JP6303782B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet excellent in punching fatigue characteristics and workability and a method for producing the same.

  Conventionally, high-strength steel plates are often used for undercarriage parts and body parts for the purpose of reducing the weight of automobiles. A high strength steel sheet used for such an automobile is required to have excellent fatigue characteristics. Particularly for high-strength steel sheets used for undercarriage parts, further improvement in punching fatigue characteristics (fatigue characteristics after punching) is required. However, the conventional high-strength steel sheet cannot sufficiently improve the fatigue characteristics after punching, and an excellent fatigue crack propagation characteristic cannot be obtained.

  In order to improve the fatigue characteristics of a steel material without a notch, it is effective to refine the microstructure. For example, Patent Documents 1 and 2 describe hot-rolled steel sheets having a good strength-ductility balance and fatigue limit ratio (fatigue limit / tensile strength) in which the average grain size of ferrite is less than 2 μm. For example, Patent Document 3 describes a steel sheet having improved fatigue resistance characteristics by reducing the average crystal grains in the surface layer portion. For example, Patent Document 4 describes a steel pipe whose fatigue characteristics are improved by refining the martensite structure. However, even if the microstructure is refined, there is no effect of reducing the propagation speed of cracks, so it is difficult to improve the fatigue characteristics after punching of steel.

  On the other hand, it has been reported that the crack propagation speed can be reduced by forming a microstructure into a composite structure. For example, in Patent Document 5, a hot rolled steel sheet having a high fatigue limit ratio and excellent fatigue crack propagation characteristics can be obtained by dispersing hard bainite or martensite in a structure having fine ferrite as a main phase. It is described. Patent Document 5 describes that the above hot-rolled steel sheet can be used not only for a smooth member but also for a member having a notch. Patent Documents 6 and 7 describe that the crack propagation speed can be reduced by increasing the aspect ratio of martensite in the composite structure.

Japanese Patent Laid-Open No. 11-92859 Japanese Patent Laid-Open No. 11-152544 JP 2004-2111199 A JP 2010-70789 A JP 04-337026 A JP 2005-320619 A Japanese Patent Application Laid-Open No. 07-90478

  By the way, the steel plate used for a motor vehicle is a thin plate, and press molding is performed at the time of processing. However, Patent Document 5 does not describe a technique for improving the press formability of a steel sheet, and does not pay special attention to the hardness and shape of bainite and martensite. For this reason, it is thought that the steel plate described in Patent Document 5 does not have good press formability and is not suitable for a steel plate for automobiles. The steel plates described in Patent Documents 6 and 7 relate to thick plates and do not have workability such as ductility and hole expansibility required when performing press molding. For this reason, it is difficult to use the steel sheets described in Patent Documents 6 and 7 as automobile steel sheets.

  The present invention has been made to solve such problems, and an object of the present invention is to provide a hot-rolled steel sheet excellent in punching fatigue characteristics and workability and a method for producing the same.

The inventors of the present invention have intensively studied to improve the punching fatigue characteristics and workability of hot-rolled steel sheets. As a result, it was found that it is important for the hot-rolled steel sheet to satisfy the following requirements (a) to (e).
(A) Martensite is present in a volume fraction of 3% or more and less than 20% in a microstructure having ferrite as a main phase.
(B) 60% or more of the martensite grains in the center of the plate thickness have an aspect ratio of 3 or more.
(C) The average length in the rolling direction of the martensite grains at the center of the plate thickness is less than 20 μm.
(D) The variation in Vickers hardness of the martensite grains at the center of the plate thickness is small. Specifically, the martensite at the center of the plate thickness satisfies the following formula (i).
σ (Hv) / E (Hv) <0.16 (i)
In the formula (i), E (Hv) is an average value of Vickers hardness of the martensite grains at the center of the plate thickness, and σ (Hv) is a standard deviation of Vickers hardness of the martensite grains at the center of the plate thickness. is there.
(E) The average aspect ratio of the ferrite grains is less than 5.

  This invention is completed based on said knowledge, The summary is the manufacturing method of the following hot-rolled steel plate and a hot-rolled steel plate.

(1) Chemical composition in mass% C: 0.04 to 0.2%,
Mn: 0.1 to 3.0%
Al: 0.001 to 2.0%,
N: 0.0001 to 0.01%
Si: 0 to 3.0%,
Ti: 0 to 0.3%,
Nb: 0 to 0.30%,
Cu: 0 to 2.0%,
Ni: 0 to 2.0%,
Mo: 0 to 1.0%,
V: 0 to 0.3%
Cr: 0 to 2.0%,
Mg: 0 to 0.01%,
Ca: 0 to 0.01%,
REM: 0-0.1%
B: 0 to 0.01%
Balance: Fe and impurities,
P, S and O as impurities are respectively P: 0.10% or less, S: 0.03% or less, O: 0.01% or less,
In the microstructure with ferrite as the main phase, martensite is present in a volume fraction of 3% to less than 20%, and 60% or more of the martensite grains in the center of the plate thickness have an aspect ratio of 3 or more. The average length in the rolling direction of the martensite grains in the center is less than 20 μm, the Vickers hardness of the martensite grains in the center of the plate thickness satisfies the following formula (i), and the average aspect ratio of the ferrite grains is less than 5 , Hot rolled steel sheet.
σ (Hv) / E (Hv) <0.16 (i)
However,
E (Hv) means the average value of the Vickers hardness of the martensite grains at the center of the plate thickness,
σ (Hv) means the standard deviation of the Vickers hardness of the martensite grains at the center of the plate thickness.

(2) The chemical composition is mass%,
Ti: a range satisfying the following formula (ii), and Nb: 0.01 to 0.1%
The hot rolled steel sheet according to (1) above, which contains one or more selected from the above.
0.005 + 48 (N / 14 + S / 32) ≦ Ti ≦ 0.3 (ii)
However, each element symbol in the formula (ii) represents the content (% by mass) of each element contained in the steel plate.

(3) The chemical composition is mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.1%
The hot rolled steel sheet according to the above (1) or (2), which contains one or more selected from the above.

(4) The chemical composition is mass%,
B: 0.0002 to 0.0020%
The hot-rolled steel sheet according to any one of (1) to (3) above.

(5) The chemical composition is mass%,
Cu: 0.02 to 1.2%,
Ni: 0.01 to 0.6%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.2%, and Cr: 0.01 to 2.0%
The hot-rolled steel sheet according to any one of (1) to (4) above, which contains one or more selected from the above.

(6) A casting process in which molten steel having any chemical composition of (1) to (5) is cast into a slab,
A heating step of heating the slab after cooling or following the casting so as to satisfy the following formula (iii):
A hot rolling step of rolling the slab after the heating so that the steel plate temperature (° C.) and the reduction rate (%) at the time of final rolling of finish rolling satisfy the following formulas (iv) and (v):
A first cooling step for cooling the steel sheet obtained by the rolling to less than 750 ° C. so that the cooling rate to 750 ° C. is 60 ° C./s or more;
Holding the steel sheet cooled to less than 750 ° C. for 5 seconds or more in a temperature range of 600 ° C. or more and less than 750 ° C. so that the temperature change is less than 50 ° C .;
Second cooling for cooling the steel plate held for 5 seconds or more to a cooling rate of 50 ° C./s or more in a temperature range of Tc ° C. or more and less than 600 ° C. to Tc ° C. or less defined by the following formula (vi). The manufacturing method of a hot-rolled steel sheet provided with the winding process which winds the process and the steel plate cooled to the said Tc degrees C or less at the temperature of Tc degrees C or less.
t ≧ 1.4 × 10 ⁻⁶ × exp {3.2 × 10 ⁴ / (Tf + 273)} (iii)
(Tr-100) ≦ steel plate temperature <Tr (iv)
3 ≦ rolling rate <40 (v)
Tc = 561-474 × C-33 × Mn-17 × Ni-17 × Cr-21 × Mo (vi)
However,
t (s) is the heating time of the slab in the soaking zone,
Tf (° C.) is the average temperature of the slab in the soaking zone,
Tr is a calculated value of 870 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V,
Each element symbol in each formula means the content (% by mass) of each element.

  (7) The method for producing a hot-rolled steel sheet according to (6), wherein the hot-rolled steel sheet is subjected to a hot-dip galvanizing process or an alloyed hot-dip galvanizing process after the winding step.

  ADVANTAGE OF THE INVENTION According to this invention, the hot-rolled steel plate provided with the outstanding punch fatigue characteristic and workability is obtained, and the fatigue life of the hot-rolled steel plate used for the undercarriage part of a motor vehicle can be extended, for example.

Diagram showing the plane and side of a fatigue specimen

  The present invention will be described in detail below. In addition, this invention can be utilized suitably for the steel plate of plate thickness 8mm or less.

1. Microstructure The microstructure of the hot-rolled steel sheet of the present invention is characterized in that it is a composite structure in which the main phase is ferrite and martensite (island martensite) is present in a volume fraction of 3% to less than 20%. . In high-strength steel sheets, a composite structure in which a high-strength second phase is dispersed in ferrite is often used in order to increase the strength. In particular, ferrite-martensite steel has a good balance between strength and ductility because ferrite improves ductility and martensite improves strength. Furthermore, martensite becomes an obstacle to fatigue crack propagation in ferrite. Specifically, many cracks propagating in the ferrite do not enter the martensite but bypass or branch so as to avoid the martensite. Thus, martensite has the effect of reducing the fatigue crack propagation rate. For this reason, ferritic and martensitic steels are excellent in punching fatigue characteristics. For these reasons, in the microstructure of the hot rolled steel sheet of the present invention, the main phase is ferrite, and martensite (island martensite) is dispersed as the second phase.

  According to the study by the present inventors, the fatigue crack propagation suppressing effect by martensite is manifested when the volume fraction of martensite in the microstructure is 3% or more. The volume fraction of martensite in the microstructure is preferably 5% or more, more preferably 7% or more. On the other hand, when the volume fraction of martensite in the microstructure is 20% or more, martensite becomes a starting point of defects called voids, and the hole expansion rate decreases. As a result, the requirement of (tensile strength) × (hole expansion ratio) ≧ 35000, which is required for automobile undercarriage parts, is not satisfied. From the above, the volume fraction of martensite was set to 3% or more and less than 20%.

  Next, the average aspect ratio of ferrite grains will be described. The aspect ratio of the ferrite grain is determined by (length of major axis of ferrite grain) / (length of minor axis of ferrite grain). When finish rolling is finished in the austenite region, the average aspect ratio of the ferrite grains is less than 5. On the other hand, when the rolling temperature in the final stage of finish rolling is low and finish rolling is finished in the two-phase region of austenite and ferrite, the ferrite grains are stretched in the rolling direction. Thereby, the average aspect ratio of the ferrite grains becomes 5 or more. In such a case, since the ferrite grains are work-hardened, the ductility of the steel sheet is lowered and (tensile strength) × (total elongation) ≧ 18000 cannot be satisfied.

  Next, the aspect ratio and Vickers hardness of the martensite grains at the center of the plate thickness will be described. In the present invention, each martensite dispersed and generated in an island shape is called a martensite grain. The aspect ratio of the martensite grain is determined by (the length of the major axis of the martensite grain) / (the length of the minor axis of the martensite grain). The larger the aspect ratio of the martensite grains, the greater the probability that there will be martensite grains that obstruct crack propagation in the direction of fatigue crack propagation, and the crack detour distance and branching frequency will increase. Thereby, in addition to preventing a fatigue crack from propagating at a high speed in one direction, the closing of the crack is promoted, so that the fatigue crack propagation speed can be effectively reduced.

  When a punching fatigue test of a hot-rolled steel sheet is performed by an axial fatigue test, generally, a fatigue crack is generated from the center part of the sheet thickness, and the crack propagates in the sheet thickness direction, leading to a fracture. In order to suppress the occurrence of such cracks and the initial crack propagation, the form of martensite grains at the center of the plate thickness is particularly important. Specifically, when the ratio of martensite grains having an aspect ratio of less than 3 (ratio to the total number of martensite grains) is high in the center portion of the plate thickness, the distance at which the crack bypasses martensite is shortened. Further, when the crack is branched so as to avoid the martensite grains, the distance at which the branched crack bypasses the martensite grains is shortened. For this reason, if the ratio of martensite grains having an aspect ratio of less than 3 is high, the effect of suppressing crack propagation is small. Therefore, in the hot-rolled steel sheet of the present invention, the ratio of martensite grains having an aspect ratio of 3 or more is set to 60% or more, preferably 80% or more, in the central portion of the plate thickness.

Next, the Vickers hardness of the martensite grains at the center of the plate thickness will be described. The above-described crack propagation suppression effect by martensite is more remarkable as the hardness of the martensite grains is higher. However, when the hardness variation of the martensite grains is large in the central portion of the plate thickness, the crack propagates while selectively destroying the relatively soft martensite grains, so that the crack propagation suppressing effect is small. Therefore, the present inventors examined the relationship between the hardness variation of martensite grains and the crack propagation suppressing effect. As a result, it was found that when the hardness variation of the martensite grains satisfies the following formula (i), the selective martensite grains are hardly broken and excellent punching fatigue characteristics can be obtained. Therefore, in the hot-rolled steel sheet of the present invention, the Vickers hardness of the martensite grains at the center of the sheet thickness satisfies the following formula (i).
σ (Hv) / E (Hv) <0.16 (i)
In the formula (i), E (Hv) is an average value of Vickers hardness of the martensite grains at the center of the plate thickness, and σ (Hv) is a standard deviation of Vickers hardness of the martensite grains at the center of the plate thickness. is there.

  Next, the length in the rolling direction of the martensite grains at the center of the plate thickness will be described. When ferritic / martensitic steel is deformed, soft ferrite preferentially undergoes plastic deformation. For this reason, the stress which a martensite bears increases with deformation of ferrite martensite steel, and a big distortion arises in a martensite near the interface with a ferrite. When the stress which martensite bears or the strain near the interface becomes a certain level or more, the martensite breaks and a plurality of defects called voids are generated in the steel. The steel plate is broken by connecting the plurality of voids. Therefore, a steel plate in which voids are likely to occur is weak against local deformation and has poor hole expandability.

  When martensite is formed so as to extend in the rolling direction, stress and strain at the time of deformation of the steel sheet are concentrated on the martensite. Thereby, since voids are generated at an early stage, the hole expandability tends to be lowered. Further, since the plastic thickness restraint is strong in the central part of the plate thickness as compared with the surface layer part, voids are easily generated. Therefore, the length of the martensite grain at the center of the plate thickness is particularly important. According to the study by the present inventors, the occurrence of voids can be sufficiently suppressed by setting the average length in the rolling direction of the martensite grains at the center of the plate thickness to less than 20 μm. Thereby, the above-mentioned requirement (tensile strength) × (hole expansion rate) ≧ 35000 required for the undercarriage parts of the automobile can be satisfied. The average length in the rolling direction of the martensite grains at the center of the plate thickness is more preferably less than 18 μm.

  The volume fraction of ferrite and martensite in the microstructure of the hot-rolled steel sheet of the present invention can be determined as follows. First, a sample is taken from a steel sheet so that a plate thickness section parallel to the rolling direction of the steel sheet becomes an observation surface. After the observation surface is polished, nital etching is performed. And the position of 1/4 thickness, 3/8 thickness, and 1/2 thickness of plate | board thickness is observed by FE-SEM among the said observation surfaces. Specifically, 10 fields of view are observed for each position described above at a magnification of 1000, and the area ratios of the ferrite phase and the martensite phase are measured. The average value of the area ratio of each phase is obtained from the measurement result (measurement result of 30 visual fields), and the obtained average value of the area ratio of each phase is defined as the volume fraction of each phase.

  The average aspect ratio of the ferrite grains can be obtained by observing 50 or more ferrite grains from the position of 1/4 thickness to the position of 1/2 thickness with the FE-SEM on the observation surface.

  The aspect ratio of the martensite grains in the central portion of the plate thickness can be obtained by observing 50 or more martensite grains at a position of 1/2 the plate thickness on the observation surface with an FE-SEM.

  The average length in the rolling direction of the martensite grains at the center of the plate thickness can be determined as follows. First, on the observation surface, 50 or more martensite grains at a position of 1/2 the plate thickness are observed with an FE-SEM, and the length of each martensite grain in the rolling direction is measured. And the average length of the rolling direction of the martensite grain of a plate | board thickness center part is obtained by calculating | requiring the average value of the measurement result.

The average value and the standard deviation of the Vickers hardness of the martensite grains at the center of the plate thickness are obtained by measuring the Vickers hardness of 50 or more martensite grains at the position of 1/2 thickness of the plate thickness on the observation surface. be able to. The test load is, for example, 10 gf (98.1 g · m / s ² ).

2. About a chemical composition The reason which prescribes | regulates the chemical composition of a hot-rolled steel plate in this invention is demonstrated below. In the description of the chemical composition of the hot-rolled steel sheet, “%” display of the content of each element means “mass%”.

C: 0.04 to 0.2%
C is one of the important elements in the present invention. C generates martensite and contributes to improving the strength of the hot-rolled steel sheet by strengthening the structure. Furthermore, C has the effect of improving the hardness of martensite and suppressing crack propagation. However, if the C content is less than 0.04%, the Vickers hardness and volume fraction of martensite cannot be sufficiently ensured, so that the punching fatigue characteristics of the hot-rolled steel sheet cannot be sufficiently improved. On the other hand, when the C content exceeds 0.2%, the volume fraction of martensite, which is the hard second phase, increases, and the hole expandability of the hot-rolled steel sheet decreases. Therefore, the C content is 0.04 to 0.2%.

Mn: 0.1 to 3.0%
Mn is contained for solid solution strengthening and quenching strengthening (generation of martensite). However, when the Mn content exceeds 3%, these effects are saturated. On the other hand, if the Mn content is less than 0.1%, the formation of pearlite and bainite cannot be sufficiently suppressed during quenching (more specifically, during cooling). That is, a sufficient volume fraction of martensite cannot be secured. Therefore, the Mn content is 0.1 to 3.0%. The Mn content is preferably 0.3% or more. The Mn content is preferably 2.5% or less.

Al: 0.001 to 2.0%
Al suppresses the formation of coarse cementite and improves the low temperature toughness of the hot-rolled steel sheet. Al can also be used as a deoxidizing material. In order to obtain these effects, it is necessary to contain 0.001% or more of Al. However, when Al is excessively contained, the number of Al-based coarse inclusions is increased, which causes a decrease in hole expansibility of hot-rolled steel sheets and surface flaws. From this, the upper limit of the Al content was set to 2.0%. A preferable upper limit value of the Al content is 1.5%, and a preferable lower limit value is 0.01%.

N: 0.0001 to 0.01%
N exists as TiN and improves the low temperature toughness of the hot-rolled steel sheet by suppressing the coarsening of crystal grains during slab heating. In order to acquire this effect, it is necessary to contain N 0.0001% or more. However, N becomes a factor of blowhole formation when welding a hot-rolled steel sheet, and there is a concern that the joint strength of the welded portion is lowered. Therefore, the N content needs to be 0.01% or less. A preferable upper limit of N content is 0.006%, and a preferable lower limit is 0.0005%.

  In addition to the above-described components, the hot-rolled steel sheet according to the present invention can contain one or more components selected from the following elements as necessary.

Si: 0 to 3.0%
Si is an element that contributes to improving the strength of the hot-rolled steel sheet, and can also be used as a deoxidizer for molten steel. In order to acquire this effect, it is preferable to contain Si 0.001% or more. In particular, when the Si content is 0.1% or more, precipitation of iron-based carbides such as cementite in the microstructure can be suppressed, and the strength and hole expandability of the hot-rolled steel sheet can be improved. Therefore, the Si content is more preferably 0.1% or more. On the other hand, even if the Si content exceeds 3.0%, the effect of improving the strength of the hot-rolled steel sheet is saturated. Therefore, the Si content is 3.0% or less. Moreover, when Si content exceeds 2.5%, the effect of precipitation suppression of an iron-type carbide will be saturated. Therefore, the Si content is preferably 2.5% or less.

Ti: 0 to 0.3%
Ti improves the strength of the hot-rolled steel sheet by precipitation strengthening. Further, Ti improves the low temperature toughness of the hot rolled steel sheet. Specifically, Ti carbonitride or solute Ti delays crystal grain growth during hot rolling, whereby crystal grains of the hot-rolled steel sheet can be refined. Thereby, the low temperature toughness of a hot-rolled steel sheet is improved. However, if the Ti content exceeds 0.3%, the above effects are saturated and the economic efficiency is lowered. Therefore, when Ti is contained, the Ti content is 0.3% or less. In order to obtain the above effect, Ti is preferably contained in an amount of “0.005 + 48 (N / 14 + S / 32)” or more (“N” and “S” are the contents of N and S ( Mass%) respectively)). That is, when Ti is contained, it is preferable to satisfy the following formula (ii). If the Ti content exceeds 0.15%, the tundish nozzle may be easily clogged during casting. For this reason, the Ti content is more preferably 0.15% or less.
0.005 + 48 (N / 14 + S / 32) ≦ Ti ≦ 0.3 (ii)
Each element symbol in formula (ii) means the content (% by mass) of each element.

Nb: 0 to 0.30%
Nb improves the low temperature toughness of the hot rolled steel sheet. Specifically, Nb carbonitride or solute Nb delays crystal grain growth during hot rolling, whereby crystal grains of the hot-rolled steel sheet can be refined. Thereby, the low temperature toughness of a hot-rolled steel sheet is improved. However, when the Nb content exceeds 0.30%, the above effect is saturated and the economic efficiency is lowered. Therefore, the Nb content in the case of inclusion is 0.30% or less. If the Nb content exceeds 0.1%, transformation from completely non-recrystallized austenite to martensite may occur, and the average length of martensite grains in the center of the plate thickness may be increased. Therefore, the preferable upper limit of Nb content is 0.1%. In order to acquire said effect, it is preferable to contain 0.01% or more of Nb.

Cu: 0 to 2.0%
Cu improves the strength of the hot-rolled steel sheet by precipitation strengthening and solid solution strengthening. However, if the Cu content exceeds 2.0%, the above effects are saturated and the economic efficiency is lowered. Therefore, the Cu content in the case of inclusion is 2.0% or less. On the other hand, if the Cu content exceeds 1.2%, Cu is melted after concentration in the scale, which may cause defects during rolling. Therefore, the preferable upper limit of Cu content is 1.2%. In order to acquire said effect, it is preferable to contain Cu 0.01% or more, and it is more preferable to contain 0.02% or more.

Ni: 0 to 2.0%
Ni improves the strength of the hot-rolled steel sheet by solid solution strengthening. However, if the Ni content exceeds 2.0%, the above effect is saturated and the economy is lowered. Therefore, the Ni content in the case of inclusion is 2.0% or less. If the Ni content exceeds 0.6%, wrinkles easily occur during hot rolling. Therefore, the preferable upper limit of Ni content is 0.6%. In order to acquire said effect, it is preferable to contain 0.01% or more of Ni.

Mo: 0 to 1.0%
Mo improves the strength of the hot-rolled steel sheet by precipitation strengthening and solid solution strengthening. However, if the Mo content exceeds 1.0%, the above effects are saturated and the economic efficiency is lowered. Therefore, the Mo content in the case of inclusion is 1.0% or less. A preferable upper limit of the Mo content is 0.7%. In order to acquire said effect, it is preferable to contain 0.01% or more of Mo.

V: 0 to 0.3%
V improves the strength of the hot-rolled steel sheet by precipitation strengthening and solid solution strengthening. However, if the V content exceeds 0.3%, the above effect is saturated and the economic efficiency is lowered. Therefore, the V content when contained is 0.3% or less. A preferable upper limit of the V content is 0.2%. In order to acquire said effect, it is preferable to contain V 0.01% or more.

Cr: 0 to 2.0%
Cr improves the strength of the hot-rolled steel sheet by solid solution strengthening. However, if the Cr content exceeds 2.0%, the above effect is saturated and the economic efficiency is lowered. Therefore, the Cr content when contained is 2.0% or less. A preferable upper limit of the Cr content is 1.2%. In order to acquire said effect, it is preferable to contain Cr 0.01% or more.

Mg: 0 to 0.01%
Non-metallic inclusions in the hot-rolled steel sheet serve as a starting point for destruction and cause the workability of the hot-rolled steel sheet to deteriorate. Mg controls the form of this nonmetallic inclusion and improves the workability of the hot-rolled steel sheet. However, if the Mg content exceeds 0.01%, the above effect is saturated and the economy is lowered. Therefore, the Mg content when contained is 0.01% or less. A preferable upper limit of the Mg content is 0.007%. The above-described effect is remarkably exhibited by containing Mg in an amount of 0.0005% or more.

Ca: 0 to 0.01%
Ca, like Mg, controls the form of non-metallic inclusions and improves the workability of the hot-rolled steel sheet. However, when the Ca content exceeds 0.01%, the above effect is saturated and the economic efficiency is lowered. Therefore, when Ca is contained, the Ca content is 0.01% or less. A preferable upper limit of the Ca content is 0.007%. The above-mentioned effect is remarkably exhibited by containing Ca of 0.0005% or more.

REM: 0 to 0.1%
Similar to Mg and Ca, REM (rare earth element) controls the form of non-metallic inclusions and improves the workability of the hot-rolled steel sheet. However, if the REM content exceeds 0.1%, the above effect is saturated and the economic efficiency is lowered. Therefore, the REM content when contained is 0.01% or less. A preferable upper limit of the REM content is 0.06%. The above-mentioned effect is remarkably exhibited by containing REM 0.0005% or more. Note that REM is a general term for 17 elements including Y and Sc combined with 15 lanthanoid elements. In the present invention, one or more of these 17 elements can be contained in the steel material. The REM content means the total content of these elements.

B: 0 to 0.01%
B segregates at the crystal grain boundaries to increase the grain boundary strength, thereby improving the low temperature toughness of the hot-rolled steel sheet. However, if the B content exceeds 0.01%, the above effect is saturated and the economic efficiency is lowered. Therefore, the B content when contained is 0.01% or less. In order to acquire said effect, it is preferable to contain B 0.0002% or more, It is more preferable to contain 0.0005% or more, It is further more preferable to contain 0.0007% or more. Further, the B content is preferably 0.0050% or less, more preferably 0.0030% or less, and further preferably 0.0020% or less.

  The hot-rolled steel sheet according to the present invention contains the above elements, and the balance is made of Fe and impurities. “Impurity” means a component that is mixed due to raw materials such as ore and scrap and other factors when industrially producing steel materials. Among the following impurities, it is necessary to strictly limit the content of P, S and O.

P: 0.10% or less P is an impurity contained in the hot metal. P segregates at the grain boundaries and lowers the low temperature toughness of the hot-rolled steel sheet as its content increases. For this reason, the lower the P content, the better. If the P content exceeds 0.10%, the workability and weldability of the hot-rolled steel sheet will be adversely affected. Therefore, the P content is 0.10% or less. In particular, considering the weldability, the P content is preferably 0.03% or less.

S: 0.03% or less S is an impurity contained in the hot metal. If the content of S is too large, it not only causes cracking during hot rolling, but also generates inclusions such as MnS that reduce hole expandability. For this reason, the S content should be reduced as much as possible, but it is acceptable if it is 0.03% or less. Therefore, the S content is 0.03% or less. However, when a certain degree of hole expandability is required, the S content is preferably 0.01% or less, and more preferably 0.005% or less.

O (oxygen): 0.01% or less O has a tendency to form an oxide and reduce the formability of the hot-rolled steel sheet, so the content thereof needs to be suppressed. In particular, when the O content exceeds 0.01%, the above tendency becomes remarkable, so the O content is set to 0.01% or less. On the other hand, since it is not economically preferable that the O content is less than 0.0001%, the O content is preferably 0.0001% or more.

Zr, Sn, Co, Zn and W: 0 to 1% in total
Even if the total content of Zr, Sn, Co, Zn and W is 1% or less, the effect of the present invention is not impaired. Note that Sn may cause wrinkles during hot rolling. Therefore, the Sn content is desirably 0.05% or less.

  The hot-rolled steel sheet of the present invention having the above-described microstructure and chemical composition can improve the corrosion resistance by subjecting the surface to a hot-dip galvanizing treatment to form a hot-dip galvanized layer. After the hot dip galvanizing treatment, an alloying treatment may be further performed to form an alloyed galvanized layer. The plating layer is not limited to pure zinc. For example, an element such as Si, Mg, Al, Fe, Mn, Ca, or Zr may be added to the plating layer to further improve the corrosion resistance. Even if such a plating layer is formed, the excellent punching fatigue characteristics and workability of the hot-rolled steel sheet of the present invention are not impaired. These characteristics of the hot-rolled steel sheet of the present invention include that the hot-rolled steel sheet has a surface treatment layer formed by organic film formation, laminate film processing, organic salt treatment, inorganic salt treatment, non-chromate treatment, or the like. It will not be damaged.

3. Manufacturing method of hot-rolled steel sheet In order to realize the excellent punching fatigue characteristics and workability of the hot-rolled steel sheet, as described above, it is important that the hot-rolled steel sheet satisfies the following requirements (a) to (d) It is.
(A) Martensite is present in a volume fraction of 3% or more and less than 20% in a microstructure having ferrite as a main phase.
(B) 60% or more of the martensite grains in the center of the plate thickness have an aspect ratio of 3 or more.
(C) The average length in the rolling direction of the martensite grains at the center of the plate thickness is less than 20 μm.
(D) The variation in Vickers hardness of the martensite grains at the center of the plate thickness is small. Specifically, the martensite at the center of the plate thickness satisfies the above-described formula (i).
(E) The average aspect ratio of the ferrite grains is less than 5.

  Hereinafter, an example of a method for producing a hot-rolled steel sheet that satisfies the requirements (a) to (e) will be described.

  The manufacturing method of the hot rolled steel sheet according to the present invention includes, for example, a casting process, a heating process, a hot rolling process, a first cooling process, a holding process, a second cooling process, and a winding process. In addition, the process before a casting process is not specifically limited. For example, various secondary smelting is performed subsequent to smelting in a blast furnace or an electric furnace to obtain molten steel having a chemical composition defined in the present invention. In the casting process, the molten steel is cast by a method such as normal continuous casting or thin slab casting to obtain a slab. In addition, as long as the chemical composition of molten steel can be controlled within the scope of the present invention, scrap may be used as a raw material.

  In the heating step, the slab obtained by casting is heated to a predetermined temperature. For example, when the slab is obtained by continuous casting, it may be heated after being cooled to a low temperature (for example, room temperature) after casting, or may be heated subsequent to casting without being cooled.

Mn is segregated in the microstructure of the central portion of the slab after casting. For this reason, when the heating of the slab is not sufficient at the time of hot rolling, the state in which Mn is segregated in the central part of the plate thickness of the steel sheet is maintained after rolling. Since Mn stabilizes austenite, a region where austenite tends to remain is formed along the segregated Mn when the rolled steel sheet is cooled. Thereby, martensite transformed from austenite at low temperature is likely to exist along the segregated Mn, and the length in the rolling direction of the martensite grains at the center of the plate thickness is increased. As a result of extensive studies by the present inventors, it was found that the average length in the rolling direction of the martensite grains can be reduced to 20 μm or less by heating the slab so as to satisfy the following formula (iii) in the heating step. Therefore, in the method for producing a hot-rolled steel sheet according to the present invention, the slab is heated so as to satisfy the formula (iii) in the heating step.
t ≧ 1.4 × 10 ⁻⁶ × exp {3.2 × 10 ⁴ / (Tf + 273)} (iii)
t (s): Slab heating time in the soaking zone Tf (° C.): Average temperature of the slab in soaking zone The average temperature of the slab is calculated based on, for example, the surface temperature of the slab.

  In the heating step, by heating the slab so as to satisfy the formula (iii), the slab can be sufficiently heated and the diffusion of Mn can be promoted. Thereby, it is thought that the length in the rolling direction of a martensite grain can be reduced. Although the effect of the present invention is exhibited even if the upper limit of the slab heating temperature is not particularly defined, it is economically undesirable to make the heating temperature excessively high. For this reason, the upper limit of the slab heating temperature is preferably less than 1300 ° C.

After the heating process, the slab is hot rolled (hot rolling process). The hot rolling process includes, for example, a rough rolling process and a finish rolling process. Specifically, in the hot rolling step, the slab taken out from the heating furnace is roughly rolled without particularly taking time to obtain a rough bar. In the finish rolling step subsequent to the rough rolling step, the slab (rough bar) is rolled so that the steel sheet temperature (° C.) and the reduction rate (%) at the time of final reduction satisfy the following formulas (iv) and (v).
(Tr-100) ≦ steel plate temperature <Tr (iv)
3 ≦ rolling rate <40 (v)
However, Tr (° C.) = 870 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V. Each element symbol in the formula means the content (% by mass) of the element.

  The above-mentioned final reduction condition is extremely important for controlling the aspect ratio of the martensite grains at the center of the plate thickness of the steel sheet. Specifically, the aspect ratio of the martensite grains in the center portion of the plate thickness is increased by rolling in a temperature range below Tr ° C. The cause is considered as follows. That is, when rolling in the above temperature range, recrystallization of austenite grains is suppressed, and a decrease in the aspect ratio of austenite grains is suppressed. The shape of the austenite grains having an increased aspect ratio is considered to be inherited by the martensite grains transformed from the austenite grains. As a result of diligent research by the present inventors, it has been found that the effect of increasing the aspect ratio is particularly exerted when the rolling reduction ratio of the final rolling is 3% or more. On the other hand, when the reduction ratio of the final reduction is 40% or more, a great burden is imposed on the rolling mill. Therefore, the rolling reduction of the final rolling is 3% or more and less than 40%. The reduction ratio of the final reduction is preferably 10% or more.

  In addition, when it rolls in the temperature range below (Tr-100) degreeC, it becomes rolling in the two-phase area | region of a ferrite and austenite, and the average aspect-ratio of a ferrite grain will be 5 or more. Since ferrite is work-hardened by rolling in such a two-phase region, the ductility of the steel sheet is lowered. Therefore, as described above, the steel plate temperature at the time of final reduction was set to (Tr-100) ° C. or higher. The steel plate temperature at the time of final reduction is preferably less than (Tr-50) ° C. Moreover, when it rolls above Tr degreeC, the recrystallization of austenite is accelerated | stimulated and the aspect-ratio of an austenite grain falls. This effect is considered to extend to martensite transformed from austenite, and the aspect ratio of the martensite grains in the central portion of the plate thickness is considered to be small. Therefore, as described above, the final reduction temperature is set to less than Tr ° C.

  In the first cooling step, the steel sheet after finish rolling is cooled to less than 750 ° C. so that the cooling rate from the final reduction temperature to 750 ° C. is 60 ° C./s or more. The cause is not clear, but when the cooling rate is less than 60 ° C./s, the average length in the rolling direction of the martensite grains at the center of the plate thickness is 20 μm or more. However, the reason why the length of the martensite grains in the rolling direction is shortened by setting the cooling rate to 60 ° C./s or more is considered as follows. That is, when the cooling rate is 60 ° C./s or more, the dislocations introduced under the final reduction of the finish rolling are difficult to recover, and the dislocations act as ferrite transformation nuclei. For this reason, the untransformed austenite grains at the center of the plate thickness are divided by the ferrite grains. As a result, the length in the rolling direction of the martensite grains transformed from the austenite grains is considered to be shortened.

  In the field of thick plates, there have been examples of controlling the aspect ratio of martensite grains for the purpose of suppressing fatigue crack propagation, but both workability such as hole expandability and suppression of fatigue crack propagation are compatible. There is no literature which reported. In a thick plate, rolling strain does not easily reach the center of the plate thickness, and dislocations are difficult to introduce in finish rolling. Further, in the case of a thick plate, since the plate thickness is thick, the cooling rate at the central portion of the plate thickness is difficult to increase. Therefore, even if dislocations are introduced in finish rolling, the recovery of the dislocations is considered to proceed. As a result, in the thick plate, it is presumed that dislocations acting as nuclei for ferrite transformation cannot be sufficiently secured, and the length of martensite grains cannot be reduced.

  In the holding step, the steel sheet cooled to less than 750 ° C. is held for 5 seconds or more in a temperature range of 600 ° C. or more and less than 750 ° C. This holding step is performed in order to make the main phase of the microstructure of the steel sheet into ferrite, or to make the martensite volume fraction less than 20%. When the holding time in the above temperature range is 5 seconds or less, ferrite does not become the main phase, or the volume fraction of martensite becomes 20% or more, and the ductility or hole expansion rate decreases.

  The temperature change of the steel sheet in the holding step is less than 50 ° C. When the temperature change of the steel sheet is 50 ° C. or higher in the holding step, the hardness variation of the martensite grains becomes large, and it becomes difficult to satisfy the above-described formula (i). The cause of this is not clear, but when ferrite grains having greatly different generation temperatures are present in the microstructure, a mixed grain state in which coarse ferrite grains and fine ferrite grains are mixed is formed. This may affect the variation in the carbon concentration of the martensite grains present around the ferrite grains, and may increase the hardness variation of the martensite grains.

In the second cooling step, the steel plate held for 5 seconds or more is cooled to Tc ° C. or less defined by the following formula (vi). In the second cooling step, when the cooling rate from 600 ° C. to Tc ° C. is less than 50 ° C./s, bainite or pearlite is generated in the microstructure. Thereby, it becomes difficult for the volume fraction or hardness of martensite to satisfy the above-described requirements of the present invention, and the punching fatigue characteristics of the steel sheet are lowered. For this reason, in the manufacturing method of the hot rolled steel sheet according to the present invention, the cooling rate in the temperature range of Tc ° C. or higher and lower than 600 ° C. is set to 50 ° C./s or higher.
Tc = 561-474 × C-33 × Mn-17 × Ni-17 × Cr-21 × Mo (vi)
Each element symbol in the formula (vi) means the content (% by mass) of each element.

  In the winding process, the steel sheet cooled to Tc ° C. or lower is wound at a temperature of Tc ° C. or lower. Thereby, the hot-rolled steel sheet according to the present invention is obtained. In addition, when the temperature of the steel plate at the time of winding exceeds Tc ° C., bainite or pearlite is generated in the microstructure. Thereby, it becomes difficult for the volume fraction or hardness of martensite to satisfy the above-described requirements of the present invention, and the punching fatigue characteristics of the steel sheet are lowered.

  In addition, after completion of all the above steps (that is, after the winding step), skin pass rolling with a rolling reduction of 0.1% to 2% is performed for the purpose of improving the ductility by correcting the shape of the hot-rolled steel sheet and introducing movable dislocations. It is desirable to apply to hot rolled steel sheets. Moreover, after completion | finish of all the processes, you may pickle a hot-rolled steel plate as needed for the purpose of the removal of the scale adhering to the surface of a hot-rolled steel plate. Furthermore, after pickling, the hot-rolled steel sheet may be subjected to skin pass rolling or cold rolling with a reduction rate of 10% or less inline or offline. Further, after all the steps are completed, a hot dip galvanizing treatment or an alloying hot dip galvanizing treatment may be performed.

  As described above, the hot-rolled steel sheet according to the present invention can be manufactured through a continuous casting process, a rough rolling process, and, if necessary, a pickling process, which are included in a normal hot-rolled steel sheet manufacturing process. However, even if some of these steps are omitted to produce a hot-rolled steel sheet, the excellent punching fatigue characteristics and workability that are the effects of the present invention can be ensured.

  After manufacturing a hot-rolled steel sheet by the above-described manufacturing method, heat treatment may be performed in a temperature range of 100 to 600 ° C. online or offline for the purpose of improving the ductility of the hot-rolled steel sheet. Even in this case, excellent punching fatigue characteristics and ductility which are the effects of the present invention can be ensured.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  First, slabs (steel A to M and steel a to e) having chemical compositions shown in Table 1 were prepared through predetermined steps (melting, secondary smelting, casting, and the like). Steels A to M satisfy the chemical composition requirements defined in the present invention, and steels a to e do not satisfy the chemical composition requirements defined in the present invention.

  A plurality of hot-rolled steel plates (steel plates Nos. 1 to 81) were produced from the steels A to M and a to e described above under the conditions shown in Table 2. In Table 2, the final reduction temperature means the steel plate temperature at the final reduction of the finish rolling, the final reduction rate means the reduction rate of the final reduction, and the first cooling rate is the final reduction of the steel plate in the first cooling step. It means the cooling rate when cooling from the reduction temperature to 750 ° C. The holding time means the holding time of the steel plate in the holding step, the temperature change means the temperature change of the steel plate in the holding step, and the second cooling rate is from 600 ° C. to Tc ° C. in the second cooling step. It means the cooling rate when cooling to.

  More specifically, the manufacturing method of the hot-rolled steel sheet (steel Nos. 1 to 81) will be described. First, the slabs (steel A to M and a to e) after casting are directly or after being cooled to room temperature, It heated in the heating furnace on the heating conditions shown in Table 2. Next, the rolling process, the first cooling process, the holding process, the second cooling process, and the winding process were performed under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 3 mm. Thereafter, the hot-rolled steel sheet was pickled and subjected to skin pass rolling with a rolling reduction of 0.5%. Steel plate No. 4, 6, 15, 34, 43 and 62 were subjected to predetermined plating treatment. Specifically, steel material No. Nos. 4, 15, 34 and 62 are alloyed hot-dip galvanized steel sheets (GA). 6 and 43 were hot-dip galvanized steel sheets (GI).

  Various test pieces were cut out from the obtained hot-rolled steel sheet and subjected to structure observation and various tests. The results are shown in Table 3. In addition, the volume fraction of martensite, the average aspect ratio of ferrite grains, the ratio of martensite grains having an aspect ratio of 3 or more at the center of the plate thickness, the average length in the rolling direction of the martensite grains at the center of the plate thickness, and the plate thickness The average value of the Vickers hardness of the martensite grains in the center and the standard deviation of the Vickers hardness of the martensite grains in the center of the plate thickness were determined by the method described above. The volume fraction of ferrite and other structures was determined in the same manner as the volume fraction of martensite. Tensile strength characteristics (tensile strength and total elongation) were measured in the JIS Z 2241 2011 No. 5 test taken in the sheet width direction (direction perpendicular to the rolling direction) from the 1/4 width or 3/4 width position of the sheet width. It evaluated based on JISZ22412011 using the piece. The hole expansion rate was evaluated by a hole expansion test. In the hole expansion test, a test piece was sampled from the same position as the position where the No. 5 test piece was sampled, and the hole expansion rate was evaluated according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. The punching fatigue strength was evaluated by a fatigue test. In the fatigue test, a fatigue test piece having the shape shown in FIG. 1 was used. The fatigue test piece was collected from the same position as the position where the test piece was taken so that the rolling direction was long. In the fatigue test, in order to bring the fatigue characteristics of the fatigue test piece closer to the fatigue characteristics in actual use of automobile parts, the center part of the test piece was punched with a cylindrical punch as in the hole expansion test. The punching clearance (ratio of the clearance between the cylindrical punch and the die with respect to the plate thickness) was 10%. The fatigue test piece was subjected to three-sided grinding to a depth of about 0.05 mm from the outermost layer. Using this fatigue test piece, a stress-controlled tensile fatigue test was performed under the conditions of a stress ratio of 0.1 and a frequency of 15 to 25 Hz. Table 3 shows the strength for 10 million times as a result of the fatigue test (punching fatigue strength).

  In the present invention, the hot-rolled steel sheet having excellent workability means a hot-rolled steel sheet satisfying (tensile strength) × (total elongation) ≧ 18000 and (tensile strength) × (hole expansion ratio) ≧ 35000. To do. Further, in the present invention, the hot-rolled steel sheet having excellent punching fatigue characteristics is a value obtained by dividing the punching fatigue strength (10 million times strength) obtained in the above fatigue test by the tensile strength obtained in the tensile test. It means a hot-rolled steel sheet having a (punch fatigue limit ratio) of 0.30 or more.

  As shown in Table 3, the hot-rolled steel sheets according to the examples of the present invention were excellent in workability and punching fatigue characteristics. Steel plate No. From the test results of 4, 6, 15, 34, 43, and 62, even when the hot-rolled steel sheet according to the present invention is subjected to hot dip galvanizing treatment or alloying hot dip galvanizing treatment, excellent workability and punching fatigue characteristics are obtained. It was found that it was maintained.

  On the other hand, steel plate No. 7, 39, 63, the slab heating time t did not satisfy the above-mentioned formula (iii). That is, the slab heating time was not sufficient. For this reason, Mn was not sufficiently diffused in the central portion of the plate thickness, and the average length in the rolling direction of the martensite grains in the central portion of the plate thickness was 20 μm or more. As a result, the hole expansion rate was low, and (tensile strength) × (hole expansion rate) ≧ 35000 could not be satisfied. That is, excellent workability could not be obtained.

  Steel plate No. In 8, 40, 64, the final rolling temperature of finish rolling was Tr ° C. or higher, so that recrystallization of austenite was promoted, and the ratio of martensite grains having an aspect ratio of 3 or more was 60% or more in the central portion of the plate thickness. I couldn't. For this reason, the punching fatigue strength was lowered, and the punching fatigue limit ratio was less than 0.30. That is, excellent punching fatigue characteristics could not be obtained.

  Steel plate No. In 9,41,65, since the final rolling temperature of finish rolling was less than (Tr-100) ° C., the average aspect ratio of the ferrite grains became 5 or more, the work hardening of the ferrite progressed, and the ductility decreased. As a result, the value of total elongation was low, and (tensile strength) × (total elongation) ≧ 18000 could not be satisfied. That is, excellent workability could not be obtained.

  Steel plate No. Nos. 10, 42, 55, and 66 had a final rolling reduction ratio of less than 3%, so that sufficient rolling strain could not be introduced into the center portion of the plate thickness. As a result, the average length in the rolling direction of the martensite grains at the center of the plate thickness was 20 μm or more. For this reason, the hole expansion rate was lowered, and (tensile strength) × (hole expansion rate) ≧ 35000 could not be satisfied. That is, excellent workability could not be obtained.

  Steel plate No. In 18, 48, 72, the cooling rate (first cooling rate) from the final rolling temperature of finish rolling to 750 ° C. was less than 60 ° C./s. For this reason, the rolling strain introduced into the central portion of the plate thickness is recovered, and the average length in the rolling direction of the martensite at the central portion of the plate thickness is 20 μm or more. As a result, the hole expansion rate was low, and (tensile strength) × (hole expansion rate) ≧ 35000 could not be satisfied. That is, excellent workability could not be obtained.

  Steel plate No. Nos. 19, 49, and 73 had a holding time at 600 ° C. or higher and lower than 750 ° C. of less than 5 seconds. For this reason, the steel plate No. In 19 and 49, ferrite was not the main phase, and sufficient ductility was not obtained. As a result, (tensile strength) × (total elongation) ≧ 18000 could not be satisfied. Steel plate No. In 73, the volume fraction of martensite was 20% or more, and the hole expansion rate was low. (Tensile strength) × (hole expansion ratio) ≧ 35000 could not be satisfied. Thus, the steel plate No. In 19, 49, 73, excellent workability could not be obtained.

  Steel plate No. 20, 50, 74 had a temperature change of 50 ° C. or more when held at 600 ° C. or more and less than 750 ° C. For this reason, the hardness variation of the martensite grains in the central portion of the plate thickness was increased, and the punching fatigue limit ratio was less than 0.30. That is, excellent punching fatigue characteristics could not be obtained.

  Steel plate No. In 21, 51, and 75, the cooling rate from 600 ° C. to Tc ° C. (second cooling rate) was less than 50 ° C./s, and thus pearlite or bainite was generated in the microstructure. For this reason, the volume fraction of martensite was not 3% or more, and the punching fatigue limit ratio was less than 0.30. That is, excellent punching fatigue characteristics could not be obtained.

  Steel plate No. In 22, 52, and 76, the winding temperature was Tc ° C. or higher, so that pearlite or bainite was generated in the microstructure. For this reason, the volume fraction of martensite was not 3% or more, and the punching fatigue limit ratio was less than 0.30. That is, excellent punching fatigue characteristics could not be obtained.

  Steel plate No. 77-81 does not satisfy the requirements defined in the present invention for chemical composition. For this reason, the steel plate No. In No. 77, the punching fatigue strength was low, and the punching fatigue limit ratio was less than 0.30. That is, excellent punching fatigue characteristics could not be obtained. Steel plate No. In 78 to 81, (tensile strength) × (hole expanding ratio) ≧ 35000 could not be satisfied. That is, excellent workability could not be obtained.

  According to the present invention, excellent punching fatigue characteristics and workability can be obtained even in a thin hot-rolled steel sheet. Therefore, the hot-rolled steel sheet according to the present invention can be suitably used as an automobile steel sheet.

Claims (7)

Chemical composition is mass%,
C: 0.04 to 0.2%,
Mn: 0.1 to 3.0%
Al: 0.001 to 2.0%,
N: 0.0001 to 0.01%
Si: 0 to 3.0%,
Ti: 0 to 0.3%,
Nb: 0 to 0.30%,
Cu: 0 to 2.0%,
Ni: 0 to 2.0%,
Mo: 0 to 1.0%,
V: 0 to 0.3%
Cr: 0 to 2.0%,
Mg: 0 to 0.01%,
Ca: 0 to 0.01%,
REM: 0-0.1%
B: 0 to 0.01%
Balance: Fe and impurities,
P, S and O as impurities are respectively P: 0.10% or less, S: 0.03% or less, O: 0.01% or less,
In the microstructure with ferrite as the main phase, martensite is present in a volume fraction of 3% to less than 20%, and 60% or more of the martensite grains in the center of the plate thickness have an aspect ratio of 3 or more. The average length in the rolling direction of the martensite grains in the center is less than 20 μm, the Vickers hardness of the martensite grains in the center of the plate thickness satisfies the following formula (i), and the average aspect ratio of the ferrite grains is less than 5 , Hot rolled steel sheet.
σ (Hv) / E (Hv) <0.16 (i)
However,
E (Hv) means the average value of the Vickers hardness of the martensite grains at the center of the plate thickness,
σ (Hv) means the standard deviation of the Vickers hardness of the martensite grains at the center of the plate thickness. The chemical composition is mass%,
Ti: a range satisfying the following formula (ii), and Nb: 0.01 to 0.1%
The hot-rolled steel sheet according to claim 1, comprising at least one selected from
0.005 + 48 (N / 14 + S / 32) ≦ Ti ≦ 0.3 (ii)
However, each element symbol in the formula (ii) represents the content (% by mass) of each element contained in the steel plate. The chemical composition is mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.1%
The hot-rolled steel sheet according to claim 1 or 2, which contains one or more selected from. The chemical composition is mass%,
B: 0.0002 to 0.0020%
The hot-rolled steel sheet according to any one of claims 1 to 3, comprising: The chemical composition is mass%,
Cu: 0.02 to 1.2%,
Ni: 0.01 to 0.6%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.2%, and Cr: 0.01 to 2.0%
The hot-rolled steel sheet according to any one of claims 1 to 4, wherein the hot-rolled steel sheet contains at least one selected from the group consisting of: A method for producing a hot-rolled steel sheet according to any one of claims 1 to 5,
A casting process for casting a molten steel having the chemical composition according to any one of claims 1 to 5 on a slab,
A heating step of heating the slab after cooling or following the casting so as to satisfy the following formula (iii):
A hot rolling step in which the heated slab is hot-rolled so that the steel plate temperature (° C.) and the reduction ratio (%) during final rolling of the finish rolling satisfy the following formulas (iv) and (v):
A first cooling step for cooling the steel sheet obtained by the rolling to less than 750 ° C. so that the cooling rate to 750 ° C. is 60 ° C./s or more;
Holding the steel sheet cooled to less than 750 ° C. for 5 seconds or more in a temperature range of 600 ° C. or more and less than 750 ° C. so that the temperature change is less than 50 ° C .;
Second cooling for cooling the steel plate held for 5 seconds or more to a cooling rate of 50 ° C./s or more in a temperature range of Tc ° C. or more and less than 600 ° C. to Tc ° C. or less defined by the following formula (vi). The manufacturing method of a hot-rolled steel sheet provided with the winding process which winds the process and the steel plate cooled to the said Tc degrees C or less at the temperature of Tc degrees C or less.
t ≧ 1.4 × 10 ⁻⁶ × exp {3.2 × 10 ⁴ / (Tf + 273)} (iii)
(Tr-100) ≦ steel plate temperature <Tr (iv)
3 ≦ rolling rate <40 (v)
Tc = 561-474 × C-33 × Mn-17 × Ni-17 × Cr-21 × Mo (vi)
However,
t (s) is the heating time of the slab in the soaking zone,
Tf (° C.) is the average temperature of the slab in the soaking zone,
Tr is a calculated value of 870 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V,
Each element symbol in each formula means the content (% by mass) of each element.   The method for producing a hot-rolled steel sheet according to claim 6, wherein after the winding step, the hot-rolled steel sheet is subjected to a hot dip galvanizing process or an alloyed hot dip galvanizing process.

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