JP2010059475A - Hot dip galvanized steel sheet and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet which is satisfactory in surface properties without any stripe patterns, and has excellent press formability. <P>SOLUTION: The surface of a steel sheet having a chemical composition containing, by mass, C, Si, Mn, P, S, sol.Al, N, sol.Ti, Nb and O within predetermined ranges, in which the contents of sol.Ti and Nb satisfy the following equations (1) to (3), and the balance Fe with impurities, and in which the content of Ti oxides in oxide inclusions is ≥50.0% expressed in terms of TiO<SB>2</SB>and the content of Nb oxides is <1.0% expressed in terms of NbO is provided with a hot dip galvanizing layer: 1.0<(Ti<SP>*</SP>/48+Nb/93)/(C/12+N<SP>*</SP>/14)(1), Ti<SP>*</SP>=max[sol.Ti-(48/14)×N, O](2) and N<SP>*</SP>=max[N-(14/48)×sol.Ti, O](3), the symbols of elements in the respective equations denote the contents of the respective elements by mass% and the max[] represents the function returning the maximum value of the argument in the brackets[]. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is formed and used in various shapes by press working or the like, and is used as a hot dip galvanized steel sheet, a hot dip galvanized steel sheet, or an alloyed hot dip galvanized steel sheet (hereinafter collectively referred to as “hot dip galvanized steel sheet”). Further, the present invention relates to a hot dip galvanized steel sheet having excellent press formability and good surface properties, and a method for producing the same.

  Now that the industrial technology field is highly divided, materials used in each technical field are required to have special and high performance. For example, even cold-rolled steel sheets used by press forming are required to have better formability with the diversification of press shapes. In particular, regarding automotive steel sheets, due to consideration for the global environment, the demand for thin-walled, high-formability cold-rolled steel sheets has increased significantly in order to reduce the weight of the vehicle body and improve fuel efficiency. In press molding, the thinner the steel sheet used, the easier it is to generate cracks and wrinkles. Therefore, a steel sheet with better deep drawability and ductility is required.

  So far, many proposals have been made on so-called Ti-IF steel sheets in which Ti is added to ultra-low carbon steel as cold-drawn steel sheets for deep drawing. In Ti-added ultra-low carbon cold-rolled steel sheets, C and N in the steel are precipitated and fixed as TiC and TiN, so that a recrystallized texture preferable for deep drawability is formed during annealing, and excellent formability is obtained. Can do. However, when hot dip galvanizing is performed using a Ti-added ultra-low carbon cold-rolled steel sheet as a raw material, a streaky pattern may be generated on the plated surface and the appearance may be impaired. This streak pattern is recognized as a difference in color tone on the plating surface due to unevenness of the plating layer along the rolling direction. Depending on the degree of the unevenness, this streak pattern is recognized even after painting. For example, a roof, a hood, a door In the case of an automobile outer panel that requires a beautiful appearance such as an outer panel or a side outer panel, it is a serious defect and is avoided.

  Several proposals have been made regarding methods for suppressing streaks in Ti-added ultra-low carbon hot-dip galvanized steel sheets. For example, Patent Document 1 discloses a technique for preventing streak unevenness by lowering the slab heating temperature before hot rolling in accordance with the Ti content, and making the crystal grain size or texture of the surface layer portion uniform. Is disclosed. However, when the slab heating temperature is low, the temperature range of hot rolling is lowered and the deformation resistance of the steel sheet is increased, which causes manufacturing problems such as the inability to roll wide materials.

  Patent Document 2 discloses a method for preventing streaks by adding Ca to change sulfide inclusions to other complex inclusions. However, since Ca is expensive and has a low yield, it causes an increase in manufacturing cost and may cause rusting.

  Patent Document 3 discloses a method for preventing streaking by increasing the hot rolling finish finishing temperature so as not to leave an unrecrystallized structure after annealing. Patent Document 4 similarly discloses a hot rolling finishing finishing temperature. A method of controlling the texture after annealing and suppressing the streak pattern by increasing is disclosed. However, the method of increasing the hot rolling finishing temperature as described above is not preferable because it causes generation of scale wrinkles.

  Further, Patent Document 5 discloses a technique of containing Nb in order to reduce the Ti amount and prevent mechanical unevenness in order to prevent uneven plating. However, since the recrystallization temperature increases due to an increase in the amount of Nb, it is necessary to anneal at a high temperature. As a result, productivity is impaired and surface flaws are liable to occur.

Regarding the manufacturing technology of the Ti—Nb-added ultra-low carbon cold-rolled steel sheet, Patent Documents 6 and 7 disclose a method for reducing the recrystallization temperature by reducing the amount of acid-soluble Al (sol. Al).
JP-A-7-228944 JP-A-5-9549 JP 2001-342522 A JP-A-10-18011 Japanese Patent Laid-Open No. 3-180429 JP 62-30822 A Japanese Patent Laid-Open No. 10-226843

  The technique disclosed in Patent Document 6 described above performs deoxidation of molten steel with Al, and the remaining sol. It is a method of reducing the recrystallization temperature by suppressing the amount of Al to a small amount, but it is prone to insufficient deoxidation, and surface defects caused by bubbles in the steel are unavoidable, and a surface property that can be applied to steel sheets for automobile exteriors is obtained. I can't.

  In the technique disclosed in Patent Document 7, deoxidation of molten steel is performed with Ti. In this method, Al is a very small amount. This method also has the advantage of preventing the occurrence of surface defects due to alumina clusters, which is often a problem with Al killed steel. However, as a result of repeated studies by the present inventors, when a Ti—Nb ultra-low carbon cold rolled steel sheet is produced by Ti deoxidation, the recrystallization temperature is lower than that produced by Al deoxidation. It has been found that the Rankford value (r value), which is an index of sex, is sometimes not good.

  The present invention has been made in order to solve such problems, and more specifically, the problem is that there is no streak pattern, the surface property is good, and the melt has excellent press formability. It is to provide a galvanized steel sheet.

  The inventors conducted a detailed investigation on the influence of additive elements and inclusion composition on the mechanical properties and surface properties of ultra-low carbon cold rolled steel sheets deoxidized mainly using Ti or mainly Ti and Al. In the present specification, “%” in the content of the steel component and the content of the inclusion composition all mean mass%.

  A series of test steels are mass%, C: less than 0.010%, Si: 0.10% or less, Mn: 2.50% or less, P: 0.10% or less, S: 0.004%, sol. Al: less than 0.002%, N: 0.005% or less, sol. It had a chemical composition consisting of Ti: 0.10% or less, Nb: 0.20% or less, O: 0.015% or less, B: 0.0020% or less, the balance Fe and impurities.

  A steel slab having such a chemical composition is heated to 1250 ° C., then hot-rolled in a temperature range of 910 ° C. or higher, wound at 650 ° C., and the resulting hot-rolled steel sheet is pickled, 82.5 The steel sheet was cold-rolled to a thickness of 0.7 mm at a rolling rate of%. The cold-rolled steel sheet was heated to 850 ° C. and held for 50 seconds using a continuous hot-dip galvanizing simulator, then cooled, hot-dip galvanized, and alloyed to obtain a hot-dip galvanized steel sheet.

  Using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray detector (EDS), the oxide inclusions present in the steel sheet after hot rolling were observed from a longitudinal section parallel to the rolling direction. The relationship with tensile properties was investigated. The oxide inclusions observed in this investigation consisted of Ti oxides, Al oxides, Nb oxides, Mn oxides and Si oxides, and oxides of impurity elements. In addition, the difference in the chemical composition of steel and the composition of an oxide type inclusion was not recognized between the steel piece and the hot dip galvanized steel sheet. Further, tensile test specimens were taken from the hot dip galvanized steel sheet in the rolling direction, 45 ° direction from the rolling direction, and the direction orthogonal to the rolling direction. Furthermore, the surface of the hot dip galvanized steel sheet was visually observed to investigate the presence or absence of streaks.

As a result of these preliminary tests, the following findings (A) to (C) were obtained.
(A) FIG. 1 is a graph showing the relationship between NbO and the amount of Si. NbO means the content (mass%) of the Nb oxide in the oxide inclusions, and the Si content means the Si content (mass%) in the steel. As shown in the figure, it can be seen that NbO decreases as the Si amount increases. 1 shows sol. It is a graph about what Ti content (it means acid-soluble Ti amount) is less than 0.0030%.

(B) FIG. 2 is a graph showing the relationship between the average r value and (Ti ^* / 48 + Nb / 93) / (C / 12 + N ^* / 14). Here, Ti ^* is a value given from the following formula (2), and N ^* is a value given from the following formula (3) or (4). The average r value is the r value in the rolling direction (r _{0 °} value), the r value in the direction 45 ° with the rolling direction (r _{45 °} value), and the r value in the direction perpendicular to the rolling direction (r _{90 °).} Value) was obtained from the following formula (5).

Ti ^* = max [sol. Ti- (48/14) × N, 0] (2)
N ^* = max [N− (14/48) × sol. Ti, 0] (3)
N ^* = max [N− (14/48) × sol. Ti- (14/11) × B, 0] (4)
Here, the element symbol in each formula represents the content of each element in steel in mass%, and max [] is a function that returns the maximum value of arguments in [].
Average r value = (r _{0 °} value + 2 × r _{45 °} value + r _{90 °} value) / 4 (5)

The black circles in the drawing indicate that NbO was less than 1.0%, and the black triangles indicate that NbO was 1.0% or more. As shown in the figure, the average r value increases with an increase in (Ti ^* / 48 + Nb / 93) (C / 12 + N ^* / 14), but when NbO is less than 1.0%, 1 It can be seen that the rate of increase is faster than the case of 0.0% or more, and the average r value level reached is also higher.

  The reason for this is not clear, but (a) oxide inclusions with NbO of less than 1.0% are fine spherical or agglomerated shapes, suppress grain growth, and refine hot-rolled steel sheets (B) an oxide inclusion with NbO less than 1.0% promotes transformation from austenite to ferrite and refines the hot-rolled steel sheet; (c) NbO has 1.0% It is presumed that the oxide inclusions that are less than this promote recrystallization, and (d) these results in the formation of a recrystallized texture preferable for deep drawability.

  (C) sol. As the amount of Ti increases, a streak pattern is generated on the surface of the hot dip galvanized steel sheet and the surface properties deteriorate. The reason for this is not clear, but sol. If the amount of Ti is large, Ti precipitates are generated non-uniformly in the vicinity of the steel plate surface in the hot rolling process, the crystal grain size on the base steel plate surface before hot dip galvanizing becomes non-uniform, and the variation in particle size is galvanized. It is presumed to be reflected in the streak pattern.

  From the above results, deoxidation was mainly performed using Ti or mainly Ti and Al by reducing the content of Nb oxide in oxide inclusions by containing a certain amount or more of Si in the steel. It is possible to stably obtain a high r value in an extremely low carbon cold rolled steel sheet, By increasing the Nb content without excessively containing Ti, it is possible to achieve both a high r value and a good surface property without streaking.

The present invention completed based on the above knowledge is as follows.
(1) By mass%, C: 0.0005% or more and less than 0.010%, Si: more than 0.020% and 0.40% or less, Mn: 2.50% or less, P: 0.10% or less, S : Less than 0.010%, sol. Al: less than 0.0050%, N: 0.005% or less, sol. Ti: 0.020% or less, Nb: 0.010% or more and 0.20% or less, and O: 0.015% or less. The content of Ti and Nb satisfies the following formulas (1) to (3), the balance has a chemical composition consisting of Fe and impurities, and the content of Ti oxide in the oxide inclusions is TiO ₂ equivalent A hot dip galvanized steel sheet comprising a hot dip galvanized layer on the surface of a steel sheet having a Nb oxide content of less than 1.0 mass% in terms of NbO.
1.0 <(Ti ^* / 48 + Nb / 93) / (C / 12 + N ^* / 14) (1)
Ti ^* = max [sol. Ti- (48/14) × N, 0] (2)
N ^* = max [N− (14/48) × sol. Ti, 0] (3)
Here, the element symbol in each formula represents the content of each element in steel in mass%, and max [] is a function that returns the maximum value of arguments in [].

  “Hot galvanizing” means not only hot dip galvanizing but also hot dip galvanizing or galvannealed galvanizing.

  (2) The chemical composition contains, in mass%, B: 0.0002% or more and 0.0020% or less instead of part of Fe, and the following formula (4) instead of the formula (3) The hot-dip galvanized steel sheet according to (1) above, wherein

N ^* = max [N− (14/48) × sol. Ti- (14/11) × B, 0] (4)
Here, the element symbol in the formula represents the content of each element in the steel in mass%, and max [] is a function that returns the maximum value of the arguments in [].

  (3) The chemical composition contains one or more selected from the group consisting of Cr, Mo, W and Ni in place of a part of Fe, and a total of 2.0% by mass or less. The hot-dip galvanized steel sheet according to (1) or (2) above, wherein

  (4) Ti is added to molten steel decarburized and refined using a vacuum degassing apparatus, and continuously cast to have the chemical composition and oxide inclusion composition described in any of (1) to (3) above. A method for producing a hot dip galvanized steel sheet, comprising hot rolling, cold rolling, recrystallization annealing, and hot dip galvanizing treatment.

  (5) After adding Al to the molten steel decarburized and refined using a vacuum degassing device to control the dissolved oxygen concentration to 0.003% by mass or more, Ti is further added, and continuous casting is performed as described in (1). Or a steel ingot having the chemical composition and oxide inclusion composition according to any one of (3), hot-rolled, cold-rolled, recrystallized annealed, and hot-dip galvanized. A method for producing a hot-dip galvanized steel sheet.

  According to the present invention, a hot-dip galvanized steel sheet having sufficient formability applicable to processing such as press forming and having no surface defects such as streaks on the steel sheet surface is obtained. Moreover, this hot dip galvanized steel sheet can be stably manufactured even in a mass production process at a large-scale steelworks. The present invention greatly contributes to the development of industries, such as contributing to the solution of global environmental problems through weight reduction of automobile bodies.

  The chemical composition and inclusion composition of the steel components in the hot dip galvanized steel sheet according to the present invention, and the steel making, rolling, hot dip galvanizing conditions, etc. in the production method capable of producing the steel sheet efficiently and stably will be described in detail below. .

1. Chemical composition of steel C: 0.0005% or more and less than 0.010% When the C content is 0.010% or more, the ductility and deep drawability of the steel sheet are significantly impaired. On the other hand, excessively low carbonization is accompanied not only by an increase in steelmaking cost, but also the precipitation of NbC becomes insufficient, so that solid solution C remains and the deep drawability deteriorates. Therefore, the content range is 0.0005% or more and less than 0.010%. A desirable range is 0.0010% or more and less than 0.0040%, and a further desirable range is 0.0010% or more and 0.0030% or less.

Si: more than 0.020% and 0.40% or less Si is an important component in the present invention, and lowers the content of Nb oxide in oxide inclusions and improves the deep drawability of the steel sheet. More than 0.020% is contained. On the other hand, since the plateability of the steel sheet is deteriorated, the upper limit of the content is set to 0.40%. A preferred range is more than 0.030% and less than 0.20%, and a more preferred range is more than 0.035% and less than 0.10%.

Mn: 2.50% or less Mn combines with the impurity S to form MnS and suppresses the harmful effects of S and has the effect of strengthening the steel sheet. On the other hand, if contained excessively, ductility and deep drawability deteriorate, so the upper limit of the content is made 2.50%. A preferred range is 0.05% or more and less than 1.00%, and a more preferred range is more than 0.15% and less than 0.50%. Further, from the viewpoint of plating properties, the lower the content, the better, and the upper limit is preferably less than 0.31%.

P: 0.10% or less P has an effect of strengthening the steel sheet without impairing deep drawability. However, if excessively contained, secondary work brittleness resistance is extremely deteriorated, so the content is made 0.10% or less. A preferred range is 0.005% or more and less than 0.050%, and a more preferred range is 0.010% or more and less than 0.015%.

S: Less than 0.010% S is an impurity inevitably contained in the steel, and segregates at the grain boundaries and embrittles the steel. Therefore, the smaller the content, the less preferable, and less than 0.010%. . A preferable upper limit is less than 0.008%, and a more preferable upper limit is less than 0.006%. However, excessively reducing the content causes an increase in manufacturing cost, so it is desirable to make the content exceed 0.001%.

sol. Al: less than 0.0050% Al in steel is in the form of oxides and the like that are not soluble in the acid used during analysis, and in the form of nitrides and solid solutions that are soluble in the acid. Sol. Indicated as Al. sol. Since the Al amount is related to the dissolved Al amount in the molten steel stage, it strongly affects the deoxidation of the steel. In the present invention, it is necessary to disperse oxide inclusions containing 50.0% or more of Ti oxide, and Al inhibits this. The Al content is less than 0.0050%. A preferable upper limit is less than 0.0030%. On the other hand, Al itself can be used for preliminary deoxidation and temperature adjustment in the manufacturing process of molten steel. It is preferable to contain Al by 0.0002% or more. A more preferable range is 0.0005% or more and less than 0.0020%.

N: 0.005% or less N is an element inevitably contained in steel, and an increase in the content deteriorates ductility, deep drawability, and normal temperature aging resistance, so 0.005% or less . A preferable range is 0.003% or less. However, excessively low nitrogen generation not only increases the steelmaking cost, but also causes insufficient precipitation of nitrides, so that solid solution N remains and causes deterioration of deep drawability. It is desirable to make it 0.001% or more.

sol. Ti: 0.020% or less and satisfying the above formulas (1), (2) and (3) or (4) Ti in steel is in the form of oxides that do not dissolve in the acid used during analysis, There are dissolved carbonitrides and solid forms, and acid-soluble Ti content is sol. Indicated as Ti. sol. Ti is an important constituent in the present invention, and the upper limit of the content is set to 0.020% or less in order to prevent streaking from occurring on the surface of the hot dip galvanized steel sheet. A preferable upper limit is 0.015% or less, and a more preferable upper limit is less than 0.004%. Further, C and N in the steel are fixed as TiC, TiN, etc., and have the effect of improving deep drawability. Therefore, the above formulas (1), (2) and (3) or the above formulas (1), (2 ) And (4).

Nb: 0.010% or more and 0.20% or less and satisfying the above formulas (1), (2) and (3) or (4) Nb is an important component in the present invention, and C in steel Is fixed as NbC, and the structure of the hot-rolled sheet is refined to develop a recrystallized texture preferable for deep drawability, and the deep drawability is improved without causing a streak pattern. If the Nb content is low, the desired effect due to the above action cannot be obtained sufficiently and the deep drawability is impaired, so that it is 0.010% or more, and the above formulas (1), (2) and (3 ) Or a range satisfying the above formulas (1), (2) and (4). The lower limit of the preferable content is 0.026% or more. On the other hand, if the Nb content is excessive, the recrystallization temperature rises too much and the deep drawability deteriorates, so the content is made 0.20% or less. It is preferable to satisfy 1.0 <(Ti ^* / 48 + Nb / 93) / (C / 12 + N ^* / 14) <10.0, and more preferably 2.0 <(Ti ^* / 48 + Nb / ⁹³⁾ is to satisfy the ^{/ (C / 12 + N *} /14)<5.0.

O: 0.015% or less If the O content exceeds 0.015%, the amount of oxide inclusions is excessively increased and surface defects are likely to occur. A preferred range is less than 0.010%. On the other hand, in order to generate an appropriate amount of oxide inclusions having a Ti oxide content of 50.0% or more and an Nb oxide content of less than 1.0% to improve deep drawability, 0 0020% or more is preferable. It is more preferable to contain 0.0030% or more.

B: If necessary, 0.0002% or more and 0.0020% or less B has the effect of segregating at the grain boundaries to strengthen the grain boundaries and improving the secondary work brittleness resistance, so 0.0002% or more It may be included. On the other hand, if the content exceeds 0.0020%, the recrystallization temperature rises and the deep drawability deteriorates. Therefore, the content is 0.0002% or more and 0.0020% or less. A preferred range is more than 0.0003% and less than 0.0010%.

One or more selected from the group consisting of Cr, Mo, W and Ni: If necessary, the total is 2.0% or less. Since these elements have an action of strengthening the steel sheet, You may make it contain 2 or more types. However, if the total content exceeds 2.0%, the ductility is remarkably deteriorated. Therefore, the total content is set to 2.0% or less. In addition, it is preferable to make the total content 0.05% or more in order to reliably exhibit the effect of strengthening the steel sheet.

Other than the elements described above, Fe and impurities.
The hot dip galvanized steel sheet of the present embodiment has the above steel composition.

2. Inclusion Composition (1) Oxide Inclusion The galvanized steel sheet according to the present invention has a content of Nb oxide in the oxide inclusion of less than 1.0% and a content of Ti oxide. It shall be 50.0% or more.

  Here, the “oxide inclusion” is generated by an oxidation reaction of an element contained in molten steel in a deoxidation process or the like, and does not include a macro inclusion contained in refractory peeling or the like. The composition of oxide inclusions is mainly composed of oxides of Nb, Ti, Al, Si and Mn, and additionally contains impurities inevitably contained. Inevitable impurities include Mg and Ca oxides, and Fe oxides that are inseparable from the Fe phase as measured by EDS described below.

The composition of the oxide inclusions is measured as follows.
After collecting a test piece from an arbitrary position of the steel plate and polishing a longitudinal section parallel to the rolling direction of the steel plate, an oxide inclusion having a major axis of 1 μm or more was observed using SEM, and Fe was added using EDS. Quantitative analysis is performed on the above elements. Based on the obtained atomic ratio of each element, the chemical composition (unit: mass%) in terms of oxide of the stoichiometric composition defined in advance for each detected element is obtained. Here, the oxides of the stoichiometric composition of the main elements constituting the inclusions are as follows. Ti: TiO ₂ , Nb: NbO, Al: Al ₂ O ₃ , Si: SiO ₂ , Mn: MnO. Further, oxides of stoichiometric composition with respect to the impurity elements are as follows. Mg: MgO, Ca: CaO. The chemical composition is measured for a plurality of oxide inclusions, and the average value is defined as the content of oxide inclusions in the steel sheet. The number of oxide inclusions to be measured is 10 or more, and the larger the number of measurements, the better.

  In addition, the SEM observation of the longitudinal section is a position more than 1/4 of the plate thickness from the boundary between the steel plate base metal and the plating layer so that the influence of the hot dip galvanized layer can be avoided and the bulk properties of the steel plate can be more accurately evaluated. To do. In addition, the region of the oxide inclusions for performing elemental analysis by EDS is a range including the central portion of the oxide inclusions in the SEM image in order to avoid the influence of MnS and the like deposited on the oxide inclusions, In order to obtain an average composition, it is preferable to set the range of ¼ or more of the area of the oxide inclusions.

(2) Nb oxide The content of Nb oxide contained in the oxide inclusions of the steel sheet according to the present invention is less than 1.0%. This is to stably improve the deep drawability of the cold-rolled steel sheet manufactured through a deoxidation process using Ti. The Nb oxide may be present in the form of NbO, NbO ₂ or the like. The content of the Nb oxide is determined by elemental analysis using SEM / EDS as described above and converted to NbO. In order to improve deep drawability, the lower the content of Nb oxide, the better. However, in order to reduce it to less than 0.1%, it is necessary to add a large amount of Ti, and the surface of the hot dip galvanized steel sheet Since it becomes easy to generate | occur | produce a pattern, it is preferable to make the minimum of content into 0.1% or more.

(3) Ti oxide The content of Ti oxide contained in the oxide inclusions of the steel sheet according to the present invention is 50.0% or more. This is because, when the content is less than 50.0%, the oxide inclusions exhibit a shape extended during rolling, and not only the deep drawability is impaired, but individual oxide inclusions are clustered. This is because a tendency is exhibited and surface flaws are likely to occur. It is preferable that the Ti oxide content is 60.0% or more. On the other hand, if the Ti oxide content is too high, the liquid phase is not included in the molten steel stage, and the immersion nozzle is likely to be clogged in the continuous casting process, so the Ti oxide content is 95.0%. The content is preferably less than 90.0%, and more preferably less than 90.0%. The content of Ti oxide, and elemental analysis similarly using SEM / EDS and the content of Nb oxide is obtained in terms of TiO _2.

(4) Other oxides By the way, when manufacturing a steel sheet according to the present invention in a mass production process of a large-scale steelworks, oxides other than Nb oxide and Ti oxide are included in the oxide inclusions. Can be done. Specifically, it is preferable to add Al in advance and partially remove oxygen in the steel before adding Ti to the molten steel in order to improve productivity and manufacturing stability. Oxide is produced. The range of the content of Al oxide in the oxide inclusion is not particularly specified, but is preferably 3.0% or more in order to enjoy the advantages of improving productivity and production stability by adding Al. On the other hand, if it is contained in a large amount, the content of Ti oxide is lowered and the deep drawability is impaired or the immersion nozzle is likely to be clogged. Therefore, the content is preferably less than 35.0%. A more preferable range of the Al oxide content is 5.0% or more and less than 30.0%.

Further, when Si or Mn is contained, the oxide inclusions contain Si oxide or Mn oxide. The content of these oxides in the oxide inclusions is not particularly specified, but if a large amount of Si oxide is contained, the oxide inclusions exhibit a shape that is extended during rolling, and the deep drawability is impaired. Therefore, the Si oxide content is preferably less than 1.0%. Moreover, since Mn oxide has the effect which prevents the obstruction | occlusion of an immersion nozzle, it is preferable to make it contain 2.0% or more. However, if a large amount is contained, the content of the Si oxide having a strong affinity with the Mn oxide is increased and the deep drawability is impaired. Therefore, the upper limit of the Mn oxide content should be less than 25.0%. Is preferred. The contents of Al, Si, and Mn oxide are obtained by elemental analysis using SEM / EDS as described above and converted to Al ₂ O ₃ , SiO _2, and MnO.
The hot dip galvanized steel sheet of the present embodiment has the above oxide inclusion composition.

3. Production Method The hot dip galvanized steel sheet according to the present invention may be produced by any production method as long as it has the above chemical composition and the above relationship can be satisfied with respect to oxide inclusions. However, by employing the following manufacturing method, it is possible to more efficiently and stably manufacture the hot-dip galvanized steel sheet according to the present invention.

(1) Steelmaking, continuous casting In the manufacturing method according to the present invention, in the steelmaking process, after rough decarburization in a steelmaking furnace such as a converter, vacuum decarburization processing is performed with a vacuum degassing apparatus such as an RH apparatus. Subsequently, component adjustment of elements other than Ti is performed, and then Ti or a Ti alloy is added for deoxidation treatment, and continuous casting is performed. Deoxidation treatment by adding Ti or Ti alloy is an oxide system in which the content of Ti oxide is 50.0% or more and the content of Nb oxide is less than 1.0% in the steel sheet. This is because it is necessary to disperse inclusions and improve the deep drawability of the steel sheet.

In order to improve productivity and manufacturing stability in a mass production process of a large-scale steelworks, it is preferable to add Al before adding Ti to perform preliminary deoxidation treatment and temperature adjustment. However, in the case of using deoxidation with Al together, the dissolved oxygen concentration before finally adding Ti needs to be 0.003% or more. This is because when the dissolved oxygen concentration is less than 0.003%, the content of Ti oxide in the oxide inclusions is reduced and the deep drawability is impaired, and the oxide inclusions in the molten steel stage This is because the content of the Al oxide in the product becomes too high, and the immersion nozzle may be blocked during continuous casting. On the other hand, if the dissolved oxygen concentration is too high, the amount of Ti or Ti alloy required for deoxidation increases too much, the cleanliness deteriorates and surface defects are likely to occur. The upper limit of the dissolved oxygen concentration is preferably 0.018%.
In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold.

(2) Hot rolling The steel ingot obtained by continuous casting is reheated, or the hot steel ingot after continuous casting is subjected to hot rolling as it is or after auxiliary heating. In order to keep the surface properties good, it is preferable to care the surface of the steel ingot cold or warm before heating. If the heating temperature is low, the rolling load increases and rolling becomes difficult, so the heating temperature is preferably higher than 1150 ° C.

The hot rolling conditions are not particularly specified, but the Ar ₃ transformation is used in order to refine the crystal grains of the hot rolled steel sheet by performing finish rolling in a low temperature range of austenite and to develop a recrystallized texture preferable for deep drawability during annealing. It is desirable to perform the final reduction in a temperature range of not less than the point (Ar ₃ transformation point + 100 ° C.) and less, and more desirably if the final reduction is performed in the range of 890 ° C. to less than 920 ° C. Moreover, in order to suppress the surface defect of scale property, it is preferable that the difference between the finish rolling start temperature and the finish rolling end temperature is 100 ° C. or more.

  In addition, in order to perform finish rolling in these temperature ranges, you may heat a rough rolling material between rough rolling and finish rolling. At this time, it is desirable to heat the rear end of the rough rolled material at a higher temperature than the front end, and to suppress the temperature variation over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or less. Thereby, the uniformity of the product characteristic in a coil improves.

  For heating the rough rolled material, for example, a solenoid-type induction heating device is provided between the rough rolling mill and the finish rolling mill, and the heating temperature rise is based on the temperature distribution in the longitudinal direction on the upstream side of the induction heating device. It is exemplified to control.

  After the hot rolling is finished, the steel plate is cooled and wound into a coil. If the winding temperature is excessively high, the yield is reduced due to the generation of scale, and therefore it is desirable to wind at a temperature lower than 700 ° C. On the other hand, in order to sufficiently precipitate Ti and Nb carbonitrides after winding and develop a recrystallized texture preferable for deep drawability, the lower limit of the winding temperature is preferably more than 610 ° C.

(3) Cold rolling, annealing, plating Cold rolling is performed according to a conventional method after descaling by pickling or the like. In order to develop a recrystallized texture preferable for deep drawability by recrystallization annealing performed after cold rolling, the rolling reduction is preferably set to 70% or more. If the reduction ratio is excessively high, the load on the rolling equipment increases, leading to a decrease in productivity. Therefore, it is preferable that the rolling reduction is less than 90% and the final plate thickness is 0.40 mm or more. A more preferable rolling reduction is less than 85%.

The cold-rolled steel sheet is subjected to a treatment such as degreasing according to a known method, if necessary, and is recrystallized and annealed. If the heating rate at the time of recrystallization annealing is too fast, ferrite becomes finer and ductility is deteriorated. For this reason, it is preferable that the heating rate to soaking temperature shall be less than 60 degreeC / s. In addition, when the annealing temperature is equal to or higher than the Ac ₃ transformation point, the recrystallization texture preferable for deep drawability is reduced by transformation. Therefore, the upper limit of the annealing temperature is preferably set to be lower than the Ac ₃ transformation point. The recrystallization annealing can be performed by any of continuous annealing, box annealing, and annealing before plating in the continuous hot dip galvanizing process.

  After annealing, a hot dip galvanizing process is performed according to a conventional method. From the viewpoint of productivity and corrosion resistance, it is preferable to perform recrystallization annealing and plating with a continuous hot dip galvanizing apparatus, and to perform alloying treatment. Further, temper rolling may be performed before or after plating.

  Thus, the hot-dip galvanized steel sheet manufactured according to the present embodiment has sufficient formability that can be applied to processing such as press molding and excellent surface properties without streaking. For this reason, this hot-dip galvanized steel sheet can be suitably used for automobile parts, particularly for automobile outer panel.

The present invention will be described more specifically with reference to examples.
Example 1
Steel having the chemical composition shown in Table 1 was melted and cast using a laboratory vacuum melting furnace. These steel ingots were made into steel pieces having a thickness of 20 mm by hot forging, heated to 1250 ° C. using an electric heating furnace, and held for 30 minutes. After the steel slab was extracted from the furnace, it was hot-rolled in a temperature range of 910 ° C. or higher using a laboratory hot rolling mill to obtain a hot-rolled steel sheet having a thickness of 4 mm. Immediately after hot rolling, it is cooled to 650 ° C. by water spray cooling to make it a winding temperature, charged in an electric heating furnace maintained at the same temperature, held for 30 minutes, and then cooled at 20 ° C./h. The furnace was cooled at a speed and the annealing was performed after winding. The obtained steel plate was pickled and used as a cold rolled base metal, and cold rolled at a reduction rate of 82.5% to obtain a cold rolled steel plate having a thickness of 0.7 mm. Using a continuous hot dip galvanizing simulator, the obtained cold rolled steel sheet was heated to 850 ° C. at a heating rate of 20 ° C./s and held for 50 seconds, then cooled to 460 ° C. and immersed in a hot dip zinc bath for 3 seconds. Then, hot dip galvanization was performed. After plating, an alloying treatment was performed for 20 seconds at 500 ° C. to obtain an alloyed hot-dip galvanized steel sheet.

  From the obtained galvannealed steel sheet, a specimen for SEM observation was collected, and the longitudinal section parallel to the rolling direction was polished and then observed using SEM. 10 to 20 oxide inclusions with a major axis of 1 μm or more existing in the range of 1/4 or more of the plate thickness from the interface between the steel plate base metal and the plating layer are randomly selected, and the EDS installed in the SEM Elemental analysis was performed, and the stoichiometric composition was assumed to be converted into an oxide amount, and the average composition of oxide inclusions was determined.

Yield stress (YS), tensile strength (TS) and total elongation were obtained by subjecting the obtained alloyed hot-dip galvanized steel sheet to temper rolling with an elongation of 1.0%, and then applying a JIS No. 5 tensile test piece from the rolling direction. It was obtained by collecting and conducting a tensile test. The r value is a tensile test performed on a JIS No. 5 tensile specimen taken from the rolling direction (0 ° direction), the direction forming 45 ° with the rolling direction (45 ° direction), and the direction orthogonal to the rolling direction (90 ° direction). Using the r value in the 0 ° direction (r _{0 °} value), the r value in the 45 ° direction (r _{45 °} value), and the r value in the 90 ° direction (r _{90 °} value), Based on the average r value.

  The surface properties were evaluated by visually observing the surface of the obtained alloyed hot-dip galvanized steel sheet and by the presence or absence of streaking.

  Table 2 shows the composition analysis and performance evaluation results of the oxide inclusions. The test results (test numbers 1, 2, 5, 8, 10) for the steel sheets within the range defined by the present invention all have good surface properties, and the average r value is 1.90 or more. Good deep drawability was exhibited.

  Test results of steel plates manufactured using steels (steel C, D, F, G, I, K) whose steel composition or oxide inclusion composition deviates from the range defined by the present invention (Trial Nos. 3, 4) , 6, 7, 9, 11), either the surface properties or the r value was inferior.

  Specifically, in the tests using steels C and D (trial numbers 3 and 4), since the Si content in the steel is small and the content of Nb oxide in the oxide inclusions is large, the r value is high. Low. Tests using steels F and K (trial numbers 6 and 11) were conducted using sol. Since the Al content is large and the Ti oxide content in the oxide inclusions is small, the r value is low. The test using steel G (Trial No. 7) was conducted using sol. Since there is too much Ti content, a streak pattern is generated on the plating surface and the surface properties are poor. In the test using steel I (trial number 9), the r value is low because the above formula (1) is not satisfied.

(Example 2)
The molten steel 290 ton was decarburized and refined with a converter, the ladle containing the undeoxidized molten steel was transferred to the RH apparatus, and vacuum decarburization was performed with the RH apparatus. After the vacuum decarburization was completed, Al was added for both the preliminary deoxidation of the undeoxidized molten steel and the temperature raising operation of the molten steel. After addition of Al, oxygen was supplied to the molten steel in the vacuum chamber at 38 Nm ³ / min, and heat was imparted to the molten steel by an appropriate oxidation reaction. Then, in consideration of the concentration already contained in the molten steel in an oxygen concentration state, various alloys other than Ti are added and adjusted, and finally Ti is added and adjusted so that the chemical composition shown in Table 3 is obtained. It was adjusted. In Al killed steel (steel P, Q), the chemical composition was adjusted by adding Ti after that in a state where Al was contained at 0.04% or more in this step.

  After performing these refining, the ladle containing the molten steel was conveyed to a continuous casting machine to obtain a slab-shaped slab having a width of 960 to 1200 mm and a thickness of 250 mm. In this continuous casting process, the change in the opening of the sliding gate that controls the flow rate of the molten steel installed on the upper part of the submerged nozzle was confirmed, and the state of nozzle clogging was evaluated.

  After the surface of the obtained slab was cleaned, it was heated under the conditions shown in Table 4, hot-rolled, pickled, and cold-rolled. Subsequently, the cold-rolled sheet was annealed in a continuous hot dip galvanizing facility, hot dip galvanized, and alloyed. Thereafter, temper rolling was performed at an elongation rate of 1.0% to obtain an galvannealed steel sheet. In some steel plates, the alloying treatment after hot dip galvanization was omitted, and hot dip galvanized steel plates were obtained.

  From the obtained galvannealed steel sheet or hot dip galvanized steel sheet, a specimen for SEM observation was collected, and the longitudinal section parallel to the rolling direction was polished and then observed using SEM. 10 to 20 oxide inclusions with a major axis of 1 μm or more existing in the range of 1/4 or more of the plate thickness from the interface between the steel plate base metal and the plating layer are randomly selected, and the EDS installed in the SEM Elemental analysis was performed, and the stoichiometric composition was assumed to be converted into an oxide amount, and the average composition of oxide inclusions was determined.

Yield stress (YS), tensile strength (TS), and total elongation were determined by taking a JIS No. 5 tensile test piece from the rolling direction and conducting a tensile test. The r value is a tensile test performed on a JIS No. 5 tensile specimen taken from the rolling direction (0 ° direction), the direction forming 45 ° with the rolling direction (45 ° direction), and the direction orthogonal to the rolling direction (90 ° direction). Using the r value in the 0 ° direction (r _{0 °} value), the r value in the 45 ° direction (r _{45 °} value), and the r value in the 90 ° direction (r _{90 °} value), Based on the average r value.

  The surface properties were evaluated by visually observing the surface of the obtained alloyed hot-dip galvanized steel sheet or hot-dip galvanized steel sheet, and the presence or absence of surface defects such as streaks and shave and sliver.

  Table 5 shows the composition analysis and performance evaluation results of the oxide inclusions. As for the test results (test numbers 12 to 15) for the steel sheets within the range defined by the present invention, the surface properties are all good, and the average r value is 1.80 or more and good deep drawability. Indicated.

  The test results (trial numbers 16, 17, 19) of cold-rolled steel sheets manufactured using steels (steel P, Q, S) whose steel composition and oxide inclusion composition deviate from the range defined by the present invention are as follows: Either the surface property and the r value or both were inferior.

  Specifically, the test using steel P (trial number 16) was conducted using sol. Since the Al content is large and the Ti oxide content in the oxide inclusions is small, the r value is low. The test using steel Q (trial number 17) was conducted using sol. Since the Al content is large and the content of Ti oxide in the oxide inclusion is small, the r value is low, and the sol. Since the amount of Ti is large, a streak pattern is generated on the plating surface, resulting in poor surface properties. In the test using steel S (Test No. 19), since the O content in the steel is large, sliver flaws are generated and the surface properties are poor.

  In trial No. 18, the steel composition was within the range defined by the present invention, but the r value was low because the dissolved oxygen concentration before Ti adjustment was low and the Ti content in the oxide inclusions was small. In addition, sliver wrinkles occurred and the surface properties were poor. Further, the increase in the opening of the sliding nozzle in the continuous casting process is large, and thus it has been difficult to stably perform a number of continuous castings.

It is a graph which shows the relationship between content (NbO) of Nb oxide in an oxide type inclusion, and Si content in steel. It is a graph which shows the relationship between an average r value and (Ti ^* / 48 + Nb / 93) / (C / 12 + N ^* / 14).

Claims (5)

In mass%, C: 0.0005% or more and less than 0.010%, Si: more than 0.020% and 0.40% or less, Mn: 2.50% or less, P: 0.10% or less, S: 0.00. Less than 010%, sol. Al: less than 0.0050%, N: 0.005% or less, sol. Ti: 0.020% or less, Nb: 0.010% or more and 0.20% or less, and O: 0.015% or less. The content of Ti and Nb satisfies the following formulas (1) to (3), the balance has a chemical composition consisting of Fe and impurities, and the content of Ti oxide in the oxide inclusions is TiO ₂ equivalent A hot dip galvanized steel sheet comprising a hot dip galvanized layer on the surface of a steel sheet having a Nb oxide content of less than 1.0 mass% in terms of NbO.
1.0 <(Ti ^* / 48 + Nb / 93) / (C / 12 + N ^* / 14) (1)
Ti ^* = max [sol. Ti- (48/14) × N, 0] (2)
N ^* = max [N− (14/48) × sol. Ti, 0] (3)
Here, the element symbol in each formula represents the content of each element in steel in mass%, and max [] is a function that returns the maximum value of arguments in []. The chemical composition contains, in mass%, B: 0.0002% or more and 0.0020% or less instead of part of Fe, and satisfies the following formula (4) instead of the formula (3) The hot-dip galvanized steel sheet according to claim 1, wherein
N ^* = max [N− (14/48) × sol. Ti- (14/11) × B, 0] (4)
Here, the element symbol in the formula represents the content of each element in the steel in mass%, and max [] is a function that returns the maximum value of the arguments in [].   The chemical composition contains at least 2.0% by mass in total of one or more selected from the group consisting of Cr, Mo, W and Ni instead of part of Fe. The hot-dip galvanized steel sheet according to claim 1 or 2.   A steel ingot having a chemical composition and an oxide inclusion composition according to any one of claims 1 to 3, wherein Ti is added to molten steel decarburized and refined using a vacuum degassing apparatus and continuously cast, A method for producing a hot dip galvanized steel sheet, characterized by hot rolling a lump, cold rolling, recrystallization annealing, and hot dip galvanizing treatment.   4. After adding Al to the molten steel decarburized and refined using a vacuum degassing apparatus to control the dissolved oxygen concentration to 0.003% by mass or more, further adding Ti and continuously casting to perform any one of claims 1 to 3 A steel ingot having a chemical composition and an oxide inclusion composition as described above, and the steel ingot is hot-rolled, cold-rolled, recrystallized, and hot-dip galvanized. Manufacturing method of galvanized steel sheet.

Images