TW201837204A - Hot rolled steel sheet including C, Si, Mn, Al, N, P, S, Ti, Nb, V, Cu, Ni, Cr, Mo, B, Mg, Ca, REM, Zr, Co, Zn, W, Sn, Fe and impurities - Google Patents

Abstract

Disclosed is a hot-rolled steel sheet, the chemical composition of which includes the following components in percentage by mass: 0.07 % to 0.22 % of C, 1.00 % to 3.20 % of Si, 0.80 % to 2.20 % of Mn, 0.010 % to 1.000 % of Al, 0.0060 % or less of N, 0.050 % or less of P, 0.005 % or less of S, 0 % to 0.150 % of Ti, 0 % to 0.100 % of Nb, 0 % to 0.300 % of V, 0 % to 2.00 % of Cu, 0 % to 2.00 % of Ni, 0 % to 2.00 % of Cr, 0 % to 1.00 % of Mo, 0 % to 0.0100 % of B, 0 % to 0.0100 % of Mg, 0 % to 0.0100 % of Ca, 0 % to 0.1000 % of REM, 0 % to 1.000 % of Zr, 0 % to 1.000 % of Co, 0 % to 1.000% of Zn, 0 % to 1.000 % of W, 0 % to 0.050 % of Sn and a residual portion of Fe and impurities. Metal structures at positions of 1/4 W or 3/4 W from an end surface of the steel plate and 1/4t or 3/4t from a surface of the steel plate include the following components in percentage by area: more than 2 % to 10 % of retained austenite, 2 % or less of martensite, 10 % to 70 % of bainite, 2 % or less of pearlite and a residual portion of ferrite. The average circle equivalent diameter of metal phases composed of the retained austenite and/or martensite is 1.0 to 5.0 [mu]m, the average value of the shortest distances between the above adjacent metal phases is 3 [mu]m or more, and a standard deviation of the ultra-fine hardness is 2.5 GPa or less.

Description

 Hot rolled steel sheet  

This invention relates to hot rolled steel sheets.

Steel sheets used in the body structure of automobiles are required to have high strength and high press workability from the viewpoint of improving safety and weight reduction. In particular, in order to improve press workability, it is a high-strength steel sheet which is required to ensure ductility during processing and to ensure impact resistance when mounted on an automobile.

As such a steel sheet, it is known to form a process-induced deformation type steel sheet containing a mixed structure of residual Worth iron (see, for example, Patent Document 1). In the following description, a process-induced abnormal steel sheet may be referred to as a TRIP (Transformation Induced Plasticity) steel sheet.

In addition, in order to meet the requirements for lightweighting of automobiles and complicated shape of parts in recent years, a mixed-structure steel sheet having higher elongation and superior local ductility than the prior art has been proposed. For example, Patent Document 2 proposes a steel sheet which is in a structure composed of a ferrite grain iron phase and a hard second phase (Mada iron, residual Worth iron), and in the cooling after hot rolling, in the ferrite iron In the phase, the alloy carbide is precipitated to strengthen the ferrite phase. In the following description, a steel material in which a soft structure such as ferrite iron and a hard structure such as granulated iron is uniformly dispersed as in Patent Document 2 is also referred to as DP (Dual Phase) steel.

Further, Patent Document 3 proposes a high-strength steel sheet excellent in elongation and local ductility, which uses a mixed structure of precipitated strengthening ferrite iron and residual Worth iron, which is in the metamorphosis from the Worthite iron to the fertilized iron. At the interface of the phase, the main is to use the appearance of the precipitation generated by the grain boundary diffusion to control the precipitation distribution to obtain the precipitated strengthening ferrite.

Patent Document 4 discloses a process-induced metamorphic composite structure steel sheet having a tensile strength of 540 MPa or more which is excellent in burring workability. Patent Document 5 discloses a hot-rolled TRIP steel having a small material variation in a coil, that is, a high-workability hot-rolled high-tensile steel sheet having excellent material uniformity. Patent Document 6 discloses a steel material which can suppress the occurrence of cracks when subjected to an impact load and can provide an impact absorbing member having a high effective flow stress. Patent Document 7 discloses a DP steel sheet which is a high-strength composite structure hot-rolled steel sheet excellent in stretch flanging, post-coating corrosion resistance, and notch fatigue characteristics. Further, Patent Document 8 discloses a high Young's modulus steel sheet excellent in hole expandability.

[Patent Document 1] Japanese Patent Laid-Open No. Hei 10-158735

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2009-84648

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2011-225941

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2002-129286

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2001-152254

[Patent Document 6] JP-A-2015-124411

[Patent Document 7] International Publication No. 2016/133222

[Patent Document 8] Japanese Patent Laid-Open Publication No. 2009-19265

With the complexity of the structure of the car body and the complexity of the shape of the part, the processing of the steel plate for automobiles is not only a conventional press processing element, but also a combination of the conventional press working elements and new machining elements, such as forging. Combination. The conventional press working elements are, for example, elements such as deep drawing processing, hole expanding, bulge forming processing, bending processing, and ironing processing.

However, in recent years, press forming, which is represented by plate forging, has been applied to the conventional press working elements in order to further apply a compressive load to further distribute the press load, and to add processing elements for forging, for example, upset processing and increase. Processing elements such as thick (thickening) processing. In other words, the die forging is a press working having a composite machining element, and includes machining elements unique to the forging process in addition to the machining elements when the steel plate is subjected to press working as in the related art.

By performing the above-described press working, the steel sheet is deformed while maintaining the original thickness of the steel sheet, or the thickness of the steel sheet is reduced (thinned), and the steel sheet is deformed to form a part. The part which is subjected to the forging process by applying a local compressive force, the thickness of the steel sheet is increased (thickness), and the steel sheet thickness of the functionally necessary portion can be efficiently deformed to ensure the parts. Strength of.

Conventional TRIP steels are known to exhibit good formability in conventional press working. However, in the conventional press working, the forming method of the element for forging processing, that is, the plate forging, has been found, and it has been found that even at a low degree of work, the steel sheet may be cracked and fractured.

In other words, in the conventional press working, in the portion where the neck thickness is necked (thickening of the thickness of the steel sheet), punching cracks occur, and even if the sheet is not forged, the sheet thickness is not required to be necked. It has been determined that cracks may occur in the material and the fracture may not be obtained.

The limit of the occurrence of cracks in such forged plates is governed by which nature of the steel plate, and how it can be improved. These are not understood. Therefore, a TRIP steel is required, and the functions of the conventional TRIP steel, that is, the functions such as deep drawing workability, hole expandability, and bulging forming workability can be effectively utilized, and cracking does not occur even when the plate forging process is performed.

The present invention has been developed in order to solve the above problems, and an object thereof is to provide a hot-rolled steel sheet which can maintain the basic function of the TRIP steel and allows the portion of the crack to be subjected to forging processing by applying a local compressive force. Improved and excellent board forgeability.

The present invention has been developed in order to solve the above problems, and the main contents thereof are the following hot rolled steel sheets.

(1) A hot-rolled steel sheet whose chemical composition is contained in mass %: C: 0.07 to 0.22%, Si: 1.00 to 3.20%, Mn: 0.80 to 2.20%, Al: 0.010 to 1.000%, N: 0.0060% Hereinafter, P: 0.050% or less, S: 0.005% or less, Ti: 0 to 0.150%, Nb: 0 to 0.100%, V: 0 to 0.300%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr :0~2.00%, Mo: 0~1.00%, B: 0~0.0100%, Mg: 0~0.0100%, Ca: 0~0.0100%, REM: 0~0.1000%, Zr: 0~1.000%, Co: 0~1.000%, Zn: 0~1.000%, W: 0~1.000%, Sn: 0~0.050%, and the remaining part: Fe and impurities, in the section perpendicular to the rolling direction of the steel sheet, when the steel sheet is When the width and the thickness are set to W and t, respectively, the metal structure at a position of 1/4 W or 3/4 W from the end surface of the steel sheet and 1/4 t or 3/4 t from the surface of the steel sheet is used. % meter, including: residual Worth iron: more than 2% and less than 10%, Ma Tian loose iron: 2% or less, toughened iron: 10 to 70%, Bora iron: 2% or less, the remaining part: fat Iron, the average circular equivalent diameter of the metal phase composed of residual Worthite iron and/or 麻田散铁 is 1.0~5.0μm, adjacent to the aforementioned metal phase The average value of the shortest distance of 3μm or more, the standard deviation of ultramicro hardness is 2.5GPa or less.

(2) The hot-rolled steel sheet according to the above (1), which has a tensile strength of 780 MPa or more and a sheet thickness of 1.0 to 4.0 mm.

According to the present invention, it is possible to obtain a hot-rolled steel sheet which can maintain the basic functions of the TRIP steel, such as deep drawing workability and bulging formability, and has excellent plate forgeability.

1‧‧‧Shear deformation generation department

2‧‧‧ gripping department

Fig. 1 is a schematic view showing a simple shear test. Figure 1 (a) shows a test piece of a simple shear test. Figure 1 (b) shows the test piece after the simple shear test.

The inventors of the present invention have conducted intensive studies in order to solve the above problems, and have obtained the following knowledge.

 (a) Equivalent plastic strain  

Plate forging is a deformation of a strain zone (high strain zone) which exceeds the strain at break of the tensile test in the past. In addition, since plate forging is a composite process, it cannot be evaluated simply by using tensile test and shear test data. Then, the inventors introduced "equivalent plastic strain" as an index and established a new evaluation method.

By using the equivalent plastic strain as an index, the tensile stress and the tensile strain at the time of the fracture after the tensile test, and the shear stress and the shear strain at the time of the fracture after the shear test can be combined. Evaluation.

The equivalent plastic strain is the relationship between the shear stress σ s and the shear plastic strain εsp in the simple shear test, and is converted into tensile stress σ and tensile strain in the uniaxial tensile test. The relationship of ε. Further, assuming that the relationship between the isotropic hardening law and the plastic work conjugate is a constant conversion coefficient (κ), the conversion can be performed as follows. After calculating the conversion coefficient (κ) by the method described later, the equivalent plastic strain is derived.

Tensile stress σ in uniaxial tensile test = shear stress σ s × κ in simple shear test

Tensile strain ε in uniaxial tensile test = shear plastic strain εsp/κ in simple shear test

 (b) Multi-segment shear test  

In order to obtain the equivalent plastic strain, it is necessary to obtain the relationship between the tensile stress and the tensile strain at the time of the tensile test, and the relationship between the shear stress and the shear strain at the time of the shear test. However, plate forging involves deformation in high strain zones. Therefore, when the test is performed once using the shear test apparatus which is generally used, the crack of the test piece progresses from the portion where the test piece is held. As a result, in most cases, the test cannot progress to deformation in the high strain zone. Therefore, it is necessary to reproduce a process of reducing the thickness (thinning and necking) of the steel sheet thickness as in the forging of the sheet.

Then, the shear test is divided into a plurality of stages, and in the shear test of each stage, the crack origin of the test piece which is generated in the portion where the test piece is held is machined to avoid the crack progress of the test piece, and these are The shear test results were serially evaluated to evaluate the test results. By using this test method, the shear test results reaching the high strain region can be obtained, and the relationship between the shear stress and the shear strain reaching the high strain region can be obtained.

On the other hand, regarding the tensile stress and the tensile strain, a conventional tensile test method can be employed. For example, JIS No. 5 test piece according to JIS Z2241 (2011) can be used.

 (c) Mechanism of cracking  

The following knowledge was obtained for the occurrence mechanism of the crack by using the multi-segment shear test described above, the evaluation method using the equivalent plastic strain, and the microscopic investigation of the steel sheet before and after the plate forging.

Holes (small voids) occur at the interface between the two phases due to the difference in strain energy between the hard phase (Mita loose iron, residual Worth iron) and the soft phase (fertilizer iron, toughened iron). Thereafter, as the strain of the plate forging increases, the pores grow, become combined with adjacent holes to become cracks, and finally cause fracture. Therefore, it is possible to suppress the occurrence of cracks by preventing the occurrence of voids and suppressing the bonding of the pores to adjacent pores. However, it is also important not to detract from the original function of TRIP steel. In the following description, the Ma Tian loose iron and the residual Worth iron are collectively referred to as a hard phase. The hard phase is identical to the "metal phase consisting of residual Worthite and/or 麻田散铁" as set out in the "Scope of Application for Proposing".

Based on the above knowledge, the following items are derived.

(i) defines the average diameter of the hard phase.

That is, since the pores occur at the boundary between the hard phase and the metal phase (other than the hard phase), the occurrence of the void can be reduced by defining the average diameter of the hard phase.

(ii) Reduce the unevenness of ultra-micro hardness.

That is, by minimizing the difference in hardness between the hard phase and the soft phase, the occurrence of voids can be reduced.

(iii) Limiting the distance of the hard phases from each other.

That is, since the holes occur at the boundary between the hard phase and the other metal phase (soft phase), by arranging the hard phases apart, the pores become difficult to bond even if they grow.

(iv) The equivalent plastic strain at the time of fracture is 0.50 (50%) or more.

It has been confirmed that the equivalent plastic strain at the time of fracture is 0.50 (50%) or more by satisfying the conditions (i) to (iii) above, and a certain degree of workability can be ensured even in the case of composite processing such as forging.

 (d) Effective cumulative strain  

In order to obtain the above structures (i) to (iv), in the multi-stage finishing rolling by continuous rolling of three or more stages (for example, 6 or 7 stages) of hot rolling, it is necessary to The final finishing rolling is performed so that the cumulative strain of the rolling (hereinafter also referred to as "effective cumulative strain") is 0.10 to 0.40.

The effective cumulative strain is an index obtained by taking into consideration the temperature at the time of rolling and the recovery, recrystallization, and grain growth of crystal grains due to the reduction ratio of the steel sheet at the time of rolling. Therefore, in determining the effective cumulative strain, a constitutive law for expressing the static recovery phenomenon as time elapses after rolling is employed. The reason for considering the static recovery of the crystal grains with the passage of time after rolling is that the release of energy accumulated in the form of strain in the crystal grains after rolling is due to the dislocation of the thermally induced crystal grains. Caused by the static reply caused by the elimination. Moreover, the elimination of the thermal differential discharge is affected by the rolling temperature and the elapsed time after rolling. Then, the static recovery is taken into consideration, and the above-mentioned index is introduced as a parameter using the temperature at the time of rolling, the rolling reduction ratio (logarithmic strain) of the steel sheet by rolling, and the time lapse after rolling. To "effectively accumulate strain."

In this way, by limiting the effective cumulative strain, the average circular equivalent diameter of the hard phase can be limited, and the distance between adjacent hard phases can be limited, so that the unevenness of the ultra-fine hardness is reduced. As a result, it is possible to suppress the growth of pores occurring at the interface between the hard phase and the soft phase, and it is difficult to bond even if the pores grow. Thereby, even if the board forging is not cracked, a steel sheet excellent in plate forgeability can be obtained.

The present invention has been developed based on the above knowledge. Hereinafter, each requirement of the present invention will be described in detail.

 (A) chemical composition  

The reason for limiting each element is as follows. In the following description, the "%" of the content means "% by mass".

 C: 0.07~0.22%  

C is an element that helps to increase strength and ensure the retention of Worth Iron. When the C content is too low, the strength cannot be sufficiently increased, and the Worstian iron remains cannot be secured. On the other hand, when the content is too large, the amount of residual Worthite iron (area ratio) increases, and the fracture strain at the time of plate forging decreases. Therefore, the C content is set to be 0.07 to 0.22%. The C content is preferably 0.08% or more, 0.10% or more, or 0.12% or more, more preferably 0.14% or more, 0.15% or more, or 0.16% or more. Further, the C content is preferably 0.20% or less or 0.18% or less, more preferably 0.17% or less.

 Si: 1.00~3.20%  

Si has a deoxidizing effect and is an element which contributes to suppressing the formation of harmful carbides and generating ferrite iron. In addition, it has the effect of suppressing the decomposition of the residual Worth iron. On the other hand, when the content is too large, not only the ductility is lowered but also the processability is lowered, and the corrosion resistance after coating is deteriorated. Therefore, the Si content is set to 1.00 to 3.20%. The Si content is preferably 1.20% or more, 1.30% or more, or 1.40% or more, more preferably 1.50% or more or 1.60% or more. Further, the Si content is preferably 3.00% or less, 2.80% or less, or 2.60% or less, more preferably 2.50% or less, 2.40% or less, or 2.30% or less.

 Mn: 0.80~2.20%  

Mn, which expands the temperature of the Worthite iron field to the low temperature side and expands the temperature range of the ferrite iron and the Vostian iron two-phase region, is an element that contributes to the stabilization of the residual Worth iron. On the other hand, when the content is too large, the hardenability is as high as necessary, and it becomes impossible to sufficiently ensure the ferrite iron, and flat embryo cracks occur at the time of casting. Therefore, the Mn content is set to 0.80 to 2.20%. The Mn content is preferably 0.90% or more, 1.00% or more, 1.20% or more, or 1.40% or more, and more preferably 1.50% or more. Further, the Mn content is preferably 2.00% or less or 1.90% or less, more preferably 1.80% or less or 1.70% or less.

 Al: 0.010~1.000%  

Al, which is the same as Si, has a deoxidizing effect and an effect of producing ferrite. On the other hand, when the content is too large, embrittlement is caused, and the tundish nozzle is easily clogged during casting. Therefore, the Al content is set to be 0.010 to 1.000%. The Al content is preferably 0.015% or more or 0.020% or more, more preferably 0.025% or more or 0.030% or more. Further, the Al content is preferably 0.800% or less, 0.700% or less, or 0.600% or less, more preferably 0.500% or less or 0.400% or less.

 N: 0.0060% or less  

N is an element which contributes to precipitation of AlN or the like and refines crystal grains. On the other hand, when the content is too large, not only solid solution nitrogen remains but the ductility is lowered, and aging deterioration is severe. Therefore, the N content is made 0.0060% or less. The lower limit of the N content is not particularly limited, and the lower limit thereof is 0%. The N content is preferably 0.0050% or less or 0.0040% or less. Further, if the content is excessively lowered, the cost at the time of refining increases, so the lower limit is 0.0010%.

 P: 0.050% or less  

P is an impurity contained in the molten iron, and segregation at the grain boundary causes local ductility to deteriorate and weldability is deteriorated. Therefore, the content is preferably as small as possible. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.030% or less or 0.020% or less. The lower limit is not particularly specified, and the lower limit is 0%. However, excessively lowering the content causes an increase in cost during refining, so the lower limit is 0.001%.

 S: 0.005% or less  

S is also an impurity contained in the molten iron, and MnS is formed to deteriorate local ductility and weldability. Therefore, the smaller the content, the better. Therefore, the S content is limited to 0.005% or less. In order to increase ductility or weldability, the S content may be set to be 0.003% or less or 0.002% or less. The lower limit is not particularly specified, and the lower limit is 0%. However, excessively lowering the content causes an increase in cost during refining, so the lower limit is 0.0005%.

 Ti: 0~0.150%  

The effect of Ti is to delay the grain growth of the carbonitride or solid solution Ti during hot rolling, thereby making the grain size of the hot rolled sheet finer and improving the low temperature toughness. Further, it exists in the form of TiC, and contributes to the increase in strength of the steel sheet by precipitation strengthening. Therefore, it can be included as necessary. However, when the content is too large, in addition to the effect of saturation, it will become the cause of blockage of the mouth during casting. Therefore, the Ti content is made 0.150% or less. The upper limit can be limited to 0.100%, 0.060% or 0.020% as necessary. The lower limit of the Ti content is 0%, but in order to sufficiently obtain the precipitation strengthening effect, the lower limit can be limited to 0.001% or 0.010%.

 Nb: 0~0.100%  

The effect of Nb is to delay the grain growth of the carbonitride or the solid solution Nb during hot rolling, thereby making the particle diameter of the hot rolled sheet fine and improving the low temperature toughness. Further, it exists in the form of NbC, and contributes to the increase in strength of the steel sheet by precipitation strengthening. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Nb content is made 0.100% or less. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower limit can be limited to 0.001% or 0.010%.

 V: 0~0.300%  

The effect of V is to increase the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the V content is made 0.300% or less. The V content may be set to 0.200% or less, 0.100% or less, or 0.060% or less as necessary. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower limit can be limited to 0.001% or 0.010%.

 Cu: 0~2.00%  

The effect of Cu is to increase the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Cu content is made 2.00% or less. Further, when Cu is contained in a large amount, damage due to scale may occur on the surface of the steel sheet. Therefore, the Cu content can be made 1.20% or less, 0.80% or less, 0.50% or less, or 0.25% or less. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower limit of the Cu content can be limited to 0.01%.

 Ni: 0~2.00%  

The effect of Ni is to increase the strength of the steel sheet by solid solution strengthening. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Ni content is made 2.00% or less. Further, when Ni is contained in a large amount, ductility may be lowered. Therefore, the Ni content can be made 0.60% or less, 0.35% or less, or 0.20% or less. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower Ni content can be limited to 0.01%.

 Cr: 0~2.00%  

The effect of Cr is to increase the strength of the steel sheet by solid solution strengthening. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Cr content is made 2.00% or less. In order to further improve economy, the upper limit can be limited to 1.00%, 0.60% or 0.30%. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower limit of the Cr content can be limited to 0.01%.

 Mo: 0~1.00%  

The effect of Mo is to increase the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Mo content is made 1.00% or less. In order to further improve economy, the upper limit can be limited to 0.60%, 0.30%, or 0.10%. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower Mo content can be limited to 0.005% or 0.01%.

 B: 0~0.0100%  

B will segregate at the grain boundary, and the low temperature toughness is improved by increasing the grain boundary strength. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the B content is made 0.0100% or less. Further, B is a strong quenching element, and when it is contained in a large amount, the fermented iron in the cooling state cannot be sufficiently progressed, and sufficient residual Worthite iron may not be obtained. Therefore, the B content can be made 0.0050% or less, 0.0020% or less, or 0.0015%. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower B content can be limited to 0.0001% or 0.0002%.

 Mg: 0~0.0100%  

Mg is a form of non-metallic inclusions which is a cause of deterioration of workability by controlling the origin of destruction, and improves workability. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Mg content is made 0.0100% or less. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower limit of the Mg content may be limited to 0.0001% or 0.0005%.

 Ca: 0~0.0100%  

Ca is a form of non-metallic inclusions that is a cause of deterioration in workability, and is improved in workability. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the Ca content is made 0.0100% or less. The lower limit is 0%, but in order to sufficiently obtain the above effects, the Ca content is preferably 0.0005% or more.

 REM: 0~0.1000%  

REM (rare earth element) is a form of non-metallic inclusions which is a cause of deterioration of workability by controlling the origin of destruction, and improves workability. Therefore, it can be included as necessary. However, when the content is too large, the effect is saturated and the economy is lowered. Therefore, the REM content is made 0.1000% or less. It may be limited to 0.0100% or 0.0060% as necessary. The lower limit is 0%, but in order to sufficiently obtain the above effects, the lower limit of the REM content can be limited to 0.0001% or 0.0005%.

Here, in the present invention, REM means a total of 17 elements of Sc, Y and lanthanoid elements, and the aforementioned REM content means the total content of these elements. The lanthanide element is industrially added in the form of a rare earth metal alloy (Misch metal).

Zr: 0~1.000%

Co: 0~1.000%

Zn: 0~1.000%

W: 0~1.000%

It has been confirmed that Zr, Co, Zn and W are each in the range of 1.000% or less, and the effects of the present invention are not impaired even if they are contained. The upper limit of these can be limited to 0.300% or 0.10%. The total content of Zr, Co, Zn and W is preferably 1.000% or less or 0.100%. The content of these is not essential, and the lower limit is 0%, and may be limited to 0.0001% as necessary.

 Sn: 0~0.050%  

It has been confirmed that Sn is a small amount, and even if it is contained, the effect of the present invention is not impaired. However, when it exceeds 0.05%, a flaw may occur during hot rolling. Therefore, the Sn content is set to be 0.050% or less. The content of Sn is not essential, and the lower limit thereof is 0%, and the lower limit can be limited to 0.001% as necessary.

In the chemical composition of the steel sheet of the present invention, the remainder is Fe and impurities.

Here, the term "impurity" means that when a steel sheet is industrially produced, a component which is mixed by a raw material such as ore, scrap, or the like, and various causes of the production process is acceptable within a range not adversely affecting the present invention.

 (B) Metal structure  

The metal structure of the steel sheet of the present invention will be described. Further, in the present invention, the metal structure means that in the cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are respectively set to W and t, 1/4 W or 3 is taken from the end surface of the steel sheet. /4W, and the structure of the position of 1/4t or 3/4t from the surface of the steel plate. In addition, "%" in the following description means "area%".

 Residual Worth Iron: more than 2% and less than 10%  

Residual Worthite iron is a structure necessary for obtaining processing-induced metamorphosis (so-called TRIP phenomenon). The residual Worthite iron is processed into a metamorphosis of the granulated iron, and is processed in the form of granulated iron, thereby ensuring workability and ensuring strength in the processed parts. In order to obtain the original function of the TRIP steel sheet, the area ratio of the residual Worth iron is set to a value exceeding 2%.

On the other hand, when there is too much residual Worthite iron, the hard phase of the hard phase of the granulated iron is abundant due to the processing induced metamorphism, which is caused by the difference in the strain energy of the ferrite iron with the soft phase. Holes occur, and as the strain of the steel plate due to forging of the plate increases, the holes combine to grow into cracks. Therefore, the area ratio of the remaining Worthfield iron is set to be 10% or less. The area ratio of the residual Worthite iron is preferably 2.5% or more, more preferably 3% or more or 4% or more. Further, the area ratio of the residual Worthite iron is preferably 9% or less, more preferably 8% or less.

 Ma Tian loose iron: 2% or less  

TRIP steel is characterized in that it can ensure the processability, and the residual Worthite iron is granulated in the field by processing induced metamorphism. Therefore, in order to ensure the workability, the hard phase of the granulated iron is as small as possible. Therefore, the area ratio of the granulated iron is set to be 2% or less. The area ratio of the granulated iron is preferably 1.5% or less, 1% or less, or 0.5% or less. However, the lower limit is not particularly specified, and the lower limit is 0%.

 Toughened iron: 10~70%  

The toughened iron of the soft phase is an important structure for ensuring the balance between strength and elongation, and has an effect of suppressing crack propagation. Based on this point of view, the area ratio of the toughened iron is set to 10% or more. In order to increase the strength, the lower limit can be limited to 20%, 30%, 35% or 40%. On the other hand, when the area ratio of the toughened iron is too high, the Worstian iron cannot be secured, and the original function of the TRIP steel cannot be ensured, so it is set to be 70% or less. The upper limit may be limited to 65%, 60%, 55% or 50% as necessary.

 Bora: 2% or less  

When a large amount of Boron iron is present, the strength is lowered, so the area ratio is set to be 2% or less. The upper limit can be limited to 1% or 0.5% as necessary. The lower the area ratio of the Borne iron, the better, preferably 0%.

 The rest: fat iron  

The soft phase of the ferrite iron is also an important organization based on the viewpoint of ensuring the balance of strength and elongation and improving workability. Therefore, the structure other than the Worthite iron, the granulated iron, the toughened iron, and the Bora iron is the ferrite iron. There is no particular limitation on the area ratio of the ferrite iron in the remaining part of the organization. However, the lower limit of the area ratio may be 10%, and the upper limit may be 88%. The lower limit may be limited to 20%, 30%, 35% or 40% as necessary, and may be limited to 80%, 70%, 60% or 55%.

Here, in the present invention, the area ratio of the metal structure is obtained as follows. As described above, the sample was first taken at a position of 1/4 W or 3/4 W from the end surface of the steel sheet and 1/4 t or 3/4 t from the surface of the steel sheet. Next, the rolling direction cross section of the sample (so-called L-direction cross section) was observed.

Specifically, the sample was etched using a nital etchant, and observed after etching using an optical microscope at a field of 300 μm × 300 μm. Next, image analysis was performed on the obtained tissue photograph, thereby obtaining the area ratio A of the ferrite iron, the area ratio B of the Borne iron, and the total area ratio C of the toughened iron, the granulated iron and the residual Worth iron.

Next, the portion etched by the oxidizing agent was subjected to Le Pera etching, and observed with an optical microscope at a field of 300 μm × 300 μm. Next, image analysis was performed on the obtained tissue photograph, thereby calculating the total area ratio D of the remaining Worthfield iron and the granulated iron. Further, the sample was subjected to surface cutting in the normal direction of the rolling surface up to a depth of 1/4 of the sheet thickness, and the volume fraction of the residual Worthite was determined by X-ray diffraction measurement. Since the volume ratio is approximately equal to the area ratio, the above-mentioned volume ratio is used as the area ratio E of the residual Worth iron. The area ratio of the toughened iron is obtained from the difference between the area ratio C and the area ratio D, and the area ratio of the granulated iron is determined from the difference between the area ratio E and the area ratio D. According to this method, the area ratios of the ferrite iron, the toughening iron, the granulated iron, the residual Worth iron, and the Bora iron can be obtained.

Further, in the present invention, the state of existence of the metal phase (hereinafter also simply referred to as "metal phase") composed of the remaining Worthite iron and/or the granulated iron is defined as follows. Further, it is preferable that the metal phase (hard phase) is mainly composed of residual Worth iron, that is, the area ratio of the remaining Worth iron is larger than the area ratio of the granulated iron.

 The average circular equivalent diameter of the metal phase: 1.0~5.0μm  

In order to secure the original function of the TRIP steel, the area of the above metal phase must be more than a certain value, so the average circular equivalent diameter of the metal phase is set to 1.0 μm or more. On the other hand, when the metal phase is too large, the strain of the steel sheet due to the forging of the sheet increases, and the pores existing at the grain boundary become easily bonded, so that the average circular equivalent diameter of the metal phase is set to 5.0 μm or less. The average circular equivalent diameter of the metal phase is preferably 1.5 μm or more, more preferably 1.8 μm or more or 2.0 μm or more. Further, the average circular equivalent diameter of the metal phase is preferably 4.8 μm or less, 4.4 μm or less, or 4.2 μm or less, more preferably 4 μm or less, 3.5 μm or less, or 3 μm or less.

The average circular equivalent diameter (diameter) of the metal phase was determined as follows. First, according to the method of measuring the area ratio D, the circle equivalent diameter is obtained from the metal photo area after the corrosion of the tissue after Le Pera etching. Next, the (simple) average value of the measured circle equivalent diameter was used as the average circle equivalent diameter.

 Average value of the shortest distance between adjacent metal phases: 3 μm or more  

The holes that occur at the interface between the hard phase and the soft phase grow, and the holes combine to form larger holes. To avoid this, the distance between the hard phases must be ensured to be a certain amount. Therefore, the average value of the distance between adjacent metal phases is set to 3 μm or more.

The average value is preferably 4 μm or more, and more preferably 5 μm or more, from the viewpoint of suppressing the occurrence of cracks caused by the growth of the pores. Although the upper limit is not particularly set, in order to secure the original function of the TRIP steel, the average value is preferably 10 μm or less.

The average value of the shortest distances of adjacent metal phases is obtained as follows. Twenty metal phases were arbitrarily selected, and the distance between them and the nearest metal phase was measured, and the average value was calculated. The shortest distance between the metal phases is obtained by performing image analysis on the observation image of the optical microscope after Le Pera etching according to the method of measuring the area ratio D.

 (C) Mechanical properties    Standard deviation of ultra-fine hardness: 2.5GPa or less  

By narrowing the difference in strain energy between the hard phase and the soft phase, it is possible to reduce the pores occurring at the interface between the two phases, and by further widening the pore spacing, it is possible to suppress the pores from being combined and growing into cracks. Therefore, the ultra-fine hardness difference corresponding to the difference in the strain energy between the hard phase and the soft phase is minimized, and the occurrence of voids can be suppressed. In the present invention, as an index of the hardness difference between the soft phase and the hard phase, the standard deviation of the ultrafine hardness in the sample cross section is employed.

The ultra-fine hardness can be measured, for example, using a Nano Indenter (TriboScope/TriboIndenter) manufactured by Hysitron. The ultra-fine hardness of 100 points or more was arbitrarily measured with a load of 1 mN, and the standard deviation of the ultra-fine hardness was calculated from the results.

In order to reduce the difference in hardness between the soft phase and the hard phase and to suppress the occurrence of voids, the standard deviation of the ultra-fine hardness is preferably as small as 2.5 GPa or less. It is preferably 2.4 GPa or less or 2.3 GPa or less.

 Tensile strength: 780MPa or more  

The steel sheet of the present invention preferably has a tensile strength of 780 MPa or more which is the same as that of the conventional TRIP steel. The upper limit of the tensile strength is not particularly limited and may be 1200 MPa, 1150 MPa or 1000 MPa. The tensile strength is a tensile strength according to JIS Z 2241 (2011).

 Product of uniform elongation and tensile strength: 9500MPa% or more  

When the uniform elongation is small, the thickness of the sheet due to necking is likely to be reduced during press forming, which is a cause of punching cracks. In order to secure press formability, it is preferable to satisfy the product of uniform elongation (u-EL) and tensile strength (TS): TS × u - EL ≧ 9500 MPa%. Wherein the uniform elongation, in the test in accordance with JIS Z 2241 (2011) prescribed in the nominal stress σ n and nominal strain relationship ε n when the nominal stress σ n with the nominal value of the strain ε n differential When the nominal strain at the point of becoming zero is ε n0 , it is expressed by the following formula.

Uniform elongation (u-EL) = ln(εn0+1)

 Equivalent plastic strain: 0.50 or more  

The equivalent plastic strain is the relationship between the shear stress σ s and the shear plastic strain εsp in the simple shear test, and is converted into tensile stress σ and tensile strain in the uniaxial tensile test. The relationship of ε, assuming the relationship between the isotropic hardening law and the plastic work conjugate, is converted using a constant conversion coefficient (κ).

Here, the isotropic hardening method means that the shape of the relief curve is assumed to be a work hardening rule that does not change the strain progression (i.e., expands in a similar shape). The relationship with the plastic work conjugate means that the work hardening is written only in the form of a function of plastic work, and when the same plastic work (σ × ε) is given regardless of the deformation form, the relationship of the same work hardening amount is expressed.

Thereby, the shear stress and the shear plastic strain at the time of the simple shear test can be respectively converted into the tensile stress and the tensile strain at the time of the uniaxial tensile test. The relationship is as follows.

Tensile stress σ (conversion) in uniaxial tensile test = shear stress σ s × κ in simple shear test

Tensile strain ε (conversion) in uniaxial tensile test = shear plastic strain ε sp/κ in simple shear test

Next, the conversion coefficient κ is obtained, and the conversion coefficient κ is used to make the relationship between the shear stress and the shear plastic strain similar to the relationship between the tensile stress and the tensile strain. For example, the conversion coefficient κ can be obtained by the following procedure. First, the relationship between the tensile strain ε (actual measurement value) and the tensile stress σ (actual measurement value) at the time of the uniaxial tensile test was determined. Then, the relationship between the shear strain εs (actual measured value) and the shear stress σ s (actual measured value) in the uniaxial shear test was obtained.

Next, let κ change, and obtain the tensile strain ε (conversion) obtained from the shear strain ε s (measured value) and the tensile stress σ (conversion) obtained from the shear stress σs (actual measurement value). The tensile stress σ (conversion) when the tensile strain ε (conversion) is between 0.2% and uniform elongation (u-EL) is determined. The error of the tensile stress σ (conversion) and the tensile stress σ (measured value) at this time is obtained, and the least square method is used to find the κ with the smallest error.

The equivalent plastic strain ε eq is defined as the conversion of the shear plastic strain ε sp (fracture) at the time of the fracture of the simple shear test to the tensile strain ε in the simple tensile test using the obtained κ . .

The steel sheet according to the present invention is characterized in that the processing property in the high strain region typified by plate forging is good, and the equivalent plastic strain ε eq satisfies 0.50 or more. The equivalent plastic strain of the conventional TRIP steel is about 0.30 at the top, and thus it can be confirmed that the steel sheet of the present invention has good forgeability.

 (D) size    Plate thickness: 1.0~4.0mm  

The steel sheet of the present invention is mainly used for automobiles and the like, and its plate thickness range is mainly 1.0 to 4.0 mm. Therefore, the plate thickness can be set to 1.0 to 4.0 mm, and the lower limit can be limited to 1.2 mm, 1.4 mm or 1.6 mm as necessary, and the upper limit is limited to 3.6 mm, 3.2 mm or 2.8 mm.

 (E) Manufacturing method  

The inventors of the present invention have confirmed from the recent studies that the hot-rolled steel sheet of the present invention can be produced by the following production steps (a) to (e). Hereinafter, each manufacturing step will be described in detail.

 (a) Melting step  

The manufacturing method before hot rolling is not particularly limited. In other words, after smelting by a blast furnace or an electric furnace or the like, various smelting is performed twice to adjust the composition of the above components. Next, a flat embryo may be produced by a method such as usual continuous casting or thin flat metal casting. In this case, as long as it can be controlled within the range of the components of the present invention, waste materials or the like may be used in the raw materials.

 (b) Hot rolling step  

The produced slab is heated and hot rolled to form a hot rolled steel sheet. The conditions in the hot rolling step are not particularly limited. For example, it is preferred to set the heating temperature before hot rolling to 1050 to 1260 °C. In the case of continuous casting, it may be once cooled to a low temperature and then heated again to perform hot rolling, and may be continuously heated by continuous casting without cooling and may be hot rolled.

After heating, rough rolling is performed on the flat blank extracted from the heating furnace, and subsequent finishing rolling. As described above, the finishing rolling is a multi-stage finishing rolling by continuous rolling of three or more stages (for example, six stages or seven stages). Further, the final finishing rolling is performed so that the cumulative strain (effective cumulative strain) of the final three-stage rolling is 0.10 to 0.40.

As described above, the effective cumulative strain is a change in crystal grain size due to the temperature at the time of rolling, the reduction ratio of the steel sheet at the time of rolling, and the crystal grain which is statically recovered by the time after the rolling of the crystal grain. Changes in the path are included in the indicators considered. The effective cumulative strain (εeff) can be obtained by the following equation.

Effective cumulative strain (εeff)=Σεi(ti,Ti)...(1)

Σ in the above formula (1) represents the sum of i=1~3.

Where i=1 denotes the rolling of the first stage of the multi-stage finishing rolling (that is, the final section rolling), i=2 represents the rolling of the second stage of the last, and i=3 represents the third stage of the last Rolling.

Here, in each of the rolls indicated by i, ε i can be expressed by the following formula.

Εi(ti,Ti)=ei/exp((ti/τR) ^2/3 )...(2)

Ti: time from the rolling of the last stage i to the start of the cooling after the final section rolling (s)

Ti: rolling temperature (K) of the last i-th stage

Ei: logarithmic strain at the time of rolling in the last i-th stage

Ei=|ln{1-(inlet side thickness of the i-th segment - the exit side thickness of the i-th segment) / (in-side plate thickness of the i-th segment)}|=|ln{(outlet side plate thickness of the i-th segment) / (into the side plate thickness of the i-th segment)}|...(3)

τR=τ0. Exp(Q/(R.Ti))...(4)

Τ0=8.46×10 ^-9 (s)

Q: constant of activation energy associated with the shifting movement of Fe = 183,200 (J/mol)

R: gas constant = 8.314 (J / (K. mol))

By specifying the effective cumulative strain derived as such, the average circular equivalent diameter of the metal phase mainly composed of the residual Worthite iron and the distance between adjacent metal phases can be limited, and the unevenness of the ultra-fine hardness can be reduced. . As a result, it is possible to suppress the growth of the pores which occur at the interface between the hard phase and the soft phase, and it is not easy to combine the pores, and it is possible to obtain a steel sheet excellent in forgeability without causing cracks even when the sheet is forged.

The finishing temperature of the finishing rolling, that is, the end temperature of the continuous hot rolling step, can be set to a temperature of Ar ₃ (° C.) or more and less than Ar ₃ (° C) + 30 ° C. Thereby, the amount of residual Worth iron can be limited, and the rolling is finished in the 2-phase zone. The Ar ₃ value can be calculated by the following formula.

Ar ₃ = 970-325 × C + 33 × Si + 287 × P + 40 × Al - 92 × (Mn + Mo + Cu) - 46 × (Cr + Ni)

Here, the element symbol in the above formula represents the content (% by mass) of each element in the hot-rolled steel sheet, and in the case where the element is not contained, 0 is substituted.

 (c) 1st (acceleration) cooling step  

After the finishing rolling is completed, the cooling of the obtained hot-rolled steel sheet is started within 0.5 s. It is cooled to a temperature of 650 to 750 ° C at an average cooling rate of 10 to 40 ° C / s, and then cooled in the atmosphere for 3 to 10 s (air cooling step). The cooling in this step and the subsequent atmosphere is to promote the fermentation of the ferrite and iron, and to carry out the necessary C distribution of the Worthite iron residue in the subsequent winding step. When the average cooling rate in the first cooling step is less than 10 ° C / s, the ferrite is easily formed. On the other hand, when it exceeds 40 ° C / s, it is not a fermented iron metamorphosis but a higher temperature toughened iron metamorphosis, which hinders the subsequent residence of the Worthite iron.

Further, when the cooling rate in the atmosphere exceeds 8 ° C / s or the air cooling time exceeds 10 s, the toughened iron becomes easy to be formed, and the area ratio of the toughened iron becomes large. On the other hand, when the cooling rate in the atmosphere is less than 4 ° C / s or the air cooling time is less than 3 s, the ferritic iron is easily formed. The cooling in the atmosphere referred to herein is that the steel sheet is air-cooled at a cooling rate of 4 to 8 ° C/s in the atmosphere.

 (d) 2nd (acceleration) cooling step  

Immediately after the air cooling step, the temperature is cooled to a temperature of 350 to 450 ° C at an average cooling rate of 30 ° C / s or more. The upper limit of the average cooling rate is not particularly limited, but the steel sheet is bent in consideration of the thermal strain due to the thermal deviation, and is preferably set to 1000 ° C / s or less.

 (e) Winding step  

Then, the cooled hot-rolled steel sheet is wound. The conditions of the winding step are not particularly limited, and the average cooling rate at which the surface temperature of the coil is 200 ° C after winding can be set to 30 to 100 ° C / h. Air cooling in the atmosphere can be performed after the second (acceleration) cooling step to the winding step. If air cooling in the atmosphere is employed, the cooling rate is not necessarily limited in particular.

Hereinafter, the present invention will be more specifically described by the examples, but the invention is not limited to the examples.

 [Example 1]  

The steel having the chemical composition shown in Table 1 was melted to prepare a flat embryo, and the flat embryo was subjected to hot rolling under the conditions shown in Table 2, and then cooled and then wound to produce a hot rolled steel sheet. The sheet thickness of the obtained hot rolled steel sheet is shown in Table 3.

 [metal organization]  

The metal structure of the obtained hot-rolled steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, when the width and thickness of the steel sheet are set to W and t in the cross section perpendicular to the rolling direction of the steel sheet, 1/4 W is taken from the end surface of the steel sheet, and the surface of the steel sheet is counted. At a position of 1/4 t, a test piece for observation of metal structure was cut out.

Next, the rolling direction cross section of the test piece (so-called L-direction cross section) was etched by a oxidizing agent, and after etching, it was observed with an optical microscope at a viewing angle of 300 μm × 300 μm. Next, the obtained tissue photograph is subjected to image analysis to determine the area ratio A of the ferrite iron, the area ratio B of the Borne iron, and the total area ratio of the toughened iron, the granulated iron and the residual Worth iron. C.

Next, the portion etched by the oxidizing agent was subjected to Le Pera etching, and observed with an optical microscope at a field of 300 μm × 300 μm. Then, image analysis was performed on the obtained tissue photograph, and the total area ratio D of the remaining Worthite iron and the granulated iron was calculated. Next, the sample was subjected to surface cutting in the normal direction of the rolling surface to a depth of 1/4 of the sheet thickness, and the volume fraction of the residual Worthite was determined by X-ray diffraction measurement. Since the volume ratio is approximately equal to the area ratio, the above-mentioned volume ratio is used as the area ratio E of the residual Worth iron. The area ratio of the toughened iron is obtained from the difference between the area ratio C and the area ratio D, and the area ratio of the granulated iron is determined from the difference between the area ratio E and the area ratio D. According to this method, the area ratios of the ferrite iron, the toughening iron, the granulated iron, the residual Worth iron, and the Bora iron can be obtained.

Further, the number and area of the metal phases were determined from the photograph of the structure after the Le Pera etching, and the circle equivalent diameter (diameter) was calculated and averaged to obtain the average circle equivalent diameter. Similarly, any 20 metal phases were selected from the photo of the corrosion after Le Pera etching, and the distance between the metal phase and the nearest metal phase was measured, and the average value was calculated.

 [mechanical characteristics]  

Tensile strength characteristics (tensile strength (TS), uniform elongation (u-EL)) in mechanical properties, when the plate width is set to W, 1/4 W or 3 in the plate width direction from one end of the plate At any position of /4W, sample No. 5 of JIS Z 2241 (2011) whose length is orthogonal to the rolling direction (width direction) was sampled, and the test piece was used and carried out in accordance with JIS Z 2241 (2011). Evaluation.

Next, a simple shear test was carried out in accordance with the following procedure, and the equivalent plastic strain was obtained from the results.

In the test piece of the simple shear test, when the plate width of the steel plate is set to W, any one of 1/4 W or 3/4 W in the plate width direction is counted from one end of the plate in a direction orthogonal to the rolling direction. (Width direction) Sampling for the length direction. Fig. 1(a) shows an example of a test piece. The test piece of the simple shear test shown in Fig. 1 was uniformly ground on both sides so that the plate thickness was 2.0 mm, and the thickness of the steel plate was 23 mm in the width direction and 38 mm in the rolling direction of the steel plate. Rectangular test piece.

The long piece side (rolling direction) of the test piece was sandwiched by the nip portions 2 on both sides so as to face each side of the film direction (width direction) by 10 mm, and a shear width of 3 mm was set in the center of the test piece (cutting) Deformation generating unit 1). In the case where the sheet thickness is less than 2.0 mm, the grinding is not performed, and the thickness of the sheet is maintained for testing. Further, in the center of the test piece, a straight line mark is drawn with a pen or the like in the direction of the film (width direction).

Next, the held long piece sides were moved in such a manner as to be opposite to each other in the longitudinal direction (rolling direction), whereby the shear stress σ s was applied to apply shear deformation to the test piece. Fig. 1(b) shows an example of a test piece after shear deformation is applied. The shear stress σ s is a nominal stress obtained by the following equation.

Shear stress σ s = shear force / (length of test piece in the rolling direction of the steel sheet × thickness of the test piece)

In the shear test, since the length and thickness of the test piece are not changed, it can be regarded as shearing the nominal stress and shearing the true stress. In the shear test, a straight line drawn in the center of the test piece was photographed by a CCD camera, and the inclination angle θ was measured (see Fig. 1 (b)). Based on the inclination angle θ , the shear strain ε s due to the shear deformation is obtained by the following equation.

Shear strain ε s=tan(θ)

In the simple shear test, a simple shear test machine (maximum displacement of 8 mm) was used. Therefore, the stroke (shift) of the testing machine is limited. Further, cracking occurred at the end portion or the nip portion of the test piece, and in the single shear test, the test may not be performed until the test piece is broken. Therefore, as described above, the "multi-segment shear test method" is employed, which is a series of operations such as a load for repeatedly performing the shear test load, a load for load removal, a straight line for cutting the end portion of the test piece, and a load for reloading. .

In order to compare these multi-stage shear test results in series and evaluate them as a continuous simple shear test result, the following is obtained: the shear strain (εs) obtained from the shear test at each stage is reduced. Considering the shear elastic strain (εsp) obtained by shear elastic strain (εse), the shear plastic strain (εs) of each step is combined into one.

Shear plastic strain ε sp=shear strain ε s-shear elastic strain ε se

Shear elastic strain ε se=σs/G

Σs: shear stress

G: shear modulus

Here, it is set to G=E/2(1+v)≒78000 (MPa).

E (Young's modulus (longitudinal elastic coefficient)) = 206000 (MPa)

Passon's ratio (v) = 0.3

In the simple shear test, the test is carried out until the test piece breaks. In this way, the relationship between the shear stress σ s and the shear plastic strain ε sp can be traced. Further, the shear plastic strain at the time of fracture of the test piece was ε spf.

According to the relationship between the shear stress σ s obtained in the above simple shear test and the shear plastic strain ε spf at the time of breakage of the test piece, the equivalent plastic strain ε eq is obtained by the above-described method using the conversion coefficient κ .

Next, the standard deviation of the ultra-fine hardness was measured. The test piece for metal structure observation was again ground, with a load of 1 mN (load 10 s, load 10 s), and a 1/4 depth position of the plate thickness t from the surface of the steel sheet in the cross section parallel to the rolling direction (1) /4t part), the measurement area of 25 μm × 25 μm was measured at intervals of 5 μm. Based on the results, the average value of the ultra-fine hardness and the standard deviation of the ultra-fine hardness were calculated. The ultrafine hardness was measured using a HyboScope/TriboIndenter manufactured by Hysitron.

The results of these measurements are shown together in Table 3.

As can be seen from Table 3, the hot-rolled steel sheet of the present invention has a tensile strength (TS) of 780 MPa or more, and a product of uniform elongation u-EL and tensile strength TS (TS × u-EL) of 9,500 MPa. More than %, and the characteristics of the balance are displayed. Further, in the hot-rolled steel sheet of the present invention, the equivalent plastic strain is 0.50 or more, and it is confirmed that it is a steel sheet which can withstand high strain region processing such as forging.

 [Industry Utilization]  

According to the present invention, it is possible to obtain a hot-rolled steel sheet which is capable of maintaining deep drawing workability, bulging formability, and the like as a basic function of TRIP steel and excellent in plate forgeability. Therefore, the hot-rolled steel sheet of the present invention can be widely applied to mechanical parts and the like. In particular, it is possible to obtain a remarkable effect by processing a steel sheet having a high strain zone processing such as plate forging.

Claims (2)

 A hot-rolled steel sheet whose chemical composition is contained in mass%: C: 0.07 to 0.22%, Si: 1.00 to 3.20%, Mn: 0.80 to 2.20%, Al: 0.010 to 1.000%, N: 0.0060% or less, P : 0.050% or less, S: 0.005% or less, Ti: 0 to 0.150%, Nb: 0 to 0.100%, V: 0 to 0.300%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0~ 2.00%, Mo: 0~1.00%, B: 0~0.0100%, Mg: 0~0.0100%, Ca: 0~0.0100%, REM: 0~0.1000%, Zr: 0~1.000%, Co: 0~1.000 %, Zn: 0~1.000%, W: 0~1.000%, Sn: 0~0.050%, and the remaining part: Fe and impurities, in the section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet When W and t are respectively set, the metal structure at a position of 1/4 W or 3/4 W from the end surface of the steel sheet and 1/4 t or 3/4 t from the surface of the steel sheet is, in terms of area %, Contains: Remaining Worth Iron: more than 2% and less than 10%, Ma Tian loose iron: 2% or less, toughened iron: 10 to 70%, Bora: 2% or less, the rest: fat iron, by The average circular equivalent diameter of the metal phase composed of the residual Worthite iron and/or the granulated iron is 1.0 to 5.0 μm, and the adjacent metal phase Average of 3μm or more short-range, ultra micro hardness standard deviation is 2.5GPa or less.    The hot-rolled steel sheet according to claim 1, wherein the tensile strength is 780 MPa or more and the thickness is 1.0 to 4.0 mm.