DOUBLE-PASS, DOUBLE-ANNEALING WELDING METHOD FOR JOINING HIGH-STRENGTH STEELS

Abstract

A double-pass seam welding method for steels with a Ceq greater than approximately 0.45. The first pass welds and immediately anneales the weld. In the second pass, the welder is disengaged and the weld undergoes a second annealing.

Description

The present invention relates to welding technology. More particularly, the present invention relates to welding techniques for joining high-strength steels, such as automotive high-strength steels (AHSS). More specifically, the present invention relates to a double-pass, double-annealed welding technique for joining AHSS-type steels. Background of the Invention This invention relates to welding and heat treatment apparatus and methods. More particularly, this invention is directed to methods for manufacturing butt or seam welds with reduced weld zone hardness and improved weld zone ductility and toughness. This invention is particularly useful in the production of high-strength welds between cold-rolled steel coils in a cold rolling mill. Used ubiquitously in all modern industries, welded ferrous alloys have become the de facto standard in the design of structural components. Current trends in many fields have shifted interest away from common, low-strength mild steels toward high- and ultra-high-strength steels. These alloys are formulated to have greater tensile strength than low-carbon steels due to the specific microstructures produced during thermomechanical processing. Examples of high-strength steels currently used in the automotive industry include dual-phase, martensitic, boron-treated, split-hardener, and transformation-induced plasticity steels. Other high-strength alloys include air-, oil-, and water-hardenable carbon steels and martensitic stainless steels.All of these are designed to form a certain percentage by volume of martensite in the materials' microstructure. The resulting distorted body-centered cubic (BCC) or body-centered tetragonal (BCT) martensitic crystal structure formed in the hardened condition imparts high strength to the metal. These materials are ideal for structural components and assemblies, meeting high strength and hardness requirements. In the highly competitive automotive market, coupled with concerns about energy and the environment, automakers are constantly seeking ways to reduce fuel consumption and CO2 emissions. This can be achieved by effectively reducing vehicle weight through the use of advanced, third-generation high-strength steel, which also improves the crash performance of car body components. Unfortunately, the tendency to form martensite and the relatively high hardenability of these and other ultra-high-strength alloys pose welding challenges. The chemistry of high-strength steel grades results in a complete transformation from ferrite to austenite at high temperatures, followed by a subsequent change to the hard martensite phase upon rapid cooling. In butt / seam welding applications, the natural cooling rate of the weld can be as high as 1000°C / s, fast enough to produce a martensitic structure in most high-strength, high-carbon alloys. The resulting martensitic structure is extremely brittle in the unquenched state.Cracking of the weld zone can occur for several reasons, including: Hydrogen-induced cold cracking, due to hydrogen trapped in the distorted BCC martensitic crystal structure. Tensile stress applied to the weld increases the risk of cracking. Thermally induced stress, due to heat input during welding, the degree of joint restraint, and volume change following martensitic transformation. Most cracking results from shrinkage deformations that occur as the weld metal cools to room temperature. If shrinkage is restricted, these deformations will induce residual tensile stresses that lead to cracking. There are two opposing forces: the stresses induced by the metal's shrinkage and the surrounding stiffness of the base material. Large weld sizes, high heat input, and deep-penetration welding procedures increase shrinkage stresses. The stresses induced by these deformations will increase with higher-strength base materials. Higher yield strengths result in higher residual stresses. Recently, steel product development has led to the development of third-generation, highly alloyed (>0.3 wt%) AHSS martensitic steels. These steels have maximum tensile strengths of around 2000 MPa or more (M2000). These products can be manufactured by casting, pickling, and annealing. Unfortunately, steel coils cannot yet be processed through continuous annealing lines because currently available welding techniques have great difficulty joining coils of such steels. After welding the coil ends, the weld zone and its surrounding area become a completely martensitic structure, resulting in very brittle welds with numerous nugget cracks. Applying even a small load to this brittle zone results in weld failure and plant downtime. Most AHSS grade coils are joined using the post-weld heat treatment (PWHT) process, which increases weld hardness. Currently, three methods are available in the literature for welding steel coils in finishing mills (coating, annealing, etc.). As used herein, Aci is the temperature at which austenite begins to form during heating, and Ac3 is the temperature at which the transformation of ferrite to austenite is completed during heating. The martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (Ms), and the parent austenite becomes mechanically unstable. Ms is the martensite finishing temperature. Method 1 is known as a “one-pass welding method” in which the welding is performed without any post-weld heat treatment. Method 2 is known as a “single-pass annealing method” in which welding is followed by immediate annealing. In this method, cooling is controlled between Ar3 (austenitic ferrite transformation temperature) and Mf (martensitic finishing temperature). Method 3 is known as the “double pass with second pass annealing” method. In this method, post-weld annealing is not applied immediately, but rather after the welding process is completed. Post-weld heat treatment (PWHT) is applied once the weld temperature drops below the Ms temperature. Welding tests were performed using the three prior art methods to weld the ends of M2000 and 0.35C-0.6Mn-0.5Si coils. The 0.35C-0.6Mn-0.5Si steel is capable of achieving an ultimate tensile strength (UTS) exceeding 1800 MPa after heat treatment in hot-forming. Unfortunately, satisfactory weld hardness was not achieved using any of these prior art techniques. The phase transformation curve (ferrite-pearlite) of M2000 on a continuous cooling temperature (CCT) diagram is further to the right than that of earlier-generation AHSS grades. Therefore, to transform the final microstructure into a more ferrite-pearlite microstructure, the elevated temperature must be held in the weld metal for a longer period.This cannot be achieved in coil joining applications within the limited welding time available during continuous production. Methods 1 and 2 produce a large volume of martensitic phases that are brittle and prone to cracking. Method 3 typically produces a strong joint in highly alloyed products by tempering the martensitic structure generated after welding. However, when using this practice, nugget cracks and overlap edge gaps develop in alloys such as M2000 moments after welding. Post-weld annealing of a crack-enriched nugget region is ineffective. Until now, apart from these three traditional methods for joining high-carbon high-alloy steels, there are no methods available for welding this type of high-alloy steel in coil joining applications. Therefore, there is a need in the art for a novel method to produce solid welds of M2000 type alloys from primer to primer that is robust enough to pass the product through the continuous annealing line. Brief Description of the Invention The present invention is a welding method. The method includes the following steps: 1) provide a first surface of a hardened ferrous alloy having a composition having a carbon equivalent (Ceq) of at least 0.45; 2) provide a second surface of a hardened ferrous alloy having a composition having a carbon equivalent (Ceq) of at least 0.45; 3) create an overlay by overlapping the first surface and the second surface; 4) welding the first surface to the second surface by heating the first surface and the second surface to a sufficiently high temperature to raise the temperatures of the first surface and the second surface at least to the melting points of the alloys to form a weld; 5) cool the weld between the Ac3 and Mf temperatures of the alloys; 6) Heat the weld to heat the weld at a rate of at least 10 C / sec to a temperature between the Msy Aci temperatures of the alloys; 7) cool the weld below the Ms temperatures of the alloys; 8) Heat the weld at a rate of at least 10°C / sec to a temperature between the Msy and Aci temperatures of the alloys; and 9) Cool the weld to room temperature. Ceq is defined as: Ceq = C + A(C) * [Si / 24 + Mn / 6 + Cu / 15 + Ni / 20 + (Cr + Mo + Nb + V) / 5 + 5B] and A(C) = 0.75 + 0.25 tanh [20(C - 0.12)]; The composition of each of the elements C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B are in percent by weight. At least one of the first surface and the second surface may be formed from a hardened ferrous alloy having a composition that has a carbon equivalent (Ceq) of at least 0.5. The first surface can be one end of a first steel coil, and the second surface can be one end of a second steel coil. The step of welding the first surface to the second surface can comprise electrical resistance seam welding. The electrical resistance seam welding step can be performed using a medium-frequency direct current (MFDC) seam welder. The medium-frequency direct current (MFDC) seam welder can include welding wheels and a post-weld induction heater. The seam welder can form the weld using two passes of the welder through overlap. The first of the two passes may comprise: 1) seam welding, which is performed by the welding wheels; and 2) immediate annealing of the seam weld using the post-weld induction heater. The second of the two passes may comprise: 1) disengaging the welding wheels; and 2) annealing the weld using the post-weld induction heater. The weld can have load and ductility ratios of at least 70%. Brief Description of the Drawings FIG. 1A is a 1000x micrograph showing the microstructure of the cold-rolled, fully hardened M2000 steel samples as received; FIG. 1B is a 3000x micrograph showing the microstructure of the cold-rolled, fully hardened M2000 steel samples as received; cnocnn / i ζπζ / β / υ FIG. 2 is an approximate schematic of a seam welder useful for performing the welding method of the present invention; FIGS. 3A and 3B are schematic diagrams of an Olsen cup test; FIG. 4 is a schematic diagram of the welding section between steel coil strips 1 and 2, welding line 8 is specifically shown; FIG. 5A is a macrograph of a cross-sectional sample of a seam weld between two fully hard M2000 sheets using welding method 1 of the prior art; FIGS. 5B and 5C are 1000x SEM images of the weld nugget region of the fully hard M2000 weld using welding method 1 of the above technique; FIG. 6A is an image showing the general weld nugget sample welded using welding method 2 of the above technique FIG. 6B is a 2,500x SEM micrograph image of the soldered weld nugget using welding method 2 of the above technique; FIGS. 7A and 7B show SEM micrographs of an M2000 sample welded using welding method 4 of the invention; FIG. 8 plots the results of the Olsen cup test of M2000 welded using the four different methods; FIG. 9A represents a generic mode of the heating / cooling cycle over time of the present welding method of the invention. FIG. 9B represents a flow diagram of welding method 4 of the invention; FIG. 10A plots the Ceq (CEN) for various grades of steel alloy; FIG. 10B is a table showing the weldability of steels with variable Ceq; FIGS. 11A - 11C are macro / micrographs of a 0.35C-0.6Mn-0.5S weld using method 1 of the previous technique; FIGS. 12A-12C are macro / micrographs of a 0.35C-0.6Mn-0.5Si weld using method 2 of the previous technique; FIGS. 13A - 13C are macro / micrographs of a 0.35C-0.6Mn-0.5S weld using method 3 of the previous technique; FIGS. 14A-14C are macro / micrographs of a 0.35C-0.6Mn-0.5S weld using the method of invention 4; FIG. 15 shows the results of the Olsen cup test of a 0.35C-0.6Mn-0.5Si weld using the four different methods; FIGS. 16A-16D are macrographs of the weld nugget after the bend test for methods 1-4 respectively. Detailed Description of the Invention Welding technique test and experimental procedures M2000 alloy cnocnn / i znz / B / v Cold-rolled M2000 fully hardened steel samples with a thickness of 1.2 mm were used as the base metal (BM) for the welding tests. The nominal and actual chemical compositions of M2000 steel, as well as its carbon equivalent (Ceq), are shown in Table 1 (the alloy may also include Ti and unavoidable impurities). The microstructure of the received steel is shown in FIGS. 1A and 1B (at 1,000xy and 3,000x respectively). cnocnn / i znz / B / v Table 1 Elements Ceq C Mn Si Cr Nb ppm B ppm Composition 0.56 0.294 0.471 1.563 0.510 280 20 The fully hard M2000 contains only ferrite and fine pearlite phases. The mechanical properties of fully hard M2000 are shown in Table 2. The UTS for a cold-rolled fully hard M2000 is almost half of what it will be after the annealing process, which is required to achieve a UTS of 2000 MPa or more. Table 2 Tensile properties Ultimate tensile strength, MPa Yield strength, MPa % total elongation 1027 955 5.5 Testing welding methods for welding M2000 All welds were performed using a medium-frequency direct current (MFDC) seam welder. A rough schematic of such a seam welder is shown in Figure 2. The welder 4 is used to weld the ends of two steel coils (1 and 2) together so that they can be processed sequentially on continuous processing lines such as pickling lines, continuous annealing lines, continuous galvanizing / annealing lines after galvanizing, etc. The welder consists of welding wheels 5 (one on top and one hidden wheel beneath the steel). The welding current passes between the welding wheels 5 through the steel sheets 1 and 2. This current heats the steel due to resistive heating. This heat melts the steel at the interface of the two sheets. The welder 4 welds the steel in the direction of the arrow in Figure 2.After the steel is heated / welded by the electrode wheels 5, the molten steel sheets are mechanically forged together by the post-flattening wheels 6 (one on top and the other hidden underneath). This forces the molten metal from the steel sheets 1 and 2 to combine, forming a single weld seam / nugget. It also flattens the overlapping edges of the sheets so there is no "step" feature at the weld interface. Finally, the welder also includes a post-weld induction heater for annealing the weld. It should be noted that, depending on the welding technique, the induction heater may be switched on or off. The three previous welding methods and the welding method of the invention were used to weld complete hard samples of M2000 and 0.35C-0.6Mn-0.5Si. The microstructure and mechanical properties of all welded samples were compared. In the first method of the prior art, the welder 4 performs only one welding pass. That is, the welding is done in a single pass without annealing (i.e., the post-weld induction heater 7 is off). In the second method of the prior art, the welding is followed by immediate annealing in a single pass. That is, the post-weld induction heater 7 is turned on, and the solder nugget is annealed directly after the welding is completed, before it has time to cool excessively. The third method of the prior art is a method called the double-pass method, where the second pass is for annealing the solder nugget. In this method, the welding is performed, and the strip is allowed to cool until the solder temperature drops to the Ms temperature of that grade; then, the annealing is performed in the second pass.That is, the initial pass welds the steel sheets together, but the post-weld induction heater 7 is switched off so that the temperature of the weld nugget drops to Mf or lower. Then, on the second pass, the welder is switched off and the post-weld induction heater 7 is switched on to anneale the weld nugget. As will be seen below, none of these three methods of the above technique could successfully create a weld strong and durable enough to allow the weld joint to pass through continuous processing lines. The inventors have created a novel fourth welding technique. This technique is a two-pass method. In the first pass, the welding is performed followed by immediate annealing. That is, the post-weld induction heater 7 is turned on, and the weld nugget is annealed immediately after the welding is completed, before it has time to cool excessively. Once the first pass is complete, the weld metal cools below the M or Mt temperature of the steel grade. Then, in the second pass, the welder 4 is turned off, and the post-weld induction heater 7 is turned on to anneale the weld nugget. Table 3 describes the welding parameters for each welding method used by the medium-frequency DC (MFDC) seam welder.Where there are two values ​​separated by a bar, the values ​​represent the first step and the second step respectively. cnocnn / i znz / B / v Table 3 Samples Methods Speed ​​m / min Current KAmps Force N / mm2 Post-flattening N / mm2 Overlap OS mm Overlap DRV mm Annealing Temp, °C Welding-1 Single pass welding 1.5 16.5 0.50 7.24 1.45 2.72 NA Welding-2 One pass w / annealing 1.5 16.5 0.50 7.24 1.45 2.72 760 Welding -3 Double pass, second pass annealing 1.5 / 1.5 16.5 0.50 7.24 1.45 2.72 760 Welding -4 Double pass, double annealing 1.5 / 1.5 16.5 0.50 7.24 1.45 2.72 760 cnocnn / i ζπζ / β / υ Welding characterizations The mechanical properties and microstructure of the four weld types applied to fully hardened M2000 were obtained. Mechanical properties were tested using ball and microhardness tests, and the microstructure was observed using optical and scanning electron microscopy. Metallographic samples were cut, mounted, and polished to a 1 µm finish using standard metallographic techniques. They were then etched with 2% nital to reveal the microstructure. Vickers microhardness profiles were taken from the polished and etched weld surfaces. Hardness indentations were made using a penetration load of 200 g and a dwell time of 15 seconds. The indentations were sufficiently spaced to avoid interference. Fifty indentations were taken across the cross-section. The Olsen cup test was performed on the specimens using a 22.2 mm diameter ball (size 10) with a punch speed of 25.4 mm / min. Figures 3A and 3B are schematic diagrams of an Olsen cup test. Figure 3A shows the test just before it began, and Figure 3B shows the test at its conclusion. The test was stopped when a crack appeared in the specimen. The punch height at the time of failure was described as the limit dome height (LDH). The LDH of the weld was compared to that of the base material by dividing the LDH of the weld by the LDH of the base material, which is known as the ductility ratio. The load ratio was calculated as the ratio of the maximum applied load on the weld to the maximum applied load on the base metal as recorded before dome fracture. The minimum desired ductility and load ratio is 70%. Figure 4 is a schematic diagram of the weld section between steel coil strip 1 and steel coil strip 2, showing weld line 8. The Olsen cup test was performed at five different locations: 11a) on the line operator's welder side; 11b) one-quarter along the strip; 11c) in the center of the strip; 11d) three-quarters along the strip; and 11e) on the drive side of the strip. The test positions are shown in Figure 4. Figure 5A is a macrograph of a cross-sectional sample of a seam weld between two fully hardened M2000 sheets using welding method 1 of the prior art (one pass, without annealing). As can be seen, the weld nugget has long, massive cracks 12. Furthermore, the notches 13 are open; there is no solid-state bonding present near the notches. Therefore, the stress under load is not evenly distributed in the vicinity of the nugget. Instead, it is concentrated at the nugget. Figures 5B and 5C are 1000x SEM images of the weld nugget region of the fully hardened M2000 weld using welding method 1 of the prior art. It can be seen from these that there is an almost 100% martensitic phase with a large amount of lath martensite 14 present in the weld microstructure with many microcracks 15. In resistance seam welding (RSW) applications, the natural cooling rate of the weld can be as high as 1000°C / sec, fast enough to produce a martensitic phase in most high-carbon AHSS grades. The following describes the cause of weld nugget cracking due to thermally induced stress during weld cooling. After the molten nugget forms on the contact surfaces of the metal sheet, the outer layer of the nugget cools rapidly due to heat conduction. Therefore, a phase transformation from face-centered cubic (FCC) to body-centered copper (BCT) occurs in the outer layer of the weld nugget, resulting in its expansion. Similarly, the layer adjacent to the outer layer undergoes the same expansion, and so on, until the entire nugget eventually transforms into a stable martensitic structure. During each expansion stage, contraction also occurs. Deformations occur as the weld metal cools to room temperature.If this contraction is restricted, the deformations will induce residual tensile stresses that cause cracking. Therefore, the simultaneous effect of expansion and contraction results in cracking in high-carbon steels and alloys. As shown in FIG. 5A, there is a gap (notch 13) at the edge of the overlap between the two steel strips. This gap is thought to be created due to the springback effect of M2000 or the expansion of the nugget during cooling. Materials like M2000 typically have a very high springback effect. Therefore, after welding, during the solidification stage, the welded strips attempt to return to their original position, creating gaps and cracks in the nugget region.Once microcracks are present in the dendritic direction due to thermal stress or the high elastic recovery effect of the materials, cracks initiated in the notched regions of the sheet meet the microcracks at the edge of the nugget and propagate along the outer surface of the nugget. Figure 6A shows a general weld nugget sample welded using welding method 2 of the previous technique (single-pass welding with immediate annealing). As can be seen in Figure 6A, there were no cracks. Furthermore, no cracks are visible in the micrograph image in Figure 6B, which is a 2,500x SEM micrograph of the weld nugget. Welding method 2 appears to help prevent cracks that develop during the cooling stage of the weld due to its slower cooling rate (resulting from the annealing). Figure 6B shows that the weld from method 2 contains ferrite (F), pearlite (P), and martensitic (M) structures in the center of the nugget. With reference to an M2000 CCT diagram, the final microstructure of weld 2 can be understood. In the single-pass simple annealing method, there is a delay of a few seconds between welding and annealing the weld region.Therefore, during weld cooling, the weld nugget temperature was thought to be between the Ms and Mf temperatures of M2000. This is confirmed by evidence of martensite presence in the microstructure. During the annealing process, the weld nugget temperature rises again to just below the ACi temperature of M2000 (-760 °C). At this point, the remaining austenite transforms into ferrite and pearlite phases during cooling. However, the volume fraction of martensite is higher in the microstructure of the single-pass annealed weld sample, which limits the weld hardness (discussed later). It should be noted that welding method 3 resulted in welding problems similar to welding method 1, because there is no annealing in the first welding pass in welding method 3. Therefore, the weld nugget has massive and long cracks and notches, and annealing the crack-rich regions does not alleviate the cracking problem. Figures 7A and 7B show SEM micrographs of an M2000 sample welded using welding method 4 of the invention (double-pass double annealing). As shown in Figure 7A, a crack-free nugget was also achieved in this weld sample, since this method goes through the same stages as the single-pass annealing method (method 2). It can be seen in the enlarged image of Figure 7B that the nugget region was enriched with carbide formation (CB), meaning that all the martensite was transformed into tempered martensite (TM) during the second-pass annealing. Furthermore, due to the second-pass annealing, all the pearlite that formed during the welding and annealing of the first pass was transformed into coarse pearlite (CP). Because the weld microstructure contains coarse pearlite and tempered martensite, the weld has improved hardness. Figure 8 shows the results of the Olsen cup test of M2000 welded using the four different methods (M1, M2, M3, and M4). Specifically, Figure 8 plots the load-to-ductility ratios as defined above for two examples each of M1-M3 and three examples of M4. The load-to-ductility ratios must be greater than 70% for the weld quality to be considered sufficiently strong to allow the weld to pass through the continuous annealing line without damage. As can be seen in Figure 8, the welds produced by methods 1, 2, and 3 did not meet the minimum requirement for the desired weld quality. However, in contrast, the welds produced using method 4 achieved almost twice the weld hardness compared to the samples welded using methods 1, 2, and 3.The greater weld hardness of the samples welded using method 4 is attributed to the two factors, (1) a crack-free weld nugget was achieved and (2) a tempered martensite and coarse pearlite microstructure was achieved. In welding method 4 of the invention (double pass, double annealing), the first annealing is performed immediately after welding. This reduces the cooling rate and thus prevents crack formation. The second strategic annealing pass tempers the brittle martensite. Figure 9A represents a generic embodiment of the heating / cooling cycle versus time of the present welding method of the invention. The weld is cooled directly from the seam welding stage to a temperature Ti at a cooling rate of Vci, where Mf < Tii < AC3. The weld is then heated to a temperature Ti at a heating rate of VHi, where Ms < Ti < Aci. The weld is then cooled to a temperature T12 at a cooling rate of VC2, where Mf < Tb < Aci. The weld is then reheated to T2 at a heating rate of VH2, where Ms < T2 < Aci. Finally, the weld is allowed to cool in air (> 10°C / sec) at ambient temperature. Table 4 gives the temperatures (in °C) and heating / cooling rates (in °C / sec) for the example alloys M2000 and 0.35C-0.6Mn0.5YES. cnocnn / i znz / B / v Table 4 Alloy Tii Ti T12 T2 Vci VH1 VC2 vh2 M2000 300 760 150 760 >15 >10 >15 >10 0.35C-0.6Mn-0.5Si 250 693 150 650 >15 >10 >15 >10 Figure 9B depicts a flow diagram of welding method 4 of the invention. First, the two steel pieces are seam-welded together, preferably by electric current welding, such as medium-frequency direct current (MFDC) seam welding. While electric current welding is the preferred method for joining the steel pieces, any known means of welding the pieces together may be used. Once the weld is formed, it is allowed to cool to a temperature Tii between the temperatures of Acs and Mf for the specific steel being welded at a cooling rate of Vct. The seam weld is then heated to a temperature Ti below the temperature Aci but above the temperature M at a heating rate of Vhi. The seam weld is then allowed to cool to a temperature T12 between Mf and Aci at a cooling rate of VC2.Next, the seam weld is reheated to a temperature T that is below the Aci temperature but above the M temperature by heating at a rate of Vh2. Finally, the seam weld is allowed to cool in air at room temperature at a cooling rate > 15 °C / sec. The inventors have discovered that method 4 of the invention is very advantageous for alloys having a composition with a relatively high carbon equivalent. The inventors use the carbon equivalent formula developed in 1983 by Yurioka et al. Their carbon equivalent equation was the well-known CEN (interchangeable in this document with Ceq), where: [GEN = C + A(C) * [Si / 24 + Mn / 6 + Cu / 15 + Ni / 20 + (Cr + Mo + Nb + V) / 5 + 5B] where: A (C) = 0.75 + 0.25 tanh [20 (C - 0.12)] The composition of each of the elements C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B are in percent by weight. Inventors have observed that as the carbon equivalent of an alloy increases, its weldability decreases. Figure 10A plots the Ceq (CEN) for various steel alloy grades. Figure 10B is a table showing the weldability of steels with varying Ceq. The present welding method is most advantageously used for welding steels with a Ceq greater than or equal to approximately 0.45. The method of the invention is most useful for alloys with a Ceq of 0.5 or greater. The Ceq of the M2000 alloy in the preceding example is nominally 0.57. As shown above, the method of the invention works well for the M2000 alloy, which has a Ceq greater than 0.5. 0.35C-0.6Mn-0.5Si In the past, many welding tests were carried out on 0.35C-0.6Mn-0.5Si full hardness primer-to-primer welds. None of these welding tests resulted in successful primer-to-primer welds. Since the chemistry and CCT of 0.35C-0.6Mn-0.5Si and M2000 are close, the inventors believe that the welding technique of the invention would work for 0.35C-0.6Mn-0.5Si. The welding method of the invention was tested on 0.35C-0.6Mn-0.5Si full hardness. The mechanical properties and microstructures of the welds and the three methods of the prior art are described below. The chemical composition of 0.35C-0.6Mn-0.5S steel as well as its carbon equivalent (Ceq) is shown in Table 5. cnocnn / i ζηζ / Β / γ Table 5 Steel Grade Ceq C Mn Si Ni Mo Cr B 0.35C-0.6Mn-0.5Si 0.74 0.340 0.597 0.532 0.400 0.171 0.330 0.032 The mechanical properties of 0.35C-0.6Mn-0.5S¡ fully hard are shown in Table 6. As with M2000, the ultimate tensile strength (UTS) for a fully hardened 0.35C-0.6Mn-0.5Si cold-rolled steel is almost half of what it will be after the annealing process, which is required to achieve a UTS of 1800 MPa or higher. Table 6 Tensile Properties Ultimate tensile strength MPa Yield strength, MPa % of total elongation 1026 951 4.6 Primer-to-primer welds of 0.35C-0.6Mn-0.5Si steel were performed using the three welding techniques of the prior art and welding method 4 of the invention. Figures 11A and 11C are macro / micrographs of a 0.35C-0.6Mn-0.5Si weld using method 1 of the prior art. Figure 11A is a macrograph of the weld showing macroscopic cracks 12 in the weld nugget. Figure 11B is a micrograph of the weld nugget at 2000x magnification showing microcracks 15. Figure 11C is a micrograph of the weld nugget at 5000x magnification showing the martensite 14 of the weld bead. Figures 12A–12C are macro / micrographs of a 0.35C–0.6Mn–0.5Si weld using Method 2 of the previous technique. Figure 12A is a macrograph of the weld showing no visible macroscopic cracks in the weld nugget. Figure 12B is a micrograph of the weld nugget at 2000x magnification showing no visible microcracks. Figure 12C is a micrograph of the weld nugget at 5000x magnification showing the martensite that has been “reformed” (after being converted to austenite) 14'. Figures 13A–13C are macro / micrographs of a 0.35C–0.6Mn–0.5S weld using Method 3 of the previous technique. Figure 13A is a macrograph of the weld showing no visible macroscopic cracks in the weld nugget. Figure 13B is a micrograph of the weld nugget at 2000x magnification showing microcracks. Figure 13C is a micrograph of the weld nugget at 5000x magnification showing the reformed martensite. Figures 14A–14C are macro / micrographs of a 0.35C-0.6Mn-0.5S weld made using the method of invention 4. Figure 14A is a macrograph of the weld showing no visible macroscopic cracks in the weld nugget. Figure 14B is a micrograph of the weld nugget at 2000x magnification showing no visible microcracks. Figure 14C is a micrograph of the weld nugget at 5000x magnification showing the formation of carbide (CB) and quenched martensite (TM).After the welds were formed, each weld was subjected to Olsen cup testing to determine if it was of sufficient quality to allow it to pass through a continuous processing line. Figure 15 shows the Olsen cup test results for 0.35C-0.6Mn-0.5Si welds made using four different methods (M1, M2, M3, and M4). Specifically, Figure 15 plots the load-to-ductility ratios, as defined above, for each of the three M1-M4 examples. The load-to-ductility ratios must be greater than 70% for the weld to be considered strong enough to allow it to pass through the continuous annealing line without damage. As can be seen in Figure 15, the welds made using methods 1, 2, and 3 did not meet the minimum weld quality requirement.The samples that were welded using the welding method of the invention (M4) demonstrated a ductility ratio of -100% (weld displacement ratio to base metal (BM)) and a load ratio of -100% (weld to fracture load ratio of BM), which are well above the minimum desired value (70%) for the desired quality. Figures 16A–16D are macrographs of the weld nugget after testing for methods 1–4, respectively. Figure 16A shows that the weld nugget produced by method 1 of the prior art developed a macroscopic crack 12' that propagated through the weld nugget. This weld is not of the desired quality and would fail when passed through a continuous processing line. Figure 16B shows that the weld nugget produced by method 2 of the prior art developed a macroscopic crack 12' that propagated through the weld nugget. This weld is also not of the desired quality and would fail when passed through a continuous processing line. Figure 16C shows that the weld nugget produced by method 3 of the prior art developed a macroscopic crack 12' that propagated through the weld nugget.Again, this weld is not of the desired quality and would fail when passing through a continuous processing line. Finally, FIG. 16D shows that the weld nugget produced by the method of the invention 4. A macroscopic crack of 12” formed, but it did not propagate through the weld nugget. This weld achieves the desired quality and would not fail when passing through a continuous processing line.

Claims

1. A welding method, characterized in that it comprises the steps of: providing a first surface of a hardened ferrous alloy having a composition having a carbon equivalent (Ceq) of at least 0.45; providing a second surface of a hardened ferrous alloy having a composition having a carbon equivalent (Ceq) of at least 0.45; creating an overlap by overlapping said first surface and said second surface; welding said first surface to said second surface by heating said first surface and said second surface to a temperature sufficiently high to raise the temperatures of said first surface and said second surface to at least the melting points of said alloys to form a weld; cooling said weld between the Acs and Mf temperatures of said alloys;heating said weld at a rate of at least 10 °C / s to a temperature between the Ms and Aci temperatures of said alloys; cooling said weld below the Ms temperatures of said alloys; heating said weld at a rate of at least 10 °C / s to a temperature between the Ms and Aci temperatures of said alloys; cooling said weld to room temperature; wherein Ceq is defined as: Ceq = C + A (C) * [Si / 24 + Mn / 6 + Cu / 15 + Ni / 20 + (Cr + Mo + Nb + V) / 5 + 5B]; and A (C) = 0.75 + 0.25 tanh [20 (C - 0.12)]; furthermore, the composition of each of the elements C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B are in percent by weight.

2. The method according to claim 1, characterized in that at least one of said first surface and second surface are formed from a hardened ferrous alloy having a composition having a carbon equivalent (Ceq) of at least 0.

5.

3. The method according to claim 1, characterized in that said first surface is an end of a first steel coil and said second surface is an end of a second steel coil.

4. The method according to claim 3, characterized in that said step of welding said first surface to said second surface comprises electrical resistance seam welding.

5. The method according to claim 4, characterized in that said electrical resistance seam welding step is performed using a medium frequency direct current (MFDC) seam welder.

6. The method according to claim 5, characterized in that said medium frequency direct current (MFDC) seam welder includes welding wheels and a post-weld induction heater.

7. The method according to claim 6, characterized in that said seam welder forms said weld using two passes of said welder through said overlap.

8. The method according to claim 7, characterized in that the first of said two passes comprises: seam welding, which is carried out by said welding wheels; and immediately annealing said seam weld using said post-weld induction heater.

9. The method according to claim 8, characterized in that the second of said two passes comprises: decoupling said welding wheels; and annealing said weld using said post-weld induction heater.

10. The method according to claim 10, characterized in that said welding has load and ductility ratios of at least 70%.