Patent Description:
In automotive industry, hot-dip galvanized (HDG) or galvannealed advanced high strength steel (AHSS) products are desired due to the combined advantages of light weight and high strength which allow for considerable reduction in automobile body weight without compromising safety requirements. AHSS grades can be used to make safety relevant body in white (BIW) components such as side impact protection beams.

In response to ever increasing demand for lighter yet stronger material solutions, AHSS grades with ultimate tensile strength levels of <NUM> MPa and above have been developed. Achieving such high strength requires quite high levels of alloying elements in the chemical composition, in particular when the steel is designed for processing in conventional continuous galvanizing lines (CGLs) with relatively low cooling capacity. The steel compositions also need to be fine-tuned for specific CGL heat treatment cycles in order to meet the desired mechanical property values e.g. strength, toughness, formability and weldability; and other practical customer requirements related to products exterior qualities e.g. dimensions, shape tolerance, flatness, gauge uniformity and surface quality.

Producing AHSS grades with ultimate tensile strength of <NUM> MPa and above requires complex alloying to increase hardenability of the steel, which severely suppresses transformation of austenite on hot rolling mill runout table such that the low-temperature microstructure is formed mainly during coil cooling. This leads to pronounced microstructural variations along the strip length, since the cooling rate in different parts of a hot rolled coil vary greatly because there is no viable means to control cooling of a coil due to unrepeatable handling and storage conditions, and seasonal variation in ambient temperature when coils are left to cool outdoors. The formed microstructures can range from ferritic-pearlitic to bainitic to even martensitic within a single coil, even when a constant coiling temperature has been achieved over the length of the strip.

The variability of microstructure and mechanical properties are not readily visible from an exterior view of the hot rolled strip. They only manifests in obvious defects, such as deteriorated flatness and gauge inconsistency, in the downstream operations that follow hot rolling. One widely recognized issue occurring in the downstream processing is the so-called cold-gauge hashing which is a periodic deviation in thickness. The periodicity of gauge spikes correlates with the microstructure and mechanical properties of the hot rolled strip that varies regularly once per revolution of the hot coil. This phenomenon causes unacceptable yield losses due to intolerably defective final products. In the worst case of uncontrolled coil cooling, the strength can become excessively high in one part of the hot rolled strip, which could cause disturbances or even strip breakages and damage to the equipment in the cold-rolling mill.

The problem of cold gauge hashing has been discussed in the following articles.

Two main set of factors were found affecting the amplitude of cold gauge hashing defects. They are steel chemistry and hot rolling mill process.

It was mentioned in (<NUM>) and (<NUM>) that the ferritic transformation on the run-out table can be accelerated by reducing the contents of austenite stabilizer (C, Mn) and transformation retarding elements (B, Cr, Mo). Hot strips with mainly ferritic microstructure are easy to cold roll with excellent results for both flatness and thickness accuracy. However, this approach is not realistic with the alloying levels of steels with ultimate tensile strength of <NUM> MPa class and above.

It was further mentioned in (<NUM>) that cold gauge hashing can be attenuated by lowering the coiling temperature, which favours phase transformation of austenite into bainite on the run-out table. However, this approach is not realistic which is dependent on the length of run-out table and the cooling system. For a relatively short run-out table, it is impossible to lower the coiling temperature without also lowering the finishing temperature since the hot rolled strip shape can be significantly distorted by intense rapid cooling. On the other hand, with high levels of alloying, the bainitic transformation could be too slow to occur on the run-out table before coiling.

It was mentioned in (<NUM>) that slow cooling of coils allows for improving longitudinal and transverse uniformity of microstructure and mechanical properties, and thereby enhancing cold rollability. However, slow coil cooling must be coupled with hot mill process timing, which is less flexible and causes decrease in productivity. Furthermore, there is no reliable means to control coil cooling rate even in a thermally insulated coil cooling box as the latent heat generated during austenite transformation affects the ambient temperature.

<CIT>, concerning a high strength galvannealed steel sheet with an ultimate tensile strength of <NUM> - <NUM> MPa, recognizes that the hot rolled sheet needs to be subjected to batch annealing before cold rolling in order to obtain a microstructure which is necessary for cold-rolling with narrow thickness deviation. However, there is no evidence showing that the thickness deviation is diminished or the problem of cold gauge hashing can be solved by the extra step of batch annealing. An extra step of batch annealing would inevitably enhance production costs and reduce productivity. Further, <CIT> discloses high-strength steel sheets having a similar composition and microstructure as claimed.

The present invention is intended to improve the production yield of high strength steel sheet products with ultimate tensile strength of at least <NUM> MPa, and to obtain a wide dimensional range for the respective final products. This is achieved by preventing the above-mentioned problem of cold gauge hashing and alleviating other issues related to strength variations or flatness defects in the hot rolled strip.

In view of the state of art, the object of the present invention is to solve the problem of providing a high strength steel sheet with ultimate tensile strength of at least <NUM> MPa and having improved flatness and gauge uniformity. The problem is solved by the combination of specific alloy designs and hot rolling mill process which attenuates the phenomenon of cold gauge hashing.

In a first aspect, the present invention provides a high strength steel sheet as defined in claim <NUM>.

Specifically, Nb is not added in order for the steel product to have a wide dimensional range. The steel product is alloyed with essential alloying elements such as C, Si, Mn, Cr, Ti, B, Ca, and Al. Other elements such as Mo, V, Cu, and Ni may be present as residual contents that are not purposefully added.

In particular, the present invention provides a high strength steel sheet with an ultimate tensile strength (Rm) of at least <NUM> MPa and a thickness accuracy better than <NUM>/<NUM> of EN <NUM>:<NUM> normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt.

Furthermore, the present invention provides a high strength steel sheet with an ultimate tensile strength (Rm) of at least <NUM> MPa and a thickness accuracy better than <NUM>/<NUM> of EN <NUM>:<NUM> normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt.

The steel sheet has a microstructure comprising a matrix consisting of, in terms of volume percentages (vol.

remainder bainite and/or fine-grained ferrite.

The microstructure has an average grain size of less than <NUM>. Preferably, the bainite and/or fine-grained ferrite have an average grain size of less than <NUM>, preferably in the range of <NUM> to <NUM>.

Preferably, the steel sheet further has at least one of the following mechanical properties:.

Preferably, the high strength steel sheet with an ultimate tensile strength (Rm) of at least <NUM> MPa has a minimum bending radius (Ri) of <NUM> t or less.

Preferably, the high strength steel sheet with an ultimate tensile strength (Rm) of at least <NUM> MPa has a total elongation (A<NUM>) of at least <NUM> %.

Preferably, the steel sheet is a strip having a thickness in the range of <NUM> - <NUM>.

In a second aspect, the present invention provides a method for manufacturing the steel sheet as defined in claim <NUM>.

The optional step of hot-dip coating is preferably hot-dip galvanizing or galvannealing.

Optionally, the method further comprises steps of temper rolling and/or levelling. Optionally, the method further comprises a step of extra batch annealing at a temperature in the range of <NUM> to <NUM>, e.g. <NUM>.

The term "steel" is defined as an iron alloy containing carbon (C).

The term "flatness" is used to indicate deviations from a horizontal flat surface in a steel strip.

The term "thickness accuracy" is used to indicate deviations from a target thickness in a steel strip in length direction.

The term "gauge" refers generally to a measure of the thickness of a metal sheet.

The term "EN <NUM>:<NUM>" refers to European Standard tolerances on dimensions and shape, which is applicable to continuously hot-dip coated steel sheet and strip.

The term "ultimate tensile strength (UTS, Rm)" refers to the limit, at which the steel fractures under tension, thus the maximum tensile stress.

The term "yield strength (YS, Rp<NUM>)" refers to <NUM> % offset yield strength defined as the amount of stress that will result in a plastic strain of <NUM> %.

The term "yield ratio (YR)" refers to the ratio of Rp<NUM> and Rm, in terms of percentage, i.e. YR = (Rp<NUM>/Rm) x <NUM>.

The term "total elongation (TEL)" refers to the percentage by which the material can be stretched before it breaks; a rough indicator of formability, usually expressed as a percentage over a fixed gauge length of the measuring extensometer. Two common gauge lengths are <NUM> (A<NUM>) and <NUM> (A<NUM>).

The term "minimum bending radius (Ri)" is used to refer to the minimum radius of bending that can be applied to a test sheet without occurrence of cracks.

The term "bendability" refers to the ratio of Ri and the sheet thickness (t).

The term "hole expansion ratio (λ)" is a key indicator to evaluate stretch flanging performance of steel sheets, which is usually obtained by hole expanding test using cylindrical or conical punch.

Our approach to solve the problem of cold gauge hashing is beneficial to the formation of an essentially bainitic microstructure with minimal or at least consistent strength variation over the whole length and width of the cooled-down hot rolled strip. Concurrently, the strength of this bainitic microstructure is targeted as low as possible to increase hot and cold rollability for obtaining the widest possible dimensional range for cold-rolled products.

Through careful study of various hot strip microstructures and resulting properties, we have found a specific combination of alloying elements, which results in prevention of the formation of soft ferritic-pearlitic microstructures in the slow-cooling core of the hot rolled coil. The contents of austenite stabilizers C, Mn and Cr, and other transformation-retarding elements B and optionally Mo are adjusted to levels beneficial to bainitic transformation. As a consequence of the combined effects of suppressing the formation of soft ferritic-pearlitic microstructures and enhancing bainitic transformation, it is possible to utilize a high coiling temperature thereby obtaining generally slower cooling rate of the hot-rolled coil in storage, resulting in lower strength of the desired bainitic microstructure as it forms. The alloy design ensures that the ultimate tensile strength of at least <NUM> MPa can be obtained after hot-dip galvanizing or galvannealing. The alloy design also achieves a good balance of strength and elongation.

Next the chemical composition is described in more details.

C alloying increases strength of steel by solid solution strengthening, and hence C content determines the strength level. C content less than <NUM> wt. % may lead to insufficient tensile strength below <NUM> MPa. C also functions as an austenite stabilizer and delays transformation of austenite, which inhibits the formation of ferritic-pearlitic microstructures.

However, C content needs to be set to not more than <NUM> wt. % to prevent excessive strengthening within the hot rolled coil in parts that are locally cooled too fast to form the desired bainitic microstructures.

Si is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process. Si alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability. The presence of Si favours good balance of strength and elongation since Si simultaneously increases formation and hardness of the ferrite and low carbide containing bainite which are the ductile components in the microstructure. Also, the presence of Si stabilizes ferrite and residual austenite.

However, zinc coating adhesion and surface quality may be deteriorated if Si is present in excessive amount more than <NUM> wt.

Preferably, the Si content is more than <NUM> wt. % and less than <NUM> wt.

Mn alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability. As an austenite stabilizer, Mn stabilizes the remaining austenite at later stages of the bainitic transformation thereby inhibiting transformation of the carbon-rich constituent into different microstructures in different parts of the hot rolled coil. Mn is beneficial to the preferred carbon-rich second phase being martensite or autotempered martensite. Transformation to degenerate pearlite is inhibited in the presence of Mn.

However, Mn content needs to be set to not more than <NUM> wt. % to prevent excessive strengthening and hardenability.

Cr alloying enhances strength and hardness by increasing austenite hardenability. At least <NUM> wt. % of chromium is required for sufficient hardenability at the HDG line.

Cr also functions as an austenite stabilizer in the same manner as Mn. Cr alloying promotes the formation of the preferred carbon-rich second phase being martensite or autotempered martensite, while inhibiting the transformation to degenerate pearlite.

However, Cr in an amount above <NUM> wt. % would increase the strength of the desired bainitic structure to an unacceptable level.

Ti is added to bind free N that is harmful to toughness by forming stable TiN, which can efficiently prevent austenite grain growth in the reheating stage at high temperatures. TiN formation also suppresses BN precipitation, thereby leaving B free to make its contribution to hardenability. Thus, Ti is usually required to ensure the effectiveness of B in the case of B alloying.

However, if Ti content is too high, coarsening of TiN and precipitation hardening due to TiC develop and impact toughness may be deteriorated. Therefore, it is necessary to restrict titanium so that it is less than <NUM> wt.

B is a transformation-retarding element that suppresses formation of diffusional transformation products such as polygonal ferrite, thereby promoting formation of low carbon bainitic structures. Thus, the presence of B prevents formation of soft ferritic-pearlitic microstructures in the slow-cooling core of the hot rolled coil.

Effective B alloying would require the presence of Ti to prevent formation of BN. Toughness is rapidly deteriorated when B content exceeds <NUM> wt.

Mo is a transformation-retarding element that has the effects of promoting the formation of low carbon bainitic structure. The presence of Mo also enhances strength and hardness by increasing austenite hardenability. In the case of B alloying, Mo is optionally required to ensure the effectiveness of B.

However, Mo is not an economically acceptable alloying element. Mo content should not exceed <NUM> wt. Excessive amount of Mo may impose limitations to the achievable dimensional range due to excessive strengthening of the bainitic microstructure beyond the desired level. If Mo is used in an amount above <NUM> wt. % toughness may be deteriorated thereby increasing risk of brittleness, and also the effect of B may be reduced.

V is a strong carbide and nitride former, but V(C,N) can also form and its solubility in austenite is high. Thus, V alloying has potential for dispersion and precipitation strengthening, because large quantities of V are dissolved and available for precipitation in ferrite.

However, addition of V more than <NUM> wt. % has negative effects on hardenability.

Cu promotes low carbon bainitic structures, causes solid solution strengthening and contributes to precipitation strengthening.

The upper limit of Cu content is set to <NUM> wt. % to prevent excessive strengthening. When added in excessive amount Cu also deteriorates toughness.

Ni is an alloying element that improves austenite hardenability and increases strength without any loss of toughness.

However nickel contents of above <NUM> wt. % would increase alloying costs too much without significant technical improvement. Excess amount of Ni may produce high viscosity iron oxide scales, which deteriorate surface quality of the steel product.

Ca is not used as alloying element due to its low solubility in steel and high vapor pressure. The optional Ca addition during a steelmaking process is for refining, deoxidation, desulphurization, and control of shape, size and distribution of oxide and sulphide inclusions.

Preferably, Ca is used in the range of <NUM> wt. % to <NUM> wt.

Al is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process. Al also removes N by forming stable AIN particles and provides grain refinement, which has the effects of promoting high toughness. Also, Al stabilizes ferrite and residual austenite.

However, Al may increase non-metallic inclusions thereby deteriorating cleanliness if used in excessive amount above <NUM> wt.

Preferably, Al is used in the range of <NUM> wt. % to <NUM> wt.

Niobium Nb is considered as a major grain refining element. Nb contributes to the strengthening and toughening of steels. Specifically, Nb is not added in the composition according to the present invention in order to allow the steel product to have a wide dimensional range since excessive strengthening may decrease hot and cold rollability of the hot-rolled sheet.

The steel product is alloyed with essential alloying elements such as C, Si, Mn, Cr, Ti, B, Ca, and Al. Other elements such as Mo, V, Cu, and Ni may be present as residual contents that are not purposefully added. The difference between residual contents and unavoidable impurities is that residual contents are controlled quantities of alloying elements, which are not considered to be impurities. A residual content as normally controlled by an industrial process does not have an essential effect upon the alloy.

Unavoidable impurities can be phosphor P, sulfur S, nitrogen N. Their contents are preferably limited as follows:.

The method for producing the intermediate hot rolled product comprises the steps of:.

a composition (I) consisting of, in terms weight percentages (wt.

remainder Fe and inevitable impurities, or
a composition (II) consisting of, in terms weight percentages (wt.

remainder Fe and inevitable impurities, or
a composition (III) consisting of, in terms weight percentages (wt.

remainder Fe and inevitable impurities;
- heating and hot-rolling to achieve a steel sheet with a finish rolling temperature of Ar<NUM> or above, typically <NUM> to <NUM>;
- accelerated cooling to a temperature of <NUM> or below;
- coiling at a temperature in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>; and
- cooling the coil to ambient temperature.

It is beneficial to apply a higher coiling temperature, which slows down the cooling rate of the hot-rolled coil in storage and therefore results in lower strength of the hot-rolled sheet. Consequently, cold rollability of the hot-rolled sheet is increased, which makes it possible to obtain a wide dimensional range for cold-rolled products. The microstructure of the intermediate hot-rolled steel sheet consists of less than <NUM>% of pearlite and polygonal ferrite with a grain size above <NUM>, and the rest majority being bainite and/or fine-grained ferrite, and martensite, at all positions along the full length of the steel sheet. Fine-grained ferrite refers here to ferritic transformation products with a grain size in the order of typically <NUM>, which may nucleate above the bainitic temperature range, but are indistinguishable from low carbide containing bainitic ferrite by visual identification from a secondary electron microscope image. The martensite having variable carbon contents may be tempered and/or auto-tempered to variable degrees.

When the hot rolled coil has been cooled to ambient temperature, further process for producing the final product includes the steps of:.

In some embodiments, the step of cooling from the annealing temperature in the range of <NUM> to <NUM> to the holding temperature in the range of <NUM> to <NUM> is continuous cooling at an average rate in the range of typically <NUM>/s to <NUM>/s.

In some embodiments, the step of cooling from the annealing temperature in the range of <NUM> to <NUM> to the holding temperature in the range of <NUM> to <NUM> is a two-step cooling comprising.

The first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.

Optionally, the method further comprises steps of temper rolling and/or levelling which are not crucial, but may improve yield strength of the final product.

Optionally, the method further comprises a step of extra batch annealing at a temperature in the range of <NUM> to <NUM>, e.g. <NUM>. The step of extra batch annealing results in bake hardening and tempering of the microstructure, which increases the yield strength of the steel while improving local formability parameters such as the hole expansion ratio.

The microstructure of the final steel sheet comprises a matrix consisting of, in terms of volume percentages (vol.

The microstructure has an average grain size of less than <NUM>.

Preferably, the bainite and/or fine-grained ferrite have an average grain size of less than <NUM>, preferably in the range of <NUM> to <NUM>.

The bainite may comprise or consist of low carbide containing bainitic ferrite which is indistinguishable from fine-grained ferrite with a grain size of less than <NUM>. Together with martensite, bainite and/or fine-grained ferrite are the main microstructural components of the final steel sheet product. The martensite having variable carbon contents may be tempered and/or auto-tempered to variable degrees.

The final steel sheet has a thickness in the range of <NUM> - <NUM>, and a thickness accuracy better than <NUM>/<NUM> of EN <NUM>:<NUM> normal thickness tolerances at all positions along the full length of the steel sheet, excluding threading and tail-out sections in batch type cold rolling mills.

Tensile strength is determined mainly by the chemical composition of steel. The steel composition (I) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least <NUM> MPa. The steel composition (II) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least <NUM> MPa. The steel composition (III) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least <NUM> MPa.

It is preferable that the steel sheet further has at least one of the following mechanical properties:.

The following examples further describe and demonstrate embodiments within the scope of the present invention. The examples are given solely for the purpose of illustration and are not to be construed as limitations of the present invention, as many variations thereof are possible without departing from the scope of the invention.

The chemical compositions used for producing the tested steel sheets are presented in Table <NUM>.

The production parameters of the inventive examples (Ex. ) and the comparative example (Comp. ) are summarized in Table <NUM>.

The mechanical properties of the tested steel sheets are demonstrated in Table <NUM>.

A steel slab having the composition grade A was prepared by conventional steel metallurgy and continuous casting. The slab was then hot rolled with a finish rolling temperature of <NUM> to produce a sheet of <NUM> thickness, then water cooled to a coiling temperature of <NUM>, and the coil was thereafter allowed to cool freely in coil storage. The hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of <NUM> % to a final thickness of <NUM>. Finally, the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of.

The steps of temper rolling and levelling are not crucial, but may improve yield strength of the final product.

Microstructures of the inventive steel Ex. <NUM> were investigated by scanning electron microscopy after preparation of cross-section samples by grinding, polishing, and etching with nital reagent. The constituent phases were visually identified and their relative fractions were determined based on surface proportions.

<FIG> shows that the microstructure in the hot rolled condition comprises mainly bainite (bainitic ferrite - dark areas), and martensite with variable carbon contents or degrees of (auto)tempering (light areas).

<FIG> shows that the microstructure in the cold-rolled and hot-dip galvanized condition comprises mainly bainite (bainitic ferrite - dark areas) and martensite (light areas).

The flatness of cold rolled steel strips was measured by BFI flatness measurement roll with <NUM> piezoelectric encoders distributed across the barrel and expressed in I-units (IU) as 2D color map. The determination of the flatness distribution is realized by the measurement of the local deflection forces of the strip with the help of a measurement roll. This process is based on the fact that a strip which is under longitudinal tension, when deflected, exerts a radial force on the deflection roll.

From the flatness maps in <FIG> it is obvious that the flatness of the cold rolled steel strip <NUM> → <NUM> x <NUM> (a) according to the invention Ex. <NUM> has been improved compared to the comparative steel strip <NUM> → <NUM> x <NUM> (b). Gray scales indicate the location and severity of buckles.

The center line thickness of cold rolled steel strips was measured by a non-contact X-ray thickness gauge.

<FIG> shows that the cold rolled steel strip <NUM> → <NUM> x <NUM> (a) according to the invention Ex. <NUM> has less thickness deviation along the body length than the comparative steel strip <NUM> → <NUM> x <NUM> (b). High variations in the head and tail are due to threading and tail-out sections of the batch type cold rolling mill. Extra variation in the body of comparative strip (b) is due to the cold gauge hashing phenomenon, which is absent in the cold rolled steel strip (a) according to the invention Ex. The dashed lines indicate <NUM>/<NUM> of EN <NUM>:<NUM> normal thickness tolerances.

The mechanical properties (Rm, Rp<NUM>, and A<NUM> values) were determined by tensile testing of longitudinal test pieces according to EN ISO <NUM>-<NUM>:<NUM>.

The minimum bending radius was determined by performing a <NUM>° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.

The hole expansion test was carried out according to ISO <NUM>:<NUM>.

The tested steel sheet has an ultimate tensile strength of <NUM> MPa (Table <NUM>).

A steel slab having the composition grade B was prepared by conventional steel metallurgy and continuous casting. The slab was then hot rolled with a finish rolling temperature of <NUM> to produce a sheet of <NUM> thickness, then water cooled to a coiling temperature of <NUM>, and the coil was thereafter allowed to cool freely in coil storage. The hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of <NUM> % to a final thickness of <NUM>. Finally, the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of.

The steps of temper rolling and/or levelling are not crucial, but may improve yield strength of the final product.

The hot-dip galvanized steel sheet of Example <NUM> was further batch annealed in a laboratory furnace at <NUM> with a holding time of <NUM> hours and tested after cooling to room temperature.

Claim 1:
A high strength steel sheet having a composition consisting of, in terms weight percentages:

<TAB>

remainder Fe and inevitable impurities, wherein the steel sheet has
an ultimate tensile strength Rm of at least <NUM> MPa, and a thickness accuracy better than <NUM>/<NUM> of EN <NUM>:<NUM> normal thickness tolerances,
wherein the steel sheet have a microstructure comprising a matrix consisting of, in terms of volume percentages:

<TAB>

remainder bainite and/or fine-grained ferrite, and
wherein the bainite and/or fine-grained ferrite have a microstructure with the average grain size of less than <NUM>.