JP5867278B2 - High-strength hot-dip galvanized steel sheet with excellent formability in normal and medium temperature ranges and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength hot dip galvanized steel sheet suitable for press forming in a normal and medium temperature range and a method for producing the same.

  In order to achieve both weight reduction and safety of automobile bodies, parts, etc., the strength of steel plates as materials is being increased. Generally, when the strength of a steel plate is increased, workability such as ductility and hole expansibility is impaired. Therefore, in order to use a high-strength steel sheet as a member for automobiles, a balance between strength and formability is required. In response to such demands, so-called TRIP steel sheets have been proposed so far that use transformation-induced plasticity of retained austenite to achieve high ductility (see, for example, Patent Document 1).

Currently, 980 MPa class and 1180 MPa class high tensile steel are also used. High strength leads to deterioration of moldability such as elongation and hole expansibility, and it is extremely difficult to form high strength high strength steel. Although there is a TRIP steel sheet of such a strength class, in general, the formability is not sufficient, so that a device for the forming method is required.
As a technique for improving the formability of a high-strength steel sheet, warm processing is attracting attention. For example, Patent Document 2 reports that warm workability is improved by controlling the average axial ratio of retained austenite within a specific range.

However, in order to obtain the retained austenite described in Patent Document 2, it is considered necessary to perform annealing twice after cold rolling (see Non-Patent Document 1). From this, it is speculated that it is inferior in economic efficiency.
In addition, processing using temperature is used in metastable austenitic stainless steel, but a large amount of expensive elements such as Ni are added to stabilize austenite (for example, see Non-Patent Document 2). Inferior in economic efficiency.
In view of the above background, there is a need for a steel sheet that is relatively inexpensive and suitable for warm processing and that is easy to manufacture.

JP 2000-345288 A JP 2004-190050 A

"Effect of second phase morphology on warm stretch flangeability of TRIP type composite steel" Iron and steel, vol. 84, no. 3, 60-65 (1998) “Processing induced transformation in stainless steel” plasticity and processing, vol. 18, no. 202, pages 938-945

  The present invention is intended to solve the above-mentioned problems, and provides a high-strength hot-dip galvanized steel sheet having high ductility and hole expansibility in a normal and medium temperature range (50 to 250 ° C.) and a method for producing the same. To do.

The present inventors perform melting, hot rolling, cold rolling, and annealing in a laboratory on various steels with different amounts of C, Si, and Mn, and obtain required ductility and hole expandability in a target temperature range. Various methods were studied.
As a result, it was found that a high-strength steel sheet capable of improving the hole expandability and ductility in the normal and middle temperature range can be manufactured after specifying the components.
The gist of the present invention thus made is as follows.

(1) The chemical composition of steel is mass%,
C: 0.08% to 0.35%,
Si: 0.01% to 2.5%,
Mn: 1.0% to 3.5%,
Al: 0.005% or more and 2.0% or less, and P: 0.05% or less,
S: 0.01% or less,
N: limited to 0.01% or less, consisting of the balance iron and inevitable impurities,
Further, as a steel microstructure , among tempered martensite, bainite, and ferrite , 90% or less each containing one or more bainite containing 16% or more of bainite is included , and martensite is 5% or less ( The austenite content remaining when the pearlite is limited to 20% or less (including 0%) and the equivalent plastic strain of 0.1 to 0.3 at 20 ° C. is applied to the austenite. the rate _f 0, when the austenite fraction which remains when given equivalent plastic strain 0.1-0.3 at 50 to 250 ° C. was _{f a,} the value of f a / _{f 0} is 1.2 Tensile strength TS, total elongation EL, hole expansion obtained by containing 5% or more of austenite, which is ˜4.0 , by area ratio, holding a steel sheet at 50-250 ° C., and conducting a tensile test and a hole expandability test TS × E for rate λ There at 20000 MPa ·% or more, hot-dip galvanized steel sheet excellent in formability at ordinary medium temperature range of TS × lambda is characterized der Rukoto 30000 mPa ·% or more.
(2) Furthermore, in steel,
Cr: 0.05% or more and 2.0% or less,
Ni: 0.05% or more and 2.0% or less,
Mo: 0.05% to 1.0%,
Cu: molten galvanized steel sheet according to containing one or two or more of 0.05% to 2.0% or less and wherein (1).
(3) Furthermore, in steel,
Nb: 0.005% to 0.1%,
Ti: 0.005% or more and 0.15% or less,
V: molten galvanized steel sheet according to, characterized in that it contains one or more of 0.01% to 1.0% or less (1) or (2).
(4) Furthermore, in steel,
B: molten galvanized steel sheet according to any one of characterized in that it contains 0.0001% to 0.01% or less (1) to (3).
(5) Furthermore, in mass% in steel,
Ca: 0.0005% to 0.01%,
Mg: 0.0005% to 0.01%,
REM: it contains one or more than 0.0005% 0.01% or less and wherein (1) molten galvanized steel sheet according to any one of the - (4).

( 6 ) The cast slab made of the chemical component according to any one of (1) to (5) is heated directly to 1200 ° C. or more after being directly or once cooled after casting, and then hot-rolled to heat at or above the Ar3 transformation point. When the hot rolling is completed, the steel sheet is wound in a temperature range of 700 ° C. or less, pickled, cold-rolled with a rolling reduction of 40 to 70%, and passed through a continuous hot-dip galvanizing line, the temperature is 750 ° C. or higher and 900 After annealing at a temperature not higher than ℃, and then holding at a temperature range of not lower than 180 ° C and not higher than 550 ° C for not less than 5 seconds and not more than 1000 seconds, the steel sheet temperature is changed from (zinc plating bath temperature -40) ° C to (zinc plating bath temperature +50) ° C. The hot-dip galvanized steel sheet according to any one of (1) to (5), wherein the hot-dip galvanized steel sheet is adjusted and immersed in a hot-dip galvanizing bath flowing at a flow rate of 10 m / min to 50 m / min . Manufacturing Law.
( 7 ) The cast slab composed of the chemical component according to any one of (1) to (5) is heated directly to 1200 ° C. or more after being directly or once cooled after casting, and then hot-rolled to heat at or above the Ar3 transformation point. When the hot rolling is completed, the steel sheet is wound in a temperature range of 700 ° C. or less, pickled, cold-rolled with a rolling reduction of 40 to 70%, and passed through a continuous hot-dip galvanizing line, the temperature is 750 ° C. or higher and 900 After annealing at a temperature of not higher than ℃, and holding at a temperature range of not lower than 180 ° C and not higher than 550 ° C for 5 sec or more and 1000 sec or less, the steel plate temperature is (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. adjusted to, after zinc by immersion in molten zinc plating bath to flow plating in the following flow rate 10 m / min or more 50 m / min, it was subjected to alloying treatment at a temperature below 600 ° C. 460 ° C. or higher, cooling to room temperature Characterized Rukoto (1) A method of manufacturing a galvanized steel sheet according to any one of the - (5).

  Here, normal temperature means a temperature of 50 to 250 ° C., and excellent moldability in the normal temperature range means that if it is in this temperature range, the productivity is improved without deteriorating productivity. It means that it is obtained.

  According to the present invention, it is a technique relating to a high-strength hot-dip galvanized steel sheet and a method for producing the same that achieve high formability in press forming in a normal and medium temperature range, and its industrial contribution is extremely remarkable.

First, the reasons for limiting the chemical components of the steel sheet suitable for press forming in the normal and middle temperature range in the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.
C: C is added as an element for stabilizing the retained austenite of the steel. If it is less than 0.08%, it is difficult to secure an amount of retained austenite necessary for improving formability, and excessive addition exceeding 0.35% significantly deteriorates ductility, weldability, toughness and the like. Therefore, the C content is 0.08 to 0.35%. A more preferable range is 0.15 to 0.3%.

  Si: Si is an element useful for increasing the strength of a steel sheet by solid solution strengthening. Moreover, since Si suppresses the formation of cementite, it suppresses material deterioration due to heating at normal and medium temperatures. In order to obtain this effect, addition of 0.01% or more is necessary, and this is set as the lower limit. However, excessive addition exceeding 2.5% significantly deteriorates ductility and toughness, so this was made the upper limit. A more preferable range is 1.0 to 1.8%.

  Mn: Mn is an element effective for improving the hardenability. It is also an austenite stabilizing element. If it is less than 1.0%, the effect of improving hardenability or the effect of stabilizing austenite is not sufficiently exhibited. However, addition over 3.5% tends to cause cracking during production, so 3.5% was made the upper limit.

  Al: Al promotes ferrite formation and improves ductility. Moreover, since the production | generation of cementite is suppressed, the material deterioration by the heating at normal temperature is suppressed. For these reasons, Al may be added. Al can also be used as a deoxidizing material. However, excessive addition increases the number of Al-based coarse inclusions, which causes deterioration of hole expansibility and surface damage. From this, the upper limit of Al addition was set to 2.0%. The lower limit is not particularly limited, but it is substantially difficult to set it to less than 0.005%, so 0.005% was set as the lower limit.

P: P is an impurity element that segregates at the grain boundaries to lower the grain boundary strength and degrade toughness, and is desirably reduced. The upper limit of the content of P is limited to 0.05% in consideration of the current refining technology and manufacturing cost.
S: S is an impurity element that degrades hot workability and toughness, and is desirably reduced. Therefore, the upper limit was limited to 0.01%.
N: N forms coarse nitrides and degrades bendability and hole expandability, so the amount added must be suppressed. This is because when N exceeds 0.01%, this tendency becomes remarkable. Therefore, the range of N content is set to 0.01% or less. In addition, it is better to use less because it causes blowholes during welding.

  Furthermore, you may add 1 type, or 2 or more types of Cr, Mo, Ni, Cu. These elements are effective elements that improve ductility and toughness. However, if the content of Cr, Ni, and Cu exceeds 2.0% and the content of Mo exceeds 1.0%, toughness may be impaired due to an increase in strength. Therefore, the upper limit of Cr, Ni and Cu is set to 2.0%, and the upper limit of Mo is set to 1.0%. Moreover, in order to improve ductility and intensity | strength, 0.05% or more of each addition is required, and this was made into the lower limit.

  Further, one or more of Ti, Nb, and V may be added. These elements are elements that form fine carbonitrides, and are effective in suppressing the coarsening of crystal grains and enhancing strength and toughness. In order to secure strength and increase toughness, it is necessary to add 0.005% or more for Ti and Nb and 0.01% or more for V. However, if these elements are added excessively, the precipitates become coarse, and the workability may be greatly deteriorated. Therefore, Nb is set to 0.1%, Ti is set to 0.15%, and V is set to 1.0%.

  B: B is an element that segregates at the grain boundaries and suppresses the grain boundary segregation of P and S. It is also an effective element for enhancing the hardenability. However, if the amount of B exceeds 0.01%, coarse precipitates are produced at the grain boundaries, which may impair hot workability and toughness. Therefore, the B content is 0.01% or less. In order to improve the ductility, toughness and hot workability by strengthening the grain boundaries, and to improve the hardenability, 0.0001% or more of B is preferably added.

  Furthermore, you may add 1 type, or 2 or more types of Ca, Mg, and REM. These elements are elements effective in controlling the form of sulfide and suppressing the deterioration of hot workability and toughness due to S. However, since the effect is saturated even if added excessively, it is preferable to add 0.01% or less of Ca, 0.01% or less of Mg, and 0.01% or less of REM. In order to improve toughness, it is preferable to add 0.0005% or more of Ca, 0.0005% or more of Mg, and 0.0005% or more of REM. Here, REM refers to La, Y, Ce and lanthanoid elements.

Next, the microstructure of the steel sheet of the present invention will be described.
The microstructure of the steel sheet of the present invention contains austenite as defined below, and contains one or more of bainite, ferrite , and tempered martensite. In addition,% showing each existence ratio is an area ratio.

  Austenite is the most important structure for achieving high formability in normal and medium temperature ranges. Austenite undergoes processing-induced transformation with molding, but the amount of transformation is different from that at normal temperature and that at room temperature, and its ratio is very closely related to formability at normal temperature. I found out.

F ₀ austenite fraction which remains when given equivalent plastic strain 0.1-0.3 at room _temperature, the austenite fraction remaining when given the same amount of equivalent plastic strain at 50 to 250 ° C. when a _{f a,} f _a / f ₀ is, it is necessary to obtain a high formability in press working have austenite contained in the range of 1.2 to 4.0 more than 5%. In order to contain such austenite in the steel sheet structure, as described later, after cold rolling and annealing, it was essential to hold for 5 to 1000 seconds in a temperature range of 180 ° C. or higher and 550 ° C. or lower.

  In order to define the characteristics of retained austenite, the amount of retained austenite when an equivalent plastic strain of 0.1 to 0.3 was applied was compared. This is because if the equivalent plastic strain is less than 0.1, there may be a case where the difference between the cold and the transformation amount at 50 to 250 ° C. is difficult to appear. Since there is a thing which will end, it was decided to compare in the range of 0.1-0.3.

It has been found that the press formability in the normal and medium temperature range varies depending on the ratio of the cold in this strain region and the amount of retained austenite at 50 to 250 ° C. When the equivalent plastic strain in this range is given, if the ratio of the remaining austenite f _a / f ₀ is less than 1.2, high ductility and hole expansibility cannot be secured in the normal and medium temperature range, so 1.2. The lower limit was set. Further, if it exceeds 4.0, transformation is suppressed too much, and austenite breaks without transformation, and the TRIP effect cannot be used effectively, so 4.0 was made the upper limit.
When austenite having such characteristics contains less than 5% in the steel sheet, high ductility and hole expandability at 50 to 250 ° C. cannot be obtained, so 5% was set as the lower limit of the content.

Bainite and tempered martensite are effective for ensuring strength. However, when 90% or more is contained in the steel sheet, the toughness is lowered, so the upper limit was made 90%. Ferrite improves the ductility of the steel sheet, but the strength may decrease. Since the strength can be secured by precipitation strengthening or solid solution strengthening, the upper limit is set to 90%.
In the scope of the claims, 16% or more of bainite is necessarily contained based on the examples.

As a structure other than ferrite, bainite, tempered martensite, and retained austenite, a martensite or pearlite structure may be contained in an amount of 20% or less. If these structures are present in the steel sheet in an amount of 20% or more, the ductility and toughness are lowered, so 20% was made the upper limit. In the claims, the upper limit of martensite was limited to 5% based on the examples.

Incidentally, identification of each phase of the microstructure, ferrite, martensite, tempered martensite, bainite, austenite, pearlite and the remaining structure and measurement of the area ratio were disclosed in Nital reagent, Japanese Patent Application Laid-Open No. 59-219473. It can be identified by reagent, SEM-EBSD method or X-ray diffraction.
In Examples described later, ferrite, martensite, tempered martensite, bainite, and pearlite are corroded in a steel plate rolling direction cross section or a rolling direction perpendicular direction cross section, and quantified by a scanning type and transmission electron microscope of 1000 to 100,000 times. Made.

In that case, the ferrite may be in the form of a lump or needle. The bainite structure includes an upper bainite structure composed of carbide or austenite contained between lath-like ferrite and lath, or a lower bainite structure containing carbide in lath-like ferrite.
Tempered martensite is also composed of lath-like martensite and carbides, but carbides having a plurality of orientation relationships are precipitated in tempered martensite, so that there is only a single orientation relationship inside. It can be easily distinguished from the lower bainite structure containing carbide. On the other hand, fresh martensite refers to martensite containing no carbide.
As described above, each structure can be distinguished from the form of the constituent phase of the structure, the presence or absence of carbides, and the orientation relationship. Each 20 visual fields are observed, and the area ratio of each tissue can be obtained by a point counting method or image analysis.

The measurement values of f ₀ and f _a can be carried out as follows. f ₀ and f _a are processed into a tensile test piece, heated to a target temperature, pre-strained, and then subjected to a chemical polishing to 1/4 thickness from the surface layer to obtain a monochromatic MoKα Residual austenite can be quantified from the (200) and (211) area strengths of ferrite and the (200), (220) and (311) area strengths of austenite.

Next, the reasons for limiting the manufacturing conditions will be described.
The steel comprising the above components is melted, cast and hot rolled by a conventional method. Furthermore, pickling and cold rolling are performed and heat treatment is performed.
Hot rolling may be performed directly on the slab after casting, or may be performed after re-heating after cooling. In the case of reheating, the heating temperature is set to 1200 ° C. or higher in order to reduce the influence of segregation and make the final structure uniform. Hot rolling is completed at a temperature equal to or higher than the Ar3 transformation point according to a conventional method, and is wound in a temperature range of 700 ° C. or lower. When the coiling temperature exceeds 700 ° C., coarse ferrite and pearlite structures exist in the hot rolled structure, so that the structure non-uniformity after annealing increases and the hole expandability of the final product deteriorates.
In cold rolling, the reduction ratio is 40% or more in order to refine the microstructure after annealing. However, if it exceeds 70%, the load increases due to work hardening, which is considered to impair productivity. Therefore, the rolling reduction of cold rolling is 40 to 70%.

  In the present invention, it is extremely important that the steel sheet obtained as described above is subjected to the following heat treatment when passing through a continuous hot dip galvanizing line.

  When heat-treating the steel sheet, the annealing temperature is a two-phase region or austenite single-phase region temperature. Therefore, annealing at 750 ° C. or more and 900 ° C. or less is essential from the component range. If the temperature is lower than 750 ° C., non-recrystallized ferrite remains and the ductility of the steel sheet is lowered. In addition, when the annealing temperature exceeds 900 ° C., the austenite grain size becomes coarse, and the toughness is lowered. Further, in order to stabilize the retained austenite, the temperature is preferably set so that the austenite is less than 40% in an equilibrium state.

  After that, it was essential to hold for 5 seconds or more and 1000 seconds or less in a temperature range of 180 ° C. or more and 550 ° C. or less. This is because retained austenite can be stabilized by maintaining in this temperature range. When the temperature is 180 ° C. or lower, a lot of martensite is produced during cooling, so that the amount of untransformed austenite becomes small, and the required amount of austenite cannot be secured as the final structure, and the ductility is lowered. Moreover, since cementite will precipitate if it hold | maintains at the temperature over 550 degreeC, austenite cannot be stabilized by C, 550 degreeC was made into the upper limit.

  After annealing and holding the steel plate as described above, the steel plate is heated or cooled to adjust the steel plate temperature within the range of (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. Zinc plating is carried out in a hot dip galvanizing bath that flows at a rate of not less than / min and not more than 50 m / min. In addition, when heating a steel plate, the heating rate of 10 degree-C / sec or more is good. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.

The inventors scum the steel sheet surface by immersing the steel sheet in a hot dip galvanizing bath flowing at a flow rate of 10 m / min or more and 50 m / min or less and giving a jet to the steel plate in the plating bath. It has been found that the adhesion of metal can be prevented, preventing non-plating and promoting the alloying.
A Zn or Al oxide film called scum floats on the surface of the plating bath. When a large amount of an external oxide film is present on the surface of the steel sheet, when the steel sheet is immersed in the plating bath, scum tends to adhere to the surface of the steel sheet, and thus non-plating is likely to occur. In addition, the scum adhering to the steel sheet delays not only non-plating but also alloying.

Such a phenomenon becomes particularly remarkable in a steel sheet containing a large amount of Si or Mn as in the present invention. Although the detailed mechanism is unknown, it is considered that non-plating and alloying delay are promoted by the reaction of oxides of Si and Mn formed on the steel sheet surface with scum, which is also an oxide.
The reason why the jet flow velocity is set to 10 m / min or more and 50 m / min or less is that if the jet flow velocity is less than 10 m / min, the effect of suppressing the non-plating due to the jet cannot be obtained. This is because not only is the effect saturated, but excessive capital investment leads to high costs.

  Thereafter, an alloying treatment may be further performed. When the alloying heat treatment is less than 460 ° C., alloying is insufficient, and when it exceeds 600 ° C., retained austenite is decomposed and cementite is generated, so that the hole expandability is deteriorated. The alloying time is determined by the balance with the alloying temperature, but a range of 10 to 40 seconds is appropriate. If it is less than 10 seconds, alloying is difficult to proceed, and if it exceeds 40 seconds, the retained austenite decomposes and cementite is produced, so that the hole expandability deteriorates.

  In addition, in high Si steel, there are cases where deterioration of plating properties and delay in alloying may be a problem, but hydrogen is 1 to 10 vol% and the balance is nitrogen in a line in a temperature range of 300 ° C. or higher during annealing. In addition, this problem can be solved by using a composition comprising inevitable impurities, setting the dew point in the preceding stage of the heating zone and in the tropical zone to -30 ° C. or higher and 10 ° C. or lower, and setting the cooling first dew point to −25 ° C. or lower.

  After galvanization and alloying heat treatment, it is desirable to perform temper rolling for final shape correction and disappearance of yield point elongation. If the elongation is less than 0.2%, the effect is not sufficient. If the elongation exceeds 1%, the yield ratio is greatly increased and the elongation is deteriorated. Therefore, it is desirable that the elongation rate is 0.2 to 1%.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
Steel having the composition shown in Table 1 was cast, and rolling, heat treatment and hot dip galvanizing were performed under the conditions shown in Tables 2 and 3, and some of the steels were further alloyed to produce steel sheets. The plating bath temperature during plating was 460 ° C. The plate thickness of the steel plate used for the characteristic evaluation was 1.4 mm.
The microstructure of the obtained steel sheet was examined, and the area ratios of bainite, tempered martensite, ferrite, retained austenite, martensite, and pearlite were measured. In addition, the appearance of the plated steel sheet was observed, and the case where there was non-plating was defined as “non-plating present”.

In order to evaluate the mechanical properties of the obtained steel sheet at normal temperature (50 to 250 ° C.), a tensile test and a hole expansion test were performed. The method of testing by holding the steel plate at 50 to 250 ° C. uses a method of testing in heated oil, or a method of testing after heating the steel plate and jig in advance.
In the tensile test, the test piece was heated to a target temperature, held for 60 seconds to make the temperature of the test piece constant, and then a tensile test was performed in accordance with JIS Z 2241. Tensile strength (TS) and total elongation (EL) were determined from the stress-strain curve of the tensile test. Moreover, the hole expansibility test was performed according to Japan Iron and Steel Federation standard JFS T 1001 after heating a test piece to normal medium temperature (50-250 degreeC), and hold | maintaining to target temperature, and calculated | required (lambda) value.

Based on the mechanical properties obtained by the tensile test and the hole expansion test, TS × EL and TS × λ, which are indexes of workability, were obtained.
In the present invention, a steel plate satisfying TS × EL of 20000 MPa ·% or more and TS × λ of 30000 MPa ·% or more is defined as a steel plate having excellent formability.

Furthermore, tensile test pieces were used for the measurement of f ₀ and f _a , f ₀ was the test piece kept at room temperature, and f _a was heated to 150 ° C. to give a pre-strain of 10% to 20%. Then, after confirming that the equivalent plastic strain was 0.1 to 0.2, a sample was taken from between the gauge points, and the surface was chemically polished from the surface layer to ¼ thickness, by a monochromated MoKα ray. From the (200) and (211) area strengths of ferrite and the (200), (220) and (311) area strengths of austenite, the retained austenite was quantified, and the austenite fraction was calculated. _0, was _{f a.}

  Experiment No. “a” to “o” are examples of the present invention, and all the characteristics are acceptable, and a steel sheet having the target characteristics is obtained. On the other hand, in Experiment No. in which the component or the production method is outside the scope of the present invention. Any of p to aj is rejected.

Claims (7)

The chemical composition of steel is mass%,
C: 0.08% to 0.35%,
Si: 0.01% to 2.5%,
Mn: 1.0% to 3.5%,
Al: 0.005% or more and 2.0% or less, and P: 0.05% or less,
S: 0.01% or less,
N: limited to 0.01% or less, consisting of the balance iron and inevitable impurities,
Further, as a steel microstructure , among tempered martensite, bainite, and ferrite , 90% or less each containing one or more bainite containing 16% or more of bainite is included , and martensite is 5% or less ( The austenite content remaining when the pearlite is limited to 20% or less (including 0%) and the equivalent plastic strain of 0.1 to 0.3 at 20 ° C. is applied to the austenite. the rate _f 0, when the austenite fraction which remains when given equivalent plastic strain 0.1-0.3 at 50 to 250 ° C. was _{f a,} the value of f a / _{f 0} is 1.2 Tensile strength TS, total elongation EL, hole expansion obtained by containing 5% or more of austenite, which is ˜4.0 , by area ratio, holding a steel sheet at 50-250 ° C., and conducting a tensile test and a hole expandability test TS × E for rate λ There at 20000 MPa ·% or more, hot-dip galvanized steel sheet excellent in formability at ordinary medium temperature range of TS × lambda is characterized der Rukoto 30000 mPa ·% or more. Furthermore, Cr: 0.05% or more and 2.0% or less in mass% in steel,
Ni: 0.05% or more and 2.0% or less,
Mo: 0.05% to 1.0%,
The hot-dip galvanized steel sheet according to claim 1 , comprising at least one of Cu: 0.05% to 2.0%. Furthermore, in steel,
Nb: 0.005% to 0.1%,
Ti: 0.005% or more and 0.15% or less,
V: molten galvanized steel sheet according to claim 1 or 2, characterized in that it contains one or more of 0.01% to 1.0% or less. Furthermore, in steel,
B: molten zinc plated steel sheet according to any one of claims 1 to 3, characterized in that it contains 0.0001% 0.01% or less. Furthermore, in steel,
Ca: 0.0005% to 0.01%,
Mg: 0.0005% to 0.01%,
REM: molten zinc plated steel sheet according to any one of claims 1 to 4, characterized by containing one or more of 0.0005% 0.01% or less. A cast slab comprising the chemical component according to any one of claims 1 to 5 is directly or after cooling once cast, and then hot-rolled to 1200 ° C or higher, and hot-rolled at an Ar3 transformation point or higher. Completed, wound in a temperature range of 700 ° C. or lower, pickled, cold-rolled with a rolling reduction of 40 to 70%, and passed through a continuous hot-dip galvanizing line at 750 ° C. or higher and 900 ° C. or lower. After annealing and then holding at a temperature range of 180 ° C. or more and 550 ° C. or less for 5 sec or more and 1000 sec or less, the steel plate temperature is adjusted to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. production side of hot-dip galvanized steel sheet according to claim 1, characterized in that the zinc by immersion in molten zinc plating bath to flow in the following flow rate 10 m / min or more 50 m / min plating . A cast slab comprising the chemical component according to any one of claims 1 to 5 is directly or after cooling once cast, and then hot-rolled to 1200 ° C or higher, and hot-rolled at an Ar3 transformation point or higher. Completed, wound in a temperature range of 700 ° C. or lower, pickled, cold-rolled with a rolling reduction of 40 to 70%, and passed through a continuous hot-dip galvanizing line at 750 ° C. or higher and 900 ° C. or lower. After annealing and then holding at a temperature range of 180 ° C. or more and 550 ° C. or less for 5 sec or more and 1000 sec or less, the steel plate temperature is adjusted to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. after zinc by immersion in molten zinc plating bath to flow plating in the following flow rate 10 m / min or more 50 m / min, was subjected to alloying treatment at a temperature of 460 ° C. or higher 600 ° C. or less, child cooled to room temperature Method for manufacturing a galvanized steel sheet according to any one of claims 1 to 5, wherein.