METHOD OF MANUFACTURING A STEEL STRIP AND COATED STEEL SHEET OBTAINABLE THEREBY

Abstract

A method manufacturing a steel strip, including 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 including molten zinc and wiping the strip after the coating using a gas knife having a knife slot from which a wiping gas is projected and the steel strip is cold rolled to a final cold rolled thickness of between 0.40 mm and 1.00 mm in a multi-stand cold rolling mill, and the coated steel sheet includes a steel substrate provided with a hot dip metal coating.

Claims

1. A method of manufacturing a steel strip of high surface quality for use in in automobile body, 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 said coating using a gas knife having a knife slot from which a wiping gas is projected, wherein: the steel strip is cold rolled to a final cold rolled thickness of between 0.40 mm and 1.00 mm in a multi-stand cold rolling mill wherein cold rolling in the last stand takes place such that: $\frac{SRF}{AWR}\ge 21000kN/m^2$ wherein SRF is the specific rolling force expressed in kN/m calculated as the rolling force in kN divided by the strip width in m, and AWR is the average work roll radius in m of the top and bottom work roll at mid roll position, and wherein GKD is the average distance between the knife slot from which the wiping gas is projected and the surface of the coated strip that is being wiped, wherein GKD ≤ 10 mm ; wherein the bath of molten metal has a composition comprising Zn, Al and Mg, wherein the strip after coating and wiping is cooled in a cooling section between the location where the strip is wiped and a downstream location where the strip is first contacted by a guiding roll; wherein an active cooling gas flow Q in m3/hr is used which is required to maintain the strip temperature within a bandwidth of 20 degrees of a target strip temperature in the range between 200° C. and 300° C. at said guiding roll; 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 of (100 - p) of Q, wherein p is set at 70 % or more; and wherein cold rolling in the last stand takes place using work rolls that have a roughness Ra which is 7 .Math.m or less but in all cases 1.0 .Math.m or more.

2. The method according to claim 1, wherein $\frac{SRF}{AWR}\ge 22000kN/m^2$ .

3. The method according to claim 1, wherein cold rolling in the last stand takes place using work rolls that have a roughness Ra which is 6 .Math.m or less, but in all cases 1.0 .Math.m or more.

4. The method according to claim 1, wherein GKD is the average distance between the knife slot from which the wiping gas is projected and the surface of the coated strip that is being wiped wherein GKD ≤ 9 mm .

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

6. The method according to according to claim 1, wherein the bath consists of 0.6 - 4.0 weight % aluminium and 0.3 - 4.0 weight % magnesium, optionally up to 0.2 weight % each of an 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.

7. The method according to claim 6, wherein the aluminium content is 0.6 - 3.0 weight % and/or the magnesium content is 0.3 - 2.0 weight %.

8. The method according to claim 1, wherein the bath consists of 0.20 - 0.90 weight % aluminium, and up to 0.2 weight % each of an 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.

9. The method according to according to claim 1, wherein the hot dip coated strip is temper rolled with an elongation of 0.5 % or more, using a temper work roll with an average diameter of 400 mm or more .

10. The method according to claim 9, wherein a temper work roll roughness Ra is used of 4.5 .Math.m or less .

11. A coated steel sheet obtained by the method of claim 1, the sheet comprising a steel substrate provided with a Zn based hot dip coating, the steel substrate having a thickness of between 0.40 mm and 1.00 mm, wherein: i) the steel substrate has a 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 remainder being iron and unavoidable impurities; ii) the coated steel sheet has a surface characteristic Sc, Sc being defined as: Sc = Sk / (0.7*t+0.3), wherein Sk in .Math.m is defined according to NEN-EN-ISO 25178-2:2012 and t is the thickness of the steel substrate in mm being between 0.40 mm and 1.00 mm, and iii) the coated steel sheet, after a 5 % Marciniak bi-axial deformation, has a waviness Wsa which is the Wsa(1-5) value in .Math.m, measured in rolling direction, according to SEP 1941, iv) wherein the combination Sc and Wsa lies within an area defined by a contour ABCDEA in an XY-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.

12. The coated steel sheet according to claim 11, wherein the combination Sc and Wsa lies within an area defined by a contour A′FCDEA′ in an XY 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′.

13. The coated steel sheet according to claim 11, wherein the combination Sc and Wsa lies within an area defined by a contour A″GCDEA″ in an XY plot of Sc and Wsa respectively, wherein: 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) for 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 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″.

14. The coated steel sheet according to claim 11, having a total coating weight on both sides together of 60-175 g/m2.

15. The coated steel sheet according to claim 11, having a surface roughness Ra between 0.9 .Math.m and 1.8 .Math.m .

16. A method according to claim 1, performed with the purpose of producing a hot dip coated steel sheet having in its end use, in deformed state, a guaranteed maximum waviness Wsa which is the Wsa(1-5) value, measured in rolling direction, according to SEP 1941, of 0.35 .Math.m or lower.

Description

[0092] In the drawings:

[0093] FIG. 1 is an XY plot of Sc and (1-5) wherein the areas defined by the contours ABCDEA, A′FCDEA’ and A″GCDEA″ are shown, as well as combinations of Sc and Wsa resulting from experiments falling inside and outside the invention; and

[0094] FIG. 2 shows a defect coil map of a coil of 4084 m length, 1460 mm width and 0.6 mm thickness. For this coil, 93.4 % was classified as acceptable for surface critical applications. The remainder of the coil had too high local density of surface defects. This corresponds to surface quality ranking “++” according to table 2. The coil was processed at the cold mill with SRF = 6130 kN/m and AWR = 474 mm; and

[0095] FIG. 3 shows a defect coil map of a coil produced directly after the one shown in FIG. 2, with the same process settings (line speed, GKD) in the galvanizing line. This coil was 4004 m in length, 1460 mm wide and 0.6 mm thick. For this coil, 75.5 % was classified as acceptable for surface critical applications. This corresponds to surface quality ranking “+” according to table 2. The coil was processed with SRF = 5052 kN/m and AWR = 430 mm.

[0096] In order to perform experiments samples were made by casting steel slabs followed by hot rolling the slabs in a hot rolling mill to provide a hot rolled steel strip, processing the hot rolled steel strip in a pickling line, cold rolling the pickled steel strip in a cold rolling mill, annealing and hot dip coating the cold rolled strip, temper rolling in a temper rolling mill, also referred to as skin passing in a skin pass rolling mill.

[0097] Unless otherwise specified in the tables or text, the settings of the manufacturing process up to and including hot rolling were according to normal practice.

[0098] In the course of the experiments, steel substrates manufactured from different steel casts were used, the steel substrates having compositions as given in the following table 1.

TABLE-US-00001 Chemical composition of steel samples Example c Mn P Si Mo Ti Nb Al N B mwt%* ppm* 2.1 3 129 9 4 4 9 8 48 24 12 2.2 4 132 11 3 2 9 8 50 28 12 2.3 3 131 15 3 4 9 8 51 28 13 2.4 4 138 13 4 1 8 7 48 17 13 3.1 3 127 10 3 2 9 7 45 22 12 3.2 3 135 14 4 1 8 8 49 28 11 4.1 3 132 10 4 2 9 8 51 17 11 4.2 3 139 8 4 3 8 7 49 21 11 4.3 3 139 8 4 3 8 7 49 21 11 4.4 4 132 10 3 3 9 8 47 18 11 5.1 4 131 13 2 2 10 8 50 15 13 5.2 3 128 16 3 2 9 7 48 21 11 5.3 3 140 9 2 2 9 8 50 23 11 5.4 3 128 16 3 2 9 7 48 21 11 8.1 4 137 10 5 2 10 8 44 30 12 6.2 4 137 10 5 2 10 8 44 30 12 8.3 3 130 8 3 2 10 8 47 22 12 6.4 3 141 8 3 3 10 8 50 22 12 7.1 1 131 11 4 4 48 0 53 18 0 7.2 2 131 8 4 3 48 0 55 20 0 7.3 2 135 9 3 3 47 0 59 22 0 7.4 3 136 9 5 5 47 0 55 28 1 8.1 2 94 7 4 4 69 0 56 29 0 8.2 1 98 7 4 5 66 0 51 30 0 9.1 1 107 7 4 6 62 0 48 24 0 9.2 1 107 7 4 6 62 0 48 24 0 9.3 1 114 10 4 4 64 0 55 25 1 9.4 1 112 8 4 8 66 0 61 23 1 9.5 1 112 8 4 8 66 0 61 23 1 9.6 1 112 8 4 8 66 0 61 23 1 10.1 3 450 42 5 3 10 8 48 21 11 10.2 3 450 42 5 3 10 8 48 21 11 10.3 3 450 42 5 3 10 8 48 21 11 10.4 3 450 42 5 3 10 8 48 21 11 10.5 3 457 41 3 5 9 8 48 18 10 11.1 4 141 16 5 2 10 7 47 29 13 11.2 4 141 13 5 2 10 7 44 17 12 11.3 3 128 14 4 3 10 7 45 24 12 11.4 3 129 9 5 2 10 8 50 22 12 12.1 4 134 10 4 2 8 8 48 17 11 12.2 3 132 10 4 3 10 8 48 20 11 12.3 3 139 11 4 4 9 8 47 23 12 12.4 4 131 13 2 2 10 8 50 15 13 12.5 2 131 8 4 3 48 0 55 20 0 12.6 3 133 7 5 5 48 0 58 33 1 12.7 3 128 9 3 4 9 7 45 22 11 12.8 2 131 8 4 3 48 0 55 20 0 12.9 1 107 7 4 6 62 0 48 24 0 12.10 2 130 7 4 5 67 0 53 24 0 13.1 3 136 11 5 5 45 0 56 34 0 13.2 2 133 9 3 4 47 0 51 23 1 13.3 2 133 9 3 4 47 0 51 23 1 13.4 2 136 6 3 4 43 0 50 38 1 13.5 2 133 9 3 4 47 0 51 23 1

* mwt % = weight % / 1000, ppm = weight % / 10000

[0099] Hot rolled sample strip material was cold rolled in one and the same cold rolling campaign and hot dip coated according to the same hot dipping regime. The main data regarding the hot dip coating process were: [0100] In the case of production of galvanised material (“GI”) a Zn bath was used with a target aluminium amount of between 0.30 % and 0.40 %; [0101] In the case of production of so called zinc magnesium coated material (“ZM”) a Zn bath was used with a target amount of Mg of between 1.45 % and 1.50 % Mg, and a target amount of aluminium of between 1.70 % and 1.75 %; in practice the amount of Mg in the bath varied between 1.40 % and 1.70 % and the amount of Al varied between 1.60 % and 1.80 %.

[0102] Unless indicated otherwise, the knife slot width was 1.2 mm. The gas knife distance GKD was varied between 7 mm and 10 mm;

[0103] Production of the hot dip coated steel sheet examples was performed in batches. Within a batch, coils of similar steel composition, thickness and width were produced after each other. Quality of the strip was determined by visual inspection supported by camera inspection of the strip to assess the amount and severity of any defects on the strip. The ranking used for the description of the examples below is given in Table 2.

TABLE-US-00002 Surface quality ranking Description -- No part of the coil suitable for surface critical applications - < 25% of the coil can be used for surface critical applications 0 25-75 % of the coil can be used for surface critical applications + > 75% of the coil can be used for surface critical applications ++ > 90% of the coil can be used for surface critical applications +++ > 95% of the coil can be used for surface critical applications

[0104] The examples of two camera inspection defect maps (see FIG. 2 and FIG. 3) show the abrupt change in surface quality that can occur when coils with different rolling force in the final stand of the cold rolling mill are produced one after another. Each dot represents a surface feature classified as a defect by the camera inspection system across the width and length of the strip surface. The map shows both 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. The majority of the defects shown is classified as dross type defects.

[0105] During the investigation into the best conditions to achieve an excellent surface quality in terms of waviness and low defect count, it was observed that the number of defects detected by the camera inspection system could vary quite heavily between strip coils and that some combinations of steel composition, thickness and pre-processing tended to be worse than others. Strip coils with a high number of defects were rejected in the inspection step. An example is given below in Table 3, where it can be seen that examples 2.1 and 2.3 with ranking -- were wholly rejected.

TABLE-US-00003 Example Coating type Thickness [mm] Width [mm] Target coating weight(sum of both sides) [g/m2] Line speed [m/min] Gas Knife Distance (GKD)[mm] Temper mill reduction [%] Defect count /m2 per side as classified by camera inspection system [indexed, example 2.1 = 100%] Surface quality ranking 2.1 ZM 0.60 1489 70 120 7 1.4 100 -- 2.2 ZM 0.65 1305 70 120 7 1.4 1 + 2.3 ZM 0.60 1489 70 120 7 1.4 65 -- 2.4 ZM 0.65 1305 70 120 7 1.2 10 +

[0106] The operators normally expect the cause of these quality deviations to lie in the hot dip coating process. They would vary the process settings in the hot dip coating line to improve the quality and bring it in accordance with the specifications. In such cases, line speed variations, bath level fluctuations and variations in furnace temperature or temper mill processing are suspected as potential causes for the deviations.

[0107] On the basis of the results of the experiments it was noticed that despite hot dip coating conditions being constant, the quality of the product varied, the variations seeming to correlate with the cold rolling regime. Coils that were processed according to one cold rolling regime demonstrated a higher number of surface defects than coils processed according to another cold rolling regime.

[0108] In order to analyse the role of the cold rolling regime on the surface quality of the hot dip coated product, the properties of the substrate surface underneath the coating, on the steel substrate surface were measured.

[0109] To accomplish this, the coating and inhibition layer of hot dip coated samples having a size of 20 mm x 20 mm was stripped from the steel substrate. This was done by placing the samples in batches of maximum 6 samples upright in a pickling solution prepared by mixing: [0110] 800 ml water; [0111] 155 ml water-based solution of hydrochloric acid containing 37.5 volumetric % of hydrochloric acid; and [0112] 1 ml Leuzolit.sup.® Extra 283-M, a commercially available over-pickling inhibitor.

[0113] The Leuzolit.sup.® inhibitor was added to ensure that the steel base is not etched or pickled by the hydrochloric acid and to ensure that pickling does not materially affect the surface texture or roughness of the steel substrate. During this pickling process gas is produced that escaped from the pickling bath through the surface of the pickling solution. Pickling was continued until the gas production had almost stopped, typically taking 10 -15 minutes.

[0114] Because the surface texture of the steel substrate may have been affected by the temper rolling of the coated steel, the inventors focussed on the so-called core roughness of the substrate, because it is more representative of the original cold rolled strip surface.

[0115] Core roughness Sk was measured according to the standard and method described above.

[0116] 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 as well as the total cold rolling reduction, substrate thickness and width were essentially the same. The substrate that presented a higher surface core roughness had the best surface quality, as followed from the lower number of defects recorded by camera inspection and visual inspection.

[0117] Details of examples illustrating this are shown in Table 4 below.

TABLE-US-00004 Example Coating type Thickness [mm] Width [mm] Target coating weight(sum of both sides)[g/m.sup.2] Core Roughness(Sk)[.Math.m] Line speed[m/min] Gas Knife Distance (GKD) [mm] Temper mill reduction[%] Defect count / m2 per side as classified by camera inspection system [indexed, example 3.1 = 100%] Surface quality ranking 3.1 ZM 0.60 1489 90 7.74 109 10 1.5 100 + 3.2 ZM 0.60 1489 90 6.05 107 10 1.5 407 0

[0118] During further production runs it was noticed that despite relevant hot dip coating conditions being constant, again the quality of the coated product correlated with the cold rolling regime, as can be seen in table 5.

TABLE-US-00005 Example Coating type Thickness [mm] Width [mm] Target coating weight(sum of both sides)[g/m.sup.2] Final stand specified work roll roughness[.Math.m] Specific Rolling Force final stand (SRF) [kN/m] Average Work roll Radius final stand (AWR) [m] Line speed[m/min] Gas Knife Distance Temper mill reduction [%] Defect count / m.sup.2 per side as classified by camera inspection system [indexed, example 4.1 = 100%] Surface quality ranking 4.1 ZM 0.60 1489 90 5.6-6.0 4900 0.237 100 11 1.5 100 0 4.2 ZM 0.60 1489 90 5.6-6.0 5100 0.218 100 11 1.5 4 + 4.3 ZM 0.60 1489 90 5.6-6.0 5100 0.218 100 11 1.5 19 + 4.4 ZM 0.60 1489 90 5.6-6.0 4900 0.237 100 11 1.5 98 0

[0119] Upon further investigation of the processing conditions on the cold rolling mill, it occurred to the inventors that there was a difference in deformation in the final stand related to the use of a different specific rolling force, the specific rolling force being defined as the total rolling force exerted divided by the width of the strip, in combination with a different work roll radius. A higher rolling force combined with a lower work roll diameter led to a better surface quality and a lower rolling force combined with a higher work roll diameter to a worse surface quality.

[0120] Further monitoring of the cold rolling regime on the final stand in particular as to the specific rolling force and the work roll radius was carried out and in order to assess the precise influence of the final stand cold rolling in the cold rolling mill, further trials were performed. Results of these trials are shown in table 6. Clearly a reduced rolling force adversely affects the surface quality of the hot dip coated steel. The number of defects is much higher for the coils with lower specific rolling force on the final stand and the best quality is achieved with the coils produced with higher specific rolling force on the cold mill final stand.

TABLE-US-00006 Example Coating type Thickness [mm] Width [mm] Target coating weight (sum of both sides)[g/m.sup.2] Final stand specified work roll roughness [.Math.m] Specific rolling force final stand (SRF) [kN/m] Average work roll Radius final stand (AWR) [m] Core Roughness [Sk] Line speed [m/min] Gas Knife Distance (GKD)[mm] Temper mill reduction [%] Defect count /m.sup.2 per side as classified by camera inspection system [indexed, example 5.1 = 100%] Surface quality ranking 5.1 ZM 0.60 1489 90 5.6-6.0 5125 0.230 6.17 130 9 1.5 100 + 5.2 ZM 0.60 1489 90 5.6-6.0 3490 0.223 4.59 130 9 1.5 643 -- 5.3 ZM 0.60 1489 90 5.6-6.0 5125 0.230 8.40 130 9 1.5 122 + 5.4 ZM 0.60 1489 90 5.6-6.0 3490 0.223 4.36 130 9 1.5 661 --

[0121] As a next step, trials were performed with increased specific rolling force on the final stand, outside the normal range the operators of the cold mill would normally use. Data of this trial are in Table 7.

TABLE-US-00007 Example Coating type Thickness [mm] Width [mm] Target coating weight (sum of both sides) [g/m.sup.2] Final stand specified roughness [.Math.m] Specific Rolling Force final stand (SRF) [kN/m] Line speed [m/min] Gas Knife Distance (GKD)[mm] Temper mill reduction [%] Defect count / m.sup.2 per side as classified by camera inspection system [indexed, example 6.1 = 100%] Surface 6.1 ZM 0.60 1489 90 5.6-6.0 5011 100 10 1.5 100 + 6.2 ZM 0.60 1489 90 5.6-6.0 5011 100 10 1.5 235 + 6.3 ZM 0.60 1489 90 5.6-6.0 6051 100 10 1.5 22 +++ 6.4 ZM 0.60 1489 90 5.6-6.0 6051 100 10 1.5 10 +++

[0122] 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.

[0123] Results of a trial with work roll diameter and roll force are given in table 8.

TABLE-US-00008 Example Coating type Width [mm] Target Coating [g/m.sup.2] Final stand original roughness [.Math.m] Specific Rolling Force final stand (SRF) [kN/m] Average Work roll Radius final stand (AWR) [m] Core Roughness [Sk] Line speed [m/min] Gas Knife Distance (GKD) [mm] Temper mill reduction [%] Defect count / m.sup.2 per side as classified by camera inspection system [indexed, example 7.1 = 100%] Surface quality ranking 7.1 ZM 1505 100 5.6-6.0 5970 0.239 9.32 100 8 1.5 100 ++ 7.2 ZM 1505 100 5.6-6.0 5934 0.217 10.30 100 8 1.5 3 +++ 7.3 ZM 1505 100 5.6-6.0 4190 0.239 7.10 100 8 1.5 1001 0 7.4 ZM 1505 100 5.6-6.0 4217 0.217 7.32 100 8 1.5 400 +

[0124] The results of the trials clearly show that a combination of high specific rolling force and smaller average work roll radius results in a lower defect count and also that the higher the specific rolling force divided by the roll radius, the better the chance on excellent surface quality.

[0125] As roughness transfer during cold rolling is higher for thicker and/or softer material and lower for thinner and/or harder material, the same final stand processing in terms of specific rolling force and average work roll radius, may result in a different core roughness for a material of a different gauge or strength. For thinner and/or stronger material it is therefore required to increase the rolling force further to obtain the same core roughness as for thicker and/or softer material. Nevertheless, increasing the specific rolling force and/or decreasing the average work roll radius of the final stand improves the chance on excellent surface quality.

[0126] The difference between roughness transfer for different materials is illustrated in Table 9 where a thinner material rolled with higher specific rolling force and similar diameter has lower core roughness. To take this into account, the surface characteristic Sc = Sk / (0.7*t+0.3) wherein t is the thickness of the steel substrate in mm, is introduced as a measure to better compare the effectiveness of the final stand rolling process for different material thicknesses.

TABLE-US-00009 Example C oating type Thickness [mm] Width [mm] Target Coating weight [g/m.sup.2] Final stand specified roughness [.Math.m] Specific Rolling Force final stand [kN/m] Average Work roll Radius final stand [m] Core Roughness (Sk) [.Math.m] Surface Characteristic (Sc)[.Math.m] 8.1 ZM 0.65 1491 80 5.6-6.0 6006 0.224 8.28 10.92 8.2 ZM 0.80 1601 70 5.6-6.0 6078 0.225 9.42 10.93

[0127] In a further attempt to maintain the low level 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 influence of the rolling force for a lower work roll roughness, the results of which are shown in tables 10 and 13.

[0128] The waviness after deformation was established by measuring in the rolling direction, Wsa(1-5) in .Math.m according to SEP 1941, after having deformed the sample in bi-axial direction by 5 % using a Marciniak tool.

[0129] This proved to work in a similar way resulting in excellent waviness after deformation as well as a low number of defect detections by the camera system. This is seen in the examples below:

TABLE-US-00010 Example Coating type Width [mm] Target Coating weight (sum of both sides) [g/m.sup.2] Final stand specified roughness [.Math.m] Specific Rolling Force final stand (SRF) [kN/m] Average Work roll Radius final stand (AWR) [m] Core Roughness (Sk) [.Math.m] Line speed [m/min] Gas Knife Distance (GKD) [mm] Temper mill reduction [%] Waviness Wsa (5% Marciniak) rd [.Math.m] Surface quality ranking 9.1 ZM 1640 70 5.6-6.0 5985 0.214 8.70 120 8 1.4 0.36 +++ 9.2 ZM 1640 70 5.6-6.0 5985 0.214 7.40 120 8 1.4 0.37 +++ 9.3 ZM 1640 70 5.6-6.0 5903 0.214 7.21 120 8 1.4 0.35 +++ 9.4 ZM 1640 70 4.2-4.7 5943 0.232 4.83 120 8 1.4 0.29 +++ 9.5 ZM 1640 70 4.2-4.7 5914 0.232 5.23 120 8 1.4 0.31 +++ 9.6 ZM 1640 70 4.2-4.7 5972 0.232 4.74 120 8 1.4 0.28 +++

[0130] The resulting waviness is not only dependent on the work roll roughness in the cold mill, but also on the gas knife distance (GKD) and cooling conditions after wiping. Trials by the inventors showed that the smaller the knife distance and the later the strip is cooled after it has left the zinc pot, the lower the resulting waviness of the coating after deformation will be. This is illustrated by the examples in Table 11 and Table 12.

TABLE-US-00011 Example Coating type Thickness [mm] Wi dth [mm] Target Coating weight (sum of both sides) [g/m2] Final stand specified roughness [.Math.m] Line speed [m/min] Gas Knife Distance (GKD) [mm] Temper mill reduction [%] Waviness Wsa (5% Marciniak) rd [.Math.m] 10.1 ZM 0.72 1780 70 4.2-4.7 100 7 1.5 0.33 10.2 ZM 0.72 1780 70 4.2-4.7 100 7 1.5 0.36 10.3 ZM 0.72 1780 70 4.2-4.7 100 9 1.5 0.41 10.4 ZM 0.72 1780 70 4.2-4.7 100 9 1.5 0.43 10.5 ZM 0.72 1780 70 4.2-4.7 90 6 1.5 0.26

TABLE-US-00012 Example Coating type Thickness [mm] Target Coating weight (sum of both sides) [g/m2] F inal stand specified roughness [.Math.m] Line speed [m/min] Knife slot width (d)[mm] Gas Knife Distance (GKD) [mm] Temper mill reduction [%] Flow Cooler 1 [m.sup.3/hr] Flow Cooler 2 [m.sup.3/hr] Flow Cooler 3 [m.sup.3/hr] Flow Cooler 4 [m.sup.3/hr] Strip Temperature at Top roll [°C] Cooling in second half of cooling tower p [%] Waviness Wsa (5% Marciniak) rd [.Math.m] 11.1 ZM 0.60 80 4.2-4.7 100 1.0 7 1.5 0 0 31525 43650 235 100 0.35 11.2 ZM 0.60 80 4.2-4.7 100 1.0 7 1.5 65000 0 12125 14550 235 28 0.40 11.3 ZM 0.60 80 4.2-4.7 100 1.0 9 1.5 0 0 31525 43650 235 100 0.41 11.4 ZM 0.60 80 4.2-4.7 100 1.0 9 1.5 65000 0 12125 14550 235 28 0.52

[0131] The experiments presented in table 12 concern variation of the percentage p of the active cooling gas flow (in m3/hr from the blowers) used in the second half of the cooling tower. The active cooling gas flow, in this document also referred to as Q, here represents the active cooling gas flow required to maintain the strip temperature within a bandwidth of 20 degrees of a target strip temperature of 230° C. at the first roll in the cooling tower that the strip passes after the gas knives. In the examples the total flow Q is the sum of the flow from coolers 1-4. Cooler 3 and 4 in this example are positioned in the second half of the cooling tower so the combined flow of cooler 3 and 4 divided by Q multiplied by 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 as the strip is allowed to cool as slow as possible in the first half.

[0132] These results show that the method can be used to produce excellent hot dip coated strip with low numbers of defects as well as very low levels of waviness after deformation.

[0133] Based on the whole investigation and the examples in Table 13 below it was concluded that the combinations of Sc and Wsa lying within contour ABCDEA as shown in FIG. 1 represent high quality hot dip coated products allowing a highly effective manufacturing method.

TABLE-US-00013 Example Coating type Thickness [mm] Width[mm] Target Coating weight (sum of both sides) [g/m2] Final stand specified roughness [.Math.m] Specific Rolling Force final stand (SRF) [kN/m] Average Work roll Radius final stand (AWR)[m] SRF/AWR [kN/m.sup.2] Core Roughness (Sk) [.Math.m] Surface Characteristic (Sc) [.Math.m] Gas Knife Distance (GKD)[mm] Cooling in second half of coolign tower p [%] Waviness Wsa (5% Marciniak) rd [.Math.m] Surface quality ranking Symbol in FIG. 1 Below 12.1 ZM 0.60 1489 90 5.6-6.0 3490 0.223 15.652 4.59 6.38 9 > 75 0.42 - - 12.2 ZM 0.60 1489 90 4.2-4.7 4300 0.215 20.000 4.10 5.70 9 > 75 0.31 - - 12.3 ZM 0.60 1489 90 4.2-4.7 4270 0.214 20.000 3.79 5.27 9 > 75 0.37 - - 12.4 ZM 0.60 1489 90 5.6-6.0 5125 0.230 22.283 6.17 8.87 9 > 75 0.32 + 0 12.5 ZM 0.75 1505 100 5.6-6.0 6066 0.239 25.379 9.04 10.95 8 > 75 0.41 ++ + 12.6 ZM 0.75 1505 100 5.6-6.0 6066 0.239 25.379 8.60 10.43 8 > 75 0.41 ++ + 12.7 ZM 0.60 1489 90 4.2-4.7 6000 0.232 28.862 4.82 6.69 7 > 75 0.32 ++ + 12.8 ZM 0.75 1505 100 5.6-6.0 5934 0.217 27.347 10.30 12.49 8 > 75 0.40 +++ X 12.9 ZM 0.75 1640 70 5.6-6.0 5985 0.214 27.966 8.70 10.55 7 > 75 0.36 +++ X 12.10 ZM 0.75 1640 70 5.6-6.0 6054 0.214 28.357 8.05 9.76 7 > 75 0.28 +++ X

[0134] The product is even better if the combination of Sc and Wsa falls within the contour A′FCDEA’ and still better if it falls within the contour A″GCDEA″.

[0135] The lines EA, EA′and EA” represent the roughness required to allow proper strip tracking in the hot dip coating line and prevent slippage and scratching.

[0136] The lines BC, FC and GC indicate a maximum waviness above which the paint appearance of the steel, when it is finally painted, is no longer sufficient for the application in high quality visible components.

[0137] The line CD represents the maximum Sc beyond which the benefit of the invention is counteracted by the fact that firstly the average roughness becomes higher than attainable for customer desired coating weights and secondly an extremely high wiping pressure is required to control the coating weight for thin coatings.

[0138] In the course of the experiments the use of the cold rolling regime according to the invention was also tested for GI and it was confirmed that also for other types of coatings than ZM, the number of defects is reduced by the invention. The results are shown in Table 14.

TABLE-US-00014 Example Coating type Thickness [mm] Width [mm] Target Coating weight (sum of both sides)[g/m.sup.2] Final stand specified roughness [.Math.m] Specific Rolling Force final stand (SRF) [kN/m] Average Work roll Radius final stand (AWR) [m] Line speed [m/min] Gas Knife Distance (GKD) [mm] Temper mill reduction [%] Defect count Im2 per side as classified by camera inspection system [indexed, example 13.1 = 100%] Surface quality ranking 13.1 GI 0.75 1505 140 8.6-6.0 4200 0.239 120 8 1.4 100 + 13.2 GI 0.75 1505 140 8.6-6.0 6000 0.215 120 8 1.4 4 +++ 13.3 GI 0.75 1505 140 8.6-6.0 4200 0.239 120 8 1.4 107 + 13.4 GI 0.75 1505 140 8.6-6.0 6000 0.215 120 8 1.4 17 +++ 13.5 GI 0.75 1505 140 8.6-6.0 4200 0.239 120 8 1.4 58 +

[0139] The invention is carried out by preference in combination with a steel substrate that has 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 of the 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 remainder being iron and unavoidable impurities.

[0140] It is remarked that in this patent document Ra of a surface stands for its roughness according to ISO-NEN 468-1982, with a cut-off of 2.5 mm.

[0141] Further, it is remarked that the Waviness value Wsa is the Wsa(1-5), in .Math.m, determined in accordance with SEP1941:2012-05 in the rolling direction of the strip (denoted herein also as “rd”), and where applicable after a 5 % Marciniak bi-axial deformation.

[0142] Finally it is remarked that in the formula’s “*” stands for multiplication, “^” for exponentiation.