METHOD FOR MANUFACTURING METAL SHEET AND RAPID QUENCHING UNIT

Abstract

A method for manufacturing a metal sheet comprising pinching the metal sheet in rapid quenching between a pair of pinch rolls in the range where the temperature of the metal sheet is from (T.sub.Ms+150) (° C.) to (T.sub.Mf−150) (° C.), wherein the Ms temperature of the metal sheet is T.sub.Ms (° C.) and the Mf temperature thereof is T.sub.Mf (° C.), as well as a rapid quenching unit comprising a pair of pinch rolls capable of use in such a method.

Claims

1. A method for manufacturing a metal sheet using a continuous annealing line including a rapid quenching unit for cooling the metal sheet by immersing the metal sheet in a liquid, the method comprising pinching the metal sheet in rapid quenching between a pair of pinch rolls placed in a liquid in a range where a temperature of the metal sheet is from (T.sub.Ms+150) (° C.) to (T.sub.Mf−150) (° C.), T.sub.Ms (° C.) is a Ms temperature at which the martensite transformation of the metal sheet starts, T.sub.Mf (° C.) is a Mf temperature at which the martensite transformation thereof finishes.

2. The method for manufacturing the metal sheet according to claim 1, wherein a pinch position of the pair of pinch rolls is set on the basis of a threading speed, a thickness, and a quenching start temperature of the metal sheet.

3. The method for manufacturing the metal sheet according to claim 1, wherein a distance d (mm) from a water surface to a rotation center of the pinch roll is given by the following formula: $\begin{array}{cc}v\times \frac{t\left(T-T_{\mathrm{Ms}}-150\right)}{1.5}\le d\le v\times \frac{t\left(T-T_{\mathrm{Mf}}+150\right)}{1.5}& \left[\mathrm{Math}..Math.1\right]\end{array}$ T.sub.Ms (° C.) is the Ms temperature of the metal sheet, T.sub.Mf (° C.) is Mf temperature of the metal sheet, v (m/s) is a feed rate of the metal sheet, t (mm) is a thickness of the metal sheet, T (° C.) is a quenching start temperature, and d (mm) is the distance from the water surface to the rotation center of each pinch roll.

4. The method for manufacturing the metal sheet according to claim 1, wherein the rapid quenching unit includes water ejecting devices for ejecting cooling water to a front surface and back surface of the metal sheet and a pair of the pinch rolls pinch the metal sheet, placed between the metal sheet and the water ejecting devices.

5. A rapid quenching unit for cooling a high-temperature metal sheet by immersing the metal sheet in a liquid, comprising a pair of pinch rolls, wherein the pinch rolls pinch the metal sheet in the range where the temperature of the metal sheet is from (T.sub.Ms+150) (° C.) to (T.sub.Mf−150) (° C.), T.sub.Ms (° C.) is a Ms temperature of the metal sheet, and T.sub.Mf (° C.) is a Mf temperature thereof.

6. The rapid quenching unit according to claim 5, further comprising water ejecting device for ejecting cooling water to a front surface and back surface of the metal sheet, wherein the pinch rolls are placed between the metal sheet and the water ejecting device.

7. The method for manufacturing the metal sheet according to claim 2, wherein a distance d (mm) from a water surface to a rotation center of the pinch roll is given by the following formula: $\begin{array}{cc}v\times \frac{t\left(T-T_{\mathrm{Ms}}-150\right)}{1.5}\le d\le v\times \frac{t\left(T-T_{\mathrm{Mf}}+150\right)}{1.5}& \left[\mathrm{Math}..Math.1\right]\end{array}$ T.sub.Ms (° C.) is the Ms temperature of the metal sheet, T.sub.Mf (° C.) is Mf temperature of the metal sheet, v (m/s) is a feed rate of the metal sheet, t (mm) is a thickness of the metal sheet, T (° C.) is a quenching start temperature, and d (mm) is the distance from the water surface to the rotation center of each pinch roll.

8. The method for manufacturing the metal sheet according to claim 2, wherein the rapid quenching unit includes water ejecting devices for ejecting cooling water to a front surface and back surface of the metal sheet and a pair of the pinch rolls pinch the metal sheet, placed between the metal sheet and the water ejecting devices.

9. The method for manufacturing the metal sheet according to claim 3, wherein the rapid quenching unit includes water ejecting devices for ejecting cooling water to a front surface and back surface of the metal sheet and a pair of the pinch rolls pinch the metal sheet, placed between the metal sheet and the water ejecting devices.

10. The method for manufacturing the metal sheet according to claim 7, wherein the rapid quenching unit includes water ejecting devices for ejecting cooling water to a front surface and back surface of the metal sheet and a pair of the pinch rolls pinch the metal sheet, placed between the metal sheet and the water ejecting devices.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is an illustration of a rapid quenching unit according to an embodiment of the present invention.

[0021] FIG. 2 is a graph showing the relationship between the position of the rotation center of a pinch roll and the camber of a steel sheet after the pinch roll passed in an example.

[0022] FIG. 3 is an illustration showing the camber used in FIG. 2.

[0023] FIG. 4 is a graph showing the relationship between the feed rate v (m/s) of a steel sheet and the distance d (mm) from the water surface to the rotation center of a pinch roll.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0024] Embodiments of the present invention are described below with reference to the attached drawings.

[0025] FIG. 1 is an illustration of a rapid quenching unit according to an embodiment of the present invention. The rapid quenching unit is used in a cooling line placed on the delivery side of a soaking zone of a continuous annealing furnace. In FIG. 1, a pair of seal rolls 3 placed at an outlet of the soaking zone of the continuous annealing furnace is shown. The rapid quenching unit includes a water tank 1 filled with water 2 (liquid); water ejecting devices 4, placed in the water tank 1, for applying cooling water to a metal sheet 5 to cool the metal sheet 5 to the water temperature; and a sink roll 6 which immerses the metal sheet 5 in the water tank 1 and which changes the transport direction of the metal sheet 5.

[0026] The water ejecting devices 4 are partly placed in the water tank 1. The water ejecting devices 4 are arranged on the front side and back side of the metal sheet 5 with a predetermined spaced therebetween. The water ejecting devices 4, which are arranged on the front and back sides thereof, each include nozzles 4a extending in a lateral direction of the metal sheet 5. The nozzles 4a are arranged in the transport direction of the metal sheet 5. The water ejecting devices 4 eject cooling water from the nozzles 4a to the metal sheet 5 to rapidly cool the metal sheet 5.

[0027] The metal sheet 5 that is below the water surface is thermally shrunk by rapidly cooling the metal sheet 5 with cooling water. In particular, when the temperature of the metal sheet 5 is reduced to the Mf temperature that is the temperature at which martensite transformation finishes from the Ms temperature that is the temperature at which martensite transformation starts, rapid thermal shrinkage and transformation expansion occur in the metal sheet 5 together to maximize the stress acting in the metal sheet 5 and the metal sheet 5 becomes out of shape.

[0028] Therefore, according to embodiments of the present invention, supposing that the Ms temperature of the metal sheet is T.sub.Ms (° C.) and the Mf temperature thereof is T.sub.Mf (° C.), pinch rolls 7 pinching the metal sheet 5 in rapid quenching are placed below the water surface in the range where the temperature of the metal sheet 5 is from (T.sub.Ms+150) (° C.) to (T.sub.Mf−150) (° C.). In particular, a pair of the pinch rolls 7 are placed in spaces between the metal sheet 5 and the nozzles 4a of the water ejecting devices 4 so as to pinch both sides of the metal sheet 5. The reason why the position of each pinch roll 7 is in a region from the Ms temperature plus 150° C. to the Mf temperature minus 150° C. is that the camber was sufficiently reduced in this range in an example described below with reference to FIG. 4.

[0029] The Ms temperature and the Mf temperature can be calculated from the composition of the metal sheet 5.

[0030] A pair of the pinch rolls 7 are preferably placed such that the center axes thereof are misaligned in the transport direction of the metal sheet 5. Placing the pinch rolls 7 such that the center axes thereof are misaligned enables the pinching force of the metal sheet 5 to be increased, thereby enabling the shape correction force to be increased.

[0031] The preferred position of each pinch roll 7 is preferably set on the basis of the sheet feed rate v (m/s), the sheet thickness t (mm), and the quenching start temperature T (° C.). Supposing that the cooling rate is 1,500/t (° C./s), the position from the water surface that the temperature of the metal sheet 5 is (T.sub.Ms+150) (° C.) can be given by Formula (1). Incidentally, the cooling rate is a value determined depending on the sheet thickness or the like. When the sheet thickness is 1 mm, the cooling rate is 1,000/t to 2,000/t (° C./s). Therefore, in embodiments of the present invention, the cooling rate is 1,500/t (° C./s), which is an intermediate value. The cooling rate can be appropriately set depending on the sheet thickness and the like.

[00002]$\begin{array}{cc}\left[\mathrm{Math}..Math.2\right].Math.& \\ v\times \frac{\left(T-\left(T_{\mathrm{Ms}}+150\right)\right)}{\frac{1500}{t}.Math..Math.\left(m\right)}=v\times \frac{t\left(T-T_{\mathrm{Ms}}-150\right)}{1.5.Math..Math.\left(\mathrm{mm}\right)}& \left(1\right)\end{array}$

[0032] Likewise, the position from the water surface that the temperature of the metal sheet 5 is (T.sub.Mf−150) (° C.) can be given by Formula (2).

[00003]$\begin{array}{cc}\left[\mathrm{Math}..Math.3\right].Math.& \\ v\times \frac{\left(T-\left(T_{\mathrm{Mf}}-150\right)\right)}{\frac{1500}{t}.Math..Math.\left(m\right)}=v\times \frac{t\left(T-T_{\mathrm{Mf}}+150\right)}{1.5.Math..Math.\left(\mathrm{mm}\right)}& \left(2\right)\end{array}$

[0033] Thus, the distance d (mm) from the water surface to the rotation center of each pinch roll 7 is preferably given by Formula (3).

[00004]$\begin{array}{cc}\left[\mathrm{Math}..Math.4\right].Math.& \\ v\times \frac{t\left(T-T_{\mathrm{Ms}}-150\right)}{1.5}\le d\le v\times \frac{t\left(T-T_{\mathrm{Mf}}+150\right)}{1.5}& \left(3\right)\end{array}$

[0034] The rotation center of the pinch roll 7 corresponds to the pinch position of the metal sheet 5 pinched between the pinch rolls 7. Referring to FIG. 1, the two pinch rolls 7, which pinch the metal sheet 5, are placed so as to be misaligned in the transport direction of the metal sheet 5. The position of each pinch roll 7 preferably satisfies the above-mentioned range.

[0035] In embodiments of the present invention, since the pinch rolls 7, which can pinch the metal sheet 5, are placed below the water surface in the range where the temperature of the metal sheet 5 is from the Ms temperature to the Mf temperature, the shape of the metal sheet 5 can be effectively corrected in such a manner that the metal sheet 5 is pinched at a position at which the highest stress acts in the metal sheet 5.

[0036] As described above, embodiments of the present invention are intended to reduce a complicated, uneven irregular shape that is caused when martensite transformation occurs during the rapid cooling of a steel sheet to expand the volume of a microstructure. Embodiments of the present invention are preferably applied to a method for manufacturing a high-strength cold-rolled steel sheet (Haiten).

[0037] In particular, embodiments of the present invention are preferably applied to a method for manufacturing a steel sheet with a tensile strength of 580 MPa or more. The upper limit of the tensile strength is not particularly limited and is, for example, 1,600 MPa or less. An example of the composition of the high-strength cold-rolled steel sheet is as follows: C is 0.04% to 0.220%, Si is 0.01% to 2.00%, Mn is 0.80% to 2.80%, P is 0.001% to 0.090%, S is 0.0001% to 0.0050%, and sol. Al is 0.005% to 0.065% on a mass basis, the remainder being Fe and inevitable impurities. At least one or more of Cr, Mo, Nb, V, Ni, Cu, and Ti are 0.5% or less as required. B and/or Sb is 0.01% or less as required.

Example

[0038] A high-tensile strength cold-rolled steel sheet having a thickness of 1.0 mm, a width of 1,000 mm, and a tensile strength of about 1,470 MPa was manufactured at a feed rate of 1.0 m/s using a rapid quenching unit shown in FIG. 1. The quenching start temperature T of the steel sheet is 740° C., the quenching finish temperature thereof is 50° C., the Ms temperature T.sub.Ms thereof is 350° C., and the Mf temperature T.sub.Mf thereof is 250° C.

[0039] FIG. 2 shows the relationship between the distance from the water surface to the rotation center of each pinch roll and the camber of the steel sheet after the roll passed. FIG. 3 shows the definition of the camber. In particular, the camber was defined as the highest position when the steel sheet was placed on the horizontal.

[0040] In FIG. 2, the horizontal axis represents the distance from the water surface of a water tank 1 to the pinch roll 7 and the vertical axis represents the camber of the steel sheet. The steel sheet is pinched between the pinch rolls 7 at a position which is 200 mm to 400 mm below the water surface and at which the temperature of the steel sheet is from the Ms temperature to the vicinity of the Mf temperature, whereby the camber is reduced to 10 mm or less.

[0041] In order to investigate the influence of the feed rate of a steel sheet, high-tensile strength cold-rolled steel sheets having a thickness of 1.0 mm, a width of 1,000 mm, and a tensile strength of about 1,470 MPa were manufactured at a feed rate of 1.0 m/s, 1.5 m/s, or 2.0 m/s using the rapid quenching unit shown in FIG. 1. The quenching start temperature is 740° C., the quenching finish temperature is 50° C., the Ms temperature T.sub.Ms is 350° C., and the Mf temperature T.sub.Mf is 250° C.

[0042] FIG. 4 is a graph showing the relationship between the feed rate v (m/s) of each steel sheet and the distance d (mm) from the water surface to the rotation center of each roll. In a combination of the feed rate v (m/s) of the steel sheet and the distance d (mm) from the water surface to the rotation center of the roll, the camber of the steel sheet was measured. A camber of less than 10 mm was rated “◯” and a camber of 10 mm or more was rated “x”.

[0043] When the relationship between the Ms temperature T.sub.Ms of a steel sheet, the Mf temperature T.sub.Mf thereof, the feed rate v (m/s) thereof, the thickness t (mm) thereof, the quenching start temperature T (° C.) thereof, and the distance d (mm) from the water surface to each pinch roll 7 was in the range vt(T−T.sub.Ms−150)/1.5≦d≦vt(T−T.sub.Mf+150)/1.5, a good result was obtained.

[0044] In this embodiment, an apparatus for water-cooling a steel sheet has been exemplified. The present invention is not necessarily limited to this. The technical concept of the present invention is broad, can be used to cool all metal sheets other than steel sheets, and can be applied to all rapid quenching units other than water-cooling units.

REFERENCE SIGNS LIST

[0045] 1 Water tank [0046] 2 Water [0047] 3 Seal rolls [0048] 4 Water ejecting devices [0049] 4a Nozzles [0050] 5 Metal sheet [0051] 6 Sink roll [0052] 7 Pinch rolls