METHOD FOR PRODUCTION OF FLAT STEEL PLATES

Abstract

 In a first aspect, the invention provides a method for the production of flat steel sheets, comprising the uncoiling of a coil of steel material, the flattening of the uncoiled steel material, the stretching of a segment of the steel material and the transverse cutting of said segment. In particular, a steel material is used with a silicon content of at most 0.60 %, preferably at most 0.50 %, a yield strength of minimum 235 N/mm2 and a tensile strength of minimum 340 N/mm2 . In a further aspect, the invention also provides a flat steel sheet, preferably obtained by applying the method.

Description

TECHNICAL DOMAIN

[0001] The invention relates to a method for the production of flat steel sheets, and to steel sheets as such. In particular, the invention relates to steel sheets appropriate for further operations such as plasma and laser cutting, cold forming, bending and/or punching.

STATE OF THE ART

[0002] Immediately after production, sheet steel is usually rolled into a so-called "coil", with several tens or hundreds of meters of sheet steel in rolled form. As known, the sheet steel will thereby take a certain curvature.

[0003] However, further production processes start from smaller, flat steel sheets. The process of "uncoiling" thus consists of unrolling such a coil, in which the unrolled sheet steel is flattened and subsequently, cut into smaller rectangular steel sheets. These metal sheets can then be used in for example further production processes such as laser and plasma cutting, cold forming, bending, punching, etc.

[0004] In particular, for these processes, stringent requirements apply as to the flatness. Indeed, such a laser or plasma cutting head moves over a very limited distance of the sheet surface. At the same time, the metal sheets may have almost no internal tension. This for avoiding that cut parts would spring back when cutting, involving safety risks and a risk of damaging the cutting head. Moreover, a deformation of the material has a negative impact on the quality of the final product.

[0005] All kinds of methods are known for the production of flat steel sheets. Important characteristics of such methods and resulting steel sheets comprise a low internal tension, a high resistance to wear, a regular and smooth surface, a high flatness, a high material strength and a good cold formability. The emphasis should also be on methods being cost-efficient.

[0006] CA 940431, for example, describes the uncoiling and cold flattening of sheet steel, between an upper and a lower set of rolls (i.e. in a so-called straightening rolling device). The rolls ensure that the sheet steel is deformed according to an alternating curvature; the sheet steel is thereby flattened. Subsequently, the flattened sheet steel can be cut according to predetermined dimensions. A cut and flattened metal sheet of specific length is obtained.

[0007] Furthermore, US 4 751 838 describes a device for flattening and stretching a segment of sheet steel. Thereto, use is made of two clamping frames which are moved apart with a sufficiently high strength, so that the clamped segment is pulled beyond its elasticity limit.

[0008] EP 3 150 294 describes a method and device for fabricating a flat steel sheet. In EP '294, the presence of a temper mill after unrolling the coil is required, for eliminating the internal tensions in the material.

[0009] CN 1 027 038 08 describes a method for the production of cold-rolled coils in which the steel has specific material characteristics. CN '808 does however not relate to the production of flat steel sheets and the associated problems.

[0010] Finally, BE 1 021 399 and EP 2 933 033 describe another production method for fabricating flat metal sheets. The sheets are amongst other things cold-rolled, by means of a so-called skin-pass roll. A disadvantage is that only residual tensions in the longitudinal direction are equalized. Moreover, the metal sheets are not treated over the complete thickness.

[0011] Still, the known methods and devices do not sufficiently meet the specific requirements as to the flatness, internal tensions, and formability for o.a. plasma cutting processes, laser cutting processes, cold formability, bending and punching.

[0012] Considering the above-mentioned, there is a need for an improved method for producing flat steel sheets. Thereby, special quality requirements apply, amongst other things as to the cold formability of the sheet material, no to very few imperfections at the sheet surface, as well as a minimum of internal tensions.

SUMMARY OF THE INVENTION

[0013] In a first aspect, the invention provides a method according to claim 1, for the production of flat steel sheets, comprising the uncoiling of a coil of steel material, the flattening of the uncoiled steel material, the stretching of a segment of the steel material and the transverse cutting of said segment. In particular, a steel material is used with a silicon content of at most 0.60 %, preferably at most 0.50 %. In a second aspect, the invention also provides a flat steel sheet with predetermined dimensions, preferably obtained by applying the method.

DESCRIPTION OF THE FIGURES

[0014] 

Fig. 1A-B, schematically show a device for realizing the present method, according to two possible embodiments.

Fig. 2 illustrates an uncoiling device according to a possible embodiment of the invention.

Fig. 3 illustrates a straightening rolling process according to a possible embodiment of the invention.

Fig. 4 illustrates a stretching process according to a possible embodiment of the invention.

DETAILED DESCRIPTION

[0015] In a first aspect, the present invention provides a method for the production of flat steel sheets, the method comprising the following steps: (i) uncoiling a coil of steel material, (ii) flattening the uncoiled steel material between an upper rolling set comprising a plurality of upper rolling bodies and a lower rolling set comprising a plurality of lower rolling bodies, (iii) stretching a segment of the steel material, beyond its yield point, and (iv) transversely cutting the stretched segment for the formation of at least one flat steel sheet with a predetermined size, which steel sheet has a yield strength of minimum 235 N/mm² and a tensile strength of minimum 340 N/mm². In particular, the steel material has a silicon content of at most 0.60 %, preferably at most 0.50 %.

[0016] Preferably, it is a steel material with a purity as high as possible. In particular, the silicon content is at most 0.60 %, preferably less than 0.60 %, more preferably less than 0.50%, more preferably less than 0.40%, and still more preferably less than 0.30%. Such a low silicon content makes the steel material extremely appropriate for subsequent operations such as laser cutting and plasma cutting.

[0017] In a possible embodiment, the steel material possesses a yield strength of approximately 235 N/mm², in which the silicon content is at most 0.030 %, and preferably lower than 0.030 %. Such steel material is extremely appropriate for subsequent operations such as laser cutting and plasma cutting. The invention is in no case limited thereto.

[0018] Preferably, the steel structure is fine-grained. The term 'fine-grained' means that the steel will show a fine grain structure at microscopic level. This term is well-known in the field.

[0019] The advantage of a fine-grained steel (for example in accordance to NBN EN_ISO_643:2020, to NEN-EN 10025-3, to NEN-EN 10025-4, or to NEN-EN 10149-2 - but not limited thereto) is that it has superior material characteristics which are very well reproducible. Fine-grained steel material has, for example, an excellent plastic formability, even with steel material with a higher yield strength (e.g. higher than 235 N/mm²). Of course, this also influences the stretching process when rolling, i.e. amongst other things the strength with which / the distance over which the segment should be stretched.

[0020] In general, there seems to be an important interaction between the coil production on the one hand (in particular the characteristics and the quality of the starting material) and the uncoiling process on the other hand. Both influence the final quality of the flat steel sheets to a considerable extent. The invention provides for an optimization of both in combination.

[0021] As to the coil production, special attention is paid to the chemical composition (as will be further described). By using new techniques, it is possible to alloy the steel material in a very precise way (so-called "micro alloys"). When producing the sheet steel, the rolling temperature and the arrangement of the rolling cylinders are also monitored and controlled meticulously. This offers a continuous band of steel material with high flatness and constant and reproducible material characteristics.

[0022] For uncoiling, the uncoil device is controlled accordingly. The stretching of the material, for example, is realized beyond the yield point. This depends, amongst other things, on the composition of the material. In any case, it is particularly important that the sheets are processed up to the core of the material (possibly the below-mentioned "middle part"). Preferably, the steel material is in any case processed over its complete thickness. Finally, a very flat steel sheet which is poor in internal tension, is obtained, which is extremely appropriate for further processes, such as punching, and such as laser and plasma cutting of long and small strips. These processes require high-quality steel sheets, with few to no internal tensions. At the same time, the stretching (and, generally, the uncoiling) barely influences the structure of the material. In a further or alternative embodiment, the steel material comprises at most 0.025 % of phosphor (optionally at most 0.020 % of phosphor) and at most 0.020 % of sulphur (optionally at most 0.015 % of sulphur). Such low contents of phosphor and sulphur ensure the steel has an excellent cold formability. Moreover, such a low content of phosphor, combined with the low content of silicon (≤0.60 %, preferably ≤0.50%, more preferably still lower) ensure the steel can be galvanized in a post-processing step. With steel material with a silicon content of 0.25 - 0.60 %, the galvanization process will however have to be adjusted accordingly.

[0023] In a further or alternative embodiment, the steel material has an aluminium content up to 0.065 %. However, the invention is not limited thereto.

[0024] According to a non-limiting embodiment, the steel material preferably has a fine-grained structure with a yield strength of approximately 235 N/mm². The material has a silicon content of at most approximately 0.030 %, a sulphur content of at most approximately 0.015 %, and a phosphor content of at most approximately 0.020 %. The invention is however not limited thereto.

[0025] Preferably, the used steel is hot-rolled steel.

[0026] By using steel material complying to the above-mentioned characteristics, on the one hand, a high-quality flat sheet is obtained, and the production process for obtaining such flat sheets is also simplified. Indeed, by using the steel with the above-mentioned characteristics, there is no need for providing a temper mill after uncoiling. A temper mill would normally be used for removing internal tension from the steel after uncoiling. In the method of the present invention, this is however not necessary, also because of the characteristics of the used steel.

[0027] In a further or alternative embodiment, after stretching, the steel material has a flatness of at most 3 mm/m, preferably at most 2 mm/m. Optionally, the steel sheet has such a structure (o.a. no internal tension) that a flatness of at most 3 mm/m can be guaranteed after further cutting processes (e.g. laser cutting). However, this is not necessarily the case. The inventors have found that, depending on amongst other things the steel material and the thickness of the steel, the steel material can or cannot be processed until its core when flattening it. The number of rolling bodies, and the diameter and positioning of the rolling bodies can influence this.

[0028] In the case in which, when flattening with the straightening rolling device, one or more zones of the material remain untreated, such untreated zones will be stretched when stretching. Potential flattening defects can thus be removed when stretching. The material is preferably treated over its complete thickness, beyond its yield point. In a possible embodiment, when flattening, an upper part and a lower part of the uncoiled steel material, which parts are situated along an upper side and a lower side, respectively, are successively stretched and compressed, preferably beyond its yield point. Said "upper part" and "lower part" should be understood as a zone in which the material is effectively treated, beyond the yield point. Optionally, the upper part and the lower part overlap. Consequently, the material is treated over the complete thickness, when flattening. As an alternative, a middle part is situated between the upper part and the lower part, in which middle part the material is not treated beyond its yield point.

[0029] In any case, the flattening leads to a substantially flat sheet segment in which, however, internal tension can still be present. Such internal tension can for example occur when the material is not treated till its core (see the above-mentioned "middle part").

[0030] In a further or alternative embodiment, when stretching, a middle part of the steel material, which middle part is situated between said upper and lower part, is still stretched beyond the yield point. As a result, substantially all the sheet material was cold formed once beyond its yield point. The resulting sheet material is substantially free of internal tensions.

[0031] In a preferred embodiment, one or more parameters linked to the steel will be measured when stretching by means of one or more sensors, as part of a control unit of the device realizing the present method. In particular, the measuring during stretching comprises amongst other things the monitoring of parameters such as the mechanical resistance of a segment of steel material when stretching it. Hereby, when stretching it, a mechanical resistance of said segment to said stretching is continuously measured, based on which it is derived whether the yield point of the steel material has been exceeded or not. This is amongst other things important when using purer types of steel than other material compositions (o.a. a very low silicon content) and/or material structures. By measuring the mechanical resistance when stretching, it can be determined when the yield point is reached.

[0032] When stretching, the process parameters (such as exerted forces) can be set in a predetermined way. It is important that the settings also depend on the type of starting material.

[0033] Moreover, the steps of (ii) flattening the steel material and (iii) stretching the steel material can be realized sequentially and/or simultaneously. In general, the invention is not limited to any order in which the flattening and stretching are realized.

[0034] In a further or alternative embodiment, the flattening and stretching are realized at a temperature situated between 0°C and 40°C, preferably at room temperature.

[0035] In a further or alternative embodiment, the step of uncoiling is realized mainly continuously. In a further or alternative embodiment, the step of stretching is realized discontinuously. The "mainly continuously" realization of the uncoiling implies that the uncoiling is not stopped for each steel sheet that has been finished. This, despite the fact that typically or inherently discontinuous subsequent processes are realized. Think about the stretching of the steel material, and the cutting of the steel material. An important advantage is that the steel sheets will have less markings. Such markings are undesirable. They can be created as a result of discontinuities in o.a. the straightening rolling process. According to a possible example, the uncoiling is continuously realized when flattening, stretching, and cutting at least two subsequent steel sheets.

[0036] In a further or alternative embodiment, when stretching said segment, a previous and adjoining segment of flattened steel material will increasingly sag. The latter allows to (mainly) continuously realize the uncoiling, while for example the stretching can be realized discontinuously.

[0037] Between two stretching movements, the stretching device preferably has a higher (or at least equal) passing speed than the straightening rolling device. Preferably, a continuous operation is obtained, in which the part, which is sagging, sags in a cyclic way to a larger and smaller extent. Preferably, the device thereto provides a looping pit positioned between the stretching device and the coil, and more preferably between and under the stretching device and the straightening roller installation. We refer to the non-limiting embodiment of Fig. 1B.

[0038] In a further or alternative embodiment, the surface of the steel material is also brushed after the flattening and/or stretching. This allows to remove the mill scale which has come loose when flattening and/or stretching.

[0039] In a further or alternative embodiment, the steel material is edge-trimmed. Hereby, the sides of the material are cut off and/or cut. Optionally, a rolling knife is used therefore. However, the invention is not limited thereto.

[0040] In a second aspect, the invention provides a flat steel sheet with predetermined dimensions, which steel sheet is obtained by applying the above-described method. The same characteristics and advantages can be repeated. Preferably, the steel sheet comprises a steel material with a silicon content of at most 0.60 %. Preferably, the steel sheet is essentially free of internal mechanical tensions.

[0041] In another aspect, the invention provides a device for the production of flat steel sheets. The device comprises an uncoiling device with means for supporting a coil, and is furthermore adapted for unrolling the sheet steel from the coil, preferably in a continuous way. Adjacently, the device provides a straightening rolling device with an upper rolling set (comprising a plurality of upper rolling bodies) and a lower rolling set (comprising a plurality of lower rolling bodies) as described above. Adjacently, the device also comprises a stretching device, adapted for engaging and stretching a segment of flattened steel material. Finally, the device also provides a cutting device with one or more cutting means, configured for cutting the sheet steel into flat steel sheets. Thereby, it should be noticed that the straightening rolling device and the stretching device can at least partially overlap. In general, the flattening and stretching of the steel material can be realized sequentially and/or simultaneously. Possibly, said device is controlled automatically from a control unit. In a further or alternative embodiment of the device, it provides a downward looping pit for receiving a part of the unrolled steel material which sags downwards. By providing a steel according to the above-mentioned characteristics, it is not necessary to provide a temper mill after the uncoiling device, contrary to for example a device described in EP 3 150 294. This is an advantage of the present invention.

[0042] In the following, the invention will be described by means of non-limiting examples and figures illustrating the invention, and not meant to be interpreted as limiting the scope of the invention.

[0043] Figure 1A shows a schematic illustration of a device for the production of flat steel sheets, in accordance with a possible embodiment of the invention.

[0044] The device 300 is adapted for receiving a steel coil 390, for producing a plurality of cut flat steel sheets 394 of predetermined dimensions. The device 300 comprises an unrolling means 310, as illustrated in figure 2, for uncoiling the steel coil 390 to a loose band 301 of steel material. The device 300 further comprises a straightening rolling device 340 and stretching device 320, configured for flattening and stretching successive segments 392 of the unrolled steel material 301. A possible embodiment of the straightening rolling device 340 has been shown in cross-section in figure 3. The stretching has been shown schematically in figure 4.

[0045] The unrolling means 310, as illustrated in figure 2, is preferably provided with supporting means 312 for supporting the coil 390.

[0046] Preferably, the straightening rolling device 340, as illustrated in figure 3, is adapted for flattening a band of steel material 301. The straightening rolling device 340 comprises a number of upper rolls 342 (also called upper rolling bodies) and lower rolls 344 (also called lower rolling bodies), adapted for collaborating and alternately bending the band of steel material 301 in an upward an downward direction. The upper and lower rolls 342, 344 extend in a transverse direction over the width of the band material 301. The straightening rolling device 340 is illustrated schematically in figure 3, with a focus on the operations applied to the band of steel material 301. Optionally, the straightening rolling device 340 can comprise further elements (e.g. so-called "back rolls"). The invention is however not limited thereto.

[0047] In a preferred embodiment, the rolls 342, 344 are spaced vertically and they are supported by a frame (not shown). Parts of the frame can be raised and lowered vertically, for adjusting the distance between the upper rolls 342 and the lower rolls 344. Moreover, the size of the distance can be regulated. Furthermore, as a result of this relative movement of the upper rolls 342 and the lower rolls 344, the sheet material 301 is bended alternately in an upward and downward direction when passing, and is thereby compressed or stretched, preferably beyond the yield point. As a result, the sheet material 301 is flattened.

[0048] Preferably, the stretching device 320 is provided with stretching means 321 capable of clamping a segment 392 at both sides. Preferably, these stretching means 321 are adapted for movement between a retracted position and an expanded position. In the expanded position, the distance between the stretching means 321 is increased. The clamped segment 392 thus stretches, preferably beyond its yield point.

[0049] The device 300 further also comprises one or more cutting means 350 which are adapted for cutting the flattened segment 392 at desired positions, for forming steel sheets 394 of predetermined dimensions. The segment is preferably cut where it has been gripped.

[0050] Furthermore, the device 300 also comprises driving means (not shown) for driving the rotational movement of the uncoiling means 310, from the movement of the segment of sheet material 392 through the straightening rolling device 340 and the stretching device 320, and for driving the stretching means 321, the different rolls 342, 344 and cutting means 350.

[0051] Optionally, further modules can be built into the device 300, such as a brush module or an edge-trimming module (e.g. with rolling knife). It is further referred to the description. However, such modules are not shown in the figures.

[0052] Figure 1B shows a schematic illustration of an alternative device 300 for the production of flat steel sheets 394, in accordance with another possible embodiment of the invention. Hereby, the device 300 comprises a looping pit 302 positioned between the straightening rolling device 340 and the stretching device 320, and preferably between and under the straightening rolling device 340 and the stretching device 320.

[0053] When stretching a segment of the steel material 392, such a looping pit 302 allows that further/previous segments 392' of the steel material sag increasingly. Preferably, a continuous operation is obtained, in which the sagging part sags in a cyclic way to a larger (dotted line) / smaller (full line) extent. This allows to (mainly) continuously realize the uncoiling, while for example the stretching can be realized discontinuously.

[0054] The device 300 can further comprise a control unit (not shown), for controlling its operation, and in particular for controlling the uncoiling means 310, the straightening rolling device 340 and the stretching device 320, including the rolls 342, 344 and cutting means 350. Furthermore, the control unit is particularly adapted for determining when the yield point of a segment 392 has been reached or exceeded, when stretching it.

[0055] In some embodiments, the control unit can comprise a number of sensors for monitoring parameters such as the mechanical resistance of a segment of steel material when stretching it.

[0056] The control unit can further comprise a processor unit for processing measuring data or input data, which have been introduced by the user, based on a predetermined logic. In this way, the production of flat steel sheets can be automated to a certain degree. The control unit can further comprise an instruction unit sending the instructions to different components, for obtaining a desired operation of the device.

[0057] In some embodiments, the control unit can be provided as a computer program product, on a computer-readable storage medium. Such a storage medium can be provided by any mechanism for storing information in the form (including a processing application or software), readable or interpretable by a machine (such as a computer).

[0058] The invention provides a method for fabricating steel sheets from steel coils, preferably from a hot-rolled process. The method ensures a complete flattening of the uncoiled material, where the internal tensions have been removed. Furthermore, the invention provides a steel sheet with preferably a fine-grained structure, and with all other desired mechanical characteristics as described above.

[0059] Furthermore, the present invention provides the possibility for producing the steel sheets in a simple way. The thus fabricated steel sheet can be used in a large number of different applications, as they are sufficiently flat and have few internal tensions.

[0060] The enumerated elements on the figures are:

300

device

301

band of steel material

302

looping pit

310

unrolling means

320

stretching device

321

stretching means

340

straightening roller

342

upper roll

344

lower roll

350

cutting means

390

steel coil

392

segment of the steel material

394

steel sheet

Example 1 : comparison with standard material

[0061] Preferably, the steel material as used in the method comprises a phosphor content of at most 0.020 % and a sulphur content of at most 0.015 %. Consequently, the resulting material possesses strongly improved stretching and bending characteristics, with respect to a corresponding standard material (see table). Indeed, the cold formability is much better.

Table 1 : minimal elongation and bending diameter at material thickness 4 mm, and at yield strength ≈ 235 N/mm2		Standard material	Material of the invention
Elongation (L0 = 5d0)	longitudinal	26 %	30 %
	transverse	24 %	30 %
bending diameter (bending test 180°)	longitudinal	4 mm	1.2 mm
	transverse	8 mm	1.2 mm

[0062] The structure of fine-grained steel allows both longitudinal and transverse bending with excellent results. The rolling direction is not determinative anymore when bending.

[0063] Thereby, said material in accordance with the invention has a fine-grained steel structure, a yield strength of approximately, but not less than 235 N/mm², and a tensile strength of approximately 340-490 N/mm². The material has a silicon content of at most 0.030 %, a sulphur content of at most 0.020 %, a phosphor content of at most 0.015 % and an aluminium content situated between 0.015 and 0.065 %. Furthermore, preferably, a choice is made as to the content of carbon, manganese, copper, titanium, and niobium.

[0064] It will be understood that the present invention is not limited to the embodiments described above and that some adjustments or changes can be added to the described examples without changing the scope of the enclosed claims.

Claims

1. A method for the production of flat steel sheets, the method comprising the following steps: (i) uncoiling a coil of steel material, (ii) flattening the uncoiled steel material between an upper rolling set comprising a plurality of upper rolling bodies and a lower rolling set comprising a plurality of lower rolling bodies, (iii) stretching a segment of the steel material, beyond its yield point, and (iv) transversely cutting the stretched segment for the formation of at least one flat steel sheet with a predetermined size, characterized in that the steel sheet and/or the steel material has a yield strength of minimum 235 N/mm² and a tensile strength of minimum 340 N/mm² and a silicon content of at most 0.60 %, preferably at most 0.50 %.

2. The method of claim 1, in which said silicon content is at most 0.030 %.

3. The method of any of the claims 1-2, in which the steel material comprises at most 0.025 %, preferably at most 0.020 % of phosphor and at most 0.020 %, preferably at most 0.015 % of sulphur.

4. The method of any one of the previous claims, in which the steel material relates to a constructional steel, with an aluminium content up to 0.065 %.

5. The method of any one of the previous claims, in which the steel material comprises a fine-grained structure.

6. The method of any one of the previous claims, characterised in that the steel material is hot-rolled steel.

7. The method of any one of the previous claims, in which, after stretching, the steel sheet has a flatness of at most 3 mm/m, preferably at most 2 mm/m.

8. The method of any one of the previous claims, in which, when flattening, an upper part and a lower part of the uncoiled steel material are successively stretched and compressed, beyond their yield point, which parts are situated along an upper side and a lower side, respectively.

9. The method of claim 8, in which, when stretching, a middle part of the steel material, situated between said upper and lower part, is stretched beyond its yield point.

10. The method of any one of the previous claims, in which, when stretching it, a mechanical resistance of said segment to said stretching is continuously measured, from which measurement it is derived whether the yield point of the steel material has been exceeded.

11. The method of any one of the previous claims, in which the step of uncoiling the coil is largely realized in a continuous way.

12. The method of any one of the previous claims, in which the step of stretching is realized discontinuously.

13. The method of any one of the previous claims, in which, when stretching said segment, a further segment of flattened steel material sags increasingly.

14. The method of any one of the previous claims, in which the surface of the steel material is brushed after flattening and/or stretching and the steel material is optionally edge-trimmed.

15. A flat steel sheet obtained by applying of the method of any one of the claims 1-14, which steel sheet is free from internal mechanical tensions.

Drawing