JP4370795B2 - Method for producing hot-dip galvanized steel sheet - Google Patents

Description

【００６３】
【表２】

【００６４】
【表３】

【００６５】
【００６６】
【発明の効果】
本発明によれば、自動車用鋼板として最適な耐二次加工脆性およびめっき表面外観に優れた７８０ＭＰａ以上の高強度溶融亜鉛めっき鋼板が得られ産業上極めて有用である。
【図面の簡単な説明】
【図１】耐二次加工脆性評価試験方法（カップ成形材による縦割れ試験）を模式的に示す図。
【図２】縦割れ遷移温度に及ぼすフェライトの平均結晶粒径（ｄＦ）、マルテンサイト、ベイナイトの平均結晶粒径（ｄｓ）の影響を示す図。
【図３】縦割れ遷移温度に及ぼすフェライトの平均結晶粒径（ｄＦ）、マルテンサイト、ベイナイトの平均結晶粒径（ｄｓ）及び鋼板強度の影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-dip galvanized steel sheet used for automobile parts and the like, and a method for producing the same, and particularly relates to a sheet having high strength and excellent secondary work brittleness resistance after press forming.
[0002]
[Prior art]
In order to protect the global environment, efforts are being made to reduce CO2 gas in various industries including automobiles and chemical manufacturers. In an automobile company, with the development of an electric vehicle, in order to reduce CO2 gas emitted from the vehicle, it is considered to improve the fuel efficiency of a gasoline vehicle by reducing the weight of the vehicle body.
[0003]
In reducing the weight of the vehicle body, it is effective to reduce the thickness of the steel plate used, but in order to prevent the deterioration of the vehicle body rigidity, it is necessary to improve the strength of the steel plate, and the application of high-strength steel plates to automobiles is being studied.
[0004]
However, when the steel sheet strength is improved, the stress concentration at the grain boundaries increases during press forming, and the press formability deteriorates.For example, it is used for sites where tensile stress is applied, such as stretch forming and stretch flange forming. When it is used, it is necessary to improve the elongation and stretch flangeability, and to improve the resistance to vertical cracking when used for a portion to which compressive stress is applied by drawing.
[0005]
In the case of an ultra-low carbon IF steel plate applied to various parts of automobiles as a high-strength steel plate with excellent deep drawability, the grain boundary is weak, and when the strength is increased with Mn, P, etc., Secondary work brittleness deteriorates, and the addition of B improves the strength of ferrite grain boundaries, reduces P and reduces grain boundary embrittlement.
[0006]
JP-A-6-57373 relates to a technique for improving the longitudinal crack resistance of a high-strength steel sheet. As a technique for producing a high r-value high-tensile cold-rolled steel sheet having excellent secondary work brittleness resistance, P addition is extremely low. In the carbon Ti—Nb—B-based component composition, a steel sheet manufacturing technique is disclosed in which the B content is adjusted within a predetermined range determined by the weighted total amount of Si, Mn, and P, and the secondary work brittleness resistance is excellent.
[0007]
[Problems to be solved by the invention]
However, the strength of the steel sheet described in JP-A-6-57373 is 367.5 to 501.8 MPa (37.5 to 51.2 kgf / mm @ 2), which will be required in the future for high strength for 780 MPa class automobiles. Steel plate manufacturing technology is not disclosed.
[0008]
Therefore, an object of the present invention is to provide a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and excellent in secondary work brittleness resistance after press forming, and a manufacturing technique thereof.
[0009]
[Means for Solving the Problems]
The inventors of the present invention have studied in detail the influence of the microstructure on the press formability using a longitudinal crack test of a cup-formed material for a high-strength steel sheet having a tensile strength of 780 MPa or more. Hard martensite, bainite In the case of a steel sheet containing a low-temperature transformation phase from isoaustenite, the stress concentration in the vicinity of the interface between the ferrite and hard phase during press forming is large, and the ferrite phase that is effective in ultra-low-carbon IF steel with a ferrite single-phase structure is effective. Strengthening and improving grain boundary strength do not improve secondary work embrittlement resistance, and as a countermeasure against this, refinement of crystal grain size is effective. If the average crystal grain size is 5 μm or less, It was found that the stress concentration on the grain boundary was relaxed and the secondary work brittleness resistance was improved.
[0010]
The present invention has been made by further study based on these findings, that is, the present invention,
1. In mass (%), C: 0.03 to 0.15%, Si ≦ 0.7%, Mn: 2.0 to 3.0%, P ≦ 0.05%, S ≦ 0.01%, sol . Al: 0.01-0.1%, N ≦ 0.005%, Nb: 0.005-0.1%, the average cooling rate after hot-rolling a slab having a component composition consisting of the balance Fe and inevitable impurities It is cooled at 5 to 500 ° C./s, wound at a temperature of 680 ° C. or lower, annealed at a temperature of 750 to 950 ° C. after pickling or pickling / cold rolling, and then continuously hot dip galvanized. A method for producing a hot-dip galvanized steel sheet having excellent secondary work brittleness resistance .
[0011]
2. The slab further contains one or more of Ti: 0.005 to 0.1%, V: 0.01 to 0.5%, and B: 0.0002 to 0.002%. 2. A method for producing a hot-dip galvanized steel sheet having excellent secondary work brittleness resistance according to 1.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The component composition, microstructure, etc. in the present invention will be described in detail below.
1. Ingredient composition C
C is effective for strengthening steel and is added by 0.03% or more. On the other hand, if it exceeds 0.15%, stretch flangeability and deep drawability deteriorate, so 0.03 to 0.15%.
[0015]
Si
Si may be added within a range of 0.7% or less in order to strengthen the steel. If it exceeds 0.7%, the adhesiveness of hot dip galvanizing is lowered, and a non-uniform plating film is formed. When the plating surface is uneven, stress concentration occurs on the steel plate surface during deep drawing, which is not preferable for secondary work embrittlement resistance after forming. Therefore, when Si is added, the content is made 0.7% or less.
[0016]
Mn
Mn is effective for strengthening the quenching of steel and is added in an amount of 2.0% or more in order to obtain a tensile strength of 780 MPa or more. On the other hand, if it exceeds 3.0%, surface defects are likely to occur in the slab, and the surface appearance after the rolling and hot dip galvanizing treatment is significantly deteriorated.
[0017]
P
Since P is effective for strengthening steel, 0.05% or less can be contained in the present invention. If it exceeds 0.05%, a non-uniform structure resulting from segregation of P during casting develops in the central portion of the plate thickness, and ductility deteriorates. Therefore, P is set to 0.05% or less.
[0018]
S
S is an impurity and, if present excessively in the steel, segregates at the grain boundaries of austenite during slab heating, and red hot brittleness is likely to occur from the surface portion of the steel sheet during hot rolling. And
[0019]
sol. Al
Al is added in an amount of 0.01% or more for deoxidation of steel. On the other hand, if it exceeds 0.1%, the surface appearance after hot dip galvanization is remarkably deteriorated, so 0.01 to 0.1%.
[0020]
N
If N is excessively present in the steel, cracking is likely to occur on the slab surface during casting of molten steel, so 0.005% or less.
[0021]
Nb
Nb is added in an amount of 0.005% or more in order to form fine carbides and to exist in a solid solution state and to refine the ferrite and the low-temperature transformation phase. On the other hand, if it exceeds 0.1%, recrystallization of ferrite and austenite during annealing is suppressed, the processed structure remains, and the ductility deteriorates, so the content is made 0.005 to 0.1%.
[0022]
In the present invention, the basic component composition is as described above , and one or more of Ti, V and B can be added according to the desired properties.
[0023]
Ti
Ti is added in an amount of 0.005% or more in order to form fine carbides and make the structure fine. On the other hand, if it exceeds 0.1%, the effect is saturated and the appearance of the plating surface deteriorates, so 0.005 to 0.1%.
[0024]
V
V is added in an amount of 0.01% or more in order to improve the hardenability of the steel and improve the strength. On the other hand, if it exceeds 0.5%, the effect is saturated, so the content is made 0.01 to 0.5%.
[0026]
B
B improves the hardenability of the steel, improves the strength, segregates at the austenite grain boundaries, and refines the low-temperature transformation phase by suppressing grain growth, so 0.0002% or more is added. On the other hand, if it exceeds 0.002%, the effect is saturated, so the content is made 0.0002 to 0.002%.
[0027]
In the present invention, “the balance is substantially Fe” means that the inclusion of an element not described as a component composition of the present invention is allowed as long as the effects of the present invention are not impaired. To do.
[0028]
2. Average grain size of both microstructured ferrite and low-temperature transformation phase: 5 μm or less The steel sheet of the present invention is a composite structure having ferrite and martensite and / or bainite, and in particular, a hard low-temperature transformation phase from ferrite and austenite in the soft phase It is characterized in that both the martensite and / or bainite average crystal grain sizes of the phases have a fine structure of 5 μm or less.
[0029]
FIG. 2 shows the results of investigating the influence of the average grain size of ferrite (parent phase) and martensite and / or bainite (second phase) on the secondary work brittleness resistance of the steel sheet in a longitudinal crack test.
[0030]
In the longitudinal crack test, the component composition of the test steel is C: 0.050 to 0.075%, Si: 0.02 to 0.26%, Mn: 2.1 to 2.5%, P: 0.01 -0.03%, S: 0.001-0.010%, sol. Al: 0.02-0.06%, N: 0.0015-0.0045%, Nb: 0.02-0.06%, Cr: 0.07-0.15%, V: 0.07- 0.14%, balance is substantially Fe, tensile strength (TS): 810 to 840 MPa, average particle diameter (dF) of ferrite (matrix): 1 to 11 μm, average of low temperature transformation phase (second phase) Crystal grain size (dS): A hot-dip galvanized steel sheet having a thickness of 1 to 9 μm (plate thickness: 1.2 mm) was used.
[0031]
From these steel plates, as shown in FIG. 1, as a test sample, a 120 mmφ blank was collected, a 75 mmφ cup was formed at a drawing ratio of 1.6, trimmed to a cup height of 30 mm, and then in a refrigerant. A cup opening test was conducted, and the critical crack critical temperature (Tc) at which vertical cracks did not occur in the side wall of the cup was determined.
[0032]
As is clear from FIG. 2, when the average grain size (dF) of the ferrite (parent phase) is 5 μm or less and the average crystal grain size (dS) of the low temperature transformation phase (second phase) is 1 to 3 μm, both are fine, Longitudinal crack critical temperature Tc is as low as −100 to −40 ° C., and good longitudinal crack critical temperature (Tc) is obtained.
[0033]
Subsequently, the effects of tensile strength, ferrite and austenite low-temperature transformation phase on the secondary work brittleness resistance were investigated by the same longitudinal crack test of the cup molding material as described above. The resulting temperature was determined. The steel plates used are mass% C: 0.06-0.14, Si: 0.01-0.25, Mn: 2.2-2.7, P: 0.01-0.03, S: 0.001-0.007, sol. It has chemical components of Al: 0.03-0.05, N: 0.0020-0.0040, Nb: 0.02-0.05, Cr: 0.07-0.20, and TS is 800- It is a hot-dip galvanized steel sheet (plate thickness: 1.2 mm) having an average particle diameter of 1100 MPa, an average particle diameter of ferrite of 1 to 9 μm, and an average particle diameter of a second phase transformed from austenite at a low temperature of 1 to 9 μm.
[0034]
The longitudinal crack test results are shown in FIG. 3, organized by tensile strength TS, ferrite average particle diameter d _F , and second phase average particle diameter d _s . As TS increases, Tc increases and secondary work brittleness resistance deteriorates. In particular, when Tc is higher than the straight line in the figure, that is, exceeds 0.1 × TS-108 with respect to TS, Tc cannot stably obtain characteristics of 0 ° C. or lower. In other words, when the average particle diameter d _F of ferrite is as large as 7 to 9 μm (black circle in the figure), Tc does not stably become 0 ° C. or less with TS of 830 MPa or more, and good secondary work brittleness resistance is obtained. I can't. Further, even when the average particle diameter d _F of the ferrite is as small as 2 to 5 μm, when the average particle diameter of the second phase is as large as 6 to 9 μm (× in the figure), Tc is stable at TS of 970 MPa or more. The temperature is not lower than 0 ° C., and good performance cannot be obtained. These are considered to be characteristic deteriorations due to large stress concentration at the ferrite / second phase interface during press molding. On the other hand, when Tc is 0.1 × TS-108 or less, good characteristics are obtained. That is, when the average particle diameter d _F of the ferrite is 3 to 5 μm and the average particle diameter of the second phase is as small as 1 to 5 μm (Δ in the figure), Tc is 0 ° C. or less in the range of 800 to 1070 MPa. Low temperature characteristics are obtained. Furthermore, when the average particle diameter d _{F of} ferrite is 1 to 3 μm (excluding the upper limit of 3 μm), and the average particle diameter of the second phase is as small as 1 to 4 μm (◯ in the figure), a TS of 800 to 1100 MPa In this range, Tc is as low as −90 to −40 ° C., and very good characteristics are obtained.
[0035]
Thus, in the hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more having a hard phase such as ferrite, martensite, and bainite, in order to improve the secondary work brittleness resistance, all of the low-temperature transformation phases of the ferrite phase austenite are averaged. In this steel plate, the longitudinal crack transition temperature Tc of the cylindrical deep drawing material having a drawing ratio of 1.6 is 0.1 × expressed by a relational expression with TS. Since it has a low temperature characteristic of TS-108 or lower, it has been clarified that it can be used in a cold region where the use environment is severe even after being press-molded into a part having a high degree of processing.
[0036]
3. Hot Rolling Conditions In the present invention, in order to obtain a fine structure having an average crystal grain size of 5 μm or less, a cooling rate after hot rolling, a coil winding temperature, and an annealing temperature are defined.
[0037]
When the average cooling rate after hot rolling is less than 5 ° C / s, the crystal grain size after winding is coarsened, and the transformation structure from ferrite and austenite is not refined even by annealing, and the average crystal grain size Is not less than 5 μm.
[0038]
On the other hand, if it exceeds 500 ° C./s, the effect of miniaturization by cooling is saturated, special equipment is required for cooling, and the equipment load increases, so the average cooling rate is 5 to 500 ° C./s.
[0039]
If the coil winding temperature exceeds 680 ° C., the structure of the plate thickness surface layer becomes coarse, and after annealing, the structure becomes non-uniform in the plate thickness direction, and a fine structure with an average crystal grain size of 5 μm or less cannot be obtained. The temperature is 680 ° C. or lower.
[0040]
In the production of the steel of the present invention, the method for melting and casting the steel is not particularly limited as long as component segregation and structural uniformity can be obtained. Hot rolling is performed immediately after casting, or after cooling and reheating, rough rolling and finish rolling, and then coiling.
[0041]
In order to measure the homogeneity of the structure in the plate thickness direction, the finish rolling finish temperature is preferably set to Ar3 point or higher.
[0042]
After hot rolling, pickling, cold rolling as necessary, and continuous hot dip galvanizing treatment. As for the annealing conditions, the annealing temperature is set to 750 to 950 ° C. so as not to become a coarse structure. More preferably, the temperature is 750 to 900 ° C. The hot dip galvanizing conditions are not particularly defined, and after the galvanization, the plating layer can be alloyed.
[0043]
【Example】
(Example 1)
The effects of the present invention were confirmed using steels having various component compositions. Table 1 shows the chemical composition of the test steel .
[0044]
Each steel was melted in a laboratory and then cast to produce a slab having a thickness of 50 mm. However, the steel 7 was melted in an electric furnace. The slab was batch-rolled to a plate thickness of 30 mm, subjected to heat treatment at 1250 ° C. × 1 hr in an atmospheric furnace, and subjected to hot rolling.
[0045]
Finish rolling was finished at 870 ° C., cooled at an average cooling rate of 30 ° C./s, and then subjected to a heat treatment equivalent to winding at 550 ° C. × 1 hr to obtain a hot rolled sheet having a thickness of 4 mm. After pickling, it is cold-rolled to a thickness of 1.2 mm, soaked at 830 ° C. for 180 seconds, cooled at an average cooling rate of 5 ° C./sec, immersed in a 460 ° C. hot dip galvanizing bath, and then zinc plated at 550 ° C. The plating layer was alloyed. Thereafter, temper rolling was performed at an elongation rate of 1.0%. The obtained galvanized steel sheet was evaluated for tensile properties, secondary work brittleness resistance, plated surface appearance and microstructure observation.
[0046]
In the tensile test, the tensile strength (TS) was 780 MPa or more with good properties (indicated by a circle in the table) and the strength was insufficient (indicated by x in the table) by a method based on JISZ2241.
[0047]
The secondary work brittleness resistance is good when the longitudinal crack transition temperature (Tc) is 0.1 × TS-108 or less in the longitudinal cracking test of the cup molding material (indicated by ○ in the table), 0.1 × TS. When it exceeded -108, it was regarded as a characteristic defect (indicated by x in the table).
[0048]
The plating surface appearance was visually observed in a range of width 100 mm × length 1500 mm, and when non-plating, dot-like and streak-like defects were observed, it was determined that the surface property was poor (x).
[0049]
The average crystal grain size of the ferrite and the low-temperature transformation phase was determined for each of 200 pieces by observing the cross-sectional structure of the steel sheet with a scanning electron microscope of 1500 times.
[0050]
Table 2 shows these results. Steel plate No. Nos. 1 to 7 use steel numbers 1 to 7, the tensile strength (TS) is 795 to 1010 MPa, the average crystal grain size of ferrite is 1 to 3 μm, and the average crystal grain size of the low temperature transformation phase is 2 to 4 μm. The crack transition temperature (Tc) is also 0.1 × TS-108 or lower, a low temperature of −90 to −35 ° C., and good secondary work brittleness resistance is obtained. Moreover, it was favorable plating surface property.
[0051]
On the other hand, Comparative Example No. Nos. 8 to 14 are comparative steel Nos. Whose component compositions are outside the scope of the present invention. It is an example using 8-14, and it is inferior to a characteristic compared with the example of the present invention.
[0052]
Comparative Example No. As for 8 and 9, TS is as low as 680 MPa and 645 MPa. Comparative Example No. In No. 10, TS was 810 MPa, ferrite, and the average crystal grain size of the low-temperature transformation phase was 3 μm and 5 μm, respectively, but unplated defects were observed. Moreover, the longitudinal crack transition temperature after deep drawing is as high as −10 ° C.
[0053]
Comparative Example No. No. 11, TS was 840 MPa, ferrite, and the average crystal grain size of the low temperature transformation phase was 5 μm or less, and the longitudinal crack transition temperature after deep drawing was as good as −50 ° C. Was observed.
[0054]
Comparative Example No. No. 12 has a TS of 830 MPa, but the average crystal grain size of the ferrite and the low-temperature transformation phase is both 6 μm, the longitudinal crack transition temperature is as high as 30 ° C., and the secondary work brittleness resistance is poor.
[0055]
Comparative Example No. No. 13 has a TS of 930 MPa, but since it contains a large amount of Nb, there is a processed structure in the ferrite, and the longitudinal crack transition temperature after deep drawing is 60 ° C. Inferior.
[0056]
Comparative Example No. Since 14 contains a large amount of C, TS is 1060 MPa, but it is inferior in deep drawability.
[0057]
(Example 2)
Next , using steel No. 7, the influence of manufacturing conditions was investigated. Steel No. 7 was rolled out in an electric furnace to form a slab having a plate thickness of 220 mm, heated for 1 hour at 1270 ° C., roughly rolled, and finish-rolled at 880 ° C.
[0058]
Among these, using one hot-rolled material, after finishing rolling, after changing the cooling rate to the coil winding temperature to an average of 2 to 500 ° C./s in the rolling direction, winding the coil at 600 ° C., A hot rolled material having a plate thickness of 4 mm was produced. The other three hot-rolled materials were finished-rolled, cooled to the coil winding temperature at an average rate of 20 ° C./s, and then wound into coils at 550, 650, and 700 ° C. A rolled coil was used. The hot-rolled coil was pickled and then cold-rolled to a thickness of 1.4 mm. The cold-rolled plate was heated to 860 ° C. and cooled at an average rate of −5 to −10 ° C./s, and then 460 ° C. Then, it was immersed in a hot dip galvanizing bath, then alloyed at 550 ° C., and cooled to room temperature. Further, after the plating, each annealed plate was subjected to temper rolling with an elongation rate of 1.0%. In each annealing coil, from the position corresponding to the location where the cooling rate and coiling temperature until coiling during hot rolling were changed, the tensile test, the longitudinal crack test of the cup molding material, the plating surface and the cross-sectional structure Samples used for observation were collected, and characteristics were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 3.
[0059]
Steel plate No. No. 21 after finish rolling, the average cooling rate to the coiling temperature is 2 ° C./s, which is slow outside the scope of the present invention, the average particle size of ferrite and the low-temperature transformation phase is large, exceeding 5 μm, and the longitudinal crack transition temperature is as high as 50 ° C. Ru Tei inferior in resistance to secondary work brittleness.
[0060]
Steel plate No. Nos. 22 to 28 are excellent in tensile strength, secondary work brittleness resistance and plating surface appearance within the scope of the present invention, after the finish rolling, the average cooling rate up to the coiling temperature and the coiling temperature.
[0061]
Steel plate No. 29 is a high present invention range coiling temperature is 700 ° C., since the average particle size 7μm ferrite is large, the vertical cracks transition temperature Ru Tei poor high resistance to secondary work embrittlement and 30 ° C..
[0062]
[Table 1]

[0063]
[Table 2]

[0064]
[Table 3]

[0065]
[0066]
【The invention's effect】
According to the present invention, a high-strength hot-dip galvanized steel sheet having a strength of 780 MPa or more, which is excellent in secondary work brittleness resistance and plated surface appearance, which is optimal as a steel sheet for automobiles, is obtained and is extremely useful industrially.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a secondary work brittleness resistance evaluation test method (longitudinal crack test using a cup molding material).
FIG. 2 is a graph showing the influence of the average crystal grain size (dF) of ferrite and the average crystal grain size (ds) of martensite and bainite on the longitudinal crack transition temperature.
FIG. 3 is a graph showing the influence of the average crystal grain size (dF) of ferrite, the average crystal grain size (ds) of martensite and bainite, and the steel sheet strength on the longitudinal crack transition temperature.

Claims (2)

In mass (%), C: 0.03 to 0.15%, Si ≦ 0.7%, Mn: 2.0 to 3.0%, P ≦ 0.05%, S ≦ 0.01%, sol . Al: 0.01-0.1%, N ≦ 0.005%, Nb: 0.005-0.1%, the average cooling rate after hot-rolling a slab having a component composition consisting of the balance Fe and inevitable impurities It is cooled at 5 to 500 ° C./s, wound at a temperature of 680 ° C. or lower, annealed at a temperature of 750 to 950 ° C. after pickling or pickling / cold rolling, and then continuously hot dip galvanized. A method for producing a hot-dip galvanized steel sheet having excellent secondary work brittleness resistance. The slab further contains one or more of Ti: 0.005 to 0.1%, V: 0.01 to 0.5%, and B: 0.0002 to 0.002%. The method for producing a hot-dip galvanized steel sheet having excellent secondary work brittleness resistance according to claim 1.

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