JP5487916B2 - High-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more excellent in impact absorption energy and a method for producing the same - Google Patents

Description

The present invention relates to a high- strength galvanized steel sheet excellent in impact absorption energy and having a maximum tensile strength of 900 MPa or more and a method for producing the same.

  In recent years, there has been an increasing demand for higher strength of steel plates used for parts such as automobiles, and high strength cold-rolled steel plates with a maximum tensile stress of 900 MPa or more are applied mainly to structural members such as side sills and impact beams. It is progressing rapidly. This is because a high-strength steel plate is applied to reduce the weight while maintaining the strength of the vehicle body and to efficiently absorb the impact when the automobile collides. In particular, since the strain rate at the time of a car crash is significantly different from the strain rate in a normal tensile test, the stress at the high strain rate corresponding to the time of a crash is also significantly different from the stress obtained in a tensile test. (See Non-Patent Document 1). The ratio of the stress at a relatively low strain rate as in a tensile test and the stress at a high strain rate as in a collision is generally a static ratio (= stress at a high strain rate / low strain rate). This is known as a measure for evaluating the impact absorption energy of a material.

  The strain rate dependence of the steel sheet strength is due to the fact that plastic deformation in iron is carried by dislocation motion, and that the dislocation motion of bcc metals including steel is governed by the thermal activation process. As a result, the stress at the time of plastic deformation of steel increases as the strain rate increases. On the other hand, it is known that stresses of fcc metals such as Al do not increase even when the strain rate is increased, and the absorbed energy at the time of collision is not so high. Steel with high stress at the time of impact compared to stress at low strain rate suppresses the load at the time of pressing low, and the absorbed energy increases at the time of collision. Suitable.

  However, for example, as disclosed in Non-Patent Document 2, it is known that increasing the strength of a steel sheet may reduce collision absorption energy. This is because the increase in steel plate strength due to precipitation strengthening and transition strengthening reduces the static / dynamic ratio. For this reason, even if the steel sheet strength is increased by using precipitation strengthening or dislocation strengthening, there is a problem that it is not possible to increase the impact absorption energy at a high strain rate.

  As steel exceeding the maximum tensile stress of 900 MPa, for example, a precipitation-strengthened hot-rolled steel sheet disclosed in Patent Documents 1 and 2 in which Ti, Nb, Mo, V, or the like is added to the steel sheet is known. The hot-rolled steel sheets described in Patent Documents 1 and 2 are steels in which ferrite is strengthened by precipitating carbides composed of Ti, Nb, Mo, and V in ferrite that is a main phase in the hot-rolling stage, and are 900 MPa or more. It is possible to ensure the maximum tensile strength. However, as described above, increasing the strength by precipitation strengthening reduces the static / dynamic ratio, and therefore, even under high-speed deformation, only an increase in strength commensurate with the increase in strength at a low strain rate can be obtained. In addition, precipitation strengthening is accomplished by consistent precipitation of the precipitates in the main phase ferrite, which makes it difficult to apply to cold-rolled steel sheets and hot-dip galvanized steel sheets that undergo cold rolling and subsequent continuous annealing. have. That is, since the parent phase ferrite is recrystallized by performing cold rolling and annealing, the orientation relationship with the precipitates is lost (it is not matched precipitation), which causes a significant decrease in strength. In addition, Nb, Ti, V, and Mo significantly delay ferrite recrystallization, so unrecrystallized ferrite remains in the structure of the steel sheet after annealing, and is essential for thin steel sheets such as elongation and hole expansion. Therefore, it has a problem that it is difficult to apply to cold rolled steel sheets and plated steel sheets.

  On the other hand, in Patent Documents 3 and 4, by making the steel sheet a martensite single phase or steel partially containing a bainite structure, it is possible to ensure a maximum tensile strength of 900 MPa or more and excellent bendability. It is disclosed that a cold-rolled steel sheet or a plated steel sheet can be manufactured. Since the martensite structure is harder than the typical steel structures of ferrite, pearlite, and bainite, it is possible to increase the strength while suppressing the addition of elements such as C and Mn. At the same time, since the steel sheet structure is uniform with the martensite single phase, it is less likely to cause deformation concentration and microvoid formation at a specific location, so that the hole expandability is excellent. However, since martensite is a structure containing many dislocations, the stress does not increase so much even if the strain rate is increased, and a significant increase in absorbed energy under high-speed deformation cannot be expected. In addition, since martensite contains a lot of dislocations, it has a structure as if a large amount of deformation has been applied, so it is inferior in elongation. As a result, although it can be applied to a member that performs only simple bending, it has been difficult to apply to a member that performs overhanging or drawing.

In addition, it is known that the static motion ratio of non-recrystallized ferrite and high strength steel sheets containing many dislocations with high strength by processing is not high.
To solve these problems, for example, a steel plate having a configuration in which a high static ratio can be obtained by using a main phase of ferrite for a steel plate of 440 to 590 MPa class as in the steel plate disclosed in Patent Document 5. It has been. However, the technique described in Patent Document 5 is not intended for a high-strength steel sheet of 900 MPa class or higher, which is a problem in the present invention.

  With respect to these problems, DP steel made of ferrite and martensite described in Patent Document 6 or TRIP steel made of ferrite, bainite and retained austenite is known to have high stress under a high strain rate. Yes. The steel sheet described in Patent Document 6 increases the volume ratio of ferrite with a high static ratio by increasing the strength using hard martensite and retained austenite that transforms into martensite after forming. In this case, both strength and static ratio are achieved. At the same time, steel sheets made of ferrite and hard structure are steel sheets that ensure ductility by using soft ferrite as the main phase and ensure strength by containing martensite and retained austenite as hard structures. It is known that the moldability is excellent. However, even in the steel sheet described in Patent Document 6, in order to increase the strength of 900 MPa or more, it is necessary to increase the hard structure fraction, that is, to decrease the fraction of ferrite having a high static ratio, and still to increase the strength. There was a problem that the accompanying collision absorption energy decreased.

That is, when trying to secure a maximum tensile strength of 900 MPa or more, it is necessary to use a strengthening mechanism such as an increase in the volume fraction of the hard structure or precipitation strengthening of the ferrite, and in this case, the static-dynamic ratio is lowered. There was a problem. For example, when increasing the strength by strengthening the structure, the ratio of the hard structure increases, and in some cases, the martensite structure becomes the main phase, so that it becomes impossible to obtain a static ratio as high as that of a 590 MPa grade steel plate. Alternatively, when the ferrite strength is increased by precipitation strengthening while maintaining the same ferrite volume ratio as that of the 590 MPa grade steel plate, the static-dynamic ratio similarly decreases.
Thus, since the hard structure containing many precipitation strengthened ferrite and dislocations has a low static ratio, it is difficult to achieve both a maximum tensile strength of 900 MPa or more and a high static ratio in a steel sheet.

JP 2005-264240 A JP 2005-154810 A JP 2002-161336 A JP 2009-30091 A Japanese Patent Laid-Open No. 11-100635 Japanese Patent Laid-Open No. 07-18372

“CAMP-ISIJ”, Vol. 9, 1996, p1112 "Proceedings of the Japan Society of Mechanical Engineers Materials Division Lecture", No. 920-72, 1992, p. 519

  As described above, in order to obtain a high strength of 900 MPa or more in conventional steel plates, it is necessary to utilize precipitation strengthening and structure strengthening by increasing the volume fraction of hard structure, and it is possible to increase stress at a low strain rate. However, even if the strain rate is increased, there is a problem that the stress does not increase so much (the static ratio is low).

The present invention has been made in view of the above problems, and can achieve both a static ratio comparable to that of a 590 MPa grade steel sheet and a tensile maximum strength of 900 MPa or higher, and has a high tensile maximum strength of 900 MPa or higher with excellent impact absorption energy. An object is to provide a high-strength galvanized steel sheet and a method for producing the same.

The present inventors have intensively studied to solve the above problems and completed the present invention.
That is, the gist of the present invention is as follows.

[1] By mass%, C: 0.07 to 0.25%, Si: 0.3 to 2.50%, Mn: 1.5 to 3.0%, P: 0.001 to 0.03% , S: 0.0001 to 0.01%, Al: 1.0% or less, N: 0.0005 to 0.0100%, O: 0.0005 to 0.007%, the balance being iron and the cast slab to have a steel component comprising unavoidable impurities, was heated to 1050 ° C. or higher after direct or once cooled, then, to complete the hot rolled at Ar ₃ transformation point or higher, then the temperature of from 400 to 670 ° C. Winding in a zone, pickling, cold rolling at a rolling reduction of 40 to 70%, and then annealing at a maximum heating temperature of 760 to 900 ° C. when passing through a continuous hot dip galvanizing line, then an average cooling rate Cool at 1 to 1000 ° C./second, then immerse in a galvanizing bath to 250 ° C. or less. After cooling, the roughness (Ra) 3.0 The following method for producing a high strength galvanized steel sheet having excellent tensile greater than or equal to the maximum intensity 900MPa collision absorption energy and performing rolling with a roll.
[2] After immersion in the galvanizing bath, alloying is performed at a temperature of 460 to 600 ° C., and then cooled to 250 ° C. or less at an average cooling rate of 1 ° C./second or more, followed by roughness (Ra). The method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to the above [1], wherein rolling is performed using a roll of 3.0 or less.
[ 3 ] Further, the steel component of the cast slab is, in mass%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.10%, V: 0.005 to 0.10% . The method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to the above [1] or [2] , comprising one or more of them.
[ 4 ] Further, the steel component of the cast slab is in mass%, B: 0.0001 to 0.01%, Cr: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Any one of the above-mentioned [1] to [3], containing one or more of Cu: 0.01 to 2.0% and Mo: 0.01 to 0.8% A method for producing a high-strength galvanized steel sheet having a maximum tensile strength of 900 MPa or more and excellent in the impact absorption energy described in the item .
[ 5 ] Further, the steel component of the cast slab contains, by mass%, one or more of Ca, Ce, Mg, and REM in a range of 0.0001 to 0.5% in total. The method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in collision absorption energy according to any one of the above [1] to [ 4 ] .

[6] A high-strength galvanized steel sheet obtained by the production method according to any one of [1] to [5] above, wherein the density of dislocations contained in the steel sheet is 8 × 10 8 inside the steel sheet. ¹¹ (Pieces / mm ² ) And the strain rate is 0.0067 (s ^-1 ) Quasi-static strength (FS1) and strain rate 1000 (s ^-1 High-strength zinc with a maximum tensile strength of 900 MPa or more excellent in impact absorption energy, characterized in that the static-dynamic ratio (= FS2 / FS1) comprising the ratio to the dynamic strength (FS2) Plated steel sheet.

According to the high- strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more excellent in collision absorption energy of the present invention, the volume ratio of a sufficient ferrite structure without increasing the volume ratio of the martensite structure that is a hard structure by the above configuration. Therefore, it is possible to stably achieve both a static ratio comparable to that of a 590 MPa class steel plate and a maximum tensile strength of 900 MPa or more. Thereby, it becomes possible to provide a high-strength galvanized steel sheet having high collision absorption energy.

In addition, according to the method for producing a high- strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to the present invention, the above-mentioned method for controlling the steel sheet components, annealing conditions and rolling conditions after annealing is performed in a hard structure. Without increasing the volume ratio of a certain martensite structure, a sufficient volume ratio of a ferrite structure is ensured, and a high impact is achieved while stably achieving a static motion ratio equivalent to that of a 590 MPa grade steel sheet and a tensile maximum strength of 900 MPa or more. high strength galvanized steel sheet that includes the absorption energy makes it possible to produce.

It is a graph which shows the collision absorption energy represented by the relationship between a strain rate and stress.

Hereinafter, an embodiment of a high- strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent production method of the present invention and a method for producing the same will be described. In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

[High-strength cold-rolled steel sheet and high-strength galvanized steel sheet]
The high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to the present invention is mass%, C: 0.07 to 0.25%, Si: 0.3 to 2.50%, Mn : 1.5-3.0%, P: 0.001-0.03%, S: 0.0001-0.01%, Al: 1.0% or less, N: 0.0005-0.0100% , O: 0.0005 to 0.007%, with the balance having steel components composed of iron and inevitable impurities, and within the steel plate, the density of dislocations contained in the steel plate is 8 × 10 ¹¹ (pieces / piece mm ²⁾ or less, static consisting ratio of the strain rate 0.0067 ^{(s -1)} quasi-static intensity at (FS1), the dynamic strength at a strain rate of 1000 ^{(s -1)} and (FS2) The dynamic ratio (= FS2 / FS1) is 1.05 or more, which is roughly configured.
The high-strength cold-rolled steel sheet according to the present invention has both a static ratio comparable to that of a 590 MPa class steel sheet and a tensile maximum strength of 900 MPa or more, and is excellent in impact absorption energy.

The inventors of the present invention have intensively studied for the purpose of increasing the static ratio in a high-strength steel plate having a maximum tensile strength of 900 MPa or more. As a result, the steel sheet structure is a multi-phase structure steel sheet composed of ferrite and hard structure, and after annealing or rolling after hot dip galvanizing, the dislocations contained in the steel sheet structure are moved to eliminate static. It has been found that the high dislocation density that causes the reduction in dynamic ratio can be reduced, and the static ratio can be reduced while maintaining high strength.
Hereinafter, the high-strength cold-rolled steel sheet according to the present invention will be described in detail.

"Steel component (chemical component composition)"
First, the reason for limiting the essential chemical component range defined in carrying out the present invention will be described. In the following description, the amount of each element added is all represented by “mass%”.

“C: Carbon” 0.07 to 0.25%
C increases the strength of martensite and is added to increase the strength of the high-strength cold-rolled steel sheet. However, when the content of C exceeds 0.25%, weldability and workability become insufficient. On the other hand, if the C content is less than 0.07%, the strength is insufficient.
Further, the C content is preferably in the range of 0.075 to 0.23%, and more preferably in the range of 0.08 to 0.21%.

"Si: silicon" 0.3-2.50%
“Al: Aluminum” 1.0% or less Si and Al are ferrite stabilizing elements, and are added for the purpose of increasing the ferrite volume fraction in the steel sheet structure. In particular, since ferrite has a low dislocation density as compared with a martensite or bainite structure, it has an excellent static ratio. For this reason, it is necessary to add Si or Al.

  When the Si content is less than 0.3%, the volume ratio of ferrite becomes insufficient, and the ductility, bendability and strength of the high-strength cold-rolled steel sheet become insufficient. When Al is contained, the same effect as that obtained when Si is contained can be obtained. However, when the above effect can be sufficiently obtained by containing only Si, Al may not be contained. . On the other hand, if the Si content exceeds 2.50% or the Al content exceeds 1.0%, weldability and workability become insufficient.

  Further, the Si content is more preferably in the range of 0.5 to 2.25%, and still more preferably in the range of 0.7 to 2.0%. Further, the Al content is more preferably in the range of 0.005 to 0.9%, and still more preferably in the range of 0.01 to 0.8%. Moreover, since Al can be used as a deoxidizing material, it may be added not only for the structure control of the steel sheet but also for deoxidation. Further, Si is preferably added as a solid solution strengthening element because it can strengthen the steel sheet.

Moreover, in the manufacturing method of the high-strength cold-rolled steel sheet which concerns on this invention mentioned later for details, when performing annealing at the low temperature of 760 degreeC-900 degreeC at the time of heating at the time of letting a continuous annealing line and a continuous hot dip galvanizing line pass. The volume ratio of ferrite and austenite during annealing varies depending on the annealing temperature. That is, when the annealing temperature is less than 760 ° C., it takes a long time for the cementite to dissolve, so that the volume ratio of austenite (martensite after cooling) becomes too small to secure a strength of 900 MPa or more. On the other hand, Si and Al increase the Ac ₃ transformation point and reduce the dependency of the ferrite volume fraction on the annealing temperature in the two-phase annealing. As a result, even if the annealing temperature changes, it becomes possible to produce a high-strength steel plate whose material does not easily change. Therefore, when the steel plate structure is a structure whose main phase is ferrite, Si and Al within the above range. It is preferable to increase the Ac ₃ transformation point to make the steel sheet structure difficult to depend on the annealing temperature.

“Mn: Manganese” 1.5-3.0%
Mn is added to increase the strength of the high-strength cold-rolled steel sheet. However, if the Mn content exceeds 3.0%, the volume ratio of martensite increases too much, the volume ratio of ferrite contributing to securing ductility becomes insufficient, and the ductility and bendability become insufficient. Further, the hardness distribution of the steel sheet surface layer due to the segregation of Mn is also increased.
On the other hand, if the Mn content is less than 1.5%, the pearlite transformation that occurs during the cooling process cannot be suppressed, and the steel sheet structure becomes a ferrite and pearlite structure, resulting in insufficient strength.
Further, the Mn content is more preferably in the range of 1.6 to 2.7%, and further preferably in the range of 1.7 to 2.4%.

“P: phosphorus” 0.001 to 0.03%
P is an element inevitably contained in the steel sheet. Further, P tends to segregate in the central part of the plate thickness of the steel sheet, causing the weld to become brittle. When the P content exceeds 0.03%, embrittlement of the welded portion becomes remarkable, so the appropriate range is limited to 0.03% or less. Moreover, although the lower limit of content of P is not specifically defined, since it is economically disadvantageous to make it less than 0.001%, this value was made into the lower limit.

“S: sulfur” 0.0001 to 0.01%
S is an impurity and adversely affects the weldability and the manufacturability during casting and hot rolling. For this reason, the upper limit of the S content was set to 0.01%. The lower limit value of the S content is not particularly defined, but if it is less than 0.0001%, it is economically disadvantageous, so this value is the lower limit value. Further, since S is combined with Mn to form coarse MnS, the S content is preferably as low as possible in order to reduce the bendability.

“N: Nitrogen” 0.0005 to 0.0100%
N forms coarse nitrides and degrades bendability and hole expansibility, so the content needs to be suppressed. When the N content exceeds 0.0100%, such a tendency becomes remarkable. Therefore, the N content range is set to 0.0100% or less. In addition, N is better because it causes blowholes during welding. Although the lower limit of the N content is not particularly defined, the effects of the present invention are exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased. This is a reasonable lower limit.

“O: Oxygen” 0.0005 to 0.007%
Since O forms an oxide and degrades bendability and hole expandability, the content needs to be suppressed. In particular, oxides often exist as inclusions, and if they are present on the punched end face or cut face, notched scratches and coarse dimples are formed on the end face, which causes stress concentration during bending and strong processing. As a starting point of crack formation, it causes a significant deterioration in hole expansibility or bendability. When the content of O exceeds 0.007%, such a tendency becomes remarkable. Therefore, the upper limit of the content of O is set to 0.007% or less. Further, setting the content of O to less than 0.0005% causes excessive cost and is not economically preferable. Therefore, this is set as the lower limit. However, even when the O content is less than 0.0005%, the tensile maximum stress of 900 MPa or more and the excellent ductility and bendability, which are the effects of the present invention, can be secured.

  In the present invention, in addition to the above essential elements, elements as described below can be selectively added. Hereinafter, the reason for limiting the addition range of the selected component element in the present invention will be described.

"Ti: Titanium" 0.005-0.10%
Ti is
It is a strengthening element and contributes to an increase in the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. When B is added, Ti is also added for the purpose of suppressing B from becoming a nitride. Here, B contributes to the structure controllability at the time of hot rolling, the structure control in the continuous annealing equipment and the continuous hot dip galvanizing equipment, and the increase in strength. There is a problem that it cannot be obtained. However, containing Ti can prevent B from becoming a nitride and can bring out the effect of B addition. Therefore, when B is added, it is desirable to add Ti together. .

However, if the Ti content exceeds 0.10%, carbonitride precipitation increases and formability deteriorates. In addition, if the Ti content is too high, recrystallization of ferrite will be significantly delayed during continuous annealing and production in continuous hot dip galvanizing equipment, so that unrecrystallized ferrite tends to remain after annealing, resulting in significant ductility. Bring about a decline. For this reason, the upper limit of the Ti content is set to 0.10%. Moreover, the said effect obtained by containing Ti will become inadequate that content of Ti is less than 0.005%.
Further, the Ti content is more preferably in the range of 0.01 to 0.09%, and still more preferably in the range of 0.015 to 0.08%.

"Nb: Niobium" 0.005-0.10%
Nb is a strengthening element and, like Ti, contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, when the Nb content exceeds 0.10%, carbonitride precipitation increases and the formability deteriorates. In addition, if the Nb content is large, recrystallization of ferrite is greatly delayed during production in continuous annealing and continuous hot dip galvanizing equipment, so that unrecrystallized ferrite tends to remain after annealing, and significant ductility Bring about a decline. For this reason, it is preferable to make the upper limit of Nb content into 0.10%. Moreover, the said effect acquired by containing Nb will become inadequate that content of Nb is less than 0.005%.
Further, the Nb content is more preferably in the range of 0.01 to 0.09%, and still more preferably in the range of 0.015 to 0.08%.

“V: Vanadium” 0.005 to 0.10%
V is a strengthening element and, like Ti and Nb, contributes to an increase in the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Moreover, the delayed fracture characteristic can be improved by containing V. From this, it is desirable to contain V in the manufacture of a steel sheet having a maximum tensile strength exceeding 900 MPa as in the present invention.

However, when the content of V exceeds 0.10%, carbonitride precipitation increases and the formability deteriorates. In addition, when the V content is large, recrystallization of ferrite is greatly delayed during production in continuous annealing and continuous hot dip galvanizing equipment, so that unrecrystallized ferrite tends to remain after annealing, resulting in significant ductility. In order to bring about a fall, it is preferable to make an upper limit into 0.10%. Moreover, the said effect obtained by containing V will become inadequate that content of V is less than 0.005%.
The V content is more preferably in the range of 0.01 to 0.09%, and still more preferably in the range of 0.015 to 0.08%.

“B: Boron” 0.0001 to 0.01%
Since B delays the ferrite transformation from austenite, it can be utilized for increasing the strength of the steel sheet. In addition, since B delays the ferrite transformation from austenite even during hot rolling, inclusion of B improves the homogeneity of the hot-rolled steel sheet as a bainite single-phase structure and improves bendability. it can. However, when the content of B is less than 0.0001%, the above-described effect obtained by containing B becomes insufficient. Moreover, when content of B exceeds 0.01%, the effect by containing B will not only be saturated, but the productivity at the time of hot rolling will be reduced.
The B content is more preferably in the range of 0.0003 to 0.007%, and still more preferably in the range of 0.0005 to 0.005%.

“Cr: chrome” 0.01 to 2.0%,
"Ni: nickel" 0.01-2.0%,
"Cu: Copper" 0.01-2.0%,
One or more of “Mo: molybdenum” 0.01 to 0.8% Cr, Ni, Cu, and Mo are elements that contribute to strength improvement, and are used in place of part of Mn. be able to. It is preferable that Cr, Ni, Cu, and Mo contain 0.01% or more of one or more of these, respectively. On the other hand, if the content of each element is too large, pickling properties, weldability, hot workability and the like may deteriorate, so the content of Cr, Ni, and Cu may be 2.0% or less. The Mo content is preferably 0.8% or less.

“A total of 0.0001 to 0.5% of one or more of Ca, Ce, Mg, and REM”
In the present invention, 0.0001 to 0.5% in total of one or more selected from Ca, Ce, Mg, and REM can be added. These Ca, Ce, Mg, and REM are elements used for controlling the form of oxides and sulfides. By containing one or two or more in total of 0.0001% or more, the oxide after deoxidation It is possible to reduce the size and the size of the sulfide present in the hot-rolled sheet, which contributes to bendability. However, if each of these elements is contained in a total content exceeding 0.5%, it will cause deterioration of molding processability, and if the content is less than 0.0001%, a sufficient effect will be obtained. It may not be obtained. For this reason, it is preferable that the content of these elements is in the range of 0.0001 to 0.5% in total.

  REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. In the present invention, REM and Ce can be added by misch metal, and in addition to La and Ce, a lanthanoid series element may be contained in combination.

"Steel structure"
In the high-strength cold-rolled steel sheet according to the present invention, it is more desirable that the steel sheet structure has ferrite as the main phase from the viewpoint of reducing the dislocation density. Ferrite, which is a typical structure of a steel sheet, has a lower dislocation density than a bainite or martensite structure. For this reason, it is desirable to use ferrite as the main phase. This is because even if the dislocations are annihilated by rolling after annealing and the dislocation density is reduced, the dislocation density in the martensite structure is high, and the static ratio is low in the martensite single-phase structure.

Here, as shown in the graph of FIG. 1, in general, in a ferrite structure having a low dislocation density, the stress increases as the strain rate increases. On the other hand, since the martensite structure having a high dislocation density has been strengthened by the introduction of dislocations, the increase in stress accompanying an increase in strain rate is small.
Therefore, in the high-strength cold-rolled steel sheet according to the present invention, the stress at the high strain rate, that is, the dynamic strength is increased even if the steel sheet strength is the same by making the steel sheet structure having ferrite as a main phase. The effect is obtained. As a result, the impact absorption energy of the high-strength cold-rolled steel sheet is improved, and as described above, it is possible to effectively absorb the impact energy generated during the collision.

In addition, when achieving compatibility with formability, the maximum tensile stress of the high-strength cold-rolled steel sheet of the present invention depends on the volume ratio of martensite, which is a strengthened structure. It is preferable to change the volume ratio of the site.
For example, if the maximum tensile stress of the steel sheet is in the range of 900 to 1130 MPa, the ferrite volume fraction is preferably in the range of 60% to 85%, and more preferably in the range of 65% to 80%. .
Further, if the tensile maximum stress of the steel sheet is in the range of 1130 to 1280 MPa, the ferrite volume fraction is preferably in the range of 55% to 80%, and more preferably in the range of 60% to 75%. .
If the maximum tensile stress of the steel sheet is in the range of 1280 to 1430 MPa, the volume ratio of ferrite is preferably in the range of 50% to 75%, and more preferably in the range of 55% to 70%.

  As described above, by controlling the volume fraction of ferrite in the above range according to the maximum tensile stress of the steel sheet, the high tensile strength (TS) 900 MPa or more and the strength-ductility balance (TS × El.) Excellent ductility of 16000 (MPa ×%) or more can be obtained, and a steel sheet having both excellent strength and ductility can be obtained. In addition, the strength-ductility balance (TS × El.) Described in the present invention is the product of the tensile maximum stress (TS) and the total elongation (El.) In the tensile test, and changes according to the maximum tensile stress. Is.

  In addition, the steel sheet structure may contain martensite, retained austenite, bainite, cementite, pearlite, and the like as structures other than ferrite as the main phase. In addition, if the dislocation density can be controlled within the range of the present invention, the high static ratio which is the effect of the present invention is exhibited without being limited to the form of ferrite as the main phase. That is, the ferrite may be any of equiaxed polygonal ferrite, acicular ferrite on a needle or plate, or bainitic ferrite.

Incidentally, ferrite, martensite, pearlite, cementite, bainite, identification and austenite and remaining structure, the measurement of the observed and the area ratio of the present position, the reagent disclosed in nital reagent and JP Sho 59-219473, JP- Quantification is possible by corroding the steel plate rolling direction cross section or the rolling direction perpendicular direction cross section and performing 1000 times optical microscope observation and 1000 to 100000 times scanning type and transmission type electron microscope observations. The structure can also be discriminated from crystal orientation analysis using FESEM (Field Emission Scanning Electron Microscope) -EBSP (Backscattered Electron Diffraction) method and micro region hardness measurement such as micro Vickers hardness measurement.

Next, the reason for limiting the dislocation density in the steel sheet structure will be described.
In order to increase the impact absorption energy of the high-strength cold-rolled steel sheet, it is indispensable not to use a strengthening mechanism that lowers the static ratio, which will be described in detail later. In particular, the dislocations introduced during the formation of the steel sheet structure reduce the static ratio, so it is necessary to reduce it as much as possible. However, when securing a strength of 900 MPa or more, the strength must be secured by precipitation strengthening or an increase in the volumetric system ratio of the hard structure, and the static-dynamic ratio tends to decrease. Therefore, by applying cold rolling under annealing after annealing, the dislocation introduced by the transformation contained in the steel sheet is moved and annihilated, thereby increasing the static ratio. Such an effect is exhibited when the average dislocation density existing in the steel sheet is 8 × 10 ¹¹ (pieces / mm ² ) or less. Moreover, the dislocation density contained in the steel sheet is more preferably 6 × 10 ¹¹ (pieces / mm ² ) or less, and further preferably 4 × 10 ¹¹ (pieces / mm ² ) or less.

  The dislocation density can be determined by X-ray diffraction or transmission electron microscope (TEM) observation. Since TEM can observe a minute region, a dislocation density of a ferrite and a hard structure can be measured for a multiphase steel sheet. However, in the TEM observation, it is necessary to process the steel plate into a thin plate shape or a thin needle shape at the sample preparation stage, and it is difficult to prepare the sample. Since it reaches and disappears, the dislocation density may decrease, and it is necessary to pay sufficient attention when preparing the sample. In addition, TEM observation has a limited field of view. On the other hand, the X-ray diffraction method can measure the average dislocation density over a wide area relatively easily. Therefore, in the present invention, a method of measuring the dislocation density using the X-ray diffraction method is adopted.

"Mechanical properties"
The high-strength cold-rolled steel sheet according to the present invention controls the static / dynamic ratio (= FS2 / FS1) to 1.05 or more with the steel sheet structure configured as described above.
The static ratio described in the present invention means a nominal stress (dynamic strength: FS2) at a nominal strain of 0.03 when deformation is performed at a strain rate of 1000 (s ⁻¹ ), and a strain rate of 0.0067. It is the value divided by the nominal stress (quasi-static strength: FS1) at the nominal strain of 0.03 when the deformation is performed at (s ⁻¹ ) (static ratio (= FS2 / FS1)).

The strain rate of 1000 s ⁻¹ is a strain rate assuming a strain rate when the automobile collides, and the strain rate of 0.0067 s ⁻¹ is a strain rate in a normal tensile test.
In the present invention, although the strain rate in a normal tensile test was 0.0067S ^-1, if the range of strain rate 0.005～0.05S ^-1, influence of strain rate on the nominal stress Relatively small. The nominal stress at strain rate 0.0067s ^-1 is tension it is preferred to be measured in the test, the nominal stress at strain rate 1000 s ^-1 is Wanba method and Hopkinson bar method which is a similar deformation mode and this It is preferable to use it. However, measurement can also be performed using the power block method.

  In the present invention, the reason why the static ratio (FS2 / FS1) is set to 1.05 or more is that a significant effect of improving the collision performance can be obtained by setting this value or more. For example, when a steel plate having a static motion ratio of 1.0 is replaced with a steel plate having a static motion ratio of 1.05, if equivalent collision performance is obtained, the weight can be reduced by 5%, and the effect is great. Because. Further, the static ratio (FS2 / FS1) is more preferably 1.07 or more, and further preferably 1.09 or more.

  In the present invention, a high-strength galvanized steel sheet can be formed by providing a galvanized layer or an alloyed galvanized layer on the surface of the above-described high-strength cold-rolled steel sheet. By forming the galvanized layer on the surface of the high-strength cold-rolled steel sheet, the steel sheet has excellent corrosion resistance. Moreover, since the alloyed galvanized layer is formed on the surface of the high-strength cold-rolled steel sheet, it has excellent corrosion resistance and excellent paint adhesion.

  According to the high-strength cold-rolled steel sheet and high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to the present invention as described above, the volume ratio of the martensite structure, which is a hard structure, is configured as described above. Therefore, a sufficient volume ratio of the ferrite structure can be ensured without increasing the ratio, so that a static motion ratio comparable to that of a 590 MPa grade steel sheet and a maximum tensile strength of 900 MPa or more can be stably achieved. Thereby, it becomes possible to provide a high-strength cold-rolled steel sheet having high collision absorption energy.

[Method for producing high-strength cold-rolled steel sheet and high-strength galvanized steel sheet]
Next, the manufacturing method of the high-strength cold-rolled steel sheet and high-strength galvanized steel sheet according to the present invention will be described.
The method for producing a high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more excellent in impact absorption energy according to the present invention is to heat a cast slab having the above chemical component composition directly or once after cooling to 1050 ° C. or higher, and then The hot rolling is completed at the Ar ₃ transformation point or higher, and then rolled up in a temperature range of 400 to 670 ° C., pickled, cold-rolled at a rolling reduction of 40 to 70%, and then subjected to a continuous annealing line. When passing the sheet, after annealing at a maximum heating temperature of 760 ° C. to 900 ° C., the sheet is cooled at an average cooling rate of 1 to 1000 ° C./second or less, and rolled using a roll having a roughness (Ra) of 3.0 or less. How to do it.

  In order to produce the high-strength cold-rolled steel sheet of the present invention, first, a slab having the above-described chemical component (composition) is cast. In the present invention, the slab used for hot rolling is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

The hot-rolled slab heating temperature needs to be 1050 ° C. or higher. If this slab heating temperature is excessively low, the finish rolling temperature will be lower than the Ar ₃ point, resulting in two-phase rolling of ferrite and austenite, the hot rolled sheet structure will be a heterogeneous mixed grain structure, and cold rolling and annealing processes Even if it passes through, a heterogeneous structure | tissue will not be eliminated, but it will be inferior to ductility and bendability.
Moreover, since the high strength cold-rolled steel sheet of the present invention ensures a tensile maximum strength of 900 MPa or more after annealing, a large amount of alloy elements are added, so that the strength at the time of finish rolling tends to be high. Lowering the slab heating temperature leads to a decrease in the finish rolling temperature, and further increases the rolling load, making rolling difficult and causing the shape of the steel sheet after rolling to become slab heated. The temperature needs to be 1050 ° C. or higher. The upper limit of the slab heating temperature is not particularly defined, and the effect of the present invention is exhibited. However, since it is not economically preferable to make the heating temperature too high, the upper limit of the heating temperature is less than 1300 ° C. It is desirable.
The Ar ₃ temperature can be calculated by the following formula.
Ar ₃ = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)

  On the other hand, the upper limit of the finishing temperature is not particularly defined, and the effect of the present invention is exhibited. However, when the finishing rolling temperature is excessively high, the slab heating temperature is excessively increased in order to secure the temperature. I have to. For this reason, the upper limit temperature of the finish rolling temperature is desirably 1000 ° C. or less.

  The winding temperature needs to be 400-670 ° C. When the coiling temperature exceeds 670 ° C., coarse ferrite and pearlite structure exists in the hot rolled structure, so that the structure heterogeneity after annealing becomes large and the bendability of the final product is deteriorated. Further, from the viewpoint of making the microstructure after annealing fine and improving the strength ductility balance, and from the viewpoint of improving the bendability by uniformly dispersing the second phase, it is more preferable to wind up at 630 ° C or lower. In addition, winding at a temperature exceeding 670 ° C. is not preferable because the thickness of the oxide formed on the steel sheet surface is excessively increased, and the pickling property is poor. Moreover, the minimum of the coiling temperature in the manufacturing method of this invention is 400 degreeC. When the coiling temperature is lower than 400 ° C., the hot-rolled sheet strength is extremely increased. Therefore, troubles such as sheet breakage and shape defects are easily induced during cold rolling. Therefore, the lower limit of the winding temperature needs to be 400 ° C.

  Note that the rough rolled sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.

  Next, pickling is performed on the hot-rolled steel sheet produced by the above method. Pickling can remove oxides on the surface of the steel sheet, improving the chemical conversion of the cold-rolled high-strength steel sheet as the final product, and improving the hot-plating performance of cold-rolled steel sheets for hot-dip galvanized or alloyed hot-dip galvanized steel sheets Is important for. Moreover, the pickling may be performed only once, or the pickling may be performed in a plurality of times.

Next, the pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 40 to 70% and passed through a continuous annealing line or a continuous hot dip galvanizing line. If the rolling reduction is less than 40%, it is difficult to keep the shape flat, and the ductility of the final product becomes poor, so this is the lower limit. On the other hand, the cold rolling exceeding 70% makes the cold rolling difficult because the cold rolling load becomes too large. Further, the rolling reduction during cold rolling is more preferably in the range of 45 to 65%.
Note that the effects of the present invention are exhibited without particular limitation on the number of rolling passes and the rolling reduction for each pass.

  Thereafter, the obtained cold-rolled steel sheet is passed through a continuous annealing line to produce a high-strength cold-rolled steel sheet. At this time, it is performed under the first condition shown below.

"First condition"
In the present invention, when the cold-rolled steel sheet is passed through a continuous annealing line, after annealing at a maximum heating temperature of 760 ° C. to 900 ° C., the steel sheet is cooled at an average cooling rate of 1 to 1000 ° C./second or less. By rolling using a roll having a roughness (Ra) of 3.0 or less, a high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy can be obtained.

"Second condition"
In the present invention, when the cold-rolled steel sheet is passed through the continuous annealing line, after annealing in the same manner as the first condition described above, the steel sheet is cooled at an average cooling rate of 1 to 1000 ° C./second, and 150 After holding in a temperature range of ˜400 ° C., rolling is performed using a roll having a roughness (Ra) of 3.0 or less, whereby a high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more and excellent in collision absorption energy. Is obtained.

In the present invention, the high-strength galvanized steel sheet is obtained by further applying zinc-based electroplating to the high-strength cold-rolled steel sheet obtained by passing through the continuous annealing line of the first condition or the second condition. It is possible to manufacture.
Furthermore, in the present invention, a high-strength galvanized steel sheet may be produced by passing the cold-rolled steel sheet obtained by the above method through a continuous galvanizing line. In this case, the process is performed under the third condition or the fourth condition as described below.

"Third condition"
In the present invention, when passing through the continuous hot dip galvanizing line, after annealing in the same manner as the first condition described above, cooling is performed at an average cooling rate of 1 to 1000 ° C./second, and then in the galvanizing bath. After immersing and cooling to 250 ° C. or less, rolling is performed using a roll having a roughness (Ra) of 3.0 or less, whereby a galvanized layer is formed on the surface of the steel sheet, and the maximum tensile strength excellent in impact absorption energy. A high-strength galvanized steel sheet of 900 MPa or more is obtained.

"4th condition"
In the present invention, when the continuous hot-dip galvanizing line is passed through, the alloying treatment is performed at a temperature of 460 to 600 ° C. after performing the steps until the immersion in the galvanizing bath in the same manner as the third condition described above. Then, after cooling to 250 ° C. or less at an average cooling rate of 1 ° C./second or more, rolling is performed using a roll having a roughness (Ra) of 3.0 or less, so that a galvanized layer is formed on the steel sheet surface. A high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more that is formed and has excellent impact absorption energy is obtained.
In the present invention, a high-strength galvanization having a Zn-Fe alloy formed by alloying a zinc plating layer on the surface and having the alloyed zinc plating layer on the surface is performed here. A steel plate is obtained.

  In the manufacturing method according to the present invention, the maximum heating temperature during annealing is set to 760 ° C to 900 ° C because cementite precipitated in the hot-rolled sheet or continuous annealing equipment or continuous hot-dip galvanizing equipment after cold rolling This is to dissolve the cementite precipitated during the heating in order to secure a sufficient volume ratio of austenite. When annealing is performed at a maximum heating temperature of less than 760 ° C., the productivity decreases because cementite dissolves for a long time, or the cementite remains undissolved, and the martensite volume fraction after cooling decreases. , 900MPa or more cannot be secured, which is not preferable. In addition, even if it anneals at the high temperature over 900 degreeC, although a problem will not arise at all, it is unpreferable from the point which is inferior to economical efficiency.

Moreover, in this invention, it is necessary to make the average cooling rate after annealing into the range of 1-1000 degrees C / sec. When the cooling rate is less than 1 ° C./second, it is not preferable because formation of an excessive pearlite structure during the cooling process cannot be suppressed and a strength of 900 MPa or more cannot be secured. On the other hand, even if the cooling rate is increased excessively, there is no problem in terms of the material. However, since excessive equipment investment is required, it is preferably set to 1000 ° C./second or less.
Moreover, although the cooling stop temperature can exhibit the effects of the present invention without being particularly defined, it is difficult to set the cooling stop temperature to room temperature or lower, so this is a substantial lower limit temperature.

Moreover, in this invention, it is necessary to roll the steel plate cooled after annealing using the roll of roughness (Ra) 3.0 or less.
Here, in the production of a cold-rolled steel sheet, skin pass rolling is often performed in order to improve the aging of the steel sheet, improve the shape, transfer the roughness, and control the yield stress. In particular, unlike IF steel, a high-strength steel plate has a problem that it is likely to be aged because a large amount of solute C is present in the steel plate, and elongation and workability are liable to deteriorate. As a result, in order to improve aging, it was common to roll at a high elongation and improve aging, but in this case, a large amount of dislocations were introduced, and the static ratio was liable to decrease. Had. Moreover, the conventional rolling conditions were not made into the conditions which considered the reduction | restoration of the dislocation (transformation dislocation) accompanying the structure change of the steel plate introduce | transduced at the time of annealing-cooling by cold rolling under light pressure.

  As a result of intensive studies on the above problems, the inventors have made the roughness of the roll of skin pass rolling 3.0 or less, and by performing cold rolling under light pressure, the dislocations introduced during annealing and cooling are reduced. It has been found that the dislocation density can be reduced by eliminating it, and the increase in dislocation density accompanying deformation during rolling, which has been a problem of conventional skin pass rolling, can also be suppressed. That is, if the roughness of the roll is more than 3.0 in terms of Ra, it is easy for stress to concentrate on a part of the steel sheet, and it is likely to be subjected to large deformation locally, and as a result, it will deform more than necessary. Dislocation density increases. In the present invention, by setting the roughness of the roll during rolling to 3.0 or less in Ra, local deformation can be suppressed, and the dislocation density in the steel sheet can be reduced and the static ratio can be increased. . Such an effect becomes conspicuous when the roughness of the roll is set to 3.0 or less in Ra, and is preferably set to 3.0 or less. Further, the roughness of the roll is more preferably 2.75 (Ra) or less, and further preferably 2.5 or less.

  Moreover, even if the roll roughness is set within the above range and cold rolling is performed under light pressure, the dislocation density increases as the elongation increases, the static ratio decreases, and the energy absorbed at the time of collision decreases. Sometimes. Therefore, the elongation during skin pass rolling is preferably suppressed to 1.5% or less, more preferably 1.2% or less, and further preferably 1.0% or less.

  Moreover, in this invention, you may heat-process at 150-400 degreeC from a viewpoint of structure | tissue control after cooling of the said conditions. This is because it is possible to make the steel sheet structure a desirable structure by controlling the phase transformation during the heat treatment, or it is possible to adjust the hardness of the hard structure by performing a tempering process during the heat treatment. Because there is.

  Further, when producing a high-strength galvanized steel sheet through a continuous hot-dip galvanizing facility, the annealing temperature, cooling rate, or rolling conditions for the steel sheet are within the scope of the present invention for the same reason as described above. There is a need to.

  In the present invention, a high-strength galvanized steel sheet may be produced by immersing in a plating bath to obtain a hot-dip plated steel sheet, cooling to 250 ° C. or lower, and rolling under the above conditions. Alternatively, an alloyed hot-dip galvanized steel sheet may be produced by performing an alloying treatment at a temperature of 460 ° C. to 600 ° C. after immersion in the plating bath. This is because it becomes possible to improve rust prevention by using hot-dip galvanized or alloyed hot-dip galvanized steel sheet.

In the present invention, as described below, the furnace atmosphere during the manufacture of high strength cold rolled steel sheet or high-strength galvanized steel sheet, and H ₂ contains 1 to 60 vol%, the remainder N _2, H ₂ The atmosphere is composed of O, O ₂ and inevitable impurities, and the logarithm log (PH ₂ O / PH ₂ ) of water pressure and hydrogen partial pressure in the atmosphere is −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −. It is preferable to set it to 0.5.
By setting the atmosphere in the furnace, before Si, Mn, Al contained in the steel sheet diffuses to the steel sheet surface, O diffused in the steel sheet reacts with Si, Mn, Al contained in the steel sheet, An oxide is formed inside the steel sheet, and the formation of oxides composed of these elements on the steel sheet surface is suppressed. Therefore, by setting the atmosphere in the furnace as described above, non-plating due to the formation of oxide on the steel sheet surface can be suppressed, and the alloying reaction can be promoted, and the oxide is formed on the steel sheet surface. It is possible to prevent the chemical conversion processability from deteriorating.
In addition, the ratio of the water pressure and the hydrogen partial pressure in the atmosphere in the annealing furnace can be adjusted by a method of blowing water vapor into the furnace. Thus, the method of adjusting the ratio between the water pressure and the hydrogen partial pressure in the atmosphere in the annealing furnace is simple and preferable.

In addition, it is not preferable that the H ₂ concentration in the furnace atmosphere exceeds 60% by volume because the cost is increased. On the other hand, when the H ₂ concentration is less than 1% by volume, Fe contained in the steel plate is oxidized, so that wettability and plating adhesion may be insufficient.
Further, by setting the logarithm log (PH ₂ O / PH ₂ ) of the moisture pressure and the hydrogen partial pressure in the furnace atmosphere to −3 ≦ log (PH ₂ O / PH ₂ ) ≦ −0.5, a large amount of Si is obtained. Even if it is the case where it is the steel contained in, sufficient plating property can be ensured. In addition, the lower limit of the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure is set to −3 or more. If it is less than −3, Si oxide (or Si oxide and Al oxide on the steel sheet surface) This is because there is a risk that wettability and plating adhesion may be reduced. On the other hand, the upper limit of the logarithm log (PH ₂ O / PH ₂ ) of the water pressure and the hydrogen partial pressure is set to −0.5 because the effect is saturated.

When a cold-rolled steel sheet or a galvanized steel sheet is manufactured using a conventional manufacturing method, the following problems may occur because the atmosphere in the annealing furnace at the time of manufacture is not optimized.
That is, in the present invention, in order to improve the ferrite volume fraction and ensure ductility, the above chemical components (composition) including Si (or Si and Al) and Mn for increasing the strength of the high-strength steel sheet are included. A slab is used. Since Si, Mn, and Al are elements that are extremely easy to oxidize compared to Fe, even in a reducing atmosphere of Fe, the surface of the steel sheet containing Si (or Si and Al) and Mn is contained. Si oxide (or Si oxide and Al oxide) and Mn oxide are formed. Such an oxide containing Si, Mn, or Al alone or in combination formed on the surface of the steel sheet causes deterioration of the chemical conversion property in the high-strength cold-rolled steel sheet. In addition, these oxides have poor wettability with molten metals such as zinc. Therefore, when forming a galvanized layer on the surface of a high-strength steel sheet to which Si (or Si and Al) is added, Cause. Further, Si and Al sometimes cause problems such as delaying alloying when producing a high-strength galvanized steel sheet subjected to alloying treatment.

  Here, as a method for suppressing the formation of oxides on the surface of the steel sheet, a method in which the atmosphere in the annealing furnace is reduced to the respective elements can be considered. On the other hand, in the present embodiment, the atmosphere in the annealing furnace is the above atmosphere, and although it is a reducing atmosphere of Fe, an atmosphere in which elements such as Si, Mn, and Al are very easily oxidized is used. This makes it possible to improve the chemical conversion processability of high-strength cold-rolled steel sheets, and in the case of producing high-strength galvanized steel sheets, it is possible to improve the wettability of high-strength galvanized steel sheets. The plating can be suppressed and the alloying reaction can be promoted.

In addition, the manufacturing method of the high intensity | strength cold-rolled steel plate or high-strength galvanized steel plate which concerns on this invention is not limited to the example mentioned above.
For example, in the manufacturing method described above, the atmosphere in the annealing furnace is controlled by controlling the moisture pressure and the hydrogen partial pressure, but the method for controlling the partial pressure of carbon dioxide and carbon monoxide, or directly in the furnace You may control the atmosphere in an annealing furnace using the method of blowing in oxygen. Even in this case, Si, Mn, and Al are contained alone or in combination in the steel plate near the surface layer, as in the case where the atmosphere in the annealing furnace is controlled by controlling the moisture pressure and the hydrogen partial pressure. An oxide can be deposited, and the same effect as described above can be obtained.

  In the method for producing a high-strength galvanized steel sheet according to the present invention, in order to improve plating adhesion, the steel sheet before annealing is plated with one or more kinds selected from Ni, Cu, Co, and Fe. You may give it.

Moreover, when manufacturing the high-strength galvanized steel sheet according to the present invention, as a process from annealing to dipping in a galvanizing bath, “after degreasing pickling, heating in a non-oxidizing atmosphere, H ₂ and N ₂ After annealing in a reducing atmosphere, cool it to near the galvanizing bath temperature and immerse it in the galvanizing bath, or adjust the atmosphere during annealing, first oxidize the steel plate surface, then reduce After cleaning the steel plate surface before plating, the steel plate is immersed in a galvanizing bath, or the total reduction furnace method is used, or after the steel plate is degreased and pickled, flux treatment is performed using ammonium chloride or the like. Then, a flux method of soaking in a galvanizing bath may be used.

  Moreover, in the manufacturing method which concerns on this invention, you may use rolling oil in the process of performing the above-mentioned cold rolling. From the viewpoint of introducing shear strain into the surface layer of the steel sheet, a higher friction coefficient on the surface of the steel sheet is preferable, but in a non-lubricated state, the rolling load increases. Therefore, cold rolling using rolling oil may be performed as long as the friction coefficient is not significantly reduced.

  According to the manufacturing method of the high strength cold-rolled steel sheet and the high-strength galvanized steel sheet with a tensile maximum strength of 900 MPa or more excellent in the impact absorption energy according to the present invention as described above, the steel sheet components, annealing conditions and rolling conditions after annealing are The above-mentioned method for controlling does not increase the volume ratio of the martensite structure, which is a hard structure, and a sufficient volume ratio of the ferrite structure is ensured, and the static ratio as high as that of a 590 MPa steel sheet and the maximum tensile strength of 900 MPa or more are achieved. It is possible to produce a high-strength cold-rolled steel sheet and a high-strength galvanized steel sheet that have high impact absorption energy while ensuring both stability and compatibility.

  Hereinafter, examples of the high-strength cold-rolled steel sheet having a tensile maximum strength of 900 MPa or more excellent in collision absorption energy and the manufacturing method thereof, and the high-strength galvanized steel sheet and manufacturing method thereof according to the present invention will be given, and the present invention will be described more specifically. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range that can be adapted to the purpose described above and below. Is also included in the technical scope of the present invention.

[Manufacture of steel sheets]
First, slabs having chemical composition shown in Tables 1 and 2 below were obtained by performing deoxidation / desulfurization of molten steel and control of chemical components in the steelmaking process. Then, the cast slab was directly hot-rolled under the conditions shown in Table 3 below to form a cold-rolled steel sheet of 1.2 mm by cold rolling, and then heat-treated in the atmosphere shown in Tables 4 and 5 below. A cold-rolled steel sheet was obtained.

  In Tables 2 to 5, CR, which indicates the type of product plate, is a cold-rolled steel sheet manufactured by a continuous annealing line, EG is an electroplated steel sheet obtained by electroplating zinc on the cold-rolled steel sheet CR, and GI is a continuous melt. A galvanized steel sheet manufactured by a galvanizing line, GA is an alloyed galvanized steel sheet manufactured by a continuous hot dip galvanizing line.

Thereafter, the obtained cold-rolled steel sheet was passed through a continuous annealing line or a continuous hot-dip galvanizing line to produce a high-strength cold-rolled steel sheet or a high-strength galvanized steel sheet.
Here, when passing the cold-rolled steel sheet through the continuous annealing line, after annealing at the maximum heating temperature shown in Table 4 and Table 5, cooling was performed at the average cooling rate shown in Table 4 and Table 5.
Thereafter, in some experimental examples, the samples were held at the holding temperature (heat treatment temperature) and the holding time (heat treatment time) shown in Tables 4 and 5 and then cooled to room temperature.
The atmosphere in the annealing furnace of the continuous annealing line is an N ₂ atmosphere containing 1% by volume of H _2, and the logarithm log (PH ₂ O / PH ₂ ) of the moisture pressure and the hydrogen partial pressure in the atmosphere in the furnace is -2. It was set to 8.

Then, about the part of the cold-rolled steel plate of the experimental example which let the continuous annealing line pass, zinc system electroplating was given by the method shown below, and the galvanized steel plate (EG) was manufactured. First, using a steel sheet controlled under predetermined conditions in a continuous annealing facility, alkaline degreasing, water washing, pickling, and water washing were sequentially performed as pretreatment for plating. For the electroplating, a plating bath made of zinc sulfate, sodium sulfate, and sulfuric acid was used, and Zn plating was performed using a liquid circulation type electroplating apparatus. At this time, the current density was set to 100 A / dm ² , and electrolytic treatment was performed until a predetermined plating thickness was obtained, thereby producing an electrogalvanized steel sheet.

Further, when passing the cold-rolled steel sheet through the continuous hot-dip galvanizing line, after annealing at the maximum heating temperature shown in Table 4 and Table 5, cooling is performed at the average cooling rate shown in Table 4 and Table 5, Then, it immersed in the zinc plating bath of the temperature shown in Table 4 and Table 5, and cooled.
Moreover, in some experimental examples, after performing the process until it was immersed in a zinc plating bath, the alloying process was performed at the temperature shown in Table 4 and Table 5, and it cooled.
In addition, when letting a continuous hot dip galvanizing line pass, the average cooling rate before and after immersing in a galvanizing bath was made the same.
The atmosphere in the annealing furnace of the continuous hot dip galvanizing line is an N ₂ atmosphere containing 1% by volume of H _2, and the logarithm log of the water pressure and the hydrogen partial pressure in the furnace atmosphere (PH ₂ O / PH ₂ ). Was -1.2.

[Evaluation test]
The steel sheet of each experimental example manufactured by the above method was subjected to the following evaluation test.

"Steel structure"
First, the structure of the steel sheet of each experimental example was observed using a transmission electron microscope (TEM), and the dislocation density of the steel sheet was measured by taking images in 20 fields of view, and the average is shown in Table 6 below.
In the measurement of ferrite, bainite, martensite, pearlite, retained austenite, and cementite, the steel sheet rolling direction cross section or the rolling direction perpendicular cross section is obtained by using the Nital reagent and the reagent disclosed in JP-A-59-219473. After corroding, 10 fields of view were observed with an optical microscope of 1000 times and 10 fields of view were observed with a scanning electron microscope of 5000 times. The cementite described here is spherical cementite present in ferrite, and does not include layered cementite constituting the pearlite structure or fine cementite contained in the bainite structure.

“Collision absorbed energy: static ratio = dynamic strength (FS2) / quasi-static strength (FS1)”
Predetermined specimens were collected from the steel plates of each experimental example to obtain a static / kinetic ratio (= FS2 / FS1), which was used as an evaluation index for collision absorption energy.
First, in measuring the quasi-static strength (FS1), the tensile strength test was performed at a strain rate of 0.0067 (s ⁻¹ ) to measure the strength at a nominal strain of 0.03. Further, in measuring the dynamic strength (FS2), the one bar method is used, and the dynamic strength is obtained at each of two points at four strain rates in the range of strain rates of 500 to 1400 (s ⁻¹ ). The dynamic strength at 1000 (s-1) was interpolated from the value of. This is because it is difficult to accurately set the strain rate to 1000 (s ⁻¹ ). By measuring each strength by such a method, a static-dynamic ratio (FS2 / FS1) comprising a ratio of the quasi-static strength (FS1) and the dynamic strength (FS2) was obtained and shown in Table 6 below.

"Tensile maximum strength (TS) and elongation (EL .: ductility)"
A No. 5 test piece described in JIS Z 2201 was processed from the steel plate of each experimental example, and the maximum tensile strength TS (MPa) and elongation (EL.) Were measured according to the test method described in JIS Z 2241. The results are shown in Table 6 below.
Table 6 below shows a list of evaluation results in this example.

[Evaluation results]
As shown in Tables 1 to 6, the high strength of the present invention example (see the remarks column in Tables 1 to 4) having steel components specified in the present invention and manufactured according to the manufacturing conditions specified in the present invention. The cold-rolled steel sheet and the high-strength galvanized steel sheet have a dislocation density of 2.0 × 10 ¹¹ (pieces / mm ² ) or less and a ratio of quasi-static strength and dynamic strength, which is an index of high-speed deformation characteristics. The static ratio (FS2 / FS1) was 1.05 or more, and the maximum tensile strength was 900 MPa or more. From these evaluation results, the high-strength cold-rolled steel sheet and high-strength galvanized steel sheet of the example of the present invention have a high collision while stably achieving both the static ratio as high as the 590 MPa class steel sheet and the maximum tensile strength of 900 MPa or more. It became clear that the absorbed energy was obtained.

  On the other hand, the cold-rolled steel sheet and the galvanized steel sheet of the comparative example do not satisfy any of the chemical component composition and each manufacturing condition specified in the present invention. Thus, any of the static ratio, the maximum tensile strength, and the elongation ratio cannot satisfy the target characteristics.

In the cold rolled steel sheet of Experimental Example A-2, the roll roughness (Ra) exceeds the specified range of the present invention, so that the dislocation density is large and the static / dynamic ratio is lower than the specified value of the present invention. The absorption energy is inferior.
The cold-rolled steel sheet of Experimental Example A-3 is inferior in tensile maximum strength (TS) because the maximum heating temperature is below the specified range of the present invention.
In the cold rolled steel sheets of Experimental Examples A-4 and A-5, the roughness (Ra) of the roll exceeds the specified range of the present invention, and furthermore, the skin pass elongation ratio is large, so that the dislocation density is large, and the static-dynamic ratio. However, it is lower than the specified value of the present invention, resulting in inferior impact absorption energy.
In the cold rolled steel sheet of Experimental Example A-6, since the roll roughness (Ra) exceeds the specified range of the present invention, the dislocation density is large and the static / dynamic ratio is lower than the specified value of the present invention. The absorption energy is inferior.
The cold-rolled steel sheet of Experimental Example A-7 has inferior tensile maximum strength (TS) because the heat treatment temperature exceeds the specified range of the present invention.

The cold-rolled steel sheet of Experimental Example B-2 is inferior in tensile maximum strength (TS) because the maximum heating temperature is below the specified range of the present invention.
In the cold-rolled steel sheet of Experimental Example B-3, the roughness (Ra) of the roll exceeds the specified range of the present invention, so the static ratio is less than the specified value of the present invention, and the impact absorption energy is inferior. It is the result.
In the cold-rolled steel sheets of Experimental Examples B-4 and B-5, the roll roughness (Ra) exceeds the specified range of the present invention, and the skin pass elongation rate is large. It is below the value, and the impact absorption energy is inferior.
The cold-rolled steel sheet of Experimental Example B-6 has inferior tensile maximum strength (TS) because the heat treatment temperature exceeds the specified range of the present invention.

In the cold rolled steel sheet of Experimental Example C-2, the roughness (Ra) of the roll exceeds the specified range of the present invention, so the dislocation density is large and the static / dynamic ratio is lower than the specified value of the present invention. The absorption energy is inferior.
In the cold rolled steel sheet of Experimental Example C-3, the roughness (Ra) of the roll exceeds the specified range of the present invention, and furthermore, the skin pass elongation rate is large, so the dislocation density is large and the static ratio is of the present invention. The result is below the specified value and the impact absorption energy is inferior.
The cold-rolled steel sheet of Experimental Example C-4 has inferior tensile maximum strength (TS) because the heat treatment temperature exceeds the specified range of the present invention.

In the cold rolled steel sheet of Experimental Example D-4, the roll roughness (Ra) exceeds the specified range of the present invention, so the dislocation density is large and the static / dynamic ratio is lower than the specified value of the present invention. The absorption energy is inferior.
The cold-rolled steel sheet of Experimental Example D-5 has a roll roughness (Ra) that exceeds the specified range of the present invention, and furthermore, since the skin pass elongation is large, the dislocation density is large and the static ratio is that of the present invention. The result is below the specified value and the impact absorption energy is inferior.
In the cold-rolled steel sheet of Experimental Example D-7, the maximum heating temperature is lower than the specified range of the present invention, so the tensile maximum strength (TS) is inferior.
The cold rolled steel sheet of Experimental Example D-8 has an inferior tensile maximum strength (TS) because the average cooling rate is below the specified range of the present invention.

The cold rolled steel sheets of Experimental Examples E-2 and E-3 have a roll roughness (Ra) exceeding the specified range of the present invention, and further, since the skin pass elongation rate is large, the dislocation density is large, However, it is lower than the specified value of the present invention, resulting in inferior impact absorption energy.
Since the maximum heating temperature of the cold rolled steel sheet of Experimental Example E-4 is below the specified range of the present invention, the maximum tensile strength (TS) is inferior.
The cold rolled steel sheet of Experimental Example E-5 has an inferior tensile maximum strength (TS) because the average cooling rate is below the specified range of the present invention.

In the cold rolled steel sheet of Experimental Example G-2, the roll roughness (Ra) exceeds the specified range of the present invention, so the dislocation density is large and the static / dynamic ratio is lower than the specified value of the present invention. The absorption energy is inferior.
In the cold rolled steel sheet of Experimental Example G-3, the roughness (Ra) of the roll exceeds the specified range of the present invention, and further, since the skin pass elongation is large, the dislocation density is large and the static ratio is of the present invention. The result is below the specified value and the impact absorption energy is inferior.
The cold-rolled steel sheet of Experimental Example G-4 has an inferior tensile maximum strength (TS) because the average cooling rate is below the specified range of the present invention.

The cold-rolled steel sheet of Experimental Example R-1 is inferior in tensile maximum strength (TS) because the C content in the chemical components of the steel sheet is below the specified range of the present invention.
The cold-rolled steel sheet of Experimental Example S-1 has a high dislocation density and a static / dynamic ratio below the specified value of the present invention because the C content in the chemical composition of the steel sheet exceeds the specified range of the present invention. The impact absorption energy is inferior.
In the cold rolled steel sheet of Experimental Example T-1, since the Si content in the chemical composition of the steel sheet is below the specified range of the present invention, the static ratio is below the specified value of the present invention, and the impact absorption energy is The result is inferior.

The cold-rolled steel sheet of Experimental Example V-1 is inferior in tensile maximum strength (TS) because the Mn content in the chemical components of the steel sheet is below the specified range of the present invention.
In the cold-rolled steel sheet of Experimental Example W-1, the Mn content in the chemical composition of the steel sheet exceeds the specified range of the present invention, so the dislocation density is large and the static / dynamic ratio is below the specified value of the present invention. The impact absorption energy is inferior.
The cold rolled steel sheet of Experimental Example X-1 is inferior in tensile maximum strength (TS) because the Al content in the chemical components of the steel sheet exceeds the specified range of the present invention.
In the cold rolled steel sheet of Experimental Example Y-1, the C and Mn contents in the chemical components of the steel sheet exceed the specified range of the present invention, so the dislocation density is large and the static ratio is below the specified value of the present invention. The impact absorption energy is inferior.

  From the results of the examples described above, the high-strength cold-rolled steel sheet and high-strength galvanized steel sheet with a tensile maximum strength of 900 MPa or more and excellent static collision absorption energy according to the present invention are as static as the 590 MPa-class steel sheet and 900 MPa or more. It is clear that high impact absorption energy is provided while maintaining the maximum tensile strength of the material stably.

Claims (6)

% By mass
C: 0.07 to 0.25%,
Si: 0.3-2.50%,
Mn: 1.5-3.0%
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%
Al: 1.0% or less,
N: 0.0005 to 0.0100%,
O: 0.0005 to 0.007%,
A cast slab containing a steel component consisting of iron and inevitable impurities in the balance , directly or once cooled and then heated to 1050 ° C. or higher, and then hot rolling is completed at the Ar ₃ transformation point or higher,
Then, it winds up in a temperature range of 400 to 670 ° C., and after pickling, cold rolling with a rolling reduction of 40 to 70%
Next, when passing through a continuous hot dip galvanizing line, after annealing at a maximum heating temperature of 760 to 900 ° C., cooling at an average cooling rate of 1 to 1000 ° C./second, and then dipping in a galvanizing bath, 250 ° C. or less A method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy, wherein the rolling is performed using a roll having a roughness (Ra) of 3.0 or less after being cooled down. After being immersed in the galvanizing bath, an alloying treatment is performed at a temperature of 460 to 600 ° C., then cooled to 250 ° C. or less at an average cooling rate of 1 ° C./second or more, and a roughness (Ra) of 3.0 The method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to claim 1 , wherein rolling is performed using the following rolls. Further, the steel component of the cast slab is in mass%,
Ti: 0.005 to 0.10%,
Nb: 0.005 to 0.10%,
V: 0.005-0.10%
The method for producing a high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to claim 1 or 2, wherein one or more of them are contained. Further, the steel component of the cast slab is in mass%,
B: 0.0001 to 0.01%
Cr: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 2.0%,
Mo: 0.01 to 0.8%
The high-strength galvanized steel sheet having a tensile maximum strength of 900 MPa or more and excellent in impact absorption energy according to any one of claims 1 to 3, wherein the high-strength galvanized steel sheet has excellent impact absorption energy. Production method. Further, the steel component of the cast slab contains, by mass%, one or more of Ca, Ce, Mg, and REM in a total range of 0.0001 to 0.5%. The method for producing a high-strength galvanized steel sheet having a maximum tensile strength of 900 MPa or more and excellent in collision absorption energy according to any one of claims 1 to 4. A high-strength galvanized steel sheet obtained by the production method according to any one of claims 1 to 5,
Inside the steel plate, the density of dislocations contained in the steel plate is 8 × 10 ¹¹ (Pieces / mm ² ) And the strain rate is 0.0067 (s ^-1 ) Quasi-static strength (FS1) and strain rate 1000 (s ^-1 High-strength zinc with a maximum tensile strength of 900 MPa or more excellent in impact absorption energy, characterized in that the static-dynamic ratio (= FS2 / FS1) comprising the ratio to the dynamic strength (FS2) Plated steel sheet.

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