phosphorus, high-strength deep-drawing steel, the production method specifically includes the following steps: steelmaking process - hot rolling process - cold rolling process - continuous annealing process.

[0039] The steelmaking process includes: converter top and bottom blowing smelting—RH—slab continuous casting. Molten iron undergoes desulfurization pretreatment before being smelted in the converter. The sulfur content of the molten iron entering the converter must be less than 0.005%, and the slag removal area must be greater than 95%. The converter tapping temperature must be ≥1660℃, and a turnover ladle must be used for tapping. The tapping temperature drop must be less than 70℃. The RH-supplying steel has [C] ≤0.04%, [Si] ≤0.03%, [Mn] ≤0.50%, and [O] : 0.040-0.070%. The RH furnace undergoes deep decarburization treatment. After decarburization, aluminum granules are added for deoxidation based on the oxygen content. After circulation for more than 4 minutes, ferromanganese, ferrosilicon, ferroniobium, etc., are added to adjust the alloy composition. After composition adjustment, vacuum circulation is ensured for ≥6 minutes. For slab continuous casting, the tundish superheat is controlled within the range of 30~45℃ during casting, and the production speed is 1.0~1.8m / min. Based on the above steelmaking process requirements, the actual slab chemical composition (mass percentage) is shown in Table 1 below, with the balance being Fe and unavoidable impurities.

[0040] Table 1: Chemical Composition (%)

[0041] element C Si Mn P S Al Ti Nb N B Example 1 0.0018 0.01 0.35 0.032 0.005 0.042 - 0.051 0.0021 0.0005 Example 2 0.0025 0.01 0.37 0.037 0.003 0.047 - 0.055 0.0022 0.0007 Example 3 0.0023 0.02 0.40 0.039 0.006 0.046 - 0.054 0.0024 0.0008 Example 4 0.0021 0.03 0.38 0.033 0.004 0.043 - 0.057 0.0023 0.0006 Example 5 0.0017 0.02 0.35 0.038 0.004 0.048 - 0.054 0.0025 0.0007 Comparative Example 1 0.0021 0.03 0.38 0.051 0.003 0.033 0.037 0.020 0.0024 0.0008 Comparative Example 2 0.0018 0.02 0.36 0.049 0.006 0.035 0.034 0.017 0.0026 0.0007 Comparative Example 3 0.0023 0.02 0.44 0.055 0.004 0.033 0.033 0.025 0.0028 0.0009 Comparative Example 4 0.0018 0.04 0.41 0.053 0.007 0.034 0.029 0.022 0.0024 0.0011 Comparative Example 5 0.0027 0.03 0.37 0.054 0.009 0.029 0.027 0.023 0.0021 0.0009

[0042] The hot rolling process includes: billet heating—high-pressure water descaling—width-fixing press—E1R1 roughing mill rolling—E2R2 roughing mill rolling—flying shear—high-pressure water descaling—F1-F7 finishing mill rolling—dense laminar flow cooling—coiling; this process uses a walking beam furnace to heat the billet (heating process is shown in Table 2), roughing uses a double-stand R1 and R2 reciprocating rolling process, and finishing uses a continuous rolling process from F1 to F7. The cooling mode uses intermittent cooling with an upper spray beam in front and centralized cooling with a lower spray beam in front. Specific hot rolling processes are shown in Table 3.

[0043] Table 2 Heating Regime for Cast Billets

[0044] Example Heating temperature / ℃ Heating time / min Isotropic temperature / ℃ Soaking time / min Furnace temperature / ℃ Example 1 1193 93 1214 54 1216 Example 2 1188 95 1219 48 1223 Example 3 1183 89 1211 49 1215 Example 4 1207 106 1223 54 1227 Example 5 1211 86 1225 45 1231 Comparative Example 1 1182 93 1203 43 1206 Comparative Example 2 1196 103 1218 56 1221 Comparative Example 3 1201 87 1209 47 1213 Comparative Example 4 1191 116 1203 53 1206 Comparative Example 5 1196 89 1207 39 1211

[0045] Table 3 Rolling Process

[0046] Example Rolled thickness / mm Finishing rolling temperature / ℃ Winding temperature / ℃ Example 1 3.0 908 587 Example 2 3.8 899 591 Example 3 4.5 913 597 Example 4 5.0 917 583 Example 5 5.5 904 596 Comparative Example 1 3.0 912 696 Comparative Example 2 3.8 907 705 Comparative Example 3 4.5 911 709 Comparative Example 4 5.0 920 711 Comparative Example 5 5.5 908 697

[0047] The cold rolling process includes: pickling and uncoiling—welding—tension leveling—pickling—rinsing—drying—edge trimming—continuous rolling mill cold rolling—coiling;

[0048] The continuous annealing process includes: continuous annealing uncoiling—welding—cleaning—entry looper—annealing furnace—exit looper—leveling—edge trimming—surface inspection—oiling—sampling—coiling—weighing—packaging; annealing is performed using a vertical continuous annealing furnace, with a reducing atmosphere and a nitrogen-hydrogen mixed protective atmosphere used for cooling inside the furnace. The specific process parameters for the continuous annealing process are shown in Table 4.

[0049] Table 4: Annealing Process

[0050] Example Cold rolled thickness / mm Heating section temperature / ℃ Temperature of the soaking zone / ℃ Temperature of slow cooling section / ℃ rapid cooling section temperature / ℃ Over-aging temperature / ℃ Final cooling section temperature / ℃ Process section speed / m / min Elongation of leveling machine / % Example 1 0.6 797 801 633 415 393 144 174 0.69 Example 2 0.8 803 799 642 423 391 145 165 0.81 Example 3 1.2 806 805 645 419 386 144 133 0.79 Example 4 1.5 794 797 639 417 388 143 104 0.88 Example 5 1.8 801 803 636 416 390 147 87 0.92 Comparative Example 1 0.6 824 825 637 425 392 148 168 0.82 Comparative Example 2 0.8 818 817 646 426 388 143 164 0.79 Comparative Example 3 1.2 823 826 643 428 391 145 131 0.91 Comparative Example 4 1.5 816 821 636 431 393 144 106 0.98 Comparative Example 5 1.8 827 824 643 432 396 143 98 1.01

[0051] The room temperature tensile mechanical properties of the finished product, obtained through hot rolling, cold rolling, and continuous annealing processes, are shown in Table 5. The microstructure is equiaxed ferrite with a grain size of 8.5-9.5. The microstructure morphology is shown in Table 5. Figure 1 .

[0052] Table 5: Room Temperature Tensile Properties of Finished Products

[0053] Classification Rolled thickness / mm Yield strength / MPa Tensile strength / MPa Elongation / % R90 N90 Example 1 0.6 189 351 42.5 2.53 0.23 Example 2 0.8 196 355 41.7 2.61 0.22 Example 3 1.2 203 357 40.8 2.45 0.23 Example 4 1.5 201 361 41.7 2.58 0.23 Example 5 1.8 209 364 40.3 2.51 0.22 Comparative Example 1 0.6 196 353 39.6 2.34 0.22 Comparative Example 2 0.8 203 356 39.2 2.35 0.23 Comparative Example 3 1.2 207 364 39.5 2.41 0.22 Comparative Example 4 1.5 213 371 38.4 2.17 0.22 Comparative Example 5 1.8 215 377 38.1 2.21 0.21

[0054] In summary, this product has passed performance testing and meets all performance requirements. User trial molding has also met molding requirements, making it suitable for widespread use.

[0055] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing Nb-based high-strength deep-drawing steel, comprising: Steelmaking process - hot rolling process - cold rolling process - continuous annealing process; wherein: the steelmaking process includes: converter top and bottom blowing smelting - RH - slab continuous casting; the hot rolling process includes: billet heating - high pressure water descaling - fixed width press - E1R1 roughing mill rolling - E2R2 roughing mill rolling - heat insulation cover - flying shear - high pressure water descaling - F1-F7 finishing mill rolling - dense laminar flow cooling - coiling; the cold rolling process includes: pickling and uncoiling - welding - tension leveling - pickling - rinsing - drying - trimming - continuous rolling mill cold rolling - coiling; the continuous annealing process includes: continuous annealing and uncoiling - welding - cleaning - inlet looper - annealing furnace - outlet looper - leveling - trimming - surface inspection - oiling - sampling - coiling - weighing - packaging; its characteristic is: 1) Converter production After KR desulfurization treatment, molten iron is smelted in a converter. The sulfur content of the molten iron entering the converter must be less than 0.005%, and the slag removal area must be greater than 95%. The tapping temperature of the converter must be ≥1660℃, and a turnover ladle must be used for tapping. The temperature drop at tapping must be less than 70℃. The RH molten steel supplied to the converter must have [C] ≤0.04%, [Si] ≤0.03%, [Mn] ≤0.45%, and [O] : 0.040-0.07%. 2) RH furnace production The RH furnace undergoes deep decarburization. After decarburization, aluminum granules are added for deoxidation based on the oxygen content. After circulating for at least 4 minutes, alloys such as ferromanganese, ferrosilicon, and ferroniobium are added to adjust the composition. After adjusting the composition, ensure vacuum circulation for at least 6 minutes. 3) Continuous casting production For slab continuous casting, the superheat of the tundish during the casting process is controlled within the range of 30-45℃, and the production speed is 1.0-1.8m / min; 4) The heating temperature during billet heating is 1180-1230℃, the heating time is 70-130min, the soaking temperature is 1180-1230℃, the soaking time is 25-60min, and the furnace exit temperature is 1190-1240℃. 5) The finishing rolling temperature is 890-940℃, and the coiling temperature is 560-600℃; 6) In the continuous annealing process, the annealing heating and soaking temperature is 780-810℃, the slow cooling section outlet temperature is ≤670℃, the rapid cooling section outlet temperature is ≤430℃, the over-aging section temperature is ≤400℃, the final cooling section outlet temperature is ≤150℃, the leveling machine elongation is 0.7-0.9%, and the process section speed is 50-180m / min; The chemical composition of the high-strength deep-drawing steel by weight percentage includes: C ≤ 0.0030%; Si ≤ 0.03%; Mn: 0.30-0.45%; P: 0.030-0.045%; S ≤ 0.015%; Alt: 0.020-0.070%; Nb: 0.050-0.060%; N ≤ 0.0040%; B: 0.0003-0.0010%, with the balance being Fe and unavoidable impurities.

2. The method for preparing Nb-based high-strength deep-drawing steel according to claim 1, characterized in that: The hot-rolled thickness is 3.0mm-5.5mm, and the cooling mode adopts front-end centralized cooling.

3. The method for preparing Nb-based high-strength deep-drawing steel according to claim 1, characterized in that: The thickness of cold-rolled steel is 0.60-1.80 mm.

4. The method for preparing Nb-based high-strength deep-drawing steel according to claim 1, characterized in that: The metallographic structure is ferrite.

5. The method for preparing Nb-based high-strength deep-drawing steel according to claim 3, characterized in that: The metallographic grain size is 9.0-11.

0.

6. The method for preparing Nb-based high-strength deep-drawing steel according to claim 1, characterized in that: To achieve uniformity of mechanical properties for products of various thickness groups, the surface quality FD of the steel strip is achieved, and the surface roughness range is 0.6-1.9μm.

7. The method for preparing Nb-based high-strength deep-drawing steel according to claim 1, characterized in that: The mechanical properties of the deep-drawing steel meet the following requirements: yield strength 180-240 MPa, tensile strength ≥340 MPa, and elongation after fracture A. 80mm ≥34%, r 90 ≥1.7,n 90 ≥0.19.

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