EP2862673B1

EP2862673B1 - High temperature and mechanical descaling process and installation

Info

Publication number: EP2862673B1
Application number: EP13188813.3A
Authority: EP
Inventors: Diana ESPINOSA; Jean-Claude Herman; Tony PREZEAU; Alexandre David; Didier FARRUGIA; Paul Yates
Original assignee: Centre de Recherches Metallurgiques CRM ASBL
Current assignee: Centre De Recherches Metallurgiques Asbl - Centrum
Priority date: 2013-10-16
Filing date: 2013-10-16
Publication date: 2016-03-09
Anticipated expiration: 2033-10-16
Also published as: EP2862673A1

Description

Field of the invention

The present invention pertains to the field of descaling processes and installations in the metal industry, in particular steel industry, i.e. intended for removing the oxide layer formed on the metal surface during hot processing of the material.
More particularly the focus of the invention is set on removing primary scale of highly alloyed steels, electrical and rail steels with a range of Si, Al and Cr additions.

Technological background and prior art

It is known that the present configuration of almost all steel processing routes is characterised by successive steps of heating and cooling. During most of these operations, a large portion of the product surface is exposed to oxidising conditions at high temperature, resulting in the formation of scale on the steel surface. During the hot processing of the material there is thereby a potential source of surface defects and thus scale must be removed as accurately as possible to avoid the formation of such defects and to present suitable product appearance to the customer.
Currently, the descaling techniques are based on use of high-pressure water (HPW) where the material is impacted by high-speed water jets. The oxide layer is cracked and crushed by the kinetic energy of the water drops. The intensive water cooling of the surface causes different shrinking of the steel and the scale material due to the difference in thermal expansion rates and a break of the layer occurs. The scale is removed from the surface by using an angled water spray which will generate a shear component within the scale and scale-metal interface. The process of HPW descaling in its current plant implementation is energy intensive, suffers from saturation (above 40-50 l/m²) and may induce, as it is difficult to control, undue heat losses at the surface of key products (rail, flat carbon steels, etc.).
Alternatively to HPW descaling is steel shot blasting, i.e. projection of abrasive steel particles to the steel surface carried out after hot rolling or at room temperature. Shot blasting of high temperature material (typically > 700°C) is not currently used due to technical issues, such as:

1. the temperature of the shots might rise too high with the repeated impacts and shots might start to soften or lose their mechanical properties;
2. shot blasting equipment, such as blasting cabinet, blasting wheel, elevator, air separator and air filter are not meant to be used at high temperatures;
3. the blasted parts deformation during shot blasting at high temperature might be too important to reach the desired final quality and
4. a risk of shots being stuck at the product surface leading to lowered quality after rolling.

The traditional way to solve these issues is to reduce temperature of the parts, as described for example in the following documents: JP 2007-136469 , JPH06277742 , JPS5531533 .
Document JP 2007-136469 discloses a solution to provide a fine surface finish and high-efficiency descaling method of a hot-rolled steel strip by which the productivity of the descaling is ensured. Shot blasting is applied at room temperature to the hot-rolled steel strip by using shot particles of <0.30 mm in an average particle diameter and, successively, shot blasting is applied by using shot particles of ≥0.40 mm in the average particle diameter. Furthermore, pickling is applied after that.
Document JPH06277742 discloses a solution to remove oxidized scale on a steel sheet and to prevent its regeneration by shot-blasting the steel sheet at a high temperature in an atmosphere of inert gas. The steel sheet is primarily cooled down to a specific temperature under an atmosphere of inert gas, and then sent into the atmosphere.
A shot-blasting machine and a cooling machine are arranged in parallel, both being surrounded by a casing. Passing lines of the steel sheet are so constituted that they are coincident with each other in the shot-blasting machine, the cooling machine and the casing. The casing is communicating with an inert gas feeder in order to be always kept under inert atmosphere. A rolled steel sheet is cooled and shot-blasted at a temperature of 400-650°C, then further cooled to a temperature ≤300°C, carried to the outside of the device and brought into contact with air to remove oxidized scale and to prevent the regeneration of scale. In this case the shot-blasting machine is located after the run-out table, well behind the rolling mill.
In document JPS5531533 , in a shot blast device for high temperature rolled material, part of the shot is injected from an injector onto a high-temperature rolled material, getting thereby high temperature and part sent to the side of a blow-off chamber. Both are then conveyed to a shot cooler to be cooled off therein. The shot which has been cooled off is then carried out of a cooled shot outlet port of the cooler at a settled rate, supplied to the injector of the injecting chamber by a conveyor at the underneath, ensuing high temperature rolled material is continuously disposed of by shot blasting.

Aims of the invention

The present invention aims at addressing the drawbacks observed in steel manufacturing when using state-of-the-art high-pressure water descaling techniques.
More precisely, the invention aims at developing new integrated routes involving water-free descaling techniques. Particularly the invention aims at substituting or completing the primary hydraulic descaling.
The invention also aims at performing a descaling process between post-reheating discharge temperature and hot rolling temperature, on products presenting primary scale thicknesses typically comprised between 500 and 1600 µm.
The invention also aims at avoiding surface quality problems coming from harsh surface temperature reduction and thus, by avoiding problems such as thermal shocks and cracks, at improving product quality.
The invention also aims at reducing energy consumption, in particular electricity, maintenance cost in the descaling process and more generally the cost of the whole process.
The invention also aims at improving descaling efficiency and quality by reducing issues of negative overlap due to non-conformance spray impingement in HPW stationary descaling header which is a major issue for the metal industry involving careful controlled set-up, regular maintenance of header and nozzles. This issue mostly for electrical steel is a source of major surface quality problems such as tiger stripes.
Based on the above clause [0016], this invention also aims at reducing the potential overcooling generated by conventional stational HPW descaler in the overlapping jet areas.
The invention claims the water-free high temperature descaling process to be integrated in the hot processing lines and thus compatible with production speeds and production output of these lines.

Summary of the invention

A first aspect of the present invention relates to a method for water-free descaling a metallurgical product continuously processed through a hot rolling mill installation, comprising successively a reheating furnace or tunnel furnace, a roughing mill and a finishing mill, said method comprising a descaling step using a metal shot blasting device, characterised in that the shot blasting device is integrated inside the hot rolling mill and in that the metallurgical product is shot blasted at a temperature greater than 1000°C.
According to preferred embodiments, the invention scope is further limited by at least one, or by a suitable combination of the following characteristics:

the descaling step is performed between the reheating furnace or tunnel furnace exit and the roughing mill entry, at a temperature preferably comprised between 1150°C and 1250°C;
the descaling step is applied to a metallurgical product having a thickness greater than 50 mm, preferably 200 mm;
the water-free descaling step is performed under ambient or non-inert atmosphere;
the water-free descaling step is performed under an atmosphere having less than 5% oxygen;
the shot is projected onto the product with an impact angle comprised between 90° and 50° (angle measured between the product surface and the blasting direction), a shot diameter comprised between 0.1 mm and 1.7 mm, a shot velocity comprised between 40 and 90 m/s, a product conveyor speed at roughing mill speed, and a coverage width being essentially the width of the part to descale;
the shot diameter is comprised between 0.3 mm and 1 mm.

A second aspect of the present invention concerns a hot rolling mill installation for water-free descaling, comprising successively a reheating furnace or tunnel furnace, a roughing mill and a finishing mill and provided with at least one metal shot blasting device integrated inside the hot rolling mill, for performing a descaling step of a flat or long hot metallurgical product continuously processed therein, characterised in that the shot blasting device is located between the reheating furnace or tunnel furnace exit and the roughing mill entry.
Preferably, the shot blasting device comprises a number of wheel blasting systems propelling shots onto the product thanks to a bladed turbine rotated by a motor.
Still preferably, the number and arrangement of the wheel blasting systems is selected so as to descale the upper, lower and/or lateral sides of the metallurgical product, according to the actual width of said product.

Short description of the drawings

FIG.1 schematically represents a typical wheel shot blasting device (a) with the wheel components detailed (b), for use in the present invention.
FIG.2 represents possible (non-exhaustive) wheel arrangements for the wheel blasting machine depending on product shape, dimension and speed: (a) one turbine, sheet returned to blast both sides ; (b) two turbines, one on each side ; (c) two turbines per face ; (d) three turbines per face; (e) 6 turbines among which 2 on top and bottom and 1 on each side; (f) 8 turbines among which 3 on top and bottom and 1 on each side.
FIG.3A shows cross section micrographs of samples after reheating at 1160°C, water descaling and hot rolling at 20% respectively.
FIG.3B shows surface aspect and cross section micrographs of samples after reheating, shot blasting and hot rolling at 20% respectively
FIG.4 shows rail steel descaling, respectively by HPW and shot blasting, 1220°C.
FIG.5 shows low Si electrical steel descaling, respectively by HPW and shot blasting, 1160°C.
FIG.6 shows high Al/Si electrical steel descaling, respectively by HPW and shot blasting, 1160°C.
FIG.7 shows high Al/Si electrical steel descaling, respectively by HPW and shot blasting, 1200°C.

Detailed description of the invention

According to the present invention, a new high temperature descaling process, based on a water-free method is proposed. Steel shot blasting at room temperature is a process currently applied for removing oxide scale after hot rolling and could be an alternative process for avoiding pickling. The novelty induced by the present invention is to transpose this technique to high temperature and primary scale. Tests simulating primary descaling have shown the possibility to remove the oxide layer from the steel product at high temperature. A clear reduction in energy consumption can be obtained, thanks to substitution of the high pressure pumps, and the temperature drop at the surface when descaling with abrasive media is lower compared to water descaling.
The cross comparison with HPW descaling definitely shows that shot blasting can be as efficient as HPW in removing primary scale, which validates the use of HSB at high steel processing temperatures (see FIG.4, FIG.5, FIG.6 and FIG.7).
An important benefit of the hot shot blasting is the robustness of the process. Once the setup of the system is arranged, impact velocity, coverage rate, number of turbines, etc., the process does not need readjustments to keep a good descaling efficiency. One of the main problems of HPW is that it depends on many parameters that could reduce the descaling efficiency if not good controlled and difficult to solve like negative overlap, overcooling, banding, etc.
An additional benefit for the hot shot blasting is related to the cost. Preliminary cost estimation based on the pilot trials performed with an industrial wheel blasting machine, shows a lower cost for HSB descaling.
Since HSB descaling has been proven to provide similar results as compared with HPW descaling, it is a consequence of the present invention that HSB can be advantageously used at the same places as HPW descaling in the hot rolling mill.
In a typical rolling mill long products (thickness up to 220 mm and length of 10 meter), blooms (with a rectangular form of 280 mm to 460 mm), slabs (220 mm to 400 mm thickness, 1600 mm to 2000 mm width and 2 to 10 m long) or billets (100 mm² to 200 mm²) are provided at the reheating furnace at a temperature of 1250°C to 1100°C. Primary descaling is performed at the exit of the reheating furnace (exit reheating typical temperature 1200°C - exit descaler typical temperature 1190°C). Then the product is passed through the roughing mill with a typical final thickness of 50-60 mm - min. 13 mm (entry roughing mill temperature 1150°C - exit 1120°C typically).

Detailed description of preferred embodiments of the invention

Steel composition

The invention is valid for all hot steel products (between 0.002 and 0.75% wt%C), with a special interest in steels alloyed with Cr and/or Al and/or Mn and/or Si. Indeed the addition of these alloying elements will influence the type of oxide formed, its thickness, properties and adherence to metal surface at the steel/oxide interface.

Characteristics of high temperature shot blasting

Shape, size, hardness and specific distribution called operating mix, due to the wear during blasting service, are the main criteria for blasting media. Shapes of media are round or angular particles and size is defined by normal dimension. Size of the shots for hot shot blasting application can vary between 0.1 and 1.7 mm. Both shape and size are defined by the SAE J444 standard designation. The hardness range depends on the shot application. The operating mix is a mixture of particles used in a blasting machine, characterised by size distribution, abrasive type and quantity as well as blasting parameters.
Related to shot hardness, the conventional grades with hardness between 35HRC and 50HRC are suited for the application. Since the abrasive/substrate contact time is very short and the temperature increment of the shots after descaling hot parts is negligible (from 11°C to 22°C in industrial pilot), and far under the tempering temperature (∼400°C), the shots keep their properties during hot shot blasting.
Other important parameters to take into account are the media velocity, type of media, mass flow rate and impact angle. The media velocity has an effect on the kinetic energy. The type of media (shape, size, mass and hardness) affects the coverage rate. The mass flow rate defines the time required to obtain the specific coverage rate and the impact angle influences the roughness, the shot in-laying, the energy transmission efficiency and affects the transfer energy from shot to work piece and it is also an important parameter for rebound and recyclability.
One of the main advantages of the blasting descaling process is the possibility of recycling the shots. The abrasive particles were studied before and after hot sample blasting.
The measurements of size, hardness and the observation of microstructure did not show significant difference, either the hardness nor the microstructure are visibly affected by the hot material, and the sieve measurement did not show abnormal wear or breakage.

Characterisation of surface state and properties

The surface properties and oxide scale state were investigated. Temperature of the surface during descaling was also assessed. Primary scale obtained by a reheating test under industrial conditions was investigated for all the steels studied in this project.
The primary scale is non-uniform and already fairly porous with spallation and through thickness cracks, in particular for the steels with higher amount of Al and Si.
Scale obtained after shot blasting test done in the shot blasting descaling process was analysed. From cross section results it could be said that for rail steels shot blasted with S230 (0.6 mm) shot size, the oxide is descaled and no significant effect of the blasting parameters on the oxide layer is observed (FIG. 4). For the high Al electrical steels, Al is enriched at the interface leading to Fe-Al spinel (FeAl₂O₄) formation. (FIG. 6 and 7). It seems also that the bigger shots (S230) have a better descalability than the smaller ones (S170, 0.425 mm).

Process efficiency comparison and implications

The aim here is to compare both descaling processes in terms of scale removal in pilot experimental conditions.
For water-based descaling the optimum methodology requires:

appropriate level of descaling energies based on grade and pump/water supply from 17 to 50 kJ/m²; to increase descaling energy and taking into account the mill layout, feedstock speed can be adjusted and potentially reduced above the minimum speed;
optimal selection of nozzles with max spray angle of 30°, standoff within 90 to 120 mm based on optimum nozzle pitch and minimum overlap/wash; the overlap to avoid overcooling could be between 7 to 15% of the spray width. Rotary descaling is a potential alternative for difficult to descale grades.

For water-free descaling the optimal parameters selected from lab trials were:

shots with nominal sizes between 0.5mm to 1mm with speed in the range of 60 m/s to 80 m/s,
conveyor speed of 2.9 m/min to 7 m/min,
max flow rate of 90 kg/min and
coverage width of 0.6 to 1 m.

The comparison of the two processes has involved modelling, experimental testing and detailed surface characterization.

Table I

**Flow rate (l/min)**	*Pressure (bar)*	Water velocity (m/s)	Shot velocity (m/s)	*HPW descaling IP (Mpa) @93mm standoff*
20	124	133	47.6	0.6
25	193	167	59.8	0.94
30	278	200	71.6	1.36
32.5	326	217	78	1.6
35	378	233	83.4	1.85
40	493	267	95.6	2.4

Table I shows the analytical equivalence between mechanical descaling and shot blasting (no thermal effect and under static impact only). The main results are related to the shot and water velocity. A shot velocity of ∼80 m/s corresponds to a HPW descaling pressure of max 320 bar for a particular nozzle giving theoretical descaling static impact pressure (IP) of the order of 1.5-1.8 Mpa. The estimated analytical penetration depth is 0.1 to 0.3 mm using water or metallic shot with 0.4 mm diameter particle.
Steel samples obtained after high pressure water descaling and shot blasting descaling were further analysed. FIG.3A and FIG.3B display the cross section obtained after respectively high pressure water descaling and shot blasting.
It could be concluded that scale test hot shot blasting descaling is at least as efficient as high pressure water descaling in removing the primary scale.

Process efficiency and cross comparison

Large experiments trials including descaling and hot rolling with large scale hot shot blasting prototype developed therefor were performed and compared with HPW descaling. The idea was to perform a large series of controlled heavy trials as close as possible to production conditions to compare efficiency of both processes.
A standard wheel blasting machine was adapted to be able to work with hot samples. On top of the blasting cabinet was positioned a blasting wheel to access comparable angles to HPW, the turbine could be adjusted in 3 directions.
The rake angle of the wheel has been custom-made designed and built for this application. A blasting wheel of 5.5 kW with capacity of 90 kg/min in terms of flow rate was enough to ensure a good efficiency and productivity for the given test conditions.
Blasting speed and flow were monitored with an automated system. All the tests were performed under dynamic conditions and the media was reused for each sample. The parameters were the optimum parameters mentioned above.
In the first set of trials, HPW in pilot descaling rig was compared to wheel blasting descaling. From visual observations it can already be said that shot blasting seems a promising way to water-free descale samples. From cross section analysis it can be confirmed that HPW descaling for the pressure considered (200 bar) and high speed resulted in poor descability as compared to lower system pressure and low speed. The hot shot blasting process using S230 shots and 60Hz turbine speed created some shot indentation to the steel surface.
In the second set of trials not only the descaling efficiency of the two processes was compared but also the scale evolution was analysed, hot rolling was performed after water and shot blasting descaling. A detailed experimental plan was conceived to allow on one single large block (typically 100 x 70 x 430 mm) to be hot shot blasted then flip by 180°, craned to the roller table of the plate mill for water descaling and then rolled in 1 to 3 passes, all within safe operating conditions and timing (max 5min). A total of 6 large blocks were rolled encompassing electrical and rail steel grades. All the large blocks were reheated with dummy bars for initial primary scale morphology in the large gas furnace of the plate mill (∼8% max excess O₂). Post rolling, these large blocks were either cooled in the large argon box or in a direct quenching unit. Scale thickness profile was determined with an Elcometer probe 355. After test, samples for microscopic analysis were taken.
The Elcometer results show that the low Si electrical steel scale thickness maps are fairly uniform with average rolled residual scale below 20µm after one rolling pass. For both the high Si electrical and rail steels higher remaining scale and non-uniform surface state is found. The results of cross section and SEM show that regions of good descalability are regions of minimal internal oxidation and entanglement especially for the high Si electrical steel. Irrespectively of turbine set-up, the scale profile seems to have a smoother and less wavy interface with shot blasting except in areas where scale remains. In conclusion, it can be said that the water-based and water-free using similar descaling energy experimented during the pilot industrial trials show a similar efficiency in terms of primary scale removal. Similar amount of scale remaining and similar morphology of scale after both processes is visible.
Finally, up-scaling and cost benefit for a possible industrial application of the new water-free descaling was performed.
The wheel blasting pilot was able to run at 0.08 - 0.12 m/s. Blasting pattern width was adjusted between 600 mm and 1 m. To calculate the upscaling of this equipment to an industrial line, the coverage rate has to be kept and the abrasive flow adjusted. If necessary the number of turbines can be adjusted. There are two ways to increase the abrasive flow: increasing the number of turbines or increasing the flow through each turbine. A combination of more turbines with higher flow is also possible. The more turbines are input, the longer the line has to be, to avoid having facing turbines. The more turbines are input, the more flexible the line can be. FIG.2 shows possible wheel arrangement for blasting the sides of a plate.
The cost estimation considered cost of energy, water and shot, type of product for full descaling coverage, variable line speed, pump type and power, and descaling time.
For example, the abrasive is recovered at the bottom of the cabinet, the media is recyclable hundreds of times and it is calculated that only a negligible portion of the abrasive will break upon each impact.
Cost estimation also accounts for wear rate of nozzle via an increase of the coefficient of discharge. Cost for water discharge, any water treatments, accumulators (case of plate mill), maintenance and operator cost, etc. were simplified. The investment cost is not included, but the value should be similar for both systems.
Results show a significantly higher annual cost for HPW descaling compared to HSB. All neglected costs are known to be higher with HPW descaling than for shot blasting. Both cost calculations are based on the test done during this project, it is an estimation and as not all the industrial values and conditions are taken into account, more testing should be done to have a more real estimated cost.

List of reference symbols

1: feed spout
2: impeller
3: control cage
4: centring plate
5: spacer
6: blade
7: bare wheel assembly

Claims

Method for water-free descaling a metallurgical product continuously processed through a hot rolling mill installation, comprising successively a reheating furnace or tunnel furnace, a roughing mill and a finishing mill, said method comprising a descaling step using a metal shot blasting device, characterised in that the metal shot blasting device is integrated inside the hot rolling mill and in that the metallurgical product is shot blasted at a temperature greater than 1000°C.
Method according to Claim 1, characterised in that the descaling step is performed between the reheating furnace or tunnel furnace exit and the roughing mill entry, at a temperature comprised between 1150°C and 1250°C.
Method according to Claim 1 or 2, characterised in that the descaling step is applied to a metallurgical product having a thickness greater than 50mm.
Method according to Claim 1, characterised in that the water-free descaling step is performed under ambient or non-inert atmosphere.
Method according to Claim 1, characterised in that the water-free descaling step is performed under an atmosphere having less than 5% oxygen.
Method according to Claim 1, characterised in that the shot is projected onto the product with an impact angle comprised between 90° and 50°, a shot diameter comprised between 0.1 mm and 1.7 mm, a shot velocity comprised between 40 and 90 m/s, a product conveyor speed at roughing mill speed, and a coverage width being the width of the part to descale.
Method according to Claim 6, characterised in that the shot diameter is comprised between 0.3 mm and 1 mm.
Hot rolling mill installation for water-free descaling, comprising successively a reheating furnace or tunnel furnace, a roughing mill and a finishing mill and provided with at least one metal shot blasting device integrated inside the hot rolling mill, for performing a descaling of a flat or long hot metallurgical product continuously processed therein, characterised in that the shot blasting device is located between the reheating furnace or tunnel furnace exit and the roughing mill entry.
Installation according to Claim 8, characterised in that the shot blasting device comprises a number of wheel blasting systems propelling shots onto the product thanks to a bladed turbine rotated by a motor.
Installation according to Claim 9, characterised in that the number and arrangement of the wheel blasting systems is selected so as to descale the upper, lower and/or lateral sides of the metallurgical product, according to the actual width of said product.