JP4980163B2 - Composite steel sheet having excellent formability and method for producing the same - Google Patents

Description

  The present invention relates to a composite structure steel plate excellent in formability such as elongation and punching hole expandability and a method for producing the same. In particular, the present invention relates to a composite structure steel sheet that is required to have durability in addition to formability, such as an automobile undercarriage part and a road wheel, and a manufacturing method thereof.

  In recent years, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of reducing the weight in order to improve the fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength, they are extremely expensive compared to steel, so their application is limited to special applications. Therefore, in order to promote weight reduction of automobiles in a wider range, application of inexpensive high-strength steel sheets is strongly demanded.

  In response to such demands for high strength, steel sheets that have both strength and deep drawability in the field of cold rolled steel sheets used for white bodies and panels, which occupy about 1/4 of the weight of the vehicle body, The development of bake-hardening steel sheets has been promoted and contributed to the weight reduction of the car body. However, at present, the object of weight reduction has shifted to structural members and suspension members that account for about 20% of the weight of the vehicle body, and the development of high-strength hot-rolled steel sheets used for these members has become an urgent task.

  However, increasing strength generally degrades material properties such as formability (workability), so the key to developing a high-strength steel sheet is how to increase strength without deteriorating material properties. In particular, as the properties required for structural members and steel plates for suspension members, elongation, punching hole expandability, fatigue durability, etc. are important, and it is important how to balance these properties at a high level with high strength. .

  For example, as the characteristics required for steel plates for high-design road wheel discs that have become widespread these days, the stretch formability, punched hole expandability, and fatigue durability are particularly emphasized. In the road wheel disc molding process, the overhanging of spokes and hats, burring in the vicinity of decorative holes (hole expansion processing) are particularly strict, and are managed according to the strictest standards for wheel member characteristics. This is because of fatigue resistance.

  Currently, as a high-strength hot-rolled steel sheet for road wheel discs, a 590 MPa class ferritic / martensitic composite structure steel sheet (so-called Dua1 Phase steel) is used which emphasizes fatigue durability of members and is excellent in fatigue characteristics. . However, in recent years, with the high design of road wheel discs, it has become necessary to form in more complex shapes, so the steel sheet for that purpose has a higher level of formability (elongation, punching hole expandability). It has come to be required. Punching hole expandability is the formability of the punched end face of a steel plate sheared by punching. The formability is determined by the ease of propagation of cracks generated on the punched end face due to deformation. Spreading ratio, λ) ”.

As shown in the schematic diagram of the punching die in FIG. 1A, the punching hole expansion ratio (λ) is first fixed by the punching die 2 and the wrinkle presser 3, and then in FIG. As shown in FIG. 1C, the punch 4 is moved to separate the material 4 into the sheared portion 5 and the sheared portion 6, and the punched hole of the obtained sheared portion 6 is expanded as shown in FIG. 1C. As shown in the figure, the hole diameter is expanded by pushing 9 by the movement 9 of the punch 7, and the expansion ratio of the hole diameter when the crack entering the punched end face 8 penetrates the plate thickness with respect to the initial hole diameter is shown. . That is, it can be expressed by punching hole expansion rate (λ) = (d−d ₀ ) / d ₀ × 100 (%). Here, d means the hole diameter when the punch moves and the crack of the punched end face 8 penetrates the plate thickness, and d ₀ means the initial hole diameter. This test is called a punched hole expansion test. For this punching hole expansion rate (λ), the punching die clearance (Cl) (s / t × 100 (%) in FIG. 1A, where t is the plate thickness and s is the gap) Is also an important influencing factor.

  In order to meet the above-mentioned needs, a hot rolled steel sheet having a TS590 MPa class and elongation = 28%, punching hole expansion ratio = 80% or more, desirably 110% or more is required.

  Furthermore, the punching in the actual part manufacturing process varies between 5 and 20% due to problems such as mold mounting accuracy and mold wear, so it is necessary to achieve the above target within the clearance range. .

  In order to further improve the formability of such a composite steel sheet, by controlling the ferrite fraction and the hardness of the second phase to an appropriate range, the strain at the interface is controlled to expand the hole. An invention that prevents microcracks and improves hole expansibility has been proposed (see, for example, Patent Document 1).

  Further, as a thin steel plate that can obtain press formability comparable to that of a mild steel plate even at a tensile strength of 380 to 540 MPa class, C: 0.01 to 0.1%, Si: 0.6 Steel plate comprising -1.8%, Mn: 0.1-2%, P≤0.1%, S≤0.03%, Al: 0.005-1%, the balance being Fe and inevitable impurities The microstructure is mainly composed of polygonal ferrite, and the volume fraction of the second phase containing carbides of 5 μm or more is 5% or less and the aspect ratio is 2 or less, and the local ductility is excellent. As a hot-rolled steel sheet excellent in work hardenability which improves the work hardenability of a thin steel plate (for example, patent document 2) and a steel plate, and makes it possible to remarkably improve component strength, it is C: 0 by mass%. 0.01 to 0.1%, Si: 0.005 to 1.0%, Mn: 0 2 to 2.5%, P: 0.005 to 0.05%, S: 0.01% or less, Al: 0.001 to 0.1%, N: 0.007 to 0.02% The balance is composed of Fe and unavoidable impurities, and the steel structure is composed of a main phase composed of ferrite and a second phase composed of one or more selected from pearlite, bainite, and martensite, The second phase has a volume fraction of 3 to 30% and an average diameter of 5 μm or less, and the ratio of the average hardness of the second phase to the average hardness of the main phase is 3.0 or less. A hot-rolled steel sheet (for example, see Patent Document 3) excellent in work hardenability has been proposed.

  However, in order to improve the formability of the steel sheet, it is mainly necessary to improve the hole expansibility and elongation (El) of the steel sheet at the same time. However, in the inventions proposed above, the balance between the hole expansibility and the elongation is required. Is not necessarily good, and there is no disclosure of a technique for stably improving the punching hole expansion rate within a wide punching clearance range, and there is a problem that it is not a sufficient material suitable for a high-design wheel disc. is there.

JP 2001-303187 A JP 2003-293082 A JP 2005-298967 A

  In view of the above, an object of the present invention is to provide a composite structure steel plate excellent in formability such as elongation and punching hole expandability, and a method for producing the same.

The present inventors have earnestly studied to simultaneously improve both the hole expandability and elongation (El) of a 590 MPa class composite structure hot-rolled steel sheet. For improvement of hole expandability, the ferrite main phase ( area fraction) 90% or more), and after optimizing the shape of the second phase (maximum major axis of the second phase ≦ 20 μm) and density ( less than 10,000 per 1 mm ² ) , the texture is further controlled, It has been found that it is important to make the surface strength of the {100} plane of the plate thickness 1 / 2t part 2.5 or less. By doing so, it is possible to suppress the generation of voids at the crack tip of the end face during the punching / hole-expansion molding and to prevent the growth of the crack. Further, it has been found that Si: 0.8 to 3.0% is effective for improving the elongation (El). That is, since Si dissolves in ferrite and promotes work hardening of ferrite, it is estimated that elongation (El) is improved.

  The present invention has been completed based on these findings, and the gist of the invention is as follows.

(1) In mass%,
C: 0.01 to 0.2%
Si: 0.8 to 3.0%,
Mn: 0.5-3%,
P ≦ 0.1%,
S ≦ 0.01%,
Al: 0.005 to 2.0%,
N ≦ 0.02%,
Including
The balance is a steel sheet composed of Fe and unavoidable impurities, and the main phase is ferrite with an area fraction of 90% or more and less than 100%, and the second phase is martensite, bainite, or both. And the density of the second phase in a cross section parallel to the rolling direction is less than 10,000 per 1 mm ² , the maximum major axis of the second phase is 20 μm or less, and {1/2 t part { 100} surface strength is 2.5 or less, a composite structure steel plate excellent in formability.

  (2) The composite steel sheet having excellent formability as described in (1) above, wherein the steel further contains Cu: 0.01 to 2% by mass.

  (3) The composite steel sheet having excellent formability as described in (1) or (2) above, wherein the steel further contains B: 0.0002 to 0.003% by mass. .

  (4) The steel is further excellent in formability according to any one of the above (1) to (3), wherein the steel further contains Ni: 0.01 to 1% by mass. Composite steel sheet.

  (5) The above-mentioned (1), wherein the steel further contains one or two kinds of Ca: 0.0005 to 0.01% and REM: 0.0005 to 0.02% in mass%. The composite structure steel plate excellent in formability of any one of (1) thru | or (4).

  (6) The steel is further mass%, Ti: 0.01 to 0.5%, Nb: 0.005 to 0.5%, Mo: 0.05 to 1%, V: 0.02 to 0 Any one of (1) to (5) above, characterized by containing one or more of 5%, Cr: 0.01 to 1%, Zr: 0.02 to 0.2% The composite structure steel plate excellent in the formability as described in the item.

  (7) Any one of the above (1) to (6), wherein a value obtained by dividing the average value of the hardness of the second phase by the average value of the hardness of the ferrite is 1.5 or more and 2 or less The composite structure steel plate excellent in the formability as described in the item.

  (8) When hot-rolling the steel slab having the component described in any one of (1) to (6) above, rough rolling is finished at a temperature of Ar3 + 250 ° C. or higher, and the strain rate is 0.2. And finish the hot finish rolling at Ar3 transformation point temperature + 40 ° C. or higher and Ar3 transformation point temperature + 140 ° C. or lower, and then cool at a cooling rate of 30 ° C./second or higher, and then 650 ° C. or higher and 750 ° C. or lower. It is characterized in that it is slowly cooled for 5 seconds or more at a cooling rate of 15 ° C./second or less in a temperature range, and then cooled at a cooling rate of 30 ° C./s or more and wound at a winding temperature of 340 ° C. or less. A method for producing a composite steel sheet having excellent formability.

  According to the present invention, a composite structure steel plate excellent in both punching hole expandability and elongation (El) can be obtained, and a composite structure steel plate excellent in formability having properties suitable for a high-design wheel disc. There is a remarkable effect that it can be provided.

The feature of the composite structure hot-rolled steel sheet having excellent punched hole expandability according to the present invention is that the ferrite main phase (90% or more in area fraction ) is used to improve the hole expandability, and the shape of the second phase. (Maximum major axis of the second phase ≦ 20 μm), after optimizing the density (less than 10,000 per 1 mm ² ), and further controlling the texture in order to suppress the occurrence of cracks at the punched end face, The surface strength of the {100} plane of the plate thickness 1 / 2t part is 2.5 or less, and Si: 0.8 to 3.0% is added to improve the elongation (El).

  First, the process of obtaining the above knowledge will be described.

  Using the ingot of the component (steel P) shown in Table 1, hot rolling is performed under the hot rolling conditions shown in FIG. 2 and Table 2 in an actual machine or a laboratory, and then various ferrite fractions are obtained. The cooling conditions were changed as described above, and cooling was performed. Finally, the steel sheet was rapidly cooled to room temperature so as to have a structure composed of ferrite and martensite to obtain a hot-rolled steel sheet. At this time, in rough rolling, the maximum strain rate was set to 0.1 to 0.3 / second.

  The tensile test of the obtained hot-rolled sheet was carried out according to the test method described in JIS Z 2241 by processing No. 5 test piece described in JIS Z 2201 from 1/2 W part of the specimen. On the other hand, the punching hole expansion rate was similarly evaluated by processing three test pieces of 150 mm × 150 mm from the ½ W portion and according to the hole expansion test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. At that time, the punching clearance was set at three levels of 5, 15, and 20%, and the hole expansion rate in a wide punching clearance range was examined.

  Further, an X-ray diffraction test piece for observing the crystal orientation from the 1 / 2W portion in the width direction toward the plate surface direction was processed, and the surface strength in the {100} direction toward the plate surface direction was obtained.

In addition, an embedding sample having a cross section in the rolling direction was prepared from the width direction ½ W portion, polished, and then subjected to nital corrosion, and the structure hardness was examined at the ¼ t portion. The tissue hardness was measured by measuring Vickers hardness by 10-point measurement with a load of 2.5 g. Further, the maximum size in the rolling direction of the second phase was measured with the total thickness. Similarly, the number of second phases (equivalent circle diameter of 1.0 μm or more) was measured in a field of 0.2 mm × 0.2 mm at a 1/4 t portion, and converted to the number (density) per 1 mm ² . Similarly, the ferrite fraction was measured from the 1/4 t part. The ferrite fraction is defined as the area fraction of the ferrite structure in the microstructure. The fraction of the second phase is 100-ferrite fraction (%).

  FIG. 3 shows the relationship between the ferrite fraction obtained from the above results, the maximum major axis (μm) size of the second phase, and the punched hole expansion ratio of the ½ W portion. In the figure, the maximum value and the minimum value of the punching hole expansion rate examined by changing the clearance for each material are shown. As is clear from FIG. 3, it can be seen that the hole expansion ratio is the best in the region where the ferrite fraction is 90% or more and the maximum major axis of the second phase is 20 μm or less.

FIG. 4 shows the relationship between the {100} orientation plane strength measured at 1 / 2t and the punched hole expansion ratio at 1 / 2W. In the figure, the punching hole expansion ratios obtained by changing the clearance for each material are shown. Here, the {100} orientation is changed by changing the rough rolling conditions, and the ferrite fraction and the shape of the second phase are not changed (ferrite fraction = 95%, maximum major axis length of the second phase). = 17 μm). Therefore, in order to improve the punching hole expansion ratio, it is necessary to suppress the texture in which the {100} orientation is parallel to the plate surface, and the {100} plane strength at the 1 / 2t portion of the plate thickness t is 2. It turns out that it is necessary to make it 5 or less. This is because the texture of this orientation develops, cracks parallel to the plate surface direction during deformation Ru is estimated that for degrading it hole expansion tends to occur. In the steel of the present invention, low-temperature rolling is aimed at miniaturizing ferrite in order to ensure strength, and therefore, a unique texture in which the {100} orientation is parallel to the plate surface is easily developed. Need to be regulated.

FIG. 5 shows the relationship between the density of the second phase and the punching hole expansion ratio of the 1/2 W part. In the figure, the punching hole expansion ratios obtained by changing the clearance for each material are shown. Here, the second phase density changes with the rolling temperature, and the ferrite fraction and texture do not change greatly. From this, it can be seen that the density of the second phase is reduced, and a predetermined hole expansion rate can be obtained by setting it to 10000 / mm ² or less. This is because the smaller the density is, the larger the interval between the second phases is, so the connection of voids generated in the second phase at the time of deformation is suppressed. it is conceivable that.

  The balance between strength and punching hole expansion ratio is sufficiently improved over the wide clearance range by the above optimization of the structure (punching hole expansion ratio ≧ 80%). The inventors have found that it is desirable to make further improvements up to about ≧ 110%, and for that purpose, in addition to the above, it is necessary to control the hardness of the second phase. Next, the results of examining the influence of the hardness of the second phase by experiments will be described.

  The steel ingots having the components shown in Table 1 are hot-rolled and cooled under the conditions shown in FIGS. 2 and 3, and the coiling temperature is changed from room temperature to 300 ° C., and only the second-phase hardness is different. Got. FIG. 6 shows the relationship between the ratio of the hardness (Hvs) and ferrite hardness (Hvα) of the second phase of the hot-rolled steel sheet and the punched hole expansion value (λ). In the figure, the punching hole expansion ratios obtained by changing the clearance for each material are shown. Here, in the obtained steel plate, TS = 590 to 620 MPa, ferrite fraction = 94%, second phase major axis = 12.5 μm, and the second phase hardness (Hvs) and ferrite hardness ( The ratio (Hvs / Hvα) to Hvα) was examined, and it can be seen that when Hvs / Hvα = 2 or less, the hole expansion value (λ) is good as described above.

  The present invention has been made based on the above, and the reasons for limiting each requirement will be described below.

  First, the reason for limiting the components of the composite structure steel plate excellent in punching hole expandability of the present invention will be described.

  C is an element necessary for ensuring the strength and obtaining a composite structure steel having a good fatigue property, and therefore needs to be added in an amount of 0.01% or more. However, if it is excessive, the second phase increases and the hole expandability deteriorates, so the upper limit is made 0.2%. From the viewpoint of optimizing hole expansibility and not significantly degrading fatigue strength, the C content is preferably 0.03% or more and 0.1% or less.

  In the present invention, Si is necessary for improving the balance between strength and elongation. In particular, Si is an element that plays an important role in improving elongation (El). I need. In order to reliably obtain the effect even if the ROT cooling condition varies, it is desirable to add 1.3% or more. However, if it is excessive, the Ar3 transformation point becomes too high, and it becomes difficult to make the finish rolling temperature higher in the actual hot rolling process, making actual rolling impossible, so the upper limit is made 3.0%. .

  Mn is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, 0.5% or more is necessary. Further, if added over 3%, slab cracking occurs, so the content is made 3% or less, but preferably 1.0 to 3%.

  P is an impurity inevitably contained in the steel, and is preferably as low as possible. If contained over 0.1%, workability and weldability are adversely affected and fatigue characteristics are also reduced. To do. Depending on the applied parts, the required fatigue characteristics may be severe, so 0.01% or less is preferable.

  S is an impurity inevitably contained in the steel as in the case of P, and is preferably as low as possible. If it is too much, an A-based inclusion that deteriorates the hole expandability is generated, and should be reduced as much as possible. .01% or less is an acceptable range. Since it may be applied to parts with severe processing, it is preferably made 0.003% or less.

  Al needs to be contained by 0.005% or more for deoxidation of molten steel. Moreover, it is preferable to add also in order to obtain intensity | strength by solid solution strengthening. However, to raise the cost, the upper limit is made 2.0%.

  Excessive N is not preferable because it promotes age hardening and deteriorates moldability, but complete removal leads to significant cost increase, so 0.02% is made the upper limit. From the viewpoint of securing moldability, it is preferably made 0.005% or less.

  Since Cu has an effect of improving fatigue properties in a solid solution state, it is added as necessary. However, if the content is less than 0.01%, the effect is small, and even if the content exceeds 2%, the effect is saturated. Therefore, the Cu content is in the range of 0.01 to 2%, preferably 0.1 to 1%.

  B has the effect of increasing the fatigue limit by being added in combination with Cu, so is added as necessary. However, if it is less than 0.0002%, it is insufficient for obtaining the effect, and if added over 0.003%, slab cracking occurs. Therefore, the addition of B is 0.0002% or more and 0.003% or less, but preferably 0.0002 to 0.001%.

  Ni is added as necessary to prevent hot brittleness due to Cu inclusion. However, if less than 0.01%, the effect is small, and even if added over 1%, the effect is saturated, so 0.01 to 1%, preferably 0.03 to 0.5% To do.

  Ca and REM are elements that are detoxified by changing the form of non-metallic inclusions that become the starting point of destruction or deteriorate workability. However, even if added less than 0.0005%, there is no effect, and if Ca exceeds 0.01%, and if REM exceeds 0.02%, the effect is saturated, so Ca = 0.005 to 0 It is preferable to add 0.01% and REM = 0.005 to 0.02%.

  Furthermore, in order to impart strength, one or more of precipitation strengthening or solid solution strengthening elements of Ti, Nb, Mo, V, Cr, and Zr may be added. However, if less than 0.01%, 0.005%, 0.05%, 0.02%, 0.01%, and 0.02%, respectively, the effect cannot be obtained. Moreover, the effect will be saturated even if it adds exceeding 0.5%, 0.5%, 1%, 0.5%, 1%, and 0.2%, respectively.

  Next, the microstructure (composite structure) limited in the present invention will be described.

In the composite structure steel sheet according to the present invention, in order to improve both the hole expansibility and the elongation (El), it is necessary to make the microstructure of the steel sheet the ferrite having the maximum area fraction. The second phase is necessary from the viewpoint of securing strength. However, it is not always necessary when sufficient strength can be obtained with only ferrite. In order to obtain higher strength, the second phase is preferably as hard as possible, and needs to have a low temperature transformation structure such as martensite and bainite.

  From the viewpoint of obtaining high strength, it is preferable to use the hardest martensite as the second phase. However, since this is hard, strain accumulates at the interface with ferrite during deformation, and voids are easily generated, and the hole expansion value is deteriorated. It is preferable to include bainite that is softer than the site or to use bainite entirely. The fraction of the second phase needs to be 10% or less because it is necessary to reduce the second phase in order to reduce strain at the interface between the second phase and ferrite during deformation and prevent voids during deformation. . In order to obtain a hole expanding property that can withstand stricter requirements, the second phase fraction is preferably 6% or less. On the other hand, if the second phase fraction is too low, strength cannot be obtained, and when there is a large amount of C and Mn, ferrite transformation is difficult in ROT cooling, so ROT cooling (to ensure a slow cooling time, etc.) ) Is difficult to perform, so the content is preferably 2% or more. From the above, the ferrite fraction (= 100−second phase fraction) is 90% or more, preferably 94% or more and 98% or less.

  In the case of the steel of the present invention, the hole expandability is governed by the generation of voids at the interface between the ferrite and the second phase. The larger the size of the second phase, the easier it is for voids at the interface between it and ferrite. Therefore, in order to improve hole expansibility, it is necessary to make the second phase of the maximum size among the second phases fine. For this reason, the maximum major axis of the second phase is 20 μm or less.

In order to improve the hole expansion property, it is necessary to suppress the growth and connection of voids generated at the interface between the ferrite and the hard phase. From the viewpoint of preventing this connection, the density of the second phase is set in the rolling direction. The density of the second phase in the parallel cross section is defined as less than 10,000 per 1 mm ² .

  Further, in the present invention, it is necessary to suppress the texture in which the {100} orientation is parallel to the plate surface, and the {100} plane strength at the 1 / 2t portion of the plate thickness t is 2.5 or less. This is because the development of the texture of this orientation makes it easier for cracks parallel to the plate surface direction to occur during deformation and deteriorates the hole expansion property. In the steel of the present invention, low-temperature rolling is aimed at miniaturizing ferrite in order to ensure strength, and therefore, a unique texture in which the {100} orientation is parallel to the plate surface is easily developed. Need to be regulated.

  In applying the steel sheet of the present invention, when applied to more severely processed parts, it is necessary to further improve the hole expansion property, and in that case, it is preferable to adjust the structure hardness. From this viewpoint, the hardness ratio (Hvs / Hvα) of the second phase and ferrite is limited to 2 or less. On the other hand, if the second phase is too soft, it is difficult to obtain a predetermined strength, so Hvs / Hvα is set to 1.5 or more. The investigation of the structure hardness of the ferrite phase, the second phase, etc. is performed with a micro Vickers hardness tester with a load of about 2.5 g and a measurement point of 10 or more. However, when the second phase is fine, it is necessary to make the load smaller than 2.5 g. In that case, it is preferable to measure the hardness with a load of 1.0 g or less using a nanoindenter or the like.

  Next, the manufacturing method of the composite structure steel plate of this invention is demonstrated.

  In the present invention, a slab obtained by casting a molten steel whose components are adjusted so as to have a desired component content may be directly sent to a hot rolling mill as a high-temperature slab, or after being cooled to room temperature, a heating furnace It may be hot-rolled after reheating at.

In the present invention, it is necessary to suppress a texture in which the {100} plane that is strengthened by hot rolling at a low temperature is directed in the plate surface direction. This is because, due to the development of this texture, a good crack parallel to the plate surface direction is generated at the crack tip of the punched end face at the time of punched hole widening molding, promoting the propagation of the crack and deteriorating the hole expandability. Since this texture is generated by accumulation of strain in hot rolling, in order to suppress this, it is necessary to perform rough rolling at a high temperature as much as possible in the hot rolling rough rolling step and at a low strain rate. Considered important. As a result, dynamic recrystallization or dynamic recovery occurs during the rolling of each pass in the rough rolling process, and the effect of weakening the texture by rolling can be obtained. From this point of view, the rough rolling conditions are such that all passes are performed at Ar3 + 250 ° C. or higher, and the strain rate is set to 0.2 / sec or lower.
The strain rate is obtained from the following equation.
ε *: path strain rate (S ⁻¹ )
ε * = (v / (R × h ₁ ) ^0.5 ) × (1 / r ^0.5 ) × ln {1 / (1-r)}
Here, h ₁ : pass entry side plate thickness (mm), h ₂ : pass exit plate thickness (mm), r = (h ₁ −h ₂ ) / h ₁ , R: roll diameter (mm), v: pass exit It means the side speed (mm / sec).

The finish rolling end (final pass) temperature (FT) needs to be Ar 3 + 140 ° C. or lower in order to refine the ferrite grains and thereby reduce the size by making the second phase fine. However, when the finish rolling finish temperature is too low and the Ar3 temperature is 40 ° C. or lower, the ferrite and the second phase become too fine and the second phase density is excessively increased. Since the maximum size of the second phase also increases and the hole expansion property may be deteriorated, the finish rolling end temperature is set to Ar3 + 40 ° C. or higher.
The Ar3 temperature is obtained from the following equation.
Ar ₃ = 868-396 × C + 25 × Si-68 × Mn-36 × Ni-21 × Cu-25 × Cr + 30 × Mo

  Further, in order to make the ferrite grains and the second phase fine, it is necessary to perform cooling to a temperature not higher than the Ar3 temperature at a cooling rate of 30 ° C./second or more after the above finish rolling. At a cooling rate of less than 30 ° C./second, the ferrite grain size cannot be made fine. In order to prevent the flattening of the second phase by making the shape of the ferrite crystal grains and the second phase isotropic, air cooling (cooling rate ≦ 15 ° C./second) is preferably performed within 1.5 seconds after finishing rolling. It is desirable.

  In the steel of the present invention, the second phase fraction needs to be 10% or less, and for that purpose, at the time of ROT cooling, a cooling rate of 15 ° C./second or less in a temperature range of 650 ° C. or more and 750 ° C. or less. In this case, it is necessary to cool for 5 seconds or more to cause the ferrite transformation to occur near the maximum amount determined by the composition.

  Thereafter, in order not to produce a low temperature transformation structure such as pearlite which deteriorates the burring property, the cooling is performed again to the coiling temperature at a cooling rate of 30 ° C./second or more.

  The coiling temperature (CT) needs to be 340 ° C. or lower in order to obtain strength as lower bainite or martensite having an appropriate and sufficient hardness. When the coiling temperature is higher than that, the second phase becomes upper bainite or pearlite, the strength cannot be obtained, and the hole expandability is not good. In order to improve the hole expanding property, it is preferable to use a lower bainite having an appropriate hardness. For this purpose, the winding temperature is preferably in the range of 250 to 340 ° C.

According to the method for producing a composite structure hot rolled steel sheet of the present invention, in combination with the steel composition, 90% or more of ferrite in the area fraction is the main phase, and the balance is martensite, bainite, or both. The density of the second phase in a cross section parallel to the rolling direction is less than 10,000 per mm ² , the maximum major axis of the second phase is 20 μm or less, and the thickness t A composite structure hot-rolled steel sheet having a {100} plane strength of ½t part of 2.5 or less and excellent punching hole expandability can be obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  After melting the steel having the chemical components shown in Table 4 in a converter, the slab produced by continuous casting was rolled and cooled under the hot rolling conditions shown in Table 5 to obtain a plate thickness of 3.2 to 3 A 9 mm hot-rolled steel sheet was obtained.

The temperature, which is an index of rolling conditions, was measured at a 1/2 W portion in the coil width direction.
The tensile test of the obtained hot-rolled sheet was processed into a No. 5 test piece described in JIS Z 2201 from 1/2 W part of the test material, and was performed according to the test method described in JIS Z 2241. On the other hand, the punching hole expansion rate was evaluated in accordance with the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996 after processing three 150 mm × 150 mm test pieces from the 1 / 2W part. The punching hole expansion test was performed by changing the punching clearance at 5%, 15%, and 20%.

  Further, an X-ray diffraction test piece for observing the crystal orientation from the 1 / 2W portion in the width direction toward the plate surface direction was processed, and the surface strength in the {100} direction toward the plate surface direction was obtained.

In addition, an embedding sample having a cross section in the rolling direction was prepared from the width direction ½ W portion, polished, and then subjected to nital corrosion, and the structure hardness was examined at the ¼ t portion. The tissue hardness was measured by measuring Vickers hardness by 10-point measurement with a load of 2.5 g. Further, the maximum size in the rolling direction of the second phase was measured with the total thickness. Similarly, the number of second phases was measured within a visual field of 0.2 mm × 0.2 mm at the 1/4 t portion, and converted into the number (density) per 1 mm ² . Similarly, the ferrite fraction was measured from the 1/4 t part. The ferrite fraction is defined as the area fraction of the ferrite structure in the microstructure. The fraction of the second phase is 100-ferrite fraction (%).

  The above survey results are shown in Table 6.

  The levels (1), (2), (12), (13), (15) to (20) are steels of the present invention that satisfy predetermined conditions, and have good strength, elongation, and hole expansion rate (λ). The balance is obtained.

  At level (3), since the strain rate of rough rolling is large, {100} is large and the hole expansion rate is small.

  In level (4), since the temperature of rough rolling is low, {100} is large and the hole expansion rate is small.

  In level (5), since the finish rolling temperature is low, the second phase density is too large and the hole expansion rate is low.

  In level (6), since the finish rolling temperature is high, the second phase is coarse and the hole expansion rate is low.

  In level (7), since the cooling rate after finish rolling is small, the second phase is coarse and the hole expansion rate is low.

  At level (8), the slow cooling rate at 750 to 650 ° C. in ROT is high, and the slow cooling time at 750 to 650 ° C. is short, so the second phase fraction is large and the hole expansion rate is low.

  In level (9), since the slow cooling time at 750 to 650 ° C. in ROT is short, the second phase fraction is large and the hole expansion ratio is low.

  At the level (10), since the cooling rate at 650 ° C. or lower in the ROT is small, pearlite is generated and the hole expansion rate is low.

  Since CT is high at level (11), the second phase is relatively soft upper bainite, the strength is insufficient, and the hole expansion rate is low.

  At level (14), since the strain rate of rough rolling is large, {100} is large and the hole expansion rate is small.

  At the level (21), C is lower than a predetermined value, so the strength is low.

  At level (22), the balance between strength and elongation is inferior because the amount of Si is lower than specified.

  In the level (23), since the amount of Mn is lower than specified, sufficient strength is not obtained.

  In level (24), since C is too high, the second phase fraction is large and the hole expansion value is low.

It is a figure which shows a punching hole expansion test. It is a figure which shows the hot rolling conditions of test steel. It is a figure which shows the relationship between a ferrite fraction, a 2nd phase particle size, and a hole expansion rate. It is a figure which shows the relationship between {100} surface intensity | strength and a hole expansion rate. It is a figure which shows the relationship between a 2nd phase density and a hole expansion rate. It is a figure which shows the relationship between the ratio of the hardness (Hvs) of a 2nd phase, and ferrite hardness (Hv (alpha)), and a hole expansion value ((lambda)).

Explanation of symbols

1 punch 2 punching die 3 wrinkle retainer 4 material 5 shearing part 6 sheared part 7 hole expanding punch 8 punching end face 9 punch movement 10 hole diameter expansion t plate thickness s gap

Claims (8)

% By mass
C: 0.01 to 0.2%
Si: 0.8 to 3.0%,
Mn: 0.5-3%,
P ≦ 0.1%,
S ≦ 0.01%,
Al: 0.005 to 2.0%,
N ≦ 0.02%,
Including
The balance is a steel sheet composed of Fe and unavoidable impurities, and the main phase is ferrite with an area fraction of 90% or more and less than 100%, and the second phase is martensite, bainite, or both. And the density of the second phase in a cross section parallel to the rolling direction is less than 10,000 per 1 mm ² , the maximum major axis of the second phase is 20 μm or less, and {1/2 t part { 100} surface strength is 2.5 or less, a composite structure steel plate excellent in formability.
The steel is further mass%,
Cu: 0.01-2%
The composite steel sheet having excellent formability according to claim 1, comprising: The steel is further mass%,
B: 0.0002 to 0.003%
The composite structure steel plate excellent in formability of Claim 1 or 2 characterized by containing. The steel is further mass%,
Ni: 0.01 to 1%
The composite structure steel plate excellent in formability of any one of Claims 1 thru | or 3 characterized by the above-mentioned. The steel is further mass%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.02%
One type or two types of these are contained, The composite-structure steel plate excellent in the moldability of any one of Claims 1 thru | or 4 characterized by the above-mentioned. The steel is further mass%,
Ti: 0.01 to 0.5%,
Nb: 0.005 to 0.5%,
Mo: 0.05 to 1%
V: 0.02-0.5%
Cr: 0.01-1%,
Zr: 0.02 to 0.2%
The composite structure steel plate excellent in formability of any one of Claims 1 thru | or 5 characterized by containing 1 type, or 2 or more types of these.   The formability according to any one of claims 1 to 6, wherein a value obtained by dividing the average value of the hardness of the second phase by the average value of the hardness of the ferrite is 1.5 or more and 2 or less. Excellent composite steel sheet.   In hot rolling the steel slab having the component according to any one of claims 1 to 6, the rough rolling is finished at a temperature of Ar3 + 250 ° C or higher, and the strain rate is 0.2 / sec or less, and Hot finish rolling is finished at an Ar3 transformation point temperature of + 40 ° C. or more and an Ar3 transformation point temperature of + 140 ° C. or less, then cooled at a cooling rate of 30 ° C./second or more, and then in a temperature range of 650 to 750 ° C. For formability characterized by performing slow cooling for 5 seconds or more at a cooling rate of 15 ° C./second or less, then cooling at a cooling rate of 30 ° C./s or more and winding at a winding temperature of 340 ° C. or less. An excellent method of manufacturing a composite steel sheet.

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