JP6676973B2 - Hot rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet and a method for producing the same.

_2. Description of the Related Art In recent years, from the viewpoint of improving fuel efficiency by reducing the weight of a vehicle body for the purpose of reducing CO ₂ emissions and stricter collision safety standards, increasing the strength of vehicle body parts has been promoted. Further, from the viewpoint of resource saving, it is desired to obtain high strength with a small alloy addition amount in a steel sheet as a material of a body part.

  Against this background, the shapes of body parts have been diversified. For this reason, the hot-rolled steel sheet is required to have not only high strength but also various properties such as press formability, weldability, and impact resistance. In particular, excellent stretch flangeability is indispensable for underbody parts and structural members, and when used for such applications, desired ductility and hole-expandability are required.

In Patent Document 1, C: 0.01 to 0.2% (in the present specification, “%” related to the chemical composition means “% by mass” unless otherwise specified), Si: 0.01 to 2% , Mn: 0.1-2%, P ≦ 0.1%, S ≦ 0.03%, Al: 0.001-0.1%, N ≦ 0.01%, the balance being Fe and impurities After rough rolling of a slab having the chemical composition consisting of the following, finish rolling is completed in a temperature range of Ar ₃ transformation point + 50 ° C. or higher, and after 0.5 seconds, the temperature range of Ar _{3 to} 500 ° C. is raised to 80 ° C. By cooling to a temperature range of 500 ° C. or lower at a cooling rate of 500 ° C./second or higher and winding, the metal structure is mainly a continuous cooling transformed structure (Zw) having an average particle size of more than 8 μm and 30 μm or less. Manufacture hot-rolled steel sheet having both BH property and stretch flangeability with tensile strength of 370-490 MPa class. Invention is disclosed.

  In the invention disclosed in Patent Document 1, the metallographic structure is controlled to a continuous cooling transformation structure to homogenize the structure, and eliminate the interface between the hard phase and the soft phase, which are the starting points of voids, thereby improving stretch flangeability (hole expansion). Nature).

Patent Document 2 discloses that C: 0.05 to 0.40%, Si: 1.0 to 3.0%, Mn: 0.6 to 3.0%, and Cr: 0.2 to 2.0%. And at least one selected from Ti: 0.005 to 0.25%, Nb: 0.003 to 0.1%, and V: 0.003 to 0.1%, with the balance being It has a chemical composition of substantially Fe, the main phase is pro-eutectoid ferrite, the second phase has a steel structure composed of martensite, acicular ferrite, and retained austenite, and the main phase is a pro-eutectoid ferrite phase. the hardness H _V 180 or more, the main phase and by the difference [Delta] H _V in hardness between the second phase and 200 or less, excellent in the balance between impact resistance and strength and elongation, spread fatigue resistance and hole An invention relating to a high-strength, high-workability hot-rolled steel sheet having excellent workability is disclosed.

  In the invention disclosed in Patent Document 2, the hardness difference between the soft phase ferrite and the hard second phase is reduced by adding a precipitation strengthening element such as Ti, Nb, or V to increase the hardness of the proeutectoid ferrite. Improve spreadability.

JP 2005-82841 A JP-A-11-189842

  Steel strengthening mechanisms include solid solution strengthening, precipitation strengthening (particle dispersion strengthening), dislocation strengthening, and crystal grain refinement strengthening, and these various strengthening methods can be combined to increase the strength of the steel. On the other hand, it is known that ductility of steel is deteriorated by increasing strength. Hitherto, a dual phase steel such as DP steel and TRIP steel having both high ductility and strength has been developed.

  On the other hand, the ductility of a material is established by the addition of uniform elongation and local elongation, and it is said that the stretch flangeability of a material has a particularly strong correlation with local elongation. Further, the stretch flangeability of a material is considered to have a high correlation with the hole expandability evaluated by a hole expanding test, and is often used as a simple evaluation method of stretch flangeability.

  It is known that DP steels and TRIP steels having a dual phase structure have low local elongation, and therefore have poor hole-expandability. This is because the soft phase and the hard phase are mixed, and voids are likely to be generated at the interface between the different phases during punching or during stretch flange deformation. For this reason, various high-strength steel sheets have been developed in which the difference in hardness (hardness ratio) between the soft phase and the hard phase is reduced to homogenize the structure and improve hole expanding properties.

  As a specific improvement technique for improving the hole expanding property, (i) suppression of generation of coarse cementite, which is a fracture starting point due to reduction in carbon content, and control of inclusions by adding REM, (ii) disclosed in Patent Document 1 As in the invention, the homogenization of the structure containing bainite or tempered martensite as a main phase, and (iii) as in the invention disclosed in Patent Document 2, the hardness of the soft phase is increased in the steel having a dual phase structure. Utilization of precipitation strengthening elements for system carbides, solid solution strengthening with substitutional elements such as Si, and the like are known.

  However, the technique (i) has a problem of an increase in decarburization cost due to a reduction in the amount of carbon and a problem of accuracy of REM addition, and thus has a problem in mass productivity. Further, in the technique (ii), the ductility of the material is deteriorated due to a decrease in uniform elongation. Furthermore, in the technique (iii), not only the alloy cost is increased, but also the hot-dip galvanizing property and the chemical conversion property may be deteriorated, and there is a problem in securing surface properties.

  An object of the present invention is to provide a hot-rolled steel sheet suitable for use as a reinforcing member of an automobile body such as an undercarriage part of an automobile or a cross member, etc. An object of the present invention is to provide a low-cost hot-rolled steel sheet having mechanical properties of 440 to 590 MPa, a hole expansion ratio of 75% or more, and a total elongation of 30% or more.

  In view of such a background, the present inventors increase the amount of solute carbon in ferrite as a technique for securing high strength, hole expandability, and ductility (total elongation) with a small amount of alloy addition. The increase in ferrite hardness due to aging was studied.

  As a result, by increasing the hardness of the ferrite by quenching and aging hardening by the carbon supersaturated in the ferrite, the hardness ratio of the ferrite to the hard second phase existing in the structure can be reduced. It has been found that the hole expanding property can be improved while having strength.

  The present inventors have further studied and found that, in order to secure the amount of supersaturated solid solution carbon in ferrite, it is important to prevent precipitation of iron carbide such as pearlite and cementite as much as possible in the cooling process after finish rolling. It was found that.

  Furthermore, in order to obtain such a structure, the present inventors ensure sufficient amount of solute carbon in the ferrite by performing primary cooling after stopping the cooling once within a predetermined temperature range after finish rolling. In the subsequent secondary cooling process, it has been found that winding may be performed in a temperature range of 200 ° C. or lower where generation of cementite and iron carbide is suppressed.

  The present invention has been completed based on these new findings, and is as described below.

(1) Chemical composition: C: 0.07 to 0.14%, Si: 0.01 to 0.2%, Mn: 0.3 to 1.2%, P: 0.10% or less, S: 0.03% or less, Al: 0.001 to 0.3%, N: 0.010% or less, the balance being Fe and impurities,
The metal structure is a double-phase structure having an area ratio of 70% or more of polygonal ferrite and 15% or less of pearlite, which is a hard second phase, wherein the ferrite particle size of the polygonal ferrite is 15 μm or less; The amount of solute carbon contained in the ferrite is 5 ppm or more, the hardness of the ferrite is 160 Hv or more, and the hardness ratio of the hard second phase to the ferrite (hard second phase hardness / ferrite hardness) is 1 .5 or less,
A mechanical property is a hot-rolled steel sheet having a tensile strength (TS) of 440 MPa or more and less than 590 MPa, a hole expansion ratio (λ) of 75% or more, and a total elongation (El) of 30% or more.

(2) a heating step of heating the billet to a temperature range of 1100 to 1300 ° C;
A homogenizing treatment step of holding the steel slab in the temperature range for 30 minutes or more after the heating step;
After the homogenization process, rough rolling is performed in a temperature range of 1000 ° C. or more to form a rough rolled plate, and then the rough rolled plate is finish-rolled, and the final pass finishing temperature of the finish rolling is set to (Ar ₃ -10) a rolling step of completing rolling at a temperature of not less than ℃,
A primary cooling step of performing cooling at an average cooling rate of 15 ° C./second or more on the steel sheet surface after the rolling step, and stopping cooling in a temperature range of 500 to 700 ° C. for 1 to 5 seconds;
A secondary cooling step of cooling the steel sheet surface at an average cooling rate of 10 to 100 ° C./sec after the primary cooling step,
2. The method for producing a hot-rolled steel sheet according to claim 1, further comprising: a winding step of winding the steel sheet surface in a temperature range of 200 ° C. or less after the secondary cooling step.

However, Ar ₃ (° C.) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P] (1)
[% C], [% Si], [% Mn] and [% P] in the equation (1) indicate the contents (% by mass) of C, Si, Mn and P, respectively.

  The “hot-rolled steel sheet” according to the present invention belongs to a hot-rolled steel sheet specified in JIS G 3113 (hot-rolled steel sheet for automobile structure and steel strip).

  The hot-rolled steel sheet according to the present invention has high tensile strength of 440 MPa or more and less than 590 MPa, and has a hole expansion ratio of 75% or more and a total elongation of 30% or more. It can be suitably used as a material for underbody members such as wheels and wheel rims, and members such as chassis and various members.

  Furthermore, the hot-rolled steel sheet according to the present invention stabilizes the balance between high strength and excellent hole-expanding properties without containing a large amount of precipitation strengthening by MC-based carbides such as Ti and Nb and solid solution strengthening elements such as Si. It is also possible to reduce environmental load and manufacturing cost.

The present invention will be described.
1. Hot-rolled steel sheet according to the present invention (1) Chemical composition (1-1) C: 0.07 to 0.14%
C is an important element that secures the strength of the hot-rolled steel sheet and strengthens the ferrite. If the C content is less than 0.07%, a tensile strength of 440 MPa or more cannot be secured. Therefore, the C content is 0.07% or more, preferably 0.08% or more. On the other hand, if the C content exceeds 0.14%, the starting point of cracks at the time of hole expansion increases because the amount of pearlite increases, and the hole expansion property of the hot-rolled steel sheet deteriorates. Therefore, the C content is 0.14% or less, preferably 0.12% or less, and more preferably 0.11% or less.

(1-2) Si: 0.01 to 0.2%
Si suppresses the formation of cementite and improves the hole expanding property of the hot-rolled steel sheet. If the Si content is less than 0.01%, this effect cannot be obtained. Therefore, the Si content is at least 0.01%, preferably at least 0.02%. On the other hand, if the Si content exceeds 0.2%, the chemical conversion property and the plating property of the hot-rolled steel sheet are impaired. For this reason, the Si content is 0.2% or less, and preferably 0.1% or less.

(1-3) Mn: 0.3 to 1.2%
Mn strengthens a hot-rolled steel sheet by solid solution strengthening. If the Mn content is less than 0.3%, a tensile strength of 440 MPa or more cannot be secured. Therefore, the Mn content is at least 0.3%, preferably at least 0.4%. On the other hand, if the Mn content exceeds 1.2%, the ductility and hole expanding property of the hot-rolled steel sheet deteriorate due to Mn segregation and formation of MnS. Therefore, the Mn content is at most 1.2%, preferably at most 1.1%.

(1-4) P: 0.10% or less P exists as an impurity in steel and deteriorates the weldability of a hot-rolled steel sheet. Therefore, the smaller the P content, the better. If the P content exceeds 0.10%, the weldability of the hot-rolled steel sheet deteriorates. For this reason, the P content is 0.10% or less, preferably 0.03% or less.

(1-5) S: 0.03% or less S is present in steel as an impurity, and easily combines with Mn to form MnS, thereby deteriorating the hole-expanding properties of a hot-rolled steel sheet. Therefore, the smaller the S content, the better. If the S content exceeds 0.03%, the hole-expandability of the hot-rolled steel sheet deteriorates due to the formation of MnS. For this reason, the S content is 0.03% or less, and preferably 0.01% or less.

(1-6) Al: 0.001 to 0.3%
Al suppresses the formation of cementite similarly to Si, and improves the hole expanding property of the hot-rolled steel sheet. If the Al content is less than 0.001%, this effect cannot be obtained. For this reason, the Al content is at least 0.001%, preferably at least 0.005%, and more preferably at least 0.01%. On the other hand, when the Al content exceeds 0.3%, nonmetallic inclusions increase, and the hole expanding property of the hot-rolled steel sheet deteriorates. Therefore, the Al content is at most 0.3%, preferably at most 0.2%.

(1-7) N: 0.010% or less N is present in steel as an impurity and combines with Al to form nonmetallic inclusions such as AlN, thereby deteriorating the hole-expanding properties of the hot-rolled steel sheet. For this reason, the N content is 0.010% or less, and preferably 0.003% or less.

  The balance other than the above is Fe and impurities. In addition, the impurity means a component mixed due to a raw material such as ore and scrap and other factors when a steel material is industrially manufactured.

(2) Metallic structure (2-1) In terms of area ratio, polyphase structure having polygonal ferrite: 70% or more and pearlite, which is a hard second phase: 15% or less, When polygonal ferrite is present as a steel structure, It acts as the main phase responsible for the deformation of the material and increases the ductility of the hot rolled steel sheet. If the amount of polygonal ferrite is less than 70% in area ratio, it is impossible to secure a total elongation (El) of 30% or more. Therefore, the polygonal ferrite has an area ratio of 70% or more, preferably 80% or more, and more preferably 85% or more.

  The higher the area ratio of the polygonal ferrite is, the higher the ductility is obtained, which is preferable. However, in order to secure the area ratio of the pearlite as the hard second phase, the area ratio of the polygonal ferrite is preferably 99% or less. , More preferably 97% or less.

  When pearlite is present in steel, it has a double-phase structure and can have high strength. However, if pearlite is present in an area ratio exceeding 15%, the area of the interface between cementite and ferrite constituting pearlite increases, and the origin of voids increases during hole expansion deformation. The property is deteriorated. Therefore, the area ratio of pearlite is 15% or less, and preferably 8% or less. In order to ensure high strength, it is preferable that the pearlite has an area ratio of 1% or more.

  The hot-rolled steel sheet according to the present invention is basically composed of polygonal ferrite, which is the main phase, and pearlite, which is the second phase. Site) in an area ratio of 5% or less.

(2-2) Ferrite grain size of polygonal ferrite: 15 μm or less The crystal grain size affects the strength of a hot-rolled steel sheet. Therefore, to increase the strength of the hot-rolled steel sheet, the smaller the ferrite grain size, the better. If the ferrite grain size of the polygonal ferrite exceeds 15 μm, the effect of strengthening the grain refinement is reduced, and a desired tensile strength cannot be obtained. For this reason, the ferrite particle size of the polygonal ferrite is 15 μm or less, preferably 10 μm or less. In the present invention, the lower limit of the ferrite grain size does not need to be particularly defined. When the hot-rolled steel sheet according to the present invention is manufactured by the manufacturing method according to the present invention described below, the lower limit of the ferrite grain size in the steel is 2 μm. It is.

(2-3) The amount of solute carbon contained in ferrite: 5 ppm or more The amount of solute carbon in ferrite is the most important parameter in the present invention, and the greater the amount of solute carbon, the greater the hardness of ferrite. . If the amount of solute carbon is less than 5 ppm, the hardness of the ferrite becomes small, and the ratio of the hardness to the hard second phase increases, so that the desired hole expanding property cannot be obtained. Therefore, the amount of solute carbon contained in the ferrite is 5 ppm or more, preferably 10 ppm or more. The amount of carbon dissolved in ferrite is at most 200 ppm.

(2-4) Ferrite hardness: 160 Hv or more, and hardness ratio of hard second phase to ferrite (hard second phase hardness / ferrite hardness): 1.5 or less The higher the ferrite hardness is, Since the hardness ratio of the hard second phase can be reduced, the ferrite hardness is preferably high. If the hardness of the ferrite is less than 160 in Hv, the hardness ratio with the hard second phase becomes large, and the desired hole expanding property cannot be obtained. For this reason, the hardness of ferrite is 160 Hv or more.

  The hard second phase defined in the present invention is pearlite, and basically contains no bainite or martensite. If the hardness ratio of the hard second phase to the ferrite is large, voids are likely to be formed at the interface between the hard second phase and the ferrite at the time of hole expansion molding, and the desired hole expansion property cannot be obtained. For this reason, the hardness ratio of pearlite to ferrite hardness (hardness of hard second phase / hardness of ferrite) is set to 1.5 or less.

(3) Mechanical properties (3-1) Tensile strength (TS): 440 MPa or more and less than 590 MPa The hot-rolled steel sheet according to the present invention has a tensile strength of 440 MPa or more and less than 590 MPa. When the tensile strength is less than 440 MPa, it is not only possible to easily achieve the hole-expanding property and the total elongation defined in the present invention, but also to use it as a reinforcing member for an automobile body which requires rigidity. Not suitable. Therefore, the hot-rolled steel sheet according to the present invention has a tensile strength of 440 MPa or more.

  On the other hand, if the tensile strength is 590 MPa or more, the total elongation (El) is degraded, so that applicable parts are limited to ensure press formability. For this reason, the hot-rolled steel sheet according to the present invention has a tensile strength of less than 590 MPa.

(3-2) Hole expansion ratio (λ): 75% or more, total elongation (El): 30% or more The hot-rolled steel sheet according to the present invention has a hole expansion ratio (λ): 75% or more, total elongation (El). : By securing mechanical properties of 30% or more, it can be used as a material for various automobile members including undercarriage members and reinforcing members of automobiles.

2. Production method according to the present invention (1) Heating step A slab having the above-mentioned chemical composition produced by continuous casting or the like is heated to a temperature range of 1100 to 1300 ° C. If the heating temperature of the steel slab is lower than 1100 ° C., the homogenization of Mn does not proceed and Mn is unevenly distributed, so that the hole-expandability of the hot-rolled steel sheet deteriorates. On the other hand, when the heating temperature of the slab exceeds 1300 ° C., not only the scale loss becomes large, but also the prior austenite grain size becomes large and the finally obtained ferrite grain size becomes coarse, so that the desired tensile strength is obtained. I can't get it. Therefore, the heating temperature of the slab is 1100 ° C. or more and 1300 ° C. or less, preferably 1200 ° C. or more and 1250 ° C. or less.

(2) Homogenization treatment step A homogenization treatment (solution treatment) is performed in which the steel slab heated in the heating step is kept in the above temperature range (1100 to 1300 ° C) for 30 minutes or more. If the holding time of the homogenization treatment is less than 30 minutes, the homogenization does not sufficiently proceed, and the hole expanding property of the hot-rolled steel sheet is deteriorated due to Mn segregation. Therefore, the holding time of the homogenization treatment is 30 minutes or more, preferably 55 minutes or more, and more preferably 60 minutes or more.

(3) Rolling Step After the slab having been subjected to the homogenization treatment is roughly rolled at a temperature of 1000 ° C. or more to form a rough rolled plate, the rough rolled plate is finish-rolled, and the final pass finishing temperature of the finish rolling is set to (Ar ₃ -10) to complete the rolling as above ° C.. However,
Ar ₃ (° C) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P] (1)
[% C], [% Si], [% Mn] and [% P] in the equation (1) indicate the contents (% by mass) of C, Si, Mn and P, respectively.

  If the temperature of the rough rolling is lower than 1000 ° C., recrystallization is suppressed, the rolling texture develops, and the hole expanding property of the hot-rolled steel sheet deteriorates. For this reason, the temperature of the rough rolling is 1000 ° C. or higher, preferably 1040 ° C. or higher, and more preferably 1050 ° C. or higher.

Following rough rolling, finish rolling is performed. The finish rolling is basically performed at _three or more Ar points in order to suppress a rolling texture that degrades the hole expanding property of the hot-rolled steel sheet. If the surface temperature of the steel sheet in the final pass of the finish rolling is lower than (Ar ₃ -10) ° C., rolling is performed in a two-phase region, so that abnormal grain growth of ferrite occurs and the ductility and hole expanding property of the hot-rolled steel sheet are reduced. to degrade. For this reason, the final pass finishing temperature of the finish rolling is (Ar ₃ −10) ° C. or more at the surface temperature of the steel sheet.

  Subsequent to rolling, two-stage cooling (primary cooling and secondary cooling) in different temperature ranges is performed.

(4) Primary Cooling Step After rolling, cooling (water cooling) is performed at a surface temperature of the steel sheet at an average cooling rate of 15 ° C./sec or more, and cooling (water cooling) is stopped in a temperature range of 500 to 700 ° C. for 1 to 5 seconds.

In primary cooling, the steel sheet surface is cooled at a high speed in order to refine crystal grains and increase strength. If the average primary cooling rate at the surface temperature of the steel sheet is less than 15 ° C./sec, the amount of pearlite generated during cooling increases, the hole expanding property of the hot-rolled steel sheet deteriorates, and the ferrite becomes excessively large. Therefore, a desired tensile strength cannot be obtained. Therefore, the average cooling rate of the primary cooling at a surface temperature of the steel sheet is at 15 ° C. / sec or more, good Mashiku is at 50 ° C. / sec or more, further preferably 60 ° C. / sec or more.

  Although the upper limit of the average cooling rate of the primary cooling need not be particularly defined, the average cooling rate of the primary cooling is usually 200 ° C./sec or less at the surface temperature of the steel sheet from the performance of the cooling device.

  After the primary cooling, the cooling (water cooling) is temporarily stopped. By stopping the cooling (water cooling), that is, stopping the injection of the cooling water, the structure is controlled to obtain high total elongation and hole-expandability.

At this time, the cooling stop temperature is 500 to 700 ° C. If the surface temperature of the steel sheet exceeds 700 ° C., the amount of pearlite increases and the hole-expandability of the hot-rolled steel sheet deteriorates. On the other hand, when the surface temperature of the hot-rolled steel sheet is less than 500 ° C., the amount of solute carbon in the ferrite decreases, and the strength of the ferrite becomes insufficient. Bainite and martensite are excessively generated as a phase, and a desired amount of ferrite cannot be obtained, so that desired ductility cannot be obtained.

  If the cooling stop time is less than 1 second, the amount of solute carbon in the ferrite decreases, and the strength of the ferrite becomes insufficient, so that the hole expandability of the hot-rolled steel sheet deteriorates. On the other hand, if the cooling stop time exceeds 5 seconds, pearlite is excessively generated, and the hole expanding property of the hot-rolled steel sheet is deteriorated. In this case, there is a possibility that the flatness of the steel sheet is lost. Therefore, the cooling stop time is set to 1 to 5 seconds.

(5) Secondary Cooling Step After primary cooling, cooling (water cooling) is performed at an average cooling rate of 10 to 100 ° C./sec at the surface temperature of the steel sheet. The secondary cooling mainly cools the surface of the steel sheet in order to control a structure serving as a fracture starting point during hole expansion forming.

  If the cooling rate of the steel sheet surface is lower than 10 ° C./sec, the cementite becomes coarse during cooling and deteriorates the hole expanding property of the hot-rolled steel sheet. On the other hand, if the cooling rate of the steel sheet surface exceeds 100 ° C./sec, bainite and martensite are formed in the hard phase, and the hardness ratio with ferrite becomes remarkably large. And the hole-expandability of the hot-rolled steel sheet deteriorates. For this reason, the cooling rate of the secondary cooling is 10 to 100 ° C./sec, preferably 30 to 80 ° C./sec on the surface of the steel sheet.

(6) Winding Step After secondary cooling, the steel sheet is wound around a coil in a temperature range of 200 ° C. or less on the surface of the steel sheet. When wound at a winding temperature of more than 200 ° C., iron carbide is formed, so the amount of solute carbon in the ferrite decreases, and the strength of the ferrite is insufficient, so that the hole expanding property of the hot-rolled steel sheet deteriorates. . For this reason, the winding temperature is 200 ° C. or less.

  Thus, the hot-rolled steel sheet according to the present invention is manufactured.

  The present invention will be described more specifically with reference to examples.

  A slab composed of steel types A to P and a to e having the chemical compositions shown in Table 1 was heated to the reheating temperature shown in Table 2, and held in this temperature range for the holding time shown in Table 2 to perform a homogenization treatment. Was.

  Then, after performing rough rolling at the rough rolling temperature shown in Table 2, it was subjected to finish rolling, and the rolling was completed at the finishing temperature shown in Table 2.

  After rolling, cooling (water cooling) was performed at the primary cooling rate shown in Table 2, and cooling (water cooling) was stopped at the cooling stop temperature and holding time shown in Table 2. After the primary cooling, secondary cooling was performed at the secondary cooling rate shown in Table 2. After the secondary cooling, the sample was wound around a coil at a winding temperature shown in Table 2 to obtain a sample No. 1 to 45 hot-rolled steel sheets were produced.

  Then, the sample No. The following tests were performed on 1 to 45 hot-rolled steel sheets.

(1) Evaluation of steel structure (1-1) Measurement method of metal structure The area ratio of ferrite and pearlite was measured by using an image processing method for a photograph obtained by observing the structure of an optical microscope. Specifically, the sample No. After performing buff finish polishing on a cross section parallel to the rolling direction of the hot-rolled steel sheets 1 to 45, a metal structure is revealed with a 4% nital solution (4% nitric acid + ethanol), and the thickness of the sheet is reduced using an optical microscope. After observing the 1 / 4t position at a magnification of 500 or 1000 times in 10 fields of view, specifying the ferrite and pearlite phases from the obtained observation image, measuring the area ratio of each phase using two-dimensional particle analysis software. did. Further, the ferrite particle diameter was an average value of the circle equivalent diameter obtained by two-dimensional particle analysis software.

(1-2) Method for measuring the amount of solute carbon Sample No. From 1 to 45 hot-rolled steel sheets, a sample having a thickness of 1.0 mm, a width of 5 mm, and a length of 110 mm was machined to obtain a solid solution carbon measurement sample. The amount of dissolved carbon was measured using an inverted hanging torsional vibration type internal friction tester, and the internal friction (Snoek peak height (Q ^-1 max)) determined from the logarithmic decrement of free vibration damping. The measurement was performed with a vibration cycle of 2.2 Hz, a temperature rising rate of 1 ° C./min, and a measurement temperature in the range of −10 ° C. to 110 ° C. The obtained Q ^-1 max was substituted into the following equation (2) to determine the amount of dissolved carbon [C].
Ar ₃ (° C) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P] (1)
[C] = 0.0043 × Tp × Q ^-1 max × 10000 (2)
[C]: amount of dissolved carbon (ppm), Tp: absolute temperature at which Snoek peak appears (K)

(1-3) Method for Measuring Hardness of Ferrite and Hard Second Phase For the ferrite and hard second phase observed in the above-described metallographic sample, an indentation load of 5 gf and a holding time were measured using a micro Vickers hardness tester. The Vickers hardness (Hv) was measured by hitting an indent under the condition of 10 seconds. The measurement was performed for each of 10 arbitrary ferrites and a hard second phase observed at a magnification of 500 times with an optical microscope, and the measurements were performed for 5 fields of view. And a second phase hardness.

(2) Evaluation of mechanical properties Using the hot-rolled steel sheets Nos. 1 to 45, the following tests were performed to evaluate the tensile properties and the hole expanding properties.

(2-1) Evaluation of tensile properties JIS No. 5B tensile test was taken from the rolling direction of the hot-rolled steel sheets 1 to 45, and the yield point YP, tensile strength TS, and total elongation El were measured in accordance with the test method specified in JIS Z2241. -2) Evaluation of hole expansion ratio Sample No. A 100 mm square base plate was cut out from 1 to 45 hot-rolled steel plates, and a 10 mm diameter punching process was performed at the center of the base plate using a universal testing machine. The punching clearance was about 12% in accordance with the Japan Iron and Steel Federation Standard (JFST1001-1996).

  A hole expanding test was performed on these base plates. The hole expansion test was performed by a method according to the Japan Iron and Steel Federation Standard (JFST1001-1996), and three measurements were performed on each sample under the same conditions, and the average value was defined as the hole expansion ratio (λ).

(2-3) Evaluation of plating property Hot-rolled steel sheets 1 to 45 were subjected to hot-dip galvanizing by removing the scale with 10% hydrochloric acid and then immersing the hot-rolled steel sheets in a hot-dip galvanizing bath whose Al concentration was adjusted to 0.13% by mass.

  With respect to the obtained hot-dip galvanized steel sheet, the plating property was determined by visually checking the presence or absence of non-plating (area where no plating was attached). Judgment of the plating property was performed by visually confirming 10 visual fields in each area of 100 mm x 100 mm for each steel sheet. If no plating was present even in one visual field, x; Was evaluated.

  Table 2 shows the results of the metal structure, mechanical properties, and surface properties together with the production conditions. The asterisks in Tables 1 and 2 indicate that the chemical composition or production conditions are out of the range specified in the present invention, or that the mechanical properties are low.

  Sample No. in Table 2 1,4,6,9-13,16,17,20,22,24,25,27,30-32,34-37,39 are examples of the present invention that satisfy all the conditions specified by the present invention. , Sample No. 2,3,5,7,8,14,15,18,19,21,23,26,28,29,33,38,40 to 45 are comparative examples that do not satisfy the conditions specified by the present invention. .

  As shown in Table 2, the sample No. 1,4,6,9-13,16,17,20,22,24,25,27,30-32,34-37,39 do not contain precipitation strengthening elements such as Ti, Nb, V, Tensile strength TS: 442 to 560 MPa, hole expansion ratio λ: 76 to 145%, total elongation: 30.5 to 38.4%, and good plating properties. For this reason, any of these examples of the present invention can be suitably used for automobile members with severe moldability, for example, underbody members such as wheels and wheel rims, and members such as chassis and various members.

  On the other hand, the sample No. In Sample No. 2, since the reheating temperature of the slab was 1050 ° C., which was lower than the lower limit of the range of the present invention, Mn segregated to form a heterogeneous structure, and the total elongation and hole-expandability were low.

  Sample No. In No. 3, since the reheating temperature of the slab was 1400 ° C., which was higher than the upper limit of the range of the present invention, the ferrite grain size was coarse and the tensile strength was low.

  Sample No. In No. 5, since the holding time of the homogenization treatment was 25 minutes, which was lower than the lower limit of the range of the present invention, Mn segregation occurred, the formation of MnS progressed, and the total elongation and hole-expandability became low.

  Sample No. In No. 7, since the temperature of the rough rolling was 950 ° C., which was lower than the lower limit of the range of the present invention, the recrystallization of old austenite grains did not proceed, the rolling texture was excessively developed, and the hole expandability was low.

Sample No. In No. 8, since the finishing temperature of the finish rolling is lower than 800 ° C. and lower than Ar ₃ −10 ° C. which is the lower limit of the present invention, the structure becomes coarse and non-uniform due to abnormal grain growth, and the tensile strength, the total elongation and the hole expanding property are reduced. Both were low.

  Sample No. In No. 14, since the primary cooling rate after the finish rolling was 13 ° C./sec, which was lower than the lower limit of the range of the present invention, pearlite was excessively generated and the hole expanding property was low. Further, the ferrite was coarsened and the tensile strength was low.

  Sample No. In No. 15, since the winding temperature after rolling was 550 ° C., which was higher than the upper limit of the range of the present invention, the amount of dissolved carbon in the ferrite was insufficient, the ferrite hardness was reduced, and the tensile strength was low. Further, since the hardness ratio of the hard second phase to ferrite was 1.6, which was higher than the upper limit of the range of the present invention, the hole expandability was low.

  Sample No. In No. 18, since the cooling stop temperature of the primary cooling was 750 ° C., which was higher than the upper limit of the range of the present invention, pearlite was excessively generated, and the hole expanding property was low.

  Sample No. No. 19 is that the cooling stop temperature of the primary cooling is 450 ° C., which is lower than the lower limit of the range of the present invention, so that the amount of dissolved carbon in the ferrite becomes insufficient and the ferrite hardness becomes low. The hardness ratio was increased, and the hole spreading property was low.

  Sample No. In No. 21, since the primary cooling rate after the finish rolling was 10 ° C / sec, which was lower than the lower limit of the range of the present invention, pearlite was excessively generated and the hole expanding property was low. Further, the ferrite was coarsened and the tensile strength was low.

  Sample No. No. 23, since the secondary cooling rate is 150 ° C./sec, which is higher than the upper limit of the range of the present invention, martensite is generated as the hard second phase, and the hardness ratio of the hard second phase to ferrite is significantly increased; Hole spreadability was low.

  Sample No. In No. 26, the holding time at the cooling stop temperature of the primary cooling was 10 seconds, which exceeded the upper limit of the range of the present invention, so that pearlite was excessively generated and the hole expanding property was low.

  Sample No. In No. 28, since the primary cooling rate after the finish rolling was 10 ° C./sec, which was lower than the lower limit of the range of the present invention, pearlite was excessively generated and the hole expanding property was low.

  Sample No. No. 29 has a winding temperature of 600 ° C., which is higher than the upper limit of the range of the present invention, so that the amount of solute carbon in the ferrite is insufficient, the ferrite hardness is lower than the lower limit of the range of the present invention, and the hard second phase with respect to ferrite. Was higher than the upper limit of the range of the present invention, and the hole expanding property was low. Also, the ferrite particle size exceeded the upper limit of the range of the present invention, and the tensile strength was low.

  Sample No. In No. 33, since the holding time of the homogenization treatment was 25 minutes, which was lower than the lower limit of the range of the present invention, Mn segregation occurred, and the formation of MnS proceeded, and the total elongation and the hole-expandability were low.

  Sample No. In No. 38, the cooling stop temperature of the primary cooling was 450 ° C., which was lower than the lower limit of the range of the present invention, so that bainite was excessively generated, and the tensile strength was 595 MPa, which was higher than the upper limit of the range of the present invention. In addition to the low ferrite content, the total elongation was low, the hardness ratio of the hard second phase to ferrite exceeded the upper limit of the range of the present invention, and the hole expandability was low.

  Sample No. In No. 40, since the secondary cooling rate was 5 ° C./sec, which was lower than the lower limit of the range of the present invention, cementite was coarsened during cooling, and the hole expanding property was inferior.

  Sample No. In No. 41, since the C content was 0.17%, which exceeded the upper limit of the range of the present invention, the amount of pearlite was excessive, and the hole expanding property was low.

  Sample No. In No. 42, since the Si content was 0.25%, which exceeded the upper limit of the range of the present invention, non-plating occurred during plating, and the surface properties were low.

  Sample No. In No. 43, since the Mn content was 1.40%, which exceeded the upper limit of the range of the present invention, Mn segregation occurred, and the total elongation and hole expanding properties were low.

  Sample No. In No. 44, since the Al content is 0.35%, which exceeds the upper limit of the range of the present invention, the amount of AlN, which is a nonmetallic inclusion, increases, and serves as a starting point of void generation at the time of hole expansion. It was low.

  Further, the sample No. In No. 45, since the N content was 0.011%, which exceeded the upper limit of the range of the present invention, AlN was excessively generated, and the hole expanding property was low.

Claims (2)

Chemical composition is in mass%
C: 0.07 to 0.14%,
Si: 0.01 to 0.2%,
Mn: 0.3-1.2%,
P: 0.10% or less,
S: 0.03% or less,
Al: 0.001 to 0.3%,
N: 0.0 03% or less,
The balance being Fe and impurities,
Metal structure, an area ratio, a duplex structure having 70% or more of polygonal Blow wells, more than 15% pearlite is a hard second phase, and inevitably formed tissue than 5%,
Ferrite grain size of the polygonal ferrite is 15 μm or less,
The amount of solute carbon contained in the ferrite is 5 ppm or more,
The hardness of the ferrite is 160 Hv or more, and the hardness ratio of the hard second phase to the ferrite (hard second phase hardness / ferrite hardness) is 1.5 or less;
A hot-rolled steel sheet having mechanical properties of a tensile strength of 440 MPa or more and less than 590 MPa, a hole expansion ratio of 75% or more, and a total elongation of 30% or more. A heating step of heating the steel slab having the chemical composition according to claim 1 to a temperature range of 1100 to 1300 ° C,
A homogenizing treatment step of holding the steel slab in the temperature range for 30 minutes or more after the heating step;
After performing the rough rolling in the temperature range of 1000 ° C. or more after the homogenization process to obtain a rough rolled plate,
A rolling step in which the rough rolling plate is subjected to finish rolling, and the final pass finishing temperature of the finish rolling is set to (Ar ₃ -10) ° C. or higher on the steel sheet surface to complete rolling.
A primary cooling step of performing cooling at an average cooling rate of 15 ° C./second or more on the steel sheet surface after the rolling step, and stopping cooling in a temperature range of 500 to 700 ° C. for 1 to 5 seconds;
A secondary cooling step of cooling the steel sheet surface at an average cooling rate of 10 to 100 ° C./sec after the primary cooling step,
The method for producing a hot-rolled steel sheet according to claim 1, further comprising: a winding step of winding the steel sheet surface in a temperature range of 200 ° C or less after the secondary cooling step.
However, Ar ₃ (° C.) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P] (1)
[% C], [% Si], [% Mn] and [% P] in the equation (1) indicate the contents (% by mass) of C, Si, Mn and P, respectively.