JP4788291B2 - Manufacturing method of high-strength hot-dip galvanized steel sheet with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet excellent in stretch flangeability suitable for construction members, machine structural parts, automobile structural parts, and the like.

High-strength hot-dip galvanized steel sheets having a tensile strength exceeding 440 MPa have advantages of excellent rust prevention and high yield strength, and are widely applied to construction members, machine structural parts, automobile structural parts, and the like. In particular, in the automobile industry, from the viewpoint of improving fuel efficiency for the purpose of reducing CO ₂ and ensuring the safety of passengers, both the improvement of vehicle body rigidity and weight reduction are actively promoted by actively utilizing high-strength steel sheets.

  In response to such industrial needs, material manufacturers are working on the development of materials with higher strength and higher workability. Among them, a high strength hot dip galvanized steel sheet having a tensile strength exceeding 590 MPa has many proposals regarding a hot dip galvanized steel sheet based on DP (Dual Phase) steel, which has a large lightening effect and is advantageous in terms of ductility. .

  On the other hand, in the case of automobile parts, flange molding cannot be omitted in assembling the parts. Therefore, no matter how high the material strength is, stretch flange formability is essential as a material for automobiles.

However, since DP steel is formed of a composite structure of soft ferrite and hard martensite, it generally has a problem that stretch flange formability is poor.
As a technique for solving such problems and obtaining a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, for example, Patent Document 1 and Patent Document 2 have been proposed.

  The technique disclosed in Patent Document 1 attempts to improve stretch flange formability by controlling the component composition in DP steel, but includes tensile tension, hole expansion ratio that is an index of stretch flange formability, and the like. However, a sufficient level of characteristics is not always obtained for the immediate needs.

In the technique disclosed in Patent Document 2, since the second phase in the steel sheet structure is mainly composed of bainite, the stretch flange formability is good, but the ductility is sacrificed and a highly versatile steel sheet is obtained. Absent.
JP-A-4-236671 JP-A-4-173945

  The present invention has been made in view of such circumstances, and is a high-strength hot-dip galvanized steel sheet that has a DP structure that is advantageous in ductility but usually inferior in stretch flange formability and has excellent stretch flange formability. It aims at providing the manufacturing method of.

In order to solve the above-mentioned problems, the present inventors have found that DP steel plated by a continuous hot dip galvanizing line has a compositional composition and hot-rolling conditions of steel, and further to stretch flangeability exerted by the steel structure obtained thereby. As a result of diligent investigation of the influence, the volume fraction of sulfide, which is the starting point of stretch flange cracking, and its form are controlled, and the structure is refined and the lamellar structure is reduced, so that DP steel can be made extremely stretch flangeable. It has been found that an excellent steel sheet can be obtained.
Specifically, as various proposals have been made so far, the stretch flangeability can be improved to some extent by reducing the S level. However, as a result of detailed examination, in DP steel exceeding 590 MPa class, It is difficult to achieve sufficient improvement in stretch flangeability by simply reducing the S content, and it is essential to refine the structure in addition to reducing S. In order to refine the structure, it is effective to add Nb. However, if the hot rolling conditions are inappropriate even if the structure is refined simply by adding Nb, the non-uniform distribution distributed in the thickness direction of the plate is not possible. A uniform structure develops, and also the desired stretch flangeability cannot be obtained, and it is necessary to appropriately define the hot rolling conditions.

The present invention has been completed based on these findings and provides the following (1) to (3).
(1) By mass%, C: 0.03 to 0.15%, Si: 0.1 to 2% , Mn: 1.7 to 3%, P: 0.05% or less, S: 0.001% Hereinafter, after casting a steel containing Nb: 0.005 to 0.1%, the balance being Fe and inevitable impurities, the hot rolling finish temperature FT is controlled to be equal to or higher than the temperature FT _min determined by the equation (1). Hot rolled, rolled into a hot rolled steel strip at 450 to 650 ° C., pickled, cold rolled, soaked at 800 to 900 ° C. in a continuous hot dip galvanizing line, and 3 ° C./sec or more to a temperature range of 600 ° C. or less at a cooling rate cooling, galvanizing, or further manufacturing how high strength galvanized steel sheet excellent in stretch flange formability, characterized in that performing the alloying treatment.
FT _min (° C.) = 500 × Nb + 20 × Si + 50 × C + 860 (1)
(However, Nb, Si, and C represent mass% of each component)
(2) in mass%, further, Cr: 0.02~0.5%, Mo: 0.02~ 0.1%, V: 1 kind or 2 kinds selected from from 0.02 to 0.5% producing how the excellent high strength galvanized steel sheet stretch flange formability according to the above (1), it characterized in that it contains more.
(3) Elongation as described in (1) or (2) above, further comprising Ti: 0.005 to 0.05% and B: 0.0002 to 0.002% by mass% producing how the excellent high strength galvanized steel sheet flangeability.

  According to the present invention, the component composition of steel is defined in an appropriate range, and it is excellent in spite of being DP steel by hot rolling above a specific temperature determined by the content of Nb, Si, C in the steel component. A high-strength hot-dip galvanized steel sheet having stretch flange formability can be obtained, and the industrial value is extremely high.

  Hereinafter, the present invention will be specifically described by dividing it into chemical components and production methods.

[Chemical composition]
First, chemical components will be described. In the following description, “%” means “% by mass”.
C: 0.03-0.15%
C is an essential element for securing the martensite volume fraction and securing a desired strength, and 0.03% or more is necessary for obtaining the effect. However, when C is contained excessively exceeding 0.15%, the hardness of the martensite phase increases, the hardness difference from the ferrite phase increases excessively, and the stretch flangeability deteriorates. For this reason, C content is made into 0.03 to 0.15%.

Si: 2% or less Si is an effective element for stably obtaining a ferrite + martensite two-phase structure, and the ductility improves as the content increases. However, if it is contained in a large amount exceeding 2%, precipitation of ferrite in a high temperature range is promoted and the structure becomes coarse, which is not preferable. For this reason, Si content shall be 2% or less.

Mn: 1.7-3%
Mn, like C, is an essential element for securing strength, and is indispensable for obtaining high-tensile steel. In addition, in the present invention, the S content is reduced to reduce the volume fraction of MnS, and MnS is precipitated at a high temperature as much as possible to relatively coarsen, thereby suppressing the crack starting point in stretch flange molding. 1.7% or more is necessary. However, if Mn is contained excessively in excess of 3%, a lamellar structure in the thickness direction due to segregation during casting develops and stretch flange formability deteriorates, which is not desirable. For this reason, content of Mn shall be 1.7 to 3%.

P: 0.05% or less P, like Si, is an essential element for stably obtaining a ferrite + martensite two-phase structure. However, if the content exceeds 0.05%, the weldability deteriorates. For this reason, content of P shall be 0.05% or less.

S: 0.001% or less In the present invention, reducing S is extremely important in improving stretch flange formability. If the S content is high, the volume fraction of MnS that is the starting point in stretch flange molding as described above increases, and the desired stretch flange formability cannot be obtained. For this reason, content of S shall be 0.001% or less.

  Al and N are inevitably contained, but the amount of the present invention is not impaired as long as it is contained in ordinary steel. If Al is 0.05% or less and N is 0.007% or less, there is no problem.

Nb: 0.005 to 0.1%
Nb is an essential element in the present invention in that the structure is refined. In order to obtain a sufficient fine effect, 0.005% or more is necessary. On the other hand, if it exceeds 0.1%, the precipitation of NbC becomes non-uniform, and conversely, the development of the lamellar structure is promoted and the stretch flangeability deteriorates. For this reason, Nb content shall be 0.005-0.1%.

One or more selected from Cr: 0.02-0.5%, Mo: 0.02-0.5%, V: 0.02-0.5% In the present invention, in addition to Mn In order to improve the hardenability and stably obtain martensite, one or more elements of Cr, Mo, V may be added as necessary. In order to obtain the above effect, it is necessary that both elements be 0.02% or more. On the other hand, if it exceeds 0.5%, not only is the above effect saturated, but also a lamellar structure in the plate thickness direction due to segregation during casting develops and stretch flangeability deteriorates, which is not preferable. For this reason, when adding Cr, Mo, and V, these content shall be 0.02 to 0.5%, respectively.

Ti: 0.005-0.05%, B: 0.0002-0.002%
In the present invention, the solid solution B is contained in the steel for the purpose of increasing the volume ratio of martensite and increasing the strength, or for the purpose of reducing the precipitation temperature of ferrite and refining the structure as necessary. Ti and B may be added in combination. The reason why Ti and B are added in combination is that N is fixed with Ti to suppress the formation of BN and to secure solid solution B. Therefore, the lower limit of the Ti content is set to 0.005% which is the minimum necessary for fixing N, and the upper limit is set to 0.05% at which the effect is saturated even if it is contained more than this. Further, when the B content is less than 0.0002%, the effect of increasing the strength or refining the structure cannot be obtained, and when the B content exceeds 0.002%, the effect is saturated. For this reason, when adding Ti and B, the Ti content is set to 0.005 to 0.05% and the B content is set to 0.0002 to 0.002%.

[Production method]
In the present invention, after the casting, hot rolling is performed by controlling the hot rolling finishing temperature FT to be equal to or higher than the temperature FT _min determined by the equation (1).
FT _min (° C.) = 500 × Nb + 20 × Si + 50 × C + 860 (1)
(However, Nb, Si, and C represent mass% of each component)
FT _min is a lower limit value of the finishing temperature obtained as a result of intensive studies by the present inventors on the influence of hot rolling conditions on the formation of a layered structure. Such a lower limit exists because when there is an element that increases the deformation resistance during hot, such as Nb, Si, or C, when the temperature falls below a certain critical temperature, the NbC is in parallel with the rolling direction during hot rolling. This is presumably because non-uniform precipitation tends to occur, the layered structure in the thickness direction after annealing becomes prominent, and the effect of reducing the S content or refining the structure cannot be obtained, and stretch flangeability deteriorates. Then, the _{FT min} rises Nb, Si, with increasing C content. This is because when Nb, Si, and C increase, the hot deformation resistance increases, the layered structure tends to be formed, and the hot rolling finishing temperature needs to be raised. In this case, the non-uniformity of NbC occurs because NbC preferentially precipitates at a portion where the accumulation of strain is large, such as the deformation zone of austenite and the vicinity of the grain boundary during hot rolling as the deformation resistance increases. . That is, FT _min means that the above-mentioned non-uniform structure is likely to develop when the hot-rolling finishing temperature is lower than this, and by setting the hot-rolling finishing temperature to be equal to or higher than FT _min , It is possible to obtain good stretch flangeability by suppressing the development. In addition, since heating temperature should just be a temperature which NbC melt | dissolves completely, what is necessary is just to heat in the range of 1100-1300 degreeC.

-Winding temperature: 450-650 ° C
Under the conditions as described above, after hot rolling and cooling, winding is performed at 450 to 650 ° C. This is because if the coiling temperature exceeds 650 ° C., the layered structure due to casting segregation tends to develop, and if it is less than 450 ° C., the precipitation of NbC becomes non-uniform and the layered structure after annealing also tends to develop. It is. In addition, although the cooling rate after hot rolling is not particularly specified, it is desirable to cool at 5 ° C./sec or more because the lamellar structure is likely to develop when the cooling is less than 5 ° C./sec.

  In addition, about the process of making hot-rolled sheet particle size small, such as utilizing high-speed cooling of 100 to 300 ° C./sec within 1 second after the end of hot rolling, or combining this with finishing hot rolling under large pressure, As long as the winding temperature is 450 to 650 ° C., the effect of the present invention is not hindered and allowed.

  The hot-rolled steel strip obtained as described above is pickled, cold-rolled, soaked at 800 to 900 ° C. in a continuous hot-dip galvanizing line, and then cooled at a cooling rate of 3 ° C./sec or more at 600 ° C. Cool to a temperature range of ℃ or less, and perform galvanization or further alloying treatment.

-Soaking temperature: 800-900 ° C
The soaking temperature of the continuous hot dip galvanizing line is set to 800 ° C. to 900 ° C. as described above. This is because if the temperature is less than 800 ° C., the layered structure resulting from the rolled structure remains and stretch flange formability deteriorates, and if it exceeds 900 ° C., the structure becomes coarse and the desired stretch flange formability cannot be obtained.

-Cooling rate: 3 ° C / sec or more The cooling rate after soaking is 3 ° C / sec or more because martensite cannot be obtained if it is less than 3 ° C / sec.

-Cooling temperature: 600 degrees C or less Moreover, when the cooling temperature after soaking exceeds 600 degreeC, pearlite will precipitate. For this reason, the cooling temperature after soaking shall be 600 degrees C or less.

  In the present invention, the slab manufacturing method can achieve the effects of the present invention by either ingot casting or continuous casting. In addition, continuous hot rolling by the hot hot rolling bar connection in hot rolling, temperature rise within 200 ° C. using an induction heater in the hot rolling process, etc. are allowed without affecting the effect of the present invention.

Examples of the present invention will be specifically described below.
First, the results of a basic study leading to the present invention will be described. Here, the relationship between the S content and hot rolling conditions and stretch flange formability was grasped. Inventive component steels 1A to 1C and comparative component steels 1D to 2E, whose components are shown in Table 1, were produced in a converter and made into slabs by continuous casting. These slabs were hot-rolled at the finishing temperature (FDT) shown in Table 2, wound at the winding temperature (CT) shown in Table 2 to form a hot-rolled steel strip having a thickness of 3 mm, and then pickled. It was cold-rolled to a thickness of 4 mm, and subsequently annealed (soaking) under the conditions shown in Table 2 in a continuous hot-dip galvanizing line, and cooled to produce an alloyed hot-dip galvanized steel sheet. The mechanical properties are also shown in Table 2. Stretch flange formability was evaluated by the hole expansion ratio λ measured by the test method defined in the Federation of Iron and Steel.

FIG. 1 shows the influence of the S content on the hole expansion ratio for the steel sheets in Table 2. From FIG. 1, the hole expansion ratio increases by reducing the S content, but in particular, by limiting the hot rolling conditions to the scope of the present invention and reducing the S content to 0.001% or less, It can be seen that the hole expansion rate is significantly improved. On the other hand, No. In Nos. 2, 5, and 7, although the steel components are all within the scope of the present invention, the hot rolling finish temperature is too low, less than FT _min , so that the hole expansion rate is remarkable despite the low S content. Rise is not recognized. No. In Nos. 13 and 14, although the S content and the hot rolling conditions are within the scope of the present invention, Nb is out of the scope of the present invention, so that a high hole expansion ratio is not obtained.

Next, this invention component steel No. which shows a component in Table 3 is shown . 5, 7, 11 (Nos. 4, 6, 8 to 10 are reference component steels) and comparative component steel Nos. 12 to 15 were steeled out in a converter and made into slabs by continuous casting. These slabs were hot-rolled at a finishing temperature (FDT) shown in Table 4 and wound at a winding temperature (CT) shown in Table 4 to form a hot-rolled steel strip having a thickness of 3 mm, and then pickled. It was cold-rolled to a thickness of 4 mm, and subsequently annealed (soaking) under the conditions shown in Table 4 in a continuous hot-dip galvanizing line, and cooled to produce hot-dip galvanized or galvannealed steel sheets. The mechanical properties are also shown in Table 4.

FIG. 2 shows the relationship between the tensile strength and the hole expansion rate for the steels listed in Tables 2 and 4. From this figure, it can be seen that the example of the present invention has an extremely high hole expansion rate compared with the comparative example in a wide strength range of 600 to 1200 MPa.

The figure which shows the relationship between S content rate of a steel plate, and a hole expansion rate. The figure which shows the relationship between the tensile strength of a steel plate, and a hole expansion rate.

Claims (3)

In mass%, C: 0.03 to 0.15%, Si: 0.1 to 2% , Mn: 1.7 to 3%, P: 0.05% or less, S: 0.001% or less, Nb : After the steel containing 0.005 to 0.1% and the balance of Fe and inevitable impurities is cast, hot rolling is performed by controlling the hot rolling finishing temperature FT to be equal to or higher than the temperature FT _min determined by the equation (1) , Rolled at 450 to 650 ° C. to form a hot rolled steel strip, pickled, cold rolled, soaked at 800 to 900 ° C. in a continuous hot dip galvanizing line, and cooled at a cooling rate of 3 ° C./sec or more. A method for producing a high-strength hot-dip galvanized steel sheet excellent in stretch flange formability, characterized by cooling to a temperature range of 600 ° C. or lower and hot-dip galvanizing or further alloying.
FT _min (° C.) = 500 × Nb + 20 × Si + 50 × C + 860 (1)
(However, Nb, Si, and C represent mass% of each component) In mass%, further, Cr: 0.02~0.5%, Mo: 0.02~ 0.1%, V: containing one or more kinds selected from 0.02 to 0.5 percent The method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability according to claim 1.   The stretch flange formability according to claim 1 or 2, further comprising Ti: 0.005 to 0.05% and B: 0.0002 to 0.002% by mass%. A method for producing an excellent high-strength hot-dip galvanized steel sheet.

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