JP5068688B2 - Hot-rolled steel sheet with excellent hole expansion - Google Patents

Hot-rolled steel sheet with excellent hole expansion Download PDF

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JP5068688B2
JP5068688B2 JP2008114265A JP2008114265A JP5068688B2 JP 5068688 B2 JP5068688 B2 JP 5068688B2 JP 2008114265 A JP2008114265 A JP 2008114265A JP 2008114265 A JP2008114265 A JP 2008114265A JP 5068688 B2 JP5068688 B2 JP 5068688B2
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JP2009263715A (en
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大介 前田
直樹 吉永
和也 大塚
邦夫 林
展弘 藤田
学 高橋
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新日本製鐵株式会社
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  The present invention relates to a hot-rolled steel sheet excellent in hole expansibility, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet subjected to surface treatment, and methods for producing them.

  Since parts such as suspension arms of automobiles are subjected to burring and the like, the hot-rolled steel sheet as a material is required to have a hole expanding property. Since the hole expanding process is a process for uniformly expanding the diameter of the punched hole, a hot-rolled steel sheet with improved in-plane anisotropy and improved hole expanding property has been proposed (for example, Patent Document 1). ). However, since these hot-rolled steel sheets have low strength, they cannot cope with the increase in strength of hot-rolled steel sheets aimed at reducing the weight of automobiles.

  On the other hand, a hot-rolled steel sheet is proposed in which the strength is increased and the local elongation, which is considered to have a correlation with the hole expansion property, is controlled (for example, Patent Documents 2 to 4). However, these are hot-rolled steel sheets that have been subjected to hot rolling while suppressing recrystallization and controlled to a texture in which a specific crystal orientation has been developed to improve shape freezeability. Therefore, there is a problem that the r value (Rankford value) in the rolling direction (L direction) and the width direction (C direction) is low, and the r value in the 45 ° direction therebetween is high.

In addition, a hot-rolled steel sheet has been proposed in which the rolling reduction of the final stand is reduced, the development of the working texture of the austenite structure due to rolling is suppressed, the in-plane anisotropy is reduced, and the stretch flangeability is improved (patent) Reference 5). However, in this method, the texture is not sufficiently randomized, so that the hole expandability when evaluated under severe test conditions is insufficient.
JP-A-5-171270 JP 2004-183057 A JP 2004-250743 A JP-A-2005-15854 JP 2000-297349 A

  The present invention aims to provide a high-strength hot-rolled steel sheet having a tensile strength of 490 MPa or more, excellent workability, particularly hole expansibility, and reduced in-plane anisotropy, and a method for producing the same. It is.

  In the present invention, the amount of Nb that delays recrystallization is set to an appropriate range, the finishing temperature of hot rolling, the rolling reduction of finishing rolling, and the temperature range are controlled, and more preferably by controlling the air cooling time, It is a hot-rolled steel sheet that promotes recrystallization, suppresses the development of rolling texture, and randomizes the crystal orientation, thereby improving strength, ductility, and hole expandability, and reducing in-plane anisotropy. The gist of the present invention is as follows.

(1) By mass%, C: 0.005 to 0.150%, Mn: 0.10 to 3.00%, Nb: 0.005 to 0.07%, Si: 2.50% or less , P: 0.150% or less, S: 0.0150% or less, Al: 0.150% or less, N: 0.0100% or less, the balance being Fe and unavoidable impurities, bainite in the metal structure A hot-rolled steel sheet having excellent hole expansibility, wherein the area ratio is 95% or more, the crystal grain size of prior austenite is 30 μm or less, and the aspect ratio is 4 or less.
(2) The heat excellent in hole expansibility according to the above (1), containing 1 type or 2 types of Ti: 0.05% or less and B: 0.0015% or less in mass% Rolled steel sheet.
(3) By mass%, Mo: 0.01-2.00%, Cr: 0.01-2.00%, W: 0.01-2.00%, Cu: 0.01-2.00% Ni: 0.01-2.00% of 1 type or 2 types or more, The hot rolled steel sheet excellent in hole expansibility as described in said (1) or (2) characterized by the above-mentioned.
(4) By mass%, one or more of Ca: 0.0005 to 0.1000%, Rem: 0.0005 to 0.1000%, V: 0.001 to 0.100% should be contained. The hot rolled steel sheet excellent in hole expansibility according to any one of (1) to (3) above.
(5) The Rankford value in the rolling direction, the Rankford value in the direction perpendicular to the rolling direction, and the Rankford value in the direction of 45 ° from the rolling direction are more than 0.7 and 1.2 or less. The hot rolled steel sheet excellent in hole expansibility according to any one of (1) to (4).
(6) A hot-dip galvanized steel sheet excellent in hole expansibility, wherein the hot-rolled steel sheet according to any one of (1) to (5) is subjected to hot-dip galvanization.
(7) Alloyed hot dip galvanized excellent in hole expansibility, characterized in that the hot rolled steel sheet according to any one of the above (1) to (5) is alloyed hot dip galvanized. steel sheet.
(8) The hot-rolled steel sheet according to any one of (1) to (5), the hot-dip galvanized steel sheet according to (6), or the galvannealed steel sheet according to (7) is optional. A steel pipe with excellent hole expandability characterized by being wound in the direction.

  According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having excellent hole expandability and reduced in-plane anisotropy, and the industrial contribution is extremely remarkable.

  The hole expandability is evaluated by the hole expansion rate when the entire circumference of the punched hole is uniformly expanded and a crack first occurs. That is, since deterioration of characteristics in a specific direction becomes a problem, there is a possibility that it is preferable to suppress the accumulation of specific crystal orientations and reduce characteristic anisotropy when hot rolling a steel sheet. is there. Then, the present inventors examined the method of improving the hole expansibility of a high strength steel plate by suppressing the development of a specific texture after hot rolling. Specifically, after completion of hot rolling in a temperature range where the metal structure is only an austenite phase (referred to as a γ range), the austenite phase is recrystallized before phase transformation starts to improve the hole expansion property. I planned.

  Nb is necessary for precipitation strengthening, but is also an element that suppresses recrystallization. Therefore, it is considered that when Nb is added excessively, a rolling texture develops during hot rolling, which hinders improvement in hole expansibility. Therefore, the present inventors first examined the amount of Nb. As a result, Nb needs to have a lower limit of 0.005% or more in order to increase the tensile strength to 490 MPa or more, and an upper limit of 0.07% or less in order to promote recrystallization. I found out.

  Next, the influence of the temperature of finish rolling on the recrystallization behavior of austenite was examined. In the laboratory, the present inventors performed a processing for master test simulating hot rolling by changing the processing temperature using steel having an Nb amount within an appropriate range. The processing for master test is performed using an apparatus that heats a cylindrical sample having a diameter of 10 mm, performs uniaxial compression processing while controlling the temperature, and cools the sample. Processing was performed at various processing temperatures with a processing rate of 50%, quenched, and ferrite transformation was suppressed, and the recrystallization rate of austenite was measured. The recrystallization rate of austenite was obtained by observing the structure with an optical microscope. As a result, as shown in FIG. 1, it was found that when the processing temperature (finishing temperature FT) was set to 930 ° C. or higher, the recrystallization rate was 90% or higher.

Furthermore, the inventors changed the processing temperature and the retention time after processing by the processing for master test, and the time until the recrystallization rate of austenite becomes 90% or more (referred to as recrystallization time). It was measured. As a result, the graph of FIG. 2 showing the relationship between the processing temperature and the recrystallization time was obtained. The straight line in FIG. 2 is an approximate expression of the processing temperature and the recrystallization time. If the processing temperature is FT [° C.] and the recrystallization time is X [s], it can be expressed by the following (Expression 1).
X = 10 (18.4−0.0194 × FT) (Formula 1)

  In addition, as shown in FIG. 2, although the straight line of (Formula 1) and an experimental result correspond substantially at 930 degreeC or more, the tendency for a difference to spread a little wide was seen below 930 degreeC. Therefore, in hot rolling, it is considered preferable to set the finishing temperature FT [° C.] to 930 ° C. or higher and further air-cool for a time equal to or longer than X [s] obtained by (Equation 1). By air cooling after hot rolling, recrystallization further proceeds and the in-plane anisotropy of the steel sheet can be significantly reduced.

  Based on the above examination results, the present inventors further examined the temperature and reduction rate of hot rolling. As a result, it was found that in hot rolling, fine recrystallized austenite grains can be obtained by increasing the rolling reduction at a lower temperature and accumulating dislocations as driving force for recrystallization. On the other hand, it was also found that when the rolling reduction at a higher temperature was increased, recrystallization occurred during rolling and the austenite grains became coarse. Specifically, in the hot rolling, it is necessary to set the rolling reduction at 1000 ° C. or less to 50% or more. Thereby, the austenite grains after hot rolling are recrystallized, and the development of the rolling texture is suppressed. The reduction ratio at 1000 ° C. or less of the present invention is defined as a numerical value expressed as a percentage by dividing the difference between the plate thickness at 1000 ° C. and the plate thickness after finish rolling by the plate thickness at 1000 ° C. Is done.

  Next, the inventors perform hot rolling at a reduction rate of 1000% or less at a temperature of 50% or more and a finishing temperature of 930 ° C or more in a laboratory, and control the cooling rate to promote bainite transformation. And the steel plate was produced. In addition, in order to simulate winding after hot rolling, after hot rolling and cooling, it hold | maintained in the furnace of predetermined temperature for 1 hour. A test piece was collected from the obtained steel sheet, and the microstructure was observed with an optical microscope, the old austenite structure was observed, a tensile test, and a hole expansion test were performed.

  The tensile test was performed according to JIS Z 2241, and the Rankford value was measured according to JIS Z 2254. The hole expansion test was performed according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996.

  As a result, the hot rolled steel sheet having excellent hole expansibility and small in-plane anisotropy has a mean grain size of 30 μm or less and an aspect ratio of 4 or less, although prior austenite may not be equiaxed. I understood it. In particular, the average grain size of the prior austenite of the hot rolled steel sheet having good hole expansibility and in-plane anisotropy is further finer, specifically, the grain size is 20 μm or less, the aspect ratio is 2 or less, Those having good characteristics had a particle size of 10 μm or less and an aspect ratio of 1.5 or less. The aspect ratio is a ratio obtained by dividing the major axis of a crystal grain by the minor axis. Moreover, when the Rankford value in the rolling direction, the Rankford value in the direction perpendicular to the rolling direction, and the Rankford value in the direction of 45 ° from the rolling direction of these steel plates are measured, they are 0.7 to 1.2 or less. It was found that the in-plane anisotropy was small.

As a result of the above studies, in hot rolling, the reduction rate at a relatively low temperature is increased, and hot rolling is completed at 930 ° C. or higher to promote recrystallization. Preferably, the air cooling time is controlled to control the prior austenite. It was confirmed that if the aspect ratio of the prior austenite is close to 1, the hole expandability is improved and the in-plane anisotropy is reduced.
Moreover, in order to obtain the above hot rolled steel sheet with good hole expansibility by the above examination, hot rolling is performed at a reduction rate of 50% or higher at 1000 ° C. or lower and a finishing temperature of 930 ° C. or higher, and Ae 3 − It was confirmed that the average cooling rate from 100 ° C. to 700 ° C. was cooled to 15 ° C./s or more, and winding was performed at 350 to 650 ° C.

Hereinafter, the present invention will be described in detail.
Nb is an important element that combines with C and forms NbC that contributes to precipitation strengthening. In order to ensure the strength, it is necessary to add 0.005% or more of Nb. However, since Nb is an element that suppresses recrystallization of austenite, the upper limit is made 0.07%. In order to promote the recrystallization of austenite, the upper limit of Nb is preferably 0.05% or less, and more preferably 0.04% or less.

  C is an element that increases the strength, and needs to be added in an amount of 0.005% or more. If the C content exceeds 0.150%, the weldability may be impaired, or the workability may be extremely deteriorated due to an increase in the hard structure, so the upper limit is made 0.150%. Further, if the C content exceeds 0.100%, the moldability deteriorates, so the C content is preferably 0.100% or less.

  Si is a deoxidizing element, and if the added amount exceeds 2.50%, the press formability deteriorates, so 2.50% is made the upper limit. Moreover, since chemical conversion processability will fall if there is much Si amount, it is preferable to set it as 1.20% or less, and also in order to make the surface pattern resulting from Si scale inconspicuous, it shall be less than 0.50%. It is preferable. In addition, when hot dip galvanizing is performed, problems such as a decrease in plating adhesion and a decrease in productivity due to a delay in the alloying reaction may occur. Therefore, the Si content is preferably 1.00% or less. . The lower limit is not specified, but if it is less than 0.001%, the manufacturing cost becomes high. Si is an element that increases the strength by solid solution strengthening. Therefore, 0.01% or more may be added according to the target strength level.

  Mn is an element contributing to solid solution strengthening and needs to be added in an amount of 0.10% or more. In order to improve the strength, the preferable lower limit is 0.5% or more. However, in order to make a segregation zone in steel by segregation and deteriorate stretch flange formability, the upper limit was made 3.0%. Further, when the strength is increased by the addition of Mn, the fatigue characteristics may be impaired, so the preferable upper limit is 2.0% or less.

  P is an impurity, and if the content exceeds 0.150%, the fatigue strength after spot welding deteriorates, the yield strength increases, and a surface shape defect is caused during pressing. Therefore, the upper limit of the P content is 0.150%. Moreover, since P is an element that delays the alloying reaction during continuous hot dip galvanization, the upper limit is preferably made 0.100% or less. Furthermore, since the secondary workability is improved by reducing P, the content is preferably made 0.050% or less. When it is necessary to increase the strength, 0.005% or more may be contained.

S is an impurity, and if it exceeds 0.0150%, it causes hot cracking or deteriorates workability, so 0.0150% is made the upper limit.
Al is a deoxidizing element, and if added in excess, weldability deteriorates, so the upper limit is made 0.150%. Although a minimum is not specifically limited, From a viewpoint of deoxidation, it is preferable to add 0.010% or more of Al.

  N is an impurity, and when coarse Nb nitride precipitates, the amount of NbC that contributes to an increase in strength decreases, so it is limited to 0.0100% or less. From this viewpoint, it is preferably 0.0050% or less, and more preferably 0.0020% or less. The lower limit is not particularly set, but the cost is increased to make it lower than 0.0005%.

Furthermore, Ti, B, Mo, Cr, W, Cu, Ni, Ca, Rem, and V may be added as necessary.
Ti is an element that combines with C to generate TiC to improve strength, and is added as necessary. Since Ti is also an element that suppresses recrystallization of austenite, the upper limit is preferably 0.05%. From this viewpoint, it is preferably 0.03% or less, more preferably 0.01% or less.

  B is an element that promotes the formation of bainite to increase the strength, and is added as necessary. Since B is also an element that suppresses recrystallization of austenite, the upper limit is preferably made 0.0015%. From this viewpoint, it is preferably 0.0010% or less, more preferably 0.0005% or less.

  Mo, Cr, W, Cu, and Ni are elements that contribute to the improvement of strength and material, and it is preferable to add one or more of each at 0.01% or more. On the other hand, when the addition amount of each element exceeds 2.00%, pickling property, weldability, hot workability and the like may be deteriorated, so a preferable upper limit is 2.00%.

  Ca, Rem, and V are elements that contribute to strength improvement and material improvement, and it is preferable to add one or more of them. If the addition amount of Ca and Rem is less than 0.0005% and the addition amount of V is less than 0.001%, sufficient effects may not be obtained. On the other hand, when Ca and Rem are added so that the addition amount exceeds 0.1000% and the addition amount of V exceeds 0.100%, ductility may be impaired. Therefore, Ca, Rem and V are preferably added in the range of 0.0005 to 0.1000%, 0.0005 to 0.1000% and 0.001 to 0.100%, respectively.

Next, the metal structure of the hot rolled steel sheet according to the present invention will be described.
The metal structure of the hot-rolled steel sheet of the present invention has an area ratio of bainite of 95% or more, preferably a bainite single phase. This is because it is possible to achieve both strength and hole expandability by making the metallographic structure a precipitation strengthened bainite. Further, since this structure is generated by transformation at a relatively high temperature, it is not necessary to cool to a low temperature during production, and this structure is preferable from the viewpoint of material stability and productivity.

  As the balance, less than 5% of ferrite, pearlite, martensite and retained austenite are acceptable. Ferrite has no problem as long as it is sufficiently precipitation strengthened, but depending on the component, it may become soft, and when the area ratio exceeds 5%, the hole expandability slightly decreases due to the hardness difference from bainite. In addition, when the area ratio exceeds 5%, pearlite may impair the strength and workability. When the area ratio of martensite and retained austenite that becomes martensite by processing-induced transformation exceeds 5%, bainite And the interface with a structure harder than bainite becomes the starting point of crack generation, and the hole expandability deteriorates. Therefore, if the area ratio of bainite is 95% or more, the area ratio of the remaining ferrite, pearlite, martensite, and residual γ is less than 5%, so that the balance between strength and hole expansibility is improved.

Next, the reason for limiting the manufacturing conditions will be described.
Steel is melted and cast by a conventional method to obtain a steel piece to be subjected to hot rolling. Although this steel slab may be a forged or rolled steel ingot, it is preferable to manufacture the steel slab by continuous casting from the viewpoint of productivity. Moreover, you may manufacture with a thin slab caster.
Usually, the steel slab is cooled after casting, so when performing hot rolling, it is heated again. Also, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting the molten steel may be employed.

  In the present invention, it is necessary to set the rolling reduction at 1000 ° C. or lower to 50% or more and the hot rolling finishing temperature to be 930 ° C. or higher. Therefore, the heating temperature of the steel slab is set to 1050 ° C. or higher. Moreover, in order to sufficiently dissolve alloy carbides such as TiC and NbC and to heat the steel slab efficiently and uniformly, the heating temperature is preferably 1100 ° C. or higher, more preferably 1150 ° C. or higher. Although the upper limit of the heating temperature is not specified, when the steel slab is heated to over 1300 ° C., the crystal grain size of the steel sheet becomes coarse and the workability may be impaired.

  In hot rolling, it is necessary to increase the rolling reduction at a lower temperature. At temperatures exceeding 1000 ° C., it is difficult to refine the crystal grain size even if the rolling reduction is increased. Therefore, the rolling reduction at 1000 ° C. or less becomes important. The rolling reduction at 1000 ° C. or less is set to 50% or more in order to sufficiently promote recrystallization after finish rolling. From this viewpoint, the rolling reduction at 1000 ° C. or less is preferably 60% or more, and more preferably 70% or more.

  The finish temperature FT [° C.] of hot hot rolling is extremely important in the present invention. When finish rolling is performed at a temperature lower than 930 ° C., recrystallization does not proceed thereafter, and the in-plane anisotropy is increased. Spreadability is reduced. Therefore, the lower limit of the finishing temperature is 930 ° C. In this respect, 940 ° C. or higher is preferable, and 950 ° C. or higher is more preferable.

Further, it is more preferable to control the air cooling time after the hot rolling is completed in order to promote recrystallization of austenite. In order to reduce the in-plane anisotropy by recrystallization after finishing rolling, the air cooling time should be set to X [s] or more determined by the following (Formula 1) by the finishing temperature FT [° C.]. Is preferred.
X = 10 (18.4−0.0194 × FT [° C.]) (Formula 1)
On the other hand, when the air cooling time is long, grain growth proceeds and the austenite grain size may become coarse. Therefore, it is preferable that the upper limit of the air cooling time is 100 s after the end of hot rolling. More preferably, it is within 10 s, and even more preferably within 5 s.

The cooling rate from Ae 3 to 700 ° C. is also important in order to suppress the ferrite transformation and make the metal structure bainite with an area ratio of 95% or more. When the cooling rate in this range is slow, the area ratio of polygonal ferrite may exceed 5%, so the cooling rate needs to be 15 ° C./s or more. From this viewpoint, the cooling rate is preferably 20 ° C./s or more, and more preferably 30 ° C./s or more.

Ae 3 [° C.] is calculated by the following (formula 2) depending on the content [mass%] of C, Mn, Si, Cu, Ni, Cr, and Mo. In addition, when not containing a selective element, it calculates as 0.
Ae 3 = 911-239C-36Mn + 40Si-28Cu-20Ni-12Cr + 63Mo (Formula 2)

  In the present invention, the coiling temperature is also important, and it is necessary to set the temperature above 350 ° C. to 650 ° C. Above 650 ° C., the area ratio of the ferrite structure increases, and the area ratio of bainite cannot be made 95% or more. From this viewpoint, the temperature is preferably 600 ° C. or lower. The reason why the lower limit is set to over 350 ° C. is that martensite increases at 350 ° C. or lower, and the hole expandability deteriorates. In order to suppress the formation of martensite, the lower limit is preferably 400 ° C. or higher.

  The hot-rolled steel sheet may be subjected to a skin pass having a reduction rate of 10% or less by pickling, in-line or off-line as necessary.

  The hot-rolled steel sheet may be hot dip galvanized or alloyed hot dip galvanized. When the steel sheet is annealed, it may be subjected to hot dip galvanization as it is in a continuous hot dip galvanizing line after cooling. The composition of the galvanizing is not particularly limited, and besides zinc, Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni, etc. may be added as necessary.

  The alloying heat treatment is performed within a range of 450 to 600 ° C. after hot dip galvanization. If it is less than 450 ° C, alloying does not proceed sufficiently, and if it exceeds 600 ° C, alloying proceeds excessively and the plated layer becomes brittle, which causes problems such as peeling of the plating due to processing such as pressing. . The alloying time is 5 s or longer. If it is less than 5 s, alloying does not proceed sufficiently. Although the upper limit is not particularly defined, it is preferably about 10 s in consideration of plating adhesion.

  The hot-rolled steel sheet may be subjected to Al-based plating or various electroplating. Further, the hot-rolled steel sheet and various plated steel sheets can be subjected to surface treatment such as organic coating, inorganic coating, and various paints according to the purpose.

  Moreover, there is no problem even if a steel pipe is manufactured using the above hot-rolled steel sheet and plated steel sheet as a raw material. The pipe making method may be any method such as UO pipe, electric resistance welding, spiral, or the like.

Steel pieces having the composition shown in Table 1 were melted to produce steel pieces. The steel slab was heated, followed by hot rough rolling and finish rolling under the conditions shown in Table 2. The rolling speed was about 300 to 1000 mpm. Moreover, among these steel plates, when hot dip galvanizing was performed after the hot rolling was completed, “melting” and when alloying hot dip galvanizing at 520 ° C. for 15 seconds was performed, “alloy” was indicated.
Ae 3 [° C.] is a transformation temperature calculated by the following (formula 2) depending on the content [mass%] of C, Mn, Si, Cu, Ni, Cr, and Mo. In addition, when not containing the selective element, it calculated as 0.
Ae 3 = 911-239C-36Mn + 40Si-28Cu-20Ni-12Cr + 63Mo (Formula 2)

Furthermore, in Table 2, SRT [° C.] is the heating temperature of the steel slab, FT [° C.] is after the final pass of rolling, that is, finishing temperature, t AC [s] is the air cooling time after hot rolling, CR [° C. / S] is the average cooling rate from Ae 3 -100 ° C. to 700 ° C., and CT [° C.] is the coiling temperature. The rolling reduction is a value obtained by dividing the difference between the plate thickness at 1000 ° C. and the finished plate thickness by the plate thickness at 1000 ° C. According to (Formula 1), X [s] was calculated from the finishing temperature FT [° C.] and listed in Table 2.
X = 10 (18.4−0.0194 × FT) Expression (1)

The structure of the obtained steel sheet was observed by corroding the mirror-polished steel sheet with a nital solution, using an optical microscope, measuring the bainite area ratio by image analysis, and discriminating the remaining structure. For steel sheets whose structure was difficult to discern by optical microscope observation, the bainite area ratio was determined by the Image Quality map obtained by the SEM-EBSP method. Further, the mirror-polished steel plate was corroded with a repeller corrosive liquid, observed with an optical microscope, the presence or absence of retained austenite and martensite was confirmed, and the area ratio was determined by image analysis.
V B [%] in Table 3 is the area ratio of bainite. Table 3 shows the type of the remaining structure. When the remaining part is composed of ferrite (F), retained austenite and martensite, the area ratio is shown in parentheses. γ + M means a mixed structure of retained austenite and martensite.

  Furthermore, the structure which appeared with the gamma corrosion solution was analyzed with an optical microscope, and the prior austenite grain size and aspect ratio were calculated by image analysis. For steel plates where the former austenite structure cannot appear with the gamma corrosion solution, the old austenite grain boundaries are determined from the optical microscope photograph (or SEM photograph) taken after the nital corrosion and the Image Quality map obtained by the SEM-EBSP method. The prior austenite (former γ) particle size and aspect ratio were calculated.

  Furthermore, the tensile test piece based on JISZ2201 was extract | collected from the steel plate, the tensile test was performed based on JISZ2241, and tensile strength (TS) and elongation (breaking elongation, EL) were measured. The Rankford value (r value) is a Rankford value (rL) in the rolling direction according to JIS Z 2254, using a No. 13 B tensile test piece of JIS Z 2201, and Rankford in the direction of 45 ° from the rolling direction. The value (rD) and the Rankford value (rC) in the direction perpendicular to the rolling direction were measured. The amount of strain was set to 5% in consideration of the uniform elongation of the steel sheet having low ductility.

  The hole expansion test was performed according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996, and the initial hole was punched with a die diameter so that the clearance would be 12.5% according to the punch of φ10 and the plate thickness. The test was performed under severe conditions of the shoulder R2 (conditions in which the λ value becomes low), and the hole expansion value λ was evaluated.

  The results are shown in Table 3. In Tables 1 to 3, the underline means outside the scope of the present invention or outside the preferred range.

  As is apparent from Table 3, when steel having the chemical composition of the present invention is hot-rolled under appropriate conditions, a high strength (TS) -hole expansion value (λ) -elongation (EL) balance must be satisfied. I was able to. Hot rolling No. No. 19 has slightly better anisotropy because the air cooling time after finish rolling is slightly shorter than X and recrystallization does not proceed sufficiently, but shows better characteristics than the comparative example shown below. Note that (TS), (λ), and (EL) are contradictory characteristics. In general, the higher the value of TS, the smaller the values of EL and λ. Therefore, a high value of (TS) × (λ) × (EL) indicates that the three contradictory characteristics of (TS), (λ), and (EL) are good regardless of the strength (TS) level. Show. As shown in Table 3, the invention steel has a better value of (TS) × (λ) × (EL) than the comparative steel, except for those with TS outside the range.

  On the other hand, hot rolling No. Nos. 25 to 27 are steel Nos. Whose chemical components are outside the scope of the present invention. It is a comparative example using P to R. Hot rolling No. In No. 25, Nb is added excessively, the recrystallization of austenite is suppressed, the aspect ratio of the austenite grains is large, the in-plane anisotropy is large, and the hole expandability is deteriorated. Hot rolling No. No. 26 has a large amount of C, martensite, which is a hard phase, increases, and the hole expandability deteriorates. Hot rolling No. Since No. 27 has a low Nb amount, the strength is lowered.

  Hot rolling No. No. 3 has a low FT. No. 11 is an example in which recrystallization did not proceed because FT was low because SRT was low. These have strong in-plane anisotropy, a low r value in the rolling direction, and a low hole expansion value. Hot rolling No. No. 12 has a small rolling reduction at 1000 ° C. or lower, so that the ferrite grain size increases, and the elongation and hole expansion values are low.

  Hot rolling No. No. 8 has a low CT and a martensite structure fraction of 5% or more, so the hole expansion value is low. Hot rolling No. No. 10 has a high CT. No. 22 has a low CR, a ferrite area ratio of 5% or more, and the hole expandability is slightly lowered.

It is a figure which shows the relationship between processing temperature and a recrystallization rate. It is a figure which shows the relationship between processing temperature and recrystallization time.

Claims (8)

  1. % By mass
    C: 0.005-0.150%,
    Mn: 0.10 to 3.00%,
    Nb: 0.005 to 0.07%
    Containing
    Si: 2.50% or less,
    P: 0.150% or less,
    S: 0.0150% or less,
    Al: 0.150% or less,
    N: limited to 0.0100% or less, the balance being Fe and inevitable impurities,
    A hot-rolled steel sheet excellent in hole expansibility, wherein the area ratio of bainite in the metal structure is 95% or more, the crystal grain size of prior austenite is 30 μm or less, and the aspect ratio is 4 or less.
  2. % By mass
    Ti: 0.05% or less,
    B: The hot rolled steel sheet excellent in hole expansibility of Claim 1 characterized by containing 1 type or 2 types of 0.0015% or less.
  3. % By mass
    Mo: 0.01-2.00%,
    Cr: 0.01 to 2.00%
    W: 0.01 to 2.00%
    Cu: 0.01-2.00%,
    Ni: 0.01-2.00%
    1 or 2 types or more of these are included, The hot-rolled steel plate excellent in the hole expansibility of Claim 1 or 2 characterized by the above-mentioned.
  4. % By mass
    Ca: 0.0005 to 0.1000%,
    Rem: 0.0005 to 0.1000%,
    V: 0.001 to 0.100%
    The hot-rolled steel sheet excellent in hole expansibility according to any one of claims 1 to 3, characterized by containing one or more of the following.
  5.   The Rankford value in the rolling direction, the Rankford value in a direction perpendicular to the rolling direction, and the Rankford value in a direction 45 ° from the rolling direction are more than 0.7 and 1.2 or less. 4. A hot-rolled steel sheet excellent in hole expansibility according to any one of 4 above.
  6.   A hot-dip galvanized steel sheet excellent in hole expansibility, wherein the hot-rolled steel sheet according to any one of claims 1 to 5 is hot-dip galvanized.
  7.   An alloyed hot-dip galvanized steel sheet excellent in hole expansibility, wherein the hot-rolled steel sheet according to any one of claims 1 to 5 is subjected to alloyed hot-dip galvanizing.
  8.   The hot-rolled steel sheet according to any one of claims 1 to 5, the hot-dip galvanized steel sheet according to claim 6, or the galvannealed steel sheet according to claim 7 is wound in an arbitrary direction. This is a steel pipe with excellent hole expandability.
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