Method of manufacturing a steel strip and coated steel sheet obtainable thereby

Abstract

Method of manufacturing a steel strip comprising the subsequent steps of hot rolling the strip into a hot rolled strip, cold rolling the hot rolled strip, and hot dip coating the cold rolled strip with a Zn-based coating by leading the strip through a bath comprising molten zinc, and wiping the strip after the coating using an air knife having a knife slot from which a wiping gas is ejected, and cold rolling the steel strip in a multi-stand cold rolling mill to a final cold rolled thickness of 0.40 mm to 1.00 mm, wherein the cold rolled and coated steel sheet comprises a steel substrate provided with a hot dip metal coating.

Description

[0001] The present invention relates to a method of manufacturing steel strip comprising the subsequent steps of hot rolling the strip into a hot rolled strip, cold rolling said hot rolled strip, and hot dip coating said cold rolled strip with a Zn-based coating by leading the strip through a bath comprising molten zinc, and wiping the strip after said coating using an air knife having a knife slot from which a wiping gas is ejected, and to a coated steel sheet obtainable by this method comprising a steel substrate provided with a hot dip metal coating.

[0002] Such methods and resulting products are well known throughout the steel industry. Steel strip suitable for hot dip coating is produced by hot rolling steel billets into hot rolled strip, which is subsequently pickled and cold rolled into cold rolled strip in a multi-stand cold rolling mill. The cold rolled strip is subsequently coated in a continuous hot dip coating line.

[0003] Continuous hot dip coating lines are widely used and utilized throughout the world. Hot dip coating was originally developed for galvanizing, i.e. zinc coating, but is now also used for applying other metals or metal alloys to steel sheets.

[0004] In continuous hot dip coating, cold rolled steel strip is passed at high speed in the form of a continuous strip through a bath of molten metal. In the bath of molten metal, the steel strip reacts with the molten metal and a coating is bonded to the surface of the strip. The strip passes through one or more submerged rolls and exits the bath in the vertical direction. Above the exit point, a set of air knives wipe off excess molten metal, allowing a controlled coating thickness, usually expressed as the coating weight per unit area on the surface of the strip. After cooling, the strip is fed to the exit end of the hot dip coating line, which usually comprises a temper mill, also known as a skin pass mill. Air or nitrogen is usually used as the wiping gas. For the production of high quality coated products, nitrogen is usually used.

[0005] Initially, hot dip coated steel sheets were used for applications that did not require high quality finish or high formability, but recently they are increasingly used for more demanding applications, such as for automobile hoods, fenders and doors.

[0006] The surface quality of coated steel sheets is affected by several types of defects. The main types of defects are dross-type defects, furnace defects and coating defects, the latter being related to the solidification and oxidation of the liquid metal during the hot dip coating process.

[0007] In order to improve the surface quality, it is important to find not only ways to reduce the type of dross and furnace defects, but also ways to reduce the type of coating defects. If such an improvement is found, this will immediately lead to a further improvement of the product, because other types of defects become more prominent and can be eliminated in a targeted manner. In addition, it can also free up problematic sheets, because other defects are no longer missed, so that in general a product with a better surface quality is sold to the market.

[0008] Several methods have been proposed to improve the surface quality of the target product, in particular as mentioned above with respect to reducing the type of coating defects. One proposal is to reduce the oxygen level in the atmosphere around the steel strip after hot dipping. Another proposal is to change the amount of certain elements in the hot dip bath, such as Al and / or Mg, or to add very specific elements to it, such as Be or Ga.

[0009] Both solutions to improve the surface quality of the coated sheet have their downsides. The first solution requires the use of a confinement box that shields the coated strip. Such a box limits the visibility of the strip and limits the space for positioning wiping devices and any other devices, including skimming equipment, all of which are required for optimal control of the hot dip coating process. The second solution is often not satisfactory, because the application properties in use, such as the susceptibility to or resistance against filamentous corrosion, are impaired.

[0010] It is an object of the present invention to provide an improved method for manufacturing a hot dip coated steel sheet with a high surface quality, in which the number of defects is low and the waviness in the final product, such as a visible part of a car body, is low.

[0011] It is a further object of the present invention to provide an improved hot dip coated steel sheet, which is in particular suitable for use in a visible part of a car body.

[0012] These objects are achieved in accordance with the independent claims. Preferred embodiments are defined in the respective dependent claims. It should be noted that the features listed in the claims can be combined in any technically meaningful manner to describe other embodiments of the invention. The following description explains the features of the invention and contains further embodiments of the invention. Furthermore, it should be noted that features described in connection with the proposed method of manufacturing a steel strip can be used to further explain the features of the proposed coated steel sheet, and vice versa.

[0013] According to the invention, in the method:

[0014] the steel strip is cold rolled in a multi-stand cold rolling mill to a final cold rolling thickness of 0.40 mm to 1.00 mm, wherein the cold rolling in the last stand is such that:

[0015] the steel strip is cold rolled in a multi-stand cold rolling mill to a final cold rolling thickness of 0.40 mm to 1.00 mm, wherein the cold rolling in the last stand is such that:

[0016] where SRF is the specific rolling force expressed in kN / m, and calculated as the rolling force in kN divided by the strip width in m, and AWR is the average working roll radius in m of the top and bottom working rolls at the intermediate roll position.

[0017] It will be clear that the top and bottom working rolls refer to the two rolls in the rolling mill stand that are in contact with the strip being rolled.

[0018] Surprisingly it was found that in the manufacture of hot dip coated steel strip of this type, not only the conditions of the hot dip coating process step can play a role in achieving the best surface quality of the product, but also the values of said parameters in the cold rolling process step play a crucial role. This parameter according to the present invention in fact sets a whole new standard for the outstanding surface quality of the manufactured hot dip coated steel product.

[0019] As a result, the operation according to the present invention not only reduces the defects considered as "coating" defects, such as the above mentioned local micro-ridges, but also results in a reduction of the presence of dross defects and many other defects when comparing different cold rolling regimes under the same hot dip coating environment. It was found that when the present invention is implemented, the defects observed by the camera inspection system are significantly reduced, which can result in an increased production volume and higher yield in the production of high quality hot dip coated steel sheets.

[0020] In other embodiments of the method according to the present invention, the cold rolling in the last stand is performed such that in the preferred order:

[0021]

[0022] or even

[0023] The higher the value of the specific rolling force at the selected working roll intermediate position divided by the average working roll radius of the top and bottom working rolls, the more pronounced the beneficial effect on the surface quality of the product after hot dip coating.

[0024] It is advantageous if the cold rolling in the last stand is performed using working rolls having a roughness Ra of 7 pm or less, preferably 6 pm or less, more preferably 5 pm or less, but in all cases 1.0 pm or more. As a result, within these roughness ranges, even better good results are achieved if the preferred ranges are used.

[0025] In case strip tracking is important, in particular in order to keep the strip well centered in the hot dip coating line, it is preferred that the roughness is 3.0 pm or more.

[0026] The surface roughness of the work rolls in the last stand can be created by grinding and subsequent electrical discharge roll texturing ("EDT"). EDT allows precise control of the roughness parameters of the work rolls, such as Ra and Rpc.

[0027] In an embodiment, the method is characterized in that a GKD < 10 mm is observed, wherein GKD is the average distance between the knife slot of the jet wiping gas and the surface of the coated strip being wiped. While it is known that GKD plays a role in hot dip coating, this is related to the production of a certain coating weight with a certain coating line speed using a wiping knife of a certain size with a certain pressure, it has proven possible to produce a good surface quality product with a GKD value of 10 mm or less.

[0028] In a preferred embodiment, GKD < 9 mm, GKD < 8 mm and GKD < 7 mm. If possible, lower values are preferred as they result in a higher quality product; in particular, this enables a lower waviness, and fewer coating defects occur.

[0029] The strip can be stabilized by a magnetic device mounted near the desired strip path between the bath and the first guide roll to contact the strip downstream of the bath. The installation of such a device, for example in the form of an electromagnetic strip stabilizer, not only provides better control of the hot dip coating layer thickness, but also enables working with a preferred lower GKD value without the risk of the strip contacting the wiping device, and enables a more uniform coating weight distribution over the width of the strip.

[0030] In an embodiment, wherein the molten metal bath has a composition comprising Zn, Al and Mg, wherein the strip after coating and wiping is cooled in a cooling section between the position where the strip is wiped and a downstream position where the strip is first contacted by a guide roll, wherein an active cooling gas flow Q in m3 / hr is required in order to maintain the strip temperature at the guide roll within a 20 degree strip temperature band of a target strip temperature in the range between 200°C and 300°C, wherein the cooling gas flow in the second half of the cooling section is a percentage p of Q and the cooling gas flow in the first half of the cooling section is a percentage (100-p) of Q, wherein p is set to 70% or more.

[0031] It was found that at higher p, a lower waviness of the coated product can be achieved. Early cooling after wiping should be prevented as much as possible, and cooling should occur as late as possible, while still reaching the maximum required temperature of the strip before the strip contacts said guide roll, often referred to as top roll. It is therefore preferred that p = 80% or more or even 90% or more.

[0032] In one embodiment, the bath consists of 0.6 - 4.0 wt.% aluminium and 0.3 - 4.0 wt.% magnesium, optionally up to 0.2 wt.% of each element belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the remainder being unavoidable impurities and zinc.

[0033] It was found that the present invention is particularly effective for this coating. The amount of elements belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi can be up to 0.1 wt.% for each element, or can be up to 0.05 wt.% for each element.

[0034] In further embodiments, the aluminium content is 0.6 - 3.0 wt.%, preferably 1.0 - 3.0 wt.%, more preferably 1.5 - 2.0 wt.% and / or the magnesium content is 0.3 - 2.0 wt.%, preferably 1.0 - 2.0 wt.%, more preferably 1.0 - 1.5 wt.%. A relatively high Mg content leads to a better corrosion protection. A lower Al and Mg content leads to a better weldability and a reduction of a surface feature known as "marble effect" which can occur due to the solidification and oxidation behaviour of the Zn-Al-Mg coating.

[0035] In alternative embodiments, the bath consists of 0.25 - 0.90 wt.% aluminium, preferably 0.25 - 0.50 wt.% aluminium, and up to 0.2 wt.% of each element belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the remainder being unavoidable impurities and zinc. The amount of elements belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi can be up to 0.1 wt.% for each element, or can be up to 0.05 wt.% for each element.

[0036] As this coating already leads to an improvement of the surface quality of the coated steel sheet itself, it is beneficial to produce it according to the method of the present invention and to obtain a product with excellent properties.

[0037] In embodiments where the hot dip coated strip is skin passed at an elongation of 0.5% or more, a skin pass work roll with an average diameter of 400 mm or more, more preferably 500 mm or more, even more preferably 600 mm or more is used. The average diameter is defined here as the average diameter of the top and bottom work rolls at the middle roll position.

[0038] These combinations of elongation and skin pass work roll diameter are beneficial for the surface quality and roughness transfer.

[0039] In a preferred embodiment, in the leveller, a levelling work roll is used having a roughness Ra of 4.5 pm or less, preferably 3.0 pm or less, more preferably 2.5 pm or less. This achieves a lower waviness and a higher peak count in the coated steel sheet that is calendered, which is beneficial for the appearance of painted parts made from the coated steel sheet.

[0040] The invention is also embodied in a coated steel sheet obtainable by the method, the sheet comprising a steel substrate provided with a hot-dip metallic coating, the steel substrate having a thickness of 0.40 mm to 1.00 mm, wherein:

[0041] i) the steel substrate has the following composition, all in wt%:

[0042] C max 0.04;

[0043] Mn 0.01-1.20;

[0044] Si 0.001-0.50;

[0045] Al 0.005-0.1 ;

[0046] P max 0.15;

[0047] S max 0.045;

[0048] N max 0.01 ;

[0049] Mo max 0.12;

[0050] Ti max 0.12;

[0051] Nb max 0.12;

[0052] Cu: max 0.10;

[0053] Cr: max 0.06;

[0054] Ni: max 0.08;

[0055] B: max 0.0025;

[0056] V: max 0.01 ;

[0057] Ca: max 0.01 ;

[0058] Co: max 0.01 ;

[0059] Sn: max 0.01 ;

[0060] the balance being iron and unavoidable impurities;

[0061] i i) the coated steel sheet has a surface property Sc, Sc being defined as:

[0062] Sc = Sk / (0.7*t + 0.3) with Sk in pm and defined according to NEN-EN-ISO 25178-2:2012, t is the thickness of the steel substrate in mm, and

[0063] i i i) the coated steel sheet has a waviness Wsa, which is the Wsa(l-5) value in pm, measured in the rolling direction, according to SEP 1941, after 5% of Marciniak biaxial deformation,

[0064] iv) wherein the combination Sc and Wsa is located within the area defined by the contour ABCDEA in the XY plot of Sc and Wsa, wherein:

[0065] A is defined as the intersection of Sc = 3.00 and Wsa = (0.2686) - (0.0543*Sc) + (0.0105*Sc^2);

[0066] AB is defined by Wsa = (0.2686) - (0.0543*Sc) + (0.0105*Sc^2) from Sc = 3.00 at A to Wsa = 0.50 at B;

[0067] BC is defined by Wsa = 0.50 from B to C, C having Sc = 14.50;

[0068] CD is defined by Sc = 14.50 for Wsa = 0.50 at C to Wsa = 0.10 at D;

[0069] DE is defined by Wsa = 0.10 for Sc = 14.50 at D to Sc = 3.00 at E; and

[0070] EA closes the contour and is defined by Sc = 3.00 from E to A.

[0071] The result is that if a steel sheet comprising a steel substrate provided with a hot-dip metallic coating according to the present application has the above-mentioned characteristics, the hot-dip coated steel product will have a very good surface quality in the final application, for example as a visible side of a car body part. Sk as used in this patent document is a surface characteristic parameter, also called "core roughness", which is measured according to NEN-EN-ISO 25178-2:2012.

[0072] In the experiments, Sk was measured with a confocal microscope, the measurement data were filtered and Sk was calculated using the WinSam 2.6 software. Details about the Sk measurement are: equipment from the supplier Nanofocus; equipment type μSurf Mobile (also called Marsurf Mobile); Objective MPIanApo N 800XS (20x / 0.60); lateral spacing [pm] 1.56; number of stitching fields 3*3; measurement area 2.1*2.1 mm; software WinSam 2.6; calculation / evaluation area 2.0*2.0 [mm]; filter polynomial 2nd order; Penetration (kfl max) + 10 [pm]; Penetration (kfl min) - 10 [pm]; number of steps 2000; step width 10 [nm].

[0073] Sk can be measured with similar equipment and similar software as commercially available.

[0074] In a preferred embodiment, the combination of Sc and Wsa lies within the following area:

[0075] the area defined by the profile A'FCDEA' in the XY coordinate plot of Sc and Wsa, respectively, wherein:

[0076] A' is defined as the intersection of Sc = 3.00 and Wsa = (0.2276) - (0.0266*Sc) + (0.0054*Sc^2);

[0077] A'F is defined by Wsa = (0.2276) - (0.0266*Sc) + (0.0054*Sc^2) for Sc = 3.00 at A to Wsa = 0.50 at F;

[0078] FC is defined by Wsa = 0.50 from F to C, C having Sc = 14.50;

[0079] CD is defined by Sc = 14.50 for Wsa = 0.50 at C to Wsa = 0.10 at D;

[0080] DE is defined by Wsa = 0.10 for Sc = 14.50 at D to Sc = 3.00 at E; and

[0081] EA' closes the profile and is defined by Sc = 3.00 from E to A'.

[0082] This results in a hot-dip coated steel product that will have an even better surface quality, especially in the final application.

[0083] In a more preferred embodiment, the combination of Sc and Wsa lies within the following area:

[0084] the area defined by the profile A"GCDEA" in the XY coordinate plot of Sc and Wsa, respectively, wherein:

[0085] A" is defined as the intersection of Sc = 3.00 and Wsa = (0.208) - (0.0118*Sc) + (0.0027*Sc^2);

[0086] A"G is defined by Wsa = (0.208) - (0.0118*Sc) + (0.0027*Sc^2) for Sc = 3.00 at A" to Wsa = 0.50 at G;

[0087] GC is defined by Wsa = 0.50 from G to C, C having Sc = 14.50;

[0088] CD is defined by Sc = 14.50 for Wsa = 0.50 at C to Wsa = 0.10 at D;

[0089] DE is defined by Wsa = 0.10 for Sc = 14.50 at D to Sc = 3.00 at E; and

[0090] EA" closes the profile and is defined by Sc = 3.00 from E to A".

[0091] This results in hot dip coated steel products that will have the best surface quality in the end application.

[0092] In a preferred embodiment, the total coating weight on both sides of the sheet is 60-175 g / m 2 , the coating weight being measured according to EN 10346:2015. The lower the coating weight, the lower the waviness that can be achieved with hot dip coating.

[0093] In one embodiment, the surface roughness Ra of the sheet is 0.9 pm to 1.8 pm, preferably 0.9 pm to 1.6 pm, and more preferably 0.9 pm to 1.4 pm, the surface roughness being measured according to ISO-NEN 468-1982 with a 2.5 mm limit. These roughness values enable a good waviness to be obtained after deformation.

[0094] The invention is also embodied in the above-described method, characterized in that the method is carried out to produce hot-dip coated steel sheets with a guaranteed maximum waviness Wsa in their final deformed state for their final use. The guaranteed maximum waviness Wsa is a Wsa (1-5) value of 0.35 μm, 0.34 μm, 0.33 μm, 0.32 μm, 0.31 μm, 0.30 μm, 0.29 μm, 0.28 μm, or lower, measured in the rolling direction according to SEP 1941. It is noteworthy that measures taken, particularly in the upstream portion of the manufacturing process (e.g., in cold rolling), have led to this objective being achieved, which is crucial for the final application, such as in the visible body of an automobile.

[0095] The invention will now be described in more detail with the aid of accompanying drawings and experimental descriptions.

[0096] In the attached diagram:

[0097] Figure 1 This is an XY coordinate graph of Sc and (1-5), showing the region defined by contours ABCDEA, A'FCDEA', and A"GCDEA", and the combination of Sc and Wsa obtained from experiments falling within and outside this invention; and

[0098] Figure 2 A defect distribution map of a 4084m long, 1460mm wide, and 0.6mm thick coil is shown. For this coil, 93.4% is classified as acceptable for surface-critical applications. The remainder of the coil has an excessively high local density of surface defects. This corresponds to a surface quality grade of "++" according to Table 2. The coil was processed in a cold rolling mill with an SRF of 6130kN / m and an AWR of 474mm; and

[0099] Figure 3 It shows in Figure 2 The diagram shown is a defect distribution map of the coil produced directly afterward, with the same process settings (line speed, GKD) in the galvanizing production line. The coil is 4004m long, 1460mm wide, and 0.6mm thick. For this coil, 75.5% is classified as acceptable for surface-critical applications. This corresponds to treating the coil with SRF = 5052kN / m and AWR = 430mm according to the surface quality grade "+" in Table 2.

[0100] To conduct the experiment, the samples were manufactured as follows: casting steel billets, then hot-rolling the billets in a hot rolling mill to provide hot-rolled steel strip, processing the hot-rolled steel strip in a pickling production line, cold-rolling the pickled steel strip in a cold rolling mill, annealing and hot-dip coating the cold-rolled strip, and leveling rolling in a leveling mill, also known as leveling cold rolling in a finishing mill.

[0101] Unless otherwise indicated in the tables or text, the settings of the manufacturing process up to and including hot rolling are in accordance with usual practice.

[0102] During the experiments, steel substrates were used, manufactured from different steel castings, having the compositions given in Table 1 below.

[0103] Table 1 : Chemical composition of steel samples

[0104]

[0105]

[0106] * mwt% = mass % / 1000, ppm = mass % / 10000

[0107] The hot rolled sample strips were cold rolled in one same cold rolling operation, and hot dip coated according to the same hot dip protocol. The main data regarding the hot dip coating process are:

[0108] In case of production of galvanized material ("GI"), a Zn bath was used with a target aluminium amount of 0.30% to 0.40%;

[0109] In case of production of so-called zinc-magnesium coated material ("ZM"), the Zn bath used had a target Mg amount of 1.45% to 1.50% Mg, and a target aluminium amount of 1.70% to 1.75%; in practice, the Mg amount in the bath varied between 1.40% and 1.70%, and the Al amount varied between 1.60% and 1.80%.

[0110] The knife slot width was 1.2 mm, unless otherwise indicated. The air knife distance GKD varied between 7 mm and 10 mm;

[0111] The production of the hot dip coated steel sheet examples was carried out in batches. Within a batch, coils of similar steel composition, thickness and width were produced successively. The quality of the strips was determined by means of visual inspection supported by camera inspection of the strips to evaluate the number and severity of any defects on the strips. The grades used to describe the following examples are given in Table 2.

[0112] Table 2

[0113] Surface Quality Rating Description -- No coil sections suitable for surface critical applications - < 25% of coil available for surface critical applications 0 25-75% of coil available for surface critical applications + > 75% of coil available for surface critical applications ++ > 90% of coil available for surface critical applications +++ > 95% of coil available for surface critical applications

[0114] Examples of two camera inspection defect distribution maps (see Figure 2 and Figure 3) shows the abrupt changes in surface quality that can occur when producing coiled material with different rolling forces one after the other in the last stands of a cold rolling mill. Each dot represents a surface feature classified as a defect by the camera inspection system over the width and length of the strip surface. The profile shows the bottom side (left) and the top side (right) of the strip. In this case, the top side is the visible side in surface critical applications. Most of the defects shown are classified as scale type defects.

[0115] During the study to achieve the best conditions to achieve an excellent surface quality in terms of waviness and low number of defects, it was observed that the number of defects detected by the camera inspection system can vary quite significantly from one coil of strip to another and that some combinations of steel composition, thickness and pre-treatment tend to be worse than others. Coils of strip with a high number of defects were rejected in the inspection step. The examples are given in Table 3 below, where it can be seen that examples 2.1 and 2.3 with a rating of - were completely rejected.

[0116] Table 3

[0117]

[0118] The reason why the operators generally expect these quality deviations is the hot dip coating process. They will change the process settings in the hot dip coating line to improve the quality and bring it in line with the specifications. In this case, the production line speed variations, bath face fluctuations and changes in the furnace temperature or in the temper mill treatment are suspected to be the potential causes of the deviations.

[0119] Based on the experimental results, it was noted that the quality of the product varies although the hot dip coating conditions are constant, the variation seems to be related to the cold rolling scheme. Coils treated according to one cold rolling scheme exhibit a higher number of surface defects than coils treated according to another cold rolling scheme.

[0120] To analyse the effect of the cold rolling scheme on the surface quality of the hot dip coated product, the properties of the steel substrate surface under the coating were measured on the surface of the steel substrate.

[0121] To achieve this, the coated and inhibited layer of the hot dip coated sample of size 20 mm x 20 mm was detached from the steel substrate. This was done by placing the samples upright in a pickling solution prepared by mixing the following ingredients in batches of up to 6 samples:

[0122] • 800 ml of water;

[0123] • 155 ml of a hydrochloric acid water-based solution containing 37.5% by volume of hydrochloric acid; and

[0124] • 1 ml of Extra 283-M, a commercially available overpickling inhibitor.

[0125] The addition of The inhibitor ensures that the steel substrate is not etched or pickled by the hydrochloric acid and that the pickling does not substantially affect the surface texture or roughness of the steel substrate. Gas is produced during the pickling process which escapes from the pickling bath through the surface of the pickling solution. The pickling is continued until gas production is almost stopped, typically for 10-15 minutes.

[0126] As the surface texture of the steel substrate can already have been affected by the flat rolling of the coated steel, the inventors focused on the so-called core roughness of the substrate as it is more representative of the original cold rolled strip surface.

[0127] The core roughness Sk was measured according to the criteria and method described above.

[0128] For the examples in Table 4, it was found that there was a significant difference in surface texture between the different quality levels, even though the hot rolling process and the total cold rolling reduction, substrate thickness and width were essentially the same. Substrates exhibiting a higher surface core roughness had the best surface quality as evidenced by the lower number of defects recorded by camera inspection and visual inspection.

[0129] The details of the examples illustrating this are shown in Table 4 below.

[0130] Table 4

[0131]

[0132] In a further production run, it was noted that the quality of the coated product was again related to the cold rolling regime, despite the fact that the relevant hot dip coating conditions were constant, as can be seen in Table 5.

[0133] Table 5

[0134]

[0135] Upon further investigation of the processing conditions of the cold rolling mill, the inventors found that there was a difference in deformation in the last stand involving the use of different specific rolling forces, defined as the total rolling force applied divided by the width of the strip, combined with different working roll radii. Higher rolling forces combined with lower working roll diameters resulted in better surface quality, while lower rolling forces combined with higher working roll diameters resulted in poorer surface quality.

[0136] Further monitoring of the cold rolling program on the last stand, particularly with respect to specific rolling force and work roll radius, was performed and further tests were conducted in order to evaluate the precise effect of cold rolling in the last stand of the cold mill on the surface quality of the hot dip coated steel. The results of these tests are shown in Table 6. It is clear that reduced rolling force adversely affects the surface quality of the hot dip coated steel. The number of defects is much higher for the coil produced with lower specific rolling force on the last stand and the best quality is obtained for the coil produced with higher specific rolling force on the last stand of the cold mill.

[0137] Table 6

[0138]

[0139] As a next step, tests were performed with increased specific rolling force on the last stand, outside the normal range used by the operators of the cold mill. The data for this test are shown in Table 7.

[0140] Table 7

[0141]

[0142] The results show that increased rolling force strongly increases the surface quality by having a much lower defect count on the surface of the strip.

[0143] The results of the tests with work roll diameter and rolling force are given in Table 8.

[0144] Table 8

[0145]

[0146] The test results clearly show that the combination of high specific rolling force and smaller average work roll radius results in lower defect counts and the higher the value of specific rolling force divided by roll radius, the better the chance of having excellent surface quality.

[0147] Since the roughness transfer during cold rolling is higher for thicker and / or softer materials and lower for thinner and / or harder materials, the same last stand processing in terms of specific rolling force and average work roll radius can result in different core roughness for different gauges or strengths of material. Therefore, for thinner and / or harder materials, further increase of the rolling force is needed to obtain the same core roughness as for thicker and / or softer materials. However, increasing the specific rolling force and / or decreasing the average work roll radius of the last stand improves the chance of obtaining excellent surface quality.

[0148] The difference between roughness transfer for different materials is shown in Table 9, where thinner materials with higher specific rolling force and similar diameter rolling have lower core roughness. To take this into account, the surface characteristic Sc = Sk / (0.7*t+0.3) is introduced as a measure to better compare the effectiveness of the last stand rolling process for different material thicknesses, where t is the thickness of the steel substrate in mm.

[0149] Table 9

[0150]

[0151] In further attempts to keep low levels of surface defects detected by the camera inspection system and to improve the waviness of the coated steel sheet after deformation, the inventors also tested the effect of rolling force for lower work roll roughness, the results of which are shown in Tables 10 and 13.

[0152] Waviness after deformation was determined by measuring Wsa(l-5) in the rolling direction in pm according to SEP 1941 after the sample has been deformed 5% in the biaxial direction using a Masingen tool.

[0153] It was proven that this works in a similar way, resulting in excellent waviness after deformation and low numbers of defects detected by the camera system. This is seen in the following examples:

[0154] Table 10

[0155]

[0156] The resulting waviness not only depends on the work roll roughness in the cold rolling mill, but also on the knife distance (GKD) and the cooling conditions after wiping. The inventors’ experiments showed that the smaller the knife distance, the later the cooling of the strip after leaving the zinc pot, the lower the resulting coating waviness after deformation. This is illustrated by the examples in Tables 11 and 12.

[0157] Table 11

[0158]

[0159] Table 12

[0160]

[0161] The experiments presented in Table 12 concern the active cooling gas flow used in the second half of the cooling tower (in m3 / min) and the knife distance (in mm) between the last zinc pot and the cooling tower. 3 / hr meter, the change in percentage p of the flow from the blower). The active cooling gas flow is also referred to herein as Q, which here represents the active cooling gas flow required to maintain the strip temperature at the first roll in the cooling tower (where the strip passes after the air knives) within a 20 degree band of the target strip temperature of 230°C. In the examples, the total flow Q is the sum of the flows from coolers 1-4, in this example coolers 3 and 4 are located in the second half of the cooling tower, so the total flow of coolers 3 and 4 divided by Q times 100 equals p. The results show that when a large percentage p of Q is applied in the second half of the cooling tower, the waviness of the strip is improved because the strip is allowed to cool as slowly as possible in the first half.

[0162] These results show that the method can be used to produce excellent hot dip coated strip with a low number of defects and with a very low level of waviness after deformation.

[0163] Based on the entire study and the examples in Table 13 below, it is concluded that the combination of Sc and Wsa located within the profile ABCDE A represents a high quality hot dip coated product that allows for an efficient manufacturing process. Figure 1 The combination of Sc and Wsa located within the profile ABCDE A represents a high quality hot dip coated product that allows for an efficient manufacturing process.

[0164] Table 13

[0165]

[0166] If the combination of Sc and Wsa falls within the profile A'FCDE A', the product is even better and if it falls within the profile A"GCDE A" it is still better.

[0167] The lines EA, EA' and EA" represent the roughness required to allow proper strip tracking in the hot dip coating line and to prevent slippage and scratching.

[0168] The lines BC, FC and GC represent the maximum waviness above which the paint appearance of the steel is no longer sufficient to meet the applications in high quality visible parts when finally painted.

[0169] The line CD represents the maximum Sc above which the benefits of the present invention are counterbalanced by the fact that firstly the average roughness becomes higher than the average roughness obtainable with the coating weight expected by the customer and secondly that a very high wiping pressure is required to control the coating weight of the thin coating.

[0170] During the experiments, the use of the cold rolling regime according to the present invention was also tested for GI and it was confirmed that for other types of coating than ZM, the number of defects was also reduced by the present invention. The results are shown in Table 14.

[0171] Table 14

[0172]

[0173] The present application is preferably carried out in combination with a steel substrate having a composition, all in weight %, having: C max 0.04 or max 0.01 or max 0.007, and / or Mn max 1.2 or max 0.80, and / or Si max 0.50 or max 0.30, and / or Al max 0.1 or max 0.08, and / or P max 0.15 or max 0.10, and / or S max 0.045 or max 0.020, and / or N max 0.01 or max 0.008 or max 0.004, and / or Ti max 0.12 or max 0.08, and / or Nb max 0.12 or max 0.03, and / or Mo max 0.12 or max 0.01, and one or more optional elements: Cu max 0.10 or max 0.08, Cr max 0.06 or max 0.04, Ni max 0.08, B max 0.0025 or max 0.0015, V max 0.01 or max 0.004, Ca max 0.01, Co max 0.01, Sn max 0.01, the balance being iron and unavoidable impurities.

[0174] It is noted that in the present patent document, the Ra of a surface represents its roughness according to ISO-NEN 468-1982, with a cut-off of 2.5 mm.

[0175] It is moreover noted that the waviness number Wsa is Wsa(l-5), in pm, measured according to SEP 1941 :2012-05 on the rolling direction of the strip (also denoted herein as "rd"), and, where applicable, after 5% Masingneux biaxial deformation.

[0176] It is finally noted that in the formulas, "*" represents a multiplication operation, and "A" represents an exponentiation operation.

Claims

1. A method of manufacturing a high surface quality steel strip suitable for use in automotive vehicle bodies, comprising the subsequent steps of hot rolling the strip into a hot rolled strip, cold rolling the hot rolled strip, and hot dip coating the cold rolled strip with a Zn-based coating by leading the strip through a bath comprising molten zinc, and wiping the strip after the coating using an air knife having a knife slot from which a wiping gas is ejected, wherein the steel strip has the following composition, all in weight %: C max 0.04; Mn 0.01-1.20; Si 0.001-0.50; Al 0.005-0.1； P max 0.15; S max 0.045; N max 0.01; Mo max 0.12; Ti max 0.12; Nb max 0.12; Cu: max 0.10; Cr: max 0.06; Ni: max 0.08; B: max 0.0025; V: max 0.01; Ca: max 0.01; Co: max 0.01; Sn: max 0.01; the balance being iron and unavoidable impurities; wherein the steel strip is cold rolled in a multi-stand cold rolling mill to a final cold rolled thickness of 0.40 mm to 1.00 mm, wherein the cold rolling in the last stand is such that: wherein SRF is the specific rolling force expressed in kN / m and calculated as the rolling force in kN divided by the strip width in m, AWR is the average working roll radius in m of the top and bottom working rolls at the intermediate roll position, and wherein GKD is the average distance between the knife slot from which the wiping gas is ejected and the surface of the coated strip being wiped, wherein GKD < 10 mm, wherein the molten metal bath has a composition comprising Zn, Al and Mg; wherein the strip after coating and wiping is cooled in a cooling section between the position where the strip is wiped and a downstream position where the strip is first contacted by a guide roller; wherein a forced cooling gas flow Q is used, which forced cooling gas flow Q is required in order to maintain the strip temperature at the guide roller within a 20 degree strip temperature band of a target strip temperature in the range between 200 °C and 300 °C; wherein the strip has a thickness of 0.5 mm or less; and wherein the strip has a width of 1000 mm or more. 3 / hr; and wherein the forced cooling gas flow Q is in the range between 0.5 m3 / hr and 2 m3 / hr. wherein the cooling gas flow in the second half of the cooling section is a percentage p of Q and the cooling gas flow in the first half of the cooling section is a percentage (100-p) of Q, wherein p is set to 70% or more; and wherein the cold rolling in the last stand is performed using working rolls having a roughness Ra of 7 pm or less but in all cases 1.0 pm or more, and the roughness Ra is measured according to ISO-NEN 468-1982 with a limit of 2.5 mm.

2. The method according to claim 1, wherein 。 3. The method according to claim 1 or 2, wherein the cold rolling in the last stand is performed using working rolls having a roughness Ra of 6 pm or less but in all cases 1.0 pm or more.

4. The method according to claim 1 or 2, wherein the cold rolling in the last stand is performed using working rolls having a roughness Ra of 5 pm or less but in all cases 3.0 pm or more.

5. The method according to claim 1 or 2, wherein GKD is the average distance between the knife slot from which the wiping gas is ejected and the surface of the coated strip being wiped, wherein GKD < 9 mm.

6. The method according to claim 5, wherein GKD < 8 mm.

7. The method according to claim 5, wherein GKD < 7 mm.

8. The method according to claim 1 or 2, wherein p is set to 80% or more.

9. The method according to claim 1 or 2, wherein p is set to 90% or more.

10. The method of claim 1 or 2, wherein the bath consists of: 0.6 - 4.0 wt.% aluminium and 0.3 - 4.0 wt.% magnesium, optionally up to 0.2 wt.% of each element belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the balance being unavoidable impurities and zinc.

11. The method according to claim 10, wherein the aluminium content is 0.6 - 3.0 wt.% and / or the magnesium content is 0.3 - 2.0 wt.%.

12. The method according to claim 10, wherein the aluminium content is 1.0 - 3.0 wt.% and / or the magnesium content is 1.0 - 2.0 wt.%.

13. The method according to claim 10, wherein the aluminium content is 1.5 - 2.0 wt.% and / or the magnesium content is 1.0 - 1.5 wt.%.

14. The method of claim 1 or 2, wherein the bath consists of: 0.20 - 0.90 wt.% aluminium, and up to 0.2 wt.% of each element belonging to the group of elements given by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the balance being unavoidable impurities and zinc.

15. The method according to claim 14, wherein the bath comprises 0.25 - 0.50 wt.% aluminium.

16. The method according to claim 1 or 2, wherein the hot dip coated strip is skin passed at an elongation of 0.5% or more, using skin pass rolls having an average diameter of 400 mm or more.

17. The method according to claim 1 or 2, wherein the hot dip coated strip is skin passed at an elongation of 0.5% or more, using skin pass rolls having an average diameter of 500 mm or more.

18. The method according to claim 1 or 2, wherein the hot dip coated strip is skin passed at an elongation of 0.5% or more, using skin pass rolls having an average diameter of 600 mm or more.

19. The method according to claim 16, wherein skin pass rolls having a roughness Ra of 4.5 pm or less are used.

20. The method according to claim 16, wherein skin pass rolls having a roughness Ra of 3.0 pm or less are used.

21. The method according to claim 16, wherein skin pass rolls having a roughness Ra of 2.5 pm or less are used.

22. A coated steel sheet comprising a steel substrate provided with a Zn-based hot dip coating, the steel substrate having a thickness of 0.40 mm to 1.00 mm, obtained by the method of any one of claims 1 to 21, wherein: i) the steel substrate has the following composition, all in wt.%: C max 0.04; Mn 0.01 - 1.20; Si 0.001 - 0.50; P max 0.15; S max 0.045; N max 0.01; Mo max 0.12; Ti max 0.12; Nb max 0.12; Cu: max 0.10; Cr: max 0.06; Ni: max 0.08; B: max 0.0025; ​ ​ ​ ​ Al 0.005-0.1； ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ V: max 0.01; Ca: max 0.01; Co: max 0.01; Sn: max 0.01; the balance being iron and unavoidable impurities; ii) the coated steel sheet has a surface property Sc, Sc being defined as: Sc = Sk / (0.7*t+0.3), with Sk in pm and defined according to NEN-EN-ISO 25178-2:2012, t is the thickness of the steel substrate in mm and is comprised between 0.40 mm and 1.00 mm, and iii) the coated steel sheet has, after 5% of Masinga biaxial deformation, a waviness Wsa which is the Wsa(l-5) value in pm, measured in the rolling direction according to SEP 1941 :2012-05, iv) wherein the combination Sc and Wsa is located in the area defined by the contour ABCDEA in the XY coordinates plot of Sc and Wsa respectively, wherein: A is defined as the intersection of Sc = 3.00 and Wsa = (0.2686)-(0.0543*Sc) + (0.0105*Sc^2); AB is defined by Wsa = (0.2686)-(0.0543*Sc) + (0.0105*Sc^2) from Sc = 3.00 at A to Wsa = 0.50 at B; BC is defined by Wsa = 0.50 from B to C, C having Sc = 14.50; CD is defined by Sc = 14.50 for Wsa = 0.50 at C to Wsa = 0.10 at D; DE is defined by Wsa = 0.10 for Sc = 14.50 at D to Sc = 3.00 at E; and EA closes the contour and is defined by Sc = 3.00 from E to A.

23. The coated steel sheet according to claim 22, wherein the combination Sc and Wsa is located in the area defined by the contour A'FCDEA' in the XY coordinates plot of Sc and Wsa respectively, wherein: A' is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2); A'F is defined by Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2) for Sc = 3.00 at A' to Wsa = 0.50 at F; FC is defined by Wsa = 0.50 from F to C, C having Sc = 14.50; CD is defined by Sc = 14.50 for Wsa = 0.50 at C to Wsa = 0.10 at D; DE is defined by Wsa = 0.10 for Sc = 14.50 at D to Sc = 3.00 at E; and EA' closes the contour and is defined by Sc = 3.00 from E to A'.

24. The coated steel sheet according to claim 22, wherein the combination Sc and Wsa is located in the area defined by the contour A"GCDEA" in the XY coordinates plot of Sc and Wsa respectively, wherein: A" is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2); A" is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2); A" is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2); A" is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2); A" is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2); and A" is defined as the intersection of Sc = 3.00 and Wsa = (0.2276)-(0.0266*Sc) + (0.0054*Sc^2). A" is defined as the intersection of Sc = 3.00 and Wsa = (0.208) - (0.0118*Sc) + (0.0027*Sc^2); A"G is defined by Wsa = (0.208) - (0.0118*Sc) + (0.0027*Sc^2) from Wsa = 0.20 to Wsa = 0.50 at G; GC is defined by Wsa = 0.50 from B to C, C having Sc = 14.50; CD is defined by Sc = 14.50 from Wsa = 0.50 at C to Wsa = 0.10 at D; DE is defined by Wsa = 0.10 from Sc = 14.50 at D to Sc = 3.00 at E; and EA" closes the profile and is defined by Sc = 3.00 from E to A".

25. The coated steel sheet according to any one of claims 22 to 24, having a total coating weight on both sides of 60-175 g / m 2 .

26. The coated steel sheet according to any one of claims 22 to 24, having a surface roughness Ra of 0.9 μm to 1.8 μm.

27. The coated steel sheet according to any one of claims 22 to 24, having a surface roughness Ra of 0.9 μm to 1.6 μm.

28. The coated steel sheet according to any one of claims 22 to 24, having a surface roughness Ra of 0.9 μm to 1.4 μm.

29. The method of claim 1 or 2, wherein, The method is performed in order to produce a hot-dip coated steel sheet having a guaranteed maximum waviness Wsa in a deformed state in its end use of 0.35 μm, 0.34 μm, 0.33 μm, 0.32 μm, 0.31 μm, 0.30 μm, 0.29 μm, 0.28 μm or lower Wsa(l-5) value, measured in the rolling direction according to SEP 1941 :2012-05.

29. The coated steel sheet according to any one of claims 22 to 28, having a surface roughness Ra of 0.9 μm to 1.8 μm, a surface roughness Rz of 3.0 μm to 6.0 μm, and a surface roughness Rmax of 7.0 μm to 9.0 μm.

30. The coated steel sheet according to any one of claims 22 to 28, having a surface roughness Ra of 0.9 μm to 1.6 μm, a surface roughness Rz of 3.0 μm to 5.0 μm, and a surface roughness Rmax of 6.0 μm to 8.0 μm.

31. The coated steel sheet according to any one of claims 22 to 28, having a surface roughness Ra of 0.9 μm to 1.4 μm, a surface roughness Rz of 2.5 μm to 4.5 μm, and a surface roughness Rmax of 5.0 μm to 7.0 μm.

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