Field of the invention

This invention relates to a method for producing a high strength sheet silicon containing steel strip with excellent surface quality of the hot-rolled product as well as of the cold-rolled and hot-dip galvanised product.

Background of the invention

Hot rolled strips of steels containing elevated levels of silicon in combination or not with other elements often has poor surface quality after hot-rolling . The poor surface quality is related to the presence of Si which forms brittle oxide scales that, after fracturing in the roll bite, are rolled into the steel surface during the finishing rolling process. Depending on the performance of the descaling operation in the hot-strip mill a typical threshold level of silicon for these surface issues to occur is 0.10 wt.%.

The steels with mentioned compositions are cast in a slab, then reheated to temperatures in a range of 1200 to 1300°C and then descaled, rough-rolled to form a transfer bar, descaled again and finish-rolled to the end thickness. The strip is then cooled in a run-out-table and coiled. The obtained product can be already of high strength and finished with a hot-dip galvanised coating; or further pickled, cold rolled, annealed and optionally coated, e.g . by hot dip galvanising .

Conventionally the surface issues are prevented by limiting the silicon content to a low level below 0.15 wt.%. CN 102764760 discloses a method for manufacturing a high surface quality hot-rolled steel plate with a silicon content below 0. 15 wt. %. US2015/0243418-A1 discloses a hot rolled silicon steel producing method wherein the heating process and the rough rolling process are changed so that the occurrence rate of edge defects during the production of the hot rolled silicon steel can be reduced, and the hot rolled silicon steel with good surface quality can be produced . In US5235840 a system for processing steel strips in a hot strip mill including an apparatus and method for minimizing oxide growth on steel strips and reducing wear on work rolls in the finishing mill is disclosed. To reduce oxide build-up in the rolls, steel strips are sprayed with coolant at selected locations throughout the finishing mill and the surface temperature of the strips is controlled to be within a temperature range to minimise oxide growth. Accordingly, wear on the work rolls due to abrasive contact with the steel strips is reduced. JPH 11156407 provides a method for manufacturing a hot rolled steel sheet of high Si excellent in surface quality with less red scale flaw by tailoring the descaling method .

The problem with these conventional approaches is that either the silicon content must be kept low, or that due to the variable thickness of the oxide that has formed, the steel surface roughness is uneven after oxide removal and remains uneven after cold rolling and annealing .

Objectives of the invention

It is an objective of the present invention to provide a method for producing a hot- rolled high strength steel containing Si between 0.10 wt. % and 3.00 wt.% with an excellent surface uniform in aspect and roughness.

It is also an objective of the present invention to provide a method for producing a cold-rolled high strength steel containing Si between 0. 10 wt.% and 3.00 wt.% with an excellent surface uniform in aspect and roughness.

It is also an objective of the present invention to provide a method for producing a coated high strength steel containing Si 0. 10 wt.% and 3.00 wt. % with an excellent surface uniform in aspect and roughness.

Description of the invention

One or more objectives are reached with a method to produce hot-rolled steel strips from continuously cast thick or thin slabs comprising Si between 0.10 and 3.00 wt.% and at least 0.8 wt.% Mn comprising heating or reheating the slab to a reheating temperature of at least 1100 °C in a reheating or homogenising furnace, optionally hot-rolling the slab to a transfer bar in a roughing mill consisting of one or more rough rolling stands, transferring the transfer bar to a tandem finishing mill consisting of a plurality of finishing mill rolling stands, descaling the transfer bar in a descaler, and rolling the descaled transfer bar in said plurality of finishing mill rolling stands to a hot-rolled strip having a final thickness wherein the temperature of the strip at exit of the last stand in the finishing mill rolling T is determined by T (°C) > 41.823 ln(wt.% Si) + 980.4 and the time between the descaler and the first finishing mill rolling stand td-Fl is determined by td-Fl (s) < -0.603 ln(wt.% Si) + 4.8731.

In the context of this invention the descaler in which the transfer bar is descaled before finish rolling is the last descaling unit before the finishing mill. In conventional hot strip mills, but also in some thin slab casting and direct rolling mills, additional descaling units are fitted, e.g . to descale the reheated slabs prior to rough rolling of the slab. However these additional units do not descale the transfer bar, but (e.g .) the reheated thick slab. Only in the case of a thin slab casting and direct rolling mill comprising only a multi-stand finishing mill and no roughing mill, such as the CSP-plant, the descaling unit for descaling the reheated slab is also the last descaler unit before the finishing mill.

Steels with Si content above 0. 10% produce oxide scales that are rich in oxygen, hence higher proportions of the higher iron oxides magnetite (Fe304) and hematite (Fe203) are present in the oxide scale compared to a conventional low carbon-low alloyed (LC-LA) steel. In a conventional LC-LA steel, the presence of these higher oxides is controlled by keeping the surface temperature down during finishing rolling. This low surface temperature is achieved by using cooling actuators during rolling, i.e. inter-stand cooling and skin cooling (chillers). These cooling actuators are automatically controlled by aiming for a finishing exit temperature and are used either to control the temperature of the strip during hot-rolling or to protect the work rolls from the sudden increase of temperature when they contact the hot strip.

The use of cooling actuators is effective for controlling the surface of LC-LA steels and it is common practice in all hot strip mills. However, the formation of higher proportions of higher oxides in the steels containing more than 0.10 wt.% Si cannot be avoided by lowering the temperatures during finish rolling . The inventors found by experiments on oxide growth that the brittle oxides will always form in these steels containing more than 0. 10 wt. % Si and therefore a lower temperature of the surface will only increase the brittleness of the total oxide layer.

The limit of plasticity of the oxide layer depends on the actual thickness of the brittle oxides available, which are proportional and depend on the steel composition. Figure 1 shows a schematic drawing of the approximate transition of ductile to brittle when reducing the temperature. The oxide scale is ductile at high temperature. If this is combined with a thin scale, then the hematite layer does not act as crack initiator. As soon as the temperature is approx, below 920°C, the magnetite (FesCU) is no longer able to accommodate the elongation and it can break, therefore below 920°C in this temperature region the magnetite layer thickness dominates the fracture mode. When the whole oxide scale is below the ductility limit for all oxides (i.e. approx, below 600°C) then the oxide will break already under a very small load. Hematite (FezCb) is brittle for the full practical range of hot-rolling temperatures.

The method according to the invention is based on promoting the presence of the ductile magnetite and wustite (FeO) and avoiding hematite. The inventors have developed a method to determine the plasticity limit based on the oxide thickness entering each stand in the finishing mill and the surface temperature. This is achieved by starting the finish rolling process immediately after descaling the transfer bar at a high speed (v) and a high surface temperature of the transfer bar (T), preferably without the use of cooling actuators in the finishing mill. These measures ensure that the temperature stays high enough during finishing rolling to ensure that the oxide is sufficiently ductile.

The method according to the invention ensures that the surface quality is not only excellent, but also uniform. This means that if the hot rolled strip is subsequently pickled and cold-rolled, that the surface remains excellent and uniform, and this positively affects the surface quality of the coated product after providing the finished product with a metallic coating . The limit line in Figure 3 was calculated based on the surface quality results. These results are obtained from the surface inspection system located after the last stand on the finishing mill. The triangles represent sampling locations on a strip that have poor surface quality after hot rolling, the circles represent a good surface quality after hot rolling. The X-axis corresponds to the calculated surface temperature of the strip at the entry of the roll bite and the Y-axes is the equivalent calculated oxide scale thickness at the corresponding roll bite. The surface temperature calculation was obtained from our own off-line simulator of the finishing mill, which takes into account the heat losses due to convection and radiation using the industrial coil parameters. The equivalent oxide scale thickness is calculated by a paralinear growth formulation (Bolt, P. H. (2004). Understanding the properties of oxide scales on hot rolled steel strip. Steel Research International, 75(6), 399-404) : dM/dt = kp/(M +a), with a = kp/ki_

M : Oxide layer thickness (expressed as mass of oxygen per unit area) [kg nv ² ] t: Time [s]

kp Parabolic oxidation rate coefficient [kg ² m ^_4 s ^_1 ]

ki_ Linear rate constant in the initial oxidation stage [kg nv ² s ^_1 ]

The constants kp and ki_ are temperature dependant following an Arrhenius form: k= ko exp(-Q/RT) ko = temperature-independent reaction rate constant [same unit]

Q = activation energy [J/mole]

R = Gas constant [8.314 J/mole K]

T = temperature [K]

The values of ko and Q for each mechanism (i.e. linear and parabolic) are based on results from thermobalance experiments for LC-LA steels in air at humidity > 20%, in a range between 700°C and 1000°C. The set of constants used for the calculation of the equivalent oxide scale thickness were: ki_o= 0.367 kg nr ² s ^_1

QL = 45000 J/mole

kpo= 323 kg ² m ^_4 s ^_1

Qp= 159700 J/mole In the same off-line simulator for the finishing mill, the surface temperature and times of rolling are used for the calculation of the oxide entering each stand as shown in the y-axes of Figure 3. It assumes that the deformation of the scale is proportional to the deformation applied by each set of rolls, and afterwards the oxide growth continues. In Figure 2 all stands are represented, therefore as many points as there are stands in the finishing mill points are represented per each position of each studied coil, which in the example of figure 2 is a seven-stand finishing mill.

The optionality of the hot-rolling of the slab to a transfer bar in a roughing mill consisting of one or more rough rolling stands relates to the differences in set-up of the available hot strip production facilities. The conventional hot strip mill generally consists of one or more reheating furnaces to reheat thick slabs with a thickness of between about 150 and 350 mm to a high temperature. These and following dimensions in relation to the hot strip production facilities are indications and not intended to be limiting. The reheated slabs are subsequently descaled for the first time to remove the furnace scale and processed in a roughing mill comprising one or more rolling stands and reduced in thickness to about 35 to 40 mm. The rough-rolled slab is now generally referred to as a transfer bar. This transfer bar is rolled to its final hot-rolled thickness in a multi-stand finishing mill. Prior to this finish rolling the slab is again descaled to remove the scale that formed on the transfer bar in the period between roughing and the delay time before finish rolling. After finish rolling the strip is cooled to the coiling temperature on the run-out table and coiled. An example of such a facility is the HSM#2 of Tata Steel in IJmuiden.

As a second alternative there is the option to cast a thin slab in a thin slab casting and direct rolling plant wherein the rolling mill consists of a roughing mill, comprising one or more rolling stands and a multi-stand finishing mill. The thin cast slab has a thickness of below 150 mm, generally in the range of 35 to 80 mm. This strip is fed directly from the caster into a homogenising furnace and (re-)heated or homogenised to a high temperature. These thin cast slabs are subsequently descaled for the first time to remove the furnace scale and processed in a roughing mill comprising one or more rolling stands and reduced in thickness. The rough-rolled slab could now be referred to as a transfer bar but is much thinner than the one in the conventional hot strip mill. This transfer bar is rolled to its final hot-rolled thickness in a multi-stand finishing mill. Prior to this finish rolling the slab is again descaled to remove the scale that formed on the transfer bar in the period between roughing and the delay time before finish rolling. After finish rolling the strip is cooled to the coiling temperature on the run-out table and coiled. An example of such a facility is the thin slab caster and direct rolling facility of Tata Steel in IJmuiden.

As a third alternative there is the option to cast a thin slab in a thin slab casting and direct rolling plant wherein the rolling mill only consists of a multi-stand finishing mill. In this alternative the thin cast slabs are descaled to remove the furnace scale and immediately processed to its final hot-rolled thickness in the multi-stand finishing mill. In the context of this invention the descaled thin cast slab is the transfer bar. In this case the descaler after the homogenisation furnace may be the only descaling between (re-)heating and finish-rolling. After finish rolling the strip is cooled to the coiling temperature on the run-out table and coiled. An example of such a facility is the CSP thin slab caster and direct rolling facility of thyssenkrupp Steel in Duisburg.

In an embodiment of the invention the steel strip has a composition comprising (in wt.%)

C: 0.010 - 0.50 P: < 0.100 Nb: < 0.30 Mn: 0.80 - 6.00 S: < 0.050 V: < 0.50

N : 0.001 - 0.030 B: < 0.0100 Ca : < 0.050 Si : 0.10 - 3.00 0: < 0.008 Ni < 2.0

Cr: < 4.00 Ti : < 0.30 Cu < 2.0

Al : < 3.00 Mo: < 1.00 W < 0.50

the remainder being iron and unavoidable impurities.

These steels allow very good mechanical properties after a hot-forming process, whereas during the hot forming above Acl or Ac3 they are very formable. It is noted that any one or more of the optional elements may also be absent, i.e. either the amount of the element is 0 wt.% or the element is present as an unavoidable impurity.

In a preferred embodiment the steel strip has a composition comprising (in wt.%)

C: 0.040 - 0.25 P: < 0.020 Nb: < 0.30 Mn: 0.80 - 3.00 S: < 0.005 V: < 0.50

N : < 0.0120 B: < 0.0050 Ca : < 0.050 Si : < 0.10-2.00 O: < 0.008 Ni < 0.050 Cr: < 1.00 Ti : < 0.30 Cu < 0.050 Al : < 1.50 Mo: < 0.50 W < 0.020 the remainder being iron and unavoidable impurities. Typical high strength steel grades are given in table A.

In a preferred embodiment the steel strip has a composition comprising (in wt.%)

C: 0.10 - 0.25 P: < 0.020 Nb: < 0.30 Mn: 1.40 - 2.40 S: < 0.005 V: < 0.50

N : < 0.0100 B: < 0.0050 Ca : < 0.050 Si : < 0.10-0.40 0: < 0.008 Ni < 0.050 Cr: < 1.00 Ti : < 0.30 Cu < 0.050 Al : < 1.50 Mo: < 0.50 W < 0.020 the remainder being iron and unavoidable impurities Since the silicon content is the main contributing factor of the chemical composition in the method according to the invention, the ranges of the other elements are interchangeable and can be considered independently in the light of the problem that is solved by this invention. Clearly, the other elements affect other properties, like the mechanical properties, but from the surface quality point of view they are independent features.

In an embodiment the surface appearance of the hot-rolled steel strip is uniform along width and length of the strip. The principle of the method is based on a centreline measurement as described, but as it is the intention that the processing conditions are as homogeneous as possible over the width and length of the strip, the preferred embodiment should guarantee the uniformity of the surface appearance over the width and length of the strip.

In an embodiment the transfer bar is descaled immediately prior to entering the first finishing mill rolling stand. This ensures that the surface of the transfer bar is free from oxide at the start of the finish rolling. To further prevent the formation of sticking oxide, it is preferable that the cast thick or thin slab is descaled immediately prior to entering the first roughing mill rolling stand.

As used herein, a hot rolled strip generally refers to a steel product, usually supplied as a coiled product. The strip thickness can be between 1 and 25 mm, depending on the supplier and the type of application for the steel. Strips can be cut into smaller segments, such as sheets. Preferably the thickness of the strip in the method according to the invention is less than about 8.0 mm, or less than 7.0 mm, or less than 6.0 mm or less than 5.0 mm. Preferably, the steel strip or sheet produced according to the invented method has a thickness of at most 4.0 mm.

The steel strip can be used in its hot rolled state, or as pickled and oiled strip, but in many cases the steel strip will be protected against corrosion by the application of a coating layer. In an embodiment the hot-rolled strip is provided with a metallic coating by hot-dip coating or by electroplating or with a coating by physical vapour deposition. In most cases the hot rolled strip is pickled before application of the coating hot-rolled scale from the surface of the strip. The coating that is applied by physical vapour deposition is usually a metallic coating, but it can also be a non-metallic coating.

In an embodiment the hot-rolled strip produced according to the invention is subsequently cold-rolled to a cold rolled strip. In most cases the hot rolled strip is pickled before application cold-rolling to remove the hot-rolled scale from the surface of the strip. In many cases the hot-rolled strip is cold-rolled to a cold rolled strip and subsequently annealed to produce an annealed strip having a recrystallised or recovered microstructure. In an embodiment the cold rolled and subsequently annealed strip is subsequently provided with a metallic coating by hot-dip coating, by electroplating or with a coating by physical vapour deposition. The coating that is applied by physical vapour deposition is usually a metallic coating, but it can also be a non-metallic coating.

In a preferable embodiment the metallic coating is selected from the group of coatings comprising or consisting of (Zn, Zn-AI, Zn-Mg-AI, Al-Si, Sn, Cr, N i) .

The method according to any one of the preceding claims wherein the hot-rolled steel strip after cooling to ambient temperatures, and prior to the optional cold-rolling, is pickled to remove the oxides present after hot-rolling and cooling of the strip from the surface to obtain a uniform and oxide free surface without residual scale.

According to a second aspect the invention is also embodied in hot-rolled strip produced according to the method according to the invention or in a cold-rolled steel strip produced according to the method according to the invention.

Examples

Table A - Typical high strength steel grades (wt.%)

In Figure 3 the results of industrial trials are presented. Steels having a silicon content of between 0.10 and 0.40 wt.% have been rolled and the resulting hot-rolled surfaces have been investigated. The inventors found that all values above the line resulted in the presence of brittle oxide scale, and the values below the line resulted in ductile oxide scale. Based on these measurements the equations (1) and (2) could be derived, and successful subsequent steel strips having a silicon content between 0. 10 and 3.00 wt. % along with a manganese content of at least 1.40 wt.% could be produced with an excellent surface quality. Any excursions to times between the finishing mill descaler and the first stand of the mill (FI) above the calculated maximum time (td-Fi ) or below the minimum surface entry temperature (T) resulted in a deterioration of the surface quality.

From the data within the ductile region in Figure 3, the limit values for a good surface quality can be found. Three main variables define the procedure to avoid oxide fracture: Si content in the steel (Si, wt. %), time between the descaler and the first stand of the finishing mill (td-Fi, s) and the average temperature along the coil at exit of the finishing mill (T, °C) . Time and temperature are measured over the centre line of the mill, and thus of the strip. The temperature over the width should preferably be as homogeneous as possible, and preferably similar to the centreline value. This way the chance that the edges and the centre of the strip have good surface quality is maximised. A strip produced as such that satisfy both of the following conditions will have good surface at the locations where these conditions are met:

T (°C) > 41.823 ln(wt.% Si) + 980.4 (1) td-Fi (s) <-0.603 ln(wt.% Si) + 4.8731 (2)

The range of Si is between 0. 10 wt.% to 3.00 wt.%. Wherever these conditions are met, a good surface quality could be expected.

When plotted in an x-y graph, as in Figure 4, the temperature of the strip at exit of the last stand in the finishing mill rolling (T) must be above the line depicted by equation ( 1) AN D the time between descaler and entry in the first stand of the finishing mill (td-Fi) must be below the line defined by equation (2) .

It should be noted that the temperature of the strip at exit of the finishing mill and thus the average temperature along the coil at exit of the finishing mill is a direct consequence of the entry temperature, and the entry speed and any cooling during and in between stands. Variables such as the final thickness of the strip and the reduction schedule in the various rolling stands of the finishing mill are used as input for determining the time between the descaler and FI and the entry temperature to arrive at the desired exit temperature.

It should also be noted that the decision whether a surface is a good or a bad surface is somewhat arbitrary. Figure 4 gives an example of a good (top image) and a bad quality (bottom image). The steel was a 0. 14%C; 2.1%Mn; 0.2%Si; 0.25%Cr steel. The steel in the top image was produced under the conditions prescribed by equations (1) and (2), and the steel in the bottom image was not. The difference in surface quality is striking . The surface quality of the steel in the top image is perfectly suited to be processed further, whereas the surface quality of the steel in the bottom image is such that repair of the surface during further processing is virtually impossible. The steel in the bottom image is very likely to be rejected or downgraded.

Brief description of the drawings

Figure 1. Schematic of the transition between ductile and brittle behaviour per iron oxide type (Adapted from Hidaka, Y. , Anraku, T. 8i Otsuka, N . Tensile deformation of iron oxides at 600-1250 C. Oxid . Met. 58, 469-485 (2002)) .

Figure 2 shows a schematic drawing of the principle behind the invention. Two strips are shown in the drawing with the T-d data for each of the 7 stands of the finishing mill, as well as the line separating good conditions from bad conditions (T= surface temperature at entry of roll bite, d= thickness of oxide before roll bite). The strip which has all stands below the line is expected to have a good surface, whereas the ones above the line will not. The numbers in the drawing represent the stands Figure 3 shows industrial data from steels having a silicon content of between 0.10 and 0.40 wt.% as a function of a calculated equivalent oxide scale thickness (dcaic) entering each stand of the finishing mill and calculated surface temperature (T) . The triangles represent bad surface, the circles represent good surface.

Figure 4 shows a graphic representation of the equations (1) and (2) as a function of the silicon content of the steel.

Figure 5 shows the surface quality of a steel produced according to the invention (top image) and of the same steel not produced according to the invention (bottom image) .