US20230227930A1 - Ultrahigh-strength dual-phase steel and manufacturing method therefor - Google Patents

Ultrahigh-strength dual-phase steel and manufacturing method therefor Download PDF

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US20230227930A1
US20230227930A1 US17/927,781 US202117927781A US2023227930A1 US 20230227930 A1 US20230227930 A1 US 20230227930A1 US 202117927781 A US202117927781 A US 202117927781A US 2023227930 A1 US2023227930 A1 US 2023227930A1
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ultra
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phase steel
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strength dual
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Wei Li
Xiaodong Zhu
Peng XUE
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Baoshan Iron and Steel Co Ltd
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Assigned to BAOSHAN IRON & STEEL CO., LTD. reassignment BAOSHAN IRON & STEEL CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: LI, WEI, XUE, Peng, ZHU, XIAODONG
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    • C21D2211/00—Microstructure comprising significant phases
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Definitions

  • the present disclosure relates to a metallic material and a method of manufacturing the same, particularly to a dual-phase steel and a method of manufacturing the same.
  • dual-phase steel has excellent properties such as low yield strength, high tensile strength and high initial work hardening rate in addition to its low production cost and high manufacturability, it is widely used in the production of automotive parts.
  • the current highest strength level is 1180DP grade characterized by a tensile strength of greater than or equal to 1200 MPa, a yield strength of about 850 MPa, and a total elongation of about 10%.
  • a critical continuous annealing process is used for the production of cold-rolled dual-phase steel whose tensile strength depends on the fraction of martensite in the annealed structure. The higher the martensite fraction, the higher the tensile strength. Thus, a higher annealing temperature is needed in the production to form a higher martensite fraction.
  • the highest strength level of dual-phase steel that can be produced commercially is 1180 MPa, namely DP 1180 steel.
  • the chemical ingredients of the hot-dip galvanized steel plate substrate and their mass percentage contents are: C: 0.1-0.2%, Mn: 1.3-2.0%, S ⁇ 0.005%, P ⁇ 0.02%, Si: 0.2-0.3%, Als: 0.4-1.0%, Nb: 0.01-0.03%, Ti: 0.04-0.08%, B: 0.001-0.004%, Mo: 0.2-0.3%, Cr: 0.05-0.10%, V: 0.01-0.02%, and a balance of Fe and unavoidable impurities.
  • the heating temperature is 1200-1320° C.
  • the heating time is 120-200 min.
  • the hot-rolling step rough rolling is performed for 3-7 passes; the temperature at the finishing mill entry is 1020-1080° C.; and the finishing rolling temperature is 820-880° C.
  • the coiling temperature is 550-650° C.
  • the production method includes steps of slab heating, hot rolling, pickling-rolling, continuous hot-dip galvanization, skin pass and passivation.
  • the soaking temperature is 760-840° C.; the holding time is 100-200 s; the slow cooling temperature is 680-740° C.; the slow cooling rate is 10-20° C./s; the rapid cooling temperature is 420-450° C.; the rapid cooling rate is 35-65° C./s; the galvanizing temperature is 458-462° C.; and the galvanizing time is 5-15 s.
  • Chinese Patent Publication No. CN108486494A published on Sep. 4, 2018 and entitled “Method for Producing Vanadium Microalloying 1300 Mpa Grade High-strength Hot-Rolled Steel Plate and Cold-Rolled Dual-Phase Steel Plate” discloses a method for producing vanadium microalloying 1300 MPa grade high-strength hot-rolled steel plate and cold-rolled dual-phase steel plate.
  • the chemical composition is: 0.10-0.30 wt% C, 1.50-4.50 wt% Mn, 0.00-0.120 wt% Al, 0.00-0.90 wt% Si, 0.05-0.50% V, P ⁇ 0.020 wt%, S ⁇ 0.02 wt%, Fe: the balance.
  • the high-strength steel combines the precipitation strengthening of nano-sized vanadium carbide particles with the martensitic transformation strengthening. The strength of the existing dual-phase steel is increased significantly, and the high production efficiency is also guaranteed.
  • the production method includes steps of steelmaking, continuous casting, hot rolling, pickling-rolling, continuous annealing, temper rolling and tension leveling.
  • the temperature for heating the slab is ⁇ 1200° C.; rough rolling is performed for 3-7 passes; the thickness of the intermediate slab after the rough rolling is 28-40 mm; the temperature at the finishing mill entry is 1020-1100° C.; the finishing rolling temperature is 820-900° C.; and the coiling temperature is 550-650° C.
  • pickling-rolling step cold rolling is performed after pickling, wherein the cold rolling reduction is ⁇ 45%.
  • the holding temperature in the soaking stage is 760-840° C., while the holding time is 60-225 s; and the holding temperature in the over-aging stage is 250-320° C., while the holding time in the over-aging stage is 300-1225 s.
  • the products whose tensile strength grade is greater than or equal to 1300 MPa as disclosed by the existing patent documents are generally galvanized, and contain high Si and high Al according to some patents, which is not conducive to surface quality and production.
  • the products contain relatively high amounts of precious alloy elements such as Cr and Mo, and thus the production cost is high.
  • One of the objects of the present disclosure is to provide an ultra-high-strength dual-phase steel.
  • a reasonable design of the chemical elements in the ultra-high-strength dual-phase steel i.e. a design of medium Si and low Al to reduce the use of alloy elements such as Si and Al, the problems with the surface quality caused by high Si and the slab defects caused by high Al are avoided.
  • precious alloy elements such as Cr and Mo are not used in the ultra-high-strength dual-phase steel according to the present disclosure, and thus the alloy cost is controlled effectively. At the same time, the contents of impurity elements P and S are reduced, which is beneficial to promotion of performances and improvement of delayed cracking.
  • the ultra-high-strength dual-phase steel has a yield strength of ⁇ 900 MPa, preferably ⁇ 930 MPa, a tensile strength of ⁇ 1300 MPa, preferably ⁇ 1320 MPa, an elongation after fracture of ⁇ 5%, preferably ⁇ 5.5%, an initial hydrogen content of ⁇ 10 ppm, preferably ⁇ 7 ppm; and it does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength, and preferably does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to 1.2 times of the tensile strength. It can be used effectively for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.
  • the present disclosure provides an ultra-high-strength dual-phase steel having a matrix structure of ferrite + martensite, wherein ferrite and martensite are distributed evenly like islands, and wherein the ultra-high-strength dual-phase steel comprises the following chemical elements in mass percentages, in addition to Fe:
  • ultra-high-strength dual-phase steel comprises the following chemical elements in mass percentages:
  • C In the ultra-high-strength dual-phase steel according to the present disclosure, C is a solid solution strengthening element, and it is a guarantee for the material to obtain high strength. However, it should be noted that the higher the C content in the steel, the harder the martensite and the greater the tendency for delayed cracking to occur. Therefore, when a product is designed, it’s better to choose a low-carbon design. In the ultra-high-strength dual-phase steel according to the present disclosure, the mass percentage of C is controlled at 0.12-0.2%.
  • the mass percentage of C may be controlled at 0.14-0.18%.
  • Si has an effect of increasing the elongation of the steel. Si also has a great influence on the structure of the steel. Particularly, it promotes purification of ferrite and formation of retained austenite. At the same time, it can improve the tempering resistance of martensite, and inhibit precipitation and growth of Fe 3 C, so that the dominated precipitates formed during tempering are ⁇ carbides.
  • the mass percentage of Si in the steel is less than 0.5%, the elongation and tempering resistance of the steel will be affected; if the mass percentage of Si is higher than 1.0%, other metallurgical quality defects will be caused. Therefore, in the ultra-high-strength dual-phase steel according to the present disclosure, the mass percentage of Si is controlled at 0.5-1.0%.
  • Mn In the ultra-high-strength dual-phase steel according to the present disclosure, Mn is an element that strongly improves the hardenability of austenite, and it can improve the strength of the steel effectively by forming more martensite. Therefore, in the ultra-high-strength dual-phase steel according to the present disclosure, the mass percentage of Mn is controlled at 2.5-3.0%.
  • the mass percentage of Mn may be controlled at 2.5-2.8%.
  • Al is a deoxygenating element. It can remove oxygen and refine grains in the steel. Therefore, in the ultra-high-strength dual-phase steel according to the present disclosure, the mass percentage of Al is controlled at 0.02-0.05%.
  • Nb and Ti are elements for precipitation of carbonitrides. They can refine grains, precipitate carbonitrides, and improve the strength of the material. They can be added separately or in combination. However, it should be noted that if the mass percentage of Nb or Ti in the steel is higher than 0.05%, the strengthening effect is not obvious. Therefore, in the ultra-high-strength dual-phase steel of the present disclosure, the mass percentage of Nb is controlled at 0.02-0.05%, and the mass percentage of Ti is controlled at 0.02-0.05%.
  • B In the ultra-high-strength dual-phase steel according to the present disclosure, B is used as a strong element for hardenability. An appropriate amount of B can increase the hardenability of the steel, and promote formation of martensite. Therefore, in the ultra-high-strength dual-phase steel according to the present disclosure, the mass percentage of B is controlled at 0.001%-0.003%.
  • the unavoidable impurities include the P, S and N elements, and the contents thereof are controlled to be at least one of the following: P ⁇ 0.01%, S ⁇ 0.002%, N ⁇ 0.004%.
  • the P, S and N elements are all unavoidable impurity elements in the steel. It’s better to lower the contents of the P, S and N elements in the steel as far as possible. S tends to form MnS inclusions which will seriously affect the hole expansion rate.
  • the P element may reduce the toughness of the steel, which is not conducive to the delayed cracking performance. An unduly high content of the N element in the steel is prone to causing cracks on the surface of the slab, which will greatly affect the performances of the steel. Therefore, in the ultra-high-strength dual-phase steel according to the present disclosure, the mass percentage of P is controlled at P ⁇ 0.01%; the mass percentage of S is controlled at S ⁇ 0.002%; and the mass percentage of N is controlled at N ⁇ 0.004%.
  • the mass percentage contents of the chemical elements satisfy at least one of the following:
  • the phase proportion (by volume) of martensite is >90%.
  • martensite further comprises coherently distributed ⁇ carbides.
  • the performances of the ultra-high-strength dual-phase steel according to the present disclosure meet at least one of the following: yield strength ⁇ 900 MPa, tensile strength ⁇ 1300 MPa, elongation after fracture ⁇ 5%, initial hydrogen content ⁇ 10 ppm; no delayed cracking when soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
  • the performances of the ultra-high-strength dual-phase steel according to the present disclosure meet at least one of the following: yield strength ⁇ 930 MPa, tensile strength ⁇ 1320 MPa, elongation after fracture ⁇ 5.5%, initial hydrogen content ⁇ 7 ppm; no delayed cracking when soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to 1.2 times of the tensile strength.
  • the ultra-high-strength dual-phase steel according to the present disclosure has a yield strength of ⁇ 930 MPa, a tensile strength of ⁇ 1320 MPa, an elongation after fracture of ⁇ 5.5%, an initial hydrogen content of ⁇ 7 ppm; and it does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to 1.2 times of the tensile strength.
  • another object of the present disclosure is to provide a method for manufacturing an ultra-high-strength dual-phase steel.
  • the ultra-high-strength dual-phase steel manufactured by this method has a yield strength of ⁇ 900 MPa, a tensile strength of ⁇ 1300 MPa, an elongation after fracture of ⁇ 5%, an initial hydrogen content of ⁇ 10 ppm, and it does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength. It can be used effectively for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.
  • the present disclosure proposes a method for manufacturing the above ultra-high-strength dual-phase steel, comprising steps of:
  • a combination of high temperature soaking and medium temperature tempering is employed for the annealing.
  • the high temperature soaking gives rise to more austenite transformation, and thus more martensite is obtained during the subsequent rapid cooling, which finally guarantees higher strength before tempering.
  • the medium temperature tempering provides the material with a moderate yield ratio on the one hand, and on the other hand, it has a better effect in improving the delayed cracking performance.
  • the ultra-high-strength dual-phase steel according to the present disclosure has a yield ratio of 0.70-0.75.
  • the manner of long-term tempering at medium to low temperature enables removal of superfluous hydrogen from the steel plate to the greatest extent, i.e. diffusion of it out of the steel plate, so that the amount of hydrogen in its original state in the steel plate can be reduced. This is not only beneficial to reduce the hardness of martensite and the diffusion of hydrogen inside the steel plate, but also very beneficial to the mechanical properties and delayed cracking performance of the steel.
  • step (1) a drawing speed in the continuous casting is controlled at 0.9-1.5 m/min.
  • step (1) the continuous casting may be performed in a secondary cooling mode with a large amount of water to minimize segregation.
  • step (2) the cast slab is controlled to be soaked at a temperature of 1220-1260° C., preferably 1220-1250° C.; then rolled with a finishing rolling temperature being controlled at 880-920° C.; then cooled at a rate of 20-70° C./s after rolling; then coiled at a coiling temperature of 600-650° C., preferably 605-645° C.; and then subjected to heat preservation treatment after coiling.
  • the heat preservation treatment is performed for 1-5 hours after coiling.
  • step (2) in order to guarantee the stability of the rolling load, the heating temperature is controlled at 1220° C. or higher. Meanwhile, the upper limit of the heating temperature is controlled to be 1260° C. in order to prevent increase of oxidative burning loss. Therefore, the cast slab is finally controlled to be soaked at a temperature of 1220-1260° C.
  • step (3) the cold rolling reduction rate is controlled at 45-65%.
  • step (3) before cold rolling at a cold rolling reduction rate controlled at 45-65%, iron oxide scale on the surface of the steel plate can be removed by pickling.
  • step (6) the temper rolling reduction rate is controlled at ⁇ 0.3%.
  • step (6) in order to guarantee the flatness of the steel plate, a certain amount of temper rolling needs to be performed, but an excessively large amount of temper rolling will increase the yield strength of the steel too much. Therefore, in the manufacturing method according to the present disclosure, the temper rolling reduction rate is controlled at ⁇ 0.3%.
  • step (7) may be performed by a conventional electro-galvanizing method.
  • double-side plating is performed, and the weight of the plating layer on one side is in the range of 10-100 g/m 2 .
  • the ultra-high-strength dual-phase steel and the manufacturing method thereof have the following advantages and beneficial effects:
  • the composition of the ultra-high-strength dual-phase steel according to the present disclosure is designed reasonably. That is, a design of medium Si and low Al is employed to reduce the use of alloy elements such as Si and Al, so that the problems with the surface quality caused by high Si and the slab defects caused by high Al are avoided.
  • the steel is free of precious alloy elements such as Cr and Mo, and the alloy content is low. The manufacturability and economic efficiency of the steel are very good. The alloy cost is controlled effectively.
  • the ultra-high-strength dual-phase steel has a yield strength of ⁇ 900 MPa, a tensile strength of ⁇ 1300 MPa, an elongation after fracture of ⁇ 5%, an initial hydrogen content of ⁇ 10 ppm; and it does not experience delayed cracking when it is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength. It can be used effectively for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.
  • the manufacturing method according to the present disclosure by adopting the medium to low temperature tempering treatment after the continuous annealing and controlling the relevant process parameters, uniform, fine and dispersive coherent ⁇ carbides can be easily precipitated during tempering of martensite on the one hand, and on the other hand, the manner of long-term tempering at medium to low temperature enables removal of superfluous hydrogen from the steel plate to the greatest extent, i.e. diffusion of it out of the steel plate, so that the amount of hydrogen in its original state in the steel plate can be reduced.
  • FIG. 1 shows the structure of the cold-rolled and annealed dual-phase steel of Example 1.
  • Table 1 lists the mass percentages of various chemical elements in the steel grades corresponding to the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.
  • Tables 2-1 and 2-2 list the specific process parameters for the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.
  • Step (2) Step (3) Drawing speed in continuous casting (m/min) Soaking temperature (°C) Finishing rolling temperature (°C) Cooling rate (°C/s) Coiling temperature (°C) Cold rolling reduction rate (%) Ex. 1 A 1.0 1250 895 25 605 55 Ex. 2 B 1.2 1245 880 30 625 60 Ex. 3 C 1.5 1220 890 45 645 45 Ex. 4 D 0.9 1234 905 50 625 50 Ex. 5 E 1.1 1224 910 35 615 52 Ex. 6 F 1.3 1240 890 60 600 48 Ex. 7 G 1.0 1250 885 65 630 62 Comp. Ex. 1 H 1.2 1230 895 40 615 50 Comp. Ex.
  • Method for measurement of hydrogen content The sample was heated to a certain temperature, and a hydrogen analyzer was used to measure the concentration of hydrogen released along with the change (rise) of the temperature, thereby judging the initial hydrogen content in the steel.
  • Table 3 lists the performance test results for the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.
  • high-strength steels having a strength of at least 1300 MPa can be manufactured according to the present disclosure.
  • Each Example according to the present disclosure has a yield strength of ⁇ 900 MPa, a tensile strength of ⁇ 1300 MPa, an elongation after fracture of ⁇ 5%, and an initial hydrogen content of ⁇ 10 ppm.
  • the ultra-high-strength dual-phase steel in each Example has an ultra-high strength and a delayed cracking performance that is significantly better than that of a comparative steel grade of the same level. No delayed cracking occurs when the steel plate is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.
  • the ultra-high-strength dual-phase steel in each Example has excellent performances. It is suitable for manufacture of automotive safety structural parts, and it is highly valuable and promising for popularization and application.

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