JP5532624B2 - High strength galvannealed steel sheet - Google Patents

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet that can be suitably used in the fields of automobiles, building materials, home appliances, and the like, and in particular, a beautiful steel sheet using a steel sheet containing one or more of Si, Mn, Al, and Cr as a base steel sheet. The present invention relates to a high-strength galvannealed steel sheet having a surface appearance and good plating adhesion.

  2. Description of the Related Art In recent years, in the field of automobiles and the like, many surface-treated steel sheets provided with rust preventive properties, particularly alloyed hot-dip galvanized steel sheets that can be manufactured at low cost and have excellent rust preventive properties are used.

  Generally, an alloyed hot-dip galvanized steel sheet uses a thin steel sheet that has been cold-rolled or heat-treated after hot-rolling a slab as a base, and the surface of the base steel sheet is degreased and / or pre-processed. Wash by pickling, or omit the pretreatment step and burn off the oil on the surface of the underlying steel plate in the preheating furnace, then perform recrystallization annealing in a non-oxidizing atmosphere or reducing atmosphere, In a molten zinc bath in which a small amount of Al (about 0.1-0.2% by mass) is added without cooling to the air after cooling the steel sheet to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere Then, it is manufactured by subsequently heat-treating in an alloying furnace.

  In recent years, there has been a demand for higher strength of the base steel sheet, and the amount of high-strength alloyed hot-dip galvanized steel sheet obtained by applying hot-dip galvanization to such base steel sheet to have rust prevention properties is increasing. Alloy elements such as Si, Mn, Al, and Cr have been added to ensure the mechanical properties of the steel sheet, but the alloyed hot-dip galvanized steel sheet based on a high-strength steel sheet containing a large amount of these elements is used. There are the following problems.

  As described above, the galvannealed steel sheet is annealed at a temperature of about 600 to 900 ° C. in a reducing atmosphere, and then galvanized, and the plated layer is further alloyed. However, the above additive elements are selectively oxidized and concentrated on the surface even in a reducing atmosphere generally used in annealing of a steel sheet prior to hot dip galvanizing to form an oxide on the surface. Since such oxides reduce the wettability with molten zinc during plating and cause non-plating, so that the wettability between the base steel plate and molten zinc decreases rapidly and non-plating occurs frequently. Become. In addition, even when non-plating is not achieved, there is a problem that plating adhesion is poor.

  Furthermore, when the easily oxidizable elements in the steel are selectively oxidized and concentrated on the surface, the Zn—Fe alloying reaction is inhibited, so that alloying is significantly delayed in the alloying process after hot dip galvanizing. As a result, productivity is significantly inhibited. In addition, if alloying treatment is performed at a higher temperature to ensure productivity, a problem of powdering resistance deterioration due to overalloying occurs, and both high productivity and good powdering resistance are achieved. It was difficult.

  For such problems, the steel sheet is heated in an oxidizing atmosphere in advance to form iron oxide on the surface and then subjected to reduction annealing, or by reduction annealing at a high dew point without prior oxidation. A technique for improving the plating property by internally oxidizing an easily oxidizable element is known.

  For example, Patent Documents 1 to 3 describe that oxides of elements in steel generated during annealing by the above technique remain in the plating layer and the surface layer of the underlying steel sheet.

Japanese Patent No. 3887308 JP 2004-315960 A JP 2006-233333 A

  The prior art improves plating properties by internally oxidizing easily oxidizable elements in steel during annealing, and it is considered important to control the amount of oxide produced. However, in Patent Documents 1 to 3, although a certain level of plating quality can be obtained, characteristics such as plating adhesion vary widely, so that good characteristics cannot be stably obtained.

  An object of the present invention is to use a steel sheet containing one or more of Si, Mn, Al, and Cr as a base steel sheet, having a beautiful appearance without unplating, and high strength alloyed hot dip galvanizing with excellent plating adhesion It is to obtain a steel plate.

  The inventors investigated in detail the relationship between the distribution and alloying conditions of the oxides present on the steel sheet surface after annealing after alloying of the plating layer. As a result, in order to solve the above problems, it is known that it is important to control the balance between the amount of oxide present in the plating layer and the amount of oxide remaining in the surface layer of the underlying steel sheet under appropriate alloying conditions. did.

  The gist of the present invention is as follows.

(1) A high-strength galvannealed steel sheet having a steel sheet containing one or more of Si, Mn, Al, and Cr as a base steel sheet, and an oxide is present in the plating layer and in the base steel sheet surface layer, The total amount of oxide in the plating layer and the surface layer of the underlying steel sheet is 0.25 g / m ² or more, and the ratio of the oxide present on the underlying steel sheet side is 40% or less by mass ratio with respect to the total oxide amount, A high-strength galvannealed steel sheet characterized in that the proportion of Fe contained in the plating layer is 13.0% or less in terms of mass ratio.

  (2) The high-strength galvannealed steel sheet according to (1), wherein the ratio of Fe contained in the plating layer is 8.0% to 13.0% by mass ratio.

  The high-strength galvannealed steel sheet of the present invention has a beautiful surface appearance without unplating, despite the underlying steel sheet containing one or more of Si, Mn, Al, and Cr, Furthermore, the plating adhesion is also excellent.

  Hereinafter, the present invention will be specifically described.

  First, the steel plate component of the present invention will be described.

  An object of the present invention is to solve an impediment to plating in a high-strength galvannealed steel sheet using a steel sheet containing one or more elements of Si, Mn, Al, and Cr as a base steel sheet. Therefore, in this invention, the base steel plate was limited to the steel plate containing 1 or more types of elements of Si, Mn, Al, and Cr.

Si: 3.0% by mass or less If Si is contained in an amount exceeding 3.0% by mass, the steel sheet itself becomes too hard, and therefore 3.0% by mass or less is preferable. Moreover, the effect of this invention can be acquired notably when Si is contained 1.0 mass% or more.

Mn: 5.0% by mass or less When Mn exceeds 5.0% by mass, it adversely affects the securing of weldability and strength ductility balance (TS (Tensile strength) × El (Elongation)). The mass% or less is preferable. Moreover, the effect of this invention can be acquired notably when it contains 0.5 mass% or more.

Al: 0.010 mass% or more and 5.0 mass% or less Al fixes oxygen in steelmaking and slab, and suppresses generation | occurrence | production of defects, such as a slab crack. The effect is recognized by the addition of 0.010% by mass. However, if the Al content exceeds 5.0% by mass, it is not preferable in terms of increasing the amount of inclusions in the steel, but if it is less than that, the present invention is not inhibited.

Cr: 0.005% by mass or more and 2.0% by mass or less Cr is contained by 0.005% by mass or more, thereby suppressing the generation of pearlite during cooling from the annealing temperature and improving elongation, and carbide and precipitation. It is effective in improving strength by precipitating the material more uniformly. However, if it exceeds 2.0 mass%, the effect is saturated and causes an increase in cost, so 2.0 mass% or less is preferable.

  In the present invention, elements other than Si, Mn, Al, and Cr are not particularly limited, and conventionally known component systems can be used.

C: 0.01% by mass or more and 2.0% by mass or less C is an element effective for increasing the strength of a steel sheet, and further has an effect on generation of retained austenite and a low-temperature transformation phase, ensuring an improvement in strength ductility balance. It is an effective element to do. However, when the C content is less than 0.01% by mass, it is difficult to obtain a desired strength ductility balance. On the other hand, when it exceeds 2.0 mass%, deterioration of weldability is caused. From the above, C is preferably in the range of 0.01% by mass to 2.0% by mass.

P: 0.1% by mass or less P is an element effective for strengthening steel, but if added in excess of 0.1% by mass, embrittlement is caused by grain boundary segregation and impact resistance is deteriorated. . On the other hand, if it exceeds 0.1% by mass, the alloying rate is significantly delayed. Therefore, the P content is 0.1% by mass or less.

S: 0.07% by mass or less S is an inclusion such as MnS, which causes deterioration in impact resistance and cracks along the metal flow of the weld. 0.07 mass% or less from the surface.

N: 0.008% by mass or less N is an element that most deteriorates the aging resistance of steel. The smaller the amount, the better. If it exceeds 0.008% by mass, deterioration of aging resistance becomes remarkable. Therefore, the N content is 0.008% by mass or less.

  Further, in addition to the above, one or more selected from Ti, Nb, V, Mo, Cu, Ni, B, Ca, and Sb are contained if the total content is in the range of 3% by mass or less. May be. The balance is Fe and inevitable impurities.

  Next, the oxide and plating layer Fe% present in the plating layer and in the surface layer of the underlying steel plate will be described.

  As already mentioned above, in the annealing step for a steel sheet containing one or more of Si, Mn, Al, and Cr, is the steel sheet heated in an oxidizing atmosphere in advance to form iron oxide on the surface and then reduced annealing is performed? In addition, by performing reduction annealing at a high dew point without prior oxidation treatment, it is possible to internally oxidize easily oxidizable elements in steel, and the steel sheet surface layer is mainly composed of oxides of one or more of Si, Mn, Al, and Cr. Is formed. When the alloying treatment is performed by hot-dip plating after the annealing, a part of the oxide present in the surface layer of the original sheet after the annealing is taken into the plating layer and the rest remains in the base iron. The total amount of oxides incorporated in the plated layer after the alloying treatment and oxides remaining in the base iron hardly change from the total amount of oxides formed on the steel sheet surface layer after annealing.

When the total amount of oxides present in the plating layer and the surface layer of the underlying steel sheet is less than 0.25 g / m ² , a beautiful plating appearance without non-plating cannot be obtained stably. Further, since the alloying reaction after plating is also suppressed, in order to obtain a desired degree of alloying, the alloying temperature must be increased or the holding time must be lengthened, which is not economically preferable. Therefore, the total amount of oxides present in the plating layer and the surface layer of the underlying steel sheet is specified to be 0.25 g / m ² or more. The upper limit of the total amount of oxide is not specified, but when the amount of oxide itself is large, the oxide at the plating interface may deteriorate the adhesion, or the oxide remaining on the base steel plate may deteriorate the mechanical properties of the steel. For this reason, the total amount of oxide is preferably 1.0 g / m ² or less.

  In order to obtain good plating properties, it is important that the total amount of oxides present on the surface layer of the original sheet after annealing and how much of the oxides are taken into the plating layer by alloying treatment. After the plating layer is alloyed, good plating adhesion can be obtained by setting the ratio of oxide remaining in the base iron to 40% or less by mass ratio with respect to the total amount of oxide. If the oxide exceeds 40% and remains in the base iron, it means that there is a large amount of oxide in the vicinity of the interface, which adversely affects the plating adhesion.

  In hot-dip galvanizing, generally, an oxide film having a thickness of about 10 nm or less is formed on the plated surface after plating by Al added to the plating bath. In the present invention, as described above, oxides present in the surface layer of the underlying steel sheet after annealing are targeted, and therefore, Al-based oxides on the plating surface are excluded.

  After alloying, the Fe concentration in the plating layer is adjusted to 13.0% or less by mass ratio. If it exceeds 13.0%, a brittle phase having a high Fe concentration is formed at the plating / base metal interface, which adversely affects powdering properties and the like. Moreover, it is preferable that Fe concentration shall be 8.0% or more by mass ratio. If it is less than 8.0%, a low melting point phase having a low Fe concentration remains on the surface layer, and the slidability during press molding deteriorates.

  Next, the manufacturing method of the high-strength galvannealed steel sheet of this invention is demonstrated.

  A steel slab having the above composition is hot-rolled, the scale of the hot-rolled sheet surface layer is removed in a pickling process, or further cold-rolled to produce a steel sheet that is a plating original sheet. The conditions of each process from production of the steel slab to cold rolling are not particularly limited. Ordinary methods are acceptable.

  The produced steel sheet is charged into a continuous hot dip galvanizing facility, annealed, hot dip galvanized, and further subjected to alloying treatment of the plated layer.

In the annealing process, the steel plate is heated in an oxidizing atmosphere in advance to form iron oxide on the surface, and then reduction annealing is performed, or reduction annealing is performed at a high dew point without prior oxidation treatment, thereby easily oxidizing elements in steel. Is internally oxidized to form a total amount of oxide of 0.25 g / m ² or more on the steel sheet surface layer.

  After reductive annealing, it is immersed in a hot dip galvanizing bath and hot dip galvanized, pulled up from the plating bath, adjusted in amount of adhesion with a gas wiper, and then alloyed with a plating layer. The steps of hot dip galvanization and gas wiper are not particularly limited, and may be a conventional method. By the alloying treatment, the oxide formed on the surface of the steel sheet after reduction annealing moves during plating, but the ratio of the oxide present on the base steel sheet side to the total amount of oxide is 40% or less by mass ratio. It is necessary to control as follows. Although the method to control is not specifically limited, For example, the alloying temperature and the alloying time may be adjusted according to the conditions of the prior oxidation treatment or the reduction annealing at a high dew point and the original plate composition. If it is difficult to change the alloying time on the production line, the alloying time may be kept constant and the alloying temperature may be appropriately selected.

Specifically, for example, in the case where the reduction process is performed by reduction annealing after heating the steel plate by forming the iron oxide on the surface by prior oxidation treatment, the steel plate temperature To (° C.) and the original plate Si concentration Csi of the preliminary oxidation treatment. (Mass%), the alloying temperature Ta (° C.) is the following formula:
Ta <a × To + b × Csi + c
The ratio of the oxides present on the base steel plate side to the total amount of oxides is selected by alloying at a temperature within the range satisfying Can be controlled to be 40% or less. Here, the coefficients a to c may be obtained in advance for each line / apparatus. For example, when pre-oxidation processing using a direct fire burner is performed, a = 0.4, b = −40, c A value of = 280 is obtained. The meaning of this relational expression is that when the alloying temperature is high, the proportion of oxide remaining in the underlying steel sheet increases, so that there is an upper limit to the appropriate alloying temperature.

  In addition, the term “Si concentration in steel” is included in the above equation because the ease of oxidation in the pre-oxidation treatment is affected by the Si concentration in the steel, and the effect of the steel plate temperature in the pre-oxidation treatment changes. is there.

However, as described so far, in the present invention, in the manufactured high-strength galvannealed steel sheet, the total amount of oxides in the plating layer and the surface layer of the base steel sheet is 0.25 g / m ² or more, and the base steel sheet It is important that the ratio of the oxide present on the side is 40% or less, and the difference in process conditions and control method for that is not essential.

  Using cold-rolled steel sheets having a steel composition of A to E shown in Table 1 as test materials, the steel sheet is heated in an oxidizing atmosphere in advance to form iron oxide on the surface, followed by reduction annealing, and further molten zinc Plating and alloying treatment were performed.

In the oxidation treatment, a direct fire burner was used, the oxidation condition was set by setting the air-fuel ratio to 1 or more, and the maximum reached temperature (oxidation temperature) was changed by controlling the output of the direct fire burner. The reduction annealing was performed in a 5 vol% hydrogen + nitrogen atmosphere (dew point: −35 ° C.) under the conditions of plate temperature: 830 ° C. and holding time: 30-60 seconds. The plating conditions are as follows: a 460 ° C. zinc plating bath containing 0.14% by mass of Al (Fe saturation), an intrusion plate temperature: 460 ° C., and an immersion time: 1 second. One side was adjusted to 45 g / m ² .

  The obtained hot-dip galvanized steel sheet was evaluated for the plating layer Fe% (mass%) in which the alloying temperature and the alloying time were changed in an induction heating furnace.

  Each evaluation method and criteria are as follows.

<Quantitative evaluation of oxide in plating layer / underlying steel plate>
The test material was immersed in an alkaline solution to remove the Al-based oxide film on the plating surface layer. Then, after dissolving the plating layer and collecting the oxide and measuring the weight, the oxide was collected by dissolving in the same manner as the base iron portion and the weight was measured. Note that alkali (NaOH) and acid (HCl) were used for dissolution of the plating layer and the base iron portion, respectively. The total amount of both was regarded as the total oxidation amount, and the ratio of the amount of the residue in the ground iron was calculated. It was confirmed by energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD) that the collected oxide was mainly an oxide containing one or more of Si, Mn, Al, and Cr.

<Evaluation of plating adhesion (powdering)>
A test piece having a width of 25 mm and a length of 40 mm was cut out from the galvannealed steel sheet, and a cellophane tape (manufactured by Nichiban, width: 24 mm) was applied to the position of the length: 20 mm, and the tape surface was bent 90 ° inward. Then, the amount of Zn adhering when the cellophane tape was peeled off by bending back was measured as a count by fluorescent X-rays. The measured Zn count number was corrected to the count number per specimen width: unit length (1 m) and evaluated according to the following criteria.
○: Good (Count: 0 to 5000)
X: Defect (count number: over 5000)
<Plating appearance>
Using the obtained alloyed hot-dip galvanized steel sheet, visually and visually observing with a magnifying glass of 10X In the case where there is a fine unplating, the case where the unplating can be visually observed is considered as non-plating.
○: No plating is present Δ: Fine plating is not present ×: Non-plating is present These evaluation results are shown in Table 2 together with the processing conditions.

  From Table 2, the galvanized steel sheet of the present invention is an alloyed hot-dip galvanized steel sheet that has no plating and excellent plating adhesion even when it is based on a steel sheet containing high Si, Mn, Al, Cr. It turns out that it is obtained.

  A cold-rolled steel sheet having a steel composition of F shown in Table 1 is used as a test material. After oxidation treatment in which the steel sheet is heated in an oxidizing atmosphere in advance to form iron oxide on the surface, reduction annealing is performed, and further hot dip galvanizing, Alloying treatment was performed.

Oxidation treatment uses an infrared heating furnace in an oxygen + nitrogen atmosphere (dew point: + 20 ° C) to change the oxidation state by changing the oxygen concentration and the maximum temperature (oxidation temperature), and reach the maximum temperature. Then, nitrogen gas was cooled without holding. Thereafter, reduction annealing was performed using an infrared heating furnace in a 10 vol% hydrogen + nitrogen atmosphere (dew point: −35 ° C.) under conditions of a plate temperature of 850 ° C. and a holding time of 30 seconds. The plating conditions are as follows: a 460 ° C. zinc plating bath containing 0.14% by mass of Al (Fe saturation), an intrusion plate temperature: 460 ° C., and an immersion time: 1 second. One side was adjusted to 45 g / m ² .

  The obtained hot-dip galvanized steel sheet was subjected to alloying treatment by being held at an alloying temperature of 580 ° C. for 15 seconds in an electric heating furnace, and the plating layer Fe% (mass%) was evaluated.

  Each evaluation method and criteria are as follows.

<Quantitative evaluation of oxide in plating layer / underlying steel plate>
About the obtained plated steel plate, the plating layer was selectively dissolved by constant potential electrolysis in the AA electrolyte solution. The AA electrolyte used in this example was 10% acetylacetone-1% tetramethylammonium chloride-methanol, and the electrolytic potential was set to a potential at which only the plating layer was dissolved without dissolving the ground iron. Furthermore, after dissolving the plating layer, the ground metal surface layer was dissolved by constant current electrolysis with an electric quantity at which the surface layer of 5 μm was dissolved in the same AA electrolyte solution. After the residue obtained by each electrochemical treatment was filtered with a Nuclepore filter having a diameter of 50 nm, the residue captured on the filter was subjected to alkali melting and then subjected to ICP analysis to determine the amount of Si and Mn. In this example, the amount of oxide was determined by converting the amount of Si into the amount of SiO _{2 and the} amount of Mn into the amount of MnO from these quantitative values. In addition, in the steel composition of the test material used for the present Example, since the content of Al and Cr is small, the conversion value calculated | required from the amount of Si and Mn was made into the oxidation amount. In this example, it was confirmed by energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD) that the collected oxide was mainly an oxide of Si and / or Mn.

  <Evaluation of plating adhesion (powdering property)> and <plating appearance> were evaluated in the same manner as in Example 1.

  These evaluation results are shown in Table 3 together with the processing conditions.

  From Table 3, even when the plated steel sheet of the present invention example is based on a high-Si, Mn-containing steel sheet, an galvannealed steel sheet having no plating and excellent plating adhesion has been obtained. I understand that. It can also be seen that in the comparative example outside the scope of the present invention, the alloying reaction is suppressed and it is difficult to obtain the desired Fe%.

  The high-strength galvannealed steel sheet of the present invention has a beautiful appearance with no unplating, and is excellent in plating adhesion. The high-strength galvannealed steel sheet of the present invention can be suitably used in the fields of automobiles, building materials, home appliances and the like.

Claims (3)

Si : 1.0 to 3.0% by mass , Mn : 0.5 to 5.0% by mass , Al : 0.010 to 5.0% by mass, C: 0.01 more mass% 2.0 mass% or less, P: 0.1 wt% or less, S: contains 0.07 mass% or less, high-strength alloy and the balance base steel sheet to Fe and unavoidable impurities der Ru steel A hot dip galvanized steel sheet, in which an oxide is present in the plating layer and the surface layer of the underlying steel sheet, and the total amount of oxides in the plating layer and the surface layer of the underlying steel sheet is 0.25 g / m ² or more, of which the underlying steel sheet The ratio of the oxide present on the side is 40% or less by mass ratio with respect to the total oxide amount, and the ratio of Fe contained in the plating layer is 13.0% or less by mass ratio High strength galvannealed steel sheet. The high-strength galvannealed steel sheet according to claim 1, wherein the steel sheet further contains Cr: 0.005 mass% or more and 2.0 mass% or less. The high-strength galvannealed steel sheet according to claim 1 or 2 , wherein the ratio of Fe contained in the plating layer is 8.0% or more and 13.0% or less by mass ratio.