JP5304435B2 - Hot-rolled steel sheet with excellent hole-expandability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet excellent in hole expansibility and a manufacturing method thereof.

  In order to achieve improvement in fuel economy and safety of automobiles, there is a strong demand for increasing the strength and thickness of automobile steel sheets. Automobile steel sheets need various formability depending on the part, but especially the suspension parts such as suspension arms are subjected to burring, etc., so the hot-rolled steel sheet is required to have high hole-expandability. Is done. Moreover, since there are many parts with complicated shapes and various processes are performed, it is required that the elongation is also excellent.

  In recent years, particularly high strength has been promoted, and a steel sheet has been proposed in which ferrite is the main phase and Ti and Nb carbides are finely dispersed to achieve both strength and workability such as hole expansion. (For example, Patent Documents 1 and 2). However, these have a limit in the increase in strength, and the processability is not sufficient.

  Moreover, the steel plate which aimed at the further high intensity | strength using carbide | carbonized_materials, such as Ti and Mo, is also proposed (for example, patent documents 3-5).

JP 2002-322543 A JP 2006-274317 A JP 2003-089848 A JP 2007-063668 A JP 2004-143518 A

  However, the steel sheet proposed in Patent Document 3 contains a large amount of Mo, and the steel sheet proposed in Patent Document 4 contains a large amount of V. Therefore, there exists a problem that an alloy cost becomes high. Furthermore, the steel plate proposed in Patent Document 5 needs to be cooled during hot rolling in order to refine crystal grains. Therefore, there exists a problem that manufacturing cost becomes high.

  The present invention does not contain a large amount of expensive elements, does not require a special production method, has high strength, particularly tensile strength of 780 MPa or more, and is excellent in workability, particularly ductility and hole expandability. The object is to provide a high-strength hot-rolled steel sheet and a method for producing the same.

The present inventors examined a method for improving the balance between strength and workability of a steel sheet in which the metal structure is substantially a ferrite single phase and the ferrite is strengthened by fine dispersion of Ti and Nb precipitates. As a result, in order to improve the balance between strength and workability, it has been found that it is effective to suppress variation in strength of each crystal grain of ferrite. Furthermore, it was found that in order to produce such a steel sheet, it is important to lower the start temperature of the ferrite transformation and precipitate Ti and Nb precipitates almost simultaneously with the start of the ferrite transformation.
This invention is made | formed based on such knowledge, The summary is as follows.

(1) By mass%, C: 0.010 to 0.100%, Mn: 1.0 to 3.0%, Nb: 0.010 to 0.100%, Ti: 0.05 to 0.15% Si: 1.50% or less, Al: 0.150% or less, P: 0.040% or less, S: 0.0150% or less, N: 0.0100% or less, the balance being Fe And Ae ₃ [° C.] obtained by the following (formula 1) is 900 ° C. or less, and the metal structure is composed of ferrite with a volume ratio of 95% or more, and the remaining pearlite, bainite, or both. A hot-rolled steel sheet with excellent hole expandability characterized by
Ae ₃ = 937-477C + 56Si-20Mn-16Cu-15Ni-5Cr
+ 38Mo + 125V + 136Ti-19Nb + 198Al + 3315B (Formula 1)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, and B are the content [% by mass] of each element.
(2) By mass%, containing one or more of V: 0.20% or less, Mo: less than 0.30%, W: 0.01 to 2.00% A hot-rolled steel sheet having excellent hole expandability as described in 1).
(3) The hot-rolled steel sheet having excellent hole expansibility according to the above (1) or (2), which contains B: 0.0020% or less in mass%.
(4) The above (1) to (3) characterized by containing one or more of Cr: 0.01 to 2.00 % and Ni : 0.01 to 2.00 % by mass%. ) Is a hot-rolled steel sheet excellent in hole expansibility.
(5) Any one of (1) to (4) above, characterized by containing one or both of REM: 0.0005 to 0.0100% and Ca: 0.0005 to 0.0100% in mass%. 2. A hot-rolled steel sheet excellent in hole expansibility according to item 1.

(6) The hot rolled steel sheet having excellent hole expansibility according to any one of claims 1 to 5, wherein the Bs point calculated by the following (formula 2) is 700 ° C or lower.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element.
(7) The hot rolled steel sheet having excellent hole expansibility according to any one of claims 1 to 5, wherein the Bs point calculated by the following (formula 2) is less than 600 ° C.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element.

(8) After heating the steel piece which has a chemical component of any one of said (1)-(5) to 1050-1300 degreeC, the finishing temperature FT [degreeC] is set to 950-1050 degreeC, and it hot-rolls. And cooling at a cooling rate of 30 ° C./s or higher, and winding at a winding temperature CT [° C.] within the range of 550 to 700 ° C. and equal to or higher than Bs [° C.] in the following (Formula 2). A method for producing a hot-rolled steel sheet excellent in hole expansibility characterized by
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
Here, C, Mn, Mo, Ni, and Cr are the content [% by mass] of each element.
(9) After heating the steel slab having the chemical component described in (6) above to 1050 to 1300 ° C., hot rolling is performed at a finishing temperature FT [° C.] of 950 to 1050 ° C. Production of a hot-rolled steel sheet excellent in hole expansibility, which is cooled at a cooling rate and wound at a winding temperature CT [° C.] of 700 ° C. or lower and Bs [° C.] or higher of the following (Formula 2) Method.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element.
(10) After heating the steel slab having the chemical component described in (7) above to 1050 to 1300 ° C., hot rolling is performed at a finishing temperature FT [° C.] of 950 to 1050 ° C. Production of a hot-rolled steel sheet excellent in hole expansibility, which is cooled at a cooling rate and wound at a winding temperature CT [° C.] of less than 600 ° C. and equal to or higher than Bs [° C.] in the following (Formula 2) Method.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element.

  According to the present invention, a high-strength heat that does not contain a large amount of expensive elements, does not require a special manufacturing method, has a tensile strength of 780 MPa or more, and is excellent in workability, particularly ductility and hole expandability. A high-strength hot-rolled steel sheet that can provide a rolled steel sheet and a method for producing the same, and that has high strength and excellent elongation and hole-expandability can be obtained, and the industrial contribution is extremely remarkable.

  Since ferrite has a low dislocation density, steel (ferrite single-phase steel) whose metal structure is composed of ferrite has high ductility and hole expandability. Therefore, it is considered that the strength, ductility and hole expandability can be improved at the same time by precipitation strengthening the ferrite single phase steel. However, the conventional precipitation-strengthened ferrite single-phase hot-rolled steel sheet has different precipitates in size, density, and amount, so that the strengthening amount is not uniform and there is a difference in hardness. This is because, after hot rolling, precipitates grow and become coarse in ferrite generated at high temperature, and fine precipitates are generated in ferrite generated at low temperature.

  That is, even when the metal structure is substantially a ferrite single phase, the precipitation behavior is non-uniform, so that the structure has a large difference in hardness and the hole expandability is not necessarily improved. Therefore, the present inventors have studied to reduce the variation in the precipitation state in each crystal grain of ferrite. As a result, it was found that when Nb and Ti carbides precipitate in ferrite during cooling after hot rolling, the size and amount of the final precipitates change depending on the formation temperature of each crystal grain of ferrite. .

Specifically, in steel components, in order to improve the hole expansion property of precipitation strengthened ferritic single phase steel, Ti and Nb are added in combination to reduce the ferrite transformation start temperature during cooling after hot rolling. This is very important.
In the manufacturing method, hot rolling is finished at a high temperature, and the cooling start rate is controlled to lower the transformation start temperature, and fine precipitates are generated almost uniformly in each crystal grain of ferrite. Was found to be possible. That is, it is necessary to suppress the formation of precipitates by increasing the cooling rate in a high temperature region during cooling, specifically, above 700 ° C.

  Furthermore, it has been found that the coiling temperature needs to be a condition in which fine precipitates are generated and bainite is not generated. That is, in order to generate fine precipitates, it is necessary to set the coiling temperature within the range of 550 to 700 ° C., and to suppress the formation of bainite, the coiling temperature is set to the Bs point or higher. It is necessary.

Hereinafter, the present invention will be described in detail.
Ti and Nb are the most important strengthening elements in the present invention, and contribute to precipitation strengthening and fine grain strengthening by suppressing the growth of ferrite crystal grains. In order to ensure the strength of 780 MPa or more, it is necessary to add Nb 0.010% or more and Ti 0.05% or more. In order to further increase the strength, 0.020% or more of Nb is preferably added, and 0.08% or more of Ti is preferably added. On the other hand, when Nb exceeds 0.100% and Ti exceeds 0.15%, the heating temperature required for solid solution increases. Moreover, when Ti and Nb are added excessively, the hole expandability deteriorates due to the precipitates remaining during heating, and the punchability may deteriorate further. Therefore, the upper limit of the Nb amount is preferably 0.080% or less, and the upper limit of the Ti amount is preferably 0.14% or less. A more preferable upper limit of the Nb amount is 0.060% or less, and a more preferable upper limit of the Ti amount is 0.12% or less.

  In the present invention, C is an important element that contributes to precipitation strengthening. In order to obtain high strength, it is necessary to add 0.010% or more of C. In order to further increase the strength, 0.030% or more of C is preferably added, and a more preferable lower limit of the amount of C is 0.050% or more. However, if the amount of C is too large, pearlite and cementite are likely to be generated, and the hole expandability is deteriorated, so the upper limit is made 0.100%. The upper limit with preferable C amount is 0.080% or less, More preferably, it is 0.075% or less.

Mn is an element that greatly changes the transformation temperature, and is an extremely important element in the present invention. If the amount of Mn added is less than 1.0%, the ferrite transformation start temperature Ae ₃ rises, so that a difference in hardness between ferrite crystal grains occurs. Further, when the amount of Mn is small, the Bs point which is a bainite forming temperature also rises, and it becomes necessary to wind up at a high temperature, and the precipitate becomes coarse and the strength is lowered. Therefore, the preferable lower limit is 1.5% or more. Furthermore, the lower limit of the preferable amount of Mn is 1.8% or more. On the other hand, if Mn is more than 3.0%, the ferrite transformation is greatly delayed, and ferrite precipitates generated early during winding grow, and ferrite precipitates generated later in winding are fine. Therefore, a difference in hardness of each crystal grain of ferrite is generated, which causes deterioration of hole expanding property. The upper limit of the preferable amount of Mn is 2.8% or less, more preferably 2.5% or less.

Furthermore, in the present invention, it is necessary to lower the start temperature of the ferrite transformation. The starting temperature of ferrite transformation during cooling is lower than the starting temperature of ferrite transformation in the equilibrium state. In the present invention, if Ae ₃ [° C.] is set to 900 ° C. or less, the hardness difference of ferrite crystal grains is reduced. Based on the test result indicating that the ferrite transformation can be reduced, the index of the ferrite transformation start temperature was set to Ae ₃ [° C.] which is the ferrite transformation start temperature in the equilibrium state.

Ae ₃ [° C.] is determined by the following (formula 1) from the content [mass%] of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, and B.
Ae ₃ = 937-477C + 56Si-20Mn-16Cu-15Ni-5Cr
+ 38Mo + 125V + 136Ti-19Nb + 198Al + 3315B (Formula 1)
In the above (Formula 1), when the selective elements Cu, Ni, Cr, Mo, V, and B are not added intentionally, they may be calculated as 0.

  Si is a deoxidizing element, and if added in excess, the moldability decreases, so the upper limit is made 1.50% or less. Preferably it is 1.00% or less. In order to suppress the decrease in the adhesion of the plating due to the generation of scale, it is preferable to limit it to 0.50% or less. Furthermore, the upper limit of the preferable Si amount is 0.20% or less. Si is an element contributing to strengthening, and it is preferable to add 0.01% or more.

Although Al is a deoxidizing element, the upper limit is made 0.150% or less in order to greatly increase the start temperature of ferrite transformation. Preferably, it is 0.050% or less. A more preferable upper limit of the Al content is 0.030% or less. The lower limit is not particularly limited, but it is difficult to reduce it to 0.0005% or less.
P is an impurity element, and if it exceeds 0.040%, embrittlement of the weld becomes significant, so the upper limit is made 0.040% or less. Although the lower limit of P is not particularly defined, it is economically disadvantageous to make it less than 0.0001%.

  S is an impurity element and has an adverse effect on weldability, manufacturability during casting, and hot rolling, so the upper limit is made 0.0150% or less. Further, when S is contained excessively, coarse MnS is formed and the hole expanding property is lowered. Therefore, in order to improve the hole expanding property, the content is preferably limited to 0.0050% or less. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%.

  N is an impurity element, and if the content exceeds 0.0100%, coarse nitrides are formed and the bendability and hole expandability are deteriorated, so the upper limit is made 0.0100% or less. Moreover, since it will cause the blowhole generation | occurrence | production at the time of welding when content of N increases, it is preferable to reduce to 0.0050% or less. The lower limit is not particularly defined, but in order to make the N content less than 0.0005%, the manufacturing cost increases.

Furthermore, you may add 1 type (s) or 2 or more types of V, Mo, W, B, Cr, Cu, and Ni for high intensity | strength.
V, like Ti and Nb, is an element that contributes to precipitation strengthening. However, if excessively contained, alloy costs increase, so the upper limit is preferably made 0.20% or less. A more preferable upper limit is 0.15% or less. On the other hand, for precipitation strengthening, it is preferable to add 0.005% or more of V.

Mo is an element that contributes to precipitation strengthening in the present invention. However, if excessively contained, the alloy cost increases, so the upper limit is preferably made less than 0.30%. On the other hand, in order to increase the strength, it is preferable to add 0.05% or more of Mo.
W, like Mo, is an element that contributes to precipitation strengthening in the present invention. If excessively contained, the alloy cost increases, so the upper limit is preferably made 2.00% or less. On the other hand, a preferable lower limit is 0.01% or more because of an increase in strength.

  B is an element that contributes to strengthening of the grain boundary. However, when the addition amount exceeds 0.0020%, ferrite transformation is delayed, and bainite and martensite may be generated to deteriorate the hole expanding property. Therefore, the upper limit of B is preferably 0.0020% or less. Moreover, in order to improve hole expansibility by strengthening grain boundaries, it is preferable to add 0.0001% or more of B.

  Cr, Cu, and Ni are elements that contribute to strengthening, and it is preferable to add one or more of them. In addition, in order to raise intensity | strength, a preferable minimum is 0.01% or more, respectively. On the other hand, if these elements are added excessively, the manufacturability may be impaired, so the preferable upper limit is 2.00% or less, respectively.

Furthermore, in order to control the form of inclusions, one or both of Ca and REM may be added.
Ca and REM are effective elements for controlling the form of oxides and sulfides, and preferable lower limit values are 0.0005% or more, respectively. On the other hand, if added excessively, the moldability may be impaired, so the preferable upper limit is 0.0100% or less, respectively.
In the present invention, REM refers to La and lanthanoid series elements, which are often added by misch metal, and contain a series of elements such as La and Ce. Metal La or Ce may be added.

  The metal structure of the hot-rolled steel sheet of the present invention is substantially a ferrite single phase. The reason why the metal structure is substantially a ferrite single phase is that in a multiphase structure with martensite or bainite, strain tends to concentrate in the vicinity of the hard phase at the time of molding, and the hole expandability deteriorates. In the present invention, the substantially single-phase ferrite microstructure means that the volume fraction of ferrite is 95% or more and the balance is one or both of pearlite and bainite. Therefore, the sum of the volume ratios of one or both of pearlite and bainite is 5% or less, and preferably 0%.

Moreover, in order to suppress the production | generation of a bainite, it is preferable to reduce the temperature which a bainite produces | generates, ie, Bs point. In order to obtain a structure having a single phase of ferrite, the Bs point is preferably set to 700 ° C. or lower. Moreover, when the coiling temperature is lowered to less than 600 ° C., the properties of the end face after punching can be improved. When the coiling temperature is less than 600 ° C., it is preferable to set the Bs point to less than 600 ° C. in order to obtain a structure that is a ferrite single phase.
Bs [° C.] can be determined from the content [mass%] of C, Mn, Mo, Ni, and Cr by (Expression 2) described later. In addition, when Mo, Ni, and Cr which are selective elements are not added intentionally, it calculates as 0.

In order to improve the hole expansion property, it is effective to reduce the hardness variation between the grains of ferrite. The hot-rolled steel sheet of the present invention has a substantially ferrite single-phase metal structure that is homogeneously precipitation-strengthened, and therefore is extremely excellent in hole expansibility. In order to obtain such a structure, in the hot-rolled steel sheet of the present invention, the start temperature of ferrite transformation is lowered to suppress the formation of low strength ferrite with coarse precipitates. Specifically, Ae _{3 is set} to 900 ° C. or lower to prevent the start of ferrite transformation in a high temperature region exceeding 900 ° C. As a result, all ferrite is generated in substantially the same temperature range, and variations in the size and amount of precipitates precipitated in the ferrite are reduced.

Hereinafter, a method for producing a hot-rolled steel sheet of the present invention having a ferrite single-phase structure in which precipitates are finely dispersed homogeneously will be described.
First, a steel piece is manufactured by melting and casting steel by a conventional method. The casting is preferably continuous casting from the viewpoint of productivity. The obtained steel slab is heated, hot-rolled, cooled and wound up.
When the heating temperature of the hot-rolled steel slab is less than 1050 ° C., Ti and Nb are not sufficiently dissolved and do not contribute to precipitation strengthening, so it is necessary to set the heating temperature to 1050 ° C. or higher. On the other hand, if the heating temperature exceeds 1300 ° C, the crystal grain size becomes coarse, so the upper limit is made 1300 ° C or less.

  The hot rolling finishing temperature FT [° C.] is extremely important for ferrite transformation and Ti and Nb precipitate formation during cooling after hot rolling. When FT exceeds 1050 ° C., recrystallized grains after rolling grow and become coarse. As a result, dislocations that form precipitates are reduced. Therefore, the precipitates are not finely dispersed and the strength is lowered, the hardness of the ferrite varies, and the hole expandability is lowered. On the other hand, when FT is lower than 950 ° C., dislocations introduced by rolling become precipitate nuclei, precipitates are formed and grow in austenite, the hardness of ferrite varies, and the hole expandability decreases. Moreover, since the solid solution amount of elements such as Nb and Ti that contribute to precipitation strengthening decreases, precipitation in ferrite becomes insufficient, and the strength decreases.

  If the cooling rate after hot rolling is 30 ° C./s or more, precipitation at over 700 ° C. is suppressed during cooling. As a result, it is possible to suppress the formation of ferrite whose hardness is reduced due to the coarsening of precipitates, and to suppress the variation in hardness depending on the part. On the other hand, when the cooling rate is less than 30 ° C./s, the strength decreases due to coarsening of precipitates, pearlite and cementite are generated, and the hole expandability deteriorates.

  The coiling temperature CT [° C.] needs to be in the range of 550 to 700 ° C. in order to make the metal structure substantially a ferrite single phase and generate fine precipitates. When CT exceeds 700 ° C., pearlite is generated, and the hole expandability deteriorates. Moreover, when CT is high, in addition to the driving force for Ti and Nb precipitation being reduced, precipitates are liable to grow and the amount of precipitation strengthening is reduced. On the other hand, when CT is less than 550 ° C., the diffusion cannot be sufficiently performed, so that the amount of precipitation decreases and the amount of strengthening decreases.

  Precipitation of the precipitate is likely to proceed particularly in the temperature range of 600 to 650 ° C. For this reason, when the coiling temperature CT [° C.] is 600 to 650 ° C., the precipitate deposited on the grain boundary may grow and become coarse. Coarse precipitates generated at the grain boundaries may cause defects called peeling as seen on the end surfaces after cutting or punching. Although peeling has a small influence on the hole expanding property, it is not preferable in appearance. Therefore, it is desirable to prevent occurrence of peeling by setting the winding temperature CT to less than 600 ° C.

Furthermore, in order to avoid the formation of bainite, it is necessary to set CT to a temperature at which bainite is generated, that is, the Bs point or higher. Bs [° C.] can be determined from the content [mass%] of C, Mn, Mo, Ni, and Cr by the following (formula 2). In addition, when Mo, Ni, and Cr which are selective elements are not added intentionally, it calculates as 0.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)

"Example 1"
Steel pieces obtained by melting and casting steel having chemical components shown in Table 1 are heated to 1200 ° C., hot-rolled at a finishing temperature (FT) shown in Table 2, and cooling rates shown in Table 2 It was cooled at (cooling speed) and wound up at the winding temperature (CT) shown in Table 2. In Table 2, the Bs point calculated by the above (Formula 2) from the component composition of Table 1 is also shown.

A sample was collected from the obtained hot-rolled steel sheet, and the metal structure was observed using an optical microscope. As the preparation of the sample, a plate thickness cross section in the rolling direction was polished as an observation surface and etched with a Nital reagent or a SULC-G reagent. The obtained microstructure was subjected to image analysis to determine the area ratio of ferrite, bainite, and pearlite.
Tensile strength (TS) and elongation at break (El) were evaluated in accordance with JIS Z 2241 using No. 5 test piece of JIS Z 2201. The hole expansion test was evaluated according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. The results are shown in Table 3. Λ in Table 3 is the hole expansion rate.

  As shown in Table 3, the production No. Examples 1 to 15 are examples of the present invention, which have high tensile strength (TS) and excellent ductility (EL) and hole expansibility (λ).

On the other hand, production No. No. 21 has a high Ae _{3 of} steel P, and the hole expansion rate is lowered due to the variation in the hardness of the ferrite. In addition, production No. Since No. 22 has little Nb content of steel Q, the intensity | strength is falling. Production No. Since No. 23 has little Ti content of the steel R, the intensity | strength is falling. Furthermore, production No. In No. 24, since the Ti content of steel S is large, the strength increases and the hole expansion rate decreases. Production No. No. 25 has a high Nb content in steel T, so the strength increases and the hole expansion rate decreases.

Production No. For 16-20, the production conditions are outside the scope of the present invention.
Production No. Since FT of 16 and 17 is not appropriate, the strength is reduced, and the hardness of the ferrite varies, resulting in a decrease in the hole expansion rate. Production No. No. 18 has a high CT, pearlite is produced, the hole expanding property is deteriorated, and the precipitation strengthening is not sufficient, so that the strength is also lowered. Production No. In No. 19, CT is lower than Bs, bainite is generated, and ductility is lowered. Production No. No. 20 has a slow cooling rate, precipitates are coarsened, the strength is lowered, pearlite is generated, and the hole expandability is deteriorated.

"Example 2"
Steel pieces obtained by melting and casting steel having chemical components shown in Table 4 are heated to 1200 ° C., hot-rolled at a finishing temperature (FT) shown in Table 5, and cooling rates shown in Table 5 The product was cooled at (cooling speed) and wound at a winding temperature (CT) shown in Table 5. In Table 5, the Bs point calculated by the above (Formula 2) from the component composition of Table 4 is also shown.

A sample was collected from the obtained hot-rolled steel sheet, and the metal structure was observed in the same manner as in Example 1 to determine the area ratio of ferrite, bainite, and pearlite. Further, in the same manner as in Example 1, tensile strength (TS), breaking elongation (El), and hole expansion rate (λ) were evaluated.
Furthermore, production No. Four samples were prepared for each of the hot-rolled steel plates 26 to 31 and the end face punched out with a clearance of 12.5% was observed to confirm the presence or absence of peeling. And when peeling is seen, measure the length of the part where peeling is seen, and divide the total length by the length of the entire circumference of the punched end face to determine the peeling rate of each sample. The average value of the four samples was calculated, and the production No. It was set as the peeling rate of each hot rolled steel sheet of 26 to 31. The results are shown in Table 6.

Production No. Nos. 26 to 31 are all wound at a winding temperature of Bs [° C.] or higher, the metal structure is ferrite having a volume ratio of 95% or higher, high tensile strength (TS), ductility (EL) and hole. Excellent spreadability (λ).
In addition, the production No. 1 was wound at a winding temperature of less than 600 ° C. Nos. 26, 28 and 30 have a low peeling rate and almost no peeling appears. On the other hand, the production No. was wound at a winding temperature of 600 ° C. In 27, 29 and 31, the peeling rate is large.

Claims (10)

% By mass
C: 0.010-0.100%
Mn: 1.0 to 3.0%
Nb: 0.010-0.100%
Ti: 0.05 to 0.15%
Containing
Si: 1.50% or less,
Al: 0.150% or less,
P: 0.040% or less,
S: 0.0150% or less,
N: limited to 0.0100% or less, the balance is made of Fe and inevitable impurities,
Ae ₃ [° C.] calculated by the following (formula 1) is 900 ° C. or less,
A hot-rolled steel sheet excellent in hole expansibility, characterized in that the metal structure is composed of ferrite having a volume ratio of 95% or more and one or both of remaining pearlite and bainite.
Ae ₃ = 937-477C + 56Si-20Mn-16Cu-15Ni-5Cr
+ 38Mo + 125V + 136Ti-19Nb + 198Al + 3315B (Formula 1)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, and B are the content [% by mass] of each element. % By mass
V: 0.20% or less,
Mo: less than 0.30%,
W: 0.01-2.00%
The hot-rolled steel sheet excellent in hole expansibility according to claim 1, comprising one or more of the following. % By mass
B: 0.0020% or less is contained, The hot rolled steel sheet excellent in hole expansibility according to claim 1 or 2. % By mass
Cr: 0.01 to 2.00 %,
Ni : 0.01 to 2.00%
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. % By mass
REM: 0.0005 to 0.0100%,
Ca: 0.0005 to 0.0100%
One or both of these are contained, The hot-rolled steel plate excellent in the hole expansibility of any one of Claims 1-4 characterized by the above-mentioned. The hot-rolled steel sheet having excellent hole expansibility according to any one of claims 1 to 5, wherein the Bs point calculated by the following (formula 2) is 700 ° C or lower.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element. The hot-rolled steel sheet excellent in hole expansibility according to any one of claims 1 to 5, wherein the Bs point calculated by the following (Formula 2) is less than 600 ° C.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element. After heating the steel slab having the chemical component according to any one of claims 1 to 5 to 1050 to 1300 ° C, hot rolling is performed at a finishing temperature FT [° C] of 950 to 1050 ° C,
Cool at a cooling rate of 30 ° C / s or more,
Manufacture of a hot-rolled steel sheet excellent in hole expansibility, characterized by being wound at a coiling temperature CT [° C.] within the range of 550 to 700 ° C. and equal to or higher than Bs [° C.] in the following (Formula 2) Method.
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
Here, C, Mn, Mo, Ni, and Cr are the content [% by mass] of each element. After heating the steel slab having the chemical component according to claim 6 to 1050 to 1300 ° C, hot rolling is performed at a finishing temperature FT [° C] of 950 to 1050 ° C,
Cool at a cooling rate of 30 ° C / s or more,
A method for producing a hot-rolled steel sheet having excellent hole-expanding properties, wherein winding is performed at a coiling temperature CT [° C.] of 700 ° C. or lower and Bs [° C.] of the following (Formula 2).
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element. After heating the steel slab having the chemical component according to claim 7 to 1050 to 1300 ° C, hot rolling is performed at a finishing temperature FT [° C] of 950 to 1050 ° C,
Cool at a cooling rate of 30 ° C / s or more,
A method for producing a hot-rolled steel sheet having excellent hole expansibility, wherein the steel sheet is wound at a coiling temperature CT [° C.] of less than 600 ° C. and equal to or higher than Bs [° C.] in the following (Formula 2).
Bs = 830-270C-90 (Mn + Mo) -37Ni-70Cr (Formula 2)
In (Formula 2), C, Mn, Mo, Ni, and Cr are content [mass%] of each element.