KR20230129178A - coiling temperature effect cold rolled strip or steel - Google Patents

Abstract

The present invention (based on weight percent) C 0.08 to 0.28; Mn 1.4 to 4.5; Cr 0.01 to 0.5; Si 0.01 to 2.5; Al 0.01 to 0.6; Si + Al > 0.1; Si + Al + Cr ≥ 0.4; Nb ≤ 0.008; Ti ≤ 0.02; Mo ≤ 0.08; Ca ≤ 0.005; V ≤ 0.02; It relates to a cold-rolled strip or sheet comprising Fe with the remainder being an impurity. The river lies within the region defined by the coordinates A, B, C, D, where Ri/t (y-axis) is plotted against TS (MPa)/YR (x-axis), where A is [1200, 2), B is [2000, 4], C is [2000, 3], and D is [1200, 1].

Description

coiling temperature effect cold rolled strip or steel

The present invention relates to high-strength steel strip and sheet suitable for automotive applications.

An increased strength level is a prerequisite for lightweight structures, especially in the automotive industry, since for a wide variety of applications, reduced body mass leads to reduced fuel consumption.

Automotive body parts are often stamped from steel sheets to form complex structural members from thin sheets. However, these parts cannot be produced from conventional high-strength steels because the formability of complex structural parts is too low. For these reasons, multiphase transformation induced plasticity aided steel (TRIP steel) has gained considerable interest over the past few years, particularly for use in automotive body structural components and as seat frame materials.

TRIP steels have a multiphase microstructure, including a metastable retained austenite phase that can produce a TRIP effect. When steel is deformed, austenite transforms to martensite, resulting in significant work hardening. This hardening effect acts to resist necking of the material and retards failure in the sheet forming operation. The microstructure of TRIP steel can greatly alter its mechanical properties.

TRIP steels have been known for a long time and have attracted much attention mainly because the matrix allows for good stretch flangability. In addition, the TRIP effect ensured by the strain-induced transformation of metastable retained austenite islands to martensite significantly improves their drawability.

When producing cold rolled TRIP steel sheet, a slab is initially provided. The slab is hot rolled into hot rolled strip in the austenitic temperature range. After that, the hot-rolled strip is coiled. Coiling resistance decreases with increasing temperature. Typically a coiling temperature of 600°C is used. The coiled strip is then batch annealed and then cold rolled. The cold rolled strip is then continuously annealed.

WO 2019/122963 A1 and WO2019123043 A1 both disclose TRIP steels with improved phosphate coverage. Excellent phosphating coverage is possible. Improved phosphate coverage was achieved by controlling alloying elements and process parameters with low coiling temperatures. All of the above inventive examples have a coiling temperature of 450°C. The reference example with higher coiling temperature did not provide sufficient phosphate coverage. A lower coiling temperature increases the cold rolling force.

EP 2707514 B1 discloses a TRIP steel with a microstructure comprising 5 to 20% polygonal ferrite, 10 to 15% retained austenite, 5 to 15% martensite and the balance bainite. According to the literature, the presence of 5 to 20% polygonal ferrite makes it possible to exceed a V-bend angle of 90° without cracking.

WO2018116155 discloses a TRIP steel. The above inventive examples disclose a lower coiling temperature of 450°C with higher batch annealing temperatures of 620°C and 650°C, and a higher coiling temperature of 560°C with a lower batch annealing temperature of 460°C, respectively. do.

EP 3 653 738 A1 contains 3 to 15% retained austenite, at least 30% tempered martensite, max. 5% fresh martensite, max. 35% bainite, 5 to 15% martensite, 5 to 15% martensite. A TRIP steel having a microstructure comprising 35% ferrite is disclosed.

Although these steels disclose some attractive properties, with respect to advanced forming operations there is a need for steel sheets or strips of >950 MPa with an improved property profile, particularly bending properties, especially those relating to strength and toughness. Additional desirable properties are reduced grain-boundary oxidation, reduced susceptibility to liquid metal embrittlement, reduced susceptibility to hydrogen embrittlement, and good phosphatability.

The present invention relates to a cold-rolled steel having a tensile strength of at least 950 MPa and good formability, which should enable production of steel sheet/strip on an industrial scale in a continuous annealing line (CAL) and a hot dip galvanizing line (HDGL). , for cold-rolled steel. SUMMARY OF THE INVENTION The present invention aims to provide a steel having a composition and microstructure capable of being processed into complex high-strength structural members in which bending properties are important.

Careful selection of alloying elements and processing parameters reduces grain boundary oxidation. Reduced grain boundary oxidation improves bendability and reduces the risk of liquid metal embrittlement and susceptibility to hydrogen embrittlement. This further promotes good phosphate processability.

Figure 1 shows a graph with a sample of the present invention within the dotted line.
Figure 2a does not show a sample of the present invention without grain boundary oxidation.
Figure 2b shows the surface of the inventive sample of Figure 2a.
Figure 3a shows grain boundary oxidation of a reference sample.
Figure 3b is an enlarged view of the grain boundary of Figure 3a.
Figure 3c shows the surface of the reference sample of Figures 3a and 3b.
Figure 4 shows the phosphate coverage of inventive samples Figures 2a and 2b.
Figure 5 shows the phosphate coverage of the reference samples of Figures 3A-3C.

The invention is described in the claims.

The steel sheet has a composition consisting of the following alloying elements (wt%):

C 0.08 to 0.28

Mn 1.4 to 4.5

Cr 0.01 to 0.5

Si 0.01 to 2.5

Al 0.01 to 0.6

Si + Al ≥ 0.1

Si+Al+Cr ≥ 0.4

Nb ≤ 0.1

Ti ≤ 0.1

Mo ≤ 0.5

Ca ≤ 0.05

V ≤ 0.1

The balance is Fe excluding impurities.

The importance of the individual elements and their interactions with one another, as well as the limits of the chemical composition of the claimed alloy, are outlined below. All percentages for the chemical composition of steel are presented as weight percent (wt. %) throughout the description. The upper and lower limits of the individual elements may be freely combined within the upper and lower limits set forth in the claims. The arithmetic precision of numerical values may be increased by one or two orders of magnitude for all values provided herein. Thus, for example, a value given as 0.1% can also be expressed as 0.10 or 0.100%. The amounts of microstructural constituents are given in % by volume (vol.%).

C: 0.08 to 0.28%

C is important for stabilizing austenite and obtaining sufficient carbon within the retained austenite phase. C is also important in obtaining the desired strength level. Generally, an increase in tensile strength of the order of 100 MPa per 0.1% C can be expected. When C is less than 0.08%, it is difficult to achieve a tensile strength of 950 MPa. When C exceeds 0.28%, weldability is impaired. Thus, the upper limit may be 0.26, 0.24, 0.22, 0.20 or 0.18%. The lower limit may be 0.10, 0.12, 0.14, or 0.16%.

Mn: 1.4 to 4.5%

Manganese is a solid solution strengthening element and lowers the Ms temperature to stabilize austenite and prevent ferrite and pearlite from forming during cooling. In addition, Mn lowers the _Ac3 temperature and is important for austenite stability. At contents below 1.5%, it may be difficult to obtain the desired amount of retained austenite, and the tensile strength of 950 MPa and the austenitizing temperature may be too high for typical industrial annealing lines. Also, at lower contents, the formation of polygonal ferrite may be difficult to avoid. However, when the amount of Mn is more than 4.5%, a problem of segregation may arise because Mn accumulates in the liquid phase and causes banding, potentially resulting in deteriorated workability. Thus, the upper limit may be 4.2, 4.0, 3.8, 3.6, 3.4, 3.2, 3.0, 2.8, 2.6, or 2.4%. The lower limit may be 1.5, 1.7, 1.9, 2.1, 2.3, or 2.5%.

Cr: 0.01 to 0.5%

Cr is effective in increasing the strength of a steel sheet. Cr is an element that forms ferrite and retards the formation of pearlite and bainite. A _c3 temperature and Ms temperature only slightly lower with increasing Cr content. Cr increases the amount of stabilized retained austenite. If it is more than 0.5%, it may damage the surface finish of the steel, so the amount of Cr is limited to 0.5%. The upper limit may be 0.45 or 0.40, 0.35, 0.30 or 0.25%. The lower limit may be 0.01, 0.03, 0.05, 0.07, 0.10, 0.15, 0,20 or 0.25%. Preferably, no intentional addition of Cr is performed according to the present invention.

Si: 0.01 to 2.5%

Si acts as a solid solution strengthening element and is important for ensuring the strength of thin steel strips. Si inhibits cementite precipitation and is essential for austenite stabilization. However, if the content is too high, too much silicon oxide will be formed on the strip surface, which may cause cladding on the rolls in CAL and, consequently, surface defects in the steel sheet produced subsequently. Thus, the upper limit is 2.5% and may be limited to 2.4, 2.2, 2.0, 1.8 or 1.6%. The lower limit may be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.60, 0.80 or 1.0%.

Al: 0.01 to 0.6%

Al promotes ferrite formation and is also commonly used as a deoxidizer. Al is not soluble in cementite like Si and therefore significantly retards cementite formation during bainite formation. Also, reduced susceptibility to galvanization and liquid metal embrittlement can be improved. The addition of Al results in a significant increase in the carbon content in the retained austenite. A major disadvantage of Al is its segregation behavior during casting. During casting, Mn is concentrated in the middle of the slab, and the Al-content decreases. Thus, a significant austenite stabilized region or band may form in the middle of the slab. This results in martensite banding at the end of machining, and low strain internal cracks are formed in the martensite band. On the other hand, Si and Cr are also enriched during casting. Therefore, the tendency to martensitic bending can be reduced by alloying with Si and Cr, since the stabilization of austenite due to Mn enrichment is offset by these elements. For this reason, the Al content is preferably limited. The upper level can be 0.6, 0.5, 0.4, 0.3, 0.2, 0.1%. The lower limit can be set to 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1%. If Al is only used for deoxidation, the upper level may be 0.09, 0.08, 0.07 or 0.06%. Lower levels can be set at 0.03 or 0.04% to achieve specific effects.

Si + Al ≥ 0.1%

Si and Al inhibit cementite precipitation during bainite formation. Therefore, their combined content is preferably at least 0.1%. The upper limit may be 2%.

Si + Al + Cr ≥ 0.4%

Certain amounts of these elements favor the formation of austenite. Therefore, their combined content should be at least > 0.4%. The lower limit may be 0.5, 0.6 or 0.7%.

Mn + Cr 1.7 to 5.0%

Manganese and chromium affect the hardenability of steel. Their combined content is preferably in the range of 1.7 to 5.0%.

optional element

Mo ≤ 0.5%

Molybdenum is a strong hardener. This may further enhance the benefits of NbC precipitates by reducing carbide coarsening kinetics. Thus, steels can contain Mo in amounts of up to 0.5%. The upper limit may be limited to 0.4, 0.3, 0.2, or 0.1%. According to the present invention no intentional addition of Mo is required. Therefore, the upper limit may be limited to ≤ 0.01%.

Nb: ≤ 0.1%

Nb is commonly used in low alloy steels to improve strength and toughness due to its effect on grain size. Nb increases the strength-elongation balance by refining the matrix microstructure and retained austenite phase due to precipitation of NbC. The steel may contain Nb in an amount of ≤ 0.1%. The upper limit may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. The intentional addition of Nb is not necessary according to the present invention. Therefore, the upper limit may be limited to ≤ 0.004%.

V: ≤ 0.1%

The function of V is similar to that of Nb in that it contributes to precipitation hardening and grain refinement. Steel may contain V in an amount of ≤ 0.1%. The upper limit may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. The intentional addition of V is not necessary according to the present invention. Therefore, the upper limit may be limited to ≤ 0.01%.

Ti: ≤ 0.1%

Because Ti affects grain size by forming carbides, nitrides or carbonitrides, it is commonly used in low-alloy steels to improve strength and toughness. In particular, Ti is a strong nitride former and can be used to bind nitrogen in steel. However, the effect tends to saturate above 0.1%. The upper limit may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. The intentional addition of Ti is not necessary according to the present invention. Therefore, the upper limit may be limited to ≤ 0.005%.

Calcium ≤ 0.05

Ca can be used for reforming non-metallic inclusions. The upper limit is 0.05%, and may be set to 0.04, 0.03, or 0.01%. The intentional addition of Ca is not necessary according to the present invention. Therefore, the upper limit may be limited to ≤ 0.005%.

impurities

Cu: ≤ 0.06%

Cu is an undesirable impurity element limited to ≤ 0.06% by careful selection of the scrap used.

Ni: ≤ 0.08%

Ni is also an undesirable impurity element limited to ≤ 0.08% by careful selection of the scrap used.

B: ≤ 0.0006%

B is an undesirable impurity element limited to ≤ 0.0006% by careful selection of the scrap used. B increases hardness but may decrease bendability and is therefore not preferred in this proposed steel. B can further make scrap recycling more difficult, and addition of B can also deteriorate workability. Therefore, intentional addition of B is not preferred according to the present invention.

Other impurity elements may be present in the steel in normal occurrences. However, it is preferred to limit the amount of P, S, As, Zr, Sn to the following optional maximum content:

P: ≤ 0.02%

S: ≤ 0.005%

As ≤ 0.010%

Zr ≤ 0.006%

Sn ≤ 0.015%

It is also preferable to control the nitrogen content in the following range:

N: ≤ 0.015%, preferably 0.003 to 0.008%

Stable fixation of nitrogen can be achieved in this range.

Oxygen and hydrogen are additionally

O: ≤ 0.0003

H: Can be limited to ≤ 0.0020.

Microstructural constituents can be expressed in volume percent (vol. %) as:

The cold rolled steel sheet of the present invention has a microstructure comprising at least 50% of tempered martensite (TM) and bainite (B). The lower limit may be limited to at least 60%, 70%, 75%, or 80%.

and, additionally, up to 10% fresh martensite (FM). The upper limit may be limited to 8% or 5%. A small amount of fresh martensite can improve edge flangeability and local ductility. The lower limit may be limited to 1% or 2%. These untempered martensite grains are often in close contact with the retained austenite grains, and therefore they are often referred to as martensite-austenite (MA) grains.

Retained austenite is a prerequisite for obtaining the desired TRIP effect. Therefore, the amount of retained austenite should be in the range of 2 to 20%, preferably 5 to 15%. The amount of retained austenite can be found in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61-64] was measured by the saturation magnetization method.

Polygonal ferrite (PF) is susceptible to hydrogen embrittlement and is therefore not a desired microstructural component. Polygonal ferrite combined with martensite is not good for bending properties. In addition, the presence of ferrite can impart formability and elongation to the steel and also to some extent resistance to fatigue failure. This can also have negative effects due to the fact that ferrite increases the hardness gap with hard phases such as martensite and bainite and reduces local ductility, resulting in a lower hole expansion ratio. Therefore, the polygonal ferrite (PF) is limited to ≤ 10%, preferably ≤ 5%, ≤ 3% or ≤ 1%. Most preferably, the steel is free of PF. Steel does not contain other types of ferrites as their bending properties are negatively affected. Also, the yield ratio is negatively affected by ferrite, which is not good for bending properties.

The mechanical properties of the claimed steel are important and the following requirements must be met:

TS tensile strength (R _m ) 950 to 1550 MPa

YS yield strength (R _p0.2 ) 550 to 1400 MPa

YR yield ratio (R _p0.2 /R _m ) ≥ 0.50, preferably ≥ 0.7

Bendability (Ri/t) ≤ 5

The R _m , R _p0.2 values are derived according to European standard EN 10002 part 1, where samples are taken in the longitudinal direction of the strip. Total elongation (A ₅₀ ) is derived according to Japanese Industrial Standard JIS Z 2241: 2011, where samples are taken in the transverse direction of the strip.

Bendability is evaluated by the ratio of the sheet thickness t to the limit bending radius Ri, which is defined as the minimum bending radius at which cracks do not occur. For this, a 90° V-shaped block is used to bend the steel sheet according to JIS Z2248. The value obtained by dividing the limit bending radius by the thickness (Ri/t) should be less than 5, preferably less than 4. Ri(t) may be further limited to 3, 2.5 or 2.

The yield ratio YR is defined as yield strength YS divided by tensile strength TS. The lower limit for YR can be 0.70, 0.75, 0.76, 0.77, or 0.78.

The river must also lie within the region defined by coordinates A, B, C, D in Figure 1, where Ri/T (y-axis) is plotted against TS/YR (x-axis), and A is [ 1200, 2), B is [2000, 4], C is [2000, 3], and D is [1200, 1]. The upper dotted line may be mathematically expressed as y=0.0025*x-1, and the lower dotted line may be expressed as y=0.0025*x-2. This gives the criterion 1 ≤ 0.0025*TS/YR - Ri/t ≤ 2. Steels meeting the criteria were found to have a good balance between strength and bendability. The lower limit may be 1.1, 1.2 or 1.3 and the upper limit may be 1.9 or 1.8.

The TS/YR value may be further limited so that the TS/YR is within 1000 to 2000 (MPa). The lower bound may be 1100, 1200, 1300, 1400, 1500, 1600, 1700, or 1800. The upper limit can be 1900, 1800, 1700, 1600, 1500, or 1400. A preferred range may be 1200 to 1400. Other ranges may be, for example, 1400 to 1600, or 1600 to 1800, or 1800 to 2000.

The hole expansion ratio (λ) [HER] is preferably > 20%. The hole expansion ratio λ is determined by a hole expansion test according to ISO/WD 16630:2009(E). In this test, a conical punch with an apex of 60° is forced into a 10 mm diameter punched hole made in a steel sheet having dimensions of 100×100 mm 2 . As soon as the first crack is determined, the test is stopped and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean value is used in the calculation.

The hole expansion ratio (λ) expressed in % is calculated as follows:

λ = (Dh - Do)/Do × 100

[Wherein, Do is the diameter of the hole at the start (10 mm), and Dh is the diameter of the hole after testing].

The cold rolled heat treated steel sheet of the present invention may optionally be coated with zinc or zinc alloy, or with aluminum or aluminum alloy to improve its corrosion resistance.

The proposed steel can be produced by producing steel slabs of conventional metallurgy by converter melting and secondary metallurgy using the proposed composition. The slab is hot rolled into hot rolled strip in the austenitic range. Preferably, by reheating the slab to a temperature of 1000° C. to 1280° C., the slab is completely rolled in an austenitic range with a hot rolling finishing temperature of 850° C. or higher to obtain a hot-rolled steel strip. Then, the hot-rolled strip is coiled at a coiling temperature in the range of 500 to 540°C. Optionally, the coiled strip is subjected to a descaling process such as pickling. The coiled strip is then batch annealed at a temperature ranging from 500 to 650° C., preferably from 550 to 650° C., for a period of 5 to 30 hours. The annealed steel strip is then cold rolled with a reduction of 35 to 90%, preferably approximately 40 to 60%. The cold-rolled steel strip is further processed in a continuous annealing line (CAL) or hot-dip galvanizing line (HDGL), where the microstructure is finely tuned. Both lines treat the steel to an immersion temperature of 800 to 1000°C, preferably 830 to 900°C, followed by holding at 200 to 500°C, preferably 350 to 450°C, preferably for a time of 150 to 1000 seconds. Perform rapid slow jet and rapid jet cooling to temperature, then cool to room temperature. The immersion temperature is higher than A _c3 to avoid the formation of inter critical ferrite. A _c3 is as defined by A _c3 = 910-203*C ^1/2 - 15.2 Ni - 30 Mn + 44.7 Si + 104 V + 31.5 Mo + 13.1 W. Preferably, the immersion temperature is at least A _c3 +20°C, more preferably A _c3 +30°C. Preferably, the immersion temperature and time are controlled to allow for 100% austenite and free of ferrite prior to cooling. The immersion time may be, for example, 40 seconds to 180 seconds.

M _S may be defined by the formula M _S =692-502*(C+0.68N) ^0.5 -37*Mn-14*Si+20*Al-11*Cr. The holding temperature may be above or below _MS .

In one embodiment, the cold rolled strip or sheet is

a) having a composition comprising (% by weight):

C 0.08 to 0.16

Mn 2.0 to 3.0

Cr 0.1 to 0.5

Si 0.5 to 1.2

Al 0.01 to 0.5

Si + Al ≥ 0.1

Si + Al + Cr ≥ 0.4

Nb ≤ 0.1

Ti ≤ 0.1

Mo ≤ 0.5

Ca ≤ 0.05

V ≤ 0.1

Fe excluding impurities as balance;

b) meets at least one of the following conditions:

TS tensile strength (R _m ) 950 to 1150 MPa

YS yield strength (R _p0.2 ) 750 to 1000 MPa

YR yield ratio (R _p0.2 /R _m ) ≥ 0.70

Bendability (Ri/t) ≤ 2

HER ≥ 20.

In another embodiment, the cold rolled strip or sheet is

a) having a composition comprising (% by weight):

C 0.15 to 0.25

Mn 1.0 to 2.0

Cr 0.1 to 0.5

Si 0.1 to 0.5

Al 0.01 to 0.5

Si + Al ≥ 0.1

Si + Al + Cr ≥ 0.4

Nb ≤ 0.1

Ti ≤ 0.1

Mo ≤ 0.5

Ca ≤ 0.05

V ≤ 0.1

Fe excluding impurities as balance;

b) meets at least one of the following conditions:

TS tensile strength (R _m ) 1300 to 1550 MPa

YS yield strength (R _p0.2 ) 1000 to 1300 MPa

YR yield ratio (R _p0.2 /R _m ) ≥ 0.70

Bendability (Ri/t) ≤ 4

HER ≥ 20.

In another embodiment, the cold rolled strip or sheet is

a) having a composition comprising (% by weight):

C 0.15 to 0.25

Mn 2.0 to 3.0

Cr 0.1 to 0.5

Si 1 to 2.0

Al 0.01 to 0.5

Si + Al ≥ 0.1

Si + Al + Cr ≥ 0.4

Nb ≤ 0.1

Ti ≤ 0.1

Mo ≤ 0.5

Ca ≤ 0.05

V ≤ 0.1

Fe excluding impurities as balance;

b) meets at least one of the following conditions:

TS tensile strength (R _m ) 1100 to 1300 MPa

YS yield strength (R _p0.2 ) 900 to 1100 MPa

YR yield ratio (R _p0.2 /Rm) ≥ 0.70

Bendability (Ri/t) ≤ 3

HER ≥ 20.

In another embodiment, the cold roll strip or sheet is

a) having a composition comprising (% by weight):

C 0.10 to 0.20

Mn 2.0 to 3.0

Cr 0.1 to 0.5

Si 0.2 to 0.9

Al 0.01 to 0.5

Si + Al ≥ 0.1

Si + Al + Cr ≥ 0.4

Nb ≤ 0.1

Ti ≤ 0.1

Mo ≤ 0.5

Ca ≤ 0.05

V ≤ 0.1

Fe excluding impurities as balance;

b) meets at least one of the following conditions:

TS tensile strength (R _m ) 900 to 1100 MPa

YS yield strength (R _p0.2 ) 600 to 800 MPa

YR yield ratio (R _p0.2 /R _m ) ≥ 0.65

Bendability (Ri/t) ≤ 2.5

HER ≥ 20.

Example

In FIG. 1, the limit bending radius (Ri) divided by the cold rolling thickness is plotted against the tensile strength divided by the yield ratio (TS/YR) for the steels of Examples 1 to 4. The inventive steel lies within the region defined by the coordinates A, B, C, D when Ri/t (y-axis) is plotted against TS/YR (x-axis), where A is [1200, 2), B is [2000, 4], C is [2000, 3], and D is [1200, 1]. The upper dotted line may be mathematically expressed as y=0.0025*x-1, and the lower dotted line may be expressed as y=0.0025*x-2.

Thus, the reference steels coiled at higher temperatures are all above the upper dotted line mathematically defined as:

y = 0.0025*x-1, where y is Ri/t and x is TS(MPa)/YR.

The inventive steels of Examples 1 to 5 are all below the upper line.

The lower dotted line is defined by the formula:

y = 0.0025*x-2, where y is Ri/t and x is TS(MPa)/YR.

The inventive steels of Examples 1 to 5 are all above the bottom line.

Within these boundaries, good bending properties are achieved with respect to strength and toughness.

Example 1

Steels I1 to I6 and reference steels R1 and R2 were produced by conventional metallurgy and secondary metallurgy by converter melting. The composition is shown in Table 1, and the additional elements were present only as impurities and were below the lowest levels specified herein. All steels have approximately the same composition.

Table 1

The steel was continuously cast and cut into slabs.

The slab was reheated and hot rolled in the austenitic range to a thickness of about 2.8 mm. The hot rolling finishing temperature was about 900°C.

The hot-rolled steel strips were then coiled, steels I1 to I6 were conducted at a coiling temperature of 530°C, and reference steels R1 and R2 were conducted at about 630°C.

The coiled hot rolled strip was pickled and batch annealed at about 624° C. for 10 hours to reduce the cold rolling force by reducing the tensile strength of the hot rolled strip.

The strip was then cold rolled to a final thickness of about 1.41 mm on a five stand cold rolling mill and finally subjected to continuous annealing on a continuous annealing line (CAL). In CAL, the strip was heated to an immersion temperature of about 850° C. and held for about 120 seconds. After annealing, the strip was slow jet cooled (SJC) to about 750° C. followed by rapid jet cooling (RJC) to a holding temperature of about 400° C. The strip was held for about 180 seconds and then cooled to room temperature.

A _c3 was approximately 800 °C for all steels, so immersion is defined by the equation A _c3 = 910-203*C ^1/2 - 15.2 Ni - 30 Mn + 44.7 Si + 104 V + 31.5 Mo + 13.1 W A _c3 as described above performed well.

Process parameters are presented in Table 2.

Table 2

Yield strength YS and tensile strength TS were derived according to European standard EN 10002 Part 1. Samples were taken along the length of the strip.

A sample of the produced strip was subjected to a V bending test according to JIS Z2248 to find out the limit bending radius (Ri). The samples were examined visually and under a light microscope at 25x magnification to investigate the occurrence of cracks. Ri/t was determined by dividing the limit bending radius (Ri) by the thickness (t) of the cold rolled strip. Ri is the largest radius at which the material does not crack after three bending tests.

The limit bending radii (Ri) of steels I1 to I6 coiled at 530°C were smaller than those of R1 and R2 coiled at 630°C.

All steels I1 to I6 satisfied the condition 1 ≤ 0.0025*TS/YR - Ri/t ≤ 2, while R1 and R2 fell short.

Mechanical properties are presented in Table 3.

Table 3

Figures 2a and 2b show the test of the inventive steel I6 coiled at 530 °C, and Figures 3a to 3c show the test of the reference steel R1 coiled at 630 °C. The reference steel R1 showed grain boundary oxidation, whereas the inventive steel I6 did not show grain boundary oxidation.

3c shows visible cracks in the sample surface of reference steel R1. These lead to breakout after pickling and cold rolling. In particular, grain boundary oxides result in generation around the grains of the present invention, which can lead to complete grain breakout. Cracks/breakouts decrease with respect to the bend ratio.

Figure 2b shows that there are no visible cracks on the surface of the sample of the inventive steel. The lack of grain boundary oxides and the absence of visible cracks in the steels of this invention improve the bend ratio and reduce the risk of liquid metal embrittlement. This further promotes good phosphate processability.

Figure 4 shows phosphate coverage for I6.

The microstructure of I6 was determined to be:

Bainite + Tempered Martensite > 85%

fresh martensite about 5%

retained austenite about 5%

Example 2

Steel I7 and reference steel R3 were produced by conventional metallurgy and secondary metallurgy by converter melting. The composition is shown in Table 4, with additional elements present only as impurities and below the lowest level specified herein. All steels have approximately the same composition. Steels I7 and R3 have higher Cr and C contents and lower Si and Mn contents than the steel of Example 1. This provides a steel with higher yield strength and higher tensile strength.

Table 4

The steels were treated with the same process as Example 1, where steel I7 was coiled at a coiling temperature of 532°C and reference steel R3 was coiled at 626°C.

In CAL, the strip was heated to an immersion temperature of about 850° C. and held for about 120 seconds. After annealing, the strip was slow jet cooled (SJC) to about 700° C. followed by rapid jet cooling (RJC) to a holding temperature of about 250° C. The strip was held for about 180 seconds and then cooled to room temperature. All other process parameters were almost identical to those of Example 1.

A _c3 was about 780 °C for all steels, so immersion is defined by the equation A _c3 = 910-203*C ^1/2 - 15.2 Ni - 30 Mn + 44.7 Si + 104 V + 31.5 Mo + 13.1 W was performed at a much higher temperature than A _c3 as described above.

Process parameters are presented in Table 5.

table 5

A sample of the prepared strip was subjected to the same test as in Example 1. The limiting bending radius (Ri) of steel I7 coiled at 532°C was smaller than the limiting bending radius (Ri) of steel R3 coiled at 626°C.

Steel I7 satisfied the condition 1 ≤ 0.0025*TS/YR - Ri/t ≤ 2, while R3 fell short.

Mechanical properties are presented in Table 6.

table 6

The microstructure of I7 was determined to be:

Bainite + Tempered Martensite about 95%

retained austenite about 5%

Example 3

Steel I8 and reference steel R4 were produced by conventional metallurgy and secondary metallurgy by converter melting. The composition is shown in Table 7, with additional elements present only as impurities and below the lowest level specified herein. All steels have approximately the same composition. Steels I8 and R4 have higher Si and C contents and lower Cr contents than the steel of Example 1. This results in a steel with a tensile strength slightly higher than that of Example 1.

table 7

The steels were treated with the same process as Example 1, where steel I8 was coiled at a coiling temperature of 535°C and reference steel R4 was coiled at 633°C. All other process parameters were almost identical to those of Example 1.

A _c3 was approximately 810 to 815 °C for steel, so immersion is defined by the equation A _c3 = 910-203*C ^1/2 - 15.2 Ni - 30 Mn + 44.7 Si + 104 V + 31.5 Mo + 13.1 W was performed at a much higher temperature than A _c3 as described above.

Process parameters are presented in Table 8.

Table 8

A sample of the prepared strip was subjected to the same test as in Example 1.

The limit bending radius (Ri) of steel I8 coiled at 535°C was smaller than that of steel R4 coiled at 633°C (Ri).

Steel I8 satisfied the condition 1 ≤ 0.0025*TS/YR - Ri/t ≤ 2, while R4 fell short.

Mechanical properties are presented in Table 9.

Table 9

The microstructure of I9 was determined to be:

Bainite + Tempered Martensite >70%

fresh martensite < 15%

retained austenite < 15%

Example 4

Steel I9 and reference steel R5 were produced by conventional metallurgy and secondary metallurgy by converter melting. The composition is shown in Table 10, with additional elements present only as impurities and below the lowest level specified herein. All steels have approximately the same composition. Steels I9 and R5 have slightly higher C contents and slightly lower Mn and Si contents than the steel of Example 1.

Table 10

In Example 4, the CAL line was replaced with a hot dip galvanizing line. Prior to the hot dip galvanizing line, the steels were treated in a process similar to Example 1, where steel I9 was coiled at a coiling temperature of 520°C and reference steel R5 was coiled at 630°C. The batch annealing temperature was 570 °C.

A _c3 was about 780 °C for steel, so immersion is as defined by the equation A _c3 = 910-203*C ^1/2 - 15.2 Ni - 30 Mn + 44.7 Si + 104 V + 31.5 Mo + 13.1 W It was carried out at a much higher temperature than A _c3 .

Process parameters are presented in Table 11.

Table 11

A sample of the prepared strip was subjected to the same test as in Example 1.

The limit bending radius (Ri) of steel I9 coiled at 520°C was smaller than the limit bending radius (Ri) of steel R5 coiled at 630°C.

Steel I9 satisfied the condition 1 ≤ 0.0025*TS/YR - Ri/t ≤ 2, while R5 fell short.

Mechanical properties are presented in Table 12.

Table 12

The microstructure of I9 was determined to be:

Bainite + Tempered Martensite about 85%

fresh martensite about 5%

retained austenite about 10%

Claims (18)

As cold-rolled steel strip or sheet,
a) having a composition consisting of (in weight percent):
C 0.08 to 0.28
Mn 1.4 to 4.5
Cr 0.01 to 0.5
Si 0.01 to 2.5
Al 0.01 to 0.6
Si + Al ≥ 0.1
Si + Al + Cr ≥ 0.4
Nb ≤ 0.1
Ti ≤ 0.1
Mo ≤ 0.5
Ca ≤ 0.05
V ≤ 0.1
Fe excluding impurities as balance;
b) the following conditions are met:
TS tensile strength (R _m ) 950 to 1550 MPa
YS yield strength (R _p0.2 ) 550 to 1400 MPa
YR yield ratio (R _p0.2 /R _m ) ≥ 0.50
bendability (Ri/t) ≤ 5;
c) is within the region defined by the coordinates A, B, C, D, where Ri/t (y-axis) is plotted against TS (MPa)/YR (x-axis), and A is [1200, 2), B is [2000, 4] and C is [2000, 3] and D is [1200, 1];
d) having a multiphase microstructure comprising (in volume %):
Tempered martensite + bainite ≥ 50
Fresh martensite ≤ 10
Retained austenite 2 to 20
Polygonal ferrite ≤ 10
Cold rolled strip or sheet. The method of claim 1, wherein the composition (% by weight)
C 0.1 to 0.25
Mn 1.4 to 3
Cr 0.01 to 0.5
Si 0.1 to 1
Al 0.01 to 0.1
Si + Al ≥ 0.1
Si + Al + Cr ≥ 0.4
Nb ≤ 0.008
Ti ≤ 0.02
Mo ≤ 0.08
Ca ≤ 0.005
V ≤ 0.02
Fe excluding impurities as balance
Including, cold rolled strip or sheet. 3. Cold-rolled strip or sheet according to claim 1 or 2, wherein the polygonal ferrite is ≤ 5, preferably ≤ 1. 4. Cold rolled strip or sheet according to any one of claims 1 to 3, wherein the composition meets at least one of the following requirements:
Si ≥ 0.4
Si+Al ≥ 0.8
Al ≤ 0.1
Mn+Cr 1.7 to 5.0. 5. Cold rolled strip or sheet according to claim 4, which meets all of the requirements of claim 4. 6. Cold rolled strip or sheet according to any one of claims 1 to 5, wherein the microstructure meets at least one of the following requirements:
Tempered martensite + bainite ≥ 60
Fresh martensite 1 to 10
Retained austenite 5 to 15
Polygonal ferrite ≤ 1. 7. Cold rolled strip or sheet according to any one of claims 1 to 6, wherein the yield ratio is > 0.70. According to claim 1,
a) having a composition comprising (% by weight):
C 0.08 to 0.16
Mn 2.0 to 3.0
Cr 0.1 to 0.5
Si 0.5 to 1.2
Al 0.01 to 0.5
Si + Al ≥ 0.1
Si + Al + Cr ≥ 0.4
Nb ≤ 0.1
Ti ≤ 0.1
Mo ≤ 0.5
Ca ≤ 0.05
V ≤ 0.1
Fe excluding impurities as balance;
a) meeting at least one of the following conditions:
TS tensile strength (R _m ) 950 to 1150 MPa
YS yield strength (R _p0.2 ) 750 to 1000 MPa
YR yield ratio (R _p0.2 /R _m ) ≥ 0.70
Bendability (Ri/t) ≤ 2
HER ≥ 20
Cold rolled strip or sheet. According to claim 1,
c) having a composition comprising (in weight percent):
C 0.15 to 0.25
Mn 1.0 to 2.0
Cr 0.1 to 0.5
Si 0.1 to 0.5
Al 0.01 to 0.5
Si + Al ≥ 0.1
Si + Al + Cr ≥ 0.4
Nb ≤ 0.1
Ti ≤ 0.1
Mo ≤ 0.5
Ca ≤ 0.05
V ≤ 0.1
Fe excluding impurities as balance;
d) meeting at least one of the following conditions:
TS tensile strength (R _m ) 1300 to 1550 MPa
YS yield strength (R _p0.2 ) 1000 to 1300 MPa
YR yield ratio (R _p0.2 /R _m ) ≥ 0.70
Bendability (Ri/t) ≤ 4
HER ≥ 20
Cold rolled strip or sheet. According to claim 1,
a) having a composition comprising (% by weight):
C 0.15 to 0.25
Mn 2.0 to 3.0
Cr 0.1 to 0.5
Si 1 to 2.0
Al 0.01 to 0.5
Si + Al ≥ 0.1
Si + Al + Cr ≥ 0.4
Nb ≤ 0.1
Ti ≤ 0.1
Mo ≤ 0.5
Ca ≤ 0.05
V ≤ 0.1
Fe excluding impurities as balance;
b) meeting at least one of the following conditions:
TS tensile strength (R _m ) 1100 to 1300 MPa
YS yield strength (R _p0.2 ) 900 to 1100 MPa
YR yield ratio (R _p0.2 /R _m ) ≥ 0.70
Bendability (Ri/t) ≤ 3
HER ≥ 20
Cold rolled strip or sheet. According to claim 1,
a) having a composition comprising (% by weight):
C 0.10 to 0.20
Mn 2.0 to 3.0
Cr 0.1 to 0.5
Si 0.2 to 0.9
Al 0.01 to 0.5
Si + Al ≥ 0.1
Si + Al + Cr ≥ 0.4
Nb ≤ 0.1
Ti ≤ 0.1
Mo ≤ 0.5
Ca ≤ 0.05
V ≤ 0.1
Fe excluding impurities as balance;
b) meeting at least one of the following conditions:
TS tensile strength (R _m ) 900 to 1100 MPa
YS yield strength (R _p0.2 ) 600 to 800 MPa
YR yield ratio (R _p0.2 /R _m ) ≥ 0.65
Bendability (Ri/t) ≤ 2.5
HER ≥ 20
Cold rolled strip or sheet. According to any one of claims 1 to 11,
Al ≤ 0.1
Phosphorus, cold rolled strip or sheet. A method for producing a cold rolled steel strip or sheet according to any one of claims 1 to 12,
a) providing a steel slab having a composition according to any one of claims 1 to 12;
b) hot rolling the slab into a hot rolled strip in the austenitic range;
c) coiling the hot rolled strip at a coiling temperature in the range of 500 to 540°C;
d) optionally, performing a descaling process on the coiled steel strip;
e) batch annealing the coiled strip at a temperature in the range of 500 to 650° C. for a period of 5 to 30 hours;
f) cold rolling the annealed steel strip at a reduction rate of 35 to 90%;
g) further processing the cold-rolled steel strip in a continuous annealing line or hot-dip galvanizing line, wherein the dipping temperature is 800° C. to 1000° C.; and
h) further cooling the steel strip to room temperature. According to claim 13,
- in step b), reheating the slab to a temperature of 1000 ° C to 1280 ° C and completely rolling the slab in the austenitic range with a hot rolling finishing temperature of at least 850 ° C to obtain a hot rolled steel strip;
- in step e), batch annealing in the range from 550 to 650 °C;
- in step g), the holding temperature is 200 to 500 ° C, preferably 350 to 450 ° C for a time of 150 to 1000 seconds,
A method that satisfies at least one of the conditions. 15. The method according to claim 13 or 14, wherein the immersion temperature in step g) is in the range of 830 °C to 900 °C. 16. The method according to any one of claims 13 to 15, wherein in step g) the immersion temperature is A _c3 = 910-203*C ^1/2 - 15.2 Ni - 30 Mn + 44.7 Si + 104 V + 31.5 Mo + 13.1 W greater than A _c3 as defined by 17. The process according to claim 16, wherein the immersion temperature in step g) is greater than A _c3 +20°C. 17. The process according to claim 16, wherein the immersion temperature in step g) is greater than A _c3 +30°C.