CN116867923A

CN116867923A - Coiling temperature-dependent cold-rolled strip or steel

Info

Publication number: CN116867923A
Application number: CN202180093344.XA
Authority: CN
Inventors: M·施瓦岑布伦纳; K·施坦德; M·格鲁伯; T·莫尔鲍尔
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2020-12-23
Filing date: 2021-12-23
Publication date: 2023-10-10
Also published as: KR20230129178A; WO2022136683A1; US20240132989A1; SE545210C2; US20240229183A9; EP4267777A1; SE2051557A1

Abstract

The application relates to a cold rolled strip or sheet comprising, in weight-%: c0.08-0.28; mn 1.4-4.5; cr 0.01-0.5; si 0.01-2.5; 0.01-0.6 of Al; si+Al is more than or equal to 0.1; si+Al+Cr is more than or equal to 0.4; nb is less than or equal to 0.008; ti is less than or equal to 0.02; mo is less than or equal to 0.08; ca is less than or equal to 0.005; v is less than or equal to 0.02; the balance being Fe except impurities. In the case of mapping Ri/t (y-axis) with respect to TS (MPa)/YR (x-axis), the steel is in the region defined by the coordinates A, B, C, D and where a is [1200,2 ], B is [2000,4], C is [2000,3], and D is [1200,1].

Description

Coiling temperature-dependent cold-rolled strip or steel

Technical Field

The present application relates to high strength steel strips and panels suitable for automotive applications.

Background

For a wide variety of applications, increased strength levels are a prerequisite for lightweight constructions, in particular in the automotive industry, since reduced body mass leads to reduced fuel consumption.

Automotive body parts are often stamped from sheet steel to form complex sheet structural members. However, such parts cannot be produced from conventional high strength steels because the formability of the complex structural parts is too low. For this reason, multiphase transformation induced plasticity (TRIP) steels have gained considerable attention over the past few years, particularly for use in automotive body structural parts and as seat frame materials.

TRIP steel has a multiphase microstructure comprising a metastable retained austenite phase capable of producing the TRIP effect. When the steel is deformed, the austenite transforms into martensite, which results in significant work hardening. This stiffening effect acts to resist necking in the material and delays failure in the sheet forming operation. The microstructure of TRIP steel can greatly change its mechanical properties.

TRIP steels have long been known and have attracted much attention, mainly because the matrix allows excellent stretch flangeability (stretch flangability). Furthermore, the TRIP effect ensured by the strain induced transformation of the 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 a hot rolled strip in an austenite temperature range. The hot rolled strip is thereafter coiled. The take-up resistance decreases with increasing temperature. A winding temperature of 600 c is typically used. The coiled strip is thereafter batch annealed and subsequently cold rolled. The cold rolled strip is thereafter continuously annealed.

WO 2019/122963 A1 and WO2019123043 A1 both disclose TRIP steels with improved phosphating coverage. Good phosphating coverage can be achieved. Improved phosphating coverage is achieved by controlling the alloying elements and process parameters, one of which is having a low coiling temperature. All the inventive examples have a coiling temperature of 450 ℃. The reference examples with higher take-up temperatures do not provide adequate phosphating coverage. The low coiling temperature increases the cold rolling force.

EP 2707514 B1 discloses a TRIP steel having a microstructure comprising: 5-20% polygonal ferrite, 10-15% retained austenite, 5-15% martensite, and the balance bainite. According to this document, the presence of polygonal ferrite between 5 and 20% makes it possible to pass a V bending angle of more than 90 ° without cracking.

WO2018116155 discloses a TRIP steel. The examples of the application disclose a lower coiling temperature of 450 ℃ in combination with a higher batch annealing temperature of 620 ℃ or 650 ℃, respectively, and a higher coiling temperature of 560 ℃ in combination with a lower batch annealing temperature of 460 ℃.

EP 3 653738 A1 discloses a TRIP steel having a microstructure comprising: 3-15% retained austenite, at least 30% tempered martensite, up to 5% fresh martensite, up to 35% bainite, 5-15% martensite, 5-35% ferrite.

Although these steels disclose a variety of attractive properties, there is still a need for steel sheets or strips with improved property profiles, in particular >950MPa of bending properties, in advanced forming operations. In particular bending properties related to strength and toughness. Further desirable properties are: reduced grain boundary oxidation, reduced susceptibility to liquid (liquid) metal embrittlement, reduced susceptibility to hydrogen embrittlement, and good phosphating (phosphatability).

Disclosure of Invention

The present application relates to a cold rolled steel (cold rolled steel) having a tensile strength of at least 950MPa and excellent formability, wherein steel sheets/strips will be producible on an industrial scale in a Continuous Annealing Line (CAL) and in a Hot Dip Galvanising Line (HDGL).

The present application aims to provide steels with compositions and microstructures that can be processed into complex high strength structural members, where bending properties are important.

Careful selection of alloying elements and process parameters reduces grain boundary oxidation. The reduced oxidation of grain boundaries improves the bendability and reduces the risk of embrittlement of the liquid metal and the susceptibility to hydrogen embrittlement. It further promotes good phosphating.

Drawings

Fig. 1 shows a graph of a sample of the present application within a dashed line.

Figure 2a shows a sample of the application without grain boundary oxidation.

Figure 2b shows the surface of the inventive sample of figure 2 a.

Figure 3a shows the grain boundary oxidation of the reference sample.

Fig. 3b is an enlarged view of the grain boundaries of fig. 3 a.

Fig. 3c shows the surface of the reference sample of fig. 3a-3 b.

FIG. 4 shows the phosphating coverage of the samples of the present application of FIGS. 2a-2 b.

FIG. 5 shows the phosphating coverage of the reference samples of FIGS. 3a-3 c.

Detailed Description

The application is described in the claims.

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

the balance being Fe except impurities.

The importance of the individual elements of the claimed alloy and their interactions with each other and the limitations of the chemical composition are briefly explained below. All percentages for the chemical composition of the steel are given throughout the description in weight% (wt.%). The upper and lower limits of the individual elements may be freely combined within the limits set forth in the claims. The arithmetic precision of a numerical value may be increased by one or two bits for all values given in the present application. Thus, a value given as, for example, 0.1% may also be expressed as 0.10 or 0.100%. The amount of microstructure constituents is given in volume% (vol.%).

C：0.08-0.28％

C stabilizes austenite and is important for obtaining sufficient carbon in the retained austenite phase. C is also important to obtain the desired intensity level. Generally, increases in tensile strength on the order of 100MPa/0.1% C are contemplated. When C is less than 0.08%, it is difficult to obtain a tensile strength of 950 MPa. If 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：1.4-4.5％

Manganese is a solid solution strengthening element by reducing M _s The temperature stabilizes the austenite and prevents ferrite and pearlite formation during cooling. In addition, mn lowers A _c3 Temperature and are important for austenite stability. At contents of less than 1.5%, it may be difficult to obtain a desired amount of retained austenite, tensile strength of 950MPa, and austenitizing temperature may be too high for conventional industrial annealing lines. Furthermore, at lower contents, it may be difficult to avoid the formation of polygonal ferrite. However, if the amount of Mn is higher than 4.5%, segregation problems may occur because Mn accumulates in a liquid phase and causes banding, resulting in potentially 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：0.01-0.5％

Cr is effective in improving the strength of the steel sheet. Cr is an element that forms ferrite and delays the formation of pearlite and bainite. A is that _c3 Temperature and M _s The temperature only slightly decreases as the Cr content increases. Cr causes an increase in the amount of stabilized retained austenite. Above 0.5%, it may impair the surface finish of the steel, and thus 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 application.

Si：0.01-2.5％

Si acts as a solid solution strengthening element and is important to ensure the strength of the thin steel strip. Si suppresses cementite precipitation and is necessary for austenite stabilization. However, if the content is too high, too much silicon oxide will be formed on the belt surface, which may lead to a coating on the rolls in CAL and thus to surface defects on the subsequently produced steel sheet. 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：0.01-0.6％

Al promotes ferrite formation and also commonly acts as a deoxidizer. Like Si, al is insoluble in cementite, so it significantly delays cementite formation during bainite formation. In addition, galvanization can be improved and the susceptibility to liquid metal embrittlement reduced. The addition of Al results in a significant increase in the carbon content in the retained austenite. The main disadvantage of Al is its segregation behaviour during casting. During casting, mn is enriched in the middle of the slab, and the Al content is reduced. Thus, a significant austenite stabilization zone or band may form in the middle of the slab. This results in a martensite banding at the end of the processing and forms low strain internal cracks in the martensite bands. On the other hand, si and Cr are also enriched during casting. Thus, the tendency of martensite banding can be reduced by alloying with Si and Cr, since the stabilization of austenite due to Mn enrichment is counteracted by these elements. For these reasons, it is preferable to limit the Al content. The upper level may be 0.6, 0.5, 0.4, 0.3, 0.2, 0.1%. The lower limit may 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 used only for deoxidation, the upper level may be 0.09, 0.08, 0.07 or 0.06%. To ensure a certain effect, the lower level may be set to 0.03 or 0.04%.

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％

An amount of these elements is beneficial for 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 1.7-5.0％

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

Optional elements

Mo≤0.5％

Molybdenum is a strong hardenability agent. Which may further enhance the benefits of NbC precipitates by reducing carbide coarsening kinetics. Thus, the steel may contain Mo in an amount 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 application, intentional addition of Mo is not necessary. Therefore, the upper limit may be limited to 0.01% or less.

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 the retained austenite phase due to precipitation of NbC. The steel may contain Nb in an amount of 0.1% or less. The upper limit may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. According to the present application, intentional addition of Nb is not necessary. Therefore, the upper limit may be limited to 0.004% or less.

V：≤0.1％

V functions similarly to Nb in that it contributes to precipitation hardening and grain refinement. The steel may contain V in an amount of 0.1% or less. The upper limit may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. According to the application, deliberate addition of V is not necessary. Therefore, the upper limit may be limited to 0.01% or less.

Ti：≤0.1％

Ti is commonly used in low alloy steels to improve strength and toughness due to its effect on grain size through formation of carbides, nitrides or carbonitrides. Specifically, ti is a strong nitride former and can be used to bind nitrogen in steel. However, the effect tends to saturate at 0.1% or more. The upper limit may be limited to 0.09, 0.07, 0.05, 0.03, or 0.01%. According to the present application, intentional addition of Ti is not necessary. Therefore, the upper limit may be limited to 0.005% or less.

Ca≤0.05

Ca can be used for modification of nonmetallic inclusions (inclusions). The upper limit is 0.05%, and may be set to 0.04, 0.03, 0.01%. According to the present application, intentional addition of Ca is not necessary. Therefore, the upper limit may be limited to 0.005% or less.

Impurity(s)

Cu：≤0.06％

Cu is an undesirable impurity element, and is limited to 0.06% or less by careful selection of the scrap used.

Ni：≤0.08％

Ni is also an undesirable impurity element, which is limited to 0.08% or less by careful selection of the scrap used.

B：≤0.0006％

B is an undesirable impurity element, which is limited to 0.0006% or less by careful selection of the scrap used. B increases the hardness, but may be at the expense of reduced bendability, and is therefore not desirable in the steels proposed by the present application. B may further make recycling of the waste more difficult and the addition of B may also deteriorate the workability. Therefore, according to the present application, intentional addition of B is not desirable.

Other impurity elements may be contained in the steel in normally occurring amounts. However, the amount of P, S, as, zr, sn is preferably limited to the following optional maximum levels:

P：≤0.02％

S：≤0.005％

As≤0.010％

Zr≤0.006％

Sn≤0.015％

it is also preferable to control the nitrogen content to the following range:

n: less than or equal to 0.015 percent, preferably 0.003 to 0.008 percent

Within this range, stable fixation of nitrogen can be achieved.

Oxygen and hydrogen can be further limited to

O：≤0.0003

H：≤0.0020

The microstructure composition is hereinafter expressed in volume% (vol.%).

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

And further, up to 10% Fresh Martensite (FM). The upper limit may be limited to 8% or 5%. A small amount of fresh martensite may improve the edge turnup and the local ductility. The lower limit may be limited to 1% or 2%. These untempered martensite particles are often in intimate contact with the retained austenite particles, and thus they are often referred to as martensite-austenite (MA) particles.

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 is 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, pages 61-64.

Polygonal Ferrite (PF) is susceptible to hydrogen embrittlement and is therefore not a desirable microstructure component. Polygonal ferrite in combination with martensite is disadvantageous for bending properties. Furthermore, the presence of ferrite may impart formability and elongation to the steel, as well as a degree of resistance to fatigue failure. It may also have a negative impact due to the fact that: ferrite increases the gap in hardness from hard phases such as martensite and bainite and reduces the local ductility, resulting in lower hole expansion rates. Therefore, polygonal Ferrite (PF) is limited to 10% or less, preferably 5% or less, 3% or less, or 1% or less. Most preferably, the steel is PF-free. The steel does not contain other kinds of ferrite, since bending properties are negatively affected. Furthermore, the yield ratio is negatively affected by ferrite which is detrimental to bending properties.

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

R _m 、R _p0.2 the values are obtained according to European standard EN 10002, section 1, in which the samples are taken in the longitudinal direction of the belt. Total elongation (A) ₅₀ ) Obtained according to japanese industrial standard JIS Z2241:2011, wherein the sample is taken in the transverse direction of the belt.

The bending properties were evaluated by the ratio of the limiting bending radius (Ri), which is defined as the minimum bending radius in the absence of cracks, to the plate thickness (t). For this purpose, a 90 ° V-shaped block was used to bend the steel sheet according to JIS Z2248. The value (Ri/t) obtained by dividing the limiting bending radius by the thickness 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 by dividing the yield strength YS by the tensile strength TS. The lower limit of YR may be 0.70, 0.75, 0.76, 0.77 or 0.78.

In the case of mapping Ri/T (y-axis) relative to TS/YR (x-axis), the steel should be further within the region defined by coordinates A, B, C, D of fig. 1, and where a is [1200,2 ], B is [2000,4], C is [2000,3], and D is [1200,1]. The upper dashed line may be expressed mathematically as y=0.0025 x-1 and the lower dashed line may be expressed as y=0.0025 x-2. This provides a standard 1.ltoreq.0.0025 x TS/YR-Ri/t.ltoreq.2. Steels meeting this criterion have been 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 such that TS/YR is within 1000-2000 (MPa). The lower limit may be 1100, 1200, 1300, 1400, 1500, 1600, 1700, or 1800. The upper limit may be 1900, 1800, 1700, 1600, 1500, or 1400. A preferred range may be 1200-1400. Other ranges may be, for example, 1400-1600, or 1600-1800, or 1800-2000.

Hole expansion ratio (lambda) [ HER ]]Preferably not less than 20%. Hole expansion ratio (λ) was determined by hole expansion test according to ISO/WD 16630:2009 (E). In this test, a conical punch with a 60 ° apex (apex) was pressed into a die having a dimension of 100x 100mm ² In 10mm diameter punched holes made in the steel sheet of (c). Once the first crack is determined, the test is stopped and the pore size is measured in two directions orthogonal to each other.The calculation is performed using an arithmetic mean.

The hole expansion ratio in% is calculated as follows:

λ＝(Dh-Do)/Do x 100

where Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameter of the hole after testing.

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

The proposed steel can be produced with the above proposed composition by producing conventionally metallurgical steel slabs via converter smelting (converter melting) and secondary metallurgy. The slab is hot rolled into a hot rolled strip in the austenitic range. Preferably, the slab is rolled completely in the austenitic range by reheating the slab to a temperature between 1000 ℃ and 1280 ℃ to obtain a hot rolled steel strip, wherein the hot rolling finish rolling temperature is greater than or equal to 850 ℃. Thereafter, the hot rolled strip is coiled at a coiling temperature in the range of 500-540 ℃. The coiled strip is optionally subjected to a descaling process, such as pickling. The coiled strip is thereafter batch annealed at a temperature in the range 500-650 ℃, preferably 550-650 ℃, for a duration of 5-30 hours. The annealed strip is thereafter cold rolled at a reduction of between 35 and 90%, preferably about 40-60%. The cold rolled steel strip is further processed in a Continuous Annealing Line (CAL) or in a Hot Dip Galvanising Line (HDGL), wherein the microstructure is fine-tuned. Both lines comprise subjecting the steel to a soaking temperature of 800-1000 c, preferably 830-900 c, preferably followed by a fast slow spray and a fast spray cooling to a holding temperature of 200-500 c, preferably 350-450 c for a period of 150-1000 seconds, followed by cooling to room temperature. Soaking temperature is A _c3 The above is to avoid formation of subcritical ferrite. A is that _c3 Through type A _c3 ＝910-203*C ^1/2 -15.2Ni-30mn+44.7si+104v+31.5mo+13.1w definition. Preferably, the soaking temperature is at least A _c3 +20 ℃, more preferably a _c3 +30℃. Preferably, the soaking temperature and time are controlled to allow 100% austenite without ferrite prior to cooling. Soaking time may be, for example, 40 seconds to 180 seconds。

M _S Can be defined by the following formula: m is M _S ＝692-502*(C+0.68N) ^0.5 -37*Mn-14*Si+20*Al-11*Cr。

The holding temperature can be at M _S Above or below.

In one embodiment, the strip or sheet is cold rolled;

a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and

b) At least one of the following conditions is satisfied:

in another embodiment, the strip or sheet is cold rolled;

a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and

b) At least one of the following conditions is satisfied:

in another embodiment, the strip or sheet is cold rolled; a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and b) at least one of the following conditions is satisfied:

in another embodiment, the strip or sheet is cold rolled;

a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and

b) At least one of the following conditions is satisfied:

examples

In FIG. 1, the ultimate bending radius (Ri) divided by the cold rolled thickness has been plotted against the tensile strength TS/YR divided by the yield ratio for the steels in examples 1-4. When Ri/t (y-axis) is plotted against TS/YR (x-axis), the steel of the present application is located in an area defined by the coordinates A, B, C, D, where A is [1200,2 ], B is [2000,4], C is [2000,3], and D is [1200,1].

The upper dashed line may be expressed mathematically as y=0.0025 x-1 and the lower dashed 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 defined mathematically as follows:

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

The steels of examples 1-5 of the present application are below the upper line.

The following dotted lines are defined as follows:

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

The steels of examples 1-5 of the present application are all above the lower line.

Within these boundaries, good bending properties are achieved in relation to strength and toughness.

Example 1

Steels I1-I6 and reference steels R1 and R2 were produced by conventional metallurgy through converter smelting and secondary metallurgy. The compositions are shown in table 1, with further elements being present only as impurities and below the minimum level specified in the present specification. All steels have approximately the same composition.

TABLE 1

Steel and method for producing same	C	N	Mn	Cr	Si	Al
							I1	0.105	0.0037	2.63	0.195	0.81	0.045
I2	0.106	0.0038	2.67	0.197	0.84	0.048
							I3	0.106	0.0038	2.67	0.197	0.84	0.048
I4	0.105	0.0037	2.63	0.195	0.81	0.045
							I5	0.118	0.0028	2.77	0.17	0.94	0.051
I6	0.118	0.0028	2.77	0.17	0.94	0.051
							R1	0.112	0.0041	2.7	0.169	0.93	0.046
R2	0.107	0.0051	2.63	0.199	0.85	0.041

The steel is continuously cast and cut into slabs.

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

Thereafter the hot rolled steel strip is coiled, the steels I1-I6 are coiled at a coiling temperature of 530 ℃, and the reference steels R1 and R2 are coiled at about 630 ℃.

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

The strip was thereafter cold rolled in a five-stand cold rolling mill to a final thickness of about 1.41mm and finally subjected to continuous annealing in a Continuous Annealing Line (CAL). In CAL, the tape was heated to a soak temperature of about 850 ℃ and held there for about 120 seconds. After annealing, the strip was slow spray cooled to about 750 ℃ (SJC) and then fast spray cooled to a holding temperature of about 400 ℃ (RJC). The tape was held for about 180 seconds and then cooled to room temperature.

For all steels, A _c3 About 800 c and is thus soaked well above a defined by the following formula _c3 Is carried out under the condition: a is that _c3 ＝910-203*C ^1/2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W。

The process parameters are shown in table 2.

TABLE 2

The yield strength YS and the tensile strength TS are obtained according to European standard EN 10002 section 1. Samples were taken in the longitudinal direction of the belt.

The sample of the produced tape was subjected to a V bending test according to JIS Z2248 to find the limit bending radius (Ri). The samples were inspected by the naked eye and under an optical microscope at 25 x magnification to investigate the occurrence of cracks. Ri/t is determined by dividing the limiting bend radius (Ri) by the thickness (t) of the cold rolled strip. Ri is the maximum radius where the material shows no cracks after three bending tests.

The limiting bending radii (Ri) of the steels I1-I6 coiled at 530 ℃ are smaller than those of R1, R2 coiled at 630 ℃.

The steels I1-I6 all meet the condition 1.ltoreq.0.0025. Times.TS/YR-Ri/t.ltoreq.2, but R1 and R2 are not reached.

The mechanical properties are shown in table 3.

TABLE 3 Table 3

Fig. 2a and 2b show the examination results of inventive steel I6 coiled at 530 c, and fig. 3a-3c show the examination results of reference steel R1 coiled at 630 c. The reference steel R1 shows grain boundary oxidation, whereas the steel I6 of the application shows no grain boundary oxidation.

Fig. 3c shows a visible crack on the sample surface of the reference steel R1. These result from chipping (break) after pickling and cold rolling. In particular, grain boundary oxides lead to cracks (outbreak) around the presented grains, which can lead to complete (full) grain collapse. The crack/break is hypoid to the bend ratio.

Fig. 2b shows that there are no visible cracks on the surface of the sample of the steel of the application. The absence and absence of visible cracks of the grain boundary oxides of the steel of the application improves the bending ratio and reduces the risk of embrittlement of the liquid metal. It further promotes good phosphating.

FIG. 4 shows the phosphating coverage of I6.

The microstructure of I6 was determined as:

bainite + tempered martensite >85%

Fresh martensite about 5%

About 5% of retained austenite

Example 2

Steel I7 and reference steel R3 are produced by conventional metallurgy through converter smelting and secondary metallurgy. The compositions are shown in table 4, with further elements being present only as impurities and below the minimum level specified in the present specification. All steels have approximately the same composition. Steels I7 and R3 had higher Cr and C contents and lower Si and Mn contents than the steel of example 1. This provides a steel with a higher yield strength and a higher tensile strength.

TABLE 4 Table 4

The steel was treated in the same process as in example 1, with steel I7 coiled at a coiling temperature of 532 ℃ and with reference to steel R3 at 626 ℃.

In CAL, the tape was heated to a soak temperature of about 850 ℃ and held there for about 120 seconds. After annealing, the strip was slow spray cooled to about 700 ℃ (SJC) and then fast spray cooled to a holding temperature (RJC) of about 250 ℃. The tape was held for about 180 seconds and then cooled to room temperature. All other process parameters were substantially the same as those of example 1.

For all steels, A _c3 About 780 ℃ and thus soaking at a temperature well above a defined by the following formula _c3 Is carried out under the condition: a is that _c3 ＝910-203*C ^1/2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W。

The process parameters are shown in table 5.

TABLE 5

Samples of the produced tape were subjected to the same tests as those of example 1.

The limiting bending radius (Ri) of the steel I7 coiled at 532 ℃ is smaller than the limiting bending radius (Ri) of the steel R3 coiled at 626 ℃.

Steel I7 satisfies condition 1.ltoreq.0.0025 x TS/YR-Ri/t.ltoreq.2, while R3 is not.

The mechanical properties are shown in table 6.

TABLE 6

The microstructure of I7 was determined as:

bainite + tempered martensite 95%

About 5% of retained austenite

Example 3

Steel I8 and reference steel R4 are produced by conventional metallurgy through converter smelting and secondary metallurgy. The compositions are shown in table 7, with further elements being present only as impurities and below the minimum level specified in the present specification. All steels have approximately the same composition. Steels I8 and R4 had higher Si and C contents and lower Cr contents than the steel of example 1. This results in a steel with a slightly higher tensile strength than the steel of example 1.

TABLE 7

Steel and method for producing same	C	N	Mn	Cr	Si	Al
							I8	0.198	0.0037	2.51	0.029	1.49	0.054
R4	0.202	0.0053	2.53	0.027	1.45	0.056

The steel was treated in the same process as in example 1, wherein steel I8 was coiled at a coiling temperature of 535 ℃ and reference steel R4 was coiled at 633 ℃. All other process parameters were substantially the same as those of example 1.

For the steel, A _c3 About 810-815 deg.c and thus soaking at a temperature well above a defined by the following formula _c3 Is carried out under the condition: a is that _c3 ＝910-203*C ^1/2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W。

The process parameters are shown in table 8.

TABLE 8

The limiting bend radius (Ri) of steel I8 coiled at 535 ℃ is smaller than the limiting bend radius (Ri) of steel R4 coiled at 633 ℃.

Steel I8 satisfies condition 1.ltoreq.0.0025 x TS/YR-Ri/t.ltoreq.2, while R4 is not.

The mechanical properties are shown in table 9.

TABLE 9

The microstructure of I9 was determined as:

bainite + tempered martensite >70%

Fresh martensite <15%

Retained austenite <15%

Example 4

Steel I9 and reference steel R5 are produced by conventional metallurgy through converter smelting and secondary metallurgy. The compositions are shown in table 10, with further elements being present only as impurities and below the minimum levels specified in the present specification. All steels have approximately the same composition. Steels I9 and R5 have slightly higher C content and slightly lower Mn and Si content than the steel of example 1.

Table 10

Steel and method for producing same	C	N	Mn	Cr	Si	Al
							I9	0.155	0.0061	2.33	0.24	0.441	0.053
R5	0.155	0.0061	2.33	0.24	0.441	0.053

In example 4, the CAL wire was replaced by a hot dip galvanised wire. Prior to the hot dip galvanization line, the steel was treated in a similar process to example 1, wherein steel I9 was coiled at a coiling temperature of 520 ℃ and coiled at 630 ℃ with reference to steel R5. The batch annealing temperature was 570 ℃.

For the steel, A _c3 About 780 ℃ and thus soaking at a temperature well above a defined by the following formula _c3 Is carried out under the condition: a is that _c3 ＝910-203*C ^1/2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W。

The process parameters are shown in table 11.

TABLE 11

The limiting bending radius (Ri) of the steel I9 coiled at 520 ℃ is smaller than the limiting bending radius of the steel R5 coiled at 630 ℃.

Steel I9 satisfies condition 1.ltoreq.0.0025. Times. TS/YR-Ri/t.ltoreq.2, while R5 is not reached.

The mechanical properties are shown in table 12.

Table 12

The microstructure of I9 was determined as:

bainite + tempered martensite about 85%

Fresh martensite about 5%

About 10% of retained austenite

Claims

1. Cold rolled steel strip or sheet

a) Has (in weight%) a composition consisting of:

the balance of Fe except impurities,

b) The following conditions are satisfied:

c) In the case of mapping Ri/t (y-axis) with respect to TS (MPa)/YR (x-axis), within the region defined by the coordinates A, B, C, D, and where a is [1200,2 ], B is [2000,4], C is [2000,3], and D is [1200,1];

d) Having (in volume%) a multiphase microstructure comprising

Tempered martensite +

2. The cold rolled strip or sheet according to claim 1, wherein the composition comprises (in wt.%):

the balance being Fe except impurities.

3. Cold rolled strip or sheet according to claim 1 or 2, wherein the polygonal ferrite is 5 or less, preferably 1 or less.

4. Cold rolled strip or sheet according to any of the preceding claims, wherein the composition fulfils at least one of the following requirements:

5. cold rolled strip or sheet according to claim 4, which fulfils all the requirements of claim 4.

6. The cold rolled strip or sheet according to any one of the preceding claims wherein the microstructure meets at least one of the following requirements:

tempered martensite +

7. Cold rolled strip or sheet according to any of the preceding claims, wherein the yield ratio is ≡0.70.

8. The cold rolled strip or sheet according to claim 1,

a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and is also provided with

b) At least one of the following conditions is satisfied:

9. the cold rolled strip or sheet according to claim 1,

a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and is also provided with

b) At least one of the following conditions is satisfied:

10. cold rolled strip or sheet according to claim 1, a) having a composition (in wt.%) comprising:

the balance of Fe except impurities; and is also provided with

b) At least one of the following conditions is satisfied:

11. the cold rolled strip or sheet according to claim 1,

a) Has (in weight%) a composition comprising:

the balance of Fe except impurities; and is also provided with

b) At least one of the following conditions is satisfied:

12. cold rolled strip or sheet according to any of the preceding claims, wherein

Al≤0.1。

13. Method of manufacturing a cold rolled steel strip or sheet according to any one of claims 1-12, comprising the steps of:

a) Providing a steel slab having a composition according to any one of the preceding claims;

b) Hot rolling the slab to a hot rolled strip in the austenitic range;

c) Coiling the hot rolled strip at a coiling temperature in the range of 500-540 ℃;

d) Optionally descaling the coiled steel strip;

e) Batch annealing the coiled strip at a temperature in the range of 500-650 ℃ for a duration of 5-30 hours;

f) Cold rolling the annealed strip at a reduction of between 35 and 90%;

g) Further treating the cold rolled steel strip in a continuous annealing line or in a hot dip galvanising line, wherein the soaking temperature is 800-1000 ℃; and

h) The strip was further cooled to room temperature.

14. The method of claim 13, at least one of the following conditions is satisfied:

-in step b), reheating the slab to a temperature between 1000 ℃ and 1280 ℃, rolling the slab completely in the austenitic range to obtain a hot rolled steel strip, wherein the hot rolling finishing temperature is greater than or equal to 850 ℃;

-in step e), a batch annealing is performed in the range 550-650 ℃; and

in step g), the temperature is maintained at 200-500 ℃, preferably 350-450 ℃, for a period of 150 to 1000 seconds.

15. The method according to claim 13 or 14, wherein the soaking temperature in step g) is in the range of 830-900 ℃.

16. The method according to any one of claims 13-15, wherein the soaking temperature in step g) is at a defined by _c3 The following steps: a is that _c3 ＝910-203*C ^1/2 -15.2Ni-30Mn+44.7Si+104V+31.5Mo+13.1W。

17. The method of claim 16, wherein the soaking temperature in step g) is at a _c3 +20 ℃ or higher.

18. The method of claim 16, wherein the soaking temperature in step g) is at a _c3 +30 ℃ or higher.