JP2007222875A - Method and equipment for manufacturing high-strength steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of preventing shear crack and a shearing apparatus with which the shear crack is prevented which are suitable to a steel plate of ≥4 to <40 mm in thickness and ≥TS490 MPa. <P>SOLUTION: In this method, after shearing as soon as water cooling is performed after hot rolling, at least 3 mm from a shear face is removed with an edge milling device. This shearing apparatus is provided with an edge milling device on the outlet side of an end shear and before a flying inspection device. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a manufacturing method and manufacturing equipment for a high-strength thick steel plate, and more particularly to a material suitable for preventing shear cracking due to hydrogen embrittlement of a high-strength thick steel plate having a plate thickness of 4 mm or more and 40 mm or less and a tensile strength of 490 MPa or more.

  After shearing, in the vicinity of the sheared portion of the thick steel plate, deformation due to strong working during shearing, work hardening, and very high tensile residual stress are generated in the cross section after shearing due to them.

  Further, since residual hydrogen in the thick steel plate is likely to accumulate in the segregation and work-hardening portions inside the steel plate, hydrogen embrittlement cracks starting from the segregation and hardened portions are generated. When high-strength steel sheets with a thickness of 4 mm or more and 40 mm or less are melted and hot-rolled by a conventional method, the hydrogen content of the steel sheet after rolling is about 0.1 ppm, and the fine shear cracks generated on the shear plane are caused by the hydrogen. It is generated by accumulating on the shear surface where the residual stress is increased by shearing.

  Such shear cracks are likely to occur in a high-strength thick steel plate having a tensile strength of 490 MPa or more, and are not observed when the plate thickness is less than 4 mm. Moreover, since the shearing of thick steel plates exceeding 40 mm does not normally use a shearing machine, there is no concern about the occurrence of shear cracks.

  Thick steel plates that are concerned about shear cracking are subjected to slow cooling for the purpose of releasing residual hydrogen in the thick steel plates before or after shearing.

  Patent document 1 relates to a slow cooling method, and describes that when steel plates after rolling are stacked and gradually cooled, the steel plates are stacked so that the width and length of the steel plates are the same or increased sequentially from the bottom.

  Patent Document 2 describes that the critical hydrogen concentration is determined based on the yield stress prediction value of a steel sheet, and the optimum annealing time is calculated from the residual hydrogen rate by slab annealing and product annealing.

  As a phenomenon similar to shear cracking, cracks observed on end faces after bending a steel sheet when producing a thick steel pipe have been reported.

  Patent Document 3 relates to a method for manufacturing a thick-walled steel pipe, and in order to prevent end face cracks in a steel pipe whose tensile strain on the outer surface of the steel sheet is a specific value or more, the shear burr on the shear surface of the steel sheet can be removed by a grinder or cutting. Are listed.

Patent Document 4 relates to a side trimming method for a steel strip that is continuously conveyed, and shears the end of the steel strip with a rotary shearing round blade so as to leave a trim margin narrower than a predetermined width, and then the remaining trim with a cutting tool. It is described to cut the bill.
Japanese Patent Laid-Open No. 10-203212 JP-A-10-251746 JP 56-47213 A Japanese Patent Laid-Open No. 2-212028

  However, the method by slow cooling described in Patent Documents 1 and 2 is an off-line process, so that the conveyance process becomes complicated, and a prescribed slow cooling time must be observed. It is not a manufacturing method.

  The method described in Patent Document 3 is a method of removing a shear burr portion in which a steel sheet is turned in the shear direction after shearing in order to prevent cracks occurring on the end surface after bending, and about micro cracks generated on the shear surface. There is no description.

  Moreover, the side trimming method of the steel strip described in Patent Document 4 aims at preventing saw edges generated by cold rolling after hot rolling, and has an end of about 0.5 mm of a thin steel strip having a thickness of about 3 mm. It does not prevent hydrogen embrittlement cracking.

  Therefore, an object of the present invention is to provide a method and production equipment for preventing shear cracking due to hydrogen embrittlement and producing a high-strength steel plate having a sound shear surface.

  Since the high residual tensile stress generated on the hardened sheared surface after shearing is one cause of shear cracking, the present inventors have earnestly studied about a method for preventing shear cracking by removing the shearing surface. The present inventors have studied and found that shear cracks can be prevented when the hardened portion by shearing is removed so as to reduce the residual stress.

The present invention has been made based on further studies based on the obtained knowledge, that is, the present invention,
1. A method for producing a high-strength thick steel plate, characterized in that the steel plate is hot-rolled and cooled and then immediately sheared and at least 3 mm from the sheared surface is removed by a milling device within 60 minutes.
2. The method for producing a high-strength thick steel plate, wherein the high-strength thick steel plate has a plate thickness of 4 mm or more and 40 mm or less and a tensile strength of 490 MPa or more.
3. A high-strength thick steel plate manufacturing facility in which a milling device is arranged downstream of the shearing equipment between the shearing equipment and the running inspection process.

  According to the present invention, since the end portion of the thick steel plate where shear cracks are generated is reliably removed by the milling apparatus, not only shear cracks can be prevented but also the cross-sectional appearance is high, including the X120 standard with a tensile strength of 490 MPa or more. It is possible to efficiently manufacture a thick steel plate using an end shear, and an extremely useful effect in the industry is achieved.

Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing an example of an equipment layout in which a milling device suitable for carrying out the present invention is arranged. In the figure, 1 is a line arrangement after hot rolling, 2 is a hot leveler, 3a, 3b and 3c are cooling beds, Reference numeral 4 is a crop shear, 5 is a double side shear, 6a is a transfer, 7 is an end shear, 8 is a milling device, 9 is a running inspection device, and a is the flow (conveying direction) of the steel sheet.

  After hot-rolling to a predetermined plate thickness, the water-cooled steel plate is warped with a hot leveler 2 as necessary, and then cut with a crop shear 4 and the length is divided into a desired number of pieces. .

  Next, the side end portion along the rolling direction is cut off so as to obtain a desired plate width by the double side shear 5 and conveyed to the end shear 7 by the transfer 6a. The end shear 7 is cut to a desired plate length.

  In the present invention, the hardened portion is removed from the shearing surface of the steel sheet by the milling device 8 over at least 3 mm.

  The method for removing the hardened portion from the shear surface is cutting with a milling device. FIG. 2 shows an X-ray of an APIX100 standard (tensile strength of 760 MPa), a sheared surface of a thick steel plate having a thickness of 13.4 mm, a surface removed by cutting the sheared surface, and a surface removed by grinding (grinder grinding) the sheared surface. The result of the residual stress measurement is shown. In the case of cutting, the residual stress is reduced most.

  FIG. 3 shows an APIX56 standard steel plate (tensile strength: 490 MPa, plate thickness: 38 mm), hot-rolled, then water-cooled, immediately sheared, and the sheared surface was used as the surface layer. The results of a detailed investigation of the hardness in the thickness direction at a depth of 2 mm in depth are shown.

  From the figure, the hardness is extremely high at a position 1 mm deep from the shear surface, but decreases at a depth of 3 mm or more. In the hardness distribution from the steel plate surface to the back surface at a depth of 1 mm from the shear surface, the hardness of the steel plate back surface is lower than that of the steel plate surface. It is assumed that brittle fracture occurs near the lower blade of the end shear, and the increase in hardness is smaller than that near the upper blade where cleavage fracture occurs.

  On the other hand, no significant hardening is observed in the thickness direction of the steel sheet at a position 3 mm deep from the shear plane. Therefore, considering the hardness distribution from the front surface to the back surface of the steel plate, at least a portion having a depth of 3 mm is removed from the shear surface in order to remove the hardened portion after shearing.

  In addition, the presence or absence of shear cracks occurring on the shearing surface is confirmed by inspecting the shearing surface after about 24 hours have passed after shearing, and the thick steel plate where the shear cracking occurs is removed by gas cutting. Take measures such as to make a product.

  Therefore, although it is impossible to accurately grasp the elapsed time from shearing to occurrence of shear cracking, in the present invention, the effect of preventing shear cracking by removing at least a portion having a depth of 3 mm from the shearing surface is investigated. In this case, it was found that if a depth of 3 mm was removed from the sheared surface within 60 minutes after shearing with a milling device, no shear cracking occurred.

In the present invention, the steel plate is sheared by the end shear 7 immediately after being hot-rolled and water-cooled, and the above-described cutting is performed by a milling device disposed between the shearing equipment and the running inspection process within 60 minutes after the shearing. Do.

  With such a configuration, it is presumed that a synergistic effect of reducing the residual stress on the shear surface and the amount of hydrogen in steel can be obtained, and it is possible to prevent shear cracking, but details such as the degree of contribution of both are unknown.

  In the present invention, “immediately after water cooling” means within 20 minutes for a thin material having a thickness of 16 mm or less, and within 1 hour for a material having a thickness exceeding 16 mm.

  As the milling device, it is preferable to use a through-plate type milling device capable of cutting a four-round section of a thick steel plate with a desired cutting amount. Normally, the milling device 8 has a plate thickness of 40 mm and a capability of passing about 30 mpm so as to match the shearing capacity. It is possible to cut at least 3 mm from the sheared surface within 60 minutes after shearing.

A steel plate of APIX56 standard with a plate thickness of 38 mm was hot-rolled, water-cooled, sheared after 30 minutes, and after shearing, the steel plate was removed by cutting 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm from the sheared surface with a milling device after 20 minutes. 10 sheets of each were manufactured and the shear crack occurrence rate was investigated. What was not cut was made into the comparative example. The results are shown in FIG.
As can be seen from the figure, no shear cracking occurred when the portion of 3 mm or more from the shearing surface was cut off, but shear cracking occurred when the cutting depth was less than 3 mm.

The figure which shows an example of the equipment layout which has arrange | positioned the milling apparatus. The figure which shows the result of having performed the X-ray residual stress measurement about the surface which cut and removed the shear surface and the shear surface, and the surface which removed the shear surface by grinding (grinder grinding). It is a figure which shows the shear part cross-sectional hardness distribution of an API56 standard steel plate, and shows the thickness direction hardness distribution in depth 1-9mm from a shear plane. Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Line arrangement | positioning after hot rolling-water cooling 2 Hot levelers 3a, 3b, 3c Cooling bed 4 Crop shear 5 Double side shear 6a Transfer 7 End shear 8 Milling device 9 Running inspection device a Steel plate conveyance direction

Claims (3)

  A method for producing a high-strength thick steel plate, characterized in that the steel plate is hot-rolled and cooled, and then immediately sheared and at least 3 mm is removed from the sheared surface by a milling device within 60 minutes.   The method for producing a high-strength thick steel plate, wherein the high-strength thick steel plate has a plate thickness of 4 mm or more and 40 mm or less and a tensile strength of 490 MPa or more.   A high-strength thick steel plate manufacturing facility in which a milling device is arranged downstream of the shearing equipment between the shearing equipment and the running inspection process.

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