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An apparatus and method for making a seam-welded steel pipe free of untempered martensite without seam anneal. The method includes selecting a steel containing a carbon concentration below a predetermined level, for example, 0.14% or 0.12% by weight. The method also includes flooding both outside an

An apparatus and method for making a seam-welded steel pipe free of untempered martensite without seam anneal. The method includes selecting a steel containing a carbon concentration below a predetermined level, for example, 0.14% or 0.12% by weight. The method also includes flooding both outside and inside of the strip with a coolant while the weld seam is being formed, and continuing to immerse the welded strip for a sufficient time after the weld seam is formed to prevent the formation of untempered martensite. The apparatus includes a heater capable of heating the strip to a temperature sufficient to form a welded seam, a cooling module configured to supply a coolant to the welded seam both inside and outside of the strip as the weld seam is being formed, and another cooling module configured to immerse the welded strip in a coolant after the weld seam is formed for a sufficient length of time to prevent the formation of untempered martensite.

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What is claimed is: 1. A method of making a steel structure substantially free of untempered martensite, the method includes: (a) forming a welded joint between two steel portions; (b) applying a coolant to the welded joint while the welded joint is being formed; and (c) cooling the welded joint to

What is claimed is: 1. A method of making a steel structure substantially free of untempered martensite, the method includes: (a) forming a welded joint between two steel portions; (b) applying a coolant to the welded joint while the welded joint is being formed; and (c) cooling the welded joint to below martensite formation temperature after the joint is formed by continuing to subject the welded joint to the coolant for a sufficient length of time so that substantially no untempered martensite is present in the welded joint when the welded joint is cooled to below martensite formation temperature. 2. A method of making a steel structure substantially free of untempered martensite, the method includes: (a) forming a welded joint between two steel portions by electrical resistance welding or inductive welding; (b) subjecting the welded joint to a coolant while the welded joint is being formed; and (c) cooling the welded joint to below martensite formation temperature after the joint is formed by continuing to subject the welded joint to the coolant for a sufficient length of time so that substantially no untempered martensite is present in the welded joint when the welded joint is cooled to below martensite formation temperature. 3. The method of claim 1, further comprising selecting a material for the two steel portions to contain not more that 0.14 weight percent carbon. 4. The method of claim 3, wherein the selecting step comprises selecting the material for the two steel portions to contain not more that 0.12 weight percent carbon. 5. A method of making a steel pipe, the method comprising: (a) forming a steel pipe segment with a gap along a longitudinal direction of the pipe, the gap being defined between two opposing steel edges; and (b) making a seam-sealed pipe according to the method of claim 1, wherein the steel structure is the seam-sealed pipe, and the two steel portions comprise the steel edges. 6. The method of claim 5, wherein the step of subjecting the welded joint to a coolant while the welded joint is being formed comprises applying the coolant to the welded joint from both inside and outside the pipe. 7. The method of claim 6, wherein the step of forming a welded joint comprises electrical resistance welding. 8. The method of claim 6, further comprising selecting a material for the two steel portions to contain not more that 0.14 weight percent carbon. 9. The method of claim 6, wherein the selecting step comprises selecting the material for the two steel portions to contain not more that 12 weight percent carbon. 10. The method of claim 1, wherein the step of forming a welded joint comprises induction welding or electrical resistance welding. 11. The method of claim 10, wherein the step of forming a welded joint comprises induction welding. 12. The method of claim 10, wherein the step of forming a welded joint comprises electrical resistance welding.