JP3875958B2 - High strength and high ductility hot dip galvanized steel sheet with excellent workability and manufacturing method thereof - Google Patents

Description

【００７７】
【表２】

【００７８】
【表３】

【００７９】
【表４】

【００８０】
【表５】

【００８１】
【表６】

【００８２】
【発明の効果】
本発明により、延性に優れた高強度高延性溶融亜鉛めっき鋼板を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength, high-ductility hot-dip galvanized steel sheet excellent in workability suitable for building materials, home appliances, automobiles, and the like and a method for producing the same.
[0002]
[Prior art]
Hot dip galvanizing is applied for the purpose of corrosion protection of steel sheets, and is widely used in building materials, home appliances, automobiles and the like. As a manufacturing method thereof, in a continuous line, after degreasing and cleaning, heating in a non-oxidizing atmosphere,₂ And N₂ There is a Sendzimer method of cooling after annealing in a reducing atmosphere containing, cooling to near the plating bath temperature, immersing in a molten zinc bath, cooling or reheating to form an Fe-Zn alloy phase, It is frequently used for processing.
[0003]
For annealing before plating, after degreasing and washing, immediately after heating in a non-oxidizing atmosphere, H₂ And N₂ In some cases, an all-reducing furnace method in which annealing is performed in a reducing atmosphere containing selenium is also performed. In addition, a flux method is also performed in which a steel sheet is degreased and pickled, and then flux treatment is performed using ammonium chloride and the like, soaking in a plating bath, and then cooling.
[0004]
In recent years, the demand for high-strength plated steel sheets with good workability for the purpose of improving fuel economy and durability has been increasing, especially in the automobile body, and the development of high-strength steel sheets with higher workability is eagerly desired. . On the other hand, since various alloys are added to the high-strength steel sheet and the strength and ductility are increased by using the structure control, the heat treatment method has a great restriction.
[0005]
However, from the viewpoint of plating, if the alloy content in steel, especially Si content is high, or if heat treatment conditions are severely limited, the plating wettability is greatly reduced by using a normal hot dip galvanizing bath. However, since non-plating occurs, there is a problem that appearance quality deteriorates. Further, when partial alloying is required, it is necessary to perform alloying heat treatment. In the case of high-strength steel sheets, as described above, since there are a large amount of additive elements, the alloy heat treatment after plating tends to be performed at a higher temperature and longer time than in the case of mild steel. It is one of the big obstacles above.
[0006]
As a means for solving this problem, as shown in Patent Document 1 and Patent Document 2, etc., the plating property is improved by applying a specific plating. In this method, the first stage of the hot dipping line annealing furnace is used. Therefore, there is a problem that a new plating facility must be provided or a plating process must be performed in advance in the electroplating line, resulting in a significant increase in cost.
[0007]
Moreover, for the purpose of improving plating manufacturability, Patent Document 3 discloses a hot dip galvanized steel sheet having a Zn—Al—Mn—Fe based plating layer. However, although sufficient consideration is paid especially to manufacturability in the present invention, it is not an invention in which plating adhesion at the time of high working with a high strength and high ductility material is considered.
[0008]
Furthermore, as seen in Patent Document 4, although there is an invention relating to a steel sheet containing Si, Mn and Ni in the steel sheet, this is intended to improve the plateability, and the workability described in the present invention. It is not a consideration for improvement.
[0009]
On the other hand, for the purpose of enhancing the collision energy absorption capacity, Patent Document 5 discloses that ferrite is the main phase, the average particle size is 10 μm or less, and the second phase is 3 to 50% austenite or 3 to 3% by volume. A steel sheet comprising 30% martensite and having an average particle size of the second phase of 5 μm or less and selectively containing bainite is disclosed. However, the present invention does not consider the plating wetness, and is not an invention that can cope with the thinning accompanying the increase in strength in terms of corrosion resistance.
[0010]
[Patent Document 1]
JP-A-3-28359
[Patent Document 2]
Japanese Patent Laid-Open No. 3-64437
[Patent Document 3]
Japanese Patent Laid-Open No. 5-230608
[Patent Document 4]
JP 2000-850658 A
[Patent Document 5]
Japanese Patent Laid-Open No. 11-189839
[0011]
[Problems to be solved by the invention]
An object of the present invention is to solve the above problems and to provide a high-strength, high-ductility hot-dip galvanized steel sheet excellent in workability and a method for producing the same.
[0012]
[Means for Solving the Problems]
As a result of various studies, the present inventors have clarified that a steel sheet having excellent workability can be manufactured by controlling the Mn content and Ni content contained in the steel sheet within a predetermined range. . The workability described here means uniform elongation and total elongation of the steel sheet (the same applies hereinafter). Moreover, about the plating property, it discovered that it could improve by restraining the amount of Si contained in steel to a predetermined range.
[0013]
  The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
      C: 0.0001 to 0.3%,
      Si: 0.001 to less than 0.1%,
      Mn: 0.001 to 3%,
      Ni:0.79~ 5%
      Al: 0.1 to 4%
      Mo: 0.001 to 4%,
      P: 0.0001 to 0.3%,
      S: 0.01% or less
And the Mn content and the Ni content satisfy the following formula (1), and the balance is iron and inevitable impurities.In other words, the microstructure contains 50 to 97% of ferrite or ferrite and bainite as the phase with the largest volume fraction, and 3 to 50% of austenite as the second phase.On the surface of the steel plate,
      Al: 0.001 to 0.5%,
      A high-strength and high-ductility hot-dip galvanized steel sheet excellent in workability, characterized by comprising a plating layer containing Fe: 5 to 20%, the balance being Zn and inevitable impurities.
−3 × Mn + 3.5 <Ni <−4.8 × Mn + 7.2 (1)
(2) The plating layer is
      Al: 0.001 to 0.5%,
      Fe: Fe containing less than 5%, the balance consisting of Zn and inevitable impurities, the high strength and high ductility hot dip galvanized steel sheet with excellent workability described in (1).
(3) In addition, by mass% in steel,
      Cu: 0.001 to 4%,
      Cr: 0.001 to 4%,
      Co: 0.001 to 4%
(1) characterized by containing one or more ofOr (2)High-strength, high-ductility hot-dip galvanized steel sheet with excellent workability as described.
(4Further, the steel contains 0.001 to 1% in total of one or more of Nb, Ti, and V in mass%. (1) to (3) A high-strength, high-ductility hot-dip galvanized steel sheet with excellent workability.
(5) Further, the steel contains B: 0.0001 to 0.1% by mass% (1) to (4) A high-strength, high-ductility hot-dip galvanized steel sheet with excellent workability.
(6) (1) to (5The cast slab composed of any of the above components is cast, or once cooled and then heated again, the hot-rolled steel sheet wound after hot rolling is pickled and cold-rolled, and then the maximum heating temperature is Ac.₁+ 30 ° C or higher, Ac_ThreeAfter annealing at a temperature lower than or equal to ℃, it is cooled to a temperature range of 650 to 710 ° C at a cooling rate of 0.1 to 10 ° C / second, and subsequently at a cooling rate of 1 to 100 ° C / second, the temperature of the Zn plating bath to Zn plating After cooling to bath temperature + 100 ° C., hold in the temperature range of 350 ° C. to Zn plating bath temperature + 100 ° C. for 1 to 3000 seconds including the immersion time of the plating bath, then soak in the Zn plating bath, and then A method for producing a high-strength, high-ductility hot-dip galvanized steel sheet excellent in workability, characterized by cooling to room temperature.
(7) After being immersed in a Zn plating bath, holding is performed for 1 to 300 seconds in a temperature range of bath temperature to Zn plating bath temperature + 100 ° C., and cooling to room temperature (6) A method for producing a high-strength, high-ductility hot-dip galvanized steel sheet having excellent workability as described in the above.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0015]
The present inventors, in mass%, C: 0.0001 to 0.3%, Si: less than 0.001 to 0.1%, Mn: 0.001 to 3%, Ni: 0.1 to 5% , Al: 0.1 to 4%, Mo: 0.001 to 4%, P: 0.0001 to 0.3%, S: 0.01% or less, and a cold piece comprising the balance Fe and inevitable impurities The hot-rolled steel sheet is pickled and then cold-rolled.₁ + 30 ° C or higher, Ac_Three After annealing at 1 ° C to 100 ° C / second to Zn plating bath temperature to Zn plating bath temperature + 100 ° C, 350 ° C to Zn plating bath temperature + 100 ° C, including plating bath immersion time After holding for 1-3000 seconds, it was immersed in a Zn plating bath at 450-470 ° C. for 3 seconds, and further heated at 500-550 ° C. for 10-60 seconds. The composition of the plating layer was Al: 0.001 to 0.5%, Fe: 5 to 20%, and the balance was Zn and inevitable impurities. Thereafter, the plating property was investigated by measuring the area of the non-plated portion on the surface of the plated steel sheet and investigating the amount of Fe contained in the plating layer.
[0016]
As a result, it has been found that the wettability of plating is improved by making the amount of Si contained in the steel less than 0.1% by mass.
[0017]
For improvement in workability, when the Mn content (mass%) and Ni content (mass%) contained in the steel sheet satisfy the following formula (1), and the same strength is compared, It was found that what is satisfied is improved by about 2 to 5% in both the uniform elongation and the total elongation compared to the unsatisfied one.
[0018]
−3 × Mn + 3.5 <Ni <−4.8 × Mn + 7.2 (1)
Moreover, (1) Formula is effective irrespective of the intensity | strength of a steel plate, and the effect will be exhibited as long as Mn content and Ni content in a steel plate satisfy Formula (1). In addition, (1) Formula is a formula which this inventor newly discovered by the multiple regression analysis which arranged the influence of the steel plate component with respect to the intensity | strength and ductility of a steel plate.
[0019]
Next, the reason for limiting the preferable range of the steel plate component in the present invention will be described.
[0020]
C: An austenite stabilizing element, which moves from ferrite to austenite during two-phase heating and bainite transformation temperature range, and stabilizes austenite by concentrating in austenite. As a result, austenite is stabilized even at room temperature, and excellent ductility is ensured. When C is less than 0.0001% by mass, it is difficult to secure 3% or more of austenite, and therefore the lower limit was set to 0.0001% by mass. On the other hand, if C exceeds 0.3 mass%, welding becomes difficult, so the upper limit was made 0.3 mass%.
[0021]
Si: Si forms an oxide on the surface of the steel sheet during annealing and causes non-plating. This becomes significant when the Si content in the steel is 0.1% or more, so the upper limit was made less than 0.1% by mass. Moreover, even if the steel sheet is made to have a low Si content exceeding 0.001% by mass, the lower limit is set to 0.001% by mass because it is uneconomical.
[0022]
Mn: Mn is an element necessary for ensuring the strength, and if it is less than 0.001% by mass, the reinforcing effect is not exhibited, so the lower limit was made 0.001% by mass. On the other hand, if it exceeds 3% by mass, the ductility is adversely affected, so 3% by mass was made the upper limit.
[0023]
Ni: Ni is an element necessary for ensuring the strength, and if it is less than 0.1% by mass, the reinforcing effect is not exhibited, so the lower limit was made 0.1% by mass. On the other hand, if it exceeds 5% by mass, the ductility is adversely affected, so 5% by mass was made the upper limit.The lower limit of the Ni content was set to 0.79% or more based on 0.79% of the F steel in Table 1 of the examples.
[0024]
  Furthermore, the excellent workability which is the effect of this invention is exhibited by controlling Ni contained in a steel plate to the range of following (1) Formula. The workability described here refers to the uniform elongation and total elongation of the steel sheet, and is the tensile strength (TS) of the steel sheet and the total elongation (El.) In the tensile test or uniform elongation (u-El.). Expressed as a product. When the Ni content is less than the left side of the formula (1), the excellent ductility that is the effect of the present invention cannot be obtained. On the other hand, the content of Ni is of the formula (1)right sideAddition exceeding 1 is not preferable because it causes an excessive increase in cost. However, even if this value is exceeded, the excellent ductility that is the effect of the present invention is exhibited.
−3 × Mn + 3.5 <Ni <−4.8 × Mn + 7.2 (1)
  Al: In addition to being used as a deoxidizer, Al does not dissolve in cementite, so it delays cementite precipitation and delays the decomposition of austenite into ferrite and cementite. During this time, it becomes possible to concentrate C into austenite, and austenite can exist even at room temperature. However, if it is less than 0.1% by mass, the effect is not exhibited. On the other hand, if it exceeds 4%, weldability deteriorates, so the upper limit was made 4% by mass.
[0025]
Mo: Mo is important for securing retained austenite because it delays cementite precipitation and pearlite transformation. However, if it is less than 0.001% by mass, the effect is not exhibited. On the other hand, if it exceeds 4%, the ductility is adversely affected, so the upper limit was made 4% by mass.
[0026]
P: Setting P to less than 0.0001 mass% is economically disadvantageous, so this value was set as the lower limit. On the other hand, the addition exceeding 0.3% by mass adversely affects weldability and manufacturability during production and hot rolling. Therefore, the upper limit value was set to 0.3% by mass.
[0027]
S: S adversely affects weldability and manufacturability during production and hot rolling. For this reason, the upper limit value was set to 0.01% by mass or less.
[0028]
Furthermore, the steel targeted by the present invention can contain one or more of Cu, Cr and Co for the purpose of further improving the strength.
[0029]
Cu is added in an amount of 0.001% by mass or more for the purpose of strengthening, and if added in an amount exceeding 4% by mass, workability is adversely affected.
[0030]
Cr is an element added for the purpose of strengthening and suppressing the formation of carbides, and is 0.001% or more. Since addition of more than 4% adversely affects workability, this is set as the upper limit.
[0031]
Co was added in an amount of 0.001% by mass or more in order to improve the balance of strength and ductility by controlling the plating property and bainite transformation. On the other hand, there is no particular upper limit for addition, but since it is an expensive element, addition of a large amount impairs economic efficiency, so it is desirable to make it 4% by mass or less.
[0032]
Furthermore, the steel targeted by the present invention can contain one or more of Nb, Ti, and V, which are carbide forming elements, for the purpose of further improving the strength. These elements form fine carbides, nitrides or carbonitrides, and are extremely effective for strengthening steel sheets. Therefore, if necessary, one or more elements are added in a total amount of 0.001% by mass or more. it can. On the other hand, since it inhibits ductility deterioration and concentration of C in retained austenite, the upper limit of the total addition amount is set to 1% by mass.
[0033]
B can also be added as needed. B is effective for strengthening grain boundaries and increasing the strength of steel by adding 0.0001% by mass or more. However, when the amount of addition exceeds 0.1% by mass, the effect is not only saturated but also necessary. Since the steel sheet strength is increased and the workability is lowered as described above, the upper limit is set to 0.1% by mass.
[0034]
Inevitable impurities include, for example, Sn, but even if these elements are contained in a range of Sn ≦ 0.01% by mass or less, the effect of the present invention is not impaired.
[0035]
Next, a preferable microstructure of the base steel sheet will be described.
[0036]
The ductility of the high-strength hot-dip galvanized steel sheet according to the present invention depends on the volume fraction of ferrite that is the phase (main phase) having the maximum volume fraction and the volume fraction of retained austenite that is the second phase. If the volume fraction of ferrite as the main phase is less than 50% (hereinafter the same), excellent ductility cannot be exhibited. Therefore, the lower limit is set to 50% or more. In addition, when aiming to further increase the strength of the steel sheet, it may contain a bainite phase, but as a main phase from the viewpoint of ensuring ductility, a single phase of ferrite, or a composite phase of ferrite and bainite, It is desirable to contain 50% or more. On the other hand, if the ferrite or ferrite and bainite as the main phase exceeds 97%, the ratio of the hard phase (in this case, mainly shows residual austenite and martensite) that improves the strength of the steel sheet is extremely reduced. Strength cannot be improved. Therefore, the upper limit is set to 97% or less.
[0037]
In addition, when the retained austenite, which is the second phase, is less than 3%, the effect hardly appears, so the lower limit is set to 3% or more. On the other hand, if the retained austenite exceeds 50%, when extremely remarkable molding is performed, a large amount of martensite is present after molding, which may cause problems in secondary workability and impact characteristics. Then, the upper limit was set to 50%.
[0038]
Moreover, even if it contains martensite as a phase other than the retained austenite of the second phase, it does not depart from the present invention.
[0039]
The size distribution of the structure of the steel sheet is not particularly specified, but from the viewpoint of the uniformity of the structure and the strengthening of the steel sheet by refining, the grain sizes of ferrite, bainite and retained austenite are preferably 50 μm or less. However, exceeding this size does not depart from the present invention.
[0040]
Incidentally, identification of each phase of the microstructure, ferrite, bainite, austenite, martensite and the remaining structure, observation of the existing position and measurement of the volume fraction were disclosed in Nital reagent and Japanese Patent Application Laid-Open No. 59-219473. It can be quantified by observing the cross section in the rolling direction of the steel sheet or the cross section in the direction perpendicular to the rolling direction with a reagent and observing the optical microscope at 500 to 1000 times.
[0041]
In addition to the above, the steel sheet within the scope of the present invention also includes one or more of carbides, nitrides, sulfides, and oxides with an area fraction of 5% or less as the remaining structure of the microstructure.
[0042]
Next, the plating layer will be described.
[0043]
The amount of Al in the plating layer is preferably in the range of 0.001 to 0.5 mass%. When Al is less than 0.001% by mass, dross generation is remarkable and a good appearance cannot be obtained, and when Al is added exceeding 0.5% by mass, the alloying reaction is remarkably suppressed, and alloying melts. This is because it becomes difficult to form a galvanized layer.
[0044]
Further, when spot weldability and paintability are desired, these characteristics can be enhanced by alloying treatment. Specifically, after being immersed in a Zn plating bath, an alloying treatment is performed at 300 to 550 ° C., whereby Fe is taken into the plating layer, and a high-strength hot-dip galvanized steel sheet excellent in paintability and spot weldability. Can be obtained. If the amount of Fe after alloying is less than 5% by mass, spot weldability is insufficient. On the other hand, if the amount of Fe exceeds 20% by mass, the adhesion of the plating layer itself is impaired, and the plating layer breaks and drops during processing and adheres to the mold, thereby causing defects during molding. Therefore, the range of the amount of Fe in the plating layer when performing the alloying treatment is 5 to 20% by mass.
[0045]
Further, when the alloying treatment is not performed, even if the amount of Fe in the plating layer is less than 5% by mass, the corrosion resistance, ductility, and workability, which are effects other than spot welding obtained by alloying, are good.
[0046]
There are no particular restrictions on the amount of plating deposited, but the amount on one side is 5 g / m from the viewpoint of corrosion resistance.² The above is desirable. For the purpose of improving the paintability and weldability on the hot-dip Zn plated steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. However, the present invention does not depart from the present invention.
[0047]
A method for producing a high-strength hot-dip galvanized steel sheet having such a structure will be described below.
[0048]
Cast slabs satisfying the above component composition as cast or once Ar_Three(° C) After cooling to below, it is heated again, the hot-rolled steel sheet wound up after hot rolling is pickled and cold-rolled, and then the maximum heating temperature is Ac.₁ + 30 ° C or higher, Ac_Three After annealing at a temperature lower than or equal to ℃, it is cooled to a temperature range of 650 to 710 ° C at a cooling rate of 0.1 to 10 ° C / second, and subsequently at a cooling rate of 1 to 100 ° C / second, the temperature of the Zn plating bath to Zn plating After cooling to bath temperature + 100 ° C., hold in the temperature range of 350 ° C. to Zn plating bath temperature + 100 ° C. for 1 to 3000 seconds including the immersion time of the plating bath, then soak in the Zn plating bath, and then High-strength hot-dip galvanizing with excellent workability after holding for 1 to 300 seconds in the temperature range from plating bath temperature to plating bath temperature + 100 ° C, or by cooling to room temperature without holding Steel sheets can be manufactured.
[0049]
In the continuous annealing of cold-rolled steel sheets, the structure becomes a two-phase structure of ferrite and austenite.₁ Above the transformation point Ac_Three Heat to a temperature below the transformation point. The heating temperature at this time is Ac₁ When the temperature is lower than + 30 ° C., it takes too much time for the cementite to re-dissolve, and the austenite content becomes small, so the lower limit of the heating temperature is Ac.₁ + 30 ° C. Further, if the heating temperature is too high, the volume fraction of austenite becomes too large and the C fraction in the austenite decreases, so the upper limit temperature is set to Ac._Three ℃.
[0050]
If the holding time at the highest heating temperature for annealing is too short, there is a high possibility that undissolved carbides remain, and the austenite volume fraction decreases, so it is desirable that the time be 10 seconds or longer. On the other hand, if the holding time is too long, there is a high possibility that the crystal grains become coarse and the strength-ductility balance tends to deteriorate, so the upper limit is preferably set to 1000 seconds.
[0051]
Then, it cooled to the temperature range of 650-710 degreeC with the cooling rate of 0.1-10 degrees C / sec, and it cooled to the plating bath temperature-plating bath temperature +100 degreeC with the cooling rate of 1-100 degrees C / sec subsequently. Then, hold | maintain for 1-3000 seconds including the immersion time of a plating bath in the temperature range of 350 degreeC-plating bath temperature +100 degreeC.
[0052]
This is to maintain the bainite transformation without transforming the austenite generated by heating in the two-phase region to pearlite, and to pass the temperature range of 350 ° C to Zn plating bath temperature + 100 ° C over 1 to 3000 seconds. In order to obtain predetermined characteristics, the structure is ferrite (+ bainite) + residual austenite.
[0053]
After annealing, if the cooling rate to 650 to 710 ° C. is less than 0.1 ° C./second, crystal grains are coarsened, and if it exceeds 10 ° C./second, it is difficult to concentrate C in the austenite. Absent.
[0054]
The cooling stop temperature was set to a temperature range of 650 to 710 ° C. because the austenite was not transformed into pearlite and maintained until bainite transformation, and a temperature range of 350 ° C. to Zn plating bath temperature + 100 ° C. was taken for 1 to 3000 seconds. This is to obtain predetermined characteristics by passing the structure as ferrite (+ bainite) + retained austenite. When the cooling stop temperature is less than 650 ° C., austenite is transformed into pearlite, and retained austenite cannot be obtained, and predetermined characteristics cannot be obtained. When the cooling stop temperature exceeds 710 ° C., it is difficult to concentrate C in the austenite, which is not preferable.
[0055]
Further, when the cooling rate from the plating bath temperature to the plating bath temperature + 100 ° C. is less than 1 ° C./second, austenite is transformed into pearlite during the cooling, so that residual austenite does not remain, which is not desirable. On the other hand, when the cooling rate is faster than 100 ° C./second, the cooling end point temperature in the plate width direction varies, and it is not preferable because a uniform steel plate cannot be produced.
[0056]
The reason why the holding temperature before immersion in the plating bath is set to 350 ° C. to Zn plating bath temperature + 100 ° C. is that bainite transformation occurs in this temperature range and residual austenite can be left to room temperature. When the temperature is lower than 350 ° C., the austenite generated by heating in the two-phase region causes martensitic transformation and no residual austenite remains, so the lower limit temperature was set to 350 ° C. When the holding temperature is higher than the temperature of the Zn plating bath + 100 ° C., the austenite generated by heating in the two-phase region is transformed into pearlite, or cementite precipitates from the austenite, so the austenite decomposes into bainite. Therefore, the upper limit temperature was set to Zn plating bath temperature + 100 ° C.
[0057]
If the holding time including the Zn plating bath immersion time is less than 1 second, it is not preferable because the time for immersing the steel sheet in the plating bath is not sufficient, and if it exceeds 3000 seconds, the equipment becomes too large and uneconomical. Since it is not preferable, the holding time is set to 1 to 3000 seconds.
[0058]
If necessary, the holding temperature after immersion in the plating bath is set to bath temperature to Zn plating bath temperature + 100 ° C. If the temperature is higher than this temperature, austenite is transformed into pearlite, or cementite is precipitated from the austenite. Therefore, since the austenite is decomposed into bainite, the upper limit temperature is set to the Zn plating bath temperature + 100 ° C. If the holding temperature is lower than the bath temperature, it takes a long time for alloying, so the lower limit temperature was taken as the bath temperature.
[0059]
The cooling rate from the holding temperature after immersion of Zn plating or from the temperature of the Zn plating bath to room temperature is not particularly limited, but it is preferable to cool from these temperatures to 350 ° C. at 0.5 to 100 ° C./second. . If the cooling rate from these temperatures to 350 ° C. is slower than 0.5 ° C./second, the equipment becomes too large and the economy is inferior. Therefore, the lower limit value of the cooling rate is preferably 0.5 ° C./second or more. On the other hand, even if the cooling rate exceeds 100 ° C./second, the material does not cause any problem. However, excessively increasing the cooling rate causes an increase in manufacturing cost, so the upper limit is set to 100. It is preferable to set it as ° C / second.
[0060]
Even if it does not hold | maintain in the temperature range of bath temperature-plating bath temperature +100 degreeC after plating bath immersion, the high intensity | strength hot-dip galvanized steel plate excellent in workability as described in this invention can be obtained.
[0061]
In addition, the material of the high-strength and high-ductility hot-dip galvanized steel sheet having excellent workability according to the present invention is manufactured in principle through refining, steelmaking, casting, hot rolling, and cold rolling processes, which are ordinary steelmaking processes. However, even if manufactured by omitting a part or all of them, the effects of the present invention can be obtained as long as the conditions according to the present invention are satisfied.
[0062]
Further, in order to further improve the plating adhesion, the present invention does not depart from the present invention even if the steel plate is plated with Ni, Cu, Co, or Fe alone or before the annealing.
[0063]
Furthermore, regarding the annealing before plating, “after degreasing pickling, heating in a non-oxidizing atmosphere,₂And N₂After the annealing in a reducing atmosphere, cool down to near the plating bath temperature and soak in the plating bath. Adjust the atmosphere during annealing, first oxidize the steel sheet surface, and then reduce it. All-reduction furnace method of “soaking in plating bath after cleaning before plating” or “flux treatment with degreased and acid-washed steel plate and ammonium chloride etc. and soaking in plating bath” However, the effect of the present invention can be exhibited even if the treatment is performed under any conditions.
[0064]
In addition, the effect of this invention can be acquired even if it cuts and removes the oxide on the surface of a steel plate after annealing, and is immersed in a plating bath and plated.
[0065]
【Example】
Hereinafter, the present invention will be described in more detail by way of examples.
[0066]
A steel plate having a composition as shown in Table 1 is heated to a temperature range of 1200 to 1250 ° C., and Ar_Three Hot rolling was completed above the transformation temperature, and the steel strip wound up in the temperature range of 500 to 700 ° C. was pickled and then cold rolled to a thickness of 1.0 mm. Then, according to the following formula by the component (mass%) of each steel, Ac₁ And Ac_Three The transformation temperature was determined by calculation.
Ac₁ = 723-10.7 × Mn (%) + 29.1 × Si (%),
Ac_Three = 910-203 * C (%) 1/2 + 44.7 * Si (%) + 31.5 * Mo (%)-30 * Mn (%)-11 * Cr (%) + 400 * Al (%
)
These Ac₁ And Ac_Three 10% H in the annealing temperature calculated from the transformation temperature₂ -N₂ After heating and holding in the atmosphere, it is cooled to 500 to 710 ° C. at a cooling rate of 0.1 to 10 ° C./second, and then cooled to the plating bath temperature at a cooling rate of 0.1 to 10 ° C./second, After holding at 460 to 500 ° C. for 10 to 100 seconds, plating was performed by immersing in a 460 ° C. hot dip galvanizing bath with various changes in the bath composition for 3 seconds. In this case, the plating adhesion amount is 50 g / m on one side.² It was.
[0067]
Thereafter, for some of the steel plates, after holding the alloy in the temperature range of the plating bath temperature to 600 ° C. and performing the alloying treatment, the temperature range up to 350 ° C. is set to a cooling rate of 0.1 to 100 ° C./second. And then cooled to room temperature. Details of the production conditions are shown in Table 2, Table 3 (Continuation 1 of Table 2) and Table 4 (Continuation 2 of Table 2).
[0068]
The plating property was evaluated by visual observation of the state of dross entrainment on the plating surface appearance and measurement of the area of the non-plated part. The concentration in the prepared plating layer was measured using ICP emission analysis after dissolving the plating layer with 5% hydrochloric acid containing an amine-based inhibitor.
[0069]
For plating adhesion, powdering was investigated and the case where the peel width exceeded 3 mm was rejected.
[0070]
JIS No. 5 test specimens were collected from these plated steel plates and subjected to a room temperature tensile test at a gauge length of 50 mm and a tensile test speed of 10 mm / min.
[0071]
The residual austenite volume fraction Vγ was measured by X-ray diffraction using a Mo tube after chemically polishing a 7/16 inner layer of the plate thickness from the plating layer / steel plate interface, and the diffraction intensity Iα (200) of ferrite (200). ), The diffraction intensity Iα (211) of ferrite (211), the diffraction intensity Iγ (220) of austenite (220), and the diffraction intensity Iγ (311) of (311).
[0072]
Vγ (volume%)
= 0.25
× {Iγ (220) / (1.35 × Iα (200) + Iγ (220))
+ Iγ (220) / (0.69 × Iα (211) + Iγ (220))
+ Iγ (311) / (1.5 × Iα (200) + Iγ (311))
+ Iγ (311) / (0.69 × Iα (211) + Iγ (311))}
The volume fraction and distribution of the oxide were measured by observing with SEM and EPMA after polishing.
[0073]
Tables 5 and 6 (continued from Table 5) show mechanical properties, plating properties, and the like. As shown in Table 3, when the steel sheet components satisfy the predetermined range, good plating properties are obtained without any plating. The uniform elongation and total elongation of the steel of the present invention are 2 to 5% higher than the comparative steel, and are excellent in workability.
[0074]
Even if the components of the steel sheet are within the predetermined range, those in which the manufacturing conditions and the steel sheet structure do not satisfy the predetermined requirements are inferior in workability and plating adhesion, and further, the retained austenite fraction becomes low. The effect of the invention is not exhibited.
[0075]
Even if the manufacturing conditions satisfy the present invention, the effect of the present invention cannot be obtained if the component range does not satisfy a predetermined requirement.
[0076]
[Table 1]

[0077]
[Table 2]

[0078]
[Table 3]

[0079]
[Table 4]

[0080]
[Table 5]

[0081]
[Table 6]

[0082]
【The invention's effect】
According to the present invention, a high strength and high ductility hot dip galvanized steel sheet excellent in ductility can be obtained.

Claims (7)

% By mass
C: 0.0001 to 0.3%,
Si: 0.001 to less than 0.1%,
Mn: 0.001 to 3%,
Ni: 0.79-5 %,
Al: 0.1 to 4%
Mo: 0.001 to 4%,
P: 0.0001 to 0.3%,
S: contains 0.01% or less, and satisfy Mn content and Ni content of the following formula (1), Ri Do the balance iron and unavoidable impurities, microstructure, as the largest phase volume fraction On the surface of the steel sheet containing ferrite or ferrite and bainite in a volume fraction of 50 to 97% and containing 2 to 50% austenite as the second phase ,
Al: 0.001 to 0.5%,
A high-strength and high-ductility hot-dip galvanized steel sheet excellent in workability, characterized by comprising a plating layer containing Fe: 5 to 20%, the balance being Zn and inevitable impurities.
−3 × Mn + 3.5 <Ni <−4.8 × Mn + 7.2 (1) The plating layer is Al: 0.001 to 0.5%,
The high-strength and high-ductility hot-dip galvanized steel sheet with excellent workability according to claim 1, wherein Fe: less than 5%, and the balance is made of Zn and inevitable impurities. Furthermore, in steel,
Cu: 0.001 to 4%,
Cr: 0.001 to 4%,
Co: 0.001 to 4%
The high-strength and high-ductility hot-dip galvanized steel sheet having excellent workability according to claim 1 or 2 , characterized by containing at least one of The steel according to any one of claims 1 to 3 , further comprising 0.001 to 1% in total of one or more of Nb, Ti, and V in mass% in the steel. High strength, high ductility hot dip galvanized steel sheet with excellent workability. Further, the steel contains B: 0.0001 to 0.1% by mass% in steel, and has high workability and high strength and high ductility melt according to any one of claims 1 to 4. Galvanized steel sheet. The cast slab comprising the component according to any one of claims 1 to 5 is cast as it is, or after being cooled, it is heated again, and the hot-rolled steel sheet wound up after hot rolling is pickled and cold-rolled, and then After annealing at a maximum heating temperature of Ac ₁ + 30 ° C. or more and Ac ₃ ° C. or less, it is cooled to a temperature range of 650 to 710 ° C. at a cooling rate of 0.1 to 10 ° C./second, and subsequently 1 to 100 ° C. / After cooling from Zn plating bath temperature to Zn plating bath temperature + 100 ° C at a cooling rate of 2 seconds, hold for 1-3000 seconds in the temperature range of 350 ° C to Zn plating bath temperature + 100 ° C, including the immersion time of the plating bath After that, a method for producing a high-strength, high-ductility hot-dip galvanized steel sheet excellent in workability, which is immersed in a Zn plating bath and then cooled to room temperature. After immersion in Zn plating bath, performs the retention of 1 to 300 seconds at a temperature range of bath temperature ~Zn plating bath temperature + 100 ° C., and excellent workability according to claim 6, wherein the cooling to room temperature high A method for producing a high strength ductile hot dip galvanized steel sheet.