Hot-rolled high-strength steel strip with high hole expansion ratio

Abstract

The present invention relates to a high strength steel that is produced by carefully selecting the conventional alloying elements C, Mn, Si, and Al, along with the addition of trace elements, resulting in a steel with high strength and a high hole expansion ratio. The present invention also relates to a method for producing this high strength steel.

Description

[Technical Field]

[0001] The present invention relates to hot rolled steel strip having high strength and high hole expansion ratio. [Background technology]

[0002] High-strength steels are used in the automotive industry to improve in-service performance and / or reduce vehicle weight and fuel consumption. However, for example, in the case of relatively complex shaped automotive components, such as those found in automotive chassis and suspensions, high strength alone is not sufficient to improve in-service performance. For example, the value of using high-strength steels in automotive chassis components may be increased collapse strength to maintain component integrity in the event of an accident. However, the higher the strength of the steel, the more difficult it is to form the steel into automotive components without splitting the steel at the sheared edges of the blank or punched edges that form the component. This is because, in most cases, the increased strength is obtained from the presence of low-temperature transformation products in the microstructure via transformation hardening. However, this results in a large hardness differential in the final microstructure, sacrificing stretch flangeability. Therefore, the application of high strength multiphase steels, such as DP steels and TRIP steels, is limited by the formability of these steel types for certain automotive applications, e.g., chassis and suspension components with very precise and complex shapes, and the application of multiphase CP steels is also limited to some extent.

[0003] Furthermore, a common approach to reducing the weight of parts is to use high-strength steel and reduce the thickness of the steel sheet used. However, this can result in a decrease in stiffness. Stiffness is important for applications in automotive body-in-white, chassis, and suspension, and / or automotive seating and interior. For example, for automotive chassis parts, stiffness is a key performance parameter because a lack of stiffness compromises the handling and passenger safety of the vehicle. The inherent loss of stiffness due to reducing the thickness of steel used to manufacture automotive chassis parts can be recovered by optimizing the part's shape, for example, by creating deeper flanges and / or flanges with increased degrees of stretch and / or bending. In order for automotive engineers to seek to increase the stiffness of parts through shape optimization, the high-strength steel used must have excellent formability in terms of good stretchability (or tensile elongation) and excellent stretch flangeability (or hole expansion performance).

[0004] In recent years, steel suppliers have developed high-strength steel types that have both a reasonable ultimate tensile strength Rm and a reasonable total elongation A50 or A80. These mechanical properties provide information about the strength and ductility of the steel type.

[0005] However, for certain applications of high-strength steels in the automotive industry, it is also a requirement that the steel have good stretch-flangeability. Stretch-flangeability refers to the formability of the sheet not only at the flange but also at the edge of the opening. Stretch-flangeability is usually measured by expanding a circular punched hole in the sheet and is expressed as the hole expansion ratio λ. The hole expansion ratio λ is often determined in accordance with the Japan Iron and Steel Federation standard JFS T 1001. This standard will be observed below. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a hot rolled high strength steel having a high hole expansion ratio.

[0007] It is also an object of the present invention to provide a high strength steel having good elongation and a high hole expansion ratio.

[0008] A further object of the present invention is to provide a high strength steel having a tensile strength of 760 MPa or greater and a hole expansion ratio of 50% or greater.

[0009] Another object of the present invention is to provide a high strength steel having a tensile strength of 960 MPa or more and a hole expansion ratio of 40% or more.

[0010] It is a further object of the present invention to provide such a high strength steel which also has a total elongation A50 or A80 of 9±1% or more. [Means for solving the problem]

[0011] According to the present invention, C:0.02~0.13wt%; Mn:1.20~3.50wt%; Si:0.10~1.00wt%; Al_tot:0.01~0.10wt%; Ti:0.04~0.25wt%; N:0~0.010% by weight; P:0~0.10wt%; S:0~0.01% by weight; Optionally, B:0~0.005% by weight , preferably 0.0005 to 0.005% by weight ; Optionally, Cu:0~1.5wt% Cr:0~1.0wt% Mo:0~1.0wt% Ni: 0~0.50% by weight V:0~0.30wt% Nb:0~0.10wt% One or more of the following: Iron and unavoidable impurities: balance A hot rolled high strength steel strip consisting of: Ti+Nb is 0.25% by weight or less, Cr+Mo is 1.0 wt% or less, Steel, by volume percent, Bainite: 85% or more Martensite and retained austenite: 10% or less Cementite: over 0% and less than 5% Inclusions: unavoidable amounts wherein the sum of these is 100% by volume; The steel strip has the following mechanical properties: Tensile strength: 760MPa or more and 960MPa or less; Total elongation (A50): 10% or more; Hole expansion ratio (λ): 50% or more or have the following mechanical properties: Tensile strength: 960MPa or more and 1380MPa or less; Total elongation (A50): 9% or more; Hole expansion ratio (λ): 40% or more A steel strip is provided having:

[0012] The use of this composition in a hot-rolled steel with this microstructure makes it possible to provide a steel with high strength, i.e., a strength of 760 MPa or more, and a high hole expansion ratio λ. As usual, the higher the strength, the lower the formability. This also applies to the hole expansion ratio. If the hot-rolled steel according to the present invention has a moderate tensile strength, for example, a tensile strength of 760 to 960 MPa, the hole expansion ratio can be 50% or more. In the case of a high-strength steel, for example, with a tensile strength of 960 to 1380 MPa, the hole expansion ratio can be lower, for example, 40% or more.

[0013] The use of the composition according to the invention provides a microstructure consisting almost entirely of bainite. Preferably, martensite and retained austenite are absent. However, due to the hot rolling and coiling conditions, some cementite may be present, but this is less than 5%. Additionally, small amounts of carbides, precipitates and unavoidable inclusions may be present in the steel. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram illustrating the definitions of different forms of bainite (e.g., ferritic bainite (FB), granular bainite (GB), upper bainite (UB), and cementite-free bainite (CFB)) and distinct building blocks (e.g., irregular-shaped bainitic ferrite (Type 1), lath-shaped bainitic ferrite (Type 2), cementite (FeC), and martensite and / or retained austenite (M / RA)) used herein to describe the Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] High strength and formability, especially high hole expansion ratio, result from careful selection of the conventional alloying elements C, Mn, Si and Al, and the addition of trace elements, the elements used in the compositions according to the invention being described herein below.

[0016] Carbon is present in an amount of 0.02 to 0.13 wt%. C is a bainite former and is essential for achieving a final microstructure that provides sufficient strength and formability in terms of tensile elongation and hole expansion performance. To achieve sufficient strength, a suitable minimum C content is 0.02 wt%, and in preferred embodiments, at least 0.03 wt%. A low C content, in preferred embodiments, of up to 0.12 wt%, preferably up to 0.09 wt%, and more preferably up to 0.06 wt%, is beneficial for suppressing the effect of cooling rate dependence on the uniformity of the final microstructure and promoting high hole expansion performance. Furthermore, C is essential for achieving precipitation strengthening in combination with carbide-forming microalloying elements, such as titanium, niobium, or vanadium, and the carbide-forming microalloying elements are removed as much as possible to reduce the amount of cementite in the final microstructure. By optimizing other alloying elements such as Ti, Nb and / or V, a nearly uniform bainite / bainitic ferrite microstructure with little cementite can be obtained.

[0017] Manganese is present in an amount of 1.20 to 3.50 wt.%. Mn is an essential element for providing solid solution hardening and promoting a low-carbon bainite microstructure. Mn stabilizes austenite and delays the bainite transformation at certain temperatures, ensuring good hardenability. The drawback of a very high Mn content is increased central segregation in continuously cast steel slabs and a deterioration in surface quality. Therefore, preferably, the Mn content is a maximum of 2.20 wt.%.

[0018] Silicon is present in an amount of 0.10 to 1.00 wt% to improve the strength of the steel by solid solution hardening. Furthermore, Si is beneficial in suppressing the formation of cementite. However, the use of higher amounts of Si reduces the weldability and coatability of the steel. Therefore, the amount of Si is preferably at most 0.95 wt%, and in preferred embodiments at most 0.70 wt%, or even at most 0.60 wt%.

[0019] Aluminum is present in an amount of 0.01-0.10 wt%. Al is a deoxidizing element and improves the cleanliness of the steel. To be effective, at least 0.01 wt% of Al is required. However, Al can cause surface defects, so the Al content is limited to a maximum of 0.10 wt%, preferably a maximum of 0.05 wt%.

[0020] Titanium is present at 0.04 to 0.25 wt.%. This is because titanium provides hardenability and, as a carbide former, helps form as little cementite as possible while providing precipitation strengthening through the formation of small Ti-based carbides. However, Ti also combines with N, S, and C to form nitrides and carbosulfides, depending on the specific chemical composition of the steel. Therefore, at least 0.04 wt.% Ti is present to combine all of the N and S in the steel and have sufficient excess Ti to combine with the C in the steel. If more than 0.25 wt.% Ti is present, coarse Ti nitrides, carbonitrides, and carbides are formed that are difficult to dissolve during reheating of the slab before hot rolling. Furthermore, these coarse Ti nitrides, carbonitrides, and carbides result in a decrease in the hole-expansion performance of the steel. Preferably, 0.09 to 0.21 wt.% Ti is present to always have enough Ti but without risking severe coarsening. In certain embodiments, 0.09 to 0.20 wt % Ti may be present, or even 0.11 to 0.20 wt % Ti may be present, and in other embodiments, 0.12 to 0.18 wt % Ti may be present.

[0021] Boron is not required to obtain the required properties of the steel, but may be present at 0.0005-0.005 wt.%, i.e., 5-50 ppm. B is very effective in enhancing the hardenability of the steel. This means that low carbon contents and / or slow cooling rates can be used in the runout table even when no or little proeutectoid ferrite forms. B is also a very suitable alloying element for increasing yield strength. Preferably, at least 10 ppm of B is present to ensure that not all of the B becomes boron nitride. If sufficient Ti is present, titanium nitride will form first, preventing the formation of boron nitride. This maintains a boron-free state, which is favorable for optimal contribution to the hardenability of the steel.

[0022] Nitrogen is an unavoidable element and should be kept as low as possible. N should be present at a maximum of 0.010 wt%. N forms titanium nitride with Ti and acts as a dispersoid to control the austenite grain size during reheating. However, excessive N can result in excessively coarse TiN particles, impairing hole expansion performance. Preferably, the N content is 0.005 wt% (50 ppm) or less. A suitable minimum N content is 10 ppm.

[0023] Phosphorus is present as an impurity, and P must be present in a maximum of 0.10 wt%. Excessive P promotes grain boundary segregation, reducing toughness and weldability. Preferably, the P content is a maximum of 0.01 wt%.

[0024] Sulfur is also present as an impurity and should be present at a maximum of 0.01 wt.%. MnS particles are formed during casting. Coarse MnS particles are undesirable because they elongate during hot rolling, impairing hole expansion performance and reducing shear edge quality. Ti in the steel can combine with S and C, depending on the amount of Ti present, to form Ti4S2C2 particles. These Ti4S2C2 particles are coarse particles and should be avoided because they also impair hole expansion performance and shear edge quality. Preferably, S is present at a maximum of 0.005 wt.%.

[0025] Several optional elements may be present in the steel.

[0026] Copper may be present at up to 1.5 wt%. Cu may promote a low carbon bainite microstructure and provide solid solution hardening. Preferably, Cu is present at up to 0.6 wt%, more preferably at up to 0.1 wt%, provided the other elements provide the same results. In one embodiment, Cu is not added to the steel because it is not an economically preferred element, and therefore Cu is present only as an impurity.

[0027] Chromium may be present at up to 1.0 wt%. Cr improves the strength of the steel primarily through transformation strengthening by improving hardenability. Preferably, Cr is present at up to 0.9 wt%, and in certain embodiments, Cr is present at up to 0.6 wt% or even up to 0.5 wt%. In one embodiment, Cr is not added to the steel, and therefore, Cr is present only as an impurity.

[0028] Molybdenum may be present at up to 1.0 wt. %. Mo enhances hardenability and promotes a low-carbon bainite microstructure. Furthermore, Mo is a carbide-forming element and can combine with Ti, Nb, or V to form complex carbide precipitates. These Mo-based complex carbides are known to be thermally stable and less prone to subsequent coarsening. However, Mo is not an economically desirable element and is therefore used in smaller amounts, preferably up to 0.9 wt. %. In certain embodiments, Mo is present in smaller amounts, for example, up to 0.35 wt. %, or even up to 0.2 wt. % or even up to 0.1 wt. In one embodiment, Mo is not added to the steel, and thus Mo is present as an impurity. However, for other embodiments, Mo should be added, for example, up to 0.8 wt. %. Preferably, 0.005-0.7 wt. % Mo, more preferably 0.1-0.6 wt. % Mo, and even more preferably 0.2-0.5 wt. % Mo is added.

[0029] Nickel may be added up to 0.5 wt%. Ni can improve toughness and hardenability at high strength levels and mitigate the adverse effects of Cu on hot embrittlement. However, from a cost perspective, up to 0.3 wt% Ni is prudent. To prevent hot embrittlement when the Cu content exceeds 0.5 wt%, Ni can be added up to 0.5 wt%. Preferably, up to 0.3 wt%, more preferably up to 0.2 wt%, or even up to 0.1 wt% Ni is added. In one embodiment, Ni is not added to the steel, and therefore Ni is present only as an impurity.

[0030] Vanadium may be present in the steel up to 0.3 wt. %. However, V is a relatively expensive element and is primarily used to replace Ti for its precipitation strengthening effect and to form vanadium carbides to reduce cementite formation. Therefore, preferably, V is present in the steel at a maximum of 0.2 wt. %. In certain embodiments, V is present at a maximum of 0.18 wt. % or even at a maximum of 0.1 wt. %. V may also not be added to the steel at all, and thus V is present as an impurity.

[0031] Niobium may be present in the steel up to 0.10 wt.%. Nb improves the strength of the steel partly by precipitation hardening, but primarily by grain refinement. However, at large amounts of Nb, these effects saturate. Therefore, preferably, a maximum of 0.08 wt.% Nb is present. In certain embodiments, a maximum of 0.06 wt.% Nb is present, and in preferred embodiments, 0-0.04 wt.% Nb is present, preferably 0.01-0.04 wt.% Nb is present in the steel. In other embodiments, Nb is not added to the steel, and therefore, Nb is present as an impurity.

[0032] Since Ti and Nb have the same function in steel, Ti and Nb should be a maximum of 0.25 wt.%.

[0033] Similarly, Cr and Mo should be a maximum of 1.0 wt %.

[0034] To obtain high strength and a high hole expansion ratio, the microstructure of the hot-rolled steel must be composed of at least 85% bainite by volume. Preferably, the amount of bainite is as high as possible, and a small amount of cementite (less than 5%) is formed along with the formation of bainite to obtain the highest possible hole expansion ratio. Furthermore, small amounts of carbides, precipitates, and unavoidable inclusions may be present in the steel. Furthermore, the steel may contain up to 10% martensite and retained austenite, preferably up to 5% martensite and retained austenite.

[0035] The objective of the present invention is to obtain a predominantly bainite microstructure that combines a sufficient degree of strength on the one hand with a sufficient degree of hole expansion capability and tensile elongation on the other hand.

[0036] In this document, the term bainite should be understood to include ferritic bainite (FB), granular bainite (GB), upper bainite (UB) and cementite-free bainite (CFB).

[0037] FIG. 1 illustrates different forms of bainite (e.g., ferritic bainite (FB), granular bainite (GB), upper bainite (UB), and cementite-free bainite (CFB)) and distinct components (e.g., irregularly shaped bainitic ferrite (Type 1), lath-like bainitic ferrite (Type 2), cementite (FeC), and martensite and / or residual bainitic ferrite (Type 1), which are used herein to explain the examples and comparative examples. Austenite FIG. 1 is a diagram showing the definition of (M / RA).

[0038] These are all considered "composite" microstructures, and the overall microstructure may be composed of one of these "composite" microstructures, or may be composed of a mixture of two or more of these "composite" microstructures. A "composite" microstructure may then be composed of one or more phase components or "constituents." These components may be: Irregularly shaped bainitic ferrite (BF, type 1) with a relatively low internal dislocation density, Lath-like bainitic ferrite (BF, type 2) with a relatively high internal dislocation density, Cementite (Fe3C), which is present as relatively coarse particles at lath boundaries and grain boundaries, and / or to some extent at prior austenite grain boundaries, Martensite and / or retained austenite (M / RA) is.

[0039] Most of these "constituents" can be identified by electron backscatter diffraction (EBSD), which also allows for quantification of their area or volume fraction. This applies to (1) irregularly shaped bainitic ferrite (BF, type 1), which has a relatively low internal dislocation density; (2) lath-shaped bainitic ferrite (BF, type 2), which has a relatively high internal dislocation density; (3) martensite; and (4) retained austenite. Experimental methods for identifying and quantifying these "constituents" via EBSD are described in more detail in the Examples section of this document. Cementite cannot be accurately identified, much less quantified, by EBSD. To visualize cementite, optical microscopy is commonly used on polished cross sections of steel samples after etching for several seconds in a 4% picral solution. However, the limited resolution of optical microscopy and the small size of cementite grains make it impossible to quantify the amount of cementite with great precision. This also applies when using scanning electron microscopy in combination with etched steel samples, because the small size of the cementite particles and other microstructural features, such as partially etched (sub)grain boundaries, prevent accurate quantification of cementite. Therefore, optical microscopy in combination with picral etching of steel samples was primarily used to assess whether cementite was present in the microstructure. The cementite present in the total microstructure is primarily related to the presence of upper bainite (UB), which consists of lath-like bainitic ferrite (BF, Type 2), and therefore is included in the volume fraction of this component (lath-like bainitic ferrite (BF, Type 2)).

[0040] Ferrite-bainite (FB) is composed of irregularly shaped bainitic ferrite (BF, type 1) grains with a relatively low internal dislocation density. Excess carbon that cannot dissolve in the bainitic ferrite grains is consumed in the precipitation process along with carbide-forming elements (e.g., Ti, Nb, V, and / or Mo). This results in ferrite-bainite containing only irregularly shaped bainitic ferrite grains and little or no cementite and / or M+RA. This type of bainite is favored when the transformation occurs in a temperature range that provides sufficient kinetics for the precipitation of the above elements, especially Ti. The irregularly shaped bainitic ferrite grains of this type of bainite are optimally strengthened by Ti-based carbide precipitates. The high temperature range over which this type of bainite forms also explains its relatively low internal dislocation density, since this type of bainite forms primarily via a diffusion mechanism.

[0041] Granular bainite (GB) consists of irregularly shaped bainitic ferrite (BF, type 1) grains with a relatively low internal dislocation density. grainExcess carbon that cannot dissolve in the bainite is only partially consumed in the precipitation process along with carbide-forming elements (e.g., Ti, Nb, V, and / or Mo). This results in granular bainite (GB), which not only contains irregularly shaped bainitic ferrite grains but also some M+RA between the irregularly shaped bainitic ferrite grains. This type of bainite is favored when the transformation occurs in a temperature range that provides sufficient reaction rate for carbon distribution across the ferrite-austenite transformation interfaces that move during the phase transformation process. The irregularly shaped bainitic ferrite grains of this type of bainite are only partially strengthened by Ti-based carbide precipitates. The high temperature range for the formation of this type of bainite also explains its relatively low internal dislocation density, since this type of bainite is formed primarily via a diffusion mechanism. The amount of martensite and / or retained austenite needs to be limited because stress concentrations around these phase components during shearing and forming operations can lead to premature crack nucleation.

[0042] Upper bainite (UB) is composed of lath-shaped bainitic ferrite (BF, type 2) containing cementite at lath boundaries. This type of bainite favors a relatively low transformation temperature. As a result, this lath-shaped bainitic ferrite has a relatively high internal dislocation density, which is generally higher than the internal dislocation density of the irregularly shaped bainitic ferrite commonly formed at higher transformation temperatures. Lath-shaped bainitic ferrite forms primarily via a more displacive-oriented mechanism. The lower transformation temperature for forming upper bainite (UB) conflicts with optimal carbon precipitation by carbide-forming elements (e.g., Ti, Nb, V, and / or Mo) because sufficient reaction rates are not available under these conditions. As a result, upper bainite (UB) contains a significant amount of cementite at lath boundaries. Upper bainite (UB) is more resistant to crack propagation than granular bainite (GB). This is due to its fairly small (effective) crystallographic packet size (a packet corresponds to a crystallographic unit of bainite, consisting of crystallographic subunits separated from each other by low-angle boundaries (<15°) and having high-angle boundaries (≥15°) with other adjacent packets). The small crystallographic packet size of upper bainite (UB), and therefore the large number of high-angle boundaries, are beneficial for arresting crack propagation. Therefore, upper bainite (UB), consisting of lath-shaped bainitic ferrite and interlath cementite, is desirable for good hole expansion performance. Because EBSD cannot accurately detect cementite, and the cementite present in the microstructure is mainly present between the lath-shaped bainitic ferrite components of upper bainite (UB), the amount of lath-shaped bainitic ferrite measured by EBSD also includes the amount of cementite present in the microstructure.

[0043] Cementite-free bainite (CFB) is also composed of lath-shaped bainitic ferrite (BF, type 2). However, in contrast to upper bainite (UB), cementite-free bainite (CFB) does not contain cementite but instead contains martensite and / or retained austenite at the lath boundaries. Like upper bainite (UB), cementite-free bainite (CFB) favors a relatively low transformation temperature. As a result, the lath-shaped bainitic ferrite of this type of bainite has a relatively high internal dislocation density, similar to upper bainite (UB). Cementite-free bainite (CFB) is also only partially strengthened by Ti-based carbide precipitates, similar to upper bainite (UB).

[0044] As mentioned above, the objective of the present invention is to obtain a predominantly bainite microstructure that combines sufficient strength on the one hand with sufficient hole expansion capacity and tensile elongation on the other hand, the bainite microstructure being composed primarily of ferritic bainite (FB) and / or upper bainite (UB), with no or only small amounts of granular bainite (GB) or cementite-free bainite (CFB).

[0045] These bainite microstructures can be obtained by accelerated cooling after hot rolling and realizing phase transformations at low temperatures on the run-out table and / or coiler. The amount of martensite and / or retained austenite (M / RA) between the irregularly shaped bainitic ferrite grains or between the lath-like bainitic ferrite bundles should be controlled and should be limited to a maximum of 10%, preferably a maximum of 5%, more preferably a maximum of 3%, even more preferably a maximum of 2%, even more preferably a maximum of 1%, and most preferably an absence of martensite and retained austenite.

[0046] Some martensite and retained austenite (M+RA) can be tolerated and can be beneficial for strength, uniform elongation, and suppression of discontinuous yielding behavior. However, excessive martensite and retained austenite can sacrifice hole expansion performance because these phases promote the nucleation of internal microvoids and cracks during punching. If the density of these internal microvoids and cracks in the steel is too high near the edge of the punch hole, hole expansion performance is impaired because the alignment and coalescence of these microvoids and cracks promotes premature macroscopic fracture and failure.

[0047] Carbon-enriched regions to obtain the desired bainite microstructure, either by segregation during casting or primarily via carbon partitioning during phase transformation, can also be formed by iron carbide or cementite (Fe x C yThis cementite is an inherent component of upper bainite (UB) and is the result of insufficient reaction kinetics for optimal carbide precipitation at the transformation temperature for forming upper bainite (UB). Nevertheless, the excess carbon available for forming cementite can be limited in accordance with the present invention so that the amounts of carbon and carbide-forming elements (e.g., Ti, Nb, V, and Mo) are properly balanced. This is essential because too much cementite leads to poor formability in general and poor hole expansion performance in particular. However, some cementite in the overall bainite microstructure is beneficial, as small amounts of these fairly small hard phase components can help to obtain significantly improved shear edge quality. The presence of small amounts of cementite in the shear-affected zone and located on or near the resulting shear edge or punch hole edge can help provide nucleation points for localized fracture. Thus, the presence of small amounts of cementite helps promote macroscopic fracture and subsequent separation of the steel during shearing without excessive tearing, leaving a smoother surface at the shear edge in general and a smoother fracture zone at the shear edge in particular. This is beneficial for the fatigue life of the shear edge and, ultimately, the performance of automotive chassis components. However, excessive cementite leads to excessive internal damage within the steel near the shear edge. This increases the risk of void coalescence, promoting fracture propagation, and ultimately leading to premature macroscopic fracture and failure, for example, in hole expansion performance tests. In this context, a significant amount of upper bainite (UB) is beneficial because this type of bainite, with its smaller crystallographic packet size, is more resistant to crack propagation than granular bainite (GB), with its larger crystallographic packet size.

[0048] The inventors have determined that the amounts of the carbide-forming elements Ti, Nb, V and Mo, expressed in weight percent, are determined according to the following formula:

number

number

[0049] Preferably, the lower limit of this formula is 0.55, more preferably 0.75, and the upper limit is preferably 2.1, more preferably 1.8, further limiting the amount of cementite and / or the amount of martensite and retained austenite.

[0050] According to a first preferred embodiment, a high strength steel is provided that has a moderately high strength and a very good hole expansion ratio. This steel contains the following limited ranges of elements: C: 0.02 to 0.06% by weight, preferably 0.02 to 0.05% by weight; Mn: 1.30 to 2.20 wt%, preferably 1.30 to 2.00 wt%; Si:0.10~0.60wt%; Ti: 0.09 to 0.20 wt %, preferably 0.12 to 0.20 wt %; B: 0.0010 to 0.004% by weight, preferably 0.0010 to 0.003% by weight and / or Optional elements in the following limited ranges: Cu: 0 to 0.5 wt %, preferably 0 to 0.1 wt %; Cr: 0 to 0.8 wt %, preferably 0 to 0.6 wt %; Mo: 0 to 0.35% by weight, preferably 0 to 0.2% by weight, more preferably 0 to 0.1% by weight; Ni: 0 to 0.2 wt %, preferably 0 to 0.1 wt %; V: 0 to 0.18% by weight, preferably 0 to 0.1% by weight; Nb: 0 to 0.06% by weight, preferably 0 to 0.04% by weight, more preferably 0.01 to 0.04% by weight Contains one or more of the following: Mn+Cr+2Mo is 1.6% by weight or more and 2.4% by weight or less, Steel, by volume percent, Bainite: 85% or more Martensite and retained austenite: 5% or less Cementite: more than 0% and not more than 5%, preferably 0.01% or more and not more than 4%, more preferably 0.02% or more and not more than 3%, even more preferably 0.02% or more and not more than 2%, and most preferably 0.02% or more and not more than 1% Inclusions: unavoidable amounts It has a microstructure consisting of where the sum of these is 100% by volume. .

[0051] Due to the limited amount of carbon, the strength is not very high, but at the same time the amount of Mn + Cr + 2Mo must be at least 1.6 wt.%. In this way, a microstructure with at least 85% bainitic ferrite can be obtained, resulting in a very good hole expansion ratio.

[0052] All element limits for this preferred embodiment are consistent with the explanations for selecting the amount of each element above, but are selected so that the strength of the steel is not too low, as this reduces the performance of the steel in service, and so that the strength of the steel is not too high, as this generally impairs the hole expansion ratio and formability.

[0053] Preferably, the microstructure of this steel has 4% or less of martensite and retained austenite, more preferably 3% or less of martensite and retained austenite, even more preferably 2% or less of martensite and retained austenite, even more preferably 1% or less of martensite and retained austenite, and most preferably no martensite or retained austenite. In particular, martensite increases strength but, like retained austenite, reduces the hole expansion of the steel, so both phase components must be present in small amounts. The absence of martensite is best for formability.

[0054] The composition and microstructure of this preferred embodiment of the high strength steel strip according to the invention preferably has the following mechanical properties: Yield strength: 570 MPa or more and 900 MPa or less, Tensile strength: 760 MPa or more and 960 MPa or less, Total elongation (A50): 10% or more, and / or Hole expansion ratio (λ) value: 50% or more, preferably 60% or more, more preferably 70% or more, most preferably 80% or more It has.

[0055] This steel type is therefore very suitable to provide an essentially bainitic steel with a strength of 800 MPa and very good hole expansion for demanding automotive parts.

[0056] According to a second preferred embodiment, a high strength steel is provided having improved high strength and good hole expansion ratio. The steel contains the following limited ranges of elements: C: 0.03 to 0.12% by weight, preferably 0.04 to 0.09% by weight; Mn: 1.50 to 2.20 wt%, preferably 1.60 to 2.00 wt%; Si: 0.20 to 0.95 wt %, preferably 0.40 to 0.70 wt %; Ti: 0.10-0.20 wt%, preferably 0.12-0.18 wt%; B: 0.0010 to 0.004% by weight, preferably 0.0010 to 0.003% by weight and / or Optional elements in the following limited ranges: Cu: 0 to 0.5 wt %, preferably 0 to 0.1 wt %; Cr: 0 to 0.8 wt %, preferably 0 to 0.5 wt %; Mo: 0 to 0.8% by weight, preferably 0.005 to 0.7% by weight, more preferably 0.1 to 0.6% by weight, and even more preferably 0.2 to 0.5% by weight; Ni: 0 to 0.2 wt %, preferably 0 to 0.1 wt %; V: 0 to 0.18% by weight, preferably 0 to 0.1% by weight; Nb: 0 to 0.06% by weight, preferably 0 to 0.04% by weight, more preferably 0.01 to 0.04% by weight Contains one or more of the following: Mn+Cr+2Mo is 2.3% by weight or more, Steel, by volume percent, Bainite: 90% or more Martensite and retained austenite: 5% or less Cementite: more than 0% and not more than 5%, preferably 0.01% or more and not more than 4%, more preferably 0.02% or more and not more than 3%, even more preferably 0.02% or more and not more than 2%, and most preferably 0.02% or more and not more than 1% Inclusions: unavoidable amounts where the sum of these is 100% by volume.

[0057] Higher strength can be achieved due to the high amount of alloying elements, especially the high amount of C, and the amount of Mn+Cr+2Mo, which must be at least 2.3 wt%, while the microstructure contains more than 90% bainite, resulting in a somewhat lower hole expansion ratio.

[0058] Preferably, the microstructure of this steel has no more than 4% martensite and retained austenite, preferably no more than 3% martensite and retained austenite, more preferably no more than 2% martensite and retained austenite, even more preferably no more than 1% martensite and retained austenite, and even more preferably no martensite and retained austenite, again, the amount of martensite and retained austenite in particular should not be high so as not to impair the hole expansion ratio.

[0059] Preferably, Cr+2Mo≧0.20 wt%, more preferably Cr+2Mo≧0.30 wt%, and most preferably Cr+2Mo≧0.40 wt%. To reduce the amount of Mn, more Cr+2Mo is added. More Cr+2Mo needs to be added to suppress center segregation, which can impair shear edge quality or hole expansion performance.

[0060] The composition and microstructure of this preferred embodiment of the high strength steel strip according to the invention preferably has the following mechanical properties: Yield strength: 670 MPa or more and 990 MPa or less, Tensile strength: 960 MPa or more and 1380 MPa or less, Total elongation (A50): 9% or more, and / or Hole expansion ratio (λ) value: 40% or more, preferably 45% or more, more preferably 50% or more It has.

[0061] This steel type is therefore very suitable to provide an essentially bainitic steel with a strength of 1000 MPa and good hole expansion for demanding automotive parts.

[0062] Preferably, this steel type has a microstructure comprising 60% or more lath bainitic ferrite and 40% or less irregularly shaped bainitic ferrite, which, as noted above, is beneficial in providing a steel with high strength and a high hole expansion ratio.

[0063] When good hole expansion ratios are required, car or truck parts, such as automobile chassis parts, body-in-white parts, or car or truck frame or subframe parts, are preferably manufactured from steel strip as described above.

[0064] According to a second aspect of the present invention, there is provided a method for producing such high strength steel, as set out in claims 13 and 14. In particular, the coiling temperature of the production method is important, as will be seen from the examples below. [Example]

[0065] The invention will now be described with reference to the following non-limiting examples.

[0066] Example 1 Steels A to R, having the chemical compositions shown in Table 1.1, were hot rolled to a thickness of approximately 3.5 mm under the conditions shown in Tables 1.2 and 1.3 to produce steel sheets 1A to 17R and 18A to 33P, respectively. These steel sheets were produced with the aim of achieving a yield strength of 670 MPa to 990 MPa, a tensile strength of 960 MPa to 1380 MPa, a total (A50) tensile elongation of 9% or more, and a hole expansion ratio λ of 40% or more.

[0067] The forged steel block was reheated to a temperature of approximately 1240°C (RHT) and held at this temperature for approximately 45 minutes. After reheating, the forged block was hot rolled in five rolling passes to reduce the thickness from 35 mm to approximately 3.5 mm. S temperature (T IN ) The finishing rolling temperature (FRT) was 960-990°C. The finishing rolling temperature (FRT) was 875-915°C. After the final rolling pass, the hot rolled steel was transferred to a run-out table and cooled to a temperature of 450-495°C (the temperature at which accelerated cooling is stopped (T SACThe steel was actively cooled to a temperature (Stop Accelerated Cooling Temperature) at a rate of 40-100°C. The steel was then transferred to a furnace for repeated slow coil cooling. This was done at furnace temperatures (CT - coiling temperature) of 450°C (Table 1.2) and 500°C (Table 1.3).

[0068] EBSD measurements were performed on cross sections parallel to the rolling direction (RD-ND plane) mounted in conductive resin and mechanically polished to 1 μm. A final polishing step was performed with colloidal silica (OPS) to obtain a completely deformation-free surface.

[0069] The scanning electron microscope (SEM) used for EBSD measurements was a Zeiss Ultra 55 machine equipped with a field emission gun (FEG-SEM) and an EDAX PEGASUS XM 4HIKARI EBSD system. EBSD scans were collected on the RD-ND plane of the sheet. The sample was positioned at a 70° angle in the SEM. The accelerating voltage was 15 kV, and the high current option was on. A 120 μm aperture was used, and the typical working distance during scanning was 17 mm. Dynamic focus correction was used during scanning to compensate for the high tilt angle of the sample.

[0070] EBSD scans were captured using TexSEM Laboratories (TSL) software: "Orientation Imaging Microscopy (OIM) Data Collection version 7.2." Typically, the following data collection settings were used: Hikari camera (standard mode) at 5 × 5 binning combined with background subtraction. In all cases, the scan area was positioned at 1 / 4 of the sample thickness, and care was taken to avoid non-metallic inclusions in the scan area as much as possible.

[0071] In all cases, the EBSD scan size was 100 × 100 μm, the step size was 0.1 μm, and the scan speed was approximately 100 frames / s. Fe(α) and Fe(γ) were used to index the Kikuchi patterns. The Hough settings used during data collection were as follows: Binned pattern size: about 96; theta set size: 1; Rho fraction: about 90; Max Peak Count: 10; Min Peak Count: 5; Hough type settings: classic; Hough resolution setting: Low; butterfly convolution mask: 9x9; Peak symmetry: 0.5; minimum peak magnitude: 10; Max peak distance: 20.

[0072] EBSD scans were evaluated using TSL OIM Analysis software version 8.0x64 [12-14-16]. Data sets were typically rotated 90° relative to the RD axis to acquire scans at the appropriate orientation relative to the measurement direction. A standard grain dilation clean-up was performed (Grain Tolerance Angle (GTA): 5°, minimum grain size: 5 pixels, using the criterion: a grain must contain multiple rows for a single dilation iteration clean-up). A pseudo-symmetry clean-up (GTA: 5, axis angle: 30° @ 111) was then applied.

[0073] EBSD image quality (IQ) maps were used to determine the amount of martensite. Regions with low IQ were identified as MS areas. For given experimental conditions, the low IQ threshold was typically approximately 0.4 of the peak-maximum position of the IQ histogram. However, the low IQ threshold was manually checked for each scan to ensure that the area fraction of martensite did not include grain boundaries from the granular bainite or upper bainite regions.

[0074] The EBSD kernel average misorientation (KAM) map was calculated using the fifth nearest neighbor, with a maximum misorientation of 5° (all points within the kernel were used for the KAM calculation). Because the KAM is a measure of the internal dislocation density, the KAM is considered characteristic of the type of bainitic ferrite. Regions with relatively low internal dislocation density, primarily corresponding to KAM values ​​between 0 and 1°, are classified as irregularly shaped bainitic ferrite (BF, Type 1) regions (components of ferritic bainite (FB) and granular bainite (GB)). Regions with relatively high internal dislocation density, primarily corresponding to KAM values ​​between 1 and 5°, are classified as lath-shaped bainitic ferrite (BF, Type 2) and martensite. To determine the amount of lath bainitic ferrite (components of upper bainite (UB) and cementite-free bainite (CFB)), the area fraction of martensite determined by the low IQ criteria described in the previous paragraph was subtracted from the area fraction with a KAM value between 1 and 5°. Because EBSD cannot accurately detect cementite and the cementite present in the microstructure is primarily present between the lath bainitic ferrite components of upper bainite (UB), the amount of lath bainitic ferrite measured by EBSD also includes the amount of cementite present in the microstructure.

[0075] Prior to tensile testing and hole expansion performance testing, the hot-rolled sheets were sandblasted to remove the oxide layer. The reported tensile properties for Sheets 1A-17R in Table 1.2 and Sheets 18A-33P in Table 1.3 are based on the A50 tensile geometry with tensile testing parallel to the rolling direction according to EN 10002-1 / ISO 6892-1 (2009) (Rp = 0.2% offset proof or yield strength; Rm = ultimate tensile strength; YR = yield ratio defined as Rp over Rm; Ag = uniform tensile elongation; A50 = tensile elongation). Three square samples (90 × 90 mm) were cut from each sheet to determine the hole expansion ratio λ, which is a measure of stretch flangeability. 2) was cut out, and then a 10 mm diameter hole was punched in the sample with a flat punch. The hole expansion test of the sample was performed with upper burring. A 60° conical punch was pushed up from below, and the hole diameter d f The hole expansion ratio λ was calculated using the following formula:

number

[0076] Steels A to G are steels of the present invention. For these steels, the following formula:

number

number

[0077] All of Steels A to G having the compositions set out in Table 1.1 and having an atomic ratio A between 0.45 and 2.2 (endpoints included) are recognized as examples of the present invention, and the corresponding Steel Sheets 1A to 7G of the present invention in Table 1.2 and the corresponding Steel Sheets 18A to 24G of the present invention in Table 1.3 all have a yield strength of 670 MPa to 990 MPa, a tensile strength of 960 MPa to 1380 MPa, an A50 tensile elongation of 9% or more, and a hole expansion ratio λ of 40% or more.

[0078] These properties result from a microstructure consisting of a mixture of ferrite-bainite (FB) and upper bainite (UB), the latter being the dominant phase component with a volume fraction of 60% or more, typically 65-80%. Consequently, all of these microstructures show evidence of the presence of cementite based on visual inspection with an optical microscope. While precise quantification of the amount of cementite is virtually impossible, the cementite percentage in all examples is estimated to be at most 5%. The volume fraction of ferrite-bainite (FB) is fairly low in these examples, i.e., approximately 20-35%. The amount of martensite and retained austenite (M+RA) is less than 1% in all cases, and in some cases, martensite and / or retained austenite are absent. Consequently, in all of these examples, the amount of granular bainite (GB) and cementite-free bainite (CFB) is insignificant.

[0079] Steels H to R having the compositions set out in Table 1.1 and having an atomic ratio A greater than 2.2 are all considered comparative examples, while the corresponding steel sheets 8H to 17R in Table 1.2 and 25H to 33P in Table 1.3 have excessively high yield strengths, or tensile strengths less than 960 MPa, or excessively low formability in that the A50 tensile elongation is less than 9% or the hole expansion ratio λ is less than 40%.

[0080] These properties result from a microstructure that, like the examples, is composed of a mixture of ferrite-bainite (FB) and upper bainite (UB), but which has some essential differences from the examples, either in terms of an increased proportion of cementite (FeC) or an increased proportion of martensite and retained austenite (M+RA). These differences are evident in the following comparative examples: Comparative Examples 10J to 12L in Table 1.2 and Comparative Examples 27J to 29L in Table 1.3, and Comparative Examples 13M to 17R in Table 1.2 and Comparative Examples 30M to 33P in Table 1.3 is emphasized in.

[0081] In contrast to the Examples, Comparative Examples 10J, 11K, and 12L in Table 1.2 and Comparative Examples 27J, 28K, and 29L in Table 1.3 have cementite percentages rated above 5%, which amount of cementite is believed to impair formability, i.e., tensile elongation and / or hole expansion performance.

[0082] In the case of Comparative Examples 13M-17R in Table 1.2 and Comparative Examples 30M-33P in Table 1.3, the amount of upper bainite (UB) is quite low, typically 50-60%, while the amount of ferritic bainite (FB) is quite high, typically about 35-55%. For these comparative samples, as in the Examples, the cementite percentage is estimated to be greater than 0% and less than 5%. However, microscopic analysis shows that for Comparative Examples 13M-17R in Table 1.2 and Comparative Examples 30M-33P in Table 1.3, carbon leads to the formation of martensite and / or retained austenite. The amount of martensite and retained austenite (M+RA) is greater than 1% in all cases, and in most cases, the amount of martensite and retained austenite is (well) greater than 4%. This indicates an increased amount of granular bainite (GB) and cementite-free bainite (CFB) in these comparative examples. The decrease in the proportion of upper bainite (UB) with the increase in ferrite bainite (FB) and the increase in the amount of GB and / or CFB are believed to contribute to the lower hole expansion performance of these comparative examples than that observed in the examples in this case.

[0083] To achieve a steel having a yield strength of 670 MPa or more and 990 MPa or less, a tensile strength of 960 MPa or more and 1380 MPa or less, an A50 tensile elongation of 9% or more, and a hole expansion ratio λ of 40% or more, the microstructure of the steel is at least 90% bainite, preferably at least 95% bainite, more preferably at least 97% bainite, even more preferably at least 98% bainite, most preferably at least 99% bainite where the bainite is composed of a mixture of predominantly upper bainite (UB) and a minor contribution of ferrite-bainite (FB), strengthened by Ti-based complex carbide precipitates, in which the overall microstructure of the steel is At least 60% lath-like bainitic ferrite (BF, type 2) (0% > 5% below cementite, preferably 0.01% to 4% cementite, more preferably 0.02% to 3% cementite, even more preferably 0.02% to 2% cementite, and most preferably 0.02% to 1% cementite), up to 40% irregularly shaped bainitic ferrite (BF, type 1), and Maximum 5% martensite and retained austenite (M+RA), preferably maximum 3% martensite and retained austenite, more preferably maximum 2% martensite and retained austenite, even more preferably maximum 1% martensite and retained austenite, most preferably no martensite and retained austenite It consists of:

[0084] [Table 1]

[0085] [Table 2]

[0086] [Table 3]

[0087] Example 2 Steels A to J, having the chemical compositions shown in Table 2.1, were hot-rolled to a thickness of approximately 3.5 mm under the conditions shown in Tables 2.2, 2.3, and 2.4 to produce steel sheets 1A to 6F, 7A to 16J, and 17G to 20J, respectively. These steel sheets were manufactured with the aim of achieving a yield strength of 570 MPa to 900 MPa, a tensile strength of 760 MPa to 960 MPa, a total (A50) tensile elongation of 10% or more, and a hole expansion ratio λ of 50% or more.

[0088] The forged steel block was reheated to a temperature of approximately 1240°C (RHT) and held at this temperature for approximately 45 minutes. After reheating, the forged block was hot rolled in five rolling passes to reduce the thickness from 35 mm to approximately 3.5 mm. S temperature (T IN ) The finishing rolling temperature (FRT) was 960-990°C. After the final rolling pass, the hot rolled steel was transferred to a run-out table and cooled to a constant temperature (the temperature at which accelerated cooling is stopped (T SAC )) at a cooling rate of 40-100°C. After cooling on the run-out table, the steel was transferred to a furnace and subjected to repeated slow coil cooling at furnace temperatures (CT - coiling temperature) of 450°C (Table 2.2), 550°C (Table 2.3) and 500°C (Table 2.4). The exit run-out table temperatures (T E ) were 465 to 510°C, 540 to 580°C, and 500 to 550°C, respectively.

[0089] The EBSD procedure used to determine the amount of irregularly shaped bainitic ferrite, lath-shaped bainitic ferrite, martensite, and retained austenite is the same as that described in Example 1.

[0090] Prior to tensile testing and hole expansion performance testing, the hot-rolled sheets were sandblasted to remove the oxide layer. The reported tensile properties for Sheets 1A-6F in Table 1.2, Sheets 7A-16J in Table 2.3, and Sheets 17G-20J in Table 2.4 are based on the A50 tensile geometry from tensile tests parallel to the rolling direction according to EN 10002-1 / ISO 6892-1 (2009). (Rp = 0.2% proof stress or yield strength; Rm = ultimate tensile strength; YR = yield ratio defined as Rp over Rm; Ag = uniform tensile elongation; A50 = tensile elongation.) Three square samples (90 × 90 mm) were cut from each sheet to determine the hole expansion ratio, λ, which serves as a measure of stretch-flangeability. 2 ) was cut out, and then a 10 mm diameter hole was punched in the sample with a flat punch. The hole expansion test of the sample was carried out for the burr on the top. A 60° conical punch was pushed up from below, and when a crack was formed in the thickness direction, the hole diameter d f The hole expansion ratio λ was calculated using the following formula:

number

[0091] Steels A to I are steels of the present invention. For these steels, the following formula:

number

number

[0092] Steels A-I, having the compositions set forth in Table 2.1 and having an atomic ratio A between 0.45 and 2.2 (inclusive), are all recognized as examples of the present invention. Steel sheets 1A, 2B, and 4D-6F of the present invention, corresponding steel sheets 7A-15I of the present invention in Table 2.3, and corresponding steel sheets 17G-19I of the present invention in Table 2.4, all have yield strengths of 570 MPa to 900 MPa, tensile strengths of 760 MPa to 960 MPa, A50 tensile elongations of 10% or more, and hole expansion ratios λ of 50% or more. Steel J, having the composition set forth in Table 2.1 and an atomic ratio A much greater than 2.2, is recognized as a comparative example. Steel sheet 16J of Table 2.3 and steel sheet 20J of Table 2.4, which have hole expansion ratios λ of less than 50%, are recognized as comparative examples.

[0093] For the production of steels having a yield strength of 570 to 900 MPa, a tensile strength of 760 to 960 MPa, a total (A50) tensile elongation of 10% or more, and a hole expansion ratio λ of 50% or more, it is preferable to use a coiling temperature of 520 to 570°C. A comparison between the data corresponding to the examples shown in Tables 2.2, 2.3, and 2.4 shows that a coiling temperature of 550°C results in an A50 tensile elongation that is significantly higher than the A50 tensile elongations obtained with lower coiling temperatures of 450 or 500°C, yet still provides excellent hole expansion performance and good values ​​of yield strength and tensile strength.

[0094] Microstructures of Examples 1A to 6F coiled at 450°C (Table 2.2) The properties of all examples are derived from microstructures consisting of a mixture of ferrite-bainite (FB) and upper bainite (UB), with the latter being the dominant phase component at volume fractions above 60%, typically 60-75%. As a result, all these microstructures show evidence of the presence of cementite based on visual inspection with an optical microscope. The volume fraction of ferrite-bainite (FB) is fairly low in these examples, i.e., approximately 25-40%. The amount of martensite and retained austenite (M+RA) is well below 1% in all cases, and in some cases, martensite and / or retained austenite is absent. Consequently, in all these examples, granular bainite (GB) and cementite-free bainite (CFB) are not significantly present.

[0095] The composition of Steel C, in which no boron additions greater than 5 ppm are intended, is considered an example of the present invention. However, when used in combination with a coiling temperature of 450°C, the corresponding Steel Sheet 3C (Table 2.2) exhibits excessively low tensile strength (below 760 MPa) due to insufficient hardenability. This makes Steel Sheet 3C a comparative example of the present invention. The excessively low strength is explained by the increased presence of ferrite-bainite (FB) at the expense of upper bainite (UB) due to the lack of an intended boron addition greater than 5 ppm and subsequent poor hardenability. The strength is reduced because the dislocation density within ferrite-bainite (FB) is significantly lower than that within upper bainite (UB), and its crystallographic packet size is larger.

[0096] Microstructures of Examples 7A to 16J coiled at 550°C (Table 2.3) As mentioned above, coiling at 520-570°C is the preferred option. The properties of all examples obtained by coiling at 550°C result from a microstructure composed of a mixture of ferritic bainite (FB), granular bainite (GB), and upper bainite (UB), with the former (FB) being the dominant phase component with a volume fraction of 60% or more, typically 60-75%. The volume fraction of upper bainite (UB) is fairly low in these examples, i.e., approximately 25-40%. This low presence of upper bainite is associated with the presence of some cementite based on visual inspection with an optical microscope after etching with a 4% picral solution to selectively delineate the cementite. The amount of martensite and retained austenite (M+RA) is less than 4% in all cases, and less than 3% in most cases. The lowest amount of martensite and retained austenite (M+RA) measured for the examples is 0.5%.

[0097] Because the amounts of martensite and retained austenite are very low, the amount of granular bainite (GB) in all of these examples is estimated to be relatively small (≦25%). Because the coiling temperatures used are relatively high, the amount of cementite-free bainite is estimated to be insignificant. The relatively high coiling temperature of 550°C favors ferrite-bainite (FB) over upper bainite (UB). This high coiling temperature primarily favors Ti carbide precipitation, but also provides sufficient reaction rates for Nb and / or Mo carbide precipitation, so that much of the carbon is consumed in the carbide precipitation process, limiting the amount of carbon partitioning during the phase transformation. This results in a material with little or no martensite and retained austenite or cementite, and a ferrite-bainite strengthened with TiC or Ti-based complex carbide precipitates (e.g., containing Nb and / or Mo in addition to Ti).

[0098] Steel sheet 16J is a comparative example because the hole expansion ratio λ is less than 50%. The microstructure of this steel sheet has a slightly lower amount of ferrite-bainite (FB) than the examples in Table 2.3, and consequently a slightly higher percentage of upper bainite (UB). However, the percentages of both bainite forms are closer to those of the examples in this table. The amount of martensite and retained austenite in comparative example 16J is in the same range as the amounts of martensite and retained austenite in the examples, and is well below 2%, like many of the examples in Table 2.3. The amount of carbon that can remain in solid solution in the steel matrix is ​​very small, expected to be less than 0.02 wt.%. Excess carbon leads to the formation of (1) cementite, (2) either martensite and / or retained austenite, and / or (3) carbide precipitates with elements such as Ti, Nb, V, and / or Mo. The process conditions and alloy composition control the extent to which these microstructural elements form. Because the carbon content of Comparative Example 16J is much higher than that of all Examples (Table 2.1), and the sum of the amounts of the carbide-forming elements Ti, Nb, V, and / or Mo is much lower, the atomic ratio A of the Comparative Example is much higher than 2.2, at 3.45. This much higher atomic ratio A, along with the observation that the amounts of martensite and retained austenite in Comparative Example 16J are similar to those of the Examples, leads to the conclusion that the microstructure of Comparative Example 16J must contain substantially more cementite than all the other Examples. This is confirmed by visual inspection of the microstructures of all Examples shown in Table 2.3 after etching with a 4% picral solution to selectively delineate the cementite. While precise quantification of the amount of cementite is virtually impossible, the cementite percentage in Comparative Example 16J is estimated to be greater than 5%, while the cementite percentage in the Examples is estimated to be much less than 5%.

[0099] Microstructures of Examples 17G to 20J coiled at 500°C (Table 2.4) The properties of all examples, except for Example 19I, are derived from a microstructure composed of a mixture of ferrite-bainite (FB), granular bainite (GB), and upper bainite (UB). Example 19I also has a microstructure composed of a mixture of ferrite-bainite (FB) and upper bainite (UB), but the amount of granular bainite (GB) is not significant because the amount of martensite and retained austenite is well below 1%. The amount of ferrite-bainite (FB) is typically 40-60%, while the amount of upper bainite is typically 35-60%. This low presence of upper bainite is associated with the presence of some cementite based on visual inspection with an optical microscope after etching with a 4% picral solution to selectively delineate the cementite. The amount of martensite and retained austenite (M+RA) is less than 5% in all cases and less than 3% in most cases. The lowest amount of martensite and retained austenite (M+RA) measured for the examples is 0.4%.

[0100] Because the amounts of martensite and retained austenite are very low, the amount of granular bainite (GB) in most examples in Table 2.4 is estimated to be relatively small (≦25%). Because the coiling temperatures used are still relatively high, the amount of cementite-free bainite is estimated to be insignificant. The relatively high coiling temperature of 500°C may favor ferrite-bainite (FB) over upper bainite (UB). This high coiling temperature provides a sufficient reaction rate for at least partial carbide precipitation, primarily with Ti, but also with Nb and / or Mo, so that much of the carbon is consumed in the carbide precipitation process with these elements, limiting the amount of carbon partitioning during the phase transformation. This results in the material having little or no martensite and retained austenite or cementite, and producing ferrite-bainite partially reinforced with TiC or Ti-based complex carbide precipitates (e.g., containing Nb and / or Mo in addition to Ti).

[0101] Steel sheet 20J is a comparative example because the hole expansion ratio λ is less than 50%. The microstructure of this steel sheet has similar amounts of ferrite-bainite (FB) and upper bainite (UB) as the examples in Table 2.4. The amount of martensite and retained austenite in comparative example 20J is in the same range as the amounts of martensite and retained austenite in the examples, and is well below 3%, like many of the examples in Table 2.4. The amount of carbon that can remain in solid solution in the steel matrix is ​​very small, expected to be less than 0.02 wt.%. Excess carbon leads to the formation of either (1) cementite, (2) martensite and / or retained austenite, and / or (3) carbide precipitates with elements such as Ti, Nb, V, and / or Mo. The process conditions and alloy composition control the extent to which these microstructural elements form. Because the carbon content of Comparative Example 20J is much higher than that of all Examples (Table 2.1), and the sum of the amounts of the carbide-forming elements Ti, Nb, V, and / or Mo is much lower, the atomic ratio A of the Comparative Example is much higher than 2.2, at 3.45. This much higher atomic ratio A, combined with the observation that the amounts of martensite and retained austenite in Comparative Example 20J are similar to those in the Examples, leads to the conclusion that the microstructure of Comparative Example 20J must contain substantially more cementite than all the other Examples. This is confirmed by visual inspection of the microstructures of all Examples shown in Table 2.4 after etching with a 4% picral solution to selectively delineate the cementite. While precise quantification of the amount of cementite is virtually impossible, the cementite percentage in Comparative Example 20J is estimated to be greater than 5%, while the cementite percentage in the Examples is estimated to be much less than 5%.

[0102] To achieve a steel having a yield strength of 570 to 900 MPa, a tensile strength of 760 to 960 MPa, an A50 tensile elongation of 10% or more, and a hole expansion ratio λ of 50% or more, the microstructure of the steel is: at least 90% bainite, preferably at least 95% bainite, more preferably at least 97% bainite, even more preferably at least 98% bainite, most preferably at least 99% bainite where the bainite must contain A mixture of upper bainite (UB), ferritic bainite (FB) and possibly granular bainite (GB), strengthened by Ti-based complex carbide precipitates, or · Mainly ferrite-bainite (FB) and a mixture of small amounts of upper bainite (UB) and granular bainite (GB), strengthened by Ti-based complex carbide precipitates It consists of The overall microstructure of the steel therein preferably comprises: maximum 40% lath-like bainitic ferrite (BF, type 2) (containing more than 0% and not more than 5% cementite, preferably 0.01% to 4% cementite, more preferably 0.02% to 3% cementite, even more preferably 0.02% to 2% cementite, and most preferably 0.02% to 1% cementite), At least 60% irregularly shaped bainitic ferrite (BF, type 1), and Maximum 5% martensite and retained austenite (M+RA), preferably maximum 3% martensite and retained austenite, more preferably maximum 2% martensite and retained austenite, even more preferably maximum 1% martensite and retained austenite, most preferably no martensite and retained austenite It consists of:

[0103] [Table 4]

[0104] [Table 5]

[0105]

Table 6

[0106]

Table 7

Claims

1. C: 0.02-0.13% by weight; Mn: 1.20-2.20% by weight; Si: 0.10 to 1.00% by weight; Al_tot: 0.01 to 0.10% by weight; Ti: 0.09 to 0.18% by weight; N: 0.001 to 0.010% by weight; Mo: 0.005-0.2% by weight P: 0 to 0.10% by weight; S: 0-0.01% by weight; Optionally, B: 0 to 0.005% by weight; Optionally, Cu: 0 to 1.5% by weight Cr: 0 to 1.0% by weight Ni: 0 to 0.50% by weight V: 0 to 0.30% by weight Nb: 0 to 0.04% by weight One or more of the following: Iron and unavoidable impurities: balance A hot rolled high strength steel strip consisting of: The steel contains the following limited range of elements: C: 0.03-0.12% by weight; Mn: 1.50 to 2.20% by weight; Si: 0.20-0.95% by weight; Ti: 0.10 to 0.18% by weight; B: 0.0010 to 0.004% by weight; Mo: 0.005-0.2% by weight and / or Optional elements in the following limited ranges: Cu: 0 to 0.5% by weight; Cr: 0-0.9% by weight; Ni: 0 to 0.2% by weight; V: 0 to 0.18% by weight; Nb: 0 to 0.04% by weight Contains one or more of the following: Ti + Nb is 0.25 wt% or less, Cr+Mo is 1.0 wt% or less, Mn+Cr+2Mo is 2.3% by weight or more, Steel, in volume percent, Bainite: 90% or more Martensite and retained austenite: 5% or less Cementite: more than 0% and less than 5% Inclusions: unavoidable amount wherein the sum of these is 100% by volume; the steel strip comprises 60% or more lath-shaped bainitic ferrite and 40% or less irregularly shaped bainitic ferrite; The steel strip has the following mechanical properties: Tensile strength: 760 MPa or more and 960 MPa or less; Total elongation (A50): 10% or more; Hole expansion ratio (λ) value: 50% or more or have the following mechanical properties: Tensile strength: 960 MPa or more and 1380 MPa or less; Total elongation (A50): 9% or more; Hole expansion ratio (λ) value: 40% or more and The value of the following expression: [Equation 1] [Wherein Ti_sol is [Equation 2] is defined as: has a lower limit of 0.45 and an upper limit of 2.

2.

2. 2. The steel strip of claim 1, wherein the value of the above formula has a lower limit of 0.55 and an upper limit of 2.

1.

3. 3. Steel strip according to claim 1 or 2, wherein the microstructure of the steel has up to 4% martensite and retained austenite.

4. Steel strip according to any one of claims 1 to 3, wherein Cr+2Mo is equal to or greater than 0.20% by weight.

5. The steel strip has the following mechanical properties: Yield strength: 670 MPa or more and 990 MPa or less; Tensile strength: 960 MPa or more and 1380 MPa or less; Total elongation (A50): 9% or more; Hole expansion ratio (λ) value: 40% or more The steel strip according to any one of claims 1 to 4, having

6. 6. A car or truck part manufactured from the steel strip of any one of claims 1 to 5, such as an automobile chassis part, a body-in-white part, or a car or truck frame or subframe part.

7. A method for producing a steel strip according to any one of claims 1 to 5, comprising the following steps: casting a slab, wherein the step of casting the slab is followed by the steps of reheating the solidified slab to a temperature of 1050-1260°C and hot rolling the slab; or casting a slab or strip, wherein the step of casting the slab or strip is immediately followed by the step of hot rolling the slab or strip; hot rolling the steel slab or strip at an entrance temperature of the final rolling stand of 960-1100°C; Finishing the hot rolling at a finishing rolling temperature of 850 to 1080°C; cooling the hot rolled steel strip on a run-out table at a cooling rate of 10-250°C / s to a temperature of 550-420°C; Then, the wire is wound at 370 to 580°C. A method comprising:

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