US3028468A - Production of flat high strength steel sheets - Google Patents

Description

April 3, 1962 R. w. HOLMAN 3,028,458

PRODUCTION OF FLAT HIGH STRENGTH STEEL SHEETS Filed July 31, 1959 INVENTOR ROBERT W HOLMA/V United States Patent M 3,028,468 PRODUCTION OF FLAT HIGH STRENGTH STEEL SHEETS Robert W. Holman, Birmingham, Ala., assignor to United States Steel Corporation, a corporation of New Jersey Filed July 31, 1959, Ser. No. 830,947 2 Claims. (Cl. 219--7.5)

This application relates to improvements in the production of high strength steel sheets and more particularly to the production of flat high strength steel sheets.

It has heretofore been proposed to produce steel sheets having high strength by forming the sheets of hardenable steels and then heat treating, i.e. quenching from above the critical temperature of the steel and tempering to produce the desired combination of physical properties. However, the heat treatment, particularly in sheet gauges results in considerable warpage and distortion. Attempts to flatten the sheets by conventional stretcher levelling and the like have not been successful because of the high incidence of breakage even at elongations too small to produce the desired flatness.

It is accordingly an object of this invention to provide an improved method of flattening high strength steel sheets.

It is a further object of this invention to provide an improved method of stretcher levelling high yield strength notch sensitive steel sheets.

The foregoing and further objects will be apparent from the following specification when read in conjunction with the accompanying drawing, wherein:

FIGURE 1 is a plan view; and

FIGURE 2 is a side elevation.

Referring more particularly to the drawing, the numeral 2 indicates a suitable conveyor table for a sheet 4. The sheet 4 may be advanced by feed rolls 6 driven by suitable motors and gear trains 8. Adjustable side guides 10 are adapted to properly position the sheet 4 with respect to high frequency induction heaters 12 disposed on opposite sides of the conveyor table. The motors and feed rolls are adapted to advance the sheet past the induction heaters 12 at the desired rate to heat the edges of the sheet to slightly below, i.e. within about 100, of the critical temperature of the steel thereof.

A conventional stretcher leveler for flattening the sheets after edge treatment is shown in FIGURE 2. This comprises a pair of gripper members 20, one of which is fixedly mounted and the other movably mounted. The movably mounted member may be actuated by a hydraulic cylinder 22 to stretch a sheet 4 held by the gripper members.

In practicing my invention, a hardened and tempered sheet 4 which may be composed Of a ha de b e W6 containing about 3,028,468 Patented Apr. 3, 1962 Percent .25 carbon 12.0 chromium 1.0 molybdenum .25 vanadium The sheet is placed on the conveyor 2 and drawn between the induction heaters. The induction heaters and the speed of advance of the strip are regulated to heat the sheet edges to about 1200 F., i.e. slightly below the critical temperature to a transverse depth of about 1 to 5 times the strip thicknesses in a very short time. By heating sufficiently rapidly, the heat imparted to the edges can flow inwardly of the strip sufficiently rapidly to quench the edges.

Thereafter the edge treated strip is placed in the stretcher leveller and stretched to a suflicient elongation to flatten it. This eliminates breakage due to edge sensitivity.

While I have shown and described a specific embodiment of my invention, it will be understood that this embodiment is merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim: s

1. In the method of stretcher flattening heat-treatment hardened steel sheets characterized by freedom of edge cracking after stretcher flattening the improvement comprising quickly heating the edge portions only of said sheets to slightly below the lower critical temperature of the steel at a rate sufliciently rapid that the heated edge portions will be quenched by inflow of heat therefrom into the body of the sheet prior to stretcher flattening the sheet and then stretching said sheets to flatten them.

2. In the method of stretcher flattening heat-treatment hardened steel sheets characterized by freedom from edge cracking after stretcher flattening the improvement comprising quickly heating the edge portions only of said sheets to a depth of about 1 t0 5 times the strip thickness to a temperature slightly below the lower critical temperature of the steel at a rate sufficiently rapid that the heated edge portions will be quenched by inflow of heat therefrom into the body of the sheet prior to stretcher flattening the sheet and then stretching said sheets to flatten them.

References Cited in the file of this patent UNITED STATES PATENTS 2,448,062 Stoltz Aug. 31, 1948 2,477,411 King July 26, 1949 2,507,817 Ropp May 16, 1950 2,716,688 Lackner Aug. 30, 1955 2,873,344 Kooks Feb. 10, 1959

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