JP5446900B2 - High tensile hot-rolled steel sheet having high bake hardenability and excellent stretch flangeability and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile hot-rolled steel sheet, and more particularly, to a high-tensile hot-rolled steel sheet having high bake hardenability and excellent stretch flangeability that can significantly increase tensile strength by painting and baking after processing, and a method for producing the same.

  In recent years, bake-hardening type high-tensile hot-rolled steel sheets have been proposed that add a large amount of N to applications such as automobile structural members and underbody members, and use the strain aging during paint baking to increase the strength of the members. ing. This high-tensile hot-rolled steel sheet exhibits high bake hardenability that can not only increase the yield strength YS, which is an indicator of BH properties, but also significantly increase the tensile strength TS by painting and baking after processing. There are features.

  As an example of such a bake-hardening type high-tensile hot-rolled steel sheet, Patent Document 1 includes mass%, C: 0.02 to 0.13%, Si: 2.0% or less, Mn: 0.6 to 2.5%, sol.Al: 0.10% or less. , N: 0.0080-0.0250%, the balance is steel composed of Fe and inevitable impurities, or Ca: 0.0002-0.01%, Zr: 0.01-0.10%, rare earth elements: 0.002-0.10% and Cr: 3.0% or less The steel containing one or more of the above is subjected to hot rolling to finish finishing rolling at 850-950 ° C directly after casting or after reheating to 1100 ° C or higher, and then cooling rate of 15 ° C / second or higher Discloses a method for producing a hot-rolled steel sheet excellent in bake hardenability and workability after being cooled to 350 ° C. or lower. Patent Document 2 includes mass%, C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02. And the balance is composed of Fe and inevitable impurities, and has a structure whose main phase is ferrite with an average crystal grain size of 8 μm or less, and further has a solid solution N content of 0.003 to 0.01% by mass%. The ratio of the average solid solution N concentration Ngb existing in the range of ± 5 nm from the ferrite crystal grain interface to the average solid solution N concentration Ng existing in the ferrite crystal grain, Ngb / Ng in the range of 100,000 to 10,000 A high-tensile hot-rolled steel sheet excellent in certain bake hardenability, fatigue resistance, impact resistance and room temperature aging resistance is disclosed. Further, Patent Document 3 includes mass%, C: 0.05 to 0.12%, Si: 0.5% or less, Mn: 1.2 to 3.0%, P: 0.05% or less, Al: 0.001 to 0.1%, and N: 0.005 to 0.02. And the balance has a composition of Fe and inevitable impurities, the low-temperature transformation ferrite phase has an area ratio of 10 to 50%, and the balance substantially has a steel structure of polygonal ferrite phase, and the above A high-tensile hot-rolled steel sheet excellent in bake hardenability and ductility has been proposed in which the average crystal grain size of the two phases of the low-temperature transformation ferrite phase and the polygonal ferrite phase is 8 μm or less.

Japanese Unexamined Patent Publication No. 4-74824 JP 2000-297350 A JP2003-49243

Umemoto et al .: Heat treatment 24 (1984) p334

  However, the hot-rolled steel sheet produced by the method described in Patent Document 1 has a problem that the room temperature aging resistance is inferior and the ductility deteriorates with time. The high-tensile hot-rolled steel sheet described in Patent Document 2 has excellent room temperature aging resistance, but does not provide sufficiently high bake hardenability, and can only increase TS below 90 MPa by paint baking after processing. . The high-tensile hot-rolled steel sheet described in Patent Document 3 has excellent room temperature aging resistance and high bake hardenability, but may have a problem with stretch flangeability.

  The present invention has high bake hardenability capable of increasing TS of 90 MPa or more without any problem in normal temperature aging, and high tensile strength having excellent stretch flangeability with a hole expansion ratio λ of 85% or more as an index of stretch flangeability. It aims at providing a hot-rolled steel plate and its manufacturing method.

  Based on the high-tensile hot-rolled steel sheet described in Patent Document 3, the present inventors diligently studied to improve the stretch flangeability. It has been found that it is extremely effective to reduce the size, and that it is necessary to appropriately control the cooling conditions after hot rolling.

  The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.12%, Si: 0.5% or less, Mn: 1.2 to 3.0%, P: 0.05% or less, Al: 0.001 -0.1%, N: 0.005-0.02%, with the balance being composed of Fe and inevitable impurities, including a low-temperature transformation ferrite phase and a polygonal ferrite phase, the low-temperature transformation ferrite phase and the polygonal ferrite The area ratio of the total structure of the phase is 90% or more, the area ratio of the low-temperature transformation ferrite phase is 10 to 50%, and the average crystal grain size of the low-temperature transformation ferrite phase and the polygonal ferrite phase. Has a microstructure in which the ratio Hv (max) / Hv (min) of Hv (max) and Hv (min) of the low-temperature transformation ferrite phase is 2.2 μm or less, And a high-tensile hot-rolled steel sheet having excellent stretch flangeability.

  However, the Hv (max) and Hv (min) of the low-temperature transformation ferrite phase are the specimens from three positions corresponding to 1/4, 1/2, 3/4 in the width direction at the center in the longitudinal direction of the steel sheet. Samples were taken, and the thickness cross section of each specimen was observed with a scanning electron microscope (SEM) at 5 magnifications at 1000 magnifications for a total of 15 visual fields, and the Vickers hardness of 10 low temperature transformation ferrite phases for each visual field. The maximum hardness and the minimum hardness are obtained for each field of view, and the maximum and minimum hardnesses are arithmetically averaged by the number of fields of observation.

  In the high-tensile hot-rolled steel sheet of the present invention, the composition may further include at least one selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less by mass%. preferable. Furthermore, a composition containing at least one selected from Ti: 0.1% or less and Nb: 0.1% or less is preferable.

  The high-tensile hot-rolled steel sheet of the present invention is a steel having the above composition, heated to 1000-1300 ° C, hot-rolled at a finishing temperature of 800-1000 ° C, and averaged 50 ° C / second or more within 0.5 seconds. Primary cooling to a cooling stop temperature of 620 to 720 ° C at a cooling rate, air cooling for 2 to 10 seconds, secondary cooling at an average cooling rate of 80 ° C / second or more, and winding at a winding temperature of 400 to 500 ° C It can be manufactured by a method.

  The present invention makes it possible to produce a high-tensile hot-rolled steel sheet having high bake hardenability that can increase TS of 90 MPa or more and excellent stretch flangeability that can obtain λ of 85% or more without any problem in normal temperature aging. It was. The high-tensile hot-rolled steel sheet of the present invention is suitable for automobile structural members and underbody members.

  Hereinafter, the high-tensile hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be described in detail. Note that “%”, which is a unit of content of components, means “mass%” unless otherwise specified.

1) Composition
C: 0.05-0.12%
C is an element useful not only for increasing the strength of steel but also for suppressing the coarsening of crystal grains. However, if the amount is less than 0.05%, the effect is poor. On the other hand, if the C content exceeds 0.12%, the weldability deteriorates. Therefore, the C content is 0.05 to 0.12%.

Si: 0.5% or less
Si is an element that increases the strength of steel by solid solution strengthening, and the amount thereof is appropriately adjusted according to the required strength. However, when the amount exceeds 0.5%, not only the ductility deteriorates, but also the formation of the low temperature transformation ferrite phase is inhibited. Therefore, the Si content is 0.5% or less.

Mn: 1.2-3.0%
Mn is a solid solution strengthening element and a basic element for obtaining a high-tensile steel sheet. It also contributes effectively to the generation of low temperature transformation ferrite phase. However, if the amount is less than 1.2%, the effect is poor. On the other hand, if the amount of Mn exceeds 3.0%, not only ductility deteriorates but also weldability is adversely affected. Therefore, the amount of Mn is set to 1.2 to 3.0%.

P: 0.05% or less
P is an element that increases the strength of steel, and the amount thereof is appropriately adjusted according to the required strength. However, if the amount exceeds 0.05%, the weldability deteriorates, segregates at the grain boundaries to cause grain boundary cracks, and further, the formation of the low temperature transformation ferrite phase is inhibited. Therefore, the P content is 0.05% or less.

Al: 0.001 to 0.1%
Al is an element useful as a deoxidizing agent and needs to be contained at least 0.001% in order to obtain a sufficient deoxidizing effect. On the other hand, when the Al content exceeds 0.1%, not only the surface properties deteriorate, but also it becomes difficult to secure the solid solution N amount necessary for bake hardenability. Therefore, the Al amount is 0.001 to 0.1%. More preferably, the Al content is 0.001% or more and 0.04% or less.

N: 0.005-0.02%
N is an especially important element in the present invention, and has an effect of expressing high bake hardenability that increases TS as well as YS by solid solution in steel and paint baking after processing. In order to obtain such an effect, the N amount needs to be 0.005% or more, but if it exceeds 0.02%, the ductility deteriorates. Therefore, the N content is 0.005 to 0.02%, preferably 0.007 to 0.02%.

  The balance is Fe and inevitable impurities, but for the following reasons, at least one selected from Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Ti: 0.1% or less, It is preferable to contain at least one selected from Nb: 0.1% or less individually or simultaneously.

Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less
All of Cr, Mo and Ni not only effectively contribute to increasing the strength of the steel by solid solution strengthening, but also have the effect of stabilizing the austenite phase and promoting the formation of a low temperature transformation ferrite phase. However, if the amount of Cr, Mo and Ni exceeds 1.0%, the ductility deteriorates. Therefore, the amount of each is preferably 1.0% or less. In order to obtain the effects of Cr, Mo, and Ni as described above, each amount is more preferably 0.1% or more.

Ti: 0.1% or less, Nb: 0.1% or less
Ti and Nb contribute to the improvement of strength and toughness by forming carbides and nitrides, respectively. However, if the amounts of Ti and Nb each exceed 0.1%, solid solution N is precipitated as nitrides and the bake hardenability is lowered. Therefore, the respective amounts are preferably 0.1% or less. In order to obtain the effects of Ti and Nb as described above, the respective amounts are more preferably 0.01% or more.

2) Micro structure
2-1) Area ratio of the total structure of the low-temperature transformation ferrite phase and polygonal ferrite phase: 90% or more, and area ratio of the low-temperature transformation ferrite phase in the entire structure: 10 to 50%
The high-tensile hot-rolled steel sheet of the present invention has a microstructure substantially composed of a low-temperature transformation ferrite phase and a polygonal ferrite phase, and high strength is achieved by the low-temperature transformation ferrite phase, and a TS of 90 MPa or more is achieved. A high bake hardenability that enables an increase in the thickness is obtained, and a high ductility is achieved by the polygonal ferrite phase.

  Here, the low temperature transformation ferrite phase is a ferrite phase obtained by transforming the austenite phase at a low temperature of about 500 ° C. or lower, and means a bainitic ferrite phase or an upper bainite phase. In such a low-temperature transformation ferrite phase, the dislocation density is high, so that high strength is achieved, and solid solution N adheres mainly to the dislocation density further increased by processing during paint baking after processing. It is thought that high bake hardenability that also increases the amount of heat resistance is obtained. However, if the area ratio of the low temperature transformation ferrite phase in the entire structure is less than 10%, such an effect is not sufficiently exerted, and if it exceeds 50%, the polygonal ferrite phase is reduced and the ductility deteriorates. The area ratio of the entire phase structure needs to be 10 to 50%.

  In addition to the low-temperature transformation ferrite phase and polygonal ferrite phase, ductility deteriorates if other phases such as martensite phase and pearlite phase are included in the entire structure in excess of 10%. The area ratio of the total of the phase and the polygonal ferrite phase in the entire structure needs to be 90% or more.

  The area ratio of the low-temperature transformation ferrite phase and the polygonal ferrite phase is determined by the corrosion of the steel sheet in the direction perpendicular to the rolling direction of the hot-rolled steel sheet due to corrosion by nital, and observed at 1000 times magnification by SEM. Asked.

2-2) Average crystal grain size of low-temperature transformation ferrite phase and polygonal ferrite phase: 8 μm or less If the average crystal grain size of low-temperature transformation ferrite phase and polygonal ferrite phase exceeds 8 μm, TS can be increased by 90 MPa or more. Since high bake hardenability cannot be obtained and the normal temperature aging resistance deteriorates, the average crystal grain size of both phases needs to be 8 μm or less. When crystal grains are refined, the grain boundaries increase, so a higher dislocation density is achieved during processing, and high bake hardenability is obtained. Also, solid solution N trapped at the grain boundaries increases and room temperature aging is suppressed. It is thought that it is done.

  The average crystal grain size of the low temperature transformation ferrite phase and the polygonal ferrite phase was determined as follows. That is, the structure of the thickness cross section in the direction perpendicular to the rolling direction of the hot-rolled steel sheet was revealed by corrosion by nital, and the structure photograph taken at a magnification of 1000 by SEM was used to determine the quadrature method specified by ASTM. The larger of the values and the nominal grain size obtained by the cutting method described in Non-Patent Document 1 was defined as the average crystal grain size.

2-3) Hv (max) / Hv (min) of low-temperature transformation ferrite phase: 2.2 or less It is effective to minimize the local hardness fluctuation of the steel. In particular, in order to improve the stretch flangeability so that λ of 85% or more is obtained, the ratio Hv (max) / Hv (min) of the low-temperature transformation ferrite phase defined as follows: Hv (max) / Hv (min) needs to be 2.2 or less, preferably 1.9 or less.

  Here, Hv (max) and Hv (min) of the low-temperature transformation ferrite phase are the test pieces from three positions corresponding to 1/4, 1/2, and 3/4 in the width direction at the center in the longitudinal direction of the steel sheet. The thickness cross section of each specimen was observed by SEM at 5 fields at a magnification of 1000 times, a total of 15 fields, and the Vickers hardness of 10 low-temperature transformation ferrite phases was measured for each field. The maximum hardness and the minimum hardness are obtained for each, and the maximum hardness and the minimum hardness that are arithmetically averaged by the number of observation fields are represented.

3) Manufacturing conditions
3-1) Heating temperature before hot rolling: 1000-1300 ° C
In order to obtain high bake hardenability, it is necessary to secure a sufficient amount of solute N in the hot-rolled steel sheet after winding, but it is necessary to dissolve the nitride during heating before hot rolling. . However, if the heating temperature is less than 1000 ° C., the nitride is not completely dissolved, and a sufficient amount of dissolved N cannot be secured in the hot-rolled steel sheet. On the other hand, when the heating temperature exceeds 1300 ° C., the austenite phase becomes coarse, and it becomes difficult to make the average crystal grain size of the ferrite phase of the hot-rolled steel sheet 8 μm or less. Therefore, the heating temperature before hot rolling is 1000 to 1300 ° C, preferably 1100 to 1250 ° C.

3-2) Hot rolling finishing temperature: 800 ~ 1000 ℃
When the finishing temperature is below 800 ° C., a part of the processed structure remains, the microstructure becomes non-uniform, and stretch flangeability deteriorates. On the other hand, when the finishing temperature exceeds 1000 ° C., it becomes difficult to make the average crystal grain size of the ferrite phase of the hot-rolled steel sheet 8 μm or less. Therefore, the finishing temperature of hot rolling is set to 800 to 1000 ° C.

3-3) Primary cooling condition: Cooling to the cooling stop temperature of 620-720 ° C at an average cooling rate of 50 ° C / second or more within 0.5 seconds after hot rolling If rapid cooling is not started within 0.5 seconds after hot rolling In addition, if the average cooling rate during quenching is less than 50 ° C / second, it becomes difficult to make the average crystal grain size of the ferrite phase of the hot-rolled steel sheet 8 μm or less, and N precipitates as nitride, It is impossible to secure a sufficient amount of solute N. In addition, it is necessary to generate a polygonal ferrite phase for the purpose of increasing ductility. For this purpose, cooling is stopped at a temperature of 620 to 720 ° C. at which transformation to the polygonal ferrite phase is promoted, and the following is described. It is necessary to air-cool under conditions. Therefore, it is necessary to perform primary cooling to a cooling stop temperature of 620 to 720 ° C. at an average cooling rate of 50 ° C./second or more within 0.5 seconds after hot rolling.

  The upper limit of the average cooling rate is not particularly limited, but is preferably set to 130 ° C./second or less in order to ensure sufficient water drainage when water is used for cooling.

3-4) Air cooling time after primary cooling: 2-10 seconds
If the air cooling time after the primary cooling is less than 2 seconds, the amount of polygonal ferrite phase is insufficient and the ductility deteriorates, and if it exceeds 10 seconds, not only does the ferrite phase become coarse, but the amount of polygonal ferrite phase also increases. It becomes too much, and it becomes difficult to secure a sufficient amount of the low-temperature transformation ferrite phase thereafter. Therefore, the air cooling time after the primary cooling is 2 to 10 seconds.

3-5) Secondary cooling conditions: Cooling at an average cooling rate of 80 ° C / second or more When the average cooling rate in the secondary cooling after air cooling is less than 80 ° C / second, film boiling cooling tends to occur, and the local cooling rate Therefore, Hv (max) / Hv (min) of the low-temperature transformation ferrite phase exceeds 2.2, and the stretch flangeability is deteriorated. Therefore, the average cooling rate in the secondary cooling is set to 80 ° C./second or more which is nucleate boiling cooling.

  The upper limit of the average cooling rate is not particularly limited, but is preferably 130 ° C./second or less in order to ensure sufficient water drainage like the primary cooling.

3-6) Winding temperature: 400 ~ 500 ℃
When the coiling temperature is higher than 500 ° C., not only is it difficult to obtain a low-temperature transformed ferrite phase with an area ratio of 10% or more, but the average crystal grain size of the ferrite phase cannot be made 8 μm or less. On the other hand, when the temperature is lower than 400 ° C., a large amount of the lower bainite phase and martensite phase are generated, and the total area ratio of the low-temperature transformation ferrite phase and the polygonal ferrite phase cannot be 90% or more. Therefore, the coiling temperature is 400 to 500 ° C.

  To melt the steel having the composition of the present invention, both a converter and an electric furnace can be used. Moreover, the steel thus melted is made into a slab by ingot-bundling rolling or continuous casting. The slab is usually heated and then hot-rolled by rough rolling and finish rolling. In addition, in the case of the slab manufactured by continuous casting, you may apply the direct feed rolling which heats as it is or keeps heat in order to suppress a temperature fall. Moreover, in hot rolling, in order to ensure finishing temperature, a to-be-rolled material can also be heated by heating means, such as a sheet bar heater, during hot rolling.

  Steel Nos. A to I having the composition shown in Table 1 were melted in a converter and then made into a slab by continuous casting, and hot-rolled steel plates No. 1 to 20 having a thickness of 2.0 mm were formed according to the hot rolling conditions shown in Table 2. Manufactured.

Then, a sample for microstructural observation is taken from a position corresponding to 1/2 in the width direction at the center in the longitudinal direction of the hot-rolled steel sheet, and the area occupying the entire structure of the low-temperature transformation ferrite phase and the polygonal ferrite phase by the above method. The average crystal grain size was determined. Further, JIS No. 5 test specimens were taken from the same position of the hot-rolled steel sheet from the direction perpendicular to the rolling direction, and subjected to a tensile test at a strain rate of 10 ⁻³ / sec to obtain YS, TS, and total elongation El. Further, λ was obtained by the following method to evaluate stretch flangeability, ΔYS and ΔTS to evaluate bake hardenability, and ΔEl to evaluate normal temperature aging.
λ: After punching a 10mmφ hole in the center of the test piece using a 130mm square test piece taken from the position corresponding to 1/2 in the width direction at the center of the hot rolled steel sheet The 60 ° conical punch was pushed up from the side opposite to the burr, and the hole diameter dmm at the time when the crack penetrated the plate thickness was measured and calculated from the following equation.
λ (%) = [(d-10) / 10] × 100
ΔYS, ΔTS: JIS No. 5 test specimen was sampled from the center in the longitudinal direction of the hot-rolled steel sheet at a position corresponding to 1/2 of the width direction and perpendicular to the rolling direction. A heat treatment simulating paint baking for 5 minutes was performed, a tensile test was performed at a strain rate of 10 ⁻³ / sec, and the following formula was calculated. At this time, the stress before heat treatment after prestraining and TS before prestraining were also determined by conducting a tensile test at the same strain rate.
ΔYS = YS after heat treatment-Stress before heat treatment after prestraining ΔTS = TS after heat treatment-TS before prestraining
ΔEl: JIS No. 5 test specimen was sampled from the center of the hot-rolled steel sheet in the longitudinal direction from the position corresponding to 1/2 of the width direction and perpendicular to the rolling direction, and subjected to aging treatment at 50 ° C x 400 hours, strain rate A tensile test was performed at 10 ⁻³ / sec and calculated from the following formula. At this time, El before aging treatment was also obtained by conducting a tensile test at the same strain rate. Here, if ΔEl is 2.0% or less, it can be said that there is no problem in aging at room temperature.
ΔEl = El before aging treatment-El after aging treatment
Further, Hv (max) / Hv (min) of the low-temperature transformation ferrite phase was obtained by the above-described method. Here, Hv (max) and Hv (min) are the center part in the longitudinal direction of the steel sheet and in the width direction. Test specimens were collected from three positions corresponding to 1/4, 1/2, 3/4, and the plate thickness cross section of each test specimen was observed by SEM with 5 fields at 1000x magnification, for a total of 15 fields. The Vickers hardness of 10 low-temperature transformation ferrite phases is measured for each field, the maximum hardness and the minimum hardness are obtained for each field, and the maximum hardness [Hv (max)] and minimum hardness [Hv ( At this time, the Vickers hardness was determined according to JIS Z2244: 2009 using a micro Vickers hardness tester, and the test load (test force) at that time was 10 g (0.098 N).

  The results are shown in Table 3. Steel plates No. 1, 4, 7, 9, 12, 15, 17, and 19 according to the present invention have λ of 85% or more and ΔTS of 90 MPa or more, high bake hardenability and excellent stretch flangeability It can be seen that this is a high-tensile hot-rolled steel sheet having In addition, the high-tensile hot-rolled steel sheet according to the present invention has an excellent balance between strength and ductility when TS × El is 17000 MPa ·% or more, and ΔEl is 2.0% or less, and there is no problem with room temperature aging.

Claims (4)

  In mass%, C: 0.05 to 0.12%, Si: 0.5% or less, Mn: 1.2 to 3.0%, P: 0.05% or less, Al: 0.001 to 0. 1%, N: 0.005 to 0.02%, with the balance being composed of Fe and inevitable impurities, including a low-temperature transformation ferrite phase and a polygonal ferrite phase, The area ratio in the total structure of the polygonal ferrite phase is 90% or more, the area ratio in the entire structure of the low-temperature transformation ferrite phase is 10 to 50%, and the average of the low-temperature transformation ferrite phase and the polygonal ferrite phase It has a microstructure in which the crystal grain size is 8 μm or less and the ratio Hv (max) / Hv (min) of Hv (max) and Hv (min) of the low-temperature transformation ferrite phase is 2.2 or less. High bake hardenability A high-tensile hot-rolled steel sheet having excellent stretch flangeability; however, the Hv (max) and Hv (min) of the low-temperature transformation ferrite phase are the center part in the longitudinal direction of the steel sheet and are 1/4, 1/2 in the width direction. Specimens were sampled from three positions corresponding to 3/4, and the thickness cross section of each specimen was observed with a scanning electron microscope (SEM) at five magnifications of 1000 fields for a total of 15 fields. The Vickers hardness of each of the ten low-temperature transformation ferrite phases is measured to determine the maximum hardness and the minimum hardness for each field of view, and the maximum and minimum hardnesses are arithmetically averaged by the number of observation fields.   Furthermore, it has a composition containing at least one selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less in mass%. A high-tensile hot-rolled steel sheet having high bake hardenability and excellent stretch flangeability according to Item 1.   Furthermore, it has the composition containing at least 1 sort (s) chosen from Ti: 0.1% or less and Nb: 0.1% or less by mass%, The high of Claim 1 or 2 characterized by the above-mentioned. High-tensile hot-rolled steel sheet with bake hardenability and excellent stretch flangeability. The steel having the composition according to any one of claims 1 to 3 is heated to 1000 to 1300 ° C, hot-rolled at a finishing temperature of 800 to 1000 ° C, and then within 50 seconds at 50 ° C / second. Primary cooling to a cooling stop temperature of 620 to 720 ° C. at the above average cooling rate, air cooling for 2 to 10 seconds, secondary cooling at an average cooling rate of 80 ° C./second or more, and winding temperature of 400 to 500 ° C. The low-temperature transformation ferrite phase and the polygonal ferrite phase are included, and the total area ratio of the low-temperature transformation ferrite phase and the polygonal ferrite phase occupies 90% or more. The area ratio of the ferrite phase in the entire structure is 10 to 50%, the average crystal grain size of the low-temperature transformation ferrite phase and the polygonal ferrite phase is 8 μm or less, and the Hv (max) and Hv of the low-temperature transformation ferrite phase. The ratio Hv (max) / Hv (min ) is the method of producing a high tensile hot-rolled steel sheet having a high baking hardenability and excellent stretch-flange formability having microstructure is 2.2 or less in min). However, Hv (max) and Hv (min) of the low-temperature transformation ferrite phase are the center portions in the longitudinal direction of the steel sheet, and the test pieces are taken from three positions corresponding to 1/4, 1/2, 3/4 in the width direction. Samples were taken, and the cross-sectional thickness of each specimen was observed with a scanning electron microscope (SEM) at 5 fields at a magnification of 1000 times, a total of 15 fields, and the Vickers hardness of 10 low-temperature transformation ferrite phases for each field. The maximum hardness and the minimum hardness are obtained for each field of view, and the maximum and minimum hardnesses are arithmetically averaged by the number of fields of observation.