WO2023061960A1

WO2023061960A1 - Method to remove an oxide scale from a steel product and improved steel product

Info

Publication number: WO2023061960A1
Application number: PCT/EP2022/078153
Authority: WO
Inventors: Pieter BAART; Wanda Maria Carolina MELFO PRADA; Jose NARANJO ESPINOSA; Douglas Jesus Figuero GORDON; Samson PATOLE
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2021-10-13
Filing date: 2022-10-10
Publication date: 2023-04-20

Abstract

Method to remove an oxide scale from a steel product by treating the surface of the steel with a pulsed laser wherein a laser beam interacts with the surface of the steel product in a focal spot.

Description

METHOD TO REMOVE AN OXIDE SCALE FROM A STEEL PRODUCT AND IMPROVED

STEEL PRODUCT

The present invention relates to a method to remove an oxide scale from a steel product and thus achieve enhancement of the surface quality and an improved steel product having a removed oxide scale and thus an enhanced surface.

An oxide scale (often named ‘scale’) refers to the thin, flaky texture material that forms on the outer surface of hot rolled steels. It is a by-product of manufacturing hot rolled products such as steel plates and sheets, which occurs as the steel surface oxidizes during heating and hot rolling. It is usually a mixture of iron oxides such as Hematite Fe2C>3, Magnetite FesCL and Wustite FeO. It can be up to e.g. 40 pm thick depending on the process conditions and the steel composition.

The scale is undesirable for many downstream manufacturing applications and processes and poses challenges such as relating to the surface quality, and to the adherence of coatings. Occasionally, hot rolled dry steel products are supplied to the customer without removing the scale from them. Then the customers have to apply expensive shot blasting and cleaning operations to achieve the desired surface quality before painting the final product.

When the hot rolled steel manufacturer delivers a product with an oxide scale it cannot control the surface quality of the steel and risks offering a product with inferior surface quality. Moreover, the processes that customers need to operate in order to avoid poor quality final products represent significant extra manufacturing costs.

In order to remove the scale also referred to as “mill scale”, manufacturers employ aggressive cleaning methods such as pickling with strong acids like hydrochloric acid (HCI), sulphuric acid (H2SO4) and/or shot blasting with abrasive media. Such methods are very costly: they require consumables, continuous maintenance and the use of corrosive chemicals which are harmful to the environment. Further to this, they require waste treatment necessitating a recycling plant dedicated to extract the Fe from the acid and recover the acids.

A further disadvantage of pickling is that it is an inflexible process wherein changes on parameters like concentrations and temperature are not practical and normally only the speed of pickling can be used as control of surface quality. Chemicals used in pickling can leave residues on the surface if not removed carefully which in turn may reduce coating bond strength while creating secondary waste. Although alternatives for pickling are conceivable, such as use of other acids or weak acids, slurry blasting, brushing, blasting, and oxide reduction by gas reaction, all these methods are not effective or proved industrially and cannot compete in costs with the current pickling processes.

The invention solves the problem by removing the scale using laser cleaning technology which offers a better control of the surface quality while eliminating the use of acids and recovery plant. It is a cost-effective technical solution to customers that use shot blasting, without the danger of particles getting embedded in the surface. It is an ecological alternative to the pickling process before cold rolling steel strips or applying metallic coatings.

The invention solves the problems of the prior art and has additional advantages.

The method according to the invention involves treating the surface with a pulsed laser wherein a laser beam interacts with the surface of the steel product in a focal spot wherein there is relative movement between the focal spot and the steel product such that consecutive focal spots partially overlap and wherein at least one laser scan is applied, a laser scan being defined as a laser treatment given to the steel product, wherein the single pulse laser fluence is in the range of 5-12 J/cm2 and the total laser fluence is in the range of 8-350 J/cm2, wherein the single pulse laser fluence is calculated using the equation:

J single laser pulse energy (J) single pulse laser fluence ( — -) = —— - - - - - - — — cm² effective focal spot area (cnr) wherein the single laser pulse Energy (J) is the average laser output power divided by the pulse frequency and the Total Laser Fluence is calculated using the equation:

In the laser cleaning, process contaminants, debris and impurities are removed from the steel surface by focused laser irradiation. Laser irradiation causes thermal expansion of the near-surface layer which causes a mechanical stress and inertial force in the contamination/oxide layer. If these forces exceed the threshold adhesion force then cleaning occurs. The amount of material removed by a single laser pulse or continuous wave, depends on the laser parameters, such as wavelength, laser pulse duration, repetition rate, etc.

In itself laser technology is known and it can be found in several sections of modern society. Laser technology has been used for cutting, welding, material heat treatment, marking parts (engraving and bonding), additive manufacturing processes such as selective laser sintering and selective laser melting etc. In recent years laser cleaning technology has been explored by wide range of industrial sectors ranging from engineering, automotive and aerospace applications for paint/coatings removal, pre-treatment for improving the surface quality, however the known solutions offer limited options as far as processing conditions are concerned and have a complicated set up.

Laser technology is relatively environmentally friendly, safe and can be easily integrated on the processing units hence reducing the cost of operations. It offers an excellent alternative to pickling or shot blasting, potentially reducing manufacturing time, costs and environmental burdens with the advantage of being more flexible to guarantee surface quality.

Laser cleaning technology can remove contaminants, oxides, production residues, unify the appearance and moreover it does it with less chemicals, solvents, abrasives, water, or dust. The technology can also easily be integrated on-line into many production processes in a way not possible with other options. Thus time, costs, space and environmental burdens can be reduced. Another advantage is that surface roughness and corrosion performance and improved surface properties can be tailored whilst removing the oxide scale.

In an embodiment of the invention the single pulse laser fluence is in the range of 6-10 J/cm². In this range the oxide scale will be removed optimally.

In an embodiment of the invention the total laser fluence is in the range of 20-140 J/cm². In this range the invention works particularly well.

In an embodiment of the invention the total laser fluence (J/cm²) = e^A(0.28*[Oxide Thickness (pm)] + 0.33) +/- 20 %, wherein the Oxide Thickness is the thickness of the oxide scale. If a total laser fluence is chosen that is within +/- 20 % of the ideal value, then the removal process may be optimized and the surface will thus be of an enhanced quality.

In an embodiment of the invention the total laser fluence (J/cm²) = e^A(0.28*[Oxide Thickness (pm)] + 0.33) +/- 10 %. This leads to even better removal of the oxide scale.

In further embodiments of the invention the laser power is in the range of 30-5000 watts, the frequency is in the range of 40-900 kHz, and the spot area is in the range of 0.03-0.9 mm2. The values of these parameters represent preferred ranges for achieving the best results.

In a further embodiment the wavelength of the laser is 1080±40 nm and/or the pulse duration is in the range of 30-50 ns. Also the values of these parameters represent preferred ranges for achieving the best results.

In an embodiment the steel has a composition having the elements in wt. %: C: 0.010- 1.0; Si: 0.015-3.5; Mn=<8; Cr=<1.5; Nb=<0.5; Altotal=<1 .50; Mo=<0.5; V=<0.5; Ni=<0.5; Cu=<0.5; B=<0.01 , the remaining constituents being iron and impurities and/or trace elements. In a preferred embodiment the steel has a composition having at least one of the elements in wt. %: Mn: 0.05-2.5; Cr=<0.5; Altotal=<0.10, the remaining constituents being iron and impurities and/or trace elements. Such a steel is particularly suited to be cleaned using the method of the invention.

In an embodiment of the invention the thickness of the oxide scale to be removed is 40 pm or less, preferably 25 pm or less and more preferably 20 pm or less.

The invention is also embodied in a steel product with a laser-enhanced surface realised by laser-removal of the oxide scale, wherein the steel has a composition having the elements in wt. %: C: 0.010-1.0; Si: 0.015-3.5; Mn=<8; Cr=<1.5; Nb=<0.5; Altotal=<1.50; Mo=<0.5; V=<0.5; Ni=<0.5; Cu=<0.5; B=<0.01 , the remaining constituents being iron and impurities and/or trace elements. In a preferred embodiment the steel has a composition having at least one of the elements in wt. %: Mn: 0.05-2.5; Cr=<0.5; Altotal=<0.10, the remaining constituents being iron and impurities and/or trace elements. The examples shown in this invention demonstrate that deviations from the invention and its preferred parameter ranges may result in not removing the oxide scale from the substrate and in achieving undesirable steel substrate modifications and/or surface artifacts.

The invention is elucidated using drawings wherein:

Figure 1 shows the optimum range of the single laser pulse fluence and the total laser fluence represented as the indicated rectangular box;

Figure 2 shows the optimum total laser fluence (8-350 J/pm) that depends on oxide scale thickness with experimental data points and a calculated logarithmic relationship of oxide scale thickness and total laser fluence given by: total laser fluence (J/cm²) = e^A(0.28*[Oxide Thickness (pm)];

Figure 3 shows optical micrograph examples which further support and provide direct evidence of the optimum range for removing the oxide scale from the carbon based steel products in this invention. Sub-figures A-E show optical micrographs of a steel product with a composition having the elements in wt. %: C: 0.06; Si: 0.030; Mn: 0.60; Cr: 0.06; Nb: 0.023; Al soluble: 0.04; Mo: 0.020; V: 0.010; Ni: 0.1 ; Cu: 0.1 ; B: 0.0005, the remaining constituents being iron and impurities and/or trace elements, that has been exposed to a laser treatment with various total laser fluences. Sub-figure A represents the untreated steel substrate with an oxide scale and sub-figure D shows that the oxide scale has been removed from the substrate by lasers with optimum total laser fluence. Sub-figures B, C and E show the oxide scale was partially removed (B, C) or the surface was inadvertently modified (E) where a non-optimum total laser fluence was used.

As is shown in Figure 2, the optimum total laser fluence will increase in logarithmic fashion as oxide scale thickness increases. Any deviation from the optimum total laser fluence (8-350 J/cm² ) will not remove all the oxides from the steel substrate or may result in undesirable steel substrate modifications and surface artifacts.

The invention includes carbon based steel materials that will typically have thicknesses of oxide up to 40 pm, and then one must increase the total laser fluence required up to 350 J/cm² and must stay in the optimum range of single laser fluence 5-12 J/cm² in order to remove the oxide scale from carbon based steel products. As can be seen in Figure 2 there is a logarithmic relationship of oxide scale thickness to total laser fluence.

As can be seen in Figure 3, sample X5L9 (sub-figure D) was processed with optimal conditions, according to the invention with a single pulse laser fluence in the range of 5-12 J/cm², more specifically of 6.6 J/cm² and with an optimum total laser fluence in the range of between 8-350 J/cm² more specifically of 132 J/cm² showing that the scale was fully removed without adversely affecting the steel substrate. As indicated above, the left optical micrograph in Figure 3 (A) shows the original untreated sample with the dark grey oxide scale having an average thickness of oxide scale of 16 pm on top of the light grey steel substrate.

Samples X2L9 (B) and X4L9 (C) were processed with a single laser pulse fluence of 6.6 J/cm² but these samples had lower total laser fluence and hence show partial removal of the oxide scale meaning that the samples were not fully cleaned. Sample X6L9 (E) was processed with a single pulse fluence of 6.6 J/cm² but the total laser fluence was higher than 140 J/cm², and hence it shows strong surface modifications of the steel substrate. The results are in line with the logarithmic relationship shown in Figure 2. If the oxide thickness is about 16 pm thick one must use 125 J/cm² of total laser fluence and if a larger fluence is applied then the laser exposure undesirably modifies the surface and the subsurface.

The following was achieved in a range of experiments:

Removal of oxides up to 40 pm on a range of low carbon steels such as non-alloy and alloyed steel strip products the composition of which is recited above, structural grade materials and wear resistant materials which produced oxides upon heating in air related to its chemistries.

Removal of scale from the steel surface can be stationary or by moving the substrate and/or the laser apparatus such that there is relative movement between the steel surface and laser.

Removal of scale from the steel surface which is moving at a speed < 200 m/min while the single laser fluence in the range of 5-12 J/cm² in relation to its total laser fluence 20-140 J/cm² is applied to remove scale.

The thickness of the scale layers on carbon steel can be up to 40 pm thick and cannot be removed by a single laser pulse. To remove the full thickness of the scale multiple laser pulses are required on the same location on the strip. These multiple laser pulses can be applied via “laser pulse overlap” and “laser scans”. Laser pulse overlap is the laser spot overlap for subsequent pulses and is defined by the laser scan velocity (or substrate velocity) and laser pulse frequency. The number of laser scans represents the number of repetitive laser treatments (scans) given to the same area of the strip.

An embodiment of the invention includes the use of a pulsed laser at a fixed position above a moving steel strip projecting a line-shaped laser focus spot on the steel. The short axis of the projected line is in the strip moving direction and the long axis of the line is in the strip width direction. The total line-shaped focus spot area is chosen such that at the available pulse energy the single pulse laser fluence is 5-12 J/cm² according to the invention.

Several laser spots may be positioned in parallel such that the sum of the long axis of all laser spots matches the width of the steel strip or steel product. Here the laser spot overlap in the strip moving direction of subsequent laser pulses can be adjusted by adjusting the pulse frequency for a given laser spot short axis length and process line speed. Pulse frequency may also be adjusted to ensure a constant pulse overlap at varying process line speeds. The number of scans may be realized in different ways: In the case of a batch oxide removal process, the steel strip can move through the laser cleaning section multiple times, alternatively, in a continuous process multiple rows of parallel lasers may be installed.

The parameters are related as follows.

Total Laser fluence (J/cm²)=(pulse Laser fluence(J/cm²)X (Laser spot overlap)X (number of scans)).

Where the single pulse laser fluence is according to the equation in claim 1 , the laser spot overlap is the number of subsequent pulses on the strip calculated as: laser focus spot length in strip moving direction (m) * pulse frequency (1/s) I strip speed (m/s), and the number of scans is the number of cleaning repetitions.

Example 1 according to the invention, represented by an actual point in Figure 2 as (16,133), i.e. the marked point at the right, and supported with the SEM images in fig. 3:

Experimental data showed 100 % removal of the oxide layer using a pulsed laser setup with 30 ns pulse duration, 1084 nm wavelength, 15 W average laser power at 80 kHz, thus 15/80 k= 1.88*1 O'⁴ J pulse energy. The circular laser beam, with a focus diameter on the sample of 60 pm, gave an effective focal spot area of 2.83 * 10'⁵ cm² and a single pulse laser fluence of 6.6 J/cm² (eq.1). The scan velocity was 1.2 m/s to obtain a pulse overlap of 4. An area of approximately 1 cm² was cleaned repetitively by subsequent scans until the complete oxide layer was removed. It was found that a total of 5 scans was required to fully remove the oxide layer. In conclusion, a total laser fluence of 6.6 J/cm² * 4 overlaps * 5 scans = 133 J/cm² was required at a single pulse fluence of 6.6 J/cm² to fully remove the 16 pm thick oxide layer.

Example 2, a comparative example, not according to the invention.

Tests were done with a single pulse fluence level outside the invention. A pulsed laser setup with 50 ns pulse duration, 1080 nm wavelength, 350W average laser power at 90kHz, thus 3.89*1 O'⁴ J pulse energy. In this case a rectangular laser focus spot was used with a dimension of 1.64 mm over the long axis and 0.05 mm over the short axis. The effective focal spot area was thus 8.2 * 10'⁴ cm² and the single pulse laser fluence became 4.7 J/cm² (eq.1). The scan velocity was 0.1 m/s to obtain a pulse overlap of 45 during a single scan. The total laser fluence was 4.6 J/cm² * 45 overlaps * 1 scan = 212 J/cm² (eq.2) which was more than in example 1. It was found that with a single pulse laser fluence which is too low and outside the invention, only 9 % of the 14 pm thick oxide layer was removed.

Claims

1 . Method to remove an oxide scale from a steel product by treating the surface of the steel with a pulsed laser wherein a laser beam interacts with the surface of the steel product in a focal spot wherein there is relative movement between the focal spot and the steel product such that consecutive focal spots partially overlap and wherein at least one laser scan is applied, a laser scan being defined as a laser treatment given to the steel product, wherein the single pulse laser fluence is in the range of 5-12 J/cm² and the total laser fluence is in the range of 8-350 J/cm², wherein the single pulse laser fluence is calculated using the equation:

J single laser pulse energy (J) single pulse laser fluence ( — -) = —— - - - - - - — — cm² effective focal spot area (cnr) wherein the single laser pulse Energy (J) is the average laser output power divided by the pulse frequency and wherein the Total Laser Fluence is calculated using the equation:

2. Method according to claim 1 wherein the single pulse laser fluence is in the range of 6-10 J/cm².

3. Method according to any one of claims 1 to 2, wherein the total laser fluence is in the range of 20-140 J/cm².

4. Method according to any one of claims 1 to 3 wherein the total laser fluence (J/cm²) = e^A(0.28*[Oxide Thickness (pm)] + 0.33) +/- 20 %, wherein the Oxide Thickness is the thickness of the oxide scale.

5. Method according to claim 4 wherein the total laser fluence (J/cm²) = e^A(0.28*[Oxide Thickness (pm)] + 0.33) +/- 10 %.

6. Method according to any one of the preceding claims, wherein the laser power is in the range of 30-5000 watts, the frequency is in the range of 40-900 kHz, and the spot area is in the range of 0.03-0.9 mm². Method according to any one of the preceding claims wherein the wavelength of the laser is 1080±40 nm. Method according to any one of the preceding claims, wherein the pulse duration is in the range of 30-50 ns. Method according to any one of the preceding claims wherein the steel has a composition having the elements in wt. %: C: 0.010-1.0; Si: 0.015-3.5; Mn=<8; Cr=<1.5; Nb=<0.5; Al_totai=<1 .50; Mo=<0.5; V=<0.5; Ni=<0.5; Cu=<0.5; B=<0.01 , the remaining constituents being iron and impurities and/or trace elements. Method according to any one of the preceding claims wherein the steel has a composition having at least one of the elements in wt. %: Mn: 0.05-2.5; Cr=<0.5; Al_totai^=<0.10; the remaining constituents being iron and impurities and/or trace elements. Method according to any one of the preceding claims wherein the oxide scale to be removed comprises at least 3 volume % as FeaCU, less than 70 volume % Fe2Oa, and less than 30 volume % of other than binary iron oxides, the rest being Fe_xO, with x ranging from 0.95 to 0.80. Method according to any one of the preceding claims wherein the thickness of the oxide scale to be removed is 40 pm or less, preferably 25 pm or less, more preferably 20 pm or less . Steel product with a laser-enhanced surface realised by laser-removal of the oxide scale. Steel product according to claim 13 wherein the steel has a composition having the elements in wt. %: C: 0.010-1.0; Si: 0.015-3.5; Mn=<8; Cr=<1.5; Nb=<0.5; Al_totai^=<1.50; Mo=<0.5; V=<0.5; Ni=<0.5; Cu=<0.5; B=<0.01 , the remaining constituents being iron and impurities and/or trace elements. Steel product according to claim 14 wherein the steel has a composition having at least one of the elements in wt. %: Mn: 0.05-2.5; Cr=<0.5; Al_totai^=<0.10, the remaining constituents being iron and impurities and/or trace elements.