JP5299100B2 - Method for producing multilayer steel sheet with excellent corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a double layered steel sheet that can surely reform the surface layer region of a steel cast slab by reducing variance of density distribution of alloys in the surface layer region and that has a high weather resistance as well as excellent corrosion resistance, and to provide a method of manufacturing the same. <P>SOLUTION: The composition of additional elements in the surface layer regions 1A, 1B up to 1-20% from the front face 1a and the rear face 1b in the thickness t direction is Cu+Ni+Cr+Mo+Nb+inevitable impurities in the range of 3-30%, wherein the respective components of the additional elements in the surface layer regions 1A, 1B are thicker than in the inside of the steel sheet 1C. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

The present invention is provided with a high weather resistance, excellent method of manufacturing a multilayered steel plate corrosion resistance.

  Conventionally, in the field of steel sheets made of general carbon steel (steel composed of five main elements), for example, by adding elements such as Ni, Cr, Mo, Nb, V, etc., to impart corrosion resistance and weather resistance. Low alloy, high alloy steel and stainless steel have been developed. However, since the above-described elements are very expensive, when these elements are added to the entire steel plate made of alloy steel, there is a problem that the manufacturing cost increases.

  In order to reduce the amount of elements added as described above, as a method for producing a multilayer steel sheet in which elements are added only to the surface layer of a steel slab, for example, when continuously casting a steel slab, two types of molten steel are simultaneously used. A casting method has been proposed (see, for example, Patent Document 1). According to the method disclosed in Patent Document 1, it is possible to produce multilayer slabs of various combinations by casting two types of molten steel at the same time. However, in the method of Patent Document 1, it is necessary to prepare two kinds of components in the molten steel stage, and a ladle for containing molten steel, a tundish used when pouring molten steel from a ladle into a mold, Since it is necessary to prepare two types of nozzles and the like, there is a problem that the manufacturing cost increases.

  Further, in order to solve the problems of the conventional method disclosed in Patent Document 1 as described above, the surface layer of the steel slab is melted by either one of induction heating or plasma heating, or both methods, and melted. There has been proposed a melt reforming method in which an additive element is added alone or an alloy thereof is added to the surface layer portion of the steel slab (see, for example, Patent Documents 2 and 3). According to the methods disclosed in Patent Documents 2 and 3, it is not necessary to prepare molten steel having a plurality of components, and manufacturing equipment and the like can be configured simply. It becomes possible to

  However, according to the methods disclosed in Patent Documents 2 and 3, depending on the method and conditions of adding an alloy to the melted portion in the surface layer region of the steel slab, the element concentration in the surface layer region varies and cannot be uniformly modified. There's a problem. Specifically, when an alloy is added to the melted portion of the surface layer region of the steel slab, the addition method using the alloy powder, when the powder is placed on the surface of the steel slab, Since the position changes, the concentration of the additive element in the surface layer region varies. In addition, when the alloy powder is blown onto the surface of the steel slab, the powder is repelled by a plasma air flow or the like, and there is a problem that the additive element concentration in the surface layer region varies as described above. On the other hand, in the addition method using an alloy sheet, the above-mentioned problems do not occur, but technically difficult problems remain for practical use in the process.

  In addition, in the case of the addition method using a wire-like alloy, the wire needs to be heated and melted by heat transfer from the melting portion of the steel slab in addition to the radiant heat from the steel slab and plasma. However, when the wire supply speed Vw (unit: m / sec, g / sec, etc.) is relatively slow, the wire tip is heated and melted only by radiant heat. In this case, the size of the melted portion (hereinafter sometimes abbreviated as a droplet) at the tip of the wire is determined by the mass, surface tension, jet pressure, and when the gravity or the like becomes larger than the surface tension, the droplet Leaves the wire tip and falls to the surface of the steel slab. Since the supply of the alloy by the droplets is microscopically discontinuous, there is a problem in that the alloy concentration varies in the melted part due to the plasma. However, Patent Documents 2 and 3 do not disclose any wire supply method and conditions. Therefore, in the conventional method, the surface layer region of the steel slab cannot be uniformly reformed, and as a result, there is a problem that the yield decreases in hot rolling or the like.

JP 63-108947 A JP 2004-195512 A JP 2004-195513 A

The present invention has been made in view of the above-described problems, and can reduce the variation in the concentration distribution of the alloy in the surface layer region, can reliably modify the surface layer region of the steel slab, has high weather resistance, and has corrosion resistance. and to provide an excellent method for producing a multilayered steel plate.

The inventors of the present invention have made extensive studies to solve the above-mentioned problems. As a result, by optimizing the reforming conditions of the surface layer region in the steel slab and the alloy composition after the reforming, the multilayered steel sheet after rolling The present inventors have found that the corrosion resistance of can be greatly improved and completed the present invention.
That is, the present invention relates to the following.

[ 1 ] The surface and back surfaces of continuously cast steel slabs are irradiated with plasma from a plurality of plasma torches arranged at equal intervals to heat and melt the surface layer regions on the front and back surfaces. A surface layer reforming step for modifying the surface layer region by adding a wire-shaped alloy to the surface layer region, and a steel sheet in which the surface layer region is modified in the surface layer reforming step by rolling the steel slab The surface layer reforming step includes: ± Lo (mm) for the oscillation width of the wire, Fo (mm / sec) for the oscillation speed, and the plasma. The torch interval is PL (mm), the melting rate of the steel slab is V (mm / sec), and the length of the molten pool formed by melting the surface region of the steel slab is Lp (mm). When these are The wire is continuously melted while oscillating with the oscillation center position of the wire aligned with the center between the plasma torches so as to satisfy both of the relationships represented by the formulas (1) and (2). By supplying to the pool, the component composition of each additive element of Cu, Ni, Cr, Mo and Nb in the surface layer region where the thickness from the front surface and the back surface of the steel slab is 1 to 20% is mass%. In the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%}, each component of the additive element in the surface layer region is concentrated from the inside of the steel slab, and is excellent in corrosion resistance. A method of manufacturing a steel sheet.

[ 2 ] In the surface layer reforming step, the component composition of the additive element in the surface layer region of the steel slab is, in mass%, Cu: 0 to 1.5%, Ni: 3 to 30%, Cr: 0 to 0. 30%, Mo: 0 to 3%, Nb: 0 to 1%, and the range of the following formula {Cu + Ni + Cr + Mo + Nb + unavoidable impurities = 3 to 30%}, and further from the front and back surfaces of the steel slab The compound having excellent corrosion resistance according to [ 1 ] above, wherein the surface layer region is modified so that a change rate of a component composition of the additive element in the plate thickness direction is 80% / mm or less. A method for producing a layered steel sheet.
[ 3 ] In the surface layer reforming step, the surface layer region where each component of the additive element is concentrated from the inside of the steel slab has a thickness from 3 to 8% from the front and back surfaces of the steel slab. The method for producing a multilayer steel sheet having excellent corrosion resistance according to the above [ 1 ] or [ 2 ], wherein the range is modified as a range.
[ 4 ] The surface layer reforming step, when supplying the wire, imparts vibrations having an amplitude of ± 2 mm or more and ± 5 mm or less and a frequency of 20 Hz or more and 80 Hz or less at the wire tip. The manufacturing method of the multilayer steel plate excellent in corrosion resistance of any one of [ 1 ]-[ 3 ].

According to the multilayer steel sheet having excellent corrosion resistance according to the present invention, the variation in the concentration distribution of the alloy in the surface layer region is reduced by optimizing the component composition and concentration distribution of the additive element in the modified surface region. It is possible to provide a multilayer steel sheet having high weather resistance and excellent corrosion resistance at low cost.
Moreover, according to the manufacturing method of the multilayer steel plate excellent in corrosion resistance of the present invention, the supply speed of the wire supplied to the melted portion of the surface layer region in the steel slab (droplet addition frequency), the supply position, and the molten state of the wire Therefore, variation in alloy concentration in the surface layer region is suppressed. As a result, it is possible to manufacture a multi-layer steel sheet having high corrosion resistance and excellent corrosion resistance at a low cost with high productivity.
According to the present invention, as described above, the multi-layer steel plate having excellent corrosion resistance can be obtained with high productivity without incurring an increase in cost, and thus the industrial effect is immeasurable.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically 1st Embodiment of the multilayer steel plate excellent in the corrosion resistance of this invention, and its manufacturing method, and is sectional drawing which shows the structure of a multilayer steel plate. It is a figure which illustrates typically 1st Embodiment of the multilayer steel plate excellent in the corrosion resistance of this invention, and its manufacturing method, and is a perspective view which shows the procedure of a surface layer modification | reformation process. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically 1st Embodiment of the multilayer steel plate excellent in the corrosion resistance of this invention, and its manufacturing method, and is a side view which shows the procedure of a surface layer modification | reformation process. It is a figure which illustrates typically 1st Embodiment of the multilayer steel plate excellent in the corrosion resistance of this invention, and its manufacturing method, and is a top view which shows the procedure of a surface layer modification | reformation process. It is a figure which illustrates typically 2nd Embodiment of the multilayer steel plate excellent in corrosion resistance of this invention, and its manufacturing method, and is a perspective view which shows the procedure of a surface layer modification | reformation process. It is a figure explaining typically the 1st and 2nd embodiment of the multilayer steel plate excellent in corrosion resistance of the present invention, and its manufacturing method, and is a sectional view showing the structure of the steel slab before rolling.

  Hereinafter, an embodiment of a multilayer steel sheet excellent in corrosion resistance of the present invention and a method for producing the same will be described with reference to FIGS. Note that this embodiment is described in detail for better understanding of the gist of the invention, and thus does not limit the present invention unless otherwise specified.

  In the present invention, the surface layer region of continuously cast steel slab made of carbon steel is melted by plasma heating, and a wire made of an alloy such as Ni, Cr, Mo, Cu, Nb or the like is melted on the melted surface layer region. Each element is added to the surface layer region. Thereby, the surface layer region of the steel slab is modified, and the corrosion resistance of this region is improved. Therefore, excellent corrosion resistance can be obtained even in the multilayer steel plate obtained by rolling the steel slab. Also, only the surface layer region of the steel slab is modified as an alloy layer having corrosion resistance, and no modification treatment is performed on the inside of the steel slab, so it has superior corrosion resistance compared to the conventional multilayer steel plate and its manufacturing method. It is possible to provide a multilayer steel sheet at low cost.

[First Embodiment]
Hereinafter, a multi-layer steel sheet excellent in corrosion resistance according to the present invention (hereinafter sometimes simply referred to as a multi-layer steel sheet) and a first embodiment of the manufacturing method thereof will be described in detail with reference to FIGS. Explained.

<Multi-layer steel plate>
First, the multilayer steel plate 1 of this embodiment is demonstrated, referring FIG. In the multilayer steel sheet 1 of the present embodiment, the component composition of each additive element of Cu, Ni, Cr, Mo, and Nb in the surface layer regions 1A and 1B from the front surface 1a and the back surface 1b to 1 to 20% in the thickness t direction. Is in a range represented by the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%} in mass%, and each component of the additive element in the surface layer regions 1A and 1B is concentrated more than the steel plate interior 1C, It is roughly structured.

  First, this invention prescribes | regulates about the alloy composition modified | reformed with the addition element especially in surface layer area | region 1A, 1B of the multilayer steel plate 1, About other steel components, the component composition of steel plate inside 1C, etc. Is not particularly limited.

"Additive elements"
In the multilayer steel sheet 1 according to the present invention, at least one or more of Cu, Ni, Cr, Mo and Nb elements are added in the surface layer regions 1A and 1B. Further, the component composition in the surface layer regions 1A and 1B is in a range expressed by mass% and represented by the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%}.

  By adding elements such as Cu, Ni, Cr, Mo, and Nb described above to the steel material, an effect of increasing the corrosion resistance of the steel material can be obtained. Moreover, in the multilayer steel plate 1 which concerns on this invention, in addition to the alloy by the multiple component of said each additional element as a component composition of the additional element in surface layer area | region 1A, 1B, the compound of each said additional element and nitrogen, etc. are included. Can do.

  The upper limit of the total of each additive element is restricted by the element addition ability in the manufacturing method (process) described later in detail. If the total content of the above-mentioned additive elements including the inevitable impurities in the surface layer regions 1A and 1B, that is, the concentration amount = dilution rate exceeds 30%, the surface properties are lowered, so that rolling failure is caused in the rolling process described later. Since it may occur, the total content of additive elements is 30% or less. Further, if the total content of the above additive elements including inevitable impurities is less than 3%, it is difficult to obtain the effect of improving the corrosion resistance. Therefore, in the present invention, Cu, Ni, Cr, Mo including inevitable impurities are not obtained. And the total content of Nb is prescribed | regulated to the range of 3 to 30%.

Moreover, in the multilayer steel plate 1 of this embodiment, the component composition of each said addition element in surface layer area | region 1A, 1B is the mass%, Cu: 0-1.5%, Ni: 3-30%, Cr: 0. -30%, Mo: 0 to 3%, Nb: 0 to 1%, and the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%}, the corrosion resistance is further improved It is further preferable from the point that it becomes possible.
Hereinafter, the preferable addition amount and effect of each additive element will be described in detail. In the following description, unless otherwise specified, the content of each additive element is mass%.

"Cu: Copper" 0-1.5%
Cu is an element necessary for improving weather resistance by leaching together with Fe during formation of the rust layer, suppressing coarsening of the rust particles, and maintaining the denseness of the rust layer. In order to exhibit such an effect of densifying the rust layer, it is more preferable to add Cu at 0.1% or more. On the other hand, the higher the Cu content, the more effective in terms of improving weather resistance, but even if added over 1.5%, the effect is saturated, and the problem of cracking during hot rolling Therefore, the upper limit of the amount of Cu is 1.5%.

"Ni: Nickel" 3-30%
Ni is an element more effective than Cu in terms of densifying the rust layer, and is an essential element together with Cu. When Ni is contained in the rust layer, it elutes together with Fe in the steel, and Ni is contained almost uniformly in the rust layer, so that the ground of the Cl ion rust layer derived from the flying salt attached to the surface of the rust layer. Suppression of the iron interface is suppressed, and the inside of the rust layer is made a low Cl ion environment. Thus, Ni has the effect of promoting the densification of rust particles in a high flying salinity environment.

However, although it depends on specific conditions such as the amount of flying salt or the shaded environment, the more severe the salt environment, the higher the Ni content needs to be. Further, the higher the Ni content, the more effective the weather resistance and the corrosion resistance are improved. The upper limit of the Ni content is determined for other reasons than the weather resistance and the corrosion resistance.
The content of Ni is preferably in the range of 3 to 30% in order to stably obtain the above effects.

"Cr: Chrome" 0-30%
Cr has an effect of forming an oxide film on the surface layer of the composite steel sheet and improving the corrosion resistance. Further, the higher the Cr content, the more effective the corrosion resistance is. The upper limit of the Cr content is determined by other reasons than the weather resistance and the corrosion resistance. It is preferable that the above-described effects can be obtained stably.

"Mo: Molybdenum" 0-3%
Mo is an element effective for improving the weather resistance of the composite steel sheet. When Mo is contained in steel, it forms molybdic acid when rust is formed and is adsorbed on the surface of the rust particles, and voids formed between the agglomerated rust particles are regarded as excess negative charges, and Cl ions and sulfate ions Suppresses the penetration of anions such as Thereby, Mo has the effect | action which makes it possible to further raise the limit flying salt amount which can form a stable rust. However, since such an effect is saturated by addition of 3%, in the present invention, it is preferable to limit the Mo content to 3% or less.

"Nb: Niobium" 0 to 1%
Nb is an element effective for improving the strength and toughness of the multilayer steel sheet by forming Nb (C, N), but when it is excessively contained, the toughness deteriorates due to precipitation embrittlement. . Therefore, in the present invention, the upper limit of the content of Nb is limited to 1%, more preferably 0.25%, as a range in which the strength improvement effect can be obtained without incurring toughness deterioration.

  Note that “0%” in the range of the content of each additive element indicates a case where the element is not added. However, in the present invention, even when the element is “0%”, other elements having the same action in a predetermined amount within a range represented by the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%} By adding, the multilayer steel plate 1 provided with sufficient corrosion resistance is obtained.

  Furthermore, in this embodiment, after making content of each additive element into the said specified range, the change rate of the component composition of the additive element in the plate | board thickness t direction from the surface 1a and the back surface 1b of the multilayer steel plate 1 is 80%. / Mm or less is more preferable. Thus, by providing the concentration gradient of the additive element in the thickness direction of the multilayer steel plate 1, the additive elements are concentrated in the surface layer regions 1A and 1B, while in the steel plate interior 1C, The concentration of each additive element is very low or hardly contained. As a result, the multilayer steel plate 1 of the present embodiment can ensure sufficient corrosion resistance in the surface layer regions 1A and 1B, while the expensive additive element is in a low concentration in the steel plate interior 1C, thereby reducing the manufacturing cost. Is done. Therefore, the multilayer steel plate 1 having high weather resistance and very excellent corrosion resistance can be realized at low cost.

  In the multilayer steel plate 1 according to the present invention, the surface layer regions 1A and 1B in which the components of the respective additive elements are concentrated from the steel plate interior 1C are separated from the front surface 1a and the back surface 1b in the thickness t direction. Even when it is controlled within the range of up to 8%, the above effect can be sufficiently obtained.

In the multilayer steel plate 1 of the present embodiment, a steel slab whose surface layer region is modified (see the steel slab 10 in FIG. 6) is subjected to a rolling treatment, for example, a plate thickness of 1.2 to 1.. It can be set as the structure made into the thin steel plate of about 6 mm, and the thick steel plate of about 13.2 mm. The multilayer steel plate 1 having such a plate thickness has sufficient weather resistance and very excellent corrosion resistance due to the above-described configuration. Thereby, when the multilayer steel plate 1 is a thin steel plate having a thickness of, for example, about 1.2 to 1.6 mm, it can be suitably used for automobile bodies and the like that require weather resistance and corrosion resistance. In the case of a steel plate having a thickness of about 2 mm, it can be used for undercarriage parts of automobiles.
In addition, the field to which the multilayer steel sheet 1 according to the present invention is applied is not limited to the above-mentioned automobile field, for example, in the field of applications where weather resistance and corrosion resistance are required for steel sheets, such as shipbuilding and building construction materials. It can be suitably used.

  As described above, according to the multilayer steel sheet 1 having excellent corrosion resistance according to the present embodiment, by optimizing the component composition and concentration distribution of the additive element in the modified surface layer regions 1A and 1B, the surface layer region 1A. Since variation in the concentration distribution of the alloy in 1B is reduced, it is possible to provide the multilayer steel sheet 1 having high weather resistance and excellent corrosion resistance at a low cost. Since the multilayer steel sheet 1 of the present embodiment has a structure in which only the surface layer regions 1A and 1B are modified as an alloy layer having corrosion resistance and the interior of the steel sheet 1C is not modified, it has superior corrosion resistance compared to the conventional multilayer steel sheet. A multilayer steel sheet can be realized at low cost.

<Manufacturing method of multilayer steel sheet (modification of steel sheet surface)>
Below, the manufacturing method of the multilayer steel plate excellent in corrosion resistance of this embodiment is demonstrated, referring FIGS. 2-6 suitably (refer also to the multilayer steel plate 1 of FIG. 1). FIG. 2 is a perspective view schematically illustrating a procedure for performing the surface modification step of the steel slab 10 using the surface modification processing apparatus 50 in the manufacturing method of the present embodiment.

  The method for producing a multilayer steel sheet according to the present invention is obtained by using a plasma jet (from a plurality of plasma torches 13 arranged at equal intervals on the front surface 10a and the back surface 10b of a continuously cast steel slab 10 (see FIG. 6). By irradiating (plasma) 14, the surface layer regions 10A and 10B on the front surface 10a and the back surface 10b are heated and melted, and an alloy of wire (wire-shaped alloy) 17 is added to the melted surface layer regions 10A and 10B. A surface layer reforming step for modifying the surface layer regions 10A and 10B, and a rolling step for rolling the steel slab 10 in which the surface layer regions 10A and 10B are modified in the surface layer reforming step to form the multilayer steel plate 1 are provided. The surface layer reforming step is a method in which the oscillation width of the wire 17 is ± Lo (mm), the oscillation speed is Fo (mm / sec), and the interval of the plasma torch 13 is PL ( m), when the melting rate of the steel slab 10 is V (mm / sec), and the length of the molten pool 18 formed by melting the surface layer regions 10A and 10B of the steel slab 10 is Lp (mm) While oscillating with the oscillation center position of the wire 17 aligned with the center between the plasma torches 13 so as to satisfy both of the relationships represented by the following expressions (1) and (2), the wire 17 Is continuously supplied to the molten pool 18 so that Cu, Ni, Cr, Mo, and Nb in the surface layer regions 10A and 10B having a thickness of 1 to 20% from the front surface 10a and the back surface 10b of the steel slab 10 can be obtained. The component composition of each additive element is set to the range of the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%}, and each component of the additive element in the surface layer regions 10A and 10B is set in the steel slab interior 10 This is a method of thickening rather than C.

"Surface modification processing equipment"
First, the surface modification processing apparatus 50 used in the surface modification process of the manufacturing method of this embodiment will be described.
As shown in FIG. 2, the surface modification treatment apparatus 50 includes an induction heating coil 12 disposed on the upstream side in the conveyance direction of the steel slab 10, and a steel slab 10 on the downstream side in the conveyance direction of the induction heating coil 12. A plasma torch 23 disposed above, a wire 27 disposed upstream of the plasma torch 23 in the transport direction, and a pair of plasma control coils 25 for controlling the plasma jet 24 emitted from the plasma torch 23. It is roughly structured.

  As shown in FIG. 2, the plasma torch 13 is arranged at substantially equal intervals along a direction orthogonal to the conveying direction of the steel slab 10. The plasma jet 14 radiated from the plasma torch 23 can be oscillated. And the whole width direction except the edge part of the steel slab 10 can be fuse | melted by oscillating the several plasma jet 14. FIG.

  As shown in FIG. 2, the wire 17 is inserted into a wire-cooling water-cooling nozzle 16. And it is comprised so that the wire 17 may be supplied toward the front end side of the water cooling nozzle 16 for wire supply by the wire supply apparatus which is not shown in figure. Further, the water-cooling nozzle 16 for supplying the wire is inserted between the upstream plasma control coil 15 while being inclined from the upstream side to the downstream side in the transport direction, and the tip of the wire 17 is connected to the plasma jet 14. Positioned to touch.

  A swing device (not shown) is attached to the proximal end side of the water-cooling nozzle 16 for wire supply. With this oscillating device, the wire 17 can be oscillated in a direction perpendicular to the conveying direction with the oscillation center position as the center. The oscillation center position may be, for example, the middle of the adjacent plasma torches 13 or the center of the plasma jet 14. Moreover, it is preferable to adjust the oscillation width of the wire 17 so as to overlap with the locus of the adjacent wire 17.

 As another form when both induction heating and plasma heating are used together, the additive element added to the surface layer or an alloy thereof in the form of a wire or sheet is preheated together with the surface layer of the steel slab by induction heating. Thereafter, a method of melting the additive element (pure metal) or alloy by plasma heating may be used. This is a method in which induction heating is simply used as a preheating function, and an additive element or an alloy thereof is melt-alloyed by subsequent plasma heating.

 The steel slab obtained by the above method has an advantage that the surface thereof is a low-cost steel material having the characteristics of steel having high weather resistance and corrosion resistance. Therefore, as a processed product obtained by processing the partially modified composite steel slab, a low-cost product having both characteristics of steel partially different in properties can be obtained. The processed products mentioned here include steel plates such as thin plates and thick plates, shaped steels, steel pipes, etc., but in the present invention, all steel products obtained by processing steel slabs by a normal steel process are targeted. Also, semi-finished products such as hot rolled coils are included.

"Surface modification process"
Next, the procedure of the surface modification process provided in the manufacturing method of this embodiment will be described.
As shown in FIG. 2, in the surface reforming step of the present embodiment, for example, the surface layer region 10A (10B) of the steel slab 10 as shown in FIG. A plasma jet 14 is blown from the plasma torch 13 disposed above the piece 10 toward the surface 10a (10b) of the steel slab 10, and the surface layer region 10A (10B) of the steel slab 10 is heated and melted. At this time, the width of the plasma jet 14 is controlled and enlarged by the plasma control coil 15 disposed between the steel slab 10 and the plasma torch 13. And the wire 17 is supplied from the water cooling nozzle 16 for wire supply toward the fusion | melting part in the surface layer area | region 10A (10B) of the steel slab 10, ie, the fusion | melting pool 18. FIG. At this time, the wire 17 is heated by the plasma jet 14 or the like, and the components of the wire 17 are continuously added to the molten pool 18.

  FIG. 3 is a side view schematically showing the surface modification step of the first embodiment of the manufacturing method of the present invention. As shown in FIG. 3, in the surface modification step of the present embodiment, the wire 17 is heated by the radiant heat from the plasma jet 14 and the steel slab 10 and melts in the melting pool 8. Further, the flow rate of the molten pool 18 is fast in the direction parallel to the melt processing direction, but slow in the vertical direction. For this reason, in order to make the nickel concentration in the direction perpendicular to the melting treatment direction uniform, the operation of the oscillation 19 (oscillation width ± Lo, ie, the interval PL or more of the plasma torch 13 in the direction perpendicular to the melting treatment direction). 2Lo).

  In the supply of the wire 17 to the molten pool 18 in the manufacturing method of the present invention, the wire 17 continues while the tip of the wire 17 is not completely melted only by the radiant heat from the plasma jet 14 and the steel slab 10. It needs to be supplied to the molten pool 18. In this case, the stirring effect of the molten pool 18 by the unmelted wire 17 can be expected, and the nickel concentration in the direction parallel to the melting treatment direction becomes uniform. However, when the wire supply speed Vw is less than 15 mm / sec, the wire 17 is heated and melted before reaching the melt pool 18 due to the radiant heat from the plasma jet 14 and the steel slab 10, and the wire 17 This is not preferable because the stirring effect of the molten pool 18 may not be expected. On the other hand, when the wire supply speed Vw exceeds 70 mm / sec, the wire 17 is inserted into the molten pool 18 while the heating of the wire 17 is insufficient, the molten metal in the molten pool 18 is cooled, and the wire alloy This is not preferable because the alloy concentration may not be uniform, for example, the amount of fluid diffusion of the components may be reduced, and the alloy concentration may be increased centering around the oscillation locus portion of the wire 17.

  From the above, the supply speed Vw of the wire 17 is related to the heat input of the wire 17, and the wire 17 is continuously supplied to the melting pool 18, and the heating of the wire 17 is insufficient and the melting pool 18 is not heated. It is preferable to control to 15 to 70 mm / sec so that it is not inserted into the cable.

  In the surface modification step of this embodiment, when the wire 17 is continuously added to the molten pool 18, the wire 17 is oscillated and the molten pool 18 is supplied while stirring. At this time, the direction of the oscillation 19 is set to a direction substantially orthogonal to the conveying direction (melting direction) of the steel slab 10. Thereby, along the locus | trajectory of the front-end | tip of the wire 17 on the steel slab 10, the wire 17 can be continuously supplied to the fusion pool 18, stirring the molten pool 18 in the width direction of the steel slab 10, The uniformity of the nickel concentration in the surface layers 10A and 10B of the steel slab 10 is improved.

  FIG. 4 is a plan view schematically showing a surface modification step in the first embodiment of the present invention. As shown in FIG. 4, in this embodiment, the oscillation width of the wire 17 is ± Lo (mm), the oscillation speed of the wire 17 is Fo (mm / sec), and the interval between adjacent plasma torches 13 is PL ( mm), the melting processing speed of the steel slab 10 is V (mm / sec), the length of the molten pool 18 is Lp (mm), and the droplet dropping frequency of the wire 17 is Fw (pieces / minute). To do.

  As shown in FIG. 4, a molten pool 18 in which the surface 10 a (10 b) of the steel slab 10 is melted is formed on the steel slab 10. The molten pool 18 is formed toward the downstream side in the transport direction with the upstream end in the transport direction of the melted portion of the surface 10a (10b) of the steel slab 10 melted by the plasma jet 14 being blown. It is in a melted state up to a position of length Lp (mm) from the upstream end portion in the transport direction toward the downstream side in the transport direction.

  In the molten pool 18, several molten metal flows are formed by the plasma jet 14 blown from the plasma torch 13. Such molten metal flow may remain separated from other molten metal flows formed by adjacent plasma jets 14 or may be integrated in the middle of the molten pool 18.

  Here, when the wire 17 is supplied to the molten pool 18, the alloy component of the wire 17 is mixed and diffused into the molten metal from the supply position. In addition, since the molten pool 18 has a flow of molten metal as described above, the degree of mixing and diffusion of the alloy components increases in the flow direction of the molten metal and decreases in the flow direction of the adjacent molten metal. Further, when viewed from a predetermined surface position in the conveying direction of the steel slab, when the molten pool 18 moves and enters a molten state, alloy mixing and diffusion become active, and after the molten pool 18 moves, it solidifies. It becomes a solid state, and mixing and diffusion of alloy components stop.

  Here, when the oscillation center position of the wire 17 is adjusted to the center of each plasma torch 13, the alloy concentration can be made uniform in the flow of each molten metal as described above. In some cases, the alloy concentration cannot be made uniform during the flow of the molten metal, that is, in the direction perpendicular to the direction of melting.

Therefore, in the surface layer reforming step of the present embodiment, as shown as oscillation 19 in FIG. 4, the wire 17 is oscillated with the oscillation center position of the wire 17 being the center between the adjacent plasma torches 13. Thereby, since the wire 17 reciprocates between the flows of the adjacent molten metal, when the wire 17 is supplied to the molten pool 18, a large stirring effect straddling the molten metal flow is obtained. Thereby, the nickel concentration in the direction perpendicular to the melting treatment direction can be made uniform.
In addition, the oscillation method of the wire 17 is not specifically limited, You may control individually and may synchronize all.

  Further, in the method for modifying the surface layer of the steel slab of this embodiment, in order to increase the uniformity of the alloy concentration in the surface layer region 10A (10B) of the steel slab 10 shown in FIG. As shown, the oscillation of the surface 17a of the steel slab 10 is performed while the tip position of the wire 17 returns from the oscillation start point to the oscillation start point of the next cycle by one (one cycle) oscillation operation. It suffices if the position where the starting point is is in the molten state, ie, in the molten pool 18. Such a state is expressed by the following equation (1).

  In the above equation (2), the left side is the time of one cycle of oscillation, and the right side is the time during which an arbitrary place is melted. In the above formula (2), if the time on the left side is shorter than the right side, a virtual line in the transport direction is drawn within the range of the width (± Lo or 2Lo) of the wire 17 oscillating on the steel slab 10 While the molten pool 18 is melting, the locus of the tip of the wire 17 crosses the virtual line at least twice. Moreover, since the whole area | region except the edge part of the width direction of the surface 10a of the steel slab 10 will be covered with the oscillation width | variety of the some wire 17, the virtual line was drawn to any position on the steel slab 10 Even in this case, the above relationship holds. Thereby, the alloy concentration in the direction parallel to the melting treatment direction can be made uniform.

  However, at this time, when the oscillation width Lo is not at least half of the interval of the plasma torch, a portion where the wire is not supplied is generated. Therefore, simultaneously with the above equation (2), the following equation (2) It is also necessary to satisfy the condition expressed by

  As described above, when the wire 17 is continuously supplied to the molten pool 18, it is necessary to satisfy the oscillation conditions of the above formula (1) and the above formula (2).

  In the surface modification step of the present embodiment, the steel slab processing speed is preferably in the range of 1 to 20 mm / sec. This is because if the steel slab treatment speed is slower than 1 mm / sec, the economy is extremely deteriorated, and if it exceeds 20 mm / sec, the surface irregularities deteriorate.

  The plasma torch interval PL is more efficient in that the total number of plasma torches can be reduced as the interval increases. However, the plasma torch interval PL needs to be narrower than the plasma amplitude (flat plasma width). This plasma amplitude increases as the current flowing through the plasma control coil increases and as the plasma current decreases. The limit is about 100 mm. For this reason, the plasma torch interval PL is 100 mm or less at present, but there is a possibility that this limit can be expanded by developing a control coil. On the other hand, the lower limit of the plasma torch interval PL depends on the size of the plasma torch to be used, and is approximately about the outer diameter of the torch +10 mm. Here, the reason why a difference of 10 mm is provided between the lower limit of the plasma torch interval PL and the outer diameter of the torch is that if the plasma torch interval is narrower than this, abnormal discharge may occur between the plasma torches.

  Further, the plasma torch interval PL becomes larger as the plasma current value becomes larger because the cooling mechanism needs to be enlarged, and 50 mm is the lower limit when a current of 400 A or more is used. As a result, the plasma torch interval PL is 50 to 100 mm. Further, the oscillation width Lo varies depending on the oscillation speed Fo, but at least ± 1/2 PL is required. However, in the actual oscillation, the oscillation speed becomes zero at the turning point, and it is difficult to completely eliminate the time for this zero. In this case, unless the oscillation width Lo is in the range of ± 1/2 · (0.9) PL to ± 1 / 2PL, the alloy concentration at the place where the two wires overlap with each other increases.

  As the thickness of the surface layer regions 10A and 10B of the steel slab 10 to which the alloy is added in the surface modification step of this embodiment, in order to obtain weather resistance and corrosion resistance, the thickness of the steel slab 10 is as described above. 1% is necessary. Further, since the weather resistance and corrosion resistance increase as the surface alloy layer of the steel slab 10 becomes thicker, the upper limit of the thickness of the surface layer regions 10A and 10B is not particularly limited. If it exceeds 20%, the surface irregularities become large and the surface properties after rolling become poor, so 20% or less is preferable.

  As described above, in the surface layer reforming step provided in the manufacturing method of the present invention, the supply rate Vw of nickel or nickel alloy added to the molten portion of the surface layer of the steel slab 10 is set to the appropriate range described above. Variation in the nickel concentration at an arbitrary position in the nickel-enriched surface layer portion with respect to the average nickel concentration in the nickel-enriched surface layer portion can be suppressed to within ± 50%.

Here, the variation δ _i of the alloy i concentration is expressed by the following equation (3) according to the element concentration (hereinafter referred to as C _i.j ) at the point where the N point concentration of the surface 10a of the steel slab 10 is analyzed. Defined as shown.

  As a result, weather resistance and corrosion resistance are improved by the effect of the additive element, and therefore, it is suitable as a steel material used for shipbuilding, automobiles, building materials, bridges, and the like.

  Furthermore, in the method shown in the schematic diagram of FIG. 2, plasma heating is performed after preheating with the induction heating coil 2, but the present invention is not limited to this. For example, the preheating may be performed with another heat source such as a heating furnace, a burner, plasma, and a laser, and the main heating for melting may be performed with plasma.

In the manufacturing method of this embodiment, the component of the additive element in the surface layer regions 10A and 10B in which the thickness from the front surface 10a and the back surface 10b of the steel slab 10 is 1 to 20% by performing the surface modification step under the above conditions. It becomes possible to concentrate each component of the additive element in the surface layer regions 10A and 10B more than the steel slab interior 10C, with the composition in the range of the following formula {Cu + Ni + Cr + Mo + Nb + inevitable impurities = 3 to 30%}.
The steel slab 10 is subjected to the surface modification step of the above procedure under the condition that the concentration gradient of the additive element as described above is provided, and by rolling in the subsequent rolling step, excellent weather resistance and corrosion resistance are obtained. The multilayer steel plate 1 provided can be manufactured at a lower cost.

  Moreover, in the surface modification process of this embodiment, the component composition of the additive element in the surface layer regions 10A and 10B of the steel slab 10 shown in FIG. -30%, Cr: 0-30%, Mo: 0-3%, Nb: 0-1%, and additional elements in the thickness T direction from the front surface 10a and the rear surface 10b of the steel slab 10 By modifying the surface layer region of the steel slab so that the change rate of the component composition of the steel is 80% / mm or less, the multilayer steel sheet 1 having excellent weather resistance and corrosion resistance can be manufactured at a lower cost. Is more preferable.

  Moreover, in the said description, the thickness from the surface 10a and the back surface 10b of the steel slab 10 is 1 to 20% in surface layer area | region 10A, 10B where each component of the additive element was concentrated rather than 10C inside steel slab. However, it is also possible to adopt a method of reforming in the range of 3 to 8%. In such a case, in the steel slab 10 and the rolled multilayer steel plate 1, the surface layer regions 10A and 10B in which the additive elements are concentrated in the thickness direction are formed thin, so that sufficient weather resistance and corrosion resistance are ensured. However, the manufacturing cost can be reduced.

  In the production method according to the present invention, the surface modification step as described above is performed on both the front surface 10a and the rear surface 10b of the steel slab 10, and the modification treatment is similarly performed on the steel plate end portion. It becomes possible to obtain the multilayer steel plate 1 in which the corrosion resistance of the entire steel plate is improved.

"Rolling process"
In the method of manufacturing a multilayer steel sheet according to the present invention, as shown in FIG. 1, by rolling the steel slab 10 in which the surface layers 10A and 10B are modified in the surface modification step as shown in FIG. A rolling process for obtaining a multilayer steel sheet 1 is provided.
As the rolling method and conditions of the steel slab 10 used in the production method of the present invention, known methods and conditions conventionally employed in the steel sheet production process can be employed without any limitation, and are not particularly limited.

  In the manufacturing method according to the present invention, for example, the surface regions 10A and 10B of the steel slab 10 having a thickness of about 100 to 250 mm are alloyed with a predetermined composition in the surface layer reforming step, and then rolled in the rolling step. Apply processing. Thereby, the multilayer steel plate 1 made into the thin steel plate whose plate | board thickness is about 1.2-1.6 mm as mentioned above, for example, and about 13.2 mm can be obtained. Thus, the multilayer steel plate 1 obtained by rolling the steel slab 10 in which the surface layer regions 10A and 10B are modified has a surface layer region 1A of 1 to 20% from the front surface 1a and the back surface 1b in the plate thickness direction. 1B is an alloy component containing the additive element with the above composition, and the additive element is concentrated more than the inside 1C of the steel sheet, so that it has sufficient weather resistance and is extremely excellent in corrosion resistance.

  In the production method of the present invention, the range of the thickness direction of the surface layers 1A and 1B in the multilayer steel plate 1 after rolling the steel slab 10 with the modified surface layers 10A and 10B is slightly different depending on the rolling conditions. The ratio is approximately the same as that of the steel slab 10 before rolling. Thereby, also in the multilayer steel plate 1 obtained by performing a rolling process, surface layer 1A, 1B which has the characteristic similar to the steel cast piece 10 before rolling is provided. Thereby, the multi-layer steel sheet 1 is low in cost and has excellent weather resistance and corrosion resistance.

  According to the manufacturing method of the multilayer steel plate 1 having excellent corrosion resistance according to the present embodiment as described above, the supply speed of the wire 17 to be supplied to the molten portion of the surface layer regions 10A and 10B in the steel slab 10 (droplet addition frequency). ), Since the supply position and the molten state of the wire 17 are optimized, variations in the alloy concentration in the surface layer regions 10A and 10B are suppressed. Thereby, the multilayer steel plate 1 having high corrosion resistance and excellent corrosion resistance can be manufactured with high productivity and at low cost. In the manufacturing method of this embodiment, only the surface layer regions 1A and 1B are modified as an alloy layer having corrosion resistance, and the inner steel sheet 1C is not modified. Therefore, the multilayer steel sheet 1 having excellent corrosion resistance compared to the conventional method. Can be manufactured at low cost.

[Second Embodiment]
Next, a second embodiment of the multilayer steel sheet having excellent corrosion resistance and a method for producing the same according to the present invention will be described in detail with reference to FIGS. 1, 5, and 6 as appropriate. FIG. 5 is a perspective view schematically illustrating the manufacturing method of the present embodiment.

  First, the surface modification processing apparatus 50A used in the surface modification step of the present embodiment will be described. As shown in FIG. 5, the surface layer reforming apparatus 50 </ b> A of the present embodiment has a configuration in which a vibrator 60 is provided in addition to the configuration of the surface layer reforming apparatus 50 of the first embodiment. As a result, when the unmelted wire 17 is continuously supplied to the molten pool 18, vibration 61 is applied to the wire 17 via the water-cooled nozzle 16 to which the vibrator 60 is attached, and the wire 17 is oscillated. However, it is possible to supply. Of the constituent elements shown in FIG. 5, the same constituent elements as those shown in FIG. 2 are denoted by the same reference numerals as those in FIG. 2, and detailed descriptions thereof are omitted.

  In the surface modification step provided in the manufacturing method of the present embodiment, when the unmelted wire 17 is continuously supplied to the molten pool 18, in addition to the oscillation of the wire 17 described in the first embodiment. Further, by adding a vibration 61 to the wire 17, the supply amount of the wire 17 to the molten pool 18 is increased.

  In order to increase the supply amount of the wire 17, it is preferable to apply a vibration force (vibration 61) to the wire 17. Such vibration force varies depending on the device to be vibrated and the mounting method, but in any vibration force pattern, if the amplitude is less than ± 2 mm or the frequency is less than 20 Hz, the vibration force is insufficient, and the supply amount of the wire 17 Can not be raised. On the other hand, if the frequency of the vibration force exceeds 80 Hz, the entire apparatus resonates, and the apparatus may be damaged. Also, an amplitude exceeding ± 5 mm is more efficient when the wire is mechanically oscillated than when it is applied by vibration. From the above, the vibration of the tip of the wire 17 needs to have an amplitude of ± 2 mm or more and ± 5 mm or less and a frequency of 20 Hz or more and 80 Hz or less.

  According to the surface modification step provided in the manufacturing method of the present embodiment, variation in the alloy component concentration in the surface layer regions 10A and 10B of the steel slab 10 as shown in FIG. 6 as in the first embodiment described above. Can be suppressed within ± 50%. Further, when the wire 17 is supplied to the molten pool 18, it is possible to add more alloy to the surface layer regions 10 </ b> A and 10 b of the steel slab 10 because the oscillation is performed while applying the vibration 61 to the wire 17. Become.

  According to the manufacturing method of the present embodiment as described above, the variation in the alloy concentration in the surface layer regions 10A and 10B of the steel slab 10 is suppressed as in the manufacturing method of the first embodiment. Thus, it is possible to manufacture the multilayer steel sheet 1 having excellent properties and corrosion resistance with low productivity and low cost.

  According to the composite steel sheet excellent in corrosion resistance and the manufacturing method thereof according to the present embodiment, as described above, the multi-layer steel sheet 1 excellent in corrosion resistance can be obtained with high productivity without causing an increase in cost. The above effect is immeasurable

  Hereinafter, examples of the multilayer steel sheet excellent in the corrosion resistance according to the present invention and its manufacturing method will be described to explain the present invention more specifically, but the present invention is not originally limited to the following examples, The present invention can be implemented with appropriate modifications within a range that can be adapted to the gist of the following, and these are all included in the technical scope of the present invention.

[Example A]
In this example, first, a steel slab that had been continuously cast was cut into a size of 1200 mm in width, 250 mm in thickness, and 10 m in length. Next, as a wire, the wire diameter was 1.2 mm, and the following AC alloys wires having the following composition were used, and the steel slab surface was modified while changing the wire supply speed Vw. . The conditions of the surface layer reforming treatment at this time are shown in Tables 1 and 2 below.
A: 25% Cu-75% Ni
B: 30% Ni-70% Cr
C: 30% Ni-64Cr-4Mo-2Nb

In addition, an air vibrator / turbine type (frequency of 5000 cycles / min or more) was used as the vibrator, and the position was changed from the lower end of the water-cooled nozzle (Examples 5 to 7, Comparative Example 11, Test Example). Here, the amplitude and vibration frequency shown in Table 2 below were controlled by adjusting the pressure and the mounting position of the vibrator.
The steel slab of Examples 1-7, Comparative Examples 1-11, and a test example was manufactured by said each conditions and procedure.

  And about the steel slab of each Example obtained by the said point order, the comparative example, and the test example, the variation of Ni density | concentration in the part of a surface layer area | region was investigated. At this time, the alloy concentration was preliminarily ground with a grinder or the like on the part subjected to melting treatment and alloy addition, and a metal material discriminator called a mobile countback (here, PMI-MASTAR PRO manufactured by WAS) was used. However, the measurement was performed at intervals of 20 mm using other measurement methods and measuring instruments), and the results are also shown in Table 2 above. Here, “the Ni concentration variation is good” as used in this example is defined as the variation being within ± 50%.

Furthermore, the steel slabs of the above Examples, Comparative Examples, and Test Examples were subjected to an evaluation test for weather resistance and corrosion resistance under the conditions and procedures described below.
First, for weather resistance, a test piece having a size of 6 × 50 × 150 mm was taken from directly below the surface of the steel slab after removal of the black skin, and it was directly applied (bare material) or was subjected to a stabilization treatment film. Later (surface treatment material), in the coastal area of Futtsu City, Chiba Prefecture, exposure tests were carried out for 1, 3 and 7 years in sunshine and shaded environments at three locations where the amount of incoming salt was changed by the distance from the shore. The separation distance at this time was set to three types of 50 m, 800 m and 2000 m, but the amount of incoming salt at each position was 0.8 mdd, 0.2 mdd and 0.1 mdd, respectively, on an average on an annual basis.
Then, after the weather resistance test under the above conditions, the surface condition was visually confirmed, and the results were determined in the following two stages and are shown in Table 2 below.
(1) “◯”: Under an environment where the annual average amount of incoming salt was 0.8 mdd, the 50-year estimated corrosion weight loss was less than 0.2 mm.
(2) “×”: Under an environment where the annual average amount of incoming salt was 0.8 mdd, the estimated corrosion loss for 50 years was 0.2 mm or more.

As for corrosion resistance, the test pieces in the same manner as described above was collected, immersed for 96 hours in 0.4 MPa · 0.99 ° C. artificial seawater CO ₂ was saturated (ASTMD1141-96) (specific liquid ^volume: 20cm 3 / ^cm 2) , Soaked in pickling (0.3% potassium manganate + 18% sodium hydroxide) for 5 minutes, then immersed in boiled 10% aqueous ammonium citrate for 5 minutes to remove corrosion products, thickness before and after the test It was evaluated by comparison.
And about the corrosion-resistance test result by the said conditions, it determined in two steps shown below, and it showed in following Table 2. FIG.
(1) “◯”: The reduction rate of the thickness after the test with respect to the thickness before the test was less than 0.1 mm / year.
(2) “x”: The reduction rate of the thickness after the test with respect to the thickness before the test was 0.1 mm / year or more.

  Tables 1 and 2 below show a list of production conditions and evaluation results for steel slabs of Examples, Comparative Examples, and Test Examples.

  As shown in Table 2, the steel slabs of Examples 1 to 7 that were subjected to the modification treatment within the range defined by the present invention had a total content of additive elements in the surface layer region represented by the following formula {Cu + Ni + Cr + Mo + Nb + Inevitable impurities = 3 to 30%}, and variation in alloy concentration (additive element) is 50% or less, and variation in concentration distribution of the alloy in the surface layer region is reduced. It was confirmed that Moreover, as for the steel slabs of Examples 1 to 5 and 7, all the evaluation results of the weather resistance test are “◯ (passed)”, and the steel slabs of Example 6 have the corrosion resistance evaluation results of “ ○ ”.

On the other hand, in the steel slabs of Comparative Examples 1 to 11, since any one of the manufacturing conditions is outside the specified range of the present invention, either the weather resistance test or the corrosion resistance test is “x (failed)”. As a result.
In the steel slab of Comparative Example 1, since the oscillation width was insufficient, the nickel concentration variation was 80%, and it was confirmed that the alloy concentration in the surface layer region was not stable, and the evaluation of the weather resistance test was “×”. became.
Further, the steel slab of Comparative Example 2 cannot satisfy the above formula (1) because the oscillation speed is low, cannot be satisfied, the nickel concentration variation is 100%, and the alloy concentration in the surface layer region is stable. It was confirmed that this was not done, and the evaluation of the weather resistance test was “x”.

Moreover, since the steel slab of Comparative Example 3 had a wire drop frequency of 8 pieces / minute and less than 20 pieces / minute, the nickel concentration variation was 200%, and the alloy concentration in the surface layer region was not stable. It was confirmed that the evaluation of the weather resistance test was “x”.
Further, in the steel slab of Comparative Example 4, the wire supply is continuous, but because the oscillation speed is slow, the above formula (1) cannot be satisfied, the Ni concentration variation is 200%, and the surface layer region It was confirmed that the alloy concentration was not stable, and the evaluation of the weather resistance test was “x”.

  In the steel slab of Comparative Example 5, since the relationship between the oscillation width Lo and the plasma torch interval PL does not satisfy the above formula (2), the Ni concentration variation is 300%, and the alloy concentration in the surface region is stable. It was confirmed that this was not done, and the evaluation of the weather resistance test was “x”.

Further, the steel slab of Comparative Example 6 cannot satisfy the above formula (1) because the oscillation speed is low, and the relationship between the oscillation width Lo and the plasma torch interval PL satisfies the above formula (2). Since it did not satisfy | fill, the dispersion | variation in nickel density | concentration became 600%, and it was confirmed that the alloy density | concentration of a surface layer area | region is not stable, and evaluation of the weather resistance test was set to "x".
Further, in the steel slab of Comparative Example 7, the total content of Cu + Ni + Cr + Mo + Nb + inevitable impurities was less than 3%, the melting was incomplete, the surface was uneven, and the corrosion resistance was evaluated as “x”.
In addition, the steel slab of Comparative Example 8 cannot satisfy the above formula (1) due to the slow oscillation speed, the surface layer region is incompletely melted and the surface is uneven, and the surface region alloy Since the concentration was not stable, the evaluation of the weather resistance test was “x”.

Further, the steel slab of Comparative Example 9 cannot satisfy the above formula (1) because the processing speed V is low, the nickel concentration variation is 800%, and the alloy concentration in the surface layer region is not stable. Was confirmed, and the evaluation of the corrosion resistance test was “x”.
Moreover, since the ratio of the alloy layer (surface layer area | region) in the thickness direction from the surface and the back surface is less than the prescription | regulation of this invention, the steel cast of the comparative example 10 is evaluation of weather resistance. × ”.
Moreover, since the total content of Cu + Ni + Cr + Mo + Nb + inevitable impurities exceeded 30%, the steel slab of Comparative Example 11 had unevenness in the alloy concentration, but the surface was uneven, and the corrosion resistance evaluation was “x”.

  In addition, although the steel slab of the test example satisfied both of the above formulas (1) and (2), the vibration frequency was 14 Hz, and vibration was applied under conditions that were not in an appropriate range. This is an example where it was confirmed that unevenness was generated on the surface, variation in the Ni concentration became impossible to measure, and the alloy concentration in the surface layer region was not stable. For this reason, as for the steel slab of the test example, the evaluation of the weather resistance test was “x”.

[Example B]
In this example, first, continuous casting was completed, and the composition was 0.07% C-0.6% Mn-0.6Si-0.03% P-0.005% S-16.5% Cr ( The steel slab was cut into a size of 1200 mm width, 250 mm thickness, and 10 m length. Next, this continuous cast steel slab is melt-modified by induction heating, and alloy addition using a 30% Ni-70% Cr wire is performed. An area of 10 mm was melt processed. And each steel slab obtained was directly heated, and after hot rolling was completed at a predetermined temperature to obtain a steel plate, pickling was performed and cold rolling was performed at a predetermined reduction rate to achieve a plate thickness of 1.6 mm. As a result, a steel plate having a surface of 18-8 stainless steel could be obtained.

  The multilayer steel sheet excellent in corrosion resistance provided by the present invention has sufficient weather resistance and is extremely excellent in corrosion resistance. Therefore, when the multi-layer steel sheet of the present invention is used, for example, in the body or undercarriage part of an automobile that requires weather resistance and corrosion resistance, an automobile having excellent environmental resistance characteristics can be realized at low cost. It becomes possible. Furthermore, the field to which the multilayer steel sheet of the present invention can be applied is not limited to the above-mentioned automobile field. For example, in the field of applications where the steel sheet is required to have weather resistance and corrosion resistance, such as shipbuilding and building construction materials. Therefore, the industrial effect is immeasurable.

DESCRIPTION OF SYMBOLS 1 ... Multi-layer steel plate, 1A, 1B ... Surface layer area | region (multi-layer steel plate), 1a ... Front surface (multi-layer steel plate), 1b ... Back surface (multi-layer steel plate), 1C ... Inside a steel plate, 10 ... Steel slab, 10A, 10B ... surface layer region (steel slab), 10a ... front surface (steel slab), 10b ... back surface (steel slab), 10C ... inside steel slab, 13 ... plasma torch, 14 ... plasma jet (plasma), 17 ... wire 18 ... Melting pool, 19 ... oscillation, 60 ... vibrator, 61 ... vibration, t ... plate thickness

Claims (4)

To the surface and the back surface of the continuous cast steel slab, by irradiating plasma from the arrangement plasma torch into a plurality of equally spaced, heated and melted the surface and the back surface of the surface region, and the melted surface layer A surface layer reforming step of modifying the surface layer region by adding a wire-shaped alloy to the region, and rolling the steel slab whose surface layer region has been modified in the surface layer reforming step to form a multilayer steel plate A method for producing a multilayer steel sheet comprising a rolling step,
In the surface layer reforming step, the oscillation width of the wire is ± Lo (mm), the oscillation speed is Fo (mm / sec), the plasma torch interval is PL (mm), and the steel slab melting process speed V (mm / sec), and the length of the molten pool formed by melting the surface region of the steel slab is Lp (mm), these are expressed by the following formulas (1) and (2). By supplying the wire continuously to the molten pool while oscillating with the oscillation center position of the wire aligned with the center between the plasma torches so as to satisfy both of the relations described above, The component composition of each additive element of Cu, Ni, Cr, Mo and Nb in the surface layer region where the thickness from the front surface and the back surface of the piece is 1 to 20%, in mass%,
Cu + Ni + Cr + Mo + Nb + unavoidable impurities: 3-30%
The method for producing a multilayer steel sheet having excellent corrosion resistance, characterized in that each component of the additive element in the surface layer region is concentrated more than in the steel cast slab.

In the surface layer reforming step, the component composition of the additive element in the surface layer region of the steel slab is, in mass%,
Cu: 0 to 1.5%,
Ni: 3 to 30%,
Cr: 0 to 30%,
Mo: 0 to 3%,
Nb: 0 to 1%
And a range of
Cu + Ni + Cr + Mo + Nb + unavoidable impurities: 3-30%
And range
Furthermore, in the plate thickness direction from the front surface and the back surface of the steel slab, the surface layer region is modified so that the change rate of the composition of the additive element is 80% / mm or less, The manufacturing method of the multilayer steel plate excellent in corrosion resistance of Claim 1 . In the surface layer reforming step, the surface layer region where each component of the additive element is concentrated from the inside of the steel slab is modified so that the thickness from the front surface and the back surface of the steel slab is 3 to 8%. The manufacturing method of the multilayer steel plate excellent in corrosion resistance of Claim 1 or Claim 2 characterized by the above-mentioned. The surface reforming process, when supplying the wire, amplitude ± 2 mm or more in the wire tip, ± 5 mm or less and a frequency of 20Hz or more, and to grant the following vibration 80 Hz, according to claim 1 The manufacturing method of the multilayer steel plate excellent in corrosion resistance of any one of Claim 3 .

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