JP4959161B2 - Hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet with excellent corrosion resistance, elongation and hole expansibility - Google Patents

Description

The present invention is concerned primarily with the stamped galvanized steel sheet excellent in a suitable corrosion resistance and elongation and hole expandability underbody parts and structural materials such as automobile is used and galvannealed steel sheet .

  In order to improve the corrosion resistance and appearance of automobile parts, reflecting the trend of upgrading automobiles, hot dip galvanized steel sheets are used for many parts. High press workability and strength are required for steel plates used for such automobile members. Known hot-dip galvanized steel sheets having both press workability and high strength include those composed of a ferrite / martensite structure, a ferrite / bainite structure, or those containing retained austenite in the structure. In particular, a composite steel sheet in which martensite is dispersed in a ferrite base has been applied to automobile wheels and the like because it has a low yield ratio, high tensile strength, and excellent elongation characteristics. For example, Patent Documents 1 to 3 disclose a composite structure steel sheet, and Patent Document 4 discloses a hot dip galvanized steel sheet having a composite structure.

However, the conventional composite structure type hot dip galvanized steel sheet is usually manufactured from a slab having a thickness of about 200 mm, and the average cooling rate in the middle part of the slab (1/4 t position of the slab having a thickness t) is: Since it was as small as about 0.1 ° C./sec, the growth of dendrite was large, and therefore, the microsegregation of Mn was large. This microsegregation part is elongated during rolling to form a Mn band, and this part has high hardenability, so that martensite is generated in a band shape during cooling after hot rolling. As a result, stress concentrates on the interface between ferrite and band-shaped martensite and breakage is likely to occur. As described above, the conventional composite structure type hot dip galvanized steel sheet has a defect that the hole expandability is particularly poor because the structure is not uniform.
JP-A-6-128688 JP 2000-319756 A JP 2005-120436 A Japanese Patent Laid-Open No. 9-316592

It is an object of the present invention to provide a composite structure type hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a uniform and fine structure and excellent corrosion resistance, elongation and hole expandability.

The hot-dip galvanized steel sheet excellent in the corrosion resistance, elongation and hole expansibility of the present invention made to solve the above problems is
In mass%
C: 0.01% or more, 0.20% or less, Si: 2.0% or less, Al: 0.010% or more, 2.0% or less, Mn: 0.5% or more, 3.0% or less, P: 0.08% or less, S: 0.010% or less, N: 0.010% or less, a hot-dip galvanized steel sheet having a steel composition composed of the balance iron and inevitable impurities,
The structure is a ferrite martensite structure composed of ferrite having a phase fraction of 50% or more and martensite occupying the balance .
A hot dip galvanizing is performed on a steel sheet in which Mn microsegregation in the range of 1 / 8t to 3 / 8t of the sheet thickness t satisfies the formula (1).
0.10 ≧ σ / Mn (1)
Here, Mn is an addition amount, and σ is a standard deviation in Mn microsegregation measurement.

In the above-described invention, during the steel composition,
Nb: 0.005% or more, 0.10% or less, Ti: 0.03% or more, 0.20% or less, V: 0.005% or more, 0.10% or less, Mo: 0.02% or more, 0.5% or less, Cr: 0.1% or more, 5.0% or less, Co: 0.01% or more, 5.0% or less, W: 0.01% or more, 5.0% or less Or can contain two or more,
Further during the steel composition
One or more of Ca, Mg, Zr, and REM can be contained 0.0005% or more and 0.08% or less,
Further during the steel composition
Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less can do.

  Moreover, the alloyed hot-dip galvanized steel sheet excellent in corrosion resistance, elongation and hole expansibility of the present invention is obtained by subjecting the hot-dip galvanized steel sheet according to any one of claims 1 to 4 to alloying treatment on the steel sheet surface. An alloyed hot-dip galvanized layer is formed.

The hot-dip galvanized steel sheet of the present invention has a small Mn band because of a small Mn microsegregation. Therefore, the martensite generated in the Mn band portion can be made fine to make the steel structure uniform. Therefore, since local stress is not concentrated on the interface between martensite and ferrite, the elongation and hole expansibility are superior to those of conventional hot-dip galvanized steel sheets. Moreover, since the hot dip galvanization is given to the steel plate surface, it is excellent also in corrosion resistance.
Moreover, since the alloyed hot-dip galvanized steel sheet of the present invention is obtained by subjecting the hot-dip galvanized steel sheet to an alloying treatment, the Mn band is small, the hole expandability is excellent, and the corrosion resistance is excellent.

The hot-dip galvanized steel sheet of the present invention is characterized in that the microsegregation of Mn in the range of 1 / 8t to 3 / 8t of the sheet thickness t satisfies the formula (1).
0.10 ≧ σ / Mn (1)
Here, Mn is an addition amount, and σ is a standard deviation in Mn microsegregation measurement. The standard deviation σ was obtained from Mn concentration distribution data obtained by performing line analysis in the plate thickness direction on a sample having a plate thickness polished using EPMA (X-ray microanalyzer).

When σ is 0.10 <σ / Mn, variation in Mn concentration is large, and microsegregation of Mn is not sufficiently small. For this reason, since the microsegregation of Mn is extended in the rolling direction to form a relatively large Mn band, the structure cannot have uniform fine ferrite and martensite. Moreover, since the strength greatly varies in the plate thickness direction, a hot-dip galvanized steel plate excellent in hole expansibility cannot be obtained. Therefore, the microsegregation of Mn must satisfy the relationship of 0.10 ≧ σ / Mn. When the demand for formability is high, it is desirable that the microsegregation satisfies the formula (2). This is because the structure can be made more uniform and the hole expansibility can be improved.
0.05 ≧ σ / Mn (2)
This condition needs to be satisfied in the range of 1 / 8t to 3 / 8t of the plate thickness t with slow cooling.

The reasons for limiting the chemical components in the present invention will be described below.
C is an important element for strengthening the martensite phase and increasing the strength of the steel. If the C content is less than 0.01%, the strength cannot be sufficiently increased. On the other hand, if it exceeds 0.20%, the ductility decreases greatly, so the range of C is 0.01% or more and 0.20% or less. When the demand for hole expansibility is high, the upper limit of C is preferably 0.05%.

  Si is an important element for suppressing the formation of harmful carbides and obtaining a composite structure of the remaining martensite mainly composed of ferrite structure. However, addition of more than 2.0% lowers the ductility and also reduces the chemical conversion property, so the amount of Si added is made 2.0% or less. In addition, when the chemical conversion property requirement is high, Si is desirably 1.3% or less. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, so the lower limit of Si is desirably 0.01%.

  Al is added as a deoxidizer. For this purpose, Al needs to be added in an amount of 0.010% or more. On the other hand, even if Al is added excessively, the above effect is not only saturated, but also the steel is embrittled by an increase in alumina, so the upper limit was made 2.0%. In addition, when the request | requirement of chemical conversion property is high, it is desirable to set it as 1.0% or less.

  Mn is an important element for enhancing the hardenability and strengthening the steel. If Mn is less than 0.5%, the strength cannot be sufficiently increased. However, if Mn exceeds 3.0%, the hardenability is increased more than necessary, so that the strength is increased and the ductility is decreased. If the elongation requirement is high, the amount of Mn added is 2.0% or less.

  When P is contained in a large amount, it segregates to the grain boundary, so that the local ductility is degraded and the weldability is degraded. Therefore, the upper limit is made 0.08%. In addition, since it will lead to the cost increase at the time of refining to reduce P unnecessarily, a minimum is made into 0.001%.

  S is an element that forms MnS and significantly deteriorates local ductility and weldability. Therefore, the upper limit is made 0.010%. Moreover, it is desirable that the lower limit is 0.001% due to the problem of refining costs.

  N is important for refining crystal grains by precipitating AlN or the like. However, if the N content exceeds 0.010%, solid solution nitrogen remains and the ductility decreases, so the upper limit is made 0.010%. In addition, it is desirable that the lower limit is 0.0010% because of cost problems during refining.

  Nb, Ti and V precipitate fine nitrides or carbonitrides to strengthen the steel. Mo, Cr, Co, and W enhance the hardenability and strengthen the steel. For the purpose of strengthening steel, Nb: 0.005% or more, Ti: 0.03% or more, V: 0.005% or more, Mo: 0.02% or more, Cr: 0.1% or more, Co It is necessary to contain 1 type or 2 types or more of: 0.01% or more and W: 0.01% or more. However, Nb: more than 0.10%, Ti: more than 0.20%, V: more than 0.10%, Mo: more than 0.5%, Cr: more than 5.0%, Co: more than 5.0%, Even if W: more than 5.0% is added, the effect of increasing the strength is not only saturated but also the ductility is decreased.

  The steel can further contain one or more of Ca, Mg, Zr, and REM (rare earth elements) alone or in total of 0.0005% to 0.02%. Ca, Mg, Zr, and REM improve the impact characteristics and delayed fracture characteristics by controlling the shapes of oxides and sulfides. For this purpose, it is necessary to add one or more of these elements alone or in total of 0.0005% or more. However, excessive addition deteriorates workability, so the upper limit was made 0.05%.

  Further, the steel is Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less. More than seeds can be contained. Although these elements can also improve the hardenability and increase the strength of the steel, Cu: less than 0.04%, Ni: less than 0.02%, B: less than 0.0003% has an effect of strengthening the steel. small. On the other hand, even if Cu: more than 2.0%, Ni: more than 1.0%, and B: more than 0.007%, the effect of increasing the strength is saturated and the ductility is lowered.

  In addition to the above elements, steel contains elements inevitably mixed such as Sn and As, and is made of the remaining iron.

The hot-dip galvanized high-strength steel sheet of the present invention is composed of ferrite martensite whose structure is mainly composed of ferrite. When the amount of ferrite is small, the ductility decreases greatly, so the ferrite phase fraction is 50% or more and the balance is martensite.

Below, the manufacturing method of the hot dip galvanized steel sheet concerning this invention is demonstrated.
In producing the high strength thin steel sheet of the present invention, the cast slab is cooled at an average cooling rate of 100 ° C./min or more between the liquidus temperature and the solidus temperature. Here, the average cooling rate refers to the average cooling rate in the middle part of the slab (the position of 1/4 t of the slab of thickness t). In the present invention, casting may be performed by any method as long as the cooling rate during solidification can be higher than 100 ° C./min. For example, the thickness of the slab may be reduced in continuous casting, the size of the ingot may be reduced in ingot casting, or a surface layer portion having a high cooling rate may be cut out from a normal slab and used. For example, when the thickness of the continuous cast slab is changed, the thickness of the slab is preferably 100 to 30 mm. This is because when the thickness exceeds 100, the slab cannot be cooled at a sufficiently high cooling rate. When the thickness is less than 30 mm, the casting speed increases, causing fluctuations in the molten metal surface, breakout, etc., and stable slab casting. This is because it becomes difficult.

  When the average cooling rate between the liquidus temperature and the solidus temperature is less than 100 ° C./min, the molten steel cannot be rapidly solidified, and Mn microsegregation is reduced to 0.10 ≧ σ / It cannot be made small enough to satisfy the relationship of Mn. Therefore, the said average cooling rate shall be 100 degrees C / min or more. In particular, when high hole expansibility is required, it is desirable to set it at 200 ° C./min or more in order to further reduce microsegregation.

  The slab after cooling can be directly subjected to hot rolling. Alternatively, when it is cooled to less than 1100 ° C., it can be reheated to 1100 ° C. or higher and 1300 ° C. or lower in a heating furnace such as a tunnel furnace. This is because the deformation resistance in hot rolling is large at a temperature below 1100 ° C., and the generation of scale is large at temperatures exceeding 1300 ° C., and the surface properties of the steel sheet cannot be made favorable.

  Next, the slab is hot-rolled at a finishing temperature of 850 ° C. or higher and 970 ° C. or lower. This is because if the finishing temperature is less than 850 ° C., it becomes (α + γ) two-phase rolling, which may impair the shape of the plate, and if it exceeds 970 ° C., the austenite grain size becomes coarse and ductility decreases. .

  After hot rolling, the steel is cooled to 650 ° C. or less at an average cooling rate of 10 to 100 ° C./sec or more, and wound at a temperature of 650 ° C. or less to form a hot rolled steel sheet. When the cooling rate is less than 10 ° C./sec, pearlite that causes a decrease in ductility is likely to be generated, and the generation of pearlite can be suppressed at a cooling rate of up to 100 ° C./sec. This is because it becomes difficult to control the cooling rate. Further, when the coiling temperature exceeds 650 ° C., ferrite transformation does not proceed sufficiently and pearlite is easily generated, so the coiling temperature is set to 650 ° C. or less.

The hot-rolled steel sheet produced as described above is cold-rolled at a reduction rate of 40% or more after pickling, and the maximum temperature is 0.1 × (Ac ₃ -Ac ₁ ) + Ac ₁ or more, Ar ₃ + 50 ° C. or less. After annealing at a temperature of 0.1 to 100 ° C./sec, the steel is cooled to 300 ° C. or lower, and subsequently held in the same temperature range for 1 to 1000 seconds, thereby providing corrosion resistance, elongation and hole expansibility. It is possible to produce a hot dip galvanized steel sheet.

In the production of a cold-rolled steel sheet, if the rolling reduction is less than 40%, crystal grains after annealing cannot be made fine, so the rolling reduction is set to 40% or more.
Further, the maximum temperature of the _{annealing, 0.1 × (Ac 3 -Ac 1} ) + Ac 1 (℃) or higher, it is necessary to _Ar 3 + 50 ° C. or less. When the maximum temperature is less than 0.1 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, so that a desired amount of martensite is generated in the steel sheet. I can't. In addition, increasing the annealing temperature promotes the formation of a grain boundary oxide layer and increases the manufacturing cost, so that the upper limit of the annealing temperature is Ar ₃ + 50 ° C.

  Cooling after annealing is important for precipitating ferrite and ensuring the desired amount of untransformed austenite. Setting this cooling rate to less than 0.1 ° C / sec promotes the formation of a grain boundary oxide layer, and also has manufacturing disadvantages such as lengthening the required production line length and extremely slowing the production rate. Arise. Further, since ferrite transformation and pearlite transformation proceed and untransformed austenite cannot be left, the lower limit of the cooling rate was set to 0.1 ° C./sec. On the other hand, when the cooling rate exceeds 100 ° C./sec, the ferrite transformation cannot sufficiently proceed, so the cooling rate after annealing is 0.1-100 ° C./sec.

  Then, it can cool to 300 degrees C or less or more than 500 degreeC, and can hold | maintain for 1 to 1000 seconds in the temperature range. This is because the formation of bainite is slow at temperatures of 300 ° C. or lower or over 500 ° C., so that martensite is easily secured. Also, if the steel sheet is less than 1 second, the residual strain due to thermal shrinkage remains and the elongation decreases, and if it is maintained for more than 1000 seconds, bainite or the like is generated and the target martensite amount cannot be generated. is there.

  On the other hand, the same steel sheet can be produced by holding at 350 ° C. or more and 500 ° C. or less for less than 10 seconds. This is because, in this temperature range, bainite is easily generated, and if held for 10 seconds or longer, bainite is generated in the structure, the martensite structure fraction is reduced, and ductility is reduced.

The cold-rolled steel sheet produced as described above is immersed in a hot dip zinc plating bath for plating. The temperature of the bath is 450 to 475 ° C. If the temperature is lower than 450 ° C, the viscosity of the molten zinc is high and unsuitable for wiping, and bottom dross is likely to occur. On the other hand, if the temperature is higher than 475 ° C, zinc oxide is generated. This is because problems such as an increase in zinc and an increase in the amount of zinc evaporation occur. Martensite is generated during cooling to normal temperature after hot dip galvanization.

  As described above, after cooling the slab at a high speed, a hot-rolled steel sheet is manufactured by controlling the temperature, and after cold-rolling and annealing the hot-rolled steel sheet, by further performing hot dip galvanizing, A hot-dip galvanized steel sheet having a small microsegregation and a uniform ferrite / martensite structure and excellent corrosion resistance, elongation and hole expansibility can be obtained.

The hot dip galvanized steel sheet can be subsequently alloyed at a temperature of 500 to 580 ° C. When the alloying treatment temperature is less than 500 ° C., the alloying does not proceed or the alloying is not progressed sufficiently, and the alloyed hot-dip galvanized layer is not formed on the steel sheet surface, and the workability is inferior η phase This is because it is covered with ζ phase. In addition, when the processing temperature is higher than 580 ° C., alloying progresses too much and the plating adhesion during processing decreases. In this case, martensite is generated during cooling to room temperature after the alloying treatment.

  By alloying the hot dip galvanized steel sheet as described above, an galvannealed steel sheet having excellent corrosion resistance, elongation and hole expandability can be obtained.

Hereinafter, the present invention will be described in detail based on examples.
Steels having chemical components shown in Table 1 were melted in a converter and cast into slabs. At this time, the cooling rate of the solidus temperature was changed as shown in Table 2 from the liquidus temperature at the 1/4 t portion of the slab. These slabs were hot-rolled steel sheets, cold-rolled, and hot-dip galvanized and alloyed to produce alloyed hot-dip galvanized steel sheets, and various properties were investigated. Production conditions and material properties are shown in Table 2. The appearance before the corrosion resistance test was evaluated based on the degree of occurrence of non-plating, scratches and patterns based on the defect occurrence rate on the surface of the hot dip galvanized steel sheet. In addition, the corrosion resistance test was made by scratching the surface of the sample after plating with a cutter knife with a length of 1 cm, repeating a dry / wet cycle test up to 100 cycles, and again reassessing the appearance in five stages according to the degree of rusting. In each of the grades 1 to 5, the appearance of plating was evaluated by visual observation or using a magnifying glass or a microscope for the state of occurrence of non-plating, the state of occurrence of defects of scratches and patterns, and the form of corrosion products. The evaluation index is as follows.
Score 5: No plating, scratches or patterns, almost no rust after corrosion test.
Score 4: Non-plating, scratches and patterns, and rusting after corrosion test is very small (less than several percent in area ratio).
Score 3: Unplated, scratches and patterns, and rusting after corrosion test is small (over several percent in area ratio).
Score 2: Unplated, scratches and patterns, many rusting after corrosion test (over 50% in area ratio).
Grade 1: The plating did not get wet or rust occurred on the entire surface after the corrosion test.

Ac ₁ and Ac ₃ were determined from the following equations. (Reference: “Steel Materials Science”: W. C. Leslie, translated by Koyasu Naruyasu, Maruzen P273)
_{Ac 1 = 723-10.7 × Mn% -16.9} × Ni% + 29.1 × Si% + 16.9 × Cr% + 6.38 × W%.
Ac ₃ = 910−203 × √ (C%) − 15.2 × Ni% + 44.7 × Si% + 104 × V% + 31.5 × Mo% + 13.1 × W% −30 × Mn% −11 × Cr % + 20 × Cu% + 700 × P% + 400 × Al%.

The test results will be described below.
Steels A to J are steels whose chemical components are within the scope of the present invention. On the other hand, C and Mn of steel k are higher than the range of the present invention. Therefore, as shown in Test No. 28, the strength is high but the hole expansion rate is extremely low.
In Steel l, since N is higher than the range of the present invention, the crystal grains are refined and the amount of ferrite increases, and as shown in Test No. 29, the strength and elongation are low.
Since steel m has higher Si and Cr than the range of the present invention, as shown in test number 30, the elongation is low.
Since steel n is high in Nb and Ti, as shown in test number 31, the elongation and the hole expansion rate were low.
Since steels m and n have high Si, as shown in test numbers 30 and 31, the appearance score and the rusting score of the salt spray test word are low.

  For test numbers 7, 8, 16, 20, and 23, the steel has chemical components that are within the scope of the present invention, but in the cooling of the slab during casting, between the liquidus temperature and the solidus temperature. The cooling rate is significantly lower than 100 ° C / min. For this reason, the right side of formula (1), that is, the Mn microsegregation index σ / Mn is larger than 0.1 and a large Mn band is formed, resulting in a non-uniform structure and a steel plate with a low hole expansion rate. I have.

Test No. 2 has a low maximum heating temperature for annealing at 700 ° C. and a low cold rolling rate. For this reason, recrystallization does not proceed sufficiently and elongation is low.
Test No. 10 has a low heating temperature before hot rolling and a cold rolling reduction. For this reason, a crystal grain becomes coarse and elongation is low.

Compared to the comparative examples as described above, those having test numbers 1, 3 to 6, 9, 11 to 15, 17 to 19, 21, 22, 24 to 27 have appropriate chemical components of the test steel. Since various conditions such as slab cooling, hot rolling, annealing, and plating were within the scope of the present invention, Mn microsegregation was small, and a uniform and fine ferrite-martensite structure could be obtained. As a result, an alloyed hot-dip galvanized steel sheet excellent in corrosion resistance, elongation and hole expansibility could be produced.
FIG. 1 shows the elongation of the steel of the present invention in comparison with the comparative steel, and FIG. 2 shows the hole expansion ratio of the steel of the present invention in comparison with the comparative steel. It can be seen that the alloyed hot-dip galvanized steel sheet according to the present invention has excellent elongation and hole expansion ratio with respect to the comparative steel.

It is a graph which shows the elongation of the galvannealed steel plate concerning this invention compared with comparative steel. It is a graph which shows the hole expansion rate of the galvannealed steel plate based on this invention compared with comparative steel.

Claims (5)

In mass%
C: 0.01% or more, 0.20% or less, Si: 2.0% or less, Al: 0.010% or more, 2.0% or less, Mn: 0.5% or more, 3.0% or less, P: 0.08% or less, S: 0.010% or less, N: 0.010% or less, a hot-dip galvanized steel sheet having a steel composition composed of the balance iron and inevitable impurities,
The structure is a ferrite martensite structure composed of ferrite having a phase fraction of 50% or more and martensite occupying the balance .
Corrosion resistance, elongation and hole expansion, characterized by hot dip galvanizing applied to steel sheets whose Mn microsegregation in the range of 1 / 8t to 3 / 8t of thickness t satisfies the formula (1) Hot-dip galvanized steel sheet with excellent properties.
0.10 ≧ σ / Mn (1)
Here, Mn is an addition amount, and σ is a standard deviation in Mn microsegregation measurement. Further during the steel composition
Nb: 0.005% or more, 0.10% or less, Ti: 0.03% or more, 0.20% or less, V: 0.005% or more, 0.10% or less, Mo: 0.02% or more, 0.5% or less, Cr: 0.1% or more, 5.0% or less, Co: 0.01% or more, 5.0% or less, W: 0.01% or more, 5.0% or less Or the hot-dip galvanized steel plate excellent in corrosion resistance, elongation, and hole expansibility of Claim 1 characterized by containing 2 or more types. Further during the steel composition
It contains 0.0005% or more and 0.05% or less of one or more of Ca, Mg, Zr, and REM, and is excellent in corrosion resistance, elongation, and hole expansibility according to claim 1 or 2. Hot dip galvanized steel sheet. Further during the steel composition
Cu: 0.04% or more, 2.0% or less, Ni: 0.02% or more, 1.0% or less, B: 0.0003% or more, 0.007% or less A hot-dip galvanized steel sheet excellent in corrosion resistance, elongation and hole expansibility according to any one of claims 1 to 3.   The galvanized steel sheet according to any one of claims 1 to 4 is subjected to an alloying treatment to form an alloyed galvanized layer on the surface of the steel sheet, and is excellent in corrosion resistance, elongation and hole expansibility Alloyed galvanized steel sheet.

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