JP5732741B2 - Sn-Zn plated high-strength steel sheet for press working with excellent corrosion resistance and method for producing the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a Sn-Zn plated high-strength steel sheet for press working applied to the fields of automobiles, home appliances, and the like, and a method for producing the same. The present invention relates to a plated high-strength steel plate and a method for producing the same.

  In recent years, steel sheets for automobiles have been increased in strength for the purpose of improving fuel efficiency by reducing vehicle body weight. Similarly for steel plates for fuel tanks, the weight of the tanks and the complexity of the vehicle body design, as well as the complexity of the fuel tank shape, due to the location of the fuel tank storage and installation, requires excellent formability and high strength. Has been. Conventionally, in order to satisfy the demand for coexistence of such formability and high strength, IF (Interstitial Free) steel in which carbonitride-forming elements such as Ti and Nb are added to ultra-low carbon steel, P, High strength IF steels to which solid solution strengthening elements such as Si and Mn are added have been developed.

  However, when high-strength steel plates are used for the fuel tank, there are problems that the tensile strength of the wavy seam weld is low at low temperatures and that secondary work embrittlement is likely to occur due to grain boundary fracture after forming. It was. That is, even if the strength of the steel plate is increased, there has been a problem that the weld joint strength does not increase as appropriate for the strength increase of the steel plate, and there is a problem that the grain boundary strength of the IF steel is reduced. The seam welded part of the tank has a wrinkle shape, especially in the case of a high strength steel plate, stress is likely to concentrate, and the toughness is lowered and the tensile strength is lowered. This is because N or the like precipitates and fixes Nb or Ti as carbides or nitrides, so that the crystal grain boundaries become very clean, and the grain boundaries are easily broken after forming. These are concerns about the destruction resistance when the fuel tank, which is an important safety part, is subjected to an impact caused by a collision in a low temperature region.

  Further, there is a demand for a steel plate that does not generate corrosion products that cause clogging of the filter and does not cause perforated corrosion to gasoline and alcohol or organic acids that are produced by deterioration of gasoline. In response to this requirement, it has been proposed and applied to apply Pb—Sn alloy, Al—Si alloy, Sn—Zn alloy and Zn—Al alloy plating to the steel sheet surface. For this reason, the steel plate used as a base | substrate needs that the plating property of these alloys is favorable.

  Among these problems, several methods for avoiding the occurrence of secondary work embrittlement have been proposed (see, for example, Patent Documents 1 and 2). For example, in Patent Document 1, in order to avoid deterioration of secondary work embrittlement resistance due to grain boundary segregation, the P content is reduced as much as possible based on Ti-added IF steel, and a large amount of Mn and Si are balanced accordingly. There has been proposed a technique for obtaining a high-tensile steel sheet excellent in secondary work brittleness resistance. Patent Document 2 proposes a technique of using an ultra-low carbon steel plate and adding B in addition to Ti and Nb to increase the grain boundary strength and increase the secondary work brittleness resistance. In the technique described in Patent Document 2, the B content is optimized for the purpose of improving secondary work embrittlement resistance and preventing an increase in load during hot rolling accompanying a delay in recrystallization of austenite grains.

  Some proposals have also been made for the purpose of improving weldability (see, for example, Patent Documents 3 to 5). For example, the technique described in Patent Document 3 is intended to improve spot weldability by carburizing an ultra-low carbon steel sheet added with Ti and / or Nb during annealing to form a martensite and bainite structure on the surface layer. is there. Further, the technique described in Patent Document 4 intends to increase the strength of a spot welded joint by adding Cu to an extremely low carbon steel to widen a heat affected zone during welding. Furthermore, the technique described in Patent Document 5 is to refine Mg in the welded part and heat-affected part by the pinning effect by adding Mg to the steel and generating Mg oxide and / or Mg sulfide in the steel sheet. Non-Patent Document 1 discloses a technique for improving the toughness of the heat affected zone by finely dispersing TiN in a thick steel plate.

Furthermore, several techniques for improving the hot dipping properties of high-strength steel sheets have been proposed (see, for example, Patent Documents 6 and 7). For example, in the hot dip galvanized high-strength cold-rolled steel sheet described in Patent Document 6, the content of S that hinders hot dipping properties is 0.03% by mass or less, and the content of P is 0.01 to 0.12%. While limiting, Mn and Cr are positively added as reinforcing elements. Moreover, in the high-tensile alloyed galvanized steel sheet described in Patent Document 7, the molten alloy Zn plating property is improved by setting the correlation between the Si content and the Mn content within a specific range.
In order to improve the secondary work brittleness resistance, there is also a steel sheet that provides high strength and excellent secondary work brittleness resistance by adding B and optimizing the addition balance of Mn-P (for example, see Patent Document 7). . Further, a technique of adding B, Ti, Nb for improving secondary work brittleness resistance is also disclosed (for example, see Patent Document 8).

  Further, a technique related to a welding method for improving the tensile strength of a peculiar welded portion unique to a tank (Patent Document 9) and a technique related to a deep drawing and high-strength steel sheet for press working (for example, Patent Document 10, Patent Document 11, Patent Reference 12) is also disclosed, and in addition, a technique for ensuring the appearance of plating is disclosed.

  Moreover, in order to improve the plating property of Si-containing steel, it is possible to uniformly plate by suppressing the surface roughness or the method of generating residual stress with a roll having irregularities (for example, see Patent Document 13), There is a technique for improving plating properties (for example, see Patent Document 14). Further, there is a technique for improving plating performance by performing Ni or Ni-based alloy plating after performing grinding of 0.3 to 5 μm after pickling (see, for example, Patent Document 15). Furthermore, there is a technique for improving plating properties by generating internal oxidation such as grain boundary oxidation in a hot-rolled steel sheet and suppressing outward diffusion of Si during annealing (see, for example, Patent Document 16).

  As described above, there are techniques relating to the workability, weldability, tensile strength of the wavy welded portion, secondary work brittleness resistance, and plating property of the high-strength steel sheet.

JP-A-5-59491 JP-A-6-57373 JP-A-7-188777 JP-A-8-291364 JP 2001-288534 A JP-A-5-255807 JP 7-278745 A JP 2000-192188 A JP-A-6-256900 JP 2007-119808 A JP 2007-169739 A JP 2007-277714 A Japanese Patent Laid-Open No. 7-126747 Japanese Patent Laid-Open No. 2003-183796 JP-A-3-24255 Japanese Patent No. 3020846

"Iron and Steel" No. 65 (1979) No. 8, p. 1232

  However, the conventional techniques described above have the following problems.

  That is, the steel sheet manufactured by the method described in Patent Document 1 has good workability and secondary processing resistance, but depending on conditions, it must be annealed in a furnace with a particularly low dew point such as a hydrogen annealing furnace. Therefore, it is difficult to obtain sufficient corrosion resistance. The steel plate produced by the method described in Patent Document 2 pays attention to workability and secondary work brittleness resistance, but does not pay attention to plating property and corrosion resistance, leading to poor plating. Moreover, although the method of patent document 3 carburizes during annealing, since the plate speed, the atmosphere gas composition, and temperature are not constant in an actual production facility, the amount of carburization changes, and between the steel plates manufactured. There is a problem that the variation in material becomes large and it is difficult to produce a stable steel plate. Furthermore, the method described in Patent Document 4 has a problem in that since a large amount of Cu is added, surface defects due to Cu frequently occur and the yield decreases.

  Furthermore, the methods described in Patent Document 5 and Non-Patent Document 1 are effective in arc welding or the like where the cooling rate after welding is relatively slow, but the effect is not recognized in seam welding or the like where the cooling rate is high. There is a problem, and the steel plate described in Patent Document 5 and Patent Document 1 and the thin steel plate used in the fuel tank have different components, and further, the shape of the welded portion is different, so it cannot be said that the technology can be applied immediately. . Furthermore, Patent Document 6 considers hot dip galvanizing properties, restricts Si to 0.03% or less, and utilizes P and Mn as reinforcing elements. There is a problem that the secondary work brittleness resistance is insufficient. In consideration of this point, Patent Document 7 has insufficient hot-dip galvanizing properties but is insufficient for corrosion resistance for tanks.

  Patent Document 8 has a disadvantage that sufficient low-temperature toughness cannot be obtained because a large amount of P is added to ensure strength and the balance between P and B is not optimal. Since measures have not been studied, it is inferior in corrosion resistance. Patent Document 9 uses a large amount of Ti for improving formability, and cannot sufficiently ensure the strength and toughness of the welded portion. Also, even if the addition amount of Ti is appropriate, Nb is small, so workability is improved. In addition to problems that cannot be sufficiently secured, the plating property is not considered, and Si is also limited to an upper limit of 0.5. Patent Document 10 is a technique for improvement using laser welding, which is difficult to apply by seam welding used for fuel tank manufacturing, and is not mentioned in the weld characteristic improvement technique by improving the base material characteristics.

  Although Patent Document 11 is a technique for improving the base material characteristics, there is a problem that the corrosion resistance is low, and depending on the conditions, the toughness of the wavy seam weld is low, the steelmaking cost is high, and the workability is low. Further, Patent Document 12 has a problem that, depending on conditions, the toughness of the wavy seam welded portion is low, and in addition, the workability is reduced. In these Patent Documents 10 and 12, although the tensile strength improvement and plating appearance of the peculiar welded part peculiar to the tank are ensured, there is a problem that the corrosion resistance is lowered depending on the conditions.

  In patent document 13, in order to produce a residual stress in a steel plate, it is necessary to use the roll which provided the unevenness | corrugation, and corrosion resistance is not evaluated. On the other hand, in Patent Document 14, a large surface roughness is the cause of defective plating. The technique disclosed in Patent Document 15 grinds the surface after pickling, but additional equipment is required to grind the pickling plate uniformly over the entire length, and there remains a problem in uniformity. . Only the technique of Patent Document 16 cannot remove the Si oxide remaining in the surface layer after hot rolling.

  As described above, the conventional knowledge includes a technique for improving secondary work brittleness resistance and a technique for improving weld toughness in the field of thick steel sheets. However, since the fuel tank has a processing step such as pressing in the manufacturing process and a heat treatment step such as seam welding, not only the characteristics of the base material but also the characteristics after processing and heat treatment are important. That is, when high-strength steel is used, toughness generally decreases, so that secondary work brittleness resistance and weld toughness are important simultaneously. Furthermore, since the surface is plated to become a product, plating properties and corrosion resistance are important, but there is a problem that high-strength steel sheets are difficult to be plated and corrosion resistance is not sufficient. In particular, there is no technology to reduce the red rust occurrence rate to 50% or less in a strict test of SST for 960 hours, for example, in a situation where corrosion resistance becomes more important as weight reduction due to higher strength, that is, thinning of the steel sheet progresses. Absent.

  The present invention has been made in view of such problems, and the object of the present invention is to have press formability applicable to the automotive field, in particular, fuel tank use, excellent secondary work brittleness resistance, and excellent An object of the present invention is to provide a Sn-Zn plated high-strength steel plate having a low temperature toughness of seam welded portion and subjected to hot Sn-Zn plating for improving corrosion resistance on a high-strength steel plate of 340 MPa or more and a method for producing the same.

  In order to solve the above-mentioned problems, the present inventors diligently studied about a Sn—Zn plated high-strength steel sheet and a method for producing the same, and studied a mechanism of red rust generation in an SST corrosion resistance test.

  As a result, in addition to the decrease in plating wettability due to oxides of Si and Mn that are concentrated in the annealing process, there is a residual Si oxide that is concentrated at the scale / base metal interface due to the scale formation in the hot rolling process. Thus, it was found that after cold rolling and annealing, fine pulverized Si oxide remains, which decreases the wettability of the molten Sn—Zn plating and, as a result, decreases the corrosion resistance.

  Therefore, in order not to reduce the wettability of the molten Sn—Zn plating for the high-strength steel plate, it is only necessary to reduce the Si oxide that causes a decrease in the wettability of the plating remaining after the pickling. It was found that this can be achieved by reducing the amount of scale produced by hot rolling.

  The present invention has been completed based on these findings, and the gist of the invention is as follows.

(1) In mass%,
C: 0.0005 to 0.0050%,
Si: more than 0.3 to 1.0%,
Mn: 0.70 to 2.0%,
P: 0.05% or less,
Ti: 0.010 to 0.050%,
Nb: 0.010 to 0.040%,
B: 0.0005 to 0.0030%,
S: 0.010% or less,
Al: 0.01-0.30%,
N: 0.0010 to 0.01%
High-strength steel sheet having a composition comprising Fe and inevitable impurities as the balance, Si surface concentration of more than 0.3% to 1.5%, and the surface area ratio of the Si-containing oxide on the entire surface 1% to 8.8% Zn on the FeSn ₂ alloy layer having an area ratio of 97% or more on a high-strength steel plate having a size of 3% or less and one Si-containing oxide of 1 μm or less. A Sn-Zn plated high-strength steel sheet, which is provided with molten Sn-Zn plating, the balance of which consists of Sn and inevitable impurities, the adhesion amount of which is 10 to 150 g / m ² per side.

(2) A step of continuously casting the molten steel having the composition described in (1) to obtain a slab, a step of heating the slab at 1000 ° C. to 1300 ° C., and a finishing temperature of Ar ₃ temperature to 1000 ° C. The process of obtaining a hot-rolled steel sheet by hot rolling under the conditions of [FT] × 0.6-500 (sec.) Or more when the finishing temperature of the hot rolling is [FT] ° C. Pickling to remove the surface scale of the hot-rolled steel sheet by pickling, and to cold-roll the pickled steel sheet at a cold rolling rate of 50% or more to obtain a cold-rolled steel sheet having a predetermined thickness; A step of annealing the cold-rolled steel sheet at a temperature higher than the recrystallization temperature, a pickling process for removing the surface scale of the annealed steel sheet, and Fe-Ni pre-plating was performed on the pickled steel sheet surface before hot dipping. Later, 1 to 8.8% Zn and the balance consists of Sn and inevitable impurities, A step of performing molten Sn—Zn plating with an adhesion amount of 10 to 150 g / m ² per side, and the Si surface concentration of the steel sheet before the plating treatment is more than 0.3% to 1.5%. The manufacturing method of the Sn-Zn plating high-strength steel plate as described in said (1) characterized by these.

Any of the pickling process described in (3) above SL (2), nitrate in sulfuric acid aqueous solution of 20 to 400 g / l, sulfates, fluoro silicates, one or two or more of fluoroborate The method for producing a Sn-Zn plated high-strength steel sheet according to the above (2), which is a step of electrolytic pickling with a mixed pickling solution.

  The steel sheet of the present invention is a Sn-Zn plated high-strength steel sheet having excellent corrosion resistance of 340 MPa or more, and further has excellent press formability, secondary work brittleness resistance, and tensile strength of a wrinkled weld. It is possible to provide an excellent Sn—Zn plated high-strength steel sheet applicable to the field, particularly to fuel tank applications, and a method for producing the same. This makes it possible to increase the strength of the steel sheet and to improve the fuel efficiency by reducing the weight of the vehicle body. In particular, the fuel tank can be reduced in weight and the vehicle body design can be complicated.

  Furthermore, the fuel tank manufactured with the steel plate of the present invention exerts a great effect when automobile fuel is used, particularly when biofuel is used.

It is a schematic diagram for demonstrating the mechanism in which the Si containing oxide concentrated on the steel plate surface is formed. It is a figure which shows the change of Si surface concentration by the relationship between hot finishing temperature FT (degreeC) and pickling time (sec). It is a figure which shows the secondary processing brittleness-proof evaluation test method. It is sectional drawing which shows a peel test method.

  Hereinafter, embodiments for carrying out the present invention will be described in detail. In the following description, mass% in the composition is simply described as%.

In order to obtain a high-strength steel sheet having excellent press formability, secondary work brittleness resistance and tensile strength of a wrinkled weld, which is extremely difficult in the prior art, and further having excellent plating properties and corrosion resistance, We studied earnestly.
As a result, with the generation of scale in the hot rolling process, by reducing the Si oxide concentrated at the scale / base metal interface, it is a high-strength steel sheet with a tensile strength of 340 MPa or more, for the automotive field, especially for fuel tank applications. We have found a high-strength steel sheet that has applicable press-formability, has excellent secondary work brittleness resistance, and has a tensile strength of the wavy welded portion, and can also have excellent plating and corrosion resistance. Invented.

  First, the reason for limiting the numerical values of the components in the high-strength steel sheet of the present invention will be described. In addition,% about a component means the mass%.

C: 0.0005 to 0.0050%
C is an extremely important element in the present invention. Specifically, C is an extremely effective element for combining Nb and Ti to form carbides and achieving high strength. However, when the C content exceeds 0.0050%, even if Ti and Nb necessary for fixing C are added, the workability is lowered and the toughness of the seam welded portion in seam welding and laser welding is also lowered. . On the other hand, in the steel sheet of the present invention, even if the C content is low, it can be supplemented by other strengthening methods. However, when the C content is less than 0.0005%, it is difficult to ensure the strength and at the time of steelmaking. The decarburization cost increases. Therefore, the C content is set to 0.0005 to 0.0050%. Moreover, when extremely high workability and weld toughness are required, the C content is preferably 0.0030% or less.

Si: more than 0.3 to 1.0%
Si is an effective element for increasing the strength as a solid solution strengthening element. In order to ensure a tensile strength of 340 MPa or more and excellent secondary work brittleness resistance or toughness of a wavy seam weld, more than 0.3% is necessary. On the other hand, if the Si content is excessive, specifically, if the Si content exceeds 1.0%, the hot dipping property is impaired even if other conditions are within the scope of the present invention. Therefore, it is more than 0.3 to 1.0% of the Si content.

Mn: 0.70 to 2.0%
Mn is an element that increases the strength of the steel sheet by solid solution strengthening, similar to Si, in order to increase the strength of the steel sheet of the present invention with the aim of improving secondary work brittleness resistance, weld toughness and plating properties, and corrosion resistance. One of the important elements. Mn has a mechanism for increasing the strength by refining the structure and a mechanism for increasing the strength by solid solution strengthening. However, when the Mn content is less than 0.70%, the addition effect cannot be obtained. When supplemented with other elements, the targets cannot be achieved in all items of secondary work brittleness resistance, weld toughness and plating properties. On the other hand, if the Mn content exceeds 2.0%, the in-plane anisotropy of the r value, which is an index of deep drawability, increases, press formability is impaired, and Mn oxide is present on the surface of the steel sheet. It is produced and the plating property is impaired. Therefore, the Mn content is 0.70 to 2.0%. Further, by setting the Mn content to 1.0% or more, the structure of the steel sheet can be maintained even if the hot rolling finishing temperature is 910 ° C. or less, so the Mn content is 1.0 to 2.0%. preferable.

P: 0.05% or less P is an element effective in increasing strength by solid solution strengthening with little deterioration in workability even when added. However, P is also an element that segregates at the grain boundary to deteriorate the secondary work brittleness resistance and causes solidification segregation at the welded portion, which deteriorates the toughness of the wavy seam welded portion. Further, P segregates on the surface of the steel sheet due to the thermal history up to the plating and deteriorates the plating property. Specifically, when the P content exceeds 0.05%, these characteristics deteriorate. Therefore, the P content is restricted to 0.05% or less. The lower limit of the P content does not need to be specified, but if the P content is less than 0.005%, the refining cost increases, so the P content is preferably 0.005% or more. Moreover, from a viewpoint of ensuring strength, 0.02% or more is preferable.

Ti: 0.010 to 0.050%
Ti has a strong affinity for C and N, and forms carbonitrides during solidification or hot rolling, and has the effect of reducing C and N dissolved in the steel and improving workability. However, when the Ti content is less than 0.010%, this effect cannot be obtained. On the other hand, when the Ti content exceeds 0.050%, the strength and toughness of the welded portion of the welded joint, that is, the torsional seam welded portion toughness deteriorates. Therefore, the Ti content is set to 0.010 to 0.050%.

Nb: 0.010 to 0.040%
Nb has a strong affinity with C and N like Ti, forms carbonitride during solidification or hot rolling, reduces C and N dissolved in steel, and improves workability effective. However, this effect cannot be obtained when the Nb content is less than 0.010%. On the other hand, when the Nb content exceeds 0.040%, the recrystallization temperature increases, high temperature annealing is required, and the toughness of the welded portion of the welded joint deteriorates. Therefore, the Nb content is set to 0.010 to 0.040%.

B: 0.0005 to 0.0030%
B is an element that increases the grain boundary strength and improves the secondary work brittleness resistance by segregating at the grain boundaries. However, when the B content is less than 0.0005%, the effect cannot be obtained.

  On the other hand, if the B content exceeds 0.0030%, B segregates at the γ grain boundary during welding and suppresses ferrite transformation, and the structure of the welded part and its heat-affected zone becomes a low-temperature transformation-generated structure. The welded part and the heat-affected zone harden and the toughness deteriorates. As a result, the torsional seam welded part toughness deteriorates.

  Further, when B is added in a large amount, ferrite transformation during hot rolling is also suppressed, and a hot rolled steel sheet having a low temperature transformation formation structure is obtained, so the strength of the hot rolled steel sheet is increased and the load during cold rolling is increased. . Furthermore, if the B content exceeds 0.0030%, the recrystallization temperature becomes high, and annealing at a high temperature is required. This leads to an increase in manufacturing cost and an r value surface that is an index of deep drawability. The internal anisotropy increases and the press formability deteriorates. Therefore, the B content is set to 0.0005 to 0.0030%. In addition, the preferable range of B content is 0.0005 to 0.0015% from the reason mentioned above.

S: 0.010% or less S is an impurity that is inevitably mixed during the refining of steel, and combines with Mn and Ti to form precipitates and deteriorate the workability. Therefore, the S content is 0.010%. % Or less. In order to reduce the S content to less than 0.0001%, the manufacturing cost increases. Therefore, the S content is preferably set to 0.0001% or more.

Al: 0.01-0.30%
Al is an element used as a deoxidizing material during refining of steel, but if the Al content is less than 0.01%, the deoxidizing effect cannot be obtained. However, if the Al content exceeds 0.30%, the toughness of the wavy seam welded portion and the workability are reduced. Therefore, the Al content is set to 0.01 to 0.30%.

N: 0.0010 to 0.01%
N is an element inevitably mixed during the refining of steel. N forms nitrides of Ti, Al, and Nb and does not adversely affect workability, but deteriorates the toughness of the weld. For this reason, it is necessary to regulate N content to 0.01% or less. On the other hand, to reduce the N content to less than 0.0010%, the manufacturing cost increases. Therefore, the N content is set to 0.0010 to 0.01%.

  In addition, the remainder in the steel plate of this invention, ie, components other than each element mentioned above, is Fe and an unavoidable impurity.

  In the high-strength steel sheet of the present invention, by making the element content within a specific range as described above, with a tensile strength of 340 MPa or more, it has a press formability applicable to the automotive field, in particular, a fuel tank application, An Sn-Zn plated high-strength steel sheet excellent in low-temperature toughness, plating property, and corrosion resistance and a method for producing the same can be provided. These effects make it possible to increase the strength of the steel sheet, improve fuel efficiency by reducing the weight of the vehicle body, and in particular, reduce the weight of the fuel tank and complicate the vehicle body design. This effect is extremely large industrially.

  The present inventors diligently studied about the steel plate components of the high-strength steel plate and the wettability of the molten Sn—Zn plating, and studied the mechanism by which red rust occurs in the corrosion resistance test at SST. As a result, in molten Sn-Zn plating, in addition to the decrease in plating wettability caused by oxides of Si and Mn that are concentrated in the annealing process, as the scale is generated in the hot rolling process, it is concentrated at the scale / base metal interface. It turns out that after the cold rolling and annealing, finely pulverized Si-containing oxide remains, which reduces the plating wettability and consequently the corrosion resistance. did.

  That is, when Sn-Zn plating is performed on a steel sheet having a Si-containing oxide enriched with Si on the surface of the steel sheet, plating failure occurs in the portion where the Si oxide is present, and a portion where plating is lost is formed. Is done. For this reason, although Sn-Zn plating is performed, the corrosion resistance is lowered.

  In the present invention, the Si surface concentration of the steel sheet before Sn—Zn plating treatment is more than 0.3 to 1.5% or less, the area ratio of the Si-containing oxide on the surface is 3% or less with respect to the entire surface, and Si It has been found that a high strength steel sheet and corrosion resistance can be secured by controlling the size of one contained oxide to 1 μm or less. As described above, in order to ensure the tensile strength of 340 MPa or more and the excellent secondary work brittleness resistance and the toughness of the seam welded seam, the steel sheet (base iron) contains more than 0.3% Si. The lower limit of the Si surface concentration of the steel sheet was over 0.3%. On the other hand, when the Si surface concentration exceeds 1.5%, as described above, the Sn—Zn plating defective portion from which plating is lost increases so that the corrosion resistance is lowered. Therefore, the Si surface concentration is set to more than 0.3 to 1.5%. The Si surface concentration is measured by a device capable of quantitatively measuring the element concentration such as CMA (Computer-Aided Micro Analyzer).

  In addition, if the Si-containing oxide is present on the surface of the steel sheet, it causes Sn—Zn plating defects. However, as the existence state of the Si oxide that does not adversely affect the corrosion resistance even if plating failure occurs, the area ratio of the Si-containing oxide on the steel sheet surface is 3% or less with respect to the entire surface, and the Si-containing oxide 1 It was found that it is important to control the size of the individual to 1 μm or less. That is, if the area ratio of the Si-containing oxide on the surface of the steel sheet is 3% or less with respect to the entire surface and the size of one Si-containing oxide is 1 μm or less, the plating is removed during Sn-Zn plating. This is because even if a defective portion is generated, the size of the defective plating portion is small and small, so that the defective plating portion becomes a plating layer covered with plating and does not adversely affect the corrosion resistance. Note that the presence state of the Si oxide is performed with a device that can be observed at a high magnification, such as SEM (Scanning Electron Microscope).

  The means for controlling the presence state of the Si-containing oxide remaining on the steel sheet surface will be described.

  First, the mechanism by which concentrated Si-containing oxides are formed on the steel sheet surface will be considered. As shown in FIG. 1 (a), solid solution Si (Fe + solid solution Si) is present in the steel plate 1. When this steel plate is heated, as shown in FIG. 1B, scale (Fe oxide) 2 is generated on the surface of the steel plate. Si in the base iron is not taken into the scale, but is concentrated at the interface between the scale and the base iron to form the Si-containing oxide 3. Even if descaling is performed and the scale is removed, the Si-containing oxide remains on the surface of the base iron. This Si-containing oxide is generated during hot rolling or annealing.

  In the present invention, in order to reduce the Si-containing oxide, it can be achieved by reducing the scale production amount of hot rolling and optimizing the pickling time, and for reducing the scale production amount (scale thickness) of hot rolling. It is important to set the finishing temperature of hot rolling to a low temperature of 800 to 925 ° C. And the scale and surface-concentrated Si generated by lowering the hot rolling finishing temperature can be removed by pickling, and in particular, controlling the hot rolling finishing temperature and pickling time. Thus, the present inventors have found that the Si surface enrichment targeted by the present invention and the existence state of the Si-containing oxide remaining on the steel sheet surface can be controlled.

  FIG. 2 is a diagram showing conditions under which the Si surface concentration is 1.5% or less due to the relationship between the hot finishing temperature FT (° C.) and the pickling time (sec), and the Si surface concentration is 1.5%. ◯, the case where the Si surface enrichment is 1.3%, and the case where the Si surface enrichment is 1.1% are indicated by Δ. In addition, 5 to 20% hydrochloric acid generally used in the steel industry is used for pickling for removing the hot-rolled scale.

  As shown in FIG. 2, as the hot finish temperature FT (° C.) increases, the amount of scale generation increases, and if the pickling time is not lengthened, the Si surface concentration may be 1.5% or less. Can not. Here, pickling may be performed in a region where the Si surface concentration can be 1.5% or less, and the region is [FT] × when the hot rolling finish temperature is [FT] ° C. It can be seen that pickling for removing the surface scale of the hot-rolled steel sheet may be performed in a time of 0.6 to 500 (sec.) Or more.

  Therefore, the pickling time in the present invention is limited to [FT] × 0.6-500 (sec.) Or more when the hot rolling finish temperature is [FT] ° C. The upper limit is not particularly limited and may be determined as appropriate.

  The pickling may be performed as usual, and examples of the pickling include use of hydrochloric acid, sulfuric acid, nitric acid aqueous solution or a mixed aqueous solution thereof, but a hydrochloric acid aqueous solution having a high pickling speed is preferable. . For example, the hydrochloric acid concentration can be 5 to 20 mass%, the liquid temperature can be 80 to 95 ° C., and the pickling time can be 20 to 100 seconds. Moreover, before pickling, it is preferable to generate a crack in the hot-rolled scale (scale braking) to improve the descaling property and descaling.

  The pickled hot-rolled steel sheet is cold-rolled at a cold rolling rate of 50% or more to obtain a cold-rolled steel sheet having a predetermined thickness, annealed at a temperature higher than the recrystallization temperature, and then pickled. Even by washing, it is possible to control the Si surface enrichment targeted by the present invention and the existence state of the Si-containing oxide remaining on the steel sheet surface.

  In this case, the electrolytic pickling is performed by using an acid pickling solution in which an annealed steel sheet is mixed with one or more of nitrate, sulfate, fluorosilicate, and fluoroborate in 20 to 400 g / l sulfuric acid aqueous solution. Wash. This electrolytic pickling can also be applied to pickling after hot rolling of the present invention.

  The sulfuric acid used as the main agent in the electrolytic pickling solution of the present invention is added to improve the dissolving action and conductivity of the oxidized soot, but when it coexists with other solvents, the concentration is less than 20 g / l. The pickling efficiency is remarkably poor, and if the concentration exceeds 400 g / l, the pickling becomes peracid pickling and the surface appearance is damaged. Accordingly, in the present invention, sulfuric acid is used at 20 to 400 g / l. Then, in order to greatly improve the moderate removal of oxide film in sulfuric acid aqueous solution, fluorosilicate such as sodium fluorosilicate, potassium fluorosilicate, sodium fluoroborate, fluoroborate such as ammonium fluoroborate 1 type or 2 types or more are added 10-100 g / l, respectively. Fluorosilicate and fluoroborate do not contribute to an appropriate improvement in the removal of the oxide film when the addition amount is less than 10 g / l, and the effect is saturated when the addition amount exceeds 100 g / l. ˜100 g / l.

  In addition, nitrate improves the removal rate of the oxide film, but has the effect of suppressing peracid washing. As nitrate, sodium nitrate, potassium nitrate, ammonium nitrate, etc. can be used, and it is set as 50-200 g / l. If it is less than 50 g / l, the effect cannot be obtained, while if it exceeds 200 g / l, the effect is saturated.

  Also, sulfates represented by sodium sulfate have the effect of suppressing peracid washing in electrolytic pickling, but no effect is obtained at less than 50 g / l, while the effect is saturated at over 200 g / l. Therefore, it was set to 50 to 200 g / l.

  Next, the manufacturing method of the steel plate of this invention is demonstrated.

When manufacturing the steel plate of this invention, the raw material adjusted so that it may become the steel composition mentioned above is first thrown into a converter or an electric furnace, and a vacuum degassing process is performed to make a slab. Next, this slab is hot-rolled under the conditions of a heating temperature of 1000 ° C. or higher and 1300 ° C. or lower and a finishing temperature of Ar ₃ temperature or higher and 1000 ° C. or lower to obtain a hot rolled steel sheet. The heating of the hot rolling requires 1000 ° C. or higher to secure the rolling temperature, and in order to suppress the formation of coarse TiN that causes toughness reduction and to suppress the austenite grain coarsening, further reduce the heating cost. Therefore, the temperature is set to 1300 ° C. or lower. In particular, coarse TiN leads to a decrease in toughness of the wavy seam weld. Moreover, finishing temperature of hot rolling is below Ar ₃ temperature, the workability of the steel sheet is impaired, finishing temperature of hot rolling is the Ar ₃ temperature or more. Further, it is difficult to ensure a finishing temperature exceeding 1000 ° C. by hot rolling, and it is difficult to reduce the Si surface concentration after pickling, which is a feature of the present invention, to 1.5% or less. Therefore, the finishing temperature is set to Ar ₃ temperature or higher and 1000 ° C. or lower.

  Next, the hot-rolled steel sheet produced by the above-described method is descaled as necessary, and then cold-rolled at a cold rolling rate of 50% or more to obtain a cold-rolled steel sheet having a predetermined thickness. At this time, when the cold rolling rate is less than 50%, the strength of the steel sheet after annealing is lowered and the deep drawing workability is deteriorated. In addition, it is preferable that this cold rolling rate shall be 65 to 85%, and, thereby, the steel plate which was more excellent in intensity | strength and deep drawing workability is obtained.

  Thereafter, the cold rolled steel sheet is annealed at a temperature higher than the recrystallization temperature. At that time, when the annealing temperature is lower than the recrystallization temperature, a good texture does not develop, and the deep drawing processability deteriorates. On the other hand, since the strength of the steel sheet decreases as the annealing temperature increases, annealing is preferably performed at a temperature of 850 ° C. or lower.

Next, the surface of the steel plate after annealing is subjected to molten Sn—Zn plating to obtain a plated steel plate. From the viewpoint of corrosion resistance, this plating has an area ratio of 97% or more on a FeSn ₂ alloy layer with 1 to 8.8% Zn and the balance Sn: 91.2 to 99% and inevitable impurities and / or It is preferable to perform plating which consists of an incidental component and the adhesion amount is 10-150 g / m < ² > on one side.

  The reasons for these limitations are as follows.

First, the area ratio of the FeSn ₂ alloy layer is 97% or more, if the area ratio is less than 97%, since the possibility of Sn-Zn plated thereon is not sufficiently generated occurs, FeSn ₂ alloy layer The lower limit of the area ratio was 97%. The upper limit is preferably 100%, but 99.5% is preferable as the upper limit in actual operation. The FeSn ₂ alloy layer is generated by being immersed in a hot dipping bath, and more is generated if the immersion time is increased.

  The reason for the limitation of the Zn of the plating composition is that it is limited by the balance of corrosion resistance between the inner surface and the outer surface of the fuel tank. The outer surface of the fuel tank is painted after the fuel tank is molded because perfect rust prevention capability is required. Therefore, although the coating thickness determines the rust prevention ability, the material prevents red rust by the anticorrosion effect of the plating layer. In particular, the anticorrosive effect of the plating layer is extremely important in the part where the coating is not good. Addition of Zn to Sn-based plating lowers the potential of the plating layer and provides sacrificial anticorrosive ability. For that purpose, 1 mass% or more of Zn needs to be added. Addition of excess Zn exceeding 8.8%, which is the Sn-Zn binary eutectic point, promotes growth of coarse Zn crystals, raises the melting point, and forms an intermetallic compound layer (so-called alloy layer) under the plating. It must be 8.8% or less because it leads to excessive growth. The coarse Zn crystal has no problem in that the sacrificial anticorrosive ability of Zn appears, but on the other hand, the coarse Zn crystal part is likely to undergo selective corrosion. Moreover, since the intermetallic compound itself is very brittle during the growth of the intermetallic compound layer under the plating layer, plating cracks are likely to occur during press molding, and the anticorrosion effect of the plating layer is reduced.

  On the other hand, corrosion on the inner surface of the fuel tank is not a problem when only normal gasoline is used, but a severe corrosive environment appears due to water contamination, chlorine ion contamination, and the formation of organic carboxylic acids due to oxidative degradation of gasoline. there's a possibility that. If gasoline leaks outside the fuel tank due to piercing corrosion, it can lead to serious accidents, and these corrosions must be completely prevented. When a deteriorated gasoline containing the above-mentioned corrosion promoting component was prepared and the performance under various conditions was examined, it was confirmed that Sn—Zn alloy plating containing Zn of 8.8% or less exhibits extremely excellent corrosion resistance. It was.

  When pure Sn or Zn content does not contain Zn at all, the plating metal has no sacrificial anti-corrosion ability against the iron from the beginning when exposed to corrosive environment. There is a problem of pitting corrosion at the part and early red rust generation on the outer surface of the tank. On the other hand, when Zn is contained in a large amount exceeding 8.8%, Zn is preferentially dissolved, and a large amount of corrosion products are generated in a short time, so that there is a problem that the carburetor is easily clogged.

  Moreover, in terms of performance other than corrosion resistance, the workability of the plating layer is reduced due to an increase in the Zn content, and good press formability, which is a feature of Sn-based plating, is impaired. Furthermore, due to the increased Zn content, the solderability is greatly reduced due to an increase in the melting point of the plating layer and Zn oxide.

  Therefore, the Zn content in the Sn—Zn alloy plating according to the present invention is in the range of 1 to 8.8%, and further in the range of 3.0 to 8.8% in order to obtain a more sufficient sacrificial anticorrosive action. desirable.

If the adhesion amount of this Sn—Zn plating is less than 10 g / m ^{2 on} one side, good corrosion resistance cannot be secured, and if it exceeds 150 g / m ² , the cost increases and the thickness becomes mottled. It becomes a defect and weldability is lowered. Therefore, the adhesion amount of Sn—Zn plating was set to 10 to 150 g / m ^{2 on} one side.
In order to further improve the plating property, it is effective to perform pre-plating of Fe—Ni before plating in order to improve the corrosion resistance by refining the wettability of Sn—Zn plating and primary crystal Sn. The amount of adhesion is preferably 0.2 g / m ² or more in terms of wettability of plating, and the proportion of Ni is preferably 10 to 70% by mass from the viewpoint of refining primary crystal Sn. And as for the steel plate produced by the above-mentioned method, you may electroplate the surface further as needed.

Hereinafter, the effects of the present invention will be specifically described with reference to Examples and Comparative Examples of the present invention.
In this example, steel having the composition shown in Table 1 below was melted and heated to 1000-1300 ° C., and then the hot rolling finishing temperature was 840-975 ° C. and the winding temperature was 580-750 as shown in Table 2. It was hot-rolled under the condition of ° C. to obtain a hot-rolled sheet having a thickness of 3.5 mm. Next, the hot-rolled sheet was pickled and cold-rolled to obtain a cold-rolled sheet having a thickness of 1.0 mm.
Furthermore, the cold rolled sheet was annealed in a cycle that was held at 760 to 840 ° C. for 60 seconds to obtain an annealed steel sheet. The surface of this steel plate was electrolytically pickled with the pickling solution shown in Table 2, Fe-Ni plating was applied at 1 g / m ² , and then Sn—Zn plating was performed by a flux method. The Fe-Ni alloy plating bath used was a nickel plating Watt bath with 100 g / L of iron sulfate added. As the flux, a ZnCl2-NH4Cl aqueous solution was applied by roll coating, and the composition of Zn in the plating bath was as shown in Table 2. The bath temperature was 280 ° C., and the amount of plating adhered was adjusted as shown in Table 2 by gas wiping after plating. Furthermore, the steel plate after the hot dipping treatment was subjected to a Cr3 + main treatment to obtain high-strength steel plates of Examples and Comparative Examples. The balance in the steel composition shown in Table 1 below is Fe and inevitable impurities. Moreover, the underline in the following Table 1 shows that it is outside the scope of the present invention.

  Next, for each of the high-strength steel sheets of Examples and Comparative Examples prepared by the above-described methods, the tensile characteristics, the r value that is an index of deep drawing, the secondary work brittleness, the low temperature toughness of the seam welded portion, and the corrosion resistance evaluated. Hereinafter, the evaluation method will be described.

  Tensile properties were evaluated from tensile strength TS and elongation El using a JIS No. 5 test piece collected from each hot-dip plated steel sheet so that the tensile direction was parallel to the rolling direction. And the thing whose tensile strength TS is 440 Mpa or more and elongation El is 33% or more was set as the pass.

Evaluation of the r value was performed by collecting JIS No. 5 tensile test pieces from each hot-dip plated steel sheet in three directions, ie, parallel to the rolling direction, 45 ° direction, and perpendicular direction, and measuring the r value of each test piece. Then, when the r value parallel to the rolling direction is r ₀ , the r value in the 45 ° direction is r ₄₅ , and the r value in the perpendicular direction is r ₉₀ , the average of the r values in each direction obtained by the following formula (C) Evaluation was based on the value r _ave . In this example, a sample having a r _ave of 1.40 or more was accepted.
r _ave = (r ₀ + 2 × r ₄₅ + r ₉₀ ) / 4 (C)
The secondary work brittleness resistance is obtained by blanking the hot-dip plated steel sheet to a diameter of 95 mm, and then performing cylindrical drawing with a punch having an outer diameter of 50 mm, and as shown in FIG. The weight 6 having a weight of 5 kg was dropped from a position of 1 m in height under various temperature conditions, and the lowest temperature at which the cup did not crack (secondary work brittleness resistance) was determined. Although the secondary work brittleness temperature varies depending on the thickness of the steel sheet and the test method, in this example in which the thickness of the cold-rolled steel sheet is 1.0 mm, −50 ° C. or less was accepted.

  The toughness evaluation of the worship-shaped seam welded portion 7 was performed by bending a flange into a test piece shape shown in FIG. 4 and fixing each test piece (steel plate) 8a and 8b with a chuck at 200 mm / min. Tensile tests were conducted at various temperatures, the fracture surface after fracture was investigated, and the temperature at which the brittle fracture surface and the ductile fracture surface were 50% each was determined as the ductile brittle transition temperature. In the present Example, the thing below -40 degreeC was set as the pass.

  Further, the corrosion resistance was evaluated by the red rust occurrence rate at SST 960 hours, and 50% or less was regarded as acceptable. The above evaluation results are summarized in Table 3.

  As shown in Table 3 above, the No. of the example within the scope of the present invention. Steel plate No. 1 has a good red rust occurrence rate of 9.9% at SST 960 hours, which is an index of corrosion resistance, an elongation El of 36.6%, and an average value rave of r value of 1.65 and excellent processing characteristics. The secondary work brittleness temperature and the ductile brittle transition temperature of the wavy seam weld were good at low temperatures. Examples No. within the scope of the present invention. Steel plate No. 2 also has excellent corrosion resistance with a red rust occurrence rate of 13.0% at SST 960 hours, excellent properties such as an elongation El of 36.3% and a rave of 1.64, which are indexes of workability. Also, it was excellent in secondary work brittleness resistance and toughness of the wrinkled seam weld. Examples No. within the scope of the present invention. The steel plate No. 3 also has excellent SST red rust occurrence rate of 9.6%, excellent properties such as elongation El of 35.6% and rave of 1.67, which are indexes of workability, Sub-working brittleness and tough seam weld toughness also had excellent properties.

  Examples No. within the scope of the present invention. Steel plate No. 4 has excellent SST red rust occurrence rate of 11.3%, excellent corrosion resistance, elongation El of 37.1%, r value average value 1.69 and excellent processing characteristics. Both the next processing brittle temperature and the ductile brittle transition temperature of the wavy seam weld were good at low temperatures. Examples No. within the scope of the present invention. The steel plate No. 5 also has excellent SST red rust occurrence rate of 5.0%, excellent corrosion resistance, elongation characteristics El of 36.4% and rave of 1.67. It was also excellent in secondary processing brittleness and toughness of the wavy seam weld. Examples No. within the scope of the present invention. Steel plate No. 6 also has excellent SST red rust occurrence rate of 9.6%, excellent properties such as elongation El of 35.5% and rave of 1.64, which are indexes of workability, Sub-working brittleness and tough seam weld toughness also had excellent properties.

  Examples No. within the scope of the present invention. Steel sheet No. 7 also has excellent SST red rust occurrence rate of 43.6%, excellent properties such as elongation El of 34.3% and rave of 1.58, which are indexes of workability, Sub-working brittleness and tough seam weld toughness were also excellent. Examples No. within the scope of the present invention. Steel plate No. 8 also has excellent SST red rust occurrence rate of 8.4%, excellent properties such as elongation El of 37.6% and rave of 1.72, which are indexes of workability, and has two resistance. Sub-working brittleness and tough seam weld toughness were also excellent. Similarly, no. 9-No. No. 14 also had excellent corrosion resistance, workability, secondary work brittleness resistance and excellent comb-like seam weld toughness.

  On the other hand, the pickling time after hot rolling was [FT] × 0.6-500 (sec.) Less than No. 15, no. 16, no. 17, no. 18, no. 19, no. 20, no. 22, no. 23, no. The steel plate No. 26 had an Si surface concentration after pickling exceeding 1.5%, and the SST red rust occurrence rate exceeded 50% and was inferior in corrosion resistance. In addition, No. of the comparative example in which the C content is out of the scope of the present invention. Steel sheet No. 21 has an elongation El as an index of workability of 30.2% and an r value as low as 1.14, and the workability is inferior to the steel sheets of the above-mentioned examples. The toughness was also inferior.

  No. The steel plate No. 23 had an Mn content exceeding the upper limit of the present invention, and the elongation El and r value, which are indexes of workability, were lower than those of the above-described Examples, and the workability was inferior. No. Steel plate No. 24 is a comparative example in which the P content deviated from the scope of the present invention, and the secondary work brittleness resistance and the wavy seam weld toughness were inferior to those of the above-described examples. No. Steel plate No. 25 is a comparative example in which the Ti content is less than the range of the present invention. This steel sheet had low elongation El and r value, and was inferior in workability. No. Steel plate No. 26 is a comparative example in which the Ti content exceeds the upper limit of the present invention. This steel sheet had low elongation El and r value, and was inferior to the steel sheet of the above-mentioned example in the toroidal seam weld toughness. No. Steel plate No. 27 is a comparative example in which the Nb content is less than the range of the present invention. This hot dipped steel sheet has a low r value and elongation El and does not meet the purpose of the present invention having excellent workability.

  No. Steel plate No. 28 is a comparative example in which the B content is 0.0003%, which is less than the lower limit of the present invention. This steel plate had a secondary work brittleness resistance of −20 ° C., which was inferior to the steel plates of the above-mentioned examples. Moreover, since Zn mass% of plating is low, it does not have sufficient sacrificial anticorrosive effect and is inferior to external surface corrosion resistance. No. No. 29 steel plate is a comparative example in which the B content exceeds the range of the present invention. This steel sheet had low elongation El and r values, which are indexes of workability, and had a high ductile brittle transition temperature at the wavy seam welded portion, resulting in poor welded portion toughness. Furthermore, since the Zn mass% of the plating was high, Sn segregation did not appear and Zn segregation at the eutectic cell grain boundaries and growth of coarse Zn crystals were promoted, so the corrosion resistance was also lowered.

1 Steel sheet metal 2 Scale (Fe oxide)
3 Si oxide 4 Drawing cup 5 Frustum 6 Weight 7 Wrinkle-shaped seam weld 8a, 8b Test piece (steel plate)

Claims (3)

% By mass
C: 0.0005 to 0.0050%,
Si: more than 0.3 to 1.0%,
Mn: 0.70 to 2.0%,
P: 0.05% or less,
Ti: 0.010 to 0.050%,
Nb: 0.010 to 0.040%,
B: 0.0005 to 0.0030%,
S: 0.010% or less,
Al: 0.01-0.30%,
N: 0.0010 to 0.01%
High-strength steel sheet having a composition comprising Fe and inevitable impurities as the balance, Si surface concentration of more than 0.3% to 1.5%, and the surface area ratio of the Si-containing oxide on the entire surface 1% to 8.8% Zn on the FeSn ₂ alloy layer having an area ratio of 97% or more on a high-strength steel plate having a size of 3% or less and one Si-containing oxide of 1 μm or less. A Sn-Zn plated high-strength steel sheet, which is provided with molten Sn-Zn plating, the balance of which consists of Sn and inevitable impurities, the adhesion amount of which is 10 to 150 g / m ² per side. A process of obtaining a slab by continuously casting the molten steel having the component composition according to claim 1; a process of heating the slab at 1000 ° C. to 1300 ° C .; and a finishing temperature of Ar ₃ temperature to 1000 ° C. Hot-rolled steel sheet is obtained by hot rolling, and pickling is performed at a time of [FT] × 0.6-500 (sec.) Or more when the finishing temperature of the hot rolling is [FT] ° C. A pickling step for removing the surface scale of the hot-rolled steel plate, a step of cold-rolling the pickled steel plate at a cold rolling rate of 50% or more to obtain a cold-rolled steel plate having a predetermined thickness, and the cold-rolled steel plate After the step of annealing at a temperature higher than the recrystallization temperature, the pickling step of removing the surface scale of the annealed steel plate, the surface of the pickled steel plate is subjected to Fe-Ni pre-plating before hot dipping, 1 to 8.8% Zn and the balance consists of Sn and inevitable impurities. And characterized but has a step of performing molten Sn-Zn plating is per side 10 to 150 g / ^{m 2,} it Si surface concentration of the steel sheet before plating of 0.3 percent to 1.5% The manufacturing method of the Sn-Zn plating high-strength steel plate of Claim 1 . Any of the pickling processes according to claim 2 is a pickling solution in which one or more of nitrate, sulfate, fluorosilicate, and fluoroborate are mixed in a 20 to 400 g / l sulfuric acid aqueous solution. The method for producing a Sn-Zn plated high-strength steel sheet according to claim 2, wherein the method is electrolytic pickling.

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