WO2024013364A1

WO2024013364A1 - Galvanized steel strip and method for producing and use of said galvanized steel strip

Info

Publication number: WO2024013364A1
Application number: PCT/EP2023/069620
Authority: WO
Inventors: Pieter BAART; Maxim Peter AARNTS; Carel Hendrik Laurens Jan TEN HORN
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2022-07-14
Filing date: 2023-07-14
Publication date: 2024-01-18

Abstract

This invention relates to a skin-pass-rolled and levelled galvanized steel strip comprising a steel strip substrate and a zinc or zinc alloy coating layer on at least one of the major sides of the steel strip substrate, wherein at least one of the zinc or zinc alloy coating layer has a crystallographic twin boundary length in the high plateau areas of the surface texture topography of at least 30 mm/mm² and at most 200 mm/mm² and method for producing and use of said galvanized steel strip.

Description

GALVANIZED STEEL STRIP AND METHOD FOR PRODUCING AND USE OF SAID GALVANIZED STEEL STRIP

Field of the invention

This invention relates to a galvanized steel strip and method for producing and use of said galvanized steel strip.

Background of the invention

An important functional aspect of the surface of the metal substrate is related to press performance. In metal sheet forming, transfer of material from (metallic coated) metal sheet onto press tools and subsequent material accumulation of the material on the press tools, can initiate galling in the (metallic coated) metal sheet during forming. This disrupts a good visual appearance of the final part and may lead to rejection of the part as well as expensive cleaning of press tools. During forming of the product the surface of the substrate slides along the press tools where high contact pressure and poor lubrication may result in surface damage such as galling of the substrate and in case of an applied zinc coating, zinc pollution of the press tools. To reduce these effects, the state-of-the-art solution is to use extremely smooth tool surfaces and rather rough strip surfaces as well as applying lubrication compounds. One hypothesis is that high roughness on the strip helps to capture lubrication compounds for good lubrication properties. This strategy of improving press performance by increasing strip roughness in critical forming conditions has proven to work in press shops. However, it also has a negative effect on the painting process and appearance as high roughness generally increases waviness which is detrimental for the paint appearance.

Objectives of the invention

Therefore, it is an objective of the invention to provide a metal substrate that simultaneously provides minimal tool pollution, good galling resistance and good paint appearance.

It is another objective of the invention to provide a method to obtain such a metal substrate.

Description of the invention

One or more of the objectives is reached with a skin-pass-rolled and levelled galvanized steel strip comprising a steel strip substrate and a zinc or zinc alloy coating layer on at least one of the major sides of the steel strip substrate, wherein at least one of the zinc or zinc alloy coating layer has a crystallographic twin boundary length, CTBL, in the high plateau areas of the surface texture topography of at least 30 mm/mm² and at most 200 mm/mm² wherein the CTBL is the cumulative twin boundary length in grains with a diameter > 32 pm, representing the high plateau area, divided by the cumulative area of the grains with diameter > 32 pm, wherein the high plateau area is the area where the work roll surface has not touched the zinc or zinc alloy layer, and the valley area is the area where the work roll surface of the skin-pass rolling mill has touched the zinc or zinc alloy layer; and wherein the CTBL is measured according to the method described in the description.

In this method a skin-pass-rolled and levelled galvanized steel strip comprising a steel strip substrate and a zinc or zinc alloy coating layer on at least one of the major sides of the steel strip substrate is produced wherein at least one of the zinc or zinc alloy coating layer has a crystallographic twin boundary length (CTBL) in the high plateau areas of the surface texture topography of at least 30 mm/mm² and at most 200 mm/mm². Preferred embodiments are provided in the dependent claims. Crystallographic twin boundary length can also be referred to as twinning density.

A steel strip, whether hot-rolled or cold-rolled, can be provided with a zinc or zinc alloy coating layer on one or both major sides of the strip. The coating layers do not necessarily have to be identical on both sides.

The inventors found that the galling resistance of the galvanized steel can be improved significantly if at least one of the zinc or zinc alloy coating layer(s) has a CTBL in the high plateau areas of the surface texture topography of at least 30 mm/mm². A suitable upper boundary is 200 mm/mm². A CTBL value higher than 200 mm/mm² requires excessive elongation and has no further added value to the product.

JP4781172 B2 discloses a process for producing a hot dip-plated low-carbon steel strip which is subjected to skin-pass rolling with a total elongation of 1.5 to 4% after hot-dip plating and a light reduction with a tension leveller to suppress stretcher strains by suppressing the yield point elongation.

Skin-pass rolling (SPR) is the process of cold rolling steel strip with a small reduction in thickness (also referred to as temper rolling). In most cases the SPR- reduction is between 0.1 and 3.0% thickness reduction. The reduction results in an equivalent elongation of the strip. SPR is performed after the annealing process, which might be the recrystallisation annealing before electro-galvanising, or after the hot-dip galvanising. In the context of this invention when reference is made to "galvanising" both electro-galvanising (EG) or hot-dip galvanising (HDG) are comprised therein. The recrystallisation annealing process may be a continuous annealing process or a batch annealing process. Hot-dip galvanising combines the recrystallisation annealing of the cold-rolled strip with a subsequent hot-dip coating. In most cases the continuous recrystallisation is performed in-line with hot-dip galvanising in a continuous annealing and HDG line. The skin-pass rolling improves the mechanical properties of the coil such as increasing the yield point, suppressing yield point elongation, improve the flatness and create a specific surface texture. In figure 6 a-d the mechanism of the SPR in relation to the invention is schematically explained. The work roll of the SPR-mill has a certain surface roughness. During SPR the high points of the work roll plastically deform (schematically indicated by the oval shape representing local plastic deformation) the galvanised steel sheet and the these resulting indentations are, in the context of this invention, referred to as valleys. Between the high points of the work roll the galvanised steel sheet may not be touched and therefore not deformed (indicated by the "void" in figure 7a. The surface of the galvanised sheet is unchanged after SPR and is these unchanged or untouched areas are, in the context of this invention, referred to as high plateaus, as indicated in figure 6d and 7b.

The tension levelling process is a combination of elongating the strip and bending the strip over a series of rolls. Tension levelling can correct shape and excessive crossbow, twist and coil set. The elongation of the strip results in an equivalent reduction of the strip thickness.

The inventors now found that by combining a tension levelling (TL) step and a skin-pass rolling (SPR) step the CTBL in the high plateau areas of the surface texture topography can be increased to the level of at least 30 mm/mm², which the inventors have found to be instrumental in obtaining a better galling resistance and reduced tool pollution. The sheet metal surface consists of valleys where the surface on the work rolls of the skin-pass rolling mill after the galvanising step has touched the zinc or zinc alloy layer and it consists of high plateau areas where the work roll surface has not touched the zinc or zinc alloy layer. In the high surface plateaus where the skin-pass roll has not touched the surface, the zinc crystal size remains equal to what was obtained after solidification of the zinc layer and the twin boundary length is typically below 30 mm/mm². In the valleys, where the skin-pass roll has touched the surface, the zinc crystal size has reduced due to the contact pressure to a maximum crystal diameter of 32 pm. The combination of the tension levelling and the skin-pass rolling was found to result in the increase of the CTBL in the high plateau areas of the sheet metal surface texture topography, which in turn results in the better galling resistance and reduced tool pollution.

This is explained schematically in figure 8. The product as depicted in figure 6d is subjected to the tension levelling resulting in the introduction of twinning in the zinc layer, indicated with the dashed section. The TL does not affect the situation in the valleys. So the final product, as depicted in figure 8b and c consists of valleys formed by plastic deformation and high plateaus with a high level of twinning.

The press performance strongly depends on the coating failure mechanism. Twins can stop crack propagation within a crystal. Consequently, a CTBL leads to more smaller cracks instead of fewer larger cracks and smaller wear particles during press forming, resulting in a better galling resistance performance.

The inventors also found that, instead of using tension levelling, a second skinpass rolling (SPR) step, with relatively smooth work rolls, also increased the CTBL in the high plateau areas of the surface texture topography to the level of at least 30 mm/mm2. In the second skin-pass rolling (SPR) step the skin-pass roll touches the high plateau areas of the sheet metal surface texture topography, which was found to result in the increase of the CTBL in the high plateaus, which in turn results in the better galling resistance and reduced tool pollution. The second skin-pass rolling (SPR) step was found most efficient in increasing the CTBL when the roll surface was much smoother than the surface of first skin-pass roll, preferably with a roughness Ra < 1 pm. In this embodiment the SPR-ed product of figure 6d is SPR-ed again in the second skin-pass rolling (SPR) step, but now with a smoother work roll than the one used in the first SPR step. This smooth work roll does not affect the situation in the valleys, but only the high plateaus which results in the increase of the CTBL in the high plateaus.

It is concluded that high amount of twinning in the coating includes a failure mechanism where (only) small particles break out of the coating; particles which are small enough to be suspended in the oil and roughness valleys, minimizing tool pollution and galling.

The High Strength Low Alloy (HSLA) samples which were tension levelled (TL > 0.8%) showed no galling in the LFT test, just slight flattening of the surface. The fine zinc powder on the LFT tool looked similar to results with magnesium containing zinc- alloy coatings such as Magizinc®, which has a favourable coating failure (wear) mechanism. This outstanding galling behaviour of the tension levelled zinc or zinc-alloy coated strip (GI) was related to a high level of twinning which was observed in the large zinc grains which were "untouched" by the skin-pass mill work roll. Due to the anisotropic structure of the Zn layer and the limited deformation modes that are possible (basal slip {0001} (1120), pyramidal slip {2112} (2113) and deformation twinning {1012}(1011)) which depend on the grain size and the strain rate it was found that if the intent of the deformation is to suppress the yield point elongation the deformation is preferably applied by TL and not by SPR because the bending during TL induces more twinning in the zinc layer than a comparable deformation under pressure (SPR).

Skin-pass mill elongation (e_SPR) and tension leveller elongation (e_TL) can be exchanged without negatively affecting mechanical properties. So the method according to the invention not only gives opportunities to improve control strip roughness by varying e_SPR and e_TL, but also enables carefully chosen levels of e_TL to improve galling resistance of GI strip. In an embodiment at least one zinc or zinc alloy coating layer has a CTBL in the high plateau areas of the surface texture topography of at least 35 mm/mm², and preferably of at least 40 mm/mm² and more preferably at least 45 mm/mm². The inventors found that a higher CTBL value is beneficial for the galling behaviour. A balance must be struck between the galling behaviour, the surface texture and the mechanical properties which are all affected by the SPR and TL. A CTBL value of at least 30 is a suitable minimum CTBL value.

In a preferable embodiment a zinc or zinc alloy coating layer is provided on both of the major sides of the steel strip substrate, and wherein at least one, and preferably both of the zinc or zinc alloy coating layer(s) has a CTBL in the high plateau areas of the surface texture topography of at least 35 mm/mm², preferably of at least 40 mm/mm², more preferably of at least 45 mm/mm².

The most economical way to apply a zinc coating or zinc-alloy coating is in a hot dip coating line. Therefore it is preferred that the zinc or zinc alloy coating layer is applied in a hot-dip coating treatment in a continuous annealing and hot-dip coating line.

In an embodiment the steel strip is an interstitial-free (IF) or an ultra-low carbon (ULC) or an extra-low carbon (ELC) or a low carbon (LC) steel.

An IF steel is a specific type of ULC steel wherein the titanium content is chosen such that all nitrogen is bound to titanium.

In an embodiment the IF steel strip contains (in 1/1000 wt.%):

- C between 1.5 and 4.5;

- N between 0 and 10, preferably between 0 and 4;

- Ti between 3.42.N and (3.42.N)+6, wherein N is the nitrogen content in 1/1000 wt.%;

- Al_sol between 0 and 100;

- P between 0 and 80;

- B between 0 and 20;

- Si between 0 and 400, preferably between 0 and 80;

- Mn between 50 and 700;

- S between 0 and 20, preferably between 0 and 10;

- optionally Ca between 0 and 5;

- optionally Nb between 0 and 100;

- optionally V between 0 and 50; the balance is Fe and incidental impurities;

Preferably Ti is between 3.42-N and (3.42-N)+3.

In an embodiment the galvanised steel strip is an HSLA grade, preferably containing (in 1/1000 wt.%): - C between 25 and 180;

- N between 0 and 4;

- Ti between 0 and 60;

- Nb between 5 and 100, preferably between 10 and 90;

- Al_sol between 10 and 100;

- P between 0 and 80;

- B between 0 and 20;

- Si between 0 and 80;

- Mn between 175 and 1750;

- S between 0 and 20, preferably between 0 and 10;

- Ca at most 50;

- the balance is Fe and incidental impurities.

According to a second aspect the galvanised steel strip with improved galling resistance according to the invention is provided by a method for producing a galvanized steel strip comprising the following steps:

• providing a steel strip substrate ;

• coating the steel strip substrate with a zinc or a zinc-based alloy coating layer on at least one of the major sides of the steel strip to produce a coated steel strip;

• subjecting the coated steel strip to a skin-pass rolling and to a tension levelling treatment to produce a crystallographic twin boundary length, CTBL, of at least 30 mm/mm², preferably of at least 35 mm/mm², more preferably of at least 40 mm/mm² and even more preferably of at least 45 mm/mm² in the high plateau areas of the surface texture topography of the zinc or zinc alloy coating layer(s) wherein the CTBL is the cumulative twin boundary length in grains with a diameter > 32 pm, representing the high plateau area, divided by the cumulative area of the grains with diameter > 32 pm, wherein the high plateau area is the area where the work roll surface of the skin-pass rolling mill has not touched the zinc or zinc alloy layer and wherein the valley area is the area where the work roll surface has touched the zinc or zinc alloy layer;

• coiling the skin-pass rolled and levelled strip, or cutting the skin-pass rolled and levelled strip levelled steel strip into sheets or blanks;

• wherein the CTBL is measured according to the method described in the description.

Alternatively to the tension levelling treatment, also a second skin-pass treatment with relatively smooth rolls can be applied to produce a CTBL of at least 30 mm/mm2, preferably of at least 35 mm/mm2, more preferably of at least 40 mm/mm2 and even more preferably of at least 45 mm/mm2 in the high plateau areas of the surface texture topography of the zinc or zinc alloy coating layer(s).

If the steel strip is a hot-rolled strip and the strip is hot-dip galvanised, then the coating process is usually referred to as a heat-to-coat cycle. Reheating of the hot-rolled strip is not necessary if the strip is electrocoated.

In an embodiment both zinc or zinc alloy coating layer(s) have a crystallographic twin boundary length in the high plateau areas of the surface texture topography of at least 35 mm/mm², preferably of at least 40 mm/mm² and more preferably of at least 45 mm/mm².

Preferably the zinc alloy coating layer contains one or more alloying elements selected from the group consisting of Mg, Al each with a content of at least 0.3 wt.% and at most 10 wt.%, optionally one or more additional elements selected from the group consisting of Ni, Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the zinc alloy coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc. Preferably the zinc alloy coating layer comprises 0.2 - 5 wt.% Al and 0.3 - 5 wt.% Mg. More preferably the zinc alloy coating layer comprises 0.2 to 1.4 wt.% of Al.

In case of electrodeposition the coating layer is substantially only zinc.

The inventors have found that it is essential that the galvanised steel strip is subjected to a skin-pass rolling step and a tension levelling step. Preferably the tension levelling step is preceded by the skin-pass rolling step.

In an embodiment the steel strip substrate is provided with a zinc or a zinc-based alloy coating layer on one or both major sides of the steel strip to produce a coated steel strip by means of galvanizing, preferably by means of hot-dip galvanizing. The most economical way to apply a zinc coating or zinc-alloy coating is in a hot dip coating line.

In an embodiment the zinc or zinc alloy coating layer(s) have a CTBL in the high plateau areas of the surface texture topography of at least 30 mm/mm², preferably of at least 35 mm/mm², more preferably of at least 40 mm/mm² and even more preferably of at least 45 mm/mm². The higher this value the more favourable the properties.

In an embodiment of the invention the skin-pass rolling step precedes the tension levelling step. This has the advantage that mechanical properties can be changed without affecting the surface texture which is largely determined in the SPR step.

In an embodiment the skin-pass reduction is between 0 (excluding 0) and 3.0 %. The inventors found that values higher than 3.0% reduction results in an unacceptably high loss of elongation of the final steel product. A suitable minimum skin-pass reduction was found to be 0.50 % where an already significant increase in CTBL could be observed.

In an embodiment the e_TL is between 0 (excluding 0) and 6.0 %, preferably the e_TL is at least 0.50 %, preferably at least 0.6%. The inventors found that values higher than 6.0% e_TL results in an unacceptably high loss of elongation of the final steel product. A e_TL was found to be 0.50 % where an already significant increase in CTBL could be observed. A suitable maximum e_TL value is 3.0%.

In an embodiment the second skin-pass elongation is between 0 (excluding 0) and 6.0 %, preferably the second skin-pass reduction is at least 0.50 %. The inventors found that values higher than 6.0% SPR-elongation results in an unacceptably high loss of elongation of the final steel product. A second skin-pass elongation was found to be 0.50 % where an already significant increase in CTBL could be observed. The surface roughness of the rolls in the second skin-pass elongation must be much lower than the surface roughness of the rolls in the first skin-pass elongation to ensure sufficient contact with the high plateau areas of the surface texture topography. A suitable maximum value for roughness Ra < 1 pm. A CTBL > 35 mm/mm2 was obtained with ground work rolls with Ra 0.5 pm in the second skin-pass elongation.

In an embodiment wherein the e_SPR elongation using smooth work rolls is applied prior to the normal skin-pass rolling, the CTBL can also be increased in the high plateau areas of the surface texture topography which are obtained after the normal skin-pass rolling.

In another embodiment, the skin-pass rolling step follows the tension levelling step.

In a preferable embodiment the sum of the e_TL and the t_SPR (2(e_TL + e_SPR)) is 0.50 to 6.0 %, wherein the e_TL is at least 0.50%. In this embodiment the order of the TL and SPR may be chosen one way or the other.

In a preferable embodiment wherein the sum of the first skin-pass reduction (SPR1) and the second skin-pass reduction (SPR2) (2(e_SPRl + e_SPR2)) is 0.50 to 6.0 %, wherein the skin-pass reduction with smooth work rolls is at least 0.50%. In this embodiment the order of the SPR1 and SPR2 may be chosen one way or the other.

According to a third aspect the invention is also embodied in the use of the galvanized steel strip according to the invention to produce outer parts for automotive applications wherein at least the zinc or zinc alloy coating layer on the side that is to become the outer part of the application has a CTBL in the high plateau areas of the surface texture topography of at least 30 mm/mm², preferably of at least 35 mm/mm², more preferably of at least 40 mm/mm² and even more preferably of at least 45 mm/mm². Preferably the CTBL of said layer is at most 200 mm/mm² for reason explained above. Examples

Two different types of steel were investigated (see table 1).

Table 1: composition of the steels (all elements in wt.%)

The balance is iron and impurities consistent with a BOS steelmaking practice.

EBSD measurements

The EBSD measurements were conducted on the zinc surface. Prior to measurement the samples (10x20 mm) were polished with colloidal silica (OPS) with ION pressure until a smooth surface was obtained. This resulted in a removal of between 1 to 5 pm Zn depending on the starting surface roughness. Also scans without OPS polishing were made to verify that the polishing step does not affect the crystallographic orientations of the zinc grains or the CTBL. In that case the samples were immersed in 5% ammonia for 5 min to remove zinc-oxides from the surface.

The Scanning Electron Microscope (SEM) used for the EBSD measurements is a Zeiss Ultra 55 machine equipped with a Field Emission Gun (FEG-SEM) and an EDAX PEGASUS XM 4 HIKARI EBSD system. The samples were placed under a 70° angle in the SEM. The acceleration voltage was 25 kV with the high current option switched on. A 120 pirn aperture was used and the typically working distance was 15 mm during scanning. To compensate for the high tilt angle of the sample dynamic focus correction was used during scanning.

The EBSD scans were captured using TexSEM Laboratories (TSL) software: "Orientation Imaging Microscopy (OIM) Data Collection version 7.3". Typically, the following data collection settings were used: Hikari camera at 6 x 6 binning combined with background subtraction (standard mode). Phases used during measurement: Zinc and Iron(a). Typically three scans were made 1) overview scan 5000x5000 pm step size 2.5 pm, 2) standard scan 1000x1000 pm step size 1.0 pm, 3) detail scan 250x250 pm step size 0.25 pm. Typical frame rates lie in the range of 50-70 fps. Hough settings used during data collections were: Binned pattern size of 96; theta set size of 1; rho fraction of circa 90; maximum peak count of 9; minimum peak count of 8; Hough type set to classic; Hough resolution set to low; butterfly convolution mask of 9 x 9; peak symmetry of 0.5; minimum peak magnitude of 50; maximum peak distance of 20.

The EBSD scans were evaluated with TSL OIM Analysis software version "8.0 x64 [12-14-16]". A standard grain dilation clean-up was performed settings used: grain tolerance 15°; minimum grain size 2 pixels; grain must contain multiple rows. This resulted typically in clean-up percentages below 5% for 1000x1000 pirn² scans and below 2.5% for 250x250 ptm² scans. For grain size determination in all cases 15° was used as grain tolerance angle indicative of a grain boundary. For the twin boundary density, the 1000x1000 pirn² scans were used. For the most dominant HCP tensile twins typically the following settings were used: twin plane KI <1 0 -1 2>; angle 94.8°; plane normal <-l 2 -1 0>; twin plane deviation tolerance 1°.

Electron Back Scattering Diffraction (EBSD) measurements were done on samples to measure the zinc coating grain size and grain orientation. EBSD measurements were done on the production samples, and also, after polishing these samples to minimize disturbances in the measurement due to "shadows" of surface roughness peaks and valleys. These shadows occur because the electron beam enters the surface at a 70° angle and surface roughness inclines are steeper. The shadows help to visualize the 3D topography but disturb the EBSD measurement and may lead to false interpretation of grain size in the coating. Samples were measured before and after polishing at the same location for comparison and compared to confocal measurements at the same location.

4 samples series with steel plates which have had different process treatments or are different steel grades are as follows:

NSKP - No Skin-pass Rolling

IF which has been hot-dip-galvanized but no skin-pass rolling was applied. The solidified zinc coating layer consist for at least 97% of zinc grains having a diameter in the range 100-300 pm. This sample gives the baseline for the CTBL which may be present in the zinc coating after solidification, see Table 2.

EDT

IF which has been hot-dip-galvanized and skin-pass rolled with fine roughness work rolls to induce surface roughening compared to the NSKP sample.

During the skin-pass rolling of the EDT samples, the peaks of the work roll texture touch the strip zinc coating and create depressions I valleys in the zinc coated steel plate, the so called "touched" area. It has been observed by the inventors that the size of the zinc grains strongly reduces in the strip valleys to a diameter in the range of 10-30 pm due to applied pressure and plastic deformation of the zinc coating during skin-pass rolling. It has also been observed by the inventors that a small number of deformation twins occur in the zinc grains which have not been touched by the work roll texture, the so called "untouched" area. The inventors applied a threshold in grain size where grains with a diameter > 32 pm represent the "untouched" or "plateau" area and grains with a diameter < 32 pm represent the "touched" or "valley" area.

The EDT samples provide the range of twinning densities which are to be expected in skin-pass rolled material, see table 2. IF+

IF which has been hot-dip-galvanized and skin-pass rolled with fine roughness work rolls and also tension levelled. The strip surface roughness depends on the skinpass rolling force. The strip elongation (e_SPR) is also a function of the rolling force applied and therefore the roughness and skin-pass rolling elongation (e_SPR) are highly correlated.

The inventors found that the TL elongation strongly increases the CTBL without affecting the strip surface roughness. The CTBL is defined as the cumulative twin boundary length in grains with a diameter > 32 pm, representing the high plateau area, divided by the cumulative area of grains with a diameter > 32 pm. This is visualized in Figure 1 where the solid lines represent the grain boundaries, and the dashed lines represent the twin boundaries. The integration of the twin boundary length and grain area has been calculated using software for postprocessing EBSD measurements.

It was also found by the inventors that the final CTBL was more strongly influenced by the TL elongation (e_TL) than by the SPR elongation (e_SPR) thereby giving them a handle to control the CTBL.

HSLA+

HSLA which has been hot-dip-galvanized and skin-pass rolled with coarse roughness work rolls and also tension levelled. The samples include a condition with only SPR elongation and no TL elongation like the EDT samples, and a samples with no SPR elongation and only TL elongation.

Table 2: Results (e_SPR = Skin-Pass Rolling elongation; e_TL = Tension Levelling elongation; Galling Strip: l = no galling, 5=severe galling, Pollution tool (Zink flake size: L = large, M = medium, S = small (powder)); CTBL in un-touched "plateau" area, i.e. grain size > 32 pm; Strip surface texture depends on work roll texture).

Name Surface Ra strip e_SPR e_TL CTBL Galling Pollution texture Strip Tool

[pm] [%] [%] [/mm] [-] [-]

NSKP None 0.2 0 0 7 5 L

EDT-A Fine 1.0 1.7 0 21 4 M

EDT-B Fine 1.2 0.9 0 24 3 M

EDT-C Fine 1.3 1.5 0 29 3 L/M

IF-A+ Fine 0.9 0.6 0.6 35 3 M

IF-B+ Fine 1.1 0.9 0.6 35 3 M

IF-C+ Fine 0.9 0.6 0.9 42 2 M/S

HSLA-A Coarse 1.6 1.7 0 21 4 L

HSLA-B+ Coarse 1.2 0.6 0.4 23 4 L

HSLA-C+ Coarse 1.2 0.6 0.8 50 2 S

HSLA-D+ Coarse 0.5 0 1 48 1 S Figure 2 and the data in Table 2 show that the CTBL in the "untouched" zinc grains was increased after skin-pass rolling. The CTBL was further increased due to the tension levelling. A significant increase in CTBL was observed when using a TL elongation of >0.5% (see table 2).

IF+

IF which has been hot-dip-galvanized and firstly skin-pass rolled (SPR1) with fine roughness work rolls and secondly skin-pass rolled (SPR.2) with smooth ground work rolls. The strip surface roughness is increased in the first skin-pass rolling step (SPR1). The second skin-pass rolling step (SPR2) results in a small decrease of the strip surface roughness as the work roll surface roughness flattens small peaks on the high plateau areas, thus the area which was untouched in the first skin-pass rolling (SPR1).

The inventors found that the second skin-pass rolling step (SPR2) with smooth work rolls realized a strong increase in CTBL. The second skin-pass elongation (e_SPR2) with smooth work rolls was found more efficient than the first skin-pass elongation (e_SPRl) in increasing the CTBL; this is visualized in table 3. All samples in Table 3 where produced from the same hot-dip-galvanized coil.

Table 3: Results (e_SPR = Skin-Pass Rolling elongation; CTBL in un-touched

"plateau" area, i.e. grain size > 32 pm).

Name Surface Ra strip e_SPRl e_SPR2 CTBL texture [pm] [%] [%] [/mm]

NSKP None 0.2 0 0 7

EDT-B Fine 1.2 0.9 0 24

IF-D+ Fine 0.9 0.9 0.5 47

IF-E+ Fine 1.2 1.3 0.5 42

It was also found by the inventors that the final CTBL was more strongly influenced by the skin-pass elongation (e_SPR2) with smooth work rolls than by the skin-pass elongation (e_SPRl) with rough work rolls thereby giving them a handle to control the CTBL.

Linear Friction Test (LFT)

Strip samples were cut in a length of 300mm (in rolling direction) and a width of 50mm (in transverse direction). The samples were tested in the rolling direction (RD) in the LFT. The Linear Friction Test (LFT) simulates a draw bead where a lubricated strip is pulled in upward direction between a flat and curved tool up to 6 subsequent times. A drawing of the test setup is shown in Figure 4. Due to the high mean contact pressure (500 MPa) in the sliding (line) contact between the strip and the tools, surface damage of the (soft) zinc coating may occur. Subsequently, zinc coating particles may accumulate on the tool (tool pollution) which in its turn pulls scratches in the coating (galling).

The galling test conditions for the LFT include:

• Contact: 50 mm wide strip clamped between a cylinder with a radius of 10 mm, ground to R_a = 0.2 pm and a flat tool with a lapped surface R_a = 0.4 pm

• Load: 5 kN clamp force (Fn), corresponding to a mean contact pressure of 500 MPa in the cylinder side contact,

• Speed: 20 mm/min over a sliding length of 55 mm,

• Lubricant: Z&G MultiDraw PL61, in the amount of lg/m².on each side.

After the LFT three issues were considered in the evaluation: 1. The quality of the strip surface after pass 6 (severity of galling), the zinc build-up on the tool after pass 6 (severity of tool pollution) and the increase in the coefficient of Friction (CoF) per subsequent pass (indication of surface damage). A relative ranking of strip surfaces after 6 passes was made.

Surface topographical properties were measured with the BMT Miniprofiler MP50 stylus device with a measurement length of 50mm. A minimum of 3 stylus tracks were measured and parameters, including roughness (Ra), Peak count (RPc), Skewness (Rsk) and waviness (Wsai-s) were calculated according to corresponding norms (ISO4287, ISO 13565-2).

Figure 3 shows how the severity of galling observed on the zinc coated strip reduces when increased levels of twinning are introduced in the zinc coating. Soft steel grades such as IF grade is used for press forming of automotive exterior parts. The zinc or zinc alloy coating on these parts is sensitive to galling and has a typical rating of 3-4 in the LFT test (l = no galling, 5=severe galling). The inventors have now found that the galling resistance can be increased by inducing increased levels of CTBL in the high surface plateau areas. The improvement in galling resistance was also strongly observed in HSLA steels where even steel with 0% SPR elongation and low roughness, but with 1% TL elongation was found to have a galling rating of 1 which is the best rating possible.

Brief description of the drawings

The invention will now be explained by means of the following, non-limiting figures.

Figure 1 shows a schematic representation of the microstructure of the zinc layer as seen from above where the solid lines represent the grain boundaries, and the dashed lines represent the twin boundaries. The large grains are the "plateaus," and the small grains are the valleys.

Figure 2 show the increase in CTBL (twinning density) in the "untouched" zinc grains due to SPR and tension-levelling.

Figure 3 show the relation between galling severity and CTBL (twinning density).

Figure 4 shows the schematic set-up of the Linear friction Test.

Figure 5 show various processing routes according to the invention.

Figure 6 shows the process involving skin pass rolling of a galvanised sheet (note that all dimensions are exaggerated and not to scale to explain the principle): a. shows the surface of the rough work roll and the as-galvanised sheet. b. The galvanized sheet is being skin-pass rolled wherein the peaks on the work roll surface plastically deform the galvanised sheet where the peaks of the work roll impinge on the sheet and press valleys in the sheet. c. After skin pass rolling the galvanised sheet is left with indentations (valleys) where the peaks of the work roll have plastically deformed the sheet and with high areas (high plateaus) where the work roll has not touched the galvanised steel sheet which high areas consequently remain undeformed. The local deformation at the valleys is schematically indicated by the oval shapes. d. Shows the galvanised sheet after skin pass-rolling.

Figure 7 shows enlarged sections of the contact with the work roll (dashed rectangular in figure 6b) and after the skin pass rolling, (dashed rectangular in figure 6d).

Figure 8 shows the second deformation of the tension leveller, or after skin pass rolling with a smooth work-roll. a. Only the plateau areas of the skin-pass rolled galvanised steel sheet are touched by the tension leveller or the smooth work roll. The valleys are not touched by this step. Twinning is induced in the plateau sections as a result of the deformation of the plateau sections, indicated by the hatched areas in the plateaus. b. The final product: twinning in the plateaus as the result of the second deformation and the valleys as a result of the first deformation. c. Enlarged section.

Some abbreviations e_SPR: elongation by Skin Pass Rolling e_TL: elongation by Tension Levelling

CTBL: crystallographic twin boundary length

Claims

1. A skin-pass-rolled and levelled galvanized steel strip comprising a steel strip substrate and a zinc or zinc alloy coating layer on at least one of the major sides of the steel strip substrate, wherein at least one of the zinc or zinc alloy coating layer has a crystallographic twin boundary length, CTBL, in the high plateau areas of the surface texture topography of at least 30 mm/mm² and at most 200 mm/mm² wherein the CTBL is the cumulative twin boundary length in grains with a diameter > 32 pm, representing the high plateau area, divided by the cumulative area of the grains with diameter > 32 pm, wherein the high plateau area is the area where the work roll surface has not touched the zinc or zinc alloy layer, and the valley area is the area where the work roll surface of the skin-pass rolling mill has touched the zinc or zinc alloy layer, and wherein the CTBL is measured according to the method described in the description.

2. The galvanised steel strip according to claim 1 wherein the at least one zinc or zinc alloy coating layer has a crystallographic twin boundary length in the high plateau areas of the surface texture topography of at least 35 mm/mm², and preferably of at least 40 mm/mm² and more preferably at least 45 mm/mm².

3. The galvanised steel strip according to claim 1 or 2 wherein a zinc or zinc alloy coating layer is provided on both of the major sides of the steel strip substrate, and wherein at least one, and preferably both of the zinc or zinc alloy coating layer(s) has a crystallographic twin boundary length in the high plateau areas of the surface texture topography of at least 35 mm/mm², preferably of at least 40 mm/mm² and more preferably of at least 45 mm/mm² and even more preferably of at least 45 mm/mm².

4. The galvanized steel strip according to any one of claim 1 to 3 wherein the zinc or zinc alloy coating layer is applied in a hot-dip coating treatment.

5. The galvanised steel strip according to any one of claims 1 to 4 wherein the steel strip is an interstitial-free, IF, an ultra-low carbon, ULC, an extra-low carbon, ELC, or a low carbon, LC, steel.

6. The ultra-low carbon steel strip or interstitial-free steel strip according to claim 5 containing in 1/1000 wt.%:

C between 1.5 and 4.5;

N between 0 and 10, preferably between 0 and 4;

Ti between 3.42-N and (3.42-N)+6, wherein N represents the N-content in 1/1000 wt.%;

Al_sol between 0 and 100;

P between 0 and 80;

B between 0 and 20; Si between 0 and 400;

Mn between 50 and 700;

S between 0 and 20, preferably between 0 and 10; optionally Ca between 0 and 5; optionally Nb between 0 and 100; optionally V between 0 and 50; the balance is Fe and incidental impurities. The galvanised steel strip according to any one of claims 1 to 4 wherein the steel is a HSLA grade, preferably wherein the steel contains, in 1/1000 wt.%:

C between 25 and 180;

N between 0 and 4;

Ti between 0 and 60;

Nb between 5 and 100;

Al_sol between 10 and 100;

P between 0 and 80;

B between 0 and 20;

Si between 0 and 80;

Mn between 175 and 1750;

S between 0 and 20, preferably between 0 and 10;

Ca at most 50; the balance is Fe and incidental impurities. A method for producing a galvanized steel strip comprising the following steps:

• providing a steel strip substrate;

• subjecting the coated steel strip to a skin-pass rolling and to a tension levelling treatment to produce a crystallographic twin boundary length, CTBL, of at least 30 mm/mm², preferably of at least 35 mm/mm², more preferably of at least 40 mm/mm² and even more preferably of at least 45 mm/mm² in the high plateau areas of the surface texture topography of the zinc or zinc alloy coating layer(s) wherein the CTBL is the cumulative twin boundary length in grains with a diameter > 32 pm, representing the high plateau area, divided by the cumulative area of the grains with diameter > 32 pm, wherein the high plateau area is the area where the work roll surface of the skin-pass rolling mill has not touched the zinc or zinc alloy layer, and wherein the valley area is the area where the work roll surface has touched the zinc or zinc alloy layer; and wherein the CTBL is measured according to the method described in the description;

• wherein the tension levelling elongation e_TL is between 0 (excluding 0) and 6.0 %, preferably the tension levelling reduction is at least 0.50 %. The method for producing galvanized steel strip according to claim 8 wherein both zinc or zinc alloy coating layer(s) have a crystallographic twin boundary length in the high plateau areas of the surface texture topography of at least 35 mm/mm², preferably of at least 40 mm/mm² and more preferably of at least 45 mm/mm². The method for producing galvanized steel strip according to claims 8 or 9 wherein the steel strip substrate is coated with a zinc or a zinc-based alloy coating layer on one or both major sides of the steel strip by means of galvanizing, preferably by means of hot-dip galvanizing. The method according to any one of claims 8 to 10 wherein the skin-pass rolling step precedes the tension levelling step. The method according to any one of claims 8 to 10 wherein the skin-pass rolling step follows the tension levelling step. The method according to any one of claims 8 to 12 wherein the skin-pass reduction e_SPR is between 0 (excluding 0) and 3.0 %. The method according to any one of claims 8 to 13 wherein the tension levelling elongation e_TL is between 0 (excluding 0) and 6.0 %, preferably the tension levelling reduction is at least 0.60 %. The method according to any one of claims 8 to 14 wherein the sum of the tension levelling elongation and the skin-pass elongation ( (e_TL + e_SPR)) is 0.50 to 6.0 %, wherein the tension levelling reduction is at least 0.50%. A method according to any one of the claims 8 to 15 where the tension levelling elongation is replaced by an additional skin-pass rolling elongation using smooth work rolls having a surface roughness Ra < 1 pm. Use of galvanized steel strip according to any one of claim 1 to 7 to produce outer parts for automotive applications wherein the zinc or zinc alloy coating layer on the side that is to become the outer part of the application has a crystallographic twin boundary length in the high plateau areas of the surface texture topography of at least 30 mm/mm², preferably of at least 35 mm/mm², more preferably of at least 40 mm/mm² and even more preferably of at least 45 mm/mm².