Method for manufacturing cold-rolled, welded steel sheets, and sheets thus produced

Abstract

A method for the manufacture of a cold-rolled steel sheet of thickness e.sub.f between 0.5 mm and 3 mm is provided. At least two hot-rolled sheets of thickness e.sub.i are supplied and butt welded, so as to create a welded joint (S1) with a direction perpendicular to the direction of hot rolling. The at least two hot-rolled sheets are pickled by continuous passage through a bath, then the assembly is cold rolled, in a step (L1), to an intermediate thickness e.sub.int, the direction of cold rolling (DL.sub.1) coinciding with the direction of hot rolling. The cold rolling is carried out with a reduction ratio ${\mathrm{.Math.}}_1=\mathrm{Ln}\left(\frac{e_i}{e_{\mathrm{int}}}\right)$
such that: $0.35\le \frac{\mathrm{Ln}\left(\frac{\mathrm{ei}}{e\mathrm{int}}\right)}{\mathrm{Ln}\left(\frac{ei}{ef}\right)}\le 0.65,$
then the welded joint (S1) is removed so as to obtain at least two intermediate cold-rolled sheets. Then the two intermediate cold-rolled sheets are butt welded, so as to create a welded joint (S2), the direction of which is perpendicular to the direction of hot rolling, then the assembly of the at least two intermediate cold-rolled and welded sheets is cold-rolled, in a step (L2), to the final thickness e.sub.f, the direction (DL.sub.2) of the cold rolling step (L2) coinciding with the direction of rolling (DL.sub.1).

Claims

1. A method for the manufacture of a cold-rolled steel sheet having a thickness e.sub.f between 0.5 mm and 3 mm comprising the following successive steps: supplying at least two hot-rolled sheets having a thickness e.sub.i; butt welding the at least two hot-rolled sheets to create a first welded joint (S1) with a direction perpendicular to a direction of hot rolling; pickling the at least two hot-rolled sheets by continuous passage through a bath; cold rolling the at least two hot-rolled and welded sheets to an intermediate thickness e.sub.int, a direction of cold rolling (DL.sub.1) coinciding with the direction of hot rolling, the cold rolling being carried out with a reduction ratio ${\mathrm{.Math.}}_1=Ln\left(\frac{e_i}{e_{\mathrm{int}}}\right)$ such that: $0.35\le \frac{Ln\left(\frac{\mathrm{ei}}{e\mathrm{int}}\right)}{Ln\left(\frac{\mathrm{ei}}{ef}\right)}\le 0.65,$ then removing the first welded joint (S1) to obtain at least two intermediate cold-rolled sheets; butt welding the at least two intermediate cold-rolled sheets to create a second welded joint (S2) having a direction perpendicular to the direction of hot rolling; further cold rolling the at least two intermediate cold-rolled and welded sheets to a final thickness e.sub.f, a direction of the further cold rolling (DL.sub.2) coinciding with the direction of rolling (DL.sub.1).

2. The manufacturing method according to claim 1, wherein $0.4\le {\mathrm{.Math.}}_1=Ln\left(\frac{e_i}{e_{\mathrm{int}}}\right)\le 0.8.$

3. The manufacturing method according to claim 1, wherein a composition of the steel is that of a dual-phase steel having a tensile strength Rm greater than 600 MPa.

4. The manufacturing method according to claim 3, wherein the composition of the steel includes, the contents being expressed as weight percent: 0.05%≤C≤0.17%; 1.1%≤Mn≤2.76%; 0.07%≤Si≤0.7%; S≤0.008%; P≤0.030%; 0.015%≤Al≤0.61%; Mo≤0.13%; Cr≤0.55%; Cu<0.2%; Ni≤0.2%; Nb≤0.050%; Ti≤0.045%; V≤0.010%; B≤0.005%; Ca<0.030%; and N≤0.007%; a remainder being iron and unavoidable impurities due to processing.

5. The manufacturing method according to claim 1, wherein a composition of the steel is that of a high-formability steel having tensile strength Rm greater than 690 MPa.

6. The manufacturing method according to claim 5, wherein the composition of the steel includes, the contents being expressed as weight percent: 0.13%≤C≤0.3%; 1.8%≤Mn≤3.5%; 0.1%≤Si≤2%; 0.1%≤Al≤2%; 1%≤Si+Al≤2.5%, N≤0.010%; Ni+Cr+Mo<1%; Ti≤0.1%; Nb≤0.1%; and V≤0.1%; a remainder being iron and unavoidable impurities due to processing.

7. The manufacturing method according to claim 1, wherein the steel is a steel for press-hardening for the manufacture of parts having a tensile strength Rm greater than 1000 MPa.

8. The manufacturing method according to claim 7, wherein a composition of the steel includes, the contents being expressed as weight percent: 0.15%≤C≤0.5%; 0.4%≤Mn≤3%; 0.1≤Si≤1%; Cr≤1%; Ti≤0.2%; Al≤0.1%; B≤0.010%; a remainder being iron and unavoidable impurities due to processing.

9. The manufacturing method according to claim 8, wherein a composition of the steel includes, expressed as weight percent: 0.25%≤Nb≤2%.

10. The manufacturing method according to claim 8, wherein a composition of the steel includes, expressed as weight percent: Nb≤0.060%.

11. The manufacturing method according to claim 1, wherein a composition of the steel is that of a martensitic steel having tensile strength Rm between 1200 MPa and 1700 MPa.

12. The manufacturing method according to claim 11, wherein the composition of the steel includes, the contents being expressed as weight percent: 0.10%≤C≤0.30%; 0.40%≤Mn≤2.20%; 0.18%≤Si≤0.30%; 0.010%≤Al≤0.050; and 0.0025≤B≤0.005%; a remainder being iron and unavoidable impurities due to processing.

13. The manufacturing method according to claim 12, wherein the composition of the steel includes at least one of, the contents being expressed as weight percent: 0.020%≤Ti≤0.035%; Cu≤0.10%; Ni≤0.10%; and Cr≤0.21%.

14. The manufacturing method according to claim 1, further comprising, after the step of removing the first welded joint and prior to the step of butt welding the at least two intermediate cold-rolled sheets to create a second welded joint, the step of: coiling, storing and uncoiling the at least two intermediate cold-rolled sheets.

15. The manufacturing method according to claim 1, wherein at least one of the first or second welded joints (S1, S2) is made by flash welding.

16. The manufacturing method according to claim 1, wherein at least one of the first or second welded joints (S1, S2) is made by Laser welding.

Description

EXAMPLE 1

(1) A steel has been elaborated with a composition for the manufacture of a dual-phase type steel sheet shown in the Table below, expressed in weight percent, the remainder being iron and unavoidable impurities due to processing. This composition enables the manufacture of a dual-phase steel sheet having tensile strength Rm greater than 980 MPa.

(2) TABLE-US-00001 TABLE 1 Dual-phase Steel Composition (wt.-%) C Mn Si S P Al Mo Cr Ni Nb Ti B 0.075 2.49 0.284 0.001 0.013 0.158 0.085 0.295 0.015 0.024 0.036 0.0023

(3) Steel sheets of width 1500 mm were hot rolled to a thickness e.sub.i of 3 mm. In order to make the process continuous, these sheets were flash welded under the following conditions (S1):

(4) Spark distance: 9.5 mm

(5) Forging distance: 2.5 mm

(6) Welding cycle time: 9 s.

(7) These welded hot-rolled sheets were then cold rolled to a thickness of 1 mm by two different methods:

(8) Reference method R1: the sheets were directly cold rolled by a continuous rolling mill consisting of five rolling stands. The deformation conferred by rolling the sheet is:
Ln(3/1)˜1.10.

(9) Method according to the invention I1: the sheets were cold rolled by a continuous rolling mill consisting of five rolling stands to an intermediate thickness e.sub.int of 1.6 mm. At this stage, the deformation ε.sub.1 is equal to:

(10) 0$Ln\left(\frac{3}{1.6}\right)~0.63.$
Weld (S1) was removed by cutting, the sheets thus obtained were coiled and temporarily stored. These sheets were then uncoiled and flash welded together to create a welded joint (S2) under the following conditions:
Spark distance: 6.5 mm
Forging distance: 1.5 mm
Welding cycle time: 7 s.

(11) After the excess thickness was removed from joint (S2) by machining, this sheet of thickness 1.6 mm was then cold rolled to a final thickness e.sub.f of 1 mm. The deformation ratio conferred by this second rolling step (L2) is equal to:

(12) $Ln\left(\frac{1.6}{1}\right)~0.47.$
Thus, the ratio

(13) $\frac{Ln\left(\frac{ei}{e\mathrm{int}}\right)}{Ln\left(\frac{ei}{ef}\right)}$
is equal to: ˜0.57.

(14) The microstructures of the welded joints at various stages (initial, intermediate, and final thicknesses) as well as the variation in Vickers microhardness in the direction across these joints, under a 500 g load, were characterized. Using these characteristics, it is possible to determine the initial width of the welded joint and the width of the joint after cold rolling, and thus to deduce the local deformation ratio of the welded joint conferred by cold rolling. Table 2 shows the difference Δ between the overall deformation ratio of the sheet determined from its variation in thickness, with the local deformation ratio of the welded joint S1 or S2, according to the method of manufacture (average of three tests).

(15) TABLE-US-00002 TABLE 2 Difference Δ in deformation ratio between the sheet and the welded joint Method Il 0   Method R1 0.07

(16) For the conventional method, it is thus demonstrated that the welded joint is deformed 7% less than the adjacent sheet. Surprisingly, it is demonstrated that the method of the invention leads to a deformation ratio conferred by rolling that is nearly identical in the sheet and in the strip, thus reducing the risk of premature fracture in the welded joint due to the deformations being concentrated more particularly in this area.

(17) In addition, Table 3 compares the width of the welded joints (measured at the level of the Heat Affected Zone) and their average hardness HV.sub.0.5, measured on a sheet of 1 mm final thickness obtained either by the reference method R1 or by the method of the invention 11. For purposes of comparison, the hardness of the 1 mm thick sheet as well as the relative difference between the hardness of the welded joints and that of the sheet were also examined.

(18) The microstructure of joints (S1) and (S2) is very predominantly martensitic with a small proportion of bainite.

(19) TABLE-US-00003 TABLE 3 Hardness Difference in Width of of rolled Hardness of relative hardness/ rolled welded welded joint rolled sheet welded joint − joint (mm) (HV.sub.0.5) (HV.sub.0.5) rolled sheet Method Il 10.7 409 367 11.4% Method R1 23   438 367 19.3%

(20) It is thus demonstrated that the method of the invention results in a welded strip with a narrower joint and for which the difference in hardness is smaller compared to the base metal than in the case of the reference method, this homogeneity contributing to reducing the risk of premature fracture in the welded joint during cold rolling.

(21) Specimens 70 mm long and 5 mm wide taken parallel to the welded joints were used to measure tensile strength Rm and fracture strain A in 1 mm thick cold-rolled sheets manufactured by the reference method and the method of the invention. Results for welded joints and base sheet are presented in Table 4.

(22) TABLE-US-00004 TABLE 4 Rm (MPa) A (%) Welded rolled sheet 1400 3.4 Method Il Welded rolled sheet 1550 1 Method R1 Rolled sheet 1390 3.7

(23) Once again, the method according to the invention demonstrates that it is possible to obtain a high degree of homogeneity of mechanical properties in both the base sheet and the welded joint, which reduces the risk of fracture during cold rolling of the strip. Indeed, in the conventional method R1, the fracture strain of the welded joint is lower, which means that a local concentration of stresses could lead more easily to fracture. In the method of the invention, the plasticity reserve of the welded joint is higher and comparable to that of the base metal, so that the risk of fracture is significantly reduced.

(24) In addition, surface roughness of sheets manufactured by conventional methods and the method of the invention was measured using a 3D roughness measurement.

(25) The 3D images were processed using Mountains® software. Roughness profiles were analyzed according to ISO4287, images according to EN15178N. The results are shown in Table 5.

(26) TABLE-US-00005 TABLE 5 Ra (μm) Rolled sheet 0.83 Method Il Rolled sheet 0.88 Method R1

(27) It can be seen that the invention makes it possible to manufacture sheets whose surface roughness Ra is relatively unchanged, i.e., two passes through the rolling line did not change the roughness as compared to a single pass. Thus, we know that an increase in roughness increases emissivity during furnace annealing, which occurs after cold rolling. For example, in the case of an annealing furnace with direct flame heating that results in an oxidizing phase for the iron, a sheet with increased roughness is heated more quickly, which can affect recrystallization and precipitation kinetics and thus the final mechanical properties of the sheet. A change in roughness may therefore require annealing furnace settings to be changed.

(28) However, as we have seen, roughness is relatively unchanged for a given steel composition and thickness, sheets rolled by a conventional method and sheets rolled by the process of the invention can be passed successively through an annealing furnace without changing its settings, which has the advantage of simplifying annealing furnace management.

EXAMPLE 2

(29) A press-hardenable steel was supplied, the composition of which, expressed as weight percent, is shown in Table 5, with the remainder being iron and unavoidable impurities due to processing.

(30) TABLE-US-00006 TABLE 5 C Mn Si Al Cr Ti B N 0.22 1.16 0.26 0.03 0.17 0.035 0.003 0.005

(31) Steel sheets were hot rolled to a thickness e.sub.i of 3.5 mm. In order to make the process continuous, these sheets were flash welded under the following conditions (S1):

(32) Spark distance: 9.5 mm

(33) Forging distance: 2.5 mm

(34) Welding cycle time: 12 s

(35) Annealing time after welding: 9 s

(36) The sheets were cold rolled in a continuous rolling mill consisting of five rolling stands to an intermediate thickness e.sub.int=1.75 mm. At this stage, the deformation ε.sub.1 is equal to:

(37) $Ln\left(\frac{3.5}{1.75}\right)~0.69.$

(38) Weld (S1) was removed by cutting, the sheets thereby obtained were coiled and temporarily stored. These sheets were then uncoiled and flash welded together to create a welded joint (S2) under the following conditions:

(39) Spark distance: 6.5 mm

(40) Forging distance: 1.5 mm

(41) Welding cycle time: 7 s

(42) Post-weld annealing time: 7 s

(43) After removing the excess thickness from joint (S2) by machining, this sheet of thickness 1.75 mm was then cold rolled to a final thickness e.sub.f of 0.64 mm. The deformation ratio conferred by this second rolling stage (L2) is equal to:

(44) $Ln\left(\frac{1,75}{0,64}\right)~1.$
Thus, the ratio

(45) $\frac{Ln\left(\frac{ei}{e\mathrm{int}}\right)}{Ln\left(\frac{ei}{ef}\right)}$
is equal to: ˜0.41.

(46) In these conditions, which are those of the invention, it is stated that the process does not cause any premature failure of the strip weld and that it is possible to manufacture thin gage sheets of this press hardenable steel.

(47) The process according to the invention will be advantageously used to reduce the risk of strip failure during the manufacture of cold rolled Dual Phase and Trip Steels, of High Formability steels, of press hardening steels, cold rolled for the automotive industry.

(48) It will be also advantageously employed for the manufacture of sheets in thinner thickness ranges than those obtained directly in a single rolling step by existing facilities.