EP3529392B1

EP3529392B1 - High strength cold rolled steel sheet for automotive use

Info

Publication number: EP3529392B1
Application number: EP17808049.5A
Authority: EP
Inventors: Thomas Dr. HEBESBERGER; Florian Dr. WINKELHOFER; Michael SCHWARZENBRUNNER; Edip Ozer ARMAN
Original assignee: Voestalpine Stahl GmbH; Toyota Motor Corp
Current assignee: Voestalpine Stahl GmbH; Toyota Motor Corp
Priority date: 2016-11-25
Filing date: 2017-11-24
Publication date: 2021-02-17
Anticipated expiration: 2037-11-24
Also published as: US20190352750A1; SE540040C2; JP2020509162A; WO2018096090A1; CN110268085A; SE1651545A1; EP3529392A1; JP2023099015A; KR20190089183A

Description

TECHNICAL FIELD

The present invention relates to high strength steel sheets suitable for applications in automobiles. In particular, the invention relates to cold rolled steel sheets having a tensile strength of at least 980 MPa and an excellent formability.

BACKGROUND ART

For a great variety of applications increased strength levels are a pre-requisite for lightweight constructions in particular in the automotive industry, since car body mass reduction results in reduced fuel consumption.
Automotive body parts are often stamped out of sheet steels, forming complex structural members of thin sheet. However, such parts cannot be produced from conventional high strength steels, because of a too low formability for complex structural parts. For this reason multi phase Transformation Induced Plasticity aided steels (TRIP steels) have gained considerable interest in the last years, in particular for use in auto body structural parts and as seat frame materials.
TRIP steels possess a multi-phase microstructure, which includes a meta-stable retained austenite phase, which is capable of producing the TRIP effect. When the steel is deformed, the austenite transforms into martensite, which results in remarkable work hardening. This hardening effect, acts to resist necking in the material and postpone failure in sheet forming operations. The microstructure of a TRIP steel can greatly alter its mechanical properties. The most important aspects of the TRIP steel microstructure are the volume percentage, size and morphology of the retained austenite phase, as these properties directly affect the austenite to martensite transformation, when the steel is deformed. There are several ways by which it is possible to chemically stabilize austenite at room temperature. In low alloy TRIP steels the austenite is stabilized through its carbon content and the small size of the austenite grains. The carbon content necessary to stabilize austenite is approximately 1 wt. %. However, high carbon content in steel cannot be used in many applications because of impaired weldability.
Specific processing routs are therefore required to concentrate the carbon into the austenite in order to stabilize it at room temperature. A common TRIP steel chemistry also contains small additions of other elements to help in stabilizing the austenite as well as to aid in the creation of microstructures which partition carbon into the austenite. In order to inhibit the austenite to decompose during the bainite transformation it has generally been considered necessary that the silicon content should be about 1.5 wt. %. The most common alloying addition is 1.5 wt. % of both Si and Mn.
TRIP-aided steel with a Bainitic Ferrite matrix (TBF)-steels have been known for long and attracted a lot of interest, mainly because the bainitic ferrite matrix allows an excellent stretch flangability. Moreover, the TRIP effect ensured by the strain-induced transformation of metastable retained austenite islands into martensite, remarkably improves their drawability.
The formability of TRIP steels is heavily affected by the transformation characteristics of the retained austenite phase, which is in turn affected by the austenite chemistry, its morphology and other factors. In ISIJ International Vol. 50(2010), No. 1, p. 162 -168 aspects influencing the formability of TBF steels having a tensile strength of at least 980 MPa are discussed. However, the cold rolled materials examined in this document were annealed at 950 °C and austempered at 300-500 °C for 200 s in salt bath. Accordingly, due to the high annealing temperature these materials are not suited for the production in a conventional industrial annealing line.
However, the high Si-contents generally used in TBF-steels result in the formation of silicon oxide layers on the surface of the steel strip, which may adhere to the rolls in the continuous annealing line (CAL) and give rise to surface defects on subsequently produced steel sheets. Therefore, in recent years it has been a strive to reduce the silicon content in TBF steels.
WO2013/144377 discloses a cold rolled TBF-steel sheet alloyed with Si and Al and having a tensile strength of at least 980 MPa. WO2013/144376 discloses a cold rolled TBF-steel sheet alloyed with Si and Cr and a tensile strength of at least 980 MPa.
Although these steels disclose several attractive properties there is demand for 980 MPa steel sheets having an improved property profile with respect to advanced forming operations, where both local elongation and total elongation is of importance such as for structural members in automobile seats.
Other previously proposed arrangements are disclosed in US2012/040203A1 and US2014/000765A1 .

DISCLOSURE OF THE INVENTION

The present invention is directed to high strength (TBF) steel sheets having a tensile strength of 980 - 1100 MPa and an excellent formability, wherein it should be possible to produce the steel sheets on an industrial scale in a Continuous Annealing Line (CAL). The invention aims at providing a steel composition that can be processed to complicated structural members, where both local elongation and total elongation is of importance, in particular for automobile seat components. However, it is generally considered, that if the total elongation is increased, then the properties governed by the local elongation such as the hole expanding ratio (HER) or (λ) is deteriorated.

DETAILED DESCRIPTION

The invention is described in the claims.
The steel sheet has a composition consisting of the following alloying elements (in wt. %):

C 0.07 - 0.13

Mn 2.3 - 3.1

Si 0.65 - 1.2

Cr 0.05 - 0.5

Al ≤ 0.2

Nb ≤ 0.05

the balance consists of iron and impurities.
The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. The amount of hard phases is given in volume % (vol. %). Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims.

C: 0.07 - 0.13 %

C stabilizes the austenite and is important for obtaining sufficient carbon within the retained austenite phase. C is also important for obtaining the desired strength level. Generally, an increase of the tensile strength in the order of 100 MPa per 0.1 %C can be expected. When C is lower than 0.07 % then it is difficult to attain a tensile strength of 980 MPa. If C exceeds 0.15 %, then the weldability is impaired. The upper limit is 0.13 and may be 0.12 %. The lower limit may be 0.08, 0.09, or 0.10 %. A preferred range is 0.08 - 0.13 %.

Mn: 2.3 - 3.1 %

Manganese is a solid solution strengthening element, which stabilises the austenite by lowering the M_s temperature and prevents ferrite and pearlite to be formed during cooling. In addition, Mn lowers the A_c3 temperature and is important for the austenite stability. At a content of less than 2.3 % it might be difficult to obtain the desired amount of retained austenite, a tensile strength of 980 MPa and the austenitizing temperature might be too high for conventional industrial annealing lines. In addition, at lower contents it may be difficult to avoid the formation of polygonal ferrite. However, if the amount of Mn is higher than 3.2 %, problems with segregation may occur because Mn accumulates in the liquid phase and causes banding resulting in a potentially deteriorated workability. The upper limit is 3.1, preferably 3.0, 2.9, 2.8 or 2.7 %. The lower limit may be 2.3, 2.4, or 2.5 %.

Si: 0.65 - 1.2 %

Si acts as a solid solution strengthening element and is important for securing the strength of the thin steel sheet. Si suppresses the cementite precipitation and is essential for austenite stabilization.
However, if the content is too high, then to much silicon oxides will form on the strip surface, which may lead to cladding on the rolls in the CAL and surface defects on subsequently produced steel sheets. The upper limit is therefore 1.2 % and may be restricted to 1.1, 1.05, 1.0 or 0.95 %. The lower limit is 0.65, preferably 0.7, 0.75 or 0.80 %. A preferred range is 0.7 - 1.0 %.

Cr: 0.05 - 0.5 %

Cr is effective in increasing the strength of the steel sheet. Cr is an element that forms ferrite and retards the formation of pearlite and bainite. The A_c3 temperature and the M_s temperature are only slightly lowered with increasing Cr content. Cr results in an increased amount of stabilized retained austenite. The amount of Cr is limited to 0.7 %. The upper limit may be 0.65, 0.60, 0.55, 0.50, 0.45 or 0.40, 0.35, 0.30 or 0.25 %. The lower limit may be 0.10, or 0.15 %. A preferred range is 0.1 - 0.3 %.

Si + Cr: 0.9 - 1.3 %

It is preferred that the amount of Si + Cr is in the range of 0.9 - 1.3 % because when added in combination Si and Cr have a synergistic effect and result in an increased amount of retained austenite, which, in turn, results in an improved ductility. For these reasons the amount of Si + Cr is preferably limited to the range of 0.9 to 1.2 %.

Al: ≤ 0.2 %

Al promotes ferrite formation and is also commonly used as a deoxidizer. The M_s temperature is increased with an increasing Al content. A further drawback of Al is that it results in a drastic increase in the A_c3 temperature and therefore makes it more difficult to austenitize the steel in the CAL. For these reasons the Al content is preferably limited to less than 0.1 %, more preferably to less than 0.08 %. It is thus preferred to only use Al for deoxidation. The upper level may then be 0.09, 0.08, 0.07 or 0.06 %. For securing a certain effect the lower level may set to 0.005, 0.01, 0.02 or 0.03 %.

Nb: < 0.05 %

Nb is commonly used in low alloyed steels for improving strength and toughness, because of its influence on the grain size. Nb increases the strength elongation balance by refining the matrix microstructure and the retained austenite phase due to precipitation of NbC. The steel may contain Nb in an amount of ≤ 0.05 %, preferably ≤ 0.03 %. A deliberate addition of Nb is not necessary according to the present invention. The upper limit may therefore be restricted to ≤ 0.01 %.
The high strength TRIP-assisted bainitic ferrite (TBF) steel sheets of the present invention have microstructure mainly consisting of retained austenite inclusions embedded in the matrix.
The microstructural constituents are in the following expressed in volume % (vol. %).
The steel comprises a matrix of bainitic ferrite (BF). Hence, the amount of bainitic ferrite is generally ≥ 50 % and may be ≥ 55 %, ≥ 60 % or ≥ 65 %. The microstructure may also contain tempered martensite (TM). The constituents BF and TM may be difficult to distinguish from each other. Therefore, the total content of both constituents may be limited to 70-90 %. The amount is normally in the range of 80-90 %.
Martensite may be present in the final microstructure because, depending on its stability, some austenite may transform to martensite during cooling at the end of the overaging step. Martensite may be present in an amount of ≤ 15 %. The amount of un-tempered martensite is preferably limited to 10, 9, 8, 7, 6 or 5 %. These un-tempered martensite particles are often in close contact with the retained austenite particles and they are therefore often referred to as martensite-austenite (MA) particles.
Retained austenite is a prerequisite for obtaining the desired TRIP effect. The amount of retained austenite should therefore be in the range of 2 - 20 %, preferably 5 - 15 %. The amount of retained austenite was measured by means of the saturation magnetization method described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61-64.
Polygonal ferrite (PF) is not a desired microstructural constituent and is therefore limited to ≤ 10 %, preferably ≤ 5 %, ≤ 3 % or ≤ 1 %. Most preferably, the steel is free from PF.
The mechanical properties of the claimed steel are important and at least one of the following requirements should be fulfilled:

tensile strength (R_m) 980 - 1100 MPa

yield strength (R_p0.2) 580 - 920 MPa

total elongation (A₅₀) ≥ 13 %

hole expansion ratio (λ) ≥ 50 %

yield ratio (R_p0.2/ R_m) ≥ 0.75
Preferably, all these requirements are fulfilled at the same time.
The R_m, R_p0.2 values are derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip. The total elongation (A₅₀) is derived in accordance with the Japanese Industrial Standard JIS Z 2241: 2011, wherein the samples are taken in the transversal direction of the strip.
The mechanical properties of the steel sheets of the present invention can be largely adjusted by the alloying composition and the microstructure. The microstructure may be adjusted by the heat treatment in the CAL, in particular by the isothermal treatment temperature in the overaging step.

EXAMPLES

Table 1 disclose the composition of the examined steel sheets.

Table 1. Composition of examined steel sheets.

Example	C	Si	Mn	Cr	Al
Inv. 1	0,105	0,81	2,63	0,195	0,045
Inv. 2	0,106	0,84	2,67	0,197	0,048
Inv. 3	0,106	0,84	2,67	0,197	0,048
Inv. 4	0,105	0,81	2,63	0,195	0,045
Inv. 5	0,118	0,94	2,77	0,17	0,051

Heats of the steel alloys were produced in a continuous caster. The slabs were reheated and subjected to hot rolling to a thickness of about 2.8 mm. The hot rolling finishing temperature was about 900 °C and the coiling temperature about 550 °C. The hot rolled strips were pickled and batch annealed at about 625 °C for a time of 10 hours in order to reduce the tensile strength of the hot rolled strip and thereby reducing the cold rolling forces. The strips were thereafter cold rolled in a five stand cold rolling mill to a final thickness of about 1.4 mm and finally subjected to continuous annealing.

Table 2 discloses the hot and cold rolling parameters. The batch annealing was performed between the hot- and cold rolling steps for about 10 h.

Table 2. Hot and cold rolling parameters.

Example	Hot rolled thickness (mm)	Batch annealing temperature (°C)	Cold rolling thickness (mm)	Cold rolling reduction (%)
Inv. 1	2,80	623	1,41	50
Inv. 2	2,79	623	1,41	49
Inv. 3	2,78	625	1,41	49
Inv. 4	2,79	623	1,41	49
Inv. 5	2,79	624	1,42	49

The annealing cycle consisted of heating to a temperature of about 850 °C, soaking for about 120 s, slow gas jet cooling at a rate of about 10 °C/s to a temperature of about 750 °C, rapid gas cooling at a rate of about 40 °C/s to an overaging temperature of about 390 - 400 °C, isothermal holding at the overaging temperature and final cooling to ambient temperature. The details of the treatment in the CAL are given in Table 3.

Table 3. Parameters of the treatment in the CAL.

Example	Annealing temp. (°C)	Slow Jet Cooling temp. (°C)	Rapid Jet Cooling temp. (°C)
Inv. 1	850	750	393
Inv. 2	850	750	397
Inv. 3	846	750	397
Inv. 4	842	750	394
Inv. 5	847	750	391

The material produced according to the invention was found to have excellent mechanical properties as shown in Table 4. All examples had a matrix of bainitic ferrite and contained less than 10 % martensite and minimal amounts of ferrite.

In particular, it may be noted that all inventive examples disclose a total elongation (A₅₀) of more than 13 % at the same time as the hole expansibility (λ), as measured by the hole expansion test, exceeded 52 % for all inventive examples.

Table 4. Mechanical properties.

Example	Yield Strength R_p0.2	Tensile Strength Rm	Yield ratio (R_p0.2/R_m)	Total Elongation, A₅₀ (transversal)	Hole expanding ratio λ
	(MPa)	(MPa)		(%)	(%)
Inv. 1	838	1038	0,81	13,4	53,6
Inv. 2	806	1018	0,79	13,2	64
Inv. 3	841	1038	0,81	14,2	71,8
Inv. 4	817	1027	0,80	13,4	67,6
Inv. 5	863	1084	0,80	13,5	52,2

The R_m and R_p0. values are derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip. The elongation (A₅₀) is derived in accordance with the Japanese Industrial Standard JIS Z 2241: 2011 for samples taken in the transversal direction of the strip.
The hole expanding ratio (λ) is reported as the mean value of three samples subjected to hole expansion tests (HET). It was determined by the hole expanding test method according to ISO/TS16630:2009 (E). In this test a conical punch having an apex of 60 ° is forced into a 10 mm diameter punched hole made in a steel sheet having the size of 100 x 100 mm². The test is stopped as soon as the first crack is determined and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean value is used for the calculation.
The hole expanding ratio (λ) in % is calculated as follows: $λ = (Dh - Do) / Do \times 100$
wherein Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameter of the hole after the test.

INDUSTRIAL APPLICABILITY

The material of the present invention can be widely applied to high strength structural parts in automobiles. The high strength steel sheets are particularly well suited for the production of parts having high demands on the total elongation and at the same time a low edge crack sensitivity.

Claims

A high strength cold rolled steel sheet having
a) a composition consisting of the following elements in weight %: C 0.07 - 0.13 Mn 2.3 - 3.1 Si 0.65 - 1.2 Cr 0.05 - 0.5 Al ≤ 0.2 Nb ≤ 0.05

balance Fe apart from impurities,

b) a multiphase microstructure comprising a matrix of bainitic ferrite and ≤ 10 volume % polygonal ferrite,

c) a tensile strength (R_m) of 980 - 1100 MPa, derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip.
A high strength cold rolled steel sheet according to claim 1, fulfilling at least one of the following requirements
a) a composition fulfilling at least one of the following requirements in weight %: C 0.08 - 0.13 Mn 2.4 - 3.1 Si 0.7 - 1.1 Cr 0.05 - 0.45 Al 0.005 - 0.1 Nb ≤ 0.05

wherein the impurities fulfil at least one of the requirements: Ti ≤ 0.05 Mo ≤ 0.05 N ≤ 0.015 B ≤ 0.005

balance Fe apart from impurities,

b) a multiphase microstructure comprising at least one of in volume %: retained austenite 2 - 20 martensite ≤ 15 bainitic ferrite ≥ 50 polygonal ferrite ≤ 10

c) at least one of the following mechanical properties tensile strength (R_m) 990 - 1100 MPa yield strength (R_p0.2) 580 - 920 MPa total elongation (A₅₀) ≥ 13 % hole expansion ratio (λ) ≥ 50 % yield ratio (R_p0.2/ R_m) ≥ 0.75

wherein the R_m, R_p0.2 values are derived according to the European norm EN 10002 Part 1, wherein the samples were taken in the longitudinal direction of the strip, the total elongation (A₅₀) is derived in accordance with the Japanese Industrial Standard JIS Z 2241: 2011, wherein the samples are taken in the transversal direction of the strip and wherein the hole expanding ratio (λ) is reported as the mean value of three samples subjected to hole expansion test determined by the hole expanding test method according to ISO/TS16630:2009 (E).
A high strength cold rolled steel sheet according to claim 1 or 2 having
a) a composition fulfilling at least one of the following requirements in weight %: C 0.08 - 0.13 Mn 2.5 - 3.0 Si 0.75 - 1.05 Cr 0.1 - 0.4 Si + Cr 0.9 - 1.3 Al 0.01 - 0.08 Nb ≤ 0.01

wherein the impurities fulfil at least one of the requirements: Ti ≤ 0.02 V ≤ 0.02 Mo ≤ 0.03 N ≤ 0.008 B ≤ 0.003

balance Fe apart from impurities,

b) a multiphase microstructure comprising in volume %: retained austenite 5 - 15 martensite ≤ 10 bainitic ferrite ≥ 60 polygonal ferrite ≤ 5

c) at least one of the following mechanical properties a tensile strength (R_m) 1000 - 1100 MPa a yield strength (R_p0.2) 750 - 900 MPa a hole expansion ratio ≥ 60 % a yield ratio (R_p0.2/ R_m) 0.76 - 0.85
A high strength cold rolled steel sheet according to any of the preceding claims fulfilling at least one of the following requirements:
a) a composition fulfilling at least one of the following requirements in weight %: C 0.09 - 0.12 Mn 2.5 - 2.9 Si 0.75 - 1.0 Cr 0.1 - 0.3 Si + Cr 0.9 - 1.2 Al 0.01 - 0.05

wherein the impurities fulfil at least one of the requirements: Ti ≤ 0.01 V ≤ 0.02 Mo ≤ 0.03 N ≤ 0.008 B ≤ 0.003

balance Fe apart from impurities,

c) at least one of the following mechanical properties a tensile strength (R_m) ≥ 1020 MPa a yield strength (R_p0.2) ≥ 800 MPa a yield ratio (R_p0.2/ R_m) ≥ 0.78
A high strength cold rolled steel sheet according to claim 1 or 2 fulfilling the following requirements:
a) a composition consisting of in weight %: C 0.08 - 0.13 Mn 2.4 - 3.1 Si 0.7 - 1.1 Cr 0.05 - 0.45 Al 0.005 - 0.1 Nb ≤ 0.05

wherein the impurities fulfil the requirements: Ti ≤ 0.05 Mo ≤ 0.05 N ≤ 0.015 B ≤ 0.005

balance Fe apart from impurities,
and/or

b) a multiphase microstructure comprising in volume %: retained austenite 2 - 20 martensite ≤ 15 bainitic ferrite ≥ 50 polygonal ferrite ≤ 10

and/or

c) the following mechanical properties tensile strength (R_m) 1000 - 1100 MPa yield strength (R_p0.2) 580 - 920 MPa total elongation (A₅₀) ≥ 13 % hole expansion ratio (λ) ≥ 50 % yield ratio (R_p0.2/ R_m) ≤ 0.84
A high strength cold rolled steel sheet according to claim 3 fulfilling the following requirements:
a) a composition fulfilling the following requirements in weight %: C 0.08 - 0.13 Mn 2.5 - 3.0 Si 0.7 - 1.1 Cr 0.1 - 0.4 Si + Cr 0.9 - 1.3 Al 0.01 - 0.08

wherein the impurities fulfil the requirements: Ti ≤ 0.02 V ≤ 0.02 Mo ≤ 0.03 N ≤ 0.008 B ≤ 0.003

balance Fe apart from impurities,
and/or

b) a multiphase microstructure comprising in volume %: retained austenite 5 - 15 martensite ≤ 10 bainitic ferrite ≥ 60 polygonal ferrite ≤ 5

and/or

c) the following mechanical properties a tensile strength (R_m) 1000 - 1100 MPa a yield strength (R_p0.2) 750 - 900 MPa a hole expansion ratio ≥ 60 % a yield ratio (R_p0.2/ R_m) 0.78 - 0.83
A high strength cold rolled steel sheet according to any of the preceding claims fulfilling the following requirements:
a) a composition fulfilling the following requirements in weight %: C 0.09 - 0.12 Mn 2.5 - 2.9 Si 0.7 - 1.1 Cr 0.1 - 0.3 Si + Cr 0.9 - 1.2 Al 0.01 - 0.05

wherein the impurities fulfil the requirements: Ti ≤ 0.01 V ≤ 0.02 Mo ≤ 0.03 N ≤ 0.008 B ≤ 0.003

balance Fe apart from impurities,
and/or

*c) the following mechanical properties tensile strength (R_m) 1000 - 1100 MPa yield strength (R_p0.2) 750 - 920 MPa total elongation (A₅₀) ≥ 13 % hole expansion ratio (λ) ≥ 50 % yield ratio (R_p0.2/ R_m) 0.78 - 0.82
A high strength cold rolled steel sheet according to any of the preceding claims, wherein the thickness of the cold rolled sheet is 1.0 -1. 6 mm.
A high strength cold rolled steel sheet according to any of the preceding claims, wherein the total content of bainitic ferrite and tempered martensite is 70 - 90 volume %.
A high strength cold rolled steel sheet according to any of the preceding claims, wherein the product of the tensile strength (R_m) and the total elongation (A₅₀) is ≥ 13000 MPa%.