Method for combined pickling of a hot-rolled strip made of a steel material

Abstract

The invention relates to a method for pickling a hot-rolled strip made of a steel material according to claim 1.

Description

[0001] The invention relates to a method for pickling a hot strip made of a steel material, wherein the hot strip passes through at least two pickling baths.

[0002] The removal of surface defects, such as scale, during the production of hot-rolled steel strip is essential to ensure a virtually defect-free surface for good (further) processing in the production sequence and / or as a finished product. A nearly defect-free surface can be advantageous because it reduces the amount of flaps (flaps being residues or protrusions from hot rolling) and / or overroll from the hot-rolling stage, consequently minimizing surface defects in the mill-hardened or annealed and optionally coated hot-rolled strip. Depending on the specific design, particularly for the production of high-strength and ultra-high-strength steels, increased grain boundary oxidation may also occur alongside flaps or overroll on the surface of the hot-rolled strip.Surface defects produced during the hot strip stage can be minimized by long pickling, but can lead to a different type of defect in the form of a jagged surface due to excessively long pickling time.

[0003] A process for the electrolytic pickling of flat steel products is known from WO 2021 / 105738 A1. The key feature is that the pickling process uses alternating current, which allows for faster removal of scale from the surface of the flat steel product with a shorter pickling time compared to pickling with direct current.

[0004] The object of the present invention is to provide a pickling method with which a substantially defect-free surface of hot-rolled strip can be produced, in particular free of scale and free of damaged grain layers, such as grain boundary oxidation, inconsistencies, near-surface defects, etc., and / or with which trapped process gases on the surface can be reduced. Furthermore, compared to the prior art, surface-related problems such as grain boundary pickling, residual scale deposits, and highly wavy strips (topographic characteristics) can be avoided or at least reduced. In addition, a pickling method is to be provided by which the roughness can be adjusted to produce an optimally pickled hot-rolled strip for the subsequent cold rolling process and all further processes, such as coating, forming, painting, etc.

[0005] This problem is solved by a method having the features of claim 1. Further embodiments are described in the dependent claims.

[0006] The teaching relates to a process for pickling a hot strip made of a steel material, wherein the hot strip passes through at least two pickling baths, wherein the hot strip is pickled without current in at least a first pickling bath containing an acidic medium and in at least a second or further pickling bath containing an acidic medium with anodic polarization, following the last pickling bath with currentless pickling.

[0007] A very good result, aiming for a virtually defect-free surface, is achieved by first performing a conventional, electroless pickling process in at least one initial pickling bath containing an acidic medium. In this first pickling bath, the scale is essentially completely removed and / or abraded. Subsequent electrolytic pickling in at least one second or further, following the final electroless pickling bath, in an acidic pickling bath with anodic polarization (anodic pickling) allows for the removal of near-surface defects, particularly damaged grain layers and inconsistencies.

[0008] The invention results in a substantially defect-free and uniform surface of a pickled hot-rolled strip, such as the absence of residual scale, pickled grain boundaries, first grain layers with oxidized grain boundaries, inconsistencies, near-surface defects, trapped process gases / process media on the surface, over-pickling, and stagnation spots. These properties can be evaluated by macroscopic and microscopic metallographic analyses.

[0009] Before electroless (conventional) pickling, the surface of the hot-rolled strip can be mechanically treated. This means that the scale adhering to the surface is broken up by external mechanical action, allowing the acid to penetrate and thus remove the scale. Various methods exist for scale breaking, such as shot blasters and scale washers, which are considered state of the art and are widely implemented. Other technologies can also be used, either as alternatives or in addition to these methods.

[0010] The scale is then removed in the first two or only the first pickling tanks without current, through the action of acid and optionally through turbulence / flow (which varies depending on the pickling process). After the first / second pickling tank, most of the scale is already removed and is completely removed in at least one second or further pickling tank, which has been modified electrolytically (anodic polarization). Furthermore, the anodic polarization removes not only any remaining scale but also the first grain layers with oxidized grain boundaries.

[0011] The hot strip passes through at least two pickling baths. In the simplest configuration, it first passes through a non-electric pickling bath, followed by an anodic pickling bath. In further configurations, the hot strip can also pass through one, two, three, or more non-electric pickling baths sequentially. In another configuration, the hot strip can then pass through one, two, three, or more anodic pickling baths sequentially after the last non-electric pickling bath. Therefore, in further configurations, the hot strip can be pickled in the following sequences: 1 x non-electric + 2 x anodic; 1 x non-electric + 3 x anodic; 2 x non-electric + 1 x anodic; 3 x non-electric + 1 x anodic; 2 x non-electric + 2 x anodic. 3 x unpowered + 2 x anodic and other analog combinations.Further pickling baths, in particular non-electric or electrically charged baths, for example alkaline baths, can be used. The at least one non-electric pickling bath and the at least one anodic pickling bath can preferably be placed last, or second to last if a rinsing step with an optional drying step is provided, in the sequence of a hot strip passing through several baths.

[0012] The surface properties achieved by the described pickling process improve forming properties, prevent flaps in subsequent processes such as cold rolling (cold strip), prevent liquid metal embrittlement (LME), improve coating properties / more uniform distribution, and reduce roughness, among other benefits. In other words, the surface of the hot strip can be conditioned by the pickling process according to the invention in such a way that process and product advantages result, in particular improved fatigue strength or service life / lifespan. The material removal is controlled by the strength of the applied current. Adjusting the roughness is therefore also possible. Crucial for the material removal and the optional adjustment of the roughness are current density, time and / or strip speed, as well as sufficient electrical conductivity, independent of the composition and pH value, which are familiar to those skilled in the art.

[0013] The hot-rolled strip, formed from a steel material, is a flat steel product hot-rolled in a planar form. The invention is particularly suitable for high-strength, high-strength, and ultra-high-strength steels, which, among other things, have a high tendency to undergo low-molecular-weight metal formation (LMM), thus enabling an improvement in performance. Preferably, a hot-rolled strip is used for multiphase steels intended for the production of components by cold forming, with a tensile strength of at least 500 MPa, in particular at least 700 MPa, preferably at least 950 MPa and more, for example up to 1800 MPa, in particular up to 1600 MPa, and preferably up to 1400 MPa. The invention is particularly suitable for so-called and known DP, CP, and Q&P steels with tensile strengths between 980 and 1400 MPa. Furthermore, and alternatively, other grades can also be produced from the hot-rolled strip using suitable chemicals, in particular with tensile strengths below 500 MPa.

[0014] The steel material is a carbon steel.

[0015] The steel material may contain or consist of iron and unavoidable impurities, in wt.%: C: 0.001 to 0.9; Mn: 0.05 to 12.0; Si: 0.001 to 5.0; N: max. 0.1; S: max. 0.1; P: max. 0.5; optionally one or more of the following elements: Al: max. 3.0; Mo: max. 1.0; Ni: max. 1.0; Cr: max. 4.5; B: max. 0.01; Ca: max. 0.01; Nb: max. 0.5; Ti: max. 0.5; V: max. 0.5; W: max. 0.5; Cu: max. 1.0; Co: max. 0.5; Sn: max. 0.5; As: max. 0.2; REM: max. 0.5.

[0016] According to one embodiment, the hot-rolled strip can pass through at least one further electroless pickling bath, which is arranged between the first electroless pickling bath and the second anodic pickling bath, and in which the hot-rolled strip is pickled with an acidic medium. This allows the essentially complete removal of scale to be distributed across at least two electroless pickling baths, thereby enabling, for example, an increase in strip speeds.

[0017] According to a further or alternative embodiment, the hot-rolled strip can pass through at least one additional anodic pickling bath, which is arranged after the second anodic pickling bath and in which the hot-rolled strip is pickled with an acidic medium. This allows the essentially complete removal of near-surface defects, in particular damaged grain layers and irregularities, as well as the option to adjust the roughness, to be distributed across at least two pickling baths, thereby enabling, for example, an increase in strip speeds.

[0018] According to one embodiment, the acidic medium in the first electroless pickling bath, the second anodic pickling bath, the optional further electroless pickling bath, and the optional further anodic pickling bath can comprise an aqueous solution of an organic or inorganic acid selected from the group consisting of or containing: hydrochloric acid, phosphorous acid, phosphoric acid, perchloric acid, hypochlorous acid, nitrous acid, nitric acid, hydrofluoric acid, sulfurous acid, sulfuric acid, or a mixture of two or more of these acids, used as an aqueous solution. The inorganic acid can contain, individually or in total, a concentration between 50 and 600 g / l, the remainder being water and unavoidable impurities. The concentration can be, in particular, at least 80 g / l, preferably at least 100 g / l, and especially at most 550 g / l, preferably at most 500 g / l.

[0019] According to one embodiment, anodic polarization can be carried out with a current density between 10 and 200 A / dm². Higher current densities offer the possibility, among other things, of accelerating the pickling process compared to conventional, currentless pickling. For example, anodic polarization can be carried out with a current density between 15 and 290 A / dm², in particular between 20 and 150 A / dm², preferably between 30 and 120 A / dm², and more preferably between 30 and 100 A / dm². In a preferred embodiment, the current density in the second anodic pickling bath can be set higher compared to the optional additional anodic pickling bath.This allows, for example, a higher current density in the second anodic pickling bath, which can be selected to be greater than 50 A / dm², to achieve a higher pickling removal rate compared to a lower current density in the optional further anodic pickling bath, which can preferably be selected to be less than or equal to 50 A / dm², in order to achieve homogenization of the surface of the hot strip, in particular to achieve an electropolishing effect and to prevent oxidized grain boundaries that have not yet been removed from being reopened by subsequent processes (e.g. cold rolling, annealing and coating).

[0020] According to one embodiment, the pickling time, which corresponds to the dwell / immersion time of the hot strip during pickling, can be between 1 and 100 s in the first electroless pickling bath and in the optional second electroless pickling bath. In particular, the pickling time can be at least 2 s, preferably at least 3 s, preferably at least 5 s, and in particular a maximum of 80 s, preferably a maximum of 60 s, and more preferably a maximum of 50 s. The first electroless pickling bath and the optional second electroless pickling bath can have a temperature between 65 and 100 °C, in particular between 70 and 95 °C, and preferably between 75 and 90 °C.

[0021] According to one embodiment, the pickling time, which corresponds to the residence / immersion time of the hot strip during pickling, can be between 1 and 100 s in the second anodic pickling bath and in the optional further anodic pickling bath. In particular, the pickling time can be at least 2 s, preferably at least 3 s, preferably at least 5 s, and in particular a maximum of 80 s, preferably a maximum of 60 s, and more preferably a maximum of 50 s. The temperature of the second anodic pickling bath and the optional further anodic pickling bath can be lower than the temperature of the first electroless pickling bath and optionally the further electroless pickling bath, for example between 30 and 75 °C, in particular between 40 and 70 °C, and preferably between 45 and 65 °C.

[0022] According to one embodiment, the acidic medium in each pickling bath can contain sulfuric acid with a concentration between 100 and 400 g / l, the remainder being water and unavoidable impurities. The concentration can be, in particular, at least 130 g / l, preferably at least 150 g / l.

[0023] Since the entire near-surface chemistry is affected by the combined currentless (conventional) and anodic pickling, material is removed down to the base material, whereby the grain layers damaged by internal oxidation can be essentially completely removed.

[0024] Rinsing with water and / or an aqueous solution can be carried out between the individual pickling baths. This rinsing with water and / or an alcohol, for example selected from the group containing or consisting of methanol, ethanol, propanol, isopropanol, ethanol, in particular isopropanol, or an aqueous solution, can be interrupted. Alternatively, the rinsing is carried out in two steps: a first step with water, and a second step with an alcohol or an aqueous solution of an alcohol as specified above. Another alternative is the rinsing with water and an alcohol in a single step, preferably as a mixture of water with one of the alcohols specified above. Rinsing is preferably carried out continuously, and in particular a method selected from the group consisting of or consisting of spraying, spraying, coil coating, and preferably immersion can be used.Preferably, drying is carried out after rinsing, whereby the "rinsed" surface is preferably dried by increasing the temperature (up to a maximum of 100 °C) or, for example, by using a blower.

[0025] Alternatively, the "rinsed" surface can be air-dried, for example, especially without any further aids.

[0026] Specific embodiments of the invention are explained in detail below.

[0027] Several samples were taken from a hot-rolled strip of Q&P quality, specifically a Q&P 980, and these were pickled differently.

[0028] A series of samples (1 to 14) were pickled by, for example, successive immersion or rinsing with an electrolyte. All electrolytes contained sulfuric acid at a concentration of 250 g / l, with the remainder being water and unavoidable impurities. Key parameters such as pickling time in seconds, temperature in °C, and current density (anodic) in A / dm² are listed in Table 1 below. Table 1 Probe S1 Beizzeit S1 Temp. S2 Beizzeit S2 Temp. A1 Beizzeit A1 Temp. A1 Stromd. A2 Beizzeit A2 Temp. A2 Stromd. E 1 30 75 - - 30 50 40 - - - x 2 30 80 - - 30 55 60 - - - x 3 15 80 15 80 30 60 80 - - - x 4 40 70 - - 20 55 60 20 55 60 x 5 20 90 20 90 20 60 70 20 60 50 x 6 20 85 20 85 40 50 40 - - - x 7 15 75 15 75 30 55 60 - - - x 8 15 80 15 80 15 65 60 15 65 40 x 9 10 70 10 70 10 55 55 10 55 40 x 10 45 80 - - - - - - - - - 11 30 80 30 80 - - - - - - - 12 75 85 - - - - - - - - -

[0029] After pickling, the samples were immersed in water at approximately 20 °C (room temperature) for 2 to 4 seconds. The rinsed samples were then dried with a warm air blower for 30 seconds.

[0030] A pickling result of the differently pickled samples is in the Figures 1 to 3The images on the left show a conventionally pickled sample, and on the right a sample pickled according to the invention. It is clearly evident that the samples pickled according to the invention yield a better pickling result. Figure 1 shows SEM images of partial cross-sections of samples 10 and 1. Figures 2 and 3 Reflected light microscope images at different resolutions of a partial surface of samples 12 and 4 and 11 and 8.

[0031] Figure 4 The diagram schematically shows four examples of the arrangement or series connection of pickling baths (S1, S2, A1, A2) in which hot strips (W) can be pickled. The hot strip (W) can pass through from top to bottom: a first electric-free pickling bath (S1) and a second anodic pickling bath (A1); a first electric-free pickling bath (S1), a further electric-free pickling bath (S2) and a second anodic pickling bath (A1); a first electric-free pickling bath (S1), a second anodic pickling bath (A1) and a further anodic pickling bath (A2); a first electric pickling bath (S1), a second electric pickling bath (S2), a second anodic pickling bath (A1) and a second anodic pickling bath (A2).

Claims

1. Method for pickling a hot-rolled strip (W) formed from a steel material, wherein the hot-rolled strip (W) passes through at least two pickling baths (S1, S2, A1, A2), wherein the steel material is a carbon steel, characterized by the fact that The hot strip (W) is pickled without current in at least one first pickling bath (S1) containing an acidic medium and in at least one second or further pickling bath (A1) containing an acidic medium with anodic polarization, following the last pickling bath (S1, S2) with currentless pickling.

2. Method according to claim 1, wherein the hot strip (W) passes through at least one further electroless pickling bath (S2) which is arranged between the first electroless pickling bath (S1) and the second anodic pickling bath (A1) and in which the hot strip (W) is pickled with an acidic medium.

3. Method according to one of the preceding claims, wherein the hot strip (W) passes through at least one further anodic pickling bath (A2) which is arranged after the second anodic pickling bath (A1) and in which the hot strip (W) is pickled with an acidic medium.

4. A method according to any of the preceding claims, wherein the acidic medium in the first electroless pickling bath (S1), in the second anodic pickling bath (A1), in the optional further electroless pickling bath (S2) and in the optional further anodic pickling bath (A2) comprises an aqueous solution of an inorganic acid selected from the group consisting of or comprising: hydrochloric acid, phosphorous acid, phosphoric acid, perchloric acid, nitrous acid, nitric acid, hydrofluoric acid, sulfurous acid, sulfuric acid or a mixture of 2 or more of these acids as an aqueous solution.

5. Method according to one of the preceding claims, wherein the anodic polarization in the second anodic pickling bath (A1) and in the optional further anodic pickling bath (A2) is carried out with a current density between 10 and 200 A / dm². 2 is carried out.

6. Method according to claim 5, wherein the current density in the second anodic pickling bath (A1) is set higher compared to the optional further anodic pickling bath (A2).

7. Method according to one of the preceding claims, wherein the pickling time in the first electroless pickling bath and in the optional further electroless pickling bath (S2) is between 1 and 100 s, wherein the pickling bath (S1, S2) has a temperature between 65 and 100 °C.

8. Method according to one of the preceding claims, wherein the pickling time in the second anodic pickling bath (A1) and in the optional further anodic pickling bath (A2) is between 1 and 100 s, wherein the second anodic pickling bath (A1) and the optional further anodic pickling bath (A2) have a temperature which is lower than the temperature of the first electroless pickling bath (S1) and optionally of the further electroless pickling bath (S2).

9. Method according to one of the preceding claims, wherein the acidic medium in all pickling baths (S1, S2, A1, A2) contains sulfuric acid with a concentration between 100 and 400 g / l, the remainder being water and unavoidable impurities.

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