JP2011032549A - High-strength hot-dip galvanized steel strip with excellent moldability and less variation of material grade in steel strip, and method for production thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel strip which is inexpensive, has a tensile strength of ≥780 MPa, has less variation of material grade in a steel strip and is excellent in moldability, and also to provide a method for production thereof. <P>SOLUTION: The high-strength hot-dip galvanized steel strip contains, by mass, 0.05-0.2% C, 0.5-2.5% Si, 1.5-3.0% Mn, 0.001-0.05% P, 0.0001-0.01% S, 0.001-0.1% Al, 0.0005-0.01% N and the balance Fe with inevitable impurities. The steel strip has a microstructure composed of a ferrite phase and a martensite phase, in which the area ratio of the ferrite phase is 50% or more and that of the martensite phase is 30-50% in a whole structure. The steel strip has less variation of material grade, has excellent moldability and is characterized in that the difference between maximum and minimum tensile strengths in the strip is 60 MPa or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention is a high-strength hot-dip galvanized steel strip that is suitable mainly for structural members of automobiles, and in particular, has a high tensile strength TS of 780 MPa or more and excellent formability with small material variations in the steel strip. The present invention relates to a strength hot-dip galvanized steel strip and a method for producing the same.

  In recent years, for the purpose of ensuring the safety of passengers in the event of a collision and improving fuel efficiency by reducing the weight of the vehicle body, the application of high-strength steel sheets with a TS of 780 MPa or more and a thin plate thickness has been actively promoted. In particular, recently, the application of high strength steel sheets with extremely high strength having TS of 980 MPa class and 1180 MPa class has been studied.

  However, in general, increasing the strength of a steel sheet causes a decrease in the elongation characteristics, hole expansibility, bendability, etc. of the steel sheet, leading to a decrease in formability, and thus has both high strength and excellent formability. At present, a hot dip galvanized steel sheet having excellent corrosion resistance is desired.

In response to such a request, for example, in Patent Document 1, in mass%, C: 0.04 to 0.1%, Si: 0.4 to 2.0%, Mn: 1.5 to 3.0%, B: 0.0005 to 0.005%, P ≦ Contains 0.1%, 4N <Ti ≦ 0.05%, Nb ≦ 0.1%, the balance is Fe and inevitable impurities steel plate surface layer with alloyed galvanized layer, Fe% in alloyed hot dip galvanized layer is A high-strength galvannealed steel sheet with excellent formability and plating adhesion with a TS of 800MPa or more, in which the steel sheet structure is 5-25% and the structure of the steel sheet is a mixed structure of ferrite phase and martensite phase, has been proposed. . Patent Document 2 includes mass%, C: 0.05 to 0.15%, Si: 0.3 to 1.5%, Mn: 1.5 to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 to 0.5%, N: 0.0060% or less, the balance being Fe and inevitable impurities, further satisfying (Mn%) / (C%) ≧ 15 and (Si%) / (C%) ≧ 4, and in volume ratio in the ferrite phase A high-strength galvannealed steel sheet with good formability containing 3-20% martensite phase and retained austenite phase has been proposed. Patent Document 3 includes mass%, C: 0.04 to 0.14%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%, P: 0.02% or less, S: 0.01% or less, Al: 0.002 to 0.5%, Contains Ti: 0.005 to 0.1%, N: 0.006% or less, further satisfies (Ti%) / (S%) ≧ 5, consists of the balance Fe and inevitable impurities, the volume of martensite phase and residual austenite phase When the total ratio is 6% or more and the volume fraction of the hard phase structure of the martensite phase, residual austenite phase and bainite phase is α%,
α ≦ 50000 × {(Ti%) / 48+ (Nb%) / 93+ (Mo%) / 96+ (V%) / 51} Has been. Patent Document 4 contains, in mass%, C: 0.001 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.01 to 3%, Al: 0.001 to 4%, and the balance Fe and inevitable impurities. Is a hot dip galvanized steel sheet having a plating layer consisting of Al: 0.001 to 0.5%, Mn: 0.001 to 2%, and the balance Zn and unavoidable impurities in mass%, and the Si content of the steel : X mass%, Mn content of steel: Y mass%, Al content of steel: Z mass%, Al content of plating layer: A mass%, Mn content of plating layer: B mass% is 0 ≦ 3- (X + Y / 10 + Z / 3) -12.5 × (AB) is satisfied, the microstructure of the steel sheet is 70-97% ferrite main phase by volume ratio and its average grain size is 20 μm or less, High-strength molten zinc with excellent plating adhesion and ductility during molding, consisting of an austenite phase and / or martensite phase with a volume ratio of 3-30% as the second phase, and the average particle size of the second phase being 10 μm or less Plated steel sheets have been proposed.

JP 9-13147 A JP 11-279691 A JP 2002-69574 A JP 2003-55751 A

  However, the techniques related to the high-strength hot-dip galvanized steel sheets described in Patent Documents 1 to 4 have a problem that the material variation in the steel strip is large.

  In view of such circumstances, the present invention is inexpensive, high strength hot dip galvanizing with TS of 780 MPa or more, excellent elongation characteristics and hole expansibility, and excellent formability with small variations in material within the steel strip. An object of the present invention is to provide a steel strip and a manufacturing method thereof.

  The present inventors performed the following Test 1 and Test 2 in order to achieve the above object.

Exam 1
By mass%, C: 0.105%, Si: 1.25%, Mn: 2.15%, P: 0.018%, S: 0.0023%, Al: 0.041%, N: 0.0035%, Cr: 0.33%, Ti: 0.016%, B : Steel containing 0.0007%, the balance being Fe and inevitable impurities S1, C: 0.136%, Si: 1.62%, Mn: 2.83%, P: 0.021%, S: 0.0014%, Al: 0.032%, N : Steel S2 containing 0.0029%, the balance being Fe and inevitable impurities, C: 0.069%, Si: 1.03%, Mn: 1.88%, P: 0.015%, S: 0.0024%, Al: 0.037%, N : Steel 34 containing 0.0034%, Cr: 0.41%, Mo: 0.12%, the balance being Fe and inevitable impurities, was melted in a laboratory in a vacuum melting furnace to obtain a slab. These steel slabs are heated at 1250 ° C, hot-rolled at a finishing temperature of 880 ° C, then cooled to 550 ° C at an average cooling rate of 10-250 ° C / s after 0.5 s, and 450-625 ° C A heat treatment equivalent to winding was performed at a temperature of 1 to obtain a hot rolled steel sheet. These hot-rolled steel sheets were scaled by pickling, cold-rolled at a reduction rate of 50%, imitated hot dip galvanizing lines, heated to 700 ° C at an average heating rate of 5 ° C / s or more, and subsequently After soaking at 120 ° C for 120 s, cooling to 520 ° C at an average cooling rate of 10 ° C / s and annealing, immersed in a 475 ° C galvanizing bath containing 0.13% Al for 3 s and hot-dip galvanized, Zinc plating with an adhesion amount of 45 g / m ² was formed on the surface. Subsequently, alloying treatment of galvanization was performed at 550 ° C., and then cooled to room temperature at an average cooling rate of 15 ° C./s to obtain a hot dip galvanized steel sheet.

Then, analysis of the microstructure of the hot-rolled steel sheet and measurement of TS of the hot-dip galvanized steel sheet were performed by the following methods.
Microstructure of hot-rolled steel sheet: About the thickness cross section parallel to the rolling direction of the steel sheet, after corrosion by nital, observed at 5000 times using a scanning electron microscope (SEM), ferrite phase, pearlite phase, bainite phase and A low-temperature transformation phase such as a martensite phase was identified and analyzed by image analysis software (Image-Pro; manufactured by Cybernetics), and the area ratio of the bainite phase in the entire structure was calculated. In the production of the hot-rolled steel sheet according to the present invention, if the heat treatment temperature equivalent to winding (winding temperature) is equal to or higher than the following Tct ° C, a microstructure mainly composed of a ferrite phase and a pearlite phase is obtained. A microstructure mainly composed of a phase and a bainite phase is obtained.
Tct = 810-300 × [C] -60 × [Si] -60 × [Mn] -70 × [Cr] -80 × [Mo] -40 × [Ni] -70 × [Cu]
However, [M] represents the content (mass%) of the element M, and when the element M is an inevitable impurity, the TS measurement of the hot-dip galvanized steel sheet with [M] = 0: JIS5 in the direction perpendicular to the rolling direction Tensile test specimens were collected and subjected to a tensile test at a crosshead speed of 20 mm / min in accordance with JIS Z 2241 to measure TS.

  FIG. 1 shows the relationship between the coiling temperature and the area ratio of the bainite phase in the hot-rolled steel sheet. FIG. 2 shows the relationship between the area ratio of the low temperature transformation phase in the hot-rolled steel sheet and the TS of the hot-dip galvanized steel sheet. In any of the steels S1, S2, and S3, when the coiling temperature is less than Tct ° C, a low-temperature transformation phase mainly composed of the bainite phase is formed, and the area ratio of the bainite phase is 30% or more, which is almost constant. TS of plated steel sheet is obtained. On the other hand, when the coiling temperature is Tct ° C. or higher, a low-temperature transformation phase mainly composed of a pearlite phase is formed, the area ratio of the bainite phase is less than 30%, and the TS of the hot-dip galvanized steel sheet varies greatly.

Test 2
In mass%, C: 0.101%, Si: 1.27%, Mn: 2.22%, P: 0.011%, S: 0.0020%, Al: 0.036%, N: 0.0032%, Cr: 0.28%, Ti: 0.021%, B : Steel S4 containing 0.0011% with the balance being Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method. These steel slabs are heated at 1250 ° C, hot rolled at a finishing temperature of 890 ° C, then water cooled to 550 ° C at an average cooling rate of 200 ° C / s after 0.8 s and at a winding temperature of 500 ° C. Two levels of hot-rolled steel strip were produced: one wound and water cooled to 650 ° C and wound at a winding temperature of 600 ° C. These hot-rolled steel strips are descaled by pickling, cold-rolled at a reduction rate of 50%, heated to 730 ° C at an average heating rate of 12 ° C / s in a hot dip galvanizing line, and subsequently 110 seconds at 820 ° C. After soaking, cooling to 525 ° C at an average cooling rate of 10 ° C / s and annealing, dip galvanizing treatment is performed by immersing in a 475 ° C galvanizing bath containing 0.13% Al for 3 s. A galvanization with an adhesion amount of 45 g / m ² was formed. Subsequently, galvanizing alloying treatment was performed at 550 ° C. to produce a hot dip galvanized steel strip. At this time, the weight of the hot dip galvanized steel strip was about 15 tons, and the length of the steel strip was 1300 m.

  And TS was measured by the above methods along the longitudinal direction of the hot-dip galvanized steel strip.

  FIG. 3 shows the variation in TS in the longitudinal direction of the hot dip galvanized steel strip. When the coiling temperature is less than Tct ° C, which is 500 ° C, that is, when the area ratio of the bainite phase in the entire structure is 30% or more, the difference between the maximum TS and the minimum TS in the longitudinal direction is 60 MPa or less. It can be seen that the variation in TS within the belt is small.

  At the same time, the following i) and ii) were also revealed.

  i) After optimizing the component composition, a hot-dip galvanized steel strip having a microstructure in which the area ratio of the ferrite phase in the entire structure is 50% or more and the area ratio of the martensite phase is 30 to 50% Therefore, excellent elongation characteristics and hole expandability can be obtained with a TS of 780 MPa or more.

ii) The microstructure of the hot dip galvanized steel strip includes a bainite phase and a ferrite phase, and the area ratio of the ferrite phase in the entire structure is 0 to 70%, and the area ratio of the bainite phase is 30 to 100%. The hot-rolled steel strip having the structure is cold-rolled at a reduction rate of 40% or higher, and then heated to a temperature range of 700 ° C. or higher at an average heating rate of 5 ° C./s or higher, and subsequently (Ac ₃ transformation point −100) to (Ac ₃ transformation point -20) 30~500s soaking in a temperature range of ° C., after annealing is cooled to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C. / s, it is subjected to galvanizing treatment Obtained by.

  The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.5%, Mn: 1.5 to 3.0%, P: 0.001 to 0.05%, S: 0.0001 ~ 0.01%, Al: 0.001 ~ 0.1%, N: 0.0005 ~ 0.01%, the balance is composed of Fe and inevitable impurities, and contains ferrite phase and martensite phase. It has a microstructure in which the area ratio of the ferrite phase is 50% or more, the area ratio of the martensite phase is 30 to 50%, and the difference between the maximum tensile strength and the minimum tensile strength in the steel strip is 60 MPa. Provided is a high-strength hot-dip galvanized steel strip that is excellent in formability with small variations in material within the steel strip, characterized by the following.

  In the high-strength hot-dip galvanized steel strip of the present invention, in mass%, Cr: 0.01 to 1.5%, Ti: 0.0005 to 0.1% and B: 0.0003 to 0.003%, It is preferable that at least one element selected from Nb: 0.0005 to 0.05%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 2.0%, alone or in combination. .

  Moreover, in the high-strength hot-dip galvanized steel strip of the present invention, the galvanizing can be alloyed galvanizing.

The high-strength hot-dip galvanized steel strip of the present invention has, for example, the above component composition, and includes a ferrite phase and a bainite phase, and the area ratio of the ferrite phase in the entire structure is 0 to 70%, The hot rolled steel strip having a microstructure with an area ratio of the bainite phase of 30 to 100%, after cold rolling at a reduction rate of 40% or more, a temperature range of 700 ° C or more with an average heating rate of 5 ° C / s or more And then soaked for 30 to 500 s in the temperature range of (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) ° C., and a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s. It can manufacture by the method of cooling and cooling to galvanizing treatment.

Moreover, the high-strength hot-dip galvanized steel strip of the present invention is a steel slab having the above component composition, heated to a heating temperature of 1150 to 1300 ° C, and then hot-rolled at a finishing temperature of 800 to 950 ° C, Cool to 600 ° C or less at an average cooling rate of 50 ° C / s or more within 2 s after hot rolling, wind at a coiling temperature of less than Tct ° C as defined above, and cold at a reduction rate of 40% or more After rolling, the steel is heated to a temperature range of 700 ° C or higher at an average heating rate of 5 ° C / s or higher, and then continuously in the temperature range of (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) ° C for 30 to 500 s. It can also be manufactured by a method in which it is heated, cooled to a temperature range of 600 ° C. or lower at an average cooling rate of 3 to 30 ° C./s, annealed, and then subjected to a hot dip galvanizing treatment.

  In the manufacturing method of the high-strength hot-dip galvanized steel strip of the present invention, after the hot-dip galvanizing treatment, galvanizing alloying treatment can also be performed in a temperature range of 450 to 600 ° C.

  The steel strip having a small variation in material within the steel strip of the present invention is intended for a steel strip that becomes a product in a state of being wound in a coil shape having a weight of 5 t or more and a width of 500 mm or more. Also, the difference between the maximum tensile strength and the minimum tensile strength in the steel strip is the innermost and outermost coils of the coil corresponding to the front and rear ends in the longitudinal direction of the steel strip and both ends in the width direction of the steel strip. This was determined by measuring the TS of each divided portion by dividing at least 10 portions in the longitudinal direction and at least 5 portions in the width direction, excluding a region of 10 mm from the portion.

  According to the present invention, it is possible to manufacture a high-strength hot-dip galvanized steel strip that is inexpensive, has a TS of 780 MPa or more, excellent elongation characteristics and hole expandability, and excellent formability with small variations in material within the steel strip. It became so. By applying the high-strength hot-dip galvanized steel strip of the present invention to automobile structural members, it is possible to further improve the safety of passengers and improve fuel efficiency by significantly reducing the weight of the vehicle body.

It is a figure which shows the relationship between coiling temperature and the area ratio of the bainite phase in a hot-rolled steel plate. It is a figure which shows the relationship between the area ratio of the bainite phase in a hot-rolled steel plate, and TS of a hot-dip galvanized steel plate. It is a figure which shows the variation of TS in the longitudinal direction of a hot dip galvanized steel strip.

  Details of the present invention will be described below. Note that “%” representing the content of component elements means “% by mass” unless otherwise specified.

1) Component composition
C: 0.05-0.2%
C is an important element for strengthening steel, has high solid solution strengthening ability, and is an indispensable element for adjusting the area ratio and hardness when utilizing structure strengthening by martensite phase. . If the C content is less than 0.05%, it becomes difficult to obtain a martensite phase having a required area ratio, and the martensite phase does not harden, so that sufficient strength cannot be obtained. On the other hand, if the amount of C exceeds 0.2%, weldability deteriorates and formability is reduced due to the formation of a segregation layer. Therefore, the C content is 0.05 to 0.2%.

Si: 0.5-2.5%
Si is an extremely important element in the present invention. In the cooling process during annealing, Si promotes ferrite transformation and discharges solute C from the ferrite phase to the austenite phase to clean the ferrite phase and improve ductility. At the same time, in order to stabilize the austenite phase, a martensite phase is generated even in a hot dip galvanizing line that is difficult to rapidly cool, thereby facilitating complex organization. In particular, the stabilization of the austenite phase during the cooling process suppresses the formation of a pearlite phase or a bainite phase and promotes the formation of a martensite phase. In addition, Si dissolved in the ferrite phase promotes work hardening and increases ductility, and improves strain propagation at a portion where strain concentrates and improves bendability. In addition, Si solidifies and strengthens the ferrite phase to reduce the hardness difference between the ferrite phase and the martensite phase, suppresses the formation of cracks at the interface, improves local deformability, improves hole expansibility and bending. Contributes to the improvement of sex. In order to obtain the above effects, the Si amount needs to be 0.5% or more. On the other hand, when the Si content exceeds 2.5%, the transformation point is remarkably increased, not only the production stability is inhibited, but also an abnormal structure develops and the moldability is lowered. Therefore, the Si content is 0.5 to 2.5%.

Mn: 1.5-3.0%
Mn is effective for preventing hot embrittlement of steel and ensuring strength. In addition, the hardenability is improved to facilitate complex organization. Furthermore, the proportion of the second phase during annealing is increased, the amount of C in the untransformed austenite phase is decreased, and the local deformation is suppressed by reducing the hardness of the martensite phase in the final product. It contributes to the improvement. At the same time, Mn has the effect of suppressing the formation of pearlite and bainite phases during the cooling process, facilitating transformation from the austenite phase to the martensite phase, and allowing the martensite phase to be generated at a sufficient rate. . In order to obtain such an effect, the Mn content needs to be 1.5% or more. On the other hand, when the amount of Mn exceeds 3.0%, the moldability is deteriorated. Therefore, the Mn content is 1.5 to 3.0%.

P: 0.001 ~ 0.05%
P is an element that has a solid solution strengthening action and can be added according to a desired strength. In addition, it is an element effective for complex organization in order to promote ferrite transformation. In order to obtain such an effect, the P amount needs to be 0.001% or more. On the other hand, if the amount of P exceeds 0.05%, weldability is deteriorated and, when galvanizing is alloyed, the alloying speed is lowered and the quality of galvanizing is impaired. Therefore, the P content is 0.001 to 0.05%.

S: 0.0001 ~ 0.01%
S segregates at the grain boundaries and embrittles the steel during hot working, and also exists as a sulfide and reduces local deformability. Therefore, the amount needs to be 0.01% or less, preferably 0.003% or less, more preferably 0.001% or less. However, due to production technology constraints, the S content needs to be 0.0001% or more. Therefore, the S content is 0.0001 to 0.01%, preferably 0.0001 to 0.003%, more preferably 0.0001 to 0.001%.

Al: 0.001 to 0.1%
Al is an element effective for generating a ferrite phase and improving the balance between strength and ductility. In order to obtain such an effect, the Al content needs to be 0.001% or more. On the other hand, when the Al content exceeds 0.1%, the surface properties are deteriorated. Therefore, the Al amount is 0.001 to 0.1%.

N: 0.0005-0.01%
N is an element that degrades the aging resistance of steel. In particular, when the N content exceeds 0.01%, the deterioration of aging resistance becomes significant. The smaller the amount, the better. However, the amount of N needs to be 0.0005% or more due to restrictions on production technology. Therefore, the N content is 0.0005 to 0.01%.

  The balance is Fe and inevitable impurities, but for the following reasons, Cr: 0.01 to 1.5%, Ti: 0.0005 to 0.1% and B: 0.0003 to 0.003%, Nb: 0.0005 It is preferable that at least one element selected from ˜0.05%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 2.0% is contained alone or in combination.

Cr: 0.01-1.5%
Cr increases the ratio of the second phase during annealing, decreases the amount of C in the untransformed austenite phase, decreases the hardness of the martensite phase in the final product, suppresses local deformation, and suppresses hole expansion and bending. Contributes to the improvement of sex. At the same time, Cr has the effect of suppressing the formation of pearlite and bainite phases from the austenite phase, facilitates transformation from the austenite phase to the martensite phase, and can produce a martensite phase in a sufficient ratio. Become. In order to obtain the above effects, the Cr content needs to be 0.01% or more. On the other hand, if the Cr content exceeds 1.5%, the ratio of the second phase becomes too large, or Cr carbides are excessively produced, leading to a decrease in ductility. Therefore, the Cr content is 0.01 to 1.5%.

Ti: 0.0005-0.1%
Ti forms precipitates with C, S, and N and contributes effectively to the improvement of strength and toughness. In addition, when B is added, since N is precipitated as TiN, precipitation of BN is suppressed, and the effect of B described below is effectively exhibited. In order to obtain such an effect, the Ti amount needs to be 0.0005% or more. On the other hand, if the Ti content exceeds 0.1%, precipitation strengthening works excessively, leading to a decrease in ductility. Therefore, the Ti content is 0.0005 to 0.1%.

B: 0.0003-0.003%
B has a role of suppressing the formation of a pearlite phase and a bainite phase from the austenite phase, improving the stability of the austenite phase, and facilitating martensitic transformation in the cooling process after the hot dip galvanizing treatment. In order to obtain such an effect, the B content needs to be 0.0003% or more. On the other hand, if the amount of B exceeds 0.003%, ductility is reduced. Therefore, the B amount is set to 0.0003 to 0.003%.

Nb: 0.0005-0.05%
Nb has the effect of increasing the strength of the steel sheet, and can be added as necessary to ensure the desired strength. By adding an appropriate amount, the austenite phase generated by reverse transformation at the time of annealing in the hot dip galvanizing line is refined, so that the microstructure after annealing is also refined to increase the strength. Further, fine precipitates are formed during hot rolling or annealing in a hot dip galvanizing line to increase the strength. In order to obtain such an effect, the Nb amount needs to be 0.0005% or more. On the other hand, if the Nb content exceeds 0.05%, the structure becomes excessively fine and a suitable structure described later cannot be obtained. Therefore, the Nb content is 0.0005 to 0.05%.

Mo: 0.01-1.0%, Ni: 0.01-2.0%, Cu: 0.01-2.0%
Mo, Ni, and Cu not only serve as solid solution strengthening elements, but also stabilize the austenite phase in the cooling process during annealing to facilitate complex organization. In order to obtain such effects, the Mo content, Ni content, and Cu content must each be 0.01% or more. On the other hand, if the Mo content exceeds 1.0%, the Ni content exceeds 2.0%, and the Cu content exceeds 2.0%, the plating property, formability, and spot weldability deteriorate. Therefore, the Mo amount is 0.01 to 1.0%, the Ni amount is 0.01 to 2.0%, and the Cu amount is 0.01 to 2.0%.

2) Microstructure Area ratio of ferrite phase in the entire structure: 50% or more The high-strength hot-dip galvanized steel sheet of the present invention is a composite structure in which a hard martensite phase is mainly dispersed in a ductile soft ferrite phase. Consists of. In order to ensure sufficient ductility, a ferrite phase of 50% or more in area ratio in the entire structure is required.

Martensite phase area ratio in the entire organization: 30-50%
The area ratio of the martensite phase is one of the most important requirements in the present invention. In order to achieve a TS of 780 MPa or more, the area ratio of the martensite phase in the entire structure needs to be 30% or more. On the other hand, when the area ratio of the martensite phase exceeds 50%, sufficient ductility cannot be obtained. Therefore, the area ratio of the martensite phase in the entire structure is 30 to 50%.

  Here, the area ratio of the ferrite phase and the martensite phase in the entire structure is 10 field observations at a magnification of about 1000 to 5000 times using the scanning electron microscope (SEM) as described above, and the ferrite phase and martensite. It is the area ratio obtained by identifying the site phase and analyzing it with image analysis software (Image-Pro; manufactured by Cybernetics).

  In addition to the ferrite phase and the martensite phase, the retained austenite phase, the pearlite phase, and the bainite phase are included in the microstructure of the present invention in a total area ratio of 20% or less in the entire structure, The effect of the present invention is not impaired.

3) Manufacturing conditions Hot-rolled steel strip before cold rolling: Including the bainite phase and the ferrite phase, the area ratio of the ferrite phase occupying the entire structure is 0-70%, the area ratio of the bainite phase is 30-100%
As described above, in order to reduce the material variation in the high-strength hot-dip galvanized steel strip, the hot-rolled steel strip before cold rolling includes the ferrite phase and the bainite phase, and the area of the ferrite phase occupying the entire structure It is necessary to have a microstructure in which the rate is 0 to 70% and the area ratio of the bainite phase is 30 to 100%. This means that even if variations occur in the microstructure of the steel strip after hot rolling, cold rolling is possible if the area ratio of the ferrite phase falls within the range of 0 to 70% and the area ratio of the bainite phase falls within the range of 30 to 100%. It is thought that the transformation from the ferrite grain boundaries and the bainite phase grains to the austenite phase is promoted during the soaking of the subsequent annealing, and the material in the steel strip of the final product can be made uniform. . In addition to the ferrite phase and the bainite phase, the total area ratio of the pearlite phase and the martensite phase in the entire structure is less than 30%, that is, the total of the ferrite phase and the bainite phase in the entire structure. If the area ratio exceeds 70%, the effect of the present invention is not impaired.

  Such a hot-rolled steel strip is, for example, a steel slab having the above component composition heated to a heating temperature of 1150 to 1300 ° C, hot-rolled at a finishing temperature of 800 to 950 ° C, and within 2s after hot rolling Can be manufactured by cooling to an average cooling rate of 50 ° C / s or higher to 600 ° C or lower and winding at a winding temperature lower than Tct ° C. Below, the reason for limitation will be described.

Slab heating temperature: 1150-1300 ℃
The slab heating temperature needs to be 1150 ° C. or higher from the viewpoint of securing the temperature during hot rolling. On the other hand, if the heating temperature is too high, problems such as an increase in scale loss accompanying an increase in oxidized weight are caused, so the upper limit of the slab heating temperature is 1300 ° C.

Finishing temperature during hot rolling: 800-950 ° C
The heated slab is hot-rolled by rough rolling and finish rolling to form a hot-rolled steel sheet. At this time, if the finishing temperature is too high, the grains become coarse, the moldability of the final product is lowered, and scale defects are likely to occur. Therefore, the finishing temperature is 950 ° C. or lower. On the other hand, if the finishing temperature is less than 800 ° C, the rolling load increases, the rolling load increases, the rolling reduction rate of the austenite phase in the non-recrystallized state increases, an abnormal texture develops, and the material in the final product It is not preferable from the viewpoint of uniformity. Therefore, the finishing temperature is 800 to 950 ° C, preferably 840 to 920 ° C.

  Note that the slab is made into a sheet bar by rough rolling under normal conditions, but if the heating temperature is low, a bar heater or the like is used before finish rolling from the viewpoint of preventing troubles during hot rolling. It is preferable to heat the sheet bar.

Cooling conditions after hot rolling: Cooling to 600 ° C or less at an average cooling rate of 50 ° C / s or more within 2s.If more than 2 seconds elapse before starting cooling after hot rolling, A ferrite phase is likely to be generated non-uniformly, and a uniform microstructure of a hot-rolled steel sheet mainly composed of a ferrite phase and a bainite phase suitable in the present invention cannot be obtained. The same problem occurs when the average cooling rate is less than 50 ° C./s or when the cooling rate is not reduced to 600 ° C. or lower.

Winding temperature after hot rolling: less than Tct ° C. Tct ° C. defined as above is an empirical formula of the winding temperature empirically derived by the present inventors. By doing so, it is possible to obtain a microstructure of a hot-rolled steel strip mainly composed of a ferrite phase and a bainite phase, which is suitable in the present invention.

  The hot-rolled steel strip having a microstructure mainly composed of a ferrite phase and a bainite phase is heated to a ferrite-austenite two-phase coexistence region or an austenite single-phase region by continuous annealing. After that, it can also be produced by a method of controlling the microstructure by holding it in the bainite formation region.

Thereafter, the hot-rolled steel strip is cold-rolled at a rolling reduction of 40% or more, then heated to a temperature range of 700 ° C. or higher at an average heating rate of 5 ° C./s or more, and subsequently (Ac ₃ transformation point −100) to ( Ac ₃ transformation point -20) Soaked for 30 to 500 s in the temperature range, cooled to 600 ° C or less at an average cooling rate of 3 to 30 ° C / s, annealed, and then subjected to hot dip galvanizing treatment . Below, the reason for limitation will be described.

Rolling ratio during cold rolling: 40% or more When the rolling reduction ratio is less than 40%, the total number per unit volume of grain boundaries and dislocations that become the core of transformation to austenite phase during soaking of the subsequent annealing It becomes difficult to obtain the final microstructure as described above. In addition, non-uniformity occurs in the microstructure, and elongation characteristics and hole expansibility are reduced.

Heating conditions during annealing: Heating to a temperature range of 700 ° C or higher at an average heating rate of 5 ° C / s or higher
By heating to a temperature range of 700 ° C or higher at an average heating rate of 5 ° C / s or higher, recovery during heating and recrystallization of the ferrite phase are suppressed, and the ferrite phase and austenite phase are uniformly dispersed during the next soaking. Therefore, the hole expandability and bendability in the final product can be improved and the microstructure can be made uniform.

Soaking conditions during annealing: (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) ° C for 30 to 500 s soaking At high temperature range where the ferrite phase and austenite phase coexist, Holding in the temperature range of (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) ° C. increases the proportion of austenite phase, suppresses strength reduction due to excessive ferrite phase formation, and makes the austenite phase uniform. scatter. On the other hand, when the soaking temperature is further increased, that is, when (Ac ₃ transformation point −20) ° C. is exceeded, the ferrite phase is not sufficiently formed, the ductility is insufficient, and the desired microstructure cannot be obtained. Therefore, it is necessary to perform soaking in the temperature range of (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) ° C.

  If the soaking time is less than 30 s, the transformation to the austenite phase during heating is not sufficient, and the necessary austenite phase ratio cannot be obtained. When the soaking time exceeds 500 s, the soaking effect is saturated and productivity is hindered.

Cooling conditions during annealing: Cooling to a temperature range of 600 ° C or less at an average cooling rate of 3 to 30 ° C / s After soaking, from 3 to 30 to a temperature range of 600 ° C or less (cooling stop temperature) It is necessary to cool at an average cooling rate of ° C / s. If the average cooling rate is less than 3 ° C / s, ferrite transformation progresses during cooling and the proportion of the martensite phase decreases, leading to a decrease in strength, and the uniformity of the material is impaired by the non-uniformly generated ferrite phase. It is. When the average cooling rate exceeds 30 ° C./s, the effect of suppressing the ferrite transformation is saturated, and the ratio of the martensite phase becomes excessive, leading to a decrease in elongation characteristics and hole expandability.

  In addition, when the cooling stop temperature exceeds 600 ° C, the ratio of martensite phase significantly decreases due to the formation of ferrite phase and pearlite phase, and the area ratio in the entire structure becomes less than 30%. TS cannot be obtained, and the uniformity of the material is impaired by the heterogeneous ferrite phase and pearlite phase.

Hot dip galvanizing treatment: normal conditions After annealing, hot dip galvanization is performed under normal conditions. Moreover, galvanization can be alloyed in the temperature range of 450-600 degreeC. By alloying in the temperature range of 450 to 600 ° C., the Fe concentration during plating becomes 8 to 12%, and the adhesion of plating and the corrosion resistance after coating are improved. If the temperature is lower than 450 ° C, alloying does not proceed sufficiently, leading to a decrease in sacrificial anticorrosive action and sliding property. If the temperature exceeds 600 ° C, alloying proceeds too much and powdering properties are reduced. A large amount of phase, bainite phase, etc. is generated, leading to insufficient strength and poor hole expansibility.

  The conditions of other production methods are not particularly limited, but from the viewpoint of productivity, the series of treatments such as annealing, hot dip galvanization, and galvanization alloying treatment are preferably performed in a continuous hot dip galvanizing line. . In addition, it is preferable to use a galvanizing bath containing 0.10 to 0.20% of Al for hot dip galvanizing. After plating, wiping can be performed to adjust the amount of plating.

Steel Nos. A to I having the composition shown in Table 1 were melted by a converter and made into a slab by a continuous casting method. These steel slabs were heated at 1250 ° C to form hot-rolled steel strips under the hot-rolling conditions shown in Table 2, then pickled, cold-rolled under the cold-rolling conditions shown in Table 2, and using a continuous hot-dip galvanizing line. After annealing under the annealing conditions shown in Table 2, it was immersed in a 475 ° C galvanizing bath containing 0.13% Al for 3 s to form a galvanized coating of 45 g / m ² and alloyed at the temperatures shown in Table 2. Processing was performed to produce galvanized steel strips Nos. 1 to 22. As shown in Table 2, some galvanized steel strips were not alloyed.

And about the longitudinal direction center part and width direction center part of the hot-rolled steel strip and the galvanized steel strip, the microstructure was analyzed by said method. Further, a tensile test was performed by the above method, and TS, El, and TS × El were obtained. Further, a hole expansion test was performed by the following method to measure the hole expansion ratio λ.
Hole expansion test: A test piece of 100 mm × 100 mm was sampled and subjected to a hole expansion test three times in accordance with JFST 1001 (iron standard) to obtain an average hole expansion ratio λ (%).

  Furthermore, the hot-rolled steel strip and the galvanized steel strip are divided into 20 parts in the longitudinal direction and 8 parts in the width direction. The ratio of the points where the area ratio was less than 30% (structural compatibility ratio), and for the galvanized steel strip, the difference ΔTS between the maximum TS and the minimum TS of all the divided parts (189 points) was determined.

  The results are shown in Table 3. The galvanized steel strips of the examples of the present invention all have TS of 780 MPa or more, TS × El ≧ 18000 MPa ·%, a high balance between strength and ductility, excellent elongation characteristics, and λ is 50% or more to expand the hole. It can be seen that this is a high-strength hot-dip galvanized steel strip that has excellent properties and that has a ΔTS of 60 MPa or less and small variations in the material within the steel strip.

Claims (9)

  In mass%, C: 0.05-0.2%, Si: 0.5-2.5%, Mn: 1.5-3.0%, P: 0.001-0.05%, S: 0.0001-0.01%, Al: 0.001-0.1%, N: 0.0005- 0.01% is contained, the remainder has a composition composed of Fe and inevitable impurities, contains a ferrite phase and a martensite phase, and the area ratio of the ferrite phase in the entire structure is 50% or more, and the martens Material variation in the steel strip, which has a microstructure with an area ratio of 30-50% in the site phase, and the difference between the maximum tensile strength and the minimum tensile strength in the steel strip is 60 MPa or less. High-strength hot-dip galvanized steel strip with excellent small formability.   2. The high-strength hot-dip galvanized steel strip excellent in formability with small material variation in the steel strip according to claim 1, further comprising Cr: 0.01 to 1.5% by mass.   Furthermore, it contains at least one element of Ti: 0.0005 to 0.1%, B: 0.0003 to 0.003% in mass%, and the material variation in the steel strip according to claim 1 or 2 is small High-strength hot-dip galvanized steel strip with excellent formability.   Furthermore, the high strength hot dip galvanization excellent in formability with small variation in the material in the steel strip according to any one of claims 1 to 3, characterized by containing Nb: 0.0005 to 0.05% in mass%. Steel strip.   Furthermore, by mass%, it contains at least one element of Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 2.0%, according to any one of claims 1 to 4. A high-strength hot-dip galvanized steel strip with excellent formability with small variations in material within the steel strip.   6. The high-strength hot-dip galvanized steel strip excellent in formability with small material variation in the steel strip according to claim 1, wherein the galvanizing is alloyed galvanizing. The component composition according to any one of claims 1 to 5, and including a ferrite phase and a bainite phase, the area ratio of the ferrite phase occupying the entire structure is 0 to 70%, and the area of the bainite phase A hot-rolled steel strip having a microstructure with a rate of 30 to 100% is cold-rolled at a reduction rate of 40% or more, and then heated to a temperature range of 700 ° C or more at an average heating rate of 5 ° C / s or more. (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) Soaked for 30 to 500 s in the temperature range, cooled to a temperature range of 600 ° C. or less at an average cooling rate of 3 to 30 ° C./s, and annealed Then, the manufacturing method of the high intensity | strength hot-dip galvanized steel strip excellent in the moldability with the small dispersion | variation in the material in the steel strip characterized by performing a hot dip galvanization process. The steel slab having the component composition according to any one of claims 1 to 5, after being heated to a heating temperature of 1150 to 1300 ° C, subjected to hot rolling at a finishing temperature of 800 to 950 ° C, 2 s after the hot rolling Within 50 ℃ / s at an average cooling rate of 600 ℃ or less, winding at a coiling temperature of less than Tct ℃, cold rolling at a rolling reduction of 40% or more, and then average heating of 5 ℃ / s or more Heat to a temperature range of 700 ° C or higher at a speed, then soak in the temperature range of (Ac ₃ transformation point -100) to (Ac ₃ transformation point -20) ° C for 30 to 500 s, and average cooling of 3 to 30 ° C / s A method for producing a high-strength hot-dip galvanized steel strip excellent in formability with small variations in the material within the steel strip, characterized by cooling to a temperature range of 600 ° C or lower and annealing and then hot-dip galvanizing treatment ;
However, Tct = 810-300 × [C] -60 × [Si] -60 × [Mn] -70 × [Cr] -80 × [Mo] -40 × [Ni] -70 × [Cu] [M] represents the content (% by mass) of the element M. When the element M is an unavoidable impurity, [M] = 0.   9. After forming the hot dip galvanizing treatment, alloying treatment of galvanizing is performed in a temperature range of 450 to 600 ° C., and the formability with a small variation in material in the steel strip according to claim 7 or 8 An excellent method for producing high-strength hot-dip galvanized steel strip.

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