EP2820162A1

EP2820162A1 - Method for manufacturing a steel product

Info

Publication number: EP2820162A1
Application number: EP13706177.6A
Authority: EP
Inventors: Fouzia Hannour; Jose Reyes Flores Ramirez; Uazir Orion Bezerra DE OLIVEIRA
Original assignee: Tata Steel Nederland Technology BV
Current assignee: Tata Steel Nederland Technology BV
Priority date: 2012-02-27
Filing date: 2013-02-26
Publication date: 2015-01-07
Also published as: WO2013127515A1; HK1206074A1

Abstract

Method for manufacturing a steel product which comprises the steps of: (i) providing a steel substrate; (ii) providing a solution comprising a slag material; (iii) contacting the steel substrate with the solution comprising the slag material to form a corrosion protective layer on the surface of the steel substrate, wherein the corrosion protective layer consists mainly of a compound formed from oxides comprised in the slag material.

Description

METHOD FOR MANUFACTURING A STEEL PRODUCT

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing a steel product and the product thus produced.

BACKGROUND OF THE INVENTION

With world steel production now estimated to be over a billion tonnes per year vast quantities of steel making slag is produced that must be disposed of or recycled. Steel making slag includes blast furnace slag, blast oxygen furnace slag and electric arc furnace slag types.

Blast furnace (BF) slag is produced as a by-product during the manufacture of pig iron. Although BF slag may be recycled externally, for instance as an aggregate in cements or as a fertiliser in agriculture, BF slag is often not re-used by the steel manufacturer. In this respect BF slag may not be re-introduced into the sinter plant or into the blast furnace without further treatment due to the presence of phosphorous, which forms phosphate precipitates that are detrimental to the steel thus produced.

Blast oxygen furnace slag is produced as a by-product of the basic oxygen steel (BOS) making process in which molten pig iron from a blast furnace is charged into a basic oxygen furnace (BOF) together with scrap metal, fluxes, alloys and high purity oxygen. Oxygen is used primarily for the decarburisation and conversion of molten pig iron to liquid steel, while alloys are added to tailor the properties of the steel itself. Fluxes such as burnt lime or dolomite are used to form slag, the purpose of which is to absorb impurities in the steel and those introduced during the steel making process.

The chemical composition and physical properties of BOF slag prevents or at least limits steel manufacturers from recycling BOF slag themselves. The presence of phosphorous in BOF slag also prevents it from being re-introduced into the steel making process. However, BOF slag has the further disadvantage in that it contains Cr(lll) (Cr₂0₃) compounds, that if subjected to a high temperature treatment, for instance in a cement kiln (approximately 1400°C), oxidise to form hexavalent chromium toxic compounds that are hazardous to human health.

The relatively high CaO content in BOF slag relative to BF and EAF slag also makes BOF slag less preferred as material for use in the manufacture of concrete. This is primarily due to the increased risk of cracks forming in the concrete caused by the hydration reaction of CaO to calcium hydroxide ((CaOH₂). Electric arc furnace slag is also a by-product of the steel making process. The composition, properties and applications of EAF slag are broadly similar to those of BOF slag, although EAF slag generally has a lower content of free magnesium and calcium oxides. It is an object of the present invention to find a new use for steel making slag.

It is another object of the invention to find a new use for steel making slag in the manufacture of steel products. DESCRIPTION OF THE INVENTION

According to a first aspect of the invention there is provided a method for manufacturing a steel product which comprises the steps of:

(i) providing a steel substrate;

(ii) providing a solution comprising a slag material;

(iii) contacting the steel substrate with the solution comprising the slag material to form a corrosion protective layer on the surface of the steel substrate, wherein the corrosion protective layer consists mainly of a compound formed from one or more oxides comprised in the slag material. It has been found that steel substrates, preferably carbon steel substrates, can be protected against corrosion by contacting the steel substrate with a solution comprising a slag material. Following contact with the steel substrate, a corrosion protective layer is formed on the steel substrate surface from one or more oxides of the slag material. This corrosion protective layer acts both as a barrier layer and as a passivation layer, which protects the steel against abrasion and prevents or at least reduces corrosive electrolytes from contacting the steel substrate surface respectively. Steel substrates contacted with the solution comprising the slag material are less susceptible to corrosion relative to steel substrates contacted with a solution in which the slag material is absent. In a preferred embodiment the slag material comprises blast furnace slag, blast oxygen furnace slag and electric arc furnace slag. All of these slag materials contain calcium oxide (CaO) which increases the pH of the solution comprising the slag material. By increasing the pH, the rate of corrosion is reduced. In a preferred embodiment the solution comprising the slag material is aqueous. This has the advantage that the aqueous solution can be used instead of or in addition to aqueous solutions such as water that are used to cool and/or clean steel substrates in current steel manufacturing processes. In a preferred embodiment of the invention the solution comprising the slag material has a pH of at least 6, preferably between 7.5 and 12.5. The pH behaviour of acidic, neutral and alkaline solutions were studied both before and after a slag material (1 %) was added to the respective solutions. By adding the slag material to acidic solutions (pH 1.5) pH neutral solutions were obtained, whereas the addition of the slag material to a pH neutral solution caused the pH of the solution to change from pH neutral to alkaline (pH >12.5). When the slag material was added to an alkaline solution, the solution remained alkaline. The best corrosion protective properties were obtained when the steel substrate was contacted with a solution having an alkaline pH. This has been attributed, at least in part, to the reduced H⁺ concentration at alkaline pH, which limits the corrosive cathodic half reaction for hydrogen reduction.

The inventors also observed that the composition of the corrosion protective layer was influenced by the pH of the solution containing the slag material. For slag solutions having an alkaline pH the corrosion protective layer consisted mainly of a compound comprising Ca, Si, Mg and O as main components. For acidic slag solutions the compound comprised elements selected from the group consisting of Si, Al, P, S, Ca, V, Mg, O and Fe. The difference in the composition of the corrosion protective layer has been attributed to the solubility of the oxides in acidic and alkaline pH solutions. The above corrosion protective layers were formed on uncoated cold-rolled steel substrates.

In a preferred embodiment of the invention the slag material comprises 20 to 75 wt% CaO. When the CaO content is within the aforementioned range very good corrosion protection is obtained. The improved corrosion protection has been attributed to CaO increasing the pH of the solution and contributing to the formation of a passivation layer that prevents or at least reduces corrosive electrolytes from contacting the steel surface.

In a preferred embodiment of the invention the slag material comprises:

- 30 to 50 wt% CaO

up to 10 wt%, preferably between 1 and 5 wt% Al₂0₃

- up to 35 wt%, preferably between 10 and 20 wt% Si0₂

up to 35 %, preferably between 15 and 35 wt% FeO

The corrosion protective properties of the BOF slag are further improved when the above oxides were present in the above concentrations. This has been attributed to the elements of the above oxides interacting to form a passivation layer, while CaO and to a lesser extent alumina, contribute to increasing and then maintaining the pH of the solution.

In a preferred embodiment the slag material comprises:

- 40 to 65 wt% CaO

up to 45 wt%, preferably between 20 and 45 wt% Al₂0₃ up to 35 wt%, preferably between 1 and 15 wt% Si0₂

up to 15 %, preferably between 1 and 10 wt% FeO

In a preferred embodiment of the invention the blast oxygen furnace slag material additionally comprises

- up to 10 wt % MgO

- up to 6 wt% MnO

- up to 2 wt% Ti0₂

- up to 3 wt% P₂0₅

- up to 2 wt% V₂0₅

- up to 2 wt% Cr0₂

- up to 2 wt% Na₂0

It is understood that oxides of Mn, Mg, Ti, P, Fe, V, Na and Cr can all serve as passivation layers and/or as corrosion inhibitors thereby inhibiting corrosion of the underlying steel substrate.

In a preferred embodiment of the invention the steel substrate is contacted with the solution comprising the slag material after the steel substrate has been subjected to a milling operation, preferably the milling operation comprises hot-roll milling, cold roll milling, plate milling, bar milling and rod milling.

In a preferred embodiment the solution comprising the slag material is contacted with the milled steel substrate, the substrate having a temperature of at least 100°C, preferably between 700 and 1100^°C. This has the advantage that heat from steel substrate causes the solvent of the solution to evaporate upon contact, leaving behind a corrosion protective layer consisting mainly of a compound formed from oxides comprised in the slag material. This layer provides temporary corrosion protection to the steel substrate during storage and/or transport of the steel substrate. When the steel substrate is hot-roll milled the steel substrate has a temperature of approximately 900°C. It is therefore preferred to contact the steel substrate with the solution in a cooling section of the hot-roll mill, including the hot-roll mill of a direct sheet plant, since the steel substrate will both be cooled and provided with corrosion protective layer of slag material.

In a preferred embodiment of the invention the steel substrate is a coiled strip that is immersed in the solution comprising the slag material. This is particularly advantageous for hot-roll milled steel strip substrates. The cooling of hot-rolled coils in a cooling bath is typical within the steel manufacturing industry, especially for coils produced via the direct sheet plant (DSP) process.

These coils typically exhibit a temperature of at least 700°C before entering the cooling bath.

However, coils that are cooled in this way are known to suffer from pitting corrosion which reduces the surface quality of the steel substrate to an unacceptable level. Surprisingly, the cooling the coils in the solution comprising the slag material instead of water prevents or at least reduces the detrimental effects of pitting corrosion.

In a preferred embodiment of the invention the milled steel substrate is pickled and thereafter contacted with the solution comprising the slag material. Steel substrates such as strip and wire are pickled to remove oxides (scale) that are formed at the steel substrate surface during milling e.g. hot-roll milling. A typical pickling line comprises a scale breaker, a pickling bath containing solutions of hydrochloric acid or sulphuric acid, and a rinsing section in which the pickled steel is rinsed with water to remove traces of pickling products, pickle residues and contaminants from the steel substrate surface. In the case of a hot-rolled steel strip, the pickling operation occurs before the strip substrate is coiled.

The use of the solution comprising the slag material to rinse the pickled steel substrate in place of water results in the formation of a passivation layer on the surface of the steel substrate, which prevents or at least reduces the reoccurrence of oxide scale forming at the steel substrate surface. Since the solution comprising the slag material is typically alkaline, when the solution is contacted with an acidic solution originating from the pickling bath, for instance on the surface of the pickled steel substrate, a further advantage is realised in that the pickling solution is neutralised.

In a preferred embodiment of the invention the steel substrate is cold-roll milled to form a cold rolled strip, which strip is thereafter contacted with the solution comprising the slag material. It is preferred that the strip is contacted with the solution following a temper mill operation of the cold-roll mill. Preferably the solution is applied by spraying or by dipping the temper-milled substrate prior to the step of coiling. Advantageously, a temporary corrosion protection layer is formed on the surface of the steel substrate, which provides corrosion protection during storage and/or transport of the steel substrates. Particularly preferred cold-rolled steel substrates comprise high strength steels, advanced high strength steels and ultra high strength steels. In a preferred embodiment the steel substrate is hot-roll milled and then cold-roll milled. Thus following the hot-roll mill operation, optionally the pickling operation, and the cold-roll mill operation, the steel substrate may be contacted with the solution comprising the slag material and afforded additional corrosion protection. In a preferred embodiment of the invention the steel substrate is provided with a zinc or zinc alloy coating and contacted with the solution comprising the slag material. By providing the BOF slag material on the zinc or zinc alloy coating, improved corrosion protection is afforded to the underlying steel substrate relative to steel substrates provided with the zinc or zinc alloy coating alone. Preferably the zinc alloy comprises Zn as the main constituent, i.e. the alloy comprises more than 50% zinc, and one or more of Mg, Al, Si, Mn, Cu, Fe and Cr. Zinc alloys selected from the group consisting of Zn-Mg, Zn- n, Zn-Fe, Zn-AI, Zn-Cu, Zn-Cr, Zn-Mg-AI and Zn-Mg-AI-Si are particularly preferred. The zinc or zinc alloy coating can be applied by electro-galvanising, galvannealing or by physical vapour deposition (PVD). Hot-dip galvanising is particularly preferred since the heat from the hot-dip galvanised steel substrate can be used to remove the aqueous solvent from the BOFS solution, thereby leaving behind a corrosion protective layer of BOF slag material on the zinc or zinc alloy coating.

According to a second aspect of the invention there is provided a steel substrate manufactured according to any one of the methods of the first aspect of the invention. The advantages associated with the embodiments of the first aspect of the invention similarly apply to the embodiments of the second aspect of the invention.

In a preferred embodiment of the invention the corrosion protective layer has a dry film thickness of up to 10 μιη, preferably between 1 and 5 μηι. It is preferred not to exceed a layer thickness of 10 μιη since the corrosion protection layer is susceptible to delaminate from the surface.

In a preferred embodiment the corrosion protective layer is crystalline and comprises Ca and Si as main components. Such a corrosion protective layer is formed when a zinc or zinc alloy coated steel is immersed in an aqueous BOF solution having an alkaline pH. In another preferred embodiment the corrosion protective layer is amorphous and comprises Zn and Si as main components. Such a corrosion protective layer is formed when a zinc or zinc alloy coated steel is immersed in an aqueous BOF solution having a neutral pH e.g. pH 6. In yet another preferred embodiment the corrosion protective layer is amorphous and comprises Al, Si, Fe as main components. Such a corrosion protective layer is formed when a zinc or zinc alloy coated steel is immersed in an aqueous BOF solution having an acidic pH.

EXAMPLES

The invention will be now be elucidated by referring to the non-limitative examples below. The slag material used in the examples below is a blast oxygen furnace slag. Figure 1 shows the corrosion protective layer (1 ) that has formed on the surface of a steel substrate (2) provided with a zinc coating (3).

BOF slag analysis

Two BOF slag (BOFS) materials were obtained from one blast oxygen furnace and subsequently analysed using wavelength dispersive X-ray fluorescence (WDXRF) to determine their chemical composition (Table 1). Oxide (wt%)

BOFS Na₂0 MgO Al₂0₃ Si0₂ P2O5 CaO Ti0₂ V2O5 Cr₂0₃ MnO FeO

1 0,20 7,70 1 ,34 13,59 1 ,56 42,61 0.92 0.71 0.24 4.24 26.88

2 0.49 3.72 33.20 4.78 0.06 54.06 0.34 0.15 0.03 1.05 2.23

Table 1 : Chemical composition of BOF slag materials in wt% given by WDXRF analysis. Immersion corrosion test

A 1% BOF slag solution was prepared by mixing the slag material 1 (BOFS 1) having an average particle size range of (10 - 100 μηι) in aqueous solution. Four steel substrates were then immersed in the BOFS 1 solution and left for one week before being analysed by visual inspection for signs of corrosion. Steel substrate 1 is an uncoated cold rolled steel substrate, steel substrates 2 and 3 are hot dip-galvanised steel substrates provided with Zn and Zn-Mg coatings respectively, each coating having a thickness of 10 μητι, and steel substrate 4 is a galvannealed (zinc) steel substrate having a coating thickness of 10 μηη. Table 2 below summarises the experimental conditions that were employed during the corrosion test and the results thus obtained.

Table 2: Results of the immersion corrosion test where "O" designates no rust, "RR designates red rust and WR designates white rust". 13 000549

From Table 2 it can be seen that the presence of the BOF slag material in acidic, neutral and alkaline solutions causes solution pH to increase. For acidic solutions the pH is raised from pH 1.5 to pH 6.5, whereas for neutral solutions (demi-water and NaCI), the pH is raised from 7.5 to 12.5. The increase in pH for alkaline solutions is less significant.

Improved corrosion protection is afforded to both uncoated and coated steel substrates when the substrates are immersed in solutions containing BOF slag material, irrespective of whether the starting solution, i.e. the solution before the BOF slag material is provided, is acidic, pH neutral, or alkaline. The improved corrosion protection is believed to be due to the formation of a corrosion protective passivation layer at the steel substrate surface and a reduction in the concentration of H⁺ ions in the aqueous solution at increased pH, particularly alkaline pH, The passivation layer acts a physical barrier to corrosive electrolytes preventing them from contacting the steel, whereas the reduced concentration of H⁺ ions inhibits the corrosive cathodic half reaction for hydrogen reduction.

Selective deposition of BOF slag components

When BOF slag is added to an aqueous solution, a portion of the slag composition reacts with water (dissolution) and becomes incorporated in the aqueous solution as charged ions. Since the steel substrate or coated steel substrate also possesses a specific surface charge, it is possible to selectively deposit, via a chemisorption, specific components of the BOF slag on the steel substrate. Experiments were therefore carried out to investigate the selective deposition behaviour of aqueous acidic, neutral and alkaline BOF slag solutions. To verify the composition of the corrosion protective layers that were formed, a scanning electron microscope (Ultra 55 Zeiss FEG-SEM) equipped with an energy dispersive spectrometer (EDS) was used. The substrates, comprising the corrosion protective layer thereon, were first rinsed with de-ionised water and then air dried. These substrates were then characterized using the back-scattered electron mode of the SEM to differentiate metallic phases. In order to determine the elemental chemical composition of the corrosion protective layer, the substrates were coated with a thin layer of graphite to ensure a sufficient conductivity was obtained during SEM-EDS analysis. EDX data was obtained using a standard acceleration voltage of 15KeV at a working distance of 9.5mm. The results of the selective deposition experiments are shown in Table 3 below.

Selective deposition of Ca and Zn containing corrosion protective layers

A zinc coated steel substrate was degreased and rinsed to remove any lubricant. The zinc coated steel was then deposited in an aqueous solution (i.e. deionized water) containing 10% BOF slag. In order to obtain the desired composition, the pH of the aqueous solution was set to pH 12 using NaOH. The zinc coated steel substrate was completely immersed in the alkaline aqueous solution for 10hrs. Due to the preset pH and the existing surface charges of the substrate, calcium ions were preferentially deposited on the zinc layer forming a crystalline structure rich in zinc and calcium. Depending of the immersion time, the alloyed layer can have thicknesses ranging from 1 to 15 pm. An immersion time of 10hrs resulted in a corrosion protection layer having a thickness of approximately 5μηι. Selective deposition of Al, Si, Fe containing corrosion protective layers

A zinc coated steel is degreased and rinsed to remove any lubricant. The zinc coated steel was then deposited in an aqueous solution (i.e. deionized water) containing 10% BOF slag. In order to obtain the desired composition, the pH of the aqueous solution was adjusted to pH 2 using hydrochloric acid, sulfuric acid or nitric acid, hydrochloric acid being preferred. The zinc coated steel substrate was completely immersed in the acidified aqueous solution for 10hrs. Due to the preset pH and the existing surface charges of the substrate, aluminium, silicon, and iron ions were preferentially deposited on the zinc layer forming an amorphous structure rich in aluminium, silicon and iron and with a minor portion of calcium, vanadium and magnesium (less than 1% of the total composition). Depending of the immersion time, the amorphous layer had a thickness between 1 and 12 pm. An immersion time of 12hrs resulted in a corrosion protection layer having a thickness of approximately 6 pm.

Selective deposition of Si and Zn containing corrosion protective layers

The zinc coated steel was degreased and rinsed to remove any lubricant. Then, the steel was deposited in an aqueous solution (i.e. deionized water) containing is 10% BOF slag. In order to obtain the desired composition, the pH of the aqueous solution was kept neutral (pH 6). Thereafter, the zinc coated steel substrate was completely immersed in the aqueous solution for 10hrs. Due to the preset pH and the existing surface charges of the substrate, silicon ions were preferentially deposited on the zinc layer forming an amorphous structure rich in silicon and zinc with minor portions of titanium oxide, vanadium oxide and calcium (less than 1 % of the total composition). The zinc coated substrates were kept fully immersed for 24 hrs. After 24 hrs, an amorphous layer with a thickness of 3 microns had formed on the surface of the substrate. The process can be accelerated by adding 2% sodium chloride to the solution. The chloride ions act as catalyst to accelerate surface reactions, which results in silicon ions being deposited at much faster rate. In this particular case, a minor amount of chloride ions were incorporated into the deposited corrosion protection layer.

Table 3.

The results show that the composition of the corrosion protective layer that is formed can be controlled by adjusting the initial pH of the aqueous solution.

Claims

Method for manufacturing a steel product which comprises the steps of:

(i) providing a steel substrate;

(ii) providing a solution comprising a slag material by adding the slag material to an aqueous acidic, neutral or alkaline solution;

(iii) contacting the steel substrate with the solution comprising the slag material to form a corrosion protective layer on the surface of the steel substrate, wherein the corrosion protective layer consists mainly of a compound formed from oxides comprised in the slag material.

Method according to claim 1 , wherein the slag material comprises blast furnace slag, blast oxygen furnace slag and electric arc furnace slag.

Method according to claim 1or claim 2, wherein the solution comprising the slag material has a pH of at least 6, preferably between 7.5 and 12.5.

Method according to any one of claims 1-3 wherein the slag material comprises 20 to 75 wt% CaO.

Method according to claim 4 wherein the slag material comprises

- 30 to 50 wt% CaO

- up to 10 wt%,AI₂0₃

- up to 35 wt% Si0₂

- up to 35 wt% FeO

Method according to claim 4 wherein the slag material comprises

- 40 to 65 wt% CaO

- up to 45 wt% Al₂0₃

- up to 35 wt% Si0₂

- up to 15 wt% FeO

Method according to any one of claims 4-6 wherein the slag material additionally comprises

- up to 10 wt % MgO

- up to 6 wt%MnO

- up to 2 wt% Ti0₂

- up to 2 wt%P₂O_s

- up to 2 wt% V₂0₅

- up to 2 wt%Cr0₂

8. Method according to any one of claims 1-7 wherein the steel substrate is contacted with the solution comprising the slag material after the steel substrate has been subjected to a milling operation, preferably the milling operation comprises hot-roll milling, cold roll milling, plate milling, bar milling and rod milling.

9. Method according to claim 8 wherein the solution comprising the slag material is contacted with the milled steel substrate, the substrate having a temperature of at least 100°C, preferably between 700 and 1100°C.

10. Method according to any one of claims 8-9 wherein the steel substrate is a coiled strip that is immersed in the solution comprising the slag material.

11. Method according to claim any one of claims 8-10 wherein the milled steel substrate is pickled and thereafter contacted with the solution comprising the material.

12. Method according to claim 9 wherein the steel substrate is cold-roll milled to form a cold rolled strip, which steel strip is thereafter contacted with the solution comprising the slag material.

13. Method according to any one of claims 8-12 wherein the steel substrate is hot-dip galvanised in a galvanising line, optionally annealed and thereafter contacted with the solution comprising the slag material.

14. Steel substrate comprising a corrosion protective layer manufactured according to the method of any one of claims 1-13.

15. Steel substrate according to claim 14, wherein the corrosion protective layer has a thickness of up to 10μιτι, preferably 1-5 μηι.

16. Steel substrate according to claim 15, wherein the corrosion protective layer comprises one or more elements selected from the group consisting of Si, Al, P, S, Ca, V, Mg, O and Fe.

17. Steel substrate according to claim 15, wherein the steel substrate is a zinc or zinc alloy coated steel substrate.

18. Steel substrate according to claim 17, wherein the corrosion protective layer is crystalline and comprises Ca and Si as main components.

19. Steel substrate according to claim 17, wherein the corrosion protective layer is amorphous and comprises Zn and Si as main components.

20. Steel substrate according to claim 17, wherein the corrosion protective layer is amorphous and comprises Al, Si and Fe as main components.