JP4436275B2 - High yield ratio high strength cold rolled steel sheet, high yield ratio high strength hot dip galvanized steel sheet, high yield ratio high strength alloyed hot dip galvanized steel sheet, and methods for producing them - Google Patents

Description

  The present invention has a high yield ratio and excellent ductility, and is suitable for use in automobiles, building materials, household electrical equipment, etc., and a high yield ratio high strength cold-rolled steel sheet and a high yield ratio high strength hot-dip galvanized steel sheet and The present invention relates to a high yield ratio high strength alloyed hot dip galvanized steel sheet and a method for producing them.

In recent years, there has been a growing demand for high-strength steel sheets with good workability for the purpose of improving fuel efficiency and durability, particularly in automobile bodies. In addition, in order to respond to the market needs of safety at the time of collision and expansion of cabin space, steel sheets having a tensile strength of 780 MPa or more are being used for members such as body frame members and reinforcements.
As a steel plate for a vehicle body skeleton, the first important performance is spot weldability. The vehicle body frame member plays a role of protecting the passenger by absorbing an impact at the time of a collision. Therefore, if the strength of the spot weld is not sufficient, it breaks at the time of collision, and sufficient impact energy absorption performance cannot be obtained.
Conventionally, a high-strength steel sheet considering weldability has been proposed and put into practical use (for example, see Patent Documents 1 and 2).

As a high-strength steel sheet for body frame, the next important performance is high yield strength, that is, yield ratio.
A material with a high yield ratio is excellent in energy absorption capability in the event of a collision. In order to obtain a high yield ratio, it is useful to bainite the structure, and a steel sheet having a bainite structure as a main phase and a manufacturing method thereof have been proposed (for example, see Patent Document 3).
When the high-strength steel plate is a hot-rolled steel plate, precipitation strengthening can be used effectively, and it is not so difficult to achieve both a high yield ratio and good ductility (for example, see Patent Document 4).
On the other hand, when the high-strength steel sheet is a cold-rolled steel sheet, it is difficult to increase the strength by precipitation, and it is very difficult to achieve both a high yield ratio and good ductility.

As the steel sheet for the body frame, the last important performance is the workability of the steel sheet, that is, ductility, bendability, stretch flangeability, and the like.
For example, with regard to hole expandability, the main phase is bainite to improve hole expandability. Further, with regard to the formability, the second phase has a retained austenite. The steel which has is proposed (for example, refer nonpatent literature 1).
Furthermore, it has also been proposed that the product of “tensile strength” and “hole expansion ratio” is maximized when retained austenite with a volume ratio of 2 to 3% is generated by performing austempering at a temperature below Ms temperature. Yes.

In order to increase the ductility of a high-strength material, it is common to actively utilize a composite structure. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is remarkably reduced. Therefore, the main phase is ferrite and the second phase is martensite. A high-strength steel material whose hole expansion rate is improved by decreasing the number has been proposed (for example, see Non-Patent Document 2).
In addition, a steel sheet excellent in both properties of hole expansibility and ductility has been proposed (see, for example, Patent Document 5).
JP 2003-193194 A JP 2000-80440 A JP 2001-355043 A JP 2002-322539 A JP 2001-366043 A Sugimoto Koichi, Proceedings of the Japan Iron and Steel Institute Materials and Processes 139th Spring Lecture Meeting, Japan Iron and Steel Institute, Vol. 13, No. 3, March 1, 2000, pages 395-398 Nobuyuki Nakamura, etc., Proceedings of the Japan Iron and Steel Institute Materials and Processes 139th Spring Lecture Meeting, Japan Iron and Steel Institute, Vol. 13, No. 3, published on March 1, 2000, pages 391-394

  By the way, a steel plate having a tensile strength of 780 MPa class that is useful for a conventional vehicle body skeleton, particularly a cold-rolled steel plate that has both a high yield ratio and good ductility, and has good spot weldability, has been sufficiently studied. It is difficult to say that it has been made, and in particular, a cold-rolled steel sheet having a high yield ratio of 0.70 or more and ensuring good ductility has not been proposed yet, and it has been applied to automobile bodies, that is, the weight of automobiles. It became an obstacle to the transformation.

  The present invention has been made in view of the above circumstances, has a maximum tensile strength of 780 MPa or more, has a high yield ratio, and has high yield that combines weldability and ductility that can be adapted to an automobile body frame. It is an object of the present invention to provide a specific high-strength cold-rolled steel sheet, a high yield ratio high-strength galvanized steel sheet, a high yield ratio high-strength galvannealed steel sheet, and a method for producing them.

As a result of intensive studies, the present inventors have achieved a high yield ratio and a high ductility as a steel sheet suitable for a vehicle body skeleton member, and furthermore, in order to ensure excellent weldability, the C content is extremely high. It was important to find that the C content should not exceed 0.044%.
As for the yield ratio, a higher yield is advantageous from the viewpoint of absorbing the collision energy, but if it is too high, the shape freezeability during press molding deteriorates, so it is practically less than 0.83. The elements that play an important role together with C to ensure this are the four elements Nb, Ti, Mo, and B, and it is very important to strictly control the addition amount of Ti and Nb in a narrow range. It is important, and it has been found that a high yield ratio and good ductility can be secured.
This invention is made | formed based on said knowledge, The place made into the summary is as follows.

(1) By mass%, C: more than 0.028% to less than 0.044%, Si: less than 0.8%, Mn: 1.9 to 2.3%, P: 0.001 to 0.035% , S: 0.0001 to 0.013%, Al: 0.1% or less, N: 0.0001 to 0.008%, Ti: 0.012% to 0.029%, Nb: 0.029 to 0 0.042%, Mo: 0.05% to 0.25%, B: 0.0008% to 0.0038%, the balance is made of iron and inevitable impurities, and the yield ratio is more than 0.70 to less than 0.83. and tensile high yield ratio high-strength cold-rolled steel sheet up to strength, wherein ~900MPa less der Rukoto than 780 MPa.

(2) The high yield ratio high strength cold-rolled steel sheet according to (1) above, further comprising, by mass%, Cr: 0.01 to 0.8%.

(3) In addition, by mass%, Ca, above, characterized in that comprising 0.0001 to 0.5% one or two or selected Ce or al total (1) or (2) high yield ratio high-strength cold-rolled steel sheet of the mounting come to.

( 4 ) In the metallographic structure of the steel sheet, crystal grains extending in the rolling direction with an aspect ratio of 3 or more, crystal grains with a grain size of less than 3 μm, and rolling directions with a grain size of less than 3 μm and an aspect ratio of 3 or more. The high yield ratio high strength cold-rolled steel sheet according to any one of (1) to ( 3 ) above, wherein any one of the elongated crystal grains is contained at an area ratio of 20 to 80%.

( 5 ) Hot galvanizing is performed on the surface of the high yield ratio high strength cold-rolled steel sheet according to any one of (1) to ( 4 ), and the yield ratio is more than 0.70 to less than 0.83, and high yield ratio high-strength hot-dip galvanized steel sheet tensile maximum strength and said ~900MPa less der Rukoto than 780 MPa.

( 6 ) Alloying hot dip galvanizing is performed on the surface of the high yield ratio high strength cold-rolled steel sheet according to any one of (1) to (4), and the yield ratio is more than 0.70 to less than 0.83. and high yield ratio high-strength galvannealed steel sheet tensile maximum strength and said ~900MPa less der Rukoto than 780 MPa.

( 7 ) The cast slab comprising the steel component according to any one of (1) to ( 4 ) above is directly or once cooled and then heated to 1130 ° C or higher, and then hot-rolled at a temperature equal to or higher than the Ar3 transformation temperature. Then, it is taken out at a temperature of 750 ° C. or lower, then cold-rolled at a rolling reduction of 35 to 85%, and then subjected to continuous annealing at a maximum heating temperature of 740 ° C. or higher and 950 ° C. or lower. A method for producing a high-strength cold-rolled steel sheet having a yield ratio of more than 0.70 to less than 0.83 and a maximum tensile strength of 780 MPa to less than 900 MPa .

( 8 ) The high yield ratio high strength cold-rolled steel sheet according to the above ( 7 ), which is subjected to a heat treatment for 60 seconds or more in a temperature range of 100 to 550 ° C after the continuous annealing. Method.

( 9 ) The method for producing a high yield ratio high strength cold-rolled steel sheet as described in ( 8 ) above, wherein after the heat treatment, skin pass rolling with a rolling reduction of 0.1% or more is performed.

( 10 ) The cast slab comprising the steel component according to any one of (1) to ( 4 ) above is directly or once cooled and then heated to 1130 ° C or higher, and then hot-rolled at a temperature equal to or higher than the Ar3 transformation temperature. After that, it is taken out at a temperature of 750 ° C. or lower, then cold-rolled at a rolling reduction of 35 to 85%, and then subjected to continuous annealing at a maximum heating temperature of 740 ° C. or higher and 950 ° C. or lower. Then, it is cooled to a temperature range from a temperature lower by 40 ° C. than the temperature of the galvanizing bath to a temperature higher by 50 ° C. than the temperature of the galvanizing bath, and then immersed in the galvanizing bath . A method for producing a high-yield ratio high-strength hot-dip galvanized steel sheet having a strength exceeding 70 to less than 0.83 and having a maximum tensile strength of 780 MPa to less than 900 MPa .

( 11 ) In the method for producing a high yield ratio high strength alloyed hot dip galvanized steel sheet, wherein the surface of the high yield ratio high strength cold rolled steel sheet according to any one of (1) to (4) is subjected to alloying hot dip galvanizing. In addition, after performing the step of immersing in a galvanizing bath under the conditions described in ( 10 ) above, the alloying treatment is further performed at a temperature of 460 ° C. or higher, and the yield ratio is 0.00. A method for producing a high-yield ratio high-strength galvannealed steel sheet having a tensile strength of more than 70 and less than 0.83 and having a maximum tensile strength of 780 MPa to less than 900 MPa .

( 12 ) The method for producing a high yield ratio high-strength galvannealed steel sheet according to ( 11 ) above, wherein after the alloying treatment, skin pass rolling with a rolling reduction of 0.1% or more is performed. .

  According to the high yield ratio high strength cold-rolled steel sheet of the present invention, in mass%, C: more than 0.028% to less than 0.044%, Si: less than 0.8%, Mn: 1.9 to 2.3 %, P: 0.001 to 0.035%, S: 0.0001 to 0.013%, Al: 0.1% or less, N: 0.0001 to 0.008%, Ti: 0.012% to Containing 0.029%, Nb: 0.029-0.042%, Mo: 0.05-0.25%, B: 0.0008-0.0038%, the balance being iron and inevitable impurities The yield ratio is more than 0.70 to less than 0.83, the maximum tensile strength is 780 MPa to less than 900 MPa, the ductility is good, and the tensile strength (TS) is 780 MPa class high yield ratio high strength cold. A rolled steel sheet can be provided.

High yield ratio high strength cold-rolled steel sheet having high weldability and ductility according to the present invention, high yield ratio high strength hot-dip galvanized steel sheet, high yield ratio high-strength galvannealed steel sheet, and the best mode of their production method explain. Here, a high yield ratio high strength cold-rolled steel sheet having good ductility and tensile strength (TS) of 780 MPa class will be described as an example.
In addition, since this form is demonstrated in detail in order to make the meaning of invention be better understood, unless otherwise specified, this invention is not limited.

  The high yield ratio high-strength cold-rolled steel sheet of the present invention is in mass%, C: more than 0.028% to less than 0.044%, Si: less than 0.8%, Mn: 1.9 to 2.3%, P: 0.001-0.035%, S: 0.0001-0.013%, Al: 0.1% or less, N: 0.0001-0.008%, Ti: 0.012% -0. 029%, Nb: 0.029 to 0.042%, Mo: 0.05 to 0.25%, B: 0.0008 to 0.0038%, with the balance being iron and inevitable impurities .

Here, the reason for limiting the steel composition as described above will be described.
The reason why the C content is limited to more than 0.028% by mass and less than 0.044% by mass is that addition of more than 0.028% is necessary because C is an element effective for increasing the strength. On the other hand, if it is 0.044% by mass or more, it is difficult to achieve both a high yield ratio and good ductility, and problems may occur when molding car body skeleton parts, and productivity may deteriorate when manufacturing steel sheets. There is a case.
A preferable range of the C content is 0.032% by mass or more and less than 0.040% by mass.

  The reason for limiting the Si content to less than 0.8% by mass is that Si is effective for increasing the strength at low cost, so it is preferable to add it as necessary, and also to increase the ductility of the high-strength steel sheet. This is because there is an effect. The content of Si is preferably 0.25% by mass or more. Moreover, hole expansion is improved by suppressing the formation of relatively coarse carbides. However, addition of a large amount lowers the yield ratio and degrades the plating and alloying reactivity, which also adversely affects weldability. Therefore, the upper limit is less than 0.8% by mass. A more preferable range of the Si content is more than 0.30 mass% to 0.65 mass%.

The reason why Mn content is limited to 1.9 to 2.3% by mass is that Mn is effective in increasing the strength by strengthening the transformation structure. For example, it suppresses ferrite transformation and the main phase is bainite or In addition to providing a uniform structure by using bainitic ferrite, it is effective for suppressing carbide precipitation and pearlite generation, which are a cause of strength reduction and hole expandability deterioration, and to increase the yield ratio. It is necessary to add 1.9% by mass or more. In addition, if it is less than 1.9 mass%, high yield ratio and favorable ductility cannot be made compatible, although C is low content by combined addition with Mo, Ti, Nb, and B. On the other hand, excessive addition of Mn deteriorates weldability and promotes the formation of a large amount of martensite, which may lead to a decrease in yield ratio and a significant decrease in ductility and hole expandability due to segregation. Therefore, the upper limit is 2.3 mass%.
A more preferable range of the Mn content is 2.0% by mass to 2.3% by mass.

  The P content is limited to 0.001 to 0.035% by mass, although P is a strengthening element, but excessive addition causes hole-expandability and bendability, as well as joint strength and fatigue strength of welds. Therefore, the upper limit is made 0.035% by mass. On the other hand, since the extremely low P content is economically disadvantageous, the lower limit is set to 0.001% by mass. A more preferable range of the content of P is 0.005 to 0.013 mass%.

  The reason why the S content is limited to 0.0001 to 0.013 mass% is that the extremely low content of S is economically disadvantageous, so 0.0001 mass% is the lower limit, while 0.013 mass%. Addition in an amount exceeding mass% adversely affects the hole expandability and bendability of the steel sheet, as well as the joint strength and fatigue strength of the welded portion, so 0.013 mass% is the upper limit. A more preferable value of this upper limit is 0.005% by mass.

  The Al content is limited to 0.1% by mass or less because Al is effective as a deoxidizing element, but when added in excess, coarse Al-based inclusions, for example, alumina clusters are formed, and bendability is increased. In addition, there is a possibility of deteriorating the hole expansibility, so 0.1 mass% was made the upper limit. Further, the lower limit is not particularly limited, but it is difficult to deoxidize with Al and to make the remaining Al amount 0.003% by mass or less, so 0.003% by mass is a practical lower limit. Note that this is not the case when deoxidation is performed with an element other than Al or when an element other than Al is used in combination.

  The N content is limited to 0.0001 to 0.008% by mass. N is effective for imparting high strength and seizure curability (BH property), but the content is large. If it is too large, coarse crystal grains are formed and the bendability and hole expansibility are deteriorated, so 0.008% by mass is made the upper limit. On the other hand, a content of less than 0.0001% by mass is extremely difficult technically, so this is the lower limit. A more preferable range of the N content is 0.0010 to 0.0040% by mass.

Ti: 0.012 mass% to 0.029 mass%
Nb: 0.029-0.042 mass%
Mo: 0.05-0.25 mass%
B: 0.0008-0.0038 mass%
These four kinds of elements are important elements in the present invention, and the reason why the contents of these four kinds of elements are limited to a narrow range as described above is that high yielding can be achieved by adding these four kinds of elements simultaneously. This is because it has been clarified that the ductility required for molding into a body frame part can be secured for the first time.

  Furthermore, it is known that the addition of Si or Mn deteriorates the weldability, but it has also been found that good weldability can be secured by adding these four kinds of elements at the same time as the Si and Mn. . Regarding the manifestation of the effects as described above by such a composite addition, various steels have been developed under the problem that the present inventors create steels having both weldability, ductility, and high yield ratio. As a result of intensive studies on the above, it has been found for the first time, and the content of each of these four elements has also been determined from such a viewpoint.

If the contents of these four elements are out of the above ranges, sufficient effects cannot be obtained. A more preferable range of the content of each of these four types of elements is as follows.
Ti: 0.016 mass% to 0.025 mass%
Nb: 0.033 mass% to 0.040 mass%
Mo: 0.08 mass% to 0.19 mass%
B: 0.0016 mass% to 0.0025 mass%.

The yield ratio (YR) of the steel sheet of the present invention is more than 0.70 and less than 0.83.
This is because if the yield ratio (YR) is 0.70 or less, sufficient collision safety may not be ensured. On the other hand, if it is 0.83 or more, the shape freezing property at the time of molding deteriorates and it becomes difficult to ensure ductility, so the upper limit is made less than 0.83. A more preferable range of the yield ratio (YR) is 0.72 to 0.79, and a more preferable range is 0.73 to 0.77.
The yield ratio is evaluated by a tensile test piece whose tensile direction is the rolling direction and the vertical direction defined in Japanese Industrial Standard JIS Z 2201 “Metallic material tensile test piece”.

  The ductility of the steel sheet of the present invention varies depending on the plate thickness. Here, the ductility of the steel sheet is defined as follows. If the thickness is less than 1.2 mm, 14% or more, 1.2 to 1.4 mm thickness, 15% or more, and more than 1.4 mm thickness, 16% or more.

  The microstructure of the steel sheet of the present invention is important in the present invention. That is, in the metallographic structure of the steel sheet, crystal grains extending in the rolling direction having an aspect ratio of 3 or more, crystal grains having a grain size of less than 3 μm, and extending in the rolling direction having a grain size of less than 3 μm and an aspect ratio of 3 or more. Any one of the crystal grains needs to be contained at an area ratio of 20 to 80%. It is necessary that these structures do not recrystallize ferrite as much as possible when heated to 700 ° C. after cold rolling, that is, leave as much unrecrystallized structure as possible, and then perform heat treatment in the α + γ region or the γ region. .

If the area ratio of these structures exceeds 80%, the ductility is extremely lowered, so 80% is the upper limit. A preferable range of the area ratio is 60% or less. This structure can be confirmed by a method such as image analysis by etching a cross section perpendicular to the plate surface parallel to the rolling direction with Nital and arbitrarily taking 10 times or more of 1000-fold photographs. The aspect ratio is defined by “the length in the rolling direction / the length in the plate thickness direction” in the cross section, and the grain diameter adopts the longest diameter of the crystal grains.
Here, the phase of the crystal grains is not particularly limited, but is mainly composed of ferrite, bainite, martensite, austenite, and those containing precipitates such as carbides.

  Other structural limitations are not particularly provided, but in order to obtain a high yield ratio and good ductility, bainite or bainitic ferrite is suitable as the main phase, and the area ratio is preferably 10% or more. Here, the bainite includes both an upper bainite in which carbide is generated at the lath boundary and a lower bainite in which fine carbide is generated in the lath. Bainitic ferrite means bainite having no carbide, and for example, acicular ferrite is an example. In order to improve hole expansibility and bendability, it is desirable that the lower bainite in which carbide is finely dispersed or bainitic ferrite or ferrite without carbide is the main phase and the area ratio exceeds 25%.

The feature of the spot weldability of the steel sheet of the present invention is that the CTS is compared with the tensile load (CTS) obtained by the cross-type tensile test when welding is performed at the welding current immediately before the occurrence of the scattering even in the case where the welding current becomes the scattering occurrence region. That is, the cost of deterioration is small.
That is, when welding with scattering occurs in a normal steel sheet, the CTS greatly decreases or the CTS variation increases, whereas the steel sheet of the present invention has a low CTS decrease rate and dispersion. .

The welding current value in the scattering occurrence region is a current value obtained by adding 1.5 kA to the current value (CE) immediately before the occurrence of scattering. For example, when the average value of CTS when welding with a welding current of CE (kA) is performed five times is 1, the CTS of when the test with a welding current of (CE + 1.5) kA is performed five times The minimum value is 0.7 or more.
A preferable value of the minimum value of CTS is 0.8 or more, and a more preferable value is 0.9 or more.
CTS is evaluated according to the method specified in Japanese Industrial Standard JIS Z 3137 “Dimensions and Test Methods of Resistance Spots and Projection Welded Joints for Cross Tensile Test Specimens”.

In addition to the above composition, the high yield ratio high strength cold rolled steel sheet of the present invention is
Further, it is preferable to contain Cr: 0.01% to 0.8% by mass%.
Cr is effective for increasing the strength, and improves bendability and hole expansibility through suppression of carbide formation and generation of bainite and bainitic ferrite. Moreover, since it is an element with little deterioration of weldability for the effect with respect to high strengthening, it is preferable to add as needed.
If the content is less than 0.01% by mass, a remarkable effect cannot be obtained, so 0.01% by mass is the lower limit. On the other hand, if the content exceeds 0.8% by mass, not only the cost increases. In order to adversely affect workability and plating properties, the upper limit was set to 0.8 mass%. The preferable range of this content is 0.2 mass%-0.5 mass%.

In addition to the above composition, the high yield ratio high strength cold rolled steel sheet of the present invention is
Furthermore, it is preferable to contain 1 type or 2 types selected from the group of Ni: 0.01-2.0% and Cu: 0.001-2.0% by the mass%.
The purpose of adding Cu and Ni in the steel sheet of the present invention is to improve the plateability without adversely affecting the strength-hole expandability balance.

Since Ni has the purpose of improving hardenability in addition to improving the plating property, it should be contained in an amount of 0.01% by mass or more. On the other hand, addition of more than 2.0% by mass increases the alloy cost and the workability. The upper limit is set to 2.0% because of adverse effects such as an increase in hardness associated with martensite formation.
Since Cu has the purpose of improving the strength in addition to improving the plating property, it should be contained in an amount of 0.001% by mass or more. On the other hand, addition of an amount exceeding 2.0% by mass adversely affects workability and recyclability. Because.

In addition to the above composition, the high yield ratio high strength cold rolled steel sheet of the present invention is
Furthermore, it is preferable to contain 1 type or 2 types selected from the group of Co: 0.01-1% and W: 0.01-0.3% by mass%.
Co has a lower content limit of 0.01% by mass for a good balance of strength-hole expansibility (and bendability) by bainite transformation control. On the other hand, if it exceeds 1%, the balance of strength-hole expansibility (and bendability) is saturated, and since it is an expensive element, the addition of a large amount impairs the economy, so 1% by mass or less is preferable. .
When W is 0.01% by mass or more, a strengthening effect appears. Here, the reason why the upper limit is 0.3% by mass is that if the amount exceeds this, the workability is adversely affected.

In addition to the above composition, the high yield ratio high strength cold rolled steel sheet of the present invention is
Furthermore, you may contain 0.001 to 1% of 1 type (s) or 2 or more types selected from the group of Zr, Hf, Ta, and V in the mass%.
Since these Zr, Hf, Ta, and V are strong carbide forming elements, it is possible to further improve the balance between strength and hole expansibility by containing them in total of 0.001% or more.
On the other hand, since these additions cause deterioration of ductility and hot workability, the upper limit of the total of these contents is set to 1% by mass.

In addition to the above composition, the high yield ratio high strength cold rolled steel sheet of the present invention is
Furthermore, you may contain 0.0001-0.5% of 1 type (s) or 2 or more types selected from the group of Ca, Mg, La, Y, and Ce in the mass%.
Since these Ca, Mg, La, Y, and Ce contribute to inclusion control, particularly fine dispersion, by adding appropriate amounts, the total content of one or more of these is 0.0001% by mass or more. did. On the other hand, excessive addition reduces the manufacturability such as castability and hot workability and the ductility of the steel sheet product, so the upper limit was made 0.5 mass%.

In addition to the above composition, the high yield ratio high strength cold rolled steel sheet of the present invention is
Furthermore, by mass%, elements other than La, Y and Ce among rare earth elements, that is, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu You may contain 0.0001-0.5% of the selected 1 type (s) or 2 or more types in total.
The addition of an appropriate amount of rare earth elements (REM) other than La, Y, and Ce contributes to inclusion control, particularly fine dispersion, so the lower limit of the content was set to 0.0001% by mass. On the other hand, excessive addition is accompanied by an increase in cost, and also lowers the manufacturability such as castability and hot workability and the ductility of the steel sheet product, so the upper limit of the content was set to 0.5 mass%.

In the high yield ratio high strength cold-rolled steel sheet of the present invention, there are, for example, Sn and Sb as inevitable impurities. Even if these elements are contained in a total amount of 0.2% by mass or less, the effects of the present invention are included. Is not detrimental.
In the high yield ratio high strength cold-rolled steel sheet of the present invention, O is not particularly limited, but if it contains an appropriate amount, there is an effect of improving bendability and hole expansibility. On the other hand, when the amount is too large, these characteristics are deteriorated. Therefore, the O content is preferably in the range of 0.0005 mass% to 0.004 mass%.

If the surface of the high yield ratio high strength cold rolled steel sheet is hot dip galvanized, a high yield ratio high strength hot dip galvanized steel sheet is obtained.
Further, if this high yield ratio high strength hot dip galvanized steel sheet is subjected to an alloying treatment, a high yield ratio high strength galvannealed steel sheet is obtained.
The manufacturing method of these steel plates will be described later.

Next, the manufacturing method of the high yield ratio high strength cold-rolled steel sheet of the present invention will be described.
In this production method, the cast slab made of the steel component of the high yield ratio high strength cold rolled steel sheet of the present invention is directly or once cooled and then heated to 1130 ° C. or higher, and then hot rolled at a temperature equal to or higher than the Ar3 transformation temperature. Then, it is taken out at a temperature of 750 ° C. or lower, then cold-rolled at a rolling reduction of 35 to 85%, and then subjected to continuous annealing at a maximum heating temperature of 740 ° C. or higher and 950 ° C. or lower. It is a manufacturing method.

After performing said continuous annealing, you may perform the heat processing hold | maintained for 60 second or more in the temperature range of 100-550 degreeC.
Further, after the above heat treatment, skin pass rolling with a rolling reduction of 0.1% or more may be performed.

The component adjustment may be performed by an electric furnace or the like in addition to a normal blast furnace-converter method.
The casting method is not particularly limited, and may be produced by a normal continuous casting method, an ingot method, a thin slab casting method, or the like.
The cast slab may be cooled and reheated before hot rolling, or may be directly hot rolled without cooling. Here, when the temperature of a casting slab becomes less than 1130 degreeC, it heats again and makes it 1130 degreeC or more.
If the temperature of the casting slab is less than 1130 ° C, the bendability and hole expandability of the product deteriorate due to segregation and the like, so 1130 ° C is set as the lower limit. A preferable temperature of the cast slab is 1150 ° C. or higher, and a more preferable temperature is 1210 ° C. or higher.

The final finishing temperature of this hot rolling is not less than the Ar3 transformation temperature. The Ar3 transformation point of the steel sheet of the present invention is 650 ° C. or lower. If this final finishing temperature is lower than the Ar3 transformation temperature, ferrite grains that are stretched in the rolling direction are formed on the hot-rolled steel sheet, which is not preferable because the ductility and bendability of the steel sheet deteriorate.
After hot rolling, it is taken out at 750 ° C. or lower and wound up. When this temperature is higher than 750 ° C., a large amount of ferrite and pearlite is generated in the metal structure of the obtained hot-rolled steel sheet, so that the structure of the final product becomes non-uniform, and the bendability and hole expandability deteriorate.
The take-out temperature, that is, the winding temperature is preferably 650 ° C. or less, and more preferably 600 ° C. or less.
The lower limit is not particularly defined, but it is difficult to set it to room temperature or lower, so it is preferable to set room temperature to the lower limit.

In addition, you may join the rough rolling bars and perform hot rolling of finishing continuously. At this time, the rough rolling bar may be wound once.
The hot-rolled steel sheet manufactured in this way may be subjected to pickling and skin pass rolling as necessary. In skin pass rolling, rolling may be performed up to 4.0% for shape correction, strength adjustment, and the like. Here, if it exceeds 4.0%, the load on the skin pass rolling mill increases, so 4.0% is set as the upper limit of rolling. If the rolling rate is less than 0.1%, the effect of skin pass rolling is small and control is difficult, so 0.1% is made the lower limit. Skin pass rolling may be performed in-line or offline. In addition, skin pass rolling at a desired reduction rate may be performed at once, or may be performed in several steps.

The hot-rolled steel sheet thus obtained is cold-rolled at a reduction rate of 35 to 85%.
The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 35 to 85% and passed through a continuous annealing line or a continuous hot dip galvanizing line. Here, if the rolling reduction of cold rolling is less than 35%, it is difficult to keep the shape flat. Moreover, since the ductility of the final product becomes poor, this is the lower limit. On the other hand, when the rolling reduction of cold rolling is 85% or more, the load of cold rolling becomes very large, and the productivity is hindered. A preferable range of the cold rolling reduction is 40 to 70%.

  The maximum heating temperature when passing through the continuous annealing line is 740 to 950 ° C. If it is less than 740 ° C., the α → γ transformation does not occur or only slightly occurs, so that the transformed tissue fraction in the final structure is too small, and sufficient strength cannot be obtained. Therefore, 740 ° C. is set as the lower limit of the maximum heating temperature. On the other hand, when the maximum heating temperature is higher than 950 ° C., troubles such as deterioration of the shape of the steel sheet are induced, so 950 ° C. is set as the upper limit of the maximum heating temperature.

  The time required to pass this maximum heating temperature is not particularly limited, but one second or more is necessary to make the temperature of the steel plate uniform. However, if it exceeds 10 minutes, formation of an oxidized phase at the grain boundary is promoted, and it becomes difficult to control the yield ratio (YR). In addition, the cost may increase.

After passing through the continuous annealing line, a heat treatment may be performed for 60 seconds or longer in the range of 100 to 550 ° C. as necessary. This heat treatment may improve elongation and bendability.
If the temperature of the heat treatment is less than 100 ° C., the effect of the heat treatment is small, whereas if it exceeds 550 ° C., the bendability may be rather deteriorated due to the formation of coarse precipitates. Moreover, since it becomes economically disadvantageous, 550 degreeC is made an upper limit.
A preferred temperature range for this heat treatment is in the range of 200 to 450 ° C.

After this heat treatment, skin pass rolling may be performed.
The reduction rate of the skin pass rolling after the heat treatment is set to 0.1% or more for the same reason as described above. In order to increase the yield ratio and maintain good ductility, the rolling reduction is preferably in the range of 0.7% to 2.0%. Various plating may be performed after the heat treatment.

Next, the manufacturing method of the high yield ratio high strength hot-dip galvanized steel sheet of the present invention will be described.
In this production method, the cast slab made of the steel component of the high yield ratio high strength cold rolled steel sheet of the present invention is directly or once cooled and then heated to 1130 ° C. or higher, and then hot rolled at a temperature equal to or higher than the Ar3 transformation temperature. After that, it is taken out at a temperature of 750 ° C. or lower, then cold-rolled at a rolling reduction of 35 to 85%, and then subjected to continuous annealing at a maximum heating temperature of 740 ° C. or higher and 950 ° C. or lower. Then, it is cooled to a temperature range from a temperature lower by 40 ° C. than the temperature of the galvanizing bath to a temperature higher by 50 ° C. than the temperature of the galvanizing bath, and then immersed in the galvanizing bath.

After dipping in the above galvanizing bath, an alloying treatment may be performed at a temperature of 460 ° C. or higher.
Further, after the above alloying treatment, skin pass rolling with a rolling reduction of 0.1% or more may be performed.

  The maximum temperature reached when the continuous hot-dip galvanizing line is passed after the cold rolling is set to 740 to 950 ° C. for the same reason as when passing the continuous annealing line. In the case of a hot dip galvanizing line consisting of a so-called non-oxidizing furnace (NOF) -reducing furnace (RF), the air ratio in the non-oxidizing furnace is set to 0.9 to 1.2 to promote iron oxidation, By reducing the iron oxide generated on the surface by the subsequent reduction treatment to form metallic iron, the plating property and the alloying reactivity can be improved.

In addition, in a hot dip galvanizing line of a type without a non-oxidizing furnace (NOF), it is advantageous in terms of plating properties and alloying reactivity to have a dew point of −20 ° C. or higher.
The cooling end point temperature before immersion in the plating bath after reaching the maximum heating temperature is a temperature range from a temperature 40 ° C. lower than the galvanizing bath temperature to a temperature 50 ° C. higher than the galvanizing bath temperature, that is, “zinc plating bath temperature−40”. “° C. to“ zinc plating bath temperature + 50 ”° C.
When this cooling end point temperature is lower than “zinc plating bath temperature −40” ° C., not only the yield ratio may be less than 0.70, but also there are operational problems such as large heat removal when entering the plating bath. is there. Further, when the end point temperature of cooling is higher than “zinc plating bath temperature + 50” ° C., operational problems accompanying the increase in plating bath temperature are induced. The galvanizing bath may contain elements other than zinc as necessary.

This hot dip galvanized steel sheet may be alloyed.
When the alloying process is performed, it is performed at 460 ° C. or higher. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow, and the productivity is deteriorated. The upper limit of the alloying treatment temperature is not particularly limited, but if it exceeds 620 ° C., pearlite transformation may occur, and the yield ratio decreases, and the bendability and hole expansibility deteriorate. This is a practical upper limit.
Further, the above hot-dip galvanized steel sheet may be subjected to skin pass rolling of 0.1% or more. In order to increase the yield ratio and maintain good ductility, it is preferable to perform skin pass rolling of 0.7% to 2.0%.

The high yield ratio high strength cold-rolled steel sheet of the present invention is also excellent in weldability. As for the welding method, in addition to exhibiting particularly excellent characteristics with respect to spot welding as described above, it is also suitable for welding methods that are usually performed, such as arc welding, TIG welding, MIG welding, mash welding, laser welding, and the like. . It is also suitable for tailored blanks.
The high yield ratio high strength cold-rolled steel sheet of the present invention is also suitable for hot pressing. That is, by heating the steel plate to a temperature of 600 ° C. or higher and then press forming, a molded product having a high yield ratio can be obtained, and the subsequent weldability is also excellent.
Moreover, the high yield ratio high-strength cold-rolled steel sheet of the present invention is also excellent in resistance to hydrogen embrittlement.

Next, the steel plate of this invention is demonstrated in an Example and a comparative example.
"Experiment 1"
With a converter, steel slabs having various compositions shown in Table 1 were melted, then these steel slabs were heated to 1240 ° C., and hot rolling was completed at about 900 ° C., which is higher than the Ar3 transformation temperature, Then, it cooled to 550 degreeC and set it as the steel strip of thickness 2.8mm, and wound up with the predetermined winding speed using the winding device.
Next, after pickling these steel strips, the sheet thickness was changed to 1.2 mm by cold rolling, and subsequently heat treatment was performed under the conditions shown in Table 2.
In this heat treatment, the sample was held for 90 seconds at the maximum temperature (maximum heating temperature) shown in Table 2, and then cooled to (maximum temperature −130) ° C. at a cooling rate of 5 ° C./second. Thereafter, cooling was performed at a cooling rate of 40 ° C./second until the subsequent additional heat treatment temperature, and this additional heat treatment was performed for about 350 seconds.

After this heat treatment, skin pass rolling with a rolling reduction of 0.4% was performed. Tensile test specimens were taken from these steel sheets in accordance with Japanese Industrial Standards JIS Z 2201 “Metallic material tensile test specimens”, and the tensile properties in the direction perpendicular to the rolling direction were measured.
Here, as tensile properties, three points of maximum tensile strength (TS), yield strength (YS), and yield ratio (YR) were evaluated. In the table, “El” is the total elongation.
Next, spot welding was performed on the steel plate subjected to skin pass rolling.

Spot welding was performed under the following conditions.
Electrode (dome type): Tip diameter 6mmφ
Applied pressure: 4.3kN
Welding current: Current (CE) and (CE + 1.5) kA immediately before the occurrence of scattering
Welding time: 15 cycles Holding time: 10 cycles

After welding, in order to evaluate spot welding, a cross-type tensile test was performed in accordance with Japanese Industrial Standard JIS Z 3137 “Dimensions and Test Methods of Resistance Spots and Projection Welded Joints for Cross-Tensile Test Pieces”.
Here, when the minimum value of CTS when welding with CE as the welding current is performed 10 times is 1, when welding is performed 10 times with the welding current being scattered (CE + 1.5) kA The minimum value of CTS was less than 0.85 as “x”, 0.85 or more and less than 0.95 as “◯”, and 0.95 or more as “◎”.

For evaluation of bendability, a rectangular test piece having a length of 100 mm in the direction perpendicular to the rolling direction and a length of 30 mm in the rolling direction was sampled, and cracked when the longitudinal direction of the test piece was bent by 90 °. It was evaluated by measuring the limit bending radius at which occurrence occurred.
That is, a bending test was performed at intervals of 0.5 mm from 0.5 mm to 5.0 mm in the radius of curvature of the tip of the punch, and the minimum bending radius without occurrence of cracking was defined as “limit bending radius”.
These test results are shown in Table 2.

  According to these evaluation results, it was found that the steel sheet of the present invention has good ductility while having a high yield ratio, and is excellent in weldability and bendability.

"Experimental example 2"
The steel slabs having various compositions shown in Table 1 were subjected to cold rolling in the same manner as in “Experimental Example 1”, and were subjected to heat treatment and hot dip galvanization in a continuous alloying hot dip galvanizing facility.
In the heat treatment, the temperature was raised to 700 ° C. at a heating rate of 15 ° C./second, and then raised to the maximum temperature in about 100 seconds. At this time, the maximum attained temperature (maximum heating temperature) was varied as shown in Table 3. Thereafter, it was cooled to “maximum attained temperature −30” ° C. at a cooling rate of 1 ° C./second, and then cooled to 460 ° C. in 90 seconds.

These steel plates were subsequently immersed in a plating tank (bath composition: 0.11% Al—Zn, bath temperature: 455 ° C.) to form a galvanized film on the surface of the steel plate. Then, it heated to 530 degreeC-570 degreeC with the temperature increase rate of 30 degreeC / second, and performed the alloying process. Then, it cooled to room temperature (25 degreeC).
The amount of plating was about 55 g / m ^{2 on} both sides. The rolling reduction in skin pass rolling was 0.8%.

Next, tensile test specimens were collected from these steel plates in accordance with Japanese Industrial Standard JIS Z 2201 “Metal Material Tensile Test Specimens”, and tensile properties in a direction perpendicular to the rolling direction were measured. Table 3 shows the tensile properties, plating properties, alloying reactivity, and spot weldability of each steel plate. The spot weldability was evaluated in the same manner as in Experimental Example 1, and the plating property and the alloying reactivity were evaluated as follows.
"Plating property"
○: No plating △: There is some non-plating ×: “No alloying reactivity”
◯: No alloying unevenness on the surface appearance Δ: Some alloying unevenness on the surface appearance ×: Many alloying unevenness on the surface appearance These test results are shown in Table 3.

  According to these evaluation results, it was found that the steel sheet of the present invention was superior in the balance between the yield ratio, weldability and strength as compared with the steel sheet of the comparative example.

"Experiment 3"
No. in Table 1 For each of the steel sheets 1-1, 1-2, 4-1, 4-2, 6-1, and 6-2, the immersion was performed in the plating tank in the same manner as in Experimental Example 2, and then room temperature (25 ° C.). Air-cooled until. The amount of plating was about 50 g / m ^{2 on} both sides. Moreover, the rolling reduction of the skin pass rolling was set to 0.7%.

  According to these evaluation results, it was found that the steel sheet of the present invention was superior in the balance between the yield ratio, weldability and strength as compared with the steel sheet of the comparative example.

  The high yield ratio high strength cold-rolled steel sheet of the present invention is a steel sheet that has good ductility, bendability, etc. and a good balance between weldability and strength by controlling the yield ratio to an appropriate level. Therefore, it can be widely used as a steel material such as a steel plate for automobile body frame parts, and its industrial utility value is extremely large.

Claims (12)

C: more than 0.028% to less than 0.044%, Si: less than 0.8%, Mn: 1.9 to 2.3%, P: 0.001 to 0.035%, S: 0.0001 to 0.013%, Al: 0.1% or less, N: 0.0001 to 0.008%, Ti: 0.012% to 0.029%, Nb: 0.029 to 0.042% , Mo: 0.05 to 0.25%, B: 0.0008 to 0.0038%, the balance is made of iron and inevitable impurities , the yield ratio is more than 0.70 to less than 0.83, and high yield ratio high-strength cold-rolled steel sheet tensile maximum strength and said ~900MPa less der Rukoto than 780 MPa.   Furthermore, the high yield ratio high-strength cold-rolled steel sheet according to claim 1, characterized by containing Cr: 0.01 to 0.8% in mass%. Further, the mass% in, Ca, Ce or we selected one or two, characterized by containing 0.0001 to 0.5 percent by total claim 1 or 2 in serial mounting of high yield High strength cold-rolled steel sheet. In the metal structure of the steel sheet, crystal grains extending in the rolling direction with an aspect ratio of 3 or more, crystal grains having a grain size of less than 3 μm, crystals extending in the rolling direction with a grain size of less than 3 μm and an aspect ratio of 3 or more The high yield ratio high strength cold-rolled steel sheet according to any one of claims 1 to 3 , wherein any one of the grains is contained at an area ratio of 20 to 80%. The surface of the high yield ratio high strength cold rolled steel sheet according to any one of claims 1 to 4 , is hot dip galvanized, has a yield ratio of more than 0.70 to less than 0.83, and a maximum tensile strength of 780 MPa. high yield ratio high-strength hot-dip galvanized steel sheet, characterized in der Rukoto less than ～900MPa. The surface of the high yield ratio high strength cold-rolled steel sheet according to any one of claims 1 to 4, is subjected to alloying hot dip galvanization, the yield ratio is more than 0.70 to less than 0.83, and the maximum tensile strength. high yield ratio high-strength galvannealed steel sheet but, wherein ~900MPa less der Rukoto than 780 MPa. The cast slab comprising the steel component according to any one of claims 1 to 4 is directly or once cooled and then heated to 1130 ° C or higher, then hot-rolled at a temperature equal to or higher than the Ar3 transformation temperature, and then It is taken out at a temperature of 750 ° C. or lower, then cold-rolled at a rolling reduction of 35 to 85%, and then continuously annealed in a temperature range where the maximum heating temperature is 740 ° C. or higher and 950 ° C. or lower. The manufacturing method of the high yield ratio high strength cold-rolled steel sheet whose yield ratio is more than 0.70 to less than 0.83 and whose maximum tensile strength is 780 MPa to less than 900 MPa . The method for producing a high yield ratio high strength cold-rolled steel sheet according to claim 7, wherein after the continuous annealing, a heat treatment is performed for 60 seconds or more in a temperature range of 100 to 550 ° C. 9. The method for producing a high yield ratio high strength cold-rolled steel sheet according to claim 8 , wherein after the heat treatment, a skin pass rolling with a rolling reduction of 0.1% or more is performed. The cast slab comprising the steel component according to any one of claims 1 to 4 is directly or once cooled and then heated to 1130 ° C or higher, then hot-rolled at a temperature equal to or higher than the Ar3 transformation temperature, and then Take out at a temperature of 750 ° C. or lower, then subject to cold rolling with a rolling reduction of 35 to 85%, then subject to continuous annealing at a maximum heating temperature of 740 ° C. or higher and 950 ° C. or lower, and then galvanize It is cooled to a temperature range from a temperature 40 ° C. lower than the bath temperature to a temperature 50 ° C. higher than the galvanizing bath temperature, and then immersed in a galvanizing bath, and the yield ratio is more than 0.70 to 0.00. A method for producing a high-yield ratio high-strength hot-dip galvanized steel sheet having a tensile strength lower than 83 and lower than 83 MPa and lower than 900 MPa . A method for producing a high yield ratio, high strength alloyed hot dip galvanized steel sheet, which comprises subjecting the surface of the high yield ratio high strength cold rolled steel sheet according to any one of claims 1 to 4 to alloying hot dip galvanizing ,
After the steps up to the step of immersion in a zinc plating bath under the conditions of claim 10, further characterized by applying alloying treatment at 460 ° C. or higher, the yield ratio is 0.70 super to 0. A method for producing a high yield ratio high-strength galvannealed steel sheet having a tensile strength of less than 83 and a tensile maximum strength of 780 MPa to less than 900 MPa . 12. The method for producing a high yield ratio high strength alloyed hot dip galvanized steel sheet according to claim 11, wherein after the alloying treatment, skin pass rolling with a rolling reduction of 0.1% or more is performed.