High Strength Steel and Production Method

Patent metadata

Field	Value
Publication number	US-2018171459-A1
Application number	US-201615735337-A
Country	US
Kind code	A1
Filing date	Jun 9, 2016
Priority date	Jun 10, 2015
Publication date	Jun 21, 2018
Grant date	—

Abstract

The present invention provides a cold-rolled and annealed steel sheet of thickness between 0.7 mm and 2 mm, mechanical strength ranging from 1180 MPa to 1320 MPa, wherein the hole expansion ratio Ac % is greater than 20% and the bending angle is greater than or equal to 40°. The chemical composition includes, the contents being expressed as weight percent: 0.09%≤C≤0.11%, 2.6%≤Mn≤2.8%, 0.20%≤Si≤0.55%, 0.25%≤Cr<0.5%, 0.025%≤Ti≤0.040%, 0.0015%≤B≤0.0025%, 0.005%≤Al≤0.18%, 0.08%≤Mo≤0.15%, 0.020%≤Nb≤0.040%, 0.002%≤N≤0.007%, 0.0005%≤S≤0.005%, 0.001%≤P≤0.020%, Ca≤0.003%, the remainder being iron and inevitable impurities resulting from processing. The sheet has a microstructure including martensite or lower bainite. A sum of the surface fractions of martensite and lower is between 40% and 70%, from 15% to 45% of the surface fraction is low-carbide bainite, from 5% to less than 20% of the surface fraction is ferrite, the non-recrystallized ferrite fraction relative to the total ferrite fraction is less than 15%, and less than 5% of the surface fraction of residual austenite is in the form of islands. The fraction of former austenitic grains in which the size is less than at least one micrometer represents 40% to 60% of the total population of the former austenitic grains.

First claim

1 . Cold-rolled and annealed steel sheet having a thickness between 0.7 mm and 2 mm, mechanical strength between 1180 MPa and 1320 MPa, wherein the hole expansion ratio Ac % is greater than 20% and the bending angle is greater than or equal to 40°,
wherein the chemical composition comprises, the contents being expressed as weight percent:
0.09%≤C≤0.11%
2.6%≤Mn≤2.8%
0.20%≤Si≤0.55%
0.25%≤Cr<0.5%
0.025%≤Ti≤0.040%
0.0015%≤B≤0.0025%
0.005%≤Al≤0.18%
0.08%≤Mo≤0.15%
0.020%≤Nb≤0.040%
0.002%≤N≤0.007%
0.0005%≤S≤0.005%
0.001%≤P≤0.020%.
Ca≤0.003%

the remainder being iron and inevitable impurities arising from processing, the sheet having a microstructure comprising martensite and/or lower bainite, said martensite comprising fresh martensite and/or auto-tempered martensite, the sum of the surface fractions of martensite and lower bainite being between 40% and 70%, from 15% to 45% of the surface fraction of low-carbide bainite, from 5% to less than 20% of the surface fraction of ferrite, the non-recrystallized ferrite fraction of the total ferrite fraction being less than 15%, and less than 5% as a surface fraction of residual austenite in the form of islands, the fraction of former austenitic grains in which the size is less than at least one micrometer representing 40% to 60% of the total population of said former austenitic grains.

2 . Steel sheet according to claim 1 , characterized in that said microstructure comprises from 15% to 45% of the surface fraction of fresh martensite.

3 . Steel sheet according to claim 1 , characterized in that said microstructure comprises from 5% to 50% in surface fraction of the sum of auto-tempered martensite and lower bainite.

4 - Steel sheet according to claim 3 , characterized in that said auto-tempered martensite and said lower bainite contain carbides in the form of rods oriented in directions <111> of the martensitic and bainitic laths.

5 . Steel sheet according to any of claims 1 to 4 , characterized in that said low-carbide bainite contains fewer than 100 carbides per 100 square micrometer unit of surface area.

6 - Steel sheet according to any of characteristics 1 to 5, characterized in that it contains precipitates of the type (Ti, Nb, Mo)(C, N) of less than 5 nanometers in size, present in an amount of less than 10,000 precipitates/μm 3 .

7 . steel sheet according to any of claims 1 to 6 , characterized in that the chemical composition comprises, the content being expressed as weight percent:
2.6%≤Mn≤2.7%.

8 . Steel sheet according to any of claims 1 to 7 , characterized in that the chemical composition comprises, the content being expressed as weight percent:
0.30%≤Si≤0.5%.

9 . Steel sheet according to any of claims 1 to 8 , characterized in that the chemical composition comprises, the content being expressed as weight percent:
0.005%≤Al≤0.030%.

10 . Steel sheet according to any of claims 1 to 9 , characterized in that said sheet comprises a zinc or zinc alloy coating obtained by hardening.

11 . Steel sheet according to claim 10 , characterized in that said zinc or zinc alloy coating is a galvanized-alloyed coating, said zinc or zinc alloy coating comprising from 7 wt-% to 12 wt-% iron.

12 . Steel sheet according to any of claims 1 to 9 , characterized in that said sheet comprises a zinc or zinc alloy coating obtained by vacuum deposition.

13 . Method for manufacturing a cold-rolled and annealed sheet according to any of claims 1 to 11 , comprising the following steps:
a semi-finished product is supplied, wherein the chemical composition comprises, the contents being expressed as weight percent:
0.09%≤C≤0.11%
2.6%≤Mn≤2.8%
0.20%≤Si≤0.55%
0.25%≤Cr<0.5%
0.025%≤Ti≤0.040%
0.0015%≤B≤0.0025%
0.005%≤Al≤0.18%
0.08%≤Mo≤0.15%
0.020%≤Nb≤0.040%
0.002%≤N≤0.007%
0.0005%≤S≤0.005%
0.001%≤P≤0.020%.
Ca≤0.003%

the remainder being iron and inevitable impurities arising from processing, then
said semi-finished product is heated at a temperature T r greater than or equal to 1250° C., then
said semi-finished product is hot-rolled, the finish temperature of the rolling being greater than the temperature Ar3 at which austenitic transformation starts during cooling, to obtain a hot-rolled sheet, then
said hot-rolled sheet is cooled at a rate greater than 30° C./s to prevent the formation of ferrite and pearlite, then
said hot-rolled sheet is coiled at a temperature between 580° C. and 500° C., then
said hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet, then
said cold-rolled sheet is heated between 600° C. and Ac1, Ac1 denoting the temperature at which austenitic transformation starts during heating, at a heating rate V C between 1° C./s and 20° C./s, then
said cold-rolled sheet is brought to a temperature Tm between 780° C. and (Ac3-25° C.), and said cold-rolled sheet is held at said temperature Tm for a period of time Dm between 30 seconds and 150 seconds, it being understood that Ac3 denotes the finish temperature of austenitic transformation during heating, then
the sheet is cooled at a rate VR1 between 10° C. and 150° C./s to a temperature Te between 400° C. and 490° C., then
the sheet is held at the temperature Te for a period of time De between 5 seconds and 150 seconds, then
the sheet is continuously hot-dip coated by immersion in a zinc or zinc alloy bath at temperature TZn between 450° C. and 480° C., said temperatures Te and TZn being such that 0° C.≤(Te−TZn)≤10° C. so as to obtain a coated sheet, then
said coated sheet is optionally heated at a temperature T G between 490° C. and 550° C. for a period of time t G between 10 s and 40 s.

14 . A method for manufacturing a cold-rolled and annealed sheet according to any of claims 1 to 9 and 12 , comprising the following sequential steps:
a semi-finished product is supplied, wherein the chemical composition comprises, the contents being expressed as weight percent:
0.09%≤C≤0.11%
2.6%≤Mn≤2.8%
0.20%≤Si≤0.55%
0.25%≤Cr<0.5%
0.025%≤Ti≤0.040%
0.0015%≤B≤0.0025%
0.005%≤Al≤0.18%
0.08%≤Mo≤0.15%
0.020%≤Nb≤0.040%
0.002%≤N≤0.007%
0.0005%≤S≤0.005%
0.001%≤P≤0.020%
Ca≤0.003%

the remainder being iron and inevitable impurities arising from processing,
said semi-finished product is heated at a temperature T r greater than or equal to 1250° C.,
said semi-finished product is hot-rolled, the finish temperature for rolling being greater than Ar3, to obtain a hot-rolled sheet, then
said hot-rolled sheet is cooled at a rate greater than 30° C./s to prevent the formation of ferrite and pearlite, then
said hot-rolled sheet is coiled at a temperature between 580° C. and 500° C., then
said hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet, and then
said cold-rolled sheet is re-heated at a re-heating rate VR, between 600° C. and Ac1, Ac1 denoting the temperature at which austenitic transformation starts during heating, between 1° C./s and 20° C./s, then
said cold-rolled sheet is re-heated to a temperature Tm between 780° C. and (Ac3-25° C.), and said cold-rolled sheet is held at said temperature Tm for a period of time Dm between 30 seconds and 150 seconds, it being understood that Ac3 denotes the finish temperature of austenitic transformation during heating, then
the sheet is cooled at a rate VR2 between 10° C./s and 100° C./s to a temperature Te between 400° C. and 490° C., then
said sheet is held at the temperature Te for a period of time De from 5 seconds to 150 seconds, then
said sheet is cooled to room temperature.

15 . Method for manufacturing a cold-rolled, annealed, and coated sheet accor

Assignees

• Arcelormittal

Inventors

• Drillet Josee
• Hebert Veronique

Classifications

• C23C2/02 Primary

  Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence) · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C23C2/40

  Plates; Strips · CPC title

• C22C38/001

  containing N · CPC title

• C22C38/26

  with niobium or tantalum · CPC title