A PROCESS FOR MANUFACTURING COMPACT COILS OF ULTRA-FINE GRAINED, MARTENSITE-FREE STEEL BARS

Abstract

A process for manufacturing compact coils of ultra-fine grained, martensite-free steel bars, the process comprising the following stages:

a) rolling a steel billet by means of a roughing rolling mill producing a steel bar;

b) performing at least one first cooling stage so that the steel bar has a surface temperature higher than the martensite start temperature, and performing at least one first equalization stage in air;

c) rolling the steel bar by means of at least one intermediate rolling mill;

d) performing at least one second cooling stage always maintaining the surface temperature higher than the martensite start temperature, and performing at least one second equalization stage in air;

e) rolling the steel bar by means of a finishing rolling mill in a non-recrystallization temperature range, maintaining the whole cross-section of the steel bar within said non-recrystallization temperature range, and with a total reduction between 25 and 50% with respect to the cross-section of the steel bar at the entry of the finishing rolling mill, in order to obtain an ultra-fine-grained austenitic matrix;

f) winding the steel bar in a compact coil, by means at least one spooling device, so that the ultra-fine-grained austenitic matrix transforms in a mixture of ferrite and pearlite.

After the winding operation is completed, the compact coil can be transferred to a storage area through a transferring device, for example a walking beam, where a natural or forced or retarded cooling is applied to the coil.

Claims

1. A process for manufacturing compact coils of ultra-fine grained, martensite-free steel bars, the process comprising the following stages: a) rolling a steel billet with an initial surface temperature of 850-1200° C. by means of a roughing rolling mill, producing a steel bar; wherein the steel billet is a billet of low or medium carbon steel preferably comprising, in weight percentage, carbon lower than or equal to 0,28%, silicon lower than or equal to 0,80%, manganese lower than or equal to 1,60%, the remaining being iron and unavoidable or possible impurities; b) performing at least one first cooling stage so that the steel bar has a surface temperature higher than the martensite start temperature (Ms), and performing at least one first equalization stage in air to minimize a difference of temperature between core and surface of the steel bar until reaching the surface temperature in a range of 850-920° C.; c) rolling the steel bar by means of at least one intermediate rolling mill; d) performing at least one second cooling stage always maintaining the surface temperature higher than the martensite start temperature (Ms), and performing at least one second equalization stage in air to minimize the difference of temperature between core and surface of the steel bar until reaching the surface temperature in a range of 700-900° C.; e) rolling the steel bar by means of a finishing rolling mill in a non-recrystallization temperature range, maintaining a whole cross-section of the steel bar within said non-recrystallization temperature range, and with a total reduction between 25 and 50% with respect to a cross-section of the steel bar at an entry of the finishing rolling mill, in order to obtain an ultra-fine-grained austenitic matrix; f) winding the steel bar in a compact coil, by means at least one spooling device, at a winding temperature in a range of 500-800° C. so that the ultra-fine-grained austenitic matrix transforms in a mixture of ferrite and pearlite.

2. The process according to claim 1, wherein in step d) there are provided at least two second cooling stages and one second equalization stage in air is provided both between the at least two second cooling stages and between the last second cooling stage and the finishing rolling mill.

3. The process according to claim 1, wherein in step b) there are provided at least two first cooling stages and one first equalization stage in air is provided both between the at least two first cooling stages and between the last first cooling stage and the at least one intermediate rolling mill.

4. The process according to claim 1, wherein, between step e) and step f), there are provided at least one third cooling stage and at least one third equalization stage in air to minimize the difference of temperature between core and surface of the steel bar until reaching said winding temperature.

5. The process according to claim 4, wherein there are provided at least two third cooling stages and one third equalization stage in air is provided both between the at least two third cooling devices stages and between the last third cooling stage and the at least one spooling device; preferably wherein there are provided third cooling stages in a number comprised from two to six.

6. The process according to claim 1, wherein the at least one first cooling stage is carried out by means of a respective first cooling device, and the at least one second cooling stage is carried out by means of a respective second cooling device; preferably wherein a work pressure of first and second cooling devices is in the range of 0,2-0,6 MPa.

7. The process according to claim 1, wherein in step e) a number of finishing rolling passes is lower than or equal to four; preferably wherein in step f) first and last coil layers are wounded at 20-50° C. hotter than the rest of the coil layers.

8. The process according to claim 1, wherein surface temperatures of the steel bar are monitored by means of sensors, installed both at entry and exit of each of roughing rolling mill, intermediate rolling mill and finishing rolling mill, and operative parameters of said at least one first cooling stage and said at least one second cooling stage are managed through a closed-loop automatic control, operating in both feedforward and feedback control, based on readings of the sensors.

9. (canceled)

10. The process according to claim 1, wherein after step f) the compact coil is transferred to a storage area through a transferring device along which a natural or forced or retarded cooling is applied to the compact coil; preferably wherein a surface temperature of the coil when loaded on the transferring device is in the range of 600-700° C.

11. The process according to claim 10, wherein, after a cooling to room temperature in the storage area, the compact coil is unwound and straightened and then, preferably, a natural ageing of the steel bar is performed at room temperature.

12. The process according to claim 1, wherein the steel billet enters the roughing rolling mill coming from either a reheating furnace or directly from a continuous casting machine.

13. The process according to claim 1, wherein said low or medium carbon steel consists of, in weight percentage, carbon lower than or equal to 0,28%, silicon lower than or equal to 0,80%, manganese lower than or equal to 1,60%, the remaining being iron and unavoidable or possible impurities; preferably wherein the low or medium carbon steel comprises or consists of, in weight percentage, carbon in a range of 0,20-0,25%, silicon in a range of 0,20-0,70%, manganese in a range of 0,80-1,30%, possible vanadium in a range of 0,020-0,050%, the remaining being iron and unavoidable or possible impurities.

14. Coil of a steel bar, produced with a process according to claim 1, having a microstructure with an actual grain size equal to or higher than 9 according to standard ASTM E112, and wherein a difference of hardness (HV) measured between surface and core of the steel bar is less or equal than 40 HV, preferably in the range 10-40 HV.

15. The process according to claim 4, wherein the at least one first cooling stage (2) is carried out by means of a respective first cooling device, and the at least one second cooling stage (4) is carried out by means of a respective second cooling device; preferably wherein a work pressure of first and second cooling devices is in the range of 0,2-0.6 MPa; wherein the at least one third cooling stage is carried out by means of a respective third cooling device; and preferably wherein a work pressure of the third cooling device is in the range of 0,2-0.6 MPa.

16. The process according to claim 4, wherein surface temperatures of the steel bar are monitored by means of sensors, installed both at entry and exit of each of roughing rolling mill, intermediate rolling mill and finishing rolling mill, and operative parameters of said at least one first cooling stage and said at least one second cooling stage are managed through a closed-loop automatic control, operating in both feedforward and feedback control, based on readings of the sensors; and wherein also operative parameters of said at least one third cooling stage are managed through said closed-loop automatic control.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Reference is made in the description of the invention to the appended drawing tables, which are given by way of non-limiting examples, wherein:

[0042] FIG. 1 shows a first schematic layout of a plant for Quenching Tempering in Spooler according to the prior art;

[0043] FIG. 2 shows a second schematic layout of a plant for Soft Quenching according to the prior art;

[0044] FIG. 3 shows a first embodiment of a plant on which the process of the invention is performed;

[0045] FIG. 4 shows a second embodiment of a plant on which the process of the invention is performed;

[0046] FIG. 5 shows a third embodiment of a plant on which the process of the invention is performed;

[0047] FIG. 6 shows a fourth embodiment of a plant on which the process of the invention is performed;

[0048] FIG. 7 shows a schematic Fe-C diagram, with highlighted the Carbon and temperature range in which thermomechanical rolling is applicable;

[0049] FIG. 8 shows the cooling curves (surface temperature, average temperature and core temperature) of steel bars subjected to a known thermal treatment along the layout of FIG. 1;

[0050] FIG. 9 shows the cooling curves (surface temperature, average temperature and core temperature) of steel bars subjected to a thermal treatment according to the invention along the layout of FIG. 3;

[0051] FIG. 10 shows the cooling curves (surface temperature, average temperature and core temperature) of steel bars subjected to a thermal treatment according to the invention along the layout of FIG. 4;

[0052] FIG. 11 shows the cooling curves (surface temperature, average temperature and core temperature) of steel bars subjected to a thermal treatment according to the invention along the layout of FIG. 5;

[0053] FIG. 12 shows the cooling curves (surface temperature, average temperature and core temperature) of steel bars subjected to a thermal treatment according to the invention along the layout of FIG. 6.

DETAILED DESCRIPTION OF SOME ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

[0054] The present invention relates to a process for manufacturing compact coils of steel bars that allows production of ultra-fine grained, martensite-free and high ductility grades of spooled steel bars without addition of, or minimizing, microalloying elements with lower production costs.

[0055] In this description, the term “compact coil” means a coil having a filling coefficient higher than or equal to 65%, preferably in the range 65-74%, said filling coefficient being defined, considering the volume of the coil, as the ratio between density of the coil and theoretical density of the steel.

[0056] The term “ultra-fine grained”, instead, means that the microstructure has an average grain size equal to or higher than 9 according to standard ASTM E112.

[0057] The steel bar of the coil produced by means the process of the invention can have a size (i.e. diameter) preferably in the range of 8-40 mm.

[0058] The coil weight is in the range 1,0-10,0 tons, preferably in the range from 2,0 to 8,0 tons.

[0059] FIGS. 3-6 show some embodiments of plant layout in which the process of the invention can be performed.

[0060] In all embodiments of the invention, the process for manufacturing compact coils of ultra-fine grained, martensite-free steel bars comprises the following steps:

[0061] a) rolling a steel billet with an initial surface temperature of 850-1200° C., preferably 900-1100° C., by means of a roughing rolling mill 1, producing a steel bar;

[0062] b) performing at least one first cooling stage 2 so that the steel bar has a surface temperature higher than the martensite start temperature Ms, and performing at least one first equalization stage in air to minimize the difference of temperature between core and surface of the steel bar until reaching the surface temperature in a range of 850-920° C.;

[0063] c) rolling the steel bar by means of at least one intermediate rolling mill 3, for example only one intermediate rolling mill;

[0064] d) performing at least one second cooling stage 4 always maintaining the surface temperature higher than the martensite start temperature Ms, and performing at least one second equalization stage in air to minimize the difference of temperature between core and surface of the steel bar until reaching the surface temperature in a range of 700-900° C., preferably of 750-840° C. or 750-820° C.;

[0065] e) rolling the steel bar by means of a finishing rolling mill 5 in a non-recrystallization temperature range, maintaining the whole cross-section of the steel bar within said non-recrystallization temperature range, and with a total reduction between 25 and 50% with respect to the cross-section of the steel bar at the entry of the finishing rolling mill 5, in order to obtain an ultra-fine-grained austenitic matrix;

[0066] f) winding the steel bar in a compact coil, by means at least one spooling device 7, for example two spooling devices 7, at a winding temperature in a range of 500-800° C., preferably 500-750 or 650-730° C., so that the ultra-fine-grained austenitic matrix transforms in a mixture of ferrite and pearlite.

[0067] The process of the invention can be performed according to the above mentioned steps, in the specific case of a low/medium carbon steel, in a plant with a throughput of 90-120 t/h.

[0068] The steel billet of step a) enters the roughing rolling mill 1 coming from either a reheating furnace, for example a gas furnace or an induction heater, or directly from a continuous casting machine (not shown). The surface temperature of the steel bar at the entry of the first group of rolling stands, i.e. the roughing rolling mill 1, is in the range 850-1200° C., preferably 900-1100° C.

[0069] Preferably, the steel billet is a billet of low or medium carbon steel.

[0070] Said low/medium carbon steel comprises or consists of, in weight percentage, carbon lower than or equal to 0,28%, silicon lower than or equal to 0,80%, manganese lower than or equal to 1,60%, the remaining being iron and unavoidable or possible impurities.

[0071] Preferably, the low/medium carbon steel comprises or consists of, in weight percentage, carbon in a range of 0,20-0,25%, silicon in a range of 0,20-0,70%, manganese in a range of 0,80-1,30%, possible vanadium in a range of 0,020-0,050%, the remaining being iron and unavoidable or possible impurities.

[0072] Two non-limiting examples of the steel composition are disclosed in the following table for a steel bar having a size (diameter) of 8-40 mm.

TABLE-US-00001 Size Grade (mm) C (%) Si (%) Mn (%) V (%) HRBF400E 8-40 0.20-0.24 0.20-0.60 0.80-1.20 — HRBF500E 8-40 0.20-0.24 0.30-0.70 0.90-1.30 0.020-0.050

[0073] In an embodiment of the process, after the roughing rolling mill 1, the steel bar is cooled by means of at least one first cooling stage 2 so that the surface thereof does not reach the martensite start temperature (Ms), that can be calculated as per the formula

Ms(° C.)=512−453*C−16,9*Ni+15*Cr−9,5*Mo+217*C.sup.2−71,5*(C*Mn)−67,6*(C*Cr),

[0074] or, simply, Ms(° C.)=512−453*C+217*C.sup.2−71,5*(C*Mn).

[0075] An air equalization space is provided between said at least one first cooling stage 2 and the following intermediate rolling mill 3.

[0076] In a variant there is provided only one first cooling stage 2 (as shown in the FIGS. 3-6), and only one first equalization stage in air is provided between the first cooling stage 2 and the intermediate rolling mill 3. Alternatively, there are provided at least two first cooling stages 2, and one first equalization stage in air is provided both between the at least two first cooling stages 2 and between the last first cooling stage 2 and the intermediate rolling mill 3. For example, there are provided two first cooling stages 2, and a respective first equalization stage in air is provided both between the two subsequent first cooling stages 2 and between the last first cooling stage 2 and the intermediate rolling mill 3.

[0077] At least two second cooling stages 4 are provided after the intermediate rolling mill 3 for a higher bar surface temperature reduction, but always maintaining the surface temperature above Ms. One second equalization stage in air is provided both between the at least two second cooling stages 4 and between the last second cooling stage 4 and the finishing rolling mill 5. In said step d) no microstructural modification occurs, and both the surface and the core of the bar remain completely in the austenitic phase.

[0078] Preferably there are provided two or three second cooling stages 4, and a respective second equalization stage in air is provided between two subsequent second cooling stages 4 and between the last second cooling stage 4 and the finishing rolling mill 5. Therefore, if two second cooling stages 4 are provided there will be two second equalization stages in air. Instead, if three second cooling stages 4 are provided there will be three second equalization stages in air.

[0079] Optionally, the equalization spaces between two subsequent second cooling stages 4 can vary from 8 to 25 m, according to the plant throughput; instead, the equalization space between the last second cooling stage and the following finishing rolling mill 5 can vary between 25 and 50 m according to the plant throughput.

[0080] Preferably, the cooling-equalizing-intermediate rolling stages can be repeated multiple times, and with a variable number of second cooling stages 4 according to the plant throughput to reach the desired bar surface temperature at the entry of the finishing rolling mill 5. In this case more than one intermediate rolling mill 3 is provided. The additional intermediate rolling mills 3 are provided between two respective subsequent second cooling stages 4, in particular between the equalization space, provided after a cooling stage 4, and the subsequent cooling stage 4.

[0081] Thanks to the cooling applied in the second cooling stages 4, the surface temperature is gradually reduced until reaching the range 700-900° C., preferably of 750-840° C. or 750-820° C., at the entry of the finishing rolling mill 5.

[0082] During all the finishing rolling passes, the bar surface temperature is maintained inside the non-recrystallization range (see for example FIG. 7), for example 750-850° C. or 750-840° C. or 750-820° C. This means that the austenitic grain size is reduced applying a high reduction (25-50% total reduction on the finishing stands group), and the recrystallization and growth of austenite are suppressed by the lack of available thermal energy. FIG. 7, in particular, show a schematic Fe-C diagram, with highlighted the Carbon and temperature ranges (zone C) in which thermomechanical rolling without recrystallization is applicable.

[0083] Advantageously, the number of finishing rolling passes should be lower than or equal to four. A higher number of rolling passes may give temperature growth inside the rolled bar, that can jeopardize the microstructural process.

[0084] At the exit of the finishing rolling mill 5, as a result of the austenitic grain size refinement and of the subsequent possible controlled cooling, the final grain size is ultra-fine, resulting in values equal to or above 9, as per standard ASTM E112.

[0085] The absence of fragile phases, such as martensite and bainite, has been certified by the reduced difference of hardness (HV, preferably HV 0,5, i.e. the Vickers hardness measured with load of 4.903 N) measured between surface and core of the steel bar. Such difference is advantageously less than or equal to 40 HV, preferably in the range 10-40 HV.

[0086] Preferably, between the finishing rolling step e) and the winding step f), there are provided at least one third cooling stage 6 and at least one third equalization stage in air to minimize the difference of temperature between core and surface of the steel bar, always avoiding martensite formation, until reaching the predetermined winding temperature.

[0087] In a variant, there are provided at least two third cooling stages 6, and one third equalization stage in air is provided both between the at least two third cooling devices stages 6 and between the last third cooling stage 6 and the at least one spooling device 7.

[0088] Anyway, the one or more third cooling stages 6 are optional. These cooling stages 6 can be avoided if the bar surface temperature, coming out from the finishing rolling mill 5, is appropriate for the winding operation.

[0089] Preferably, when provided, the third cooling stages 6 are in number comprised from two to six.

[0090] The number and distance between two subsequent cooling stages 6 depend on the plant throughput. Said distance can be always equal (as shown in FIGS. 3-6) or different.

[0091] One or more cooling stages 6 can be used to obtain different winding temperatures along the same steel bar in order to uniform the cooling profile of different coil layers and limit as much as possible the spread of mechanical properties. Optionally, in the winding step first and last coil layers are wound at 20-50° C. warmer than the rest of the coil layers. The reference temperature range for the winding operation is 500-800° C., preferably 650-730° C., including the higher temperatures for the first and last coil layers.

[0092] In a first embodiment, shown in FIG. 3, no cooling stage 6 is provided. Only one cooling stage 2 and two cooling stages 4 are provided.

[0093] In a second embodiment, shown in FIG. 4, three cooling stages 6 are provided.

[0094] In a third embodiment, shown in FIG. 5, five cooling stages 6 are provided.

[0095] In a fourth embodiment, shown in FIG. 6, six cooling stages 6 are provided.

[0096] In these embodiments of FIGS. 3-6 only one cooling stage 2 and two (alternatively three) cooling stages 4 are provided.

[0097] FIGS. 9, 10, 11 and 12 show cooling curves 20, 21, 22 (surface temperature, average temperature and core temperature) of steel bars subjected to a thermal treatment according to the invention along the layout of FIGS. 3, 4, 5 and 6, respectively.

[0098] The horizontal dotted line represents a martensite start temperature Ms at about 500° C.

[0099] In all the steps of the process of the invention the surface temperature of the steel bar is always maintained above said martensite start temperature Ms, differently from the cooling curves of steel bars (see FIG. 8) subjected to a QTS thermal treatment along the layout of FIG. 1.

[0100] Preferably, at least one first cooling stage 2 is carried out by means of a respective first cooling device, at least one second cooling stage 4 is carried out by means of a respective second cooling device, and at least one possible third cooling stage 6 is carried out by means of a respective third cooling device.

[0101] As an example, first, second and third cooling stages are water cooling stages and first, second and third cooling devices are cooling boxes, for example water cooling boxes. Preferably, the work pressure used in all the cooling stages 2, 4, 6 is in the range of 0,2-0.6 MPa.

[0102] The distance between two subsequent cooling boxes can vary from 8 to 25 m, according to the plant throughput; whereas the distance between the last cooling box and the following rolling mill can vary between 25 and 50 m according to the plant throughput.

[0103] The number of cooling boxes and the distances between each other of them and between the last cooling box and the following group of rolling stands, depend on the line throughput and the steel grade to be treated. Downstream the last cooling box of the plant, two or more spooling devices 7 are provided for winding the material treated, for example on reels.

[0104] Optionally, in all the embodiments the surface temperatures of the steel bar can be monitored by means of sensors, for example pyrometers, installed both at entry and exit of each of roughing rolling mill 1, intermediate rolling mill 3 and finishing rolling mill 5. Operative parameters of said at least one first cooling stage 2, said at least one second cooling stage 4, and possibly of said at least one third cooling stage 6 can be managed through a closed-loop automatic control, operating in both feedforward and feedback control, based on readings of said sensors.

[0105] The number of cooling stages provided along the whole line makes it possible to adapt the intensity of cooling to the steel bar chemical composition, and to the final product required mechanical properties. In the same way, the chemical composition can be used to balance the necessity of achieving higher mechanical properties, but without exceeding with the cooling inside the cooling stages, or with the lowering of the rolling temperature. Micro alloyed steel billets can be used for this purpose.

[0106] During and immediately after the winding step, the ultra-fine-grained austenitic matrix transforms in a fine mixture of ferrite and pearlite. The result is a material that, compared with the spooled rebar with quenched surface, given the same final product yield strength, has a higher ductility.

[0107] After the winding operation is completed, the compact coil can be transferred to a storage area through a transferring device 8, for example a walking beam, where a natural or forced or retarded cooling is applied to the coil.

[0108] Preferably, the surface temperature of the coil when loaded on the transferring device 8 is in the range of 600-700° C.

[0109] Along the transferring device 8 the coil can be cooled by natural air convection, or its cooling profile can be modified using an appropriate equipment. The cooling profile can be accelerated using for example fans, by blowing air or air mist, installed along the transferring device 8, or it can be retarded using for example hoods, or active soaking furnaces, covering the transferring device. Modifying the coil cooling profile can be a helpful tool to further influence the morphology of the ferritic-pearlitic mixture.

[0110] Optionally, after a cooling to room temperature in the storage area, the coil can be unwound and straightened. This operation results in an increase of the yield and tensile strength (in a minor extent), and in a reduction of the fracture elongation. By means of the straightening parameters, it is possible to apply the work hardening in different extents. Anyway, the ductility of the steel bar remains satisfactory.

[0111] As an example, and in order to better understand the essence of the invention, below are provided some typical mechanical properties, that can be obtained in compliance with the GB 1499-2:2018 standard—grade HRBF400E:

TABLE-US-00002 HV (0, 5) El Agt Grain Surface- YS (MPa) UTS (MPa) (%) UTS/YS Size core Δ 430-470 580-620 >9.0 >1.25 >9.0 ≤40

[0112] where

[0113] YS=Yield Stress;

[0114] UTS=Ultimate Tensile Stress;

[0115] El=fracture elongation.

[0116] The ratio between Ultimate Tensile Stress and Yield Stress gives an idea of the ductility of the material.