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EP-4386098-B1 - HIGH STRENGTH STEEL SHEET, HIGH STRENGTH COATED OR PLATED STEEL SHEET, METHODS OF PRODUCING THESE STEEL SHEETS, AND MEMBER

EP4386098B1EP 4386098 B1EP4386098 B1EP 4386098B1EP-4386098-B1

Inventors

  • TANAKA YUJI
  • NISHIYAMA TAKESHI
  • TOMOZAWA MASANARI
  • ENDOH Kazuki
  • MINAMI HIDEKAZU
  • TOBATA Junya
  • TOJI YUKI

Dates

Publication Date
20260506
Application Date
20220621

Claims (6)

  1. A high strength steel sheet comprising a chemical composition containing, in mass%: C: 0.10 % or more and 0.30 % or less; Si: 0.20 % or more and 1.20 % or less; Mn: 2.5 % or more and 4.0 % or less; P: 0.050 % or less; S: 0.020 % or less; Al: 0.10 % or less; N: 0.01 % or less; Ti: 0.100 % or less; Nb: 0.002 % or more and 0.050 % or less; and B: 0.0005 % or more and 0.0050 % or less, and, optionally, at least one element selected from, in mass%: V: 0.100 % or less; Mo: 0.500 % or less; Cr: 1.00 % or less; Cu: 1.00 % or less; Ni: 0.50 % or less; Sb: 0.200 % or less; Sn: 0.200 % or less; Ta: 0.200 % or less; W: 0.400 % or less; Zr: 0.0200 % or less; Ca: 0.0200 % or less; Mg: 0.0200 % or less; Co: 0.020 % or less; REM: 0.0200 % or less; Te: 0.020 % or less; Hf: 0.10 % or less; or Bi: 0.200 % or less, with the balance being Fe and inevitable impurities, and satisfying the following formula (1), wherein the total area ratio of martensite and bainite is 95 % or more, the average grain size of prior austenite grains is 10 µm or less, the B concentration at a prior austenite grain boundary is 0.10 % or more in mass%, a C-concentrated region is provided along a martensitic grain boundary, the C concentration in the C-concentrated region is 4.0 times or more than the C content in the steel, the C-concentrated region has a concentration width of 3 nm or more and 100 nm or less in a direction perpendicular to the martensitic grain boundary and a length of 100 nm or more in a direction parallel to the martensitic grain boundary, and the B concentration at a prior austenite grain boundary, the average grain size of prior austenite grains and the C concentration in the C-concentrated region are measured by the methods set out in the description: % N / 14 / % Ti / 47.9 < 1.0 in the formula (1), [%N] and [%Ti] indicate the N content and the Ti content in the steel in mass%, respectively.
  2. A high strength coated or plated steel sheet having a coated or plated layer on at least one surface of the high strength steel sheet according to claim 1.
  3. A method of producing a high strength steel sheet according to claim 1, comprising: hot rolling a steel slab having the chemical composition according to claim 1 to form a hot-rolled sheet; cold rolling the hot-rolled sheet to form a cold-rolled sheet; performing an annealing process in which the cold-rolled sheet is heated to a first heating temperature of 850 °C or higher and 920 °C or lower and held for 10 seconds or longer, the temperature is then raised to a second heating temperature of 1000 °C or higher and 1200 °C or lower at an average heating rate of 50 °C/s or more, and the sheet is cooled to 500 °C or lower at an average cooling rate of 50 °C/s or more within 5 seconds after reaching the second heating temperature, after the annealing process, performing a rolling process in which the cold-rolled sheet is rolled at an elongation rate of 0.5 % or more to obtain a second cold-rolled sheet, and after the rolling process, performing a reheating process in which the second cold-rolled sheet is held at a reheating temperature of 70 °C or higher and 200 °C or lower for 600 seconds or longer to obtain a high strength steel sheet.
  4. A method of producing a high strength coated or plated steel sheet, comprising a coating or plating process in which, after the annealing process according to claim 3 and before the reheating process, at least one surface of the high strength steel sheet is subjected to coating or plating treatment to obtain a high strength coated or plated steel sheet.
  5. A member formed using the high strength steel sheet according to claim 1 for at least a portion thereof.
  6. A member formed using the high strength coated or plated steel sheet according to claim 2 for at least a portion thereof.

Description

TECHNICAL FIELD The present disclosure relates to a high strength steel sheet, a method of producing the same, and a member. BACKGROUND Automotive steel sheets are required to have higher strength to improve fuel efficiency by reducing the weight of the automotive body. High strength steel sheets with a tensile strength of 1180 MPa or higher are required for frame parts. In addition, high bendability is required for steel sheets to be subjected to press working and formed into desired shapes. Furthermore, from the viewpoint of crashworthiness of automobiles, there are some automotive parts required not to easily deform to ensure driver's and passenger's living space during a collision, in addition to strength. The use of steel sheets with a high yield ratio is desirable for such automotive parts. In addition, high toughness is required to ensure that automotive parts do not fracture in a collision. JP5728108B (PTL 1) discloses a high strength steel sheet with excellent formability and low-temperature toughness and a method of producing the same. JP6597939B (PTL 2) discloses a high strength steel sheet with excellent formability and anti-crash property, and a method of producing a high strength steel sheet with excellent formability and anti-crash property. JP6700398B (PTL 3) discloses a high yield ratio type high strength steel sheet and a method of producing the same. US 2019/194775 A1 (PTL 4) describes a steel sheet having a specified chemical composition and a method for producing the steel sheet. CITATION LIST Patent Literature PTL 1: JP5728108BPTL 2: JP6597939BPTL 3: JP6700398BPTL 4: US 2019/194775 A1 SUMMARY (Technical Problem) However, the yield ratio is not considered in PTLs 1 and 2. Toughness is not considered in PTL 3. As described above, it is difficult to produce a high strength steel sheet with a tensile strength of 1180 MPa or higher, excellent bendability and toughness, and a high yield ratio using conventional techniques. This disclosure has been made in view of these circumstances. It could be helpful to provide a high strength steel sheet with a tensile strength of 1180 MPa or higher, excellent bendability and toughness, and a high yield ratio, and a method of producing the same. In this disclosure, high strength means that a tensile strength TS measured in accordance with JIS Z2241 is 1180 MPa or higher. Excellent bendability means that a bending test specimen does not crack at the ridge of a tip thereof in a bend test conducted in accordance with JIS Z2248. Excellent toughness means that the brittle-ductile transition temperature is -40 °C or lower in a Charpy impact test conducted in accordance with JIS Z2242. A high yield ratio means that a ratio YS/TS of yield stress to tensile strength measured in accordance with JIS Z2241 is 0.80 or more. (Solution to Problem) We conducted diligent studies to accomplish the above-mentioned tasks and discovered the following. (1) Crack initiation and propagation during bending and a fracture path during brittle fracture are along the prior austenite grain boundary. Therefore, refining crystal grains to complicate the fracture path and increasing the strength of the grain boundary are effective in improving bendability. To refine prior austenite grains, it is effective to keep the annealing temperature as low as possible at a temperature equal to or higher than 850 °C, which is an austenite single phase region. On the other hand, grain boundary segregation of B is effective in strengthening the grain boundary, but the grain boundary segregation amount of B increases as being annealed at higher temperature. Therefore, to increase the grain boundary segregation amount of B while maintaining fine crystal grain size, annealing is performed at around 850 °C to obtain fine austenite grains, followed by rapid heating and rapid cooling. This promotes grain boundary segregation of B by diffusion while inhibiting crystal grain growth, thereby simultaneously achieving austenite grain size refinement and grain boundary segregation of B.(2) Dislocations present in quenched martensitic microstructure are mobile dislocations that easily generate sliding motion at low stresses, resulting in low yield stresses in the martensitic microstructure. However, when the steel sheet after quenching is slightly processed, these dislocations move close to the grain boundary, where they become entangled and form immobile dislocations. This can increase the yield ratio of the steel sheet.(3) Tempering the steel sheet at low temperatures causes carbon segregation on dislocations or cluster precipitation. Tempering at low temperatures the steel sheet after processing, in which dislocations are accumulated near the crystal grain boundary, forms a region of high C concentration (C-concentrated region) on the network along the grain boundary, which significantly increases the strength near the grain boundary. Since C is concentrated not only in the grain boundary but also in the matri