JP2019011509A - Material hot rolled steel sheet for high strength cold rolled steel sheet, and method for producing the same - Google Patents

Abstract

To provide a material hot rolled steel sheet for a high strength cold rolled steel sheet that, when made into a cold rolled steel sheet by batch annealing, has excellent material stability, high strength and excellent chemical conversion treatability, and a method for producing the material hot rolled steel sheet.SOLUTION: A material hot rolled steel sheet for a high strength cold rolled steel sheet has a chemical composition containing, in mass%, C: more than 0.050% to 0.200% or less, Si: 0.01% or more to less than 0.50%, Mn: more than 1.90% to less than 2.60%, P: 0.100% or less, S: 0.0200% or less, Al: 2.000% or less, and N: 0.0100% or less and further containing at least one selected from the group consisting of Ti: 0.005% or more and 0.100% or less and Nb: 0.005% or more and 0.100% or less, with the balance being Fe and inevitable impurities, and also has a steel structure in which in area ratios, bainite is 70% or more, polygonal ferrite is 0% or more to less than 20%, and perlite is 0.5% or more and 10% or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a material hot-rolled steel sheet for a high-strength cold-rolled steel sheet and a method for producing the same.
In particular, the present invention relates to members and parts used in the industrial field such as automobiles, electric machines, pipes and containers, and in particular, high strength cooling excellent in material stability suitable for automobile frame members and energy absorbing members. The present invention relates to a hot-rolled steel sheet for rolled steel sheets and a method for producing the same.

In recent years, from the viewpoint of conservation of the global environment, there has been a strong demand for improvement in fuel consumption for the purpose of reducing automobile CO ₂ emissions. For this reason, efforts have been made to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. For example, cold-rolled steel sheets (high-strength cold-rolled steel sheets) having a tensile strength (TS) of 590 MPa or more are increasingly used for cold-rolled steel sheets used for parts manufactured by press molding. (For example, see Patent Documents 1 and 2)

JP 2013-49901 A JP 2013-76117 A

  Generally, as the strength of a steel plate increases, fluctuations in tensile strength (TS) and total elongation (EL) within the coil tend to increase. A car manufacturer cuts a large plate (steel plate) from a coil and uses it for press molding. If the material (TS, EL, etc.) fluctuates in the coil, the material (TS or EL) of the cut large plate (steel plate) is changed. Therefore, not only is it difficult to ensure the shape and dimensional accuracy in press molding, but cracks may occur during press molding. Therefore, a cold-rolled steel sheet having a small variation in TS and EL in the coil (that is, excellent in material stability) is desired.

  Moreover, the cold-rolled steel plate used for a motor vehicle etc. is used by coating, and a chemical conversion process is performed as a pre-processing of the coating. Since the chemical conversion treatment of the cold-rolled steel sheet is one of important processes for ensuring the corrosion resistance after coating, it is required that the cold-rolled steel sheet easily forms a chemical conversion film by the chemical conversion treatment. That is, the cold-rolled steel sheet is required to have excellent chemical conversion properties.

By the way, in recent years, from the viewpoint of manufacturing lead time, the annealing treatment method for obtaining a cold-rolled steel sheet is CAL (rather than batch annealing (hereinafter also simply referred to as “batch annealing”) by BAF (Batch Annealing Furnace). Continuous annealing by continuous annealing line (hereinafter also simply referred to as “continuous annealing”) has become the mainstream.
However, in continuous annealing, the production base is limited due to large-scale equipment, and the plate thickness and width that can be passed through are limited as the strength of the steel plate increases, and the alloy elements that accompany the increase in strength of the steel plate Coil joint welding may be difficult due to the addition of a large amount.
Therefore, batch annealing may be required as an annealing treatment method for obtaining a cold-rolled steel sheet.

  Under these circumstances, the present inventors performed batch annealing on the material hot-rolled steel sheet for cold-rolled steel sheet described in Patent Documents 1 and 2 to produce a cold-rolled steel sheet. It became clear that it does not meet the level required recently. Moreover, it became clear that intensity | strength and chemical conversion treatment property may become inadequate. In the present specification, “sufficient strength (high strength)” means that TS is 590 MPa or more.

  Therefore, in view of the above circumstances, the present invention is a high-strength cold-rolled steel sheet that is excellent in material stability and has high strength and excellent chemical conversion properties when it is made into a cold-rolled steel sheet using batch annealing. It aims at providing the manufacturing method of the raw material hot-rolled steel plate for use, and the said raw material hot-rolled steel plate.

As a result of intensive studies on the above problems, the present inventors have found that the above problems can be solved by setting the component composition and the steel composition to specific aspects, and have reached the present invention.
That is, the present inventors have found that the above problem can be solved by the following configuration.

(1) By mass%, C: more than 0.050% and 0.200% or less, Si: 0.01% or more and less than 0.50%, Mn: more than 1.90% and less than 2.60%, P: 0.00. 100% or less, S: 0.0200% or less, Al: 2.000% or less, and N: 0.0100% or less, and Ti: 0.005% or more and 0.100% or less, and Nb: containing at least one selected from the group consisting of 0.005% or more and 0.100% or less, with the remaining component composition consisting of Fe and inevitable impurities;
A material hot-rolling material for high-strength cold-rolled steel sheets having an area ratio of 70% or more of bainite, 0 to 20% of polygonal ferrite, and a steel structure of pearlite of 0.5% to 10%. steel sheet.
(2) The material hot-rolled steel sheet for high-strength cold-rolled steel sheets according to (1), wherein the component composition further contains at least one selected from the following groups A to E by mass%.
[Group A] Ni: 0.01% to 1.00%, Cu: 0.005% to 1.000%, Cr: 0.01% to less than 0.5%, and Mo: 0.005 %: At least one selected from the group consisting of 0.5% to 0.500% [Group B] V: 0.005% to 0.100% and W: 0.005% to 0.100% [Group C] Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less At least one selected from the group consisting of [Group D] Sb: 0.002% to 0.200%, Sn: 0.002% to 0.010%, and Ta: 0.001% to 0.000. Selected from the group consisting of 010% or less At least one type [Group E] B: 0.0001% or more and 0.0050% or less (3) Finishing at 800 ° C. or more and 1000 ° C. or less on a steel slab having the component composition described in (1) or (2) above. By hot rolling at the rolling exit temperature, obtain a hot-rolled steel sheet,
The hot rolled steel sheet is cooled at an average cooling rate of 20 ° C./second to 120 ° C./second in the temperature range from the finish rolling exit temperature to 650 ° C., and the temperature range from 650 ° C. to the following winding temperature is set. Cool at an average cooling rate of 5 ° C / second or more and 40 ° C / second or less,
The raw hot-rolled steel sheet for high-strength cold-rolled steel sheet according to (1) or (2) is obtained by winding the hot-rolled steel sheet after cooling at a winding temperature of 400 ° C. or higher and 600 ° C. or lower. The manufacturing method of the raw material hot-rolled steel plate for high-strength cold-rolled steel plates.

As shown below, according to the present invention, a high-strength cold-rolled steel sheet having excellent material stability and high strength and excellent chemical conversion properties when formed into a cold-rolled steel sheet using batch annealing. The material hot-rolled steel sheet and the manufacturing method of the said material hot-rolled steel sheet can be provided.
In addition, the high strength cold-rolled steel sheet manufactured from the material hot-rolled steel sheet for the high-strength cold-rolled steel sheet of the present invention can improve fuel efficiency by reducing the weight of the vehicle body, for example, by applying it to a skeleton member or energy absorbing member of an automobile. Therefore, the industrial utility value is very large.

It is a perspective view which shows the coil which notched a part.

Below, the raw material hot-rolled steel sheet for high-strength cold-rolled steel sheets and its manufacturing method are demonstrated.
In the present specification, a numerical range represented by using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
Further, in the present specification, the steel sheet after hot rolling is referred to as “hot rolled steel sheet”, and the final hot rolled steel sheet obtained by winding the hot rolled steel sheet after cooling is referred to as “material hot rolled steel sheet”. .
In the present specification, cold rolling and batch annealing (pickling before cold rolling, temper rolling after batch annealing, if necessary) are performed on the raw hot-rolled steel sheet. The rolled steel sheet is also simply referred to as “cold rolled steel sheet”.
Further, in the present specification, the “material hot-rolled steel sheet for high-strength cold-rolled steel sheet” is also simply referred to as “material hot-rolled steel sheet”.

[Knowledge obtained by the present inventors]
First, the knowledge obtained by the inventors of the present invention after earnestly studying the above problems will be described below.

(1) When batch annealing is performed on the raw hot-rolled steel sheet to produce a cold-rolled steel sheet, the following points are important in order to increase the TS of the cold-rolled steel sheet to 590 MPa or more.
That is, the content of Mn in the raw hot-rolled steel sheet is set to exceed 1.90% by mass. Thereby, even if it is a very slow cooling rate after batch annealing by BAF, a desired amount of martensite can be secured.

(2) In the case of producing a cold-rolled steel sheet by subjecting the raw hot-rolled steel sheet to batch annealing, the following points are important for the cold-rolled steel sheet to exhibit good chemical conversion properties.
That is, the Si content in the raw hot-rolled steel sheet is set to less than 0.50%.

(3) In the case of producing a cold-rolled steel sheet by subjecting the material hot-rolled steel sheet to batch annealing, in order to narrow the variation of the material (ΔTS, ΔEL) in the coil of the cold-rolled steel sheet, the component composition is predetermined. And a steel structure mainly composed of bainite.
Note that ΔTS is a radial direction from the TS at a position (Hot point) (HP) 5 mm along the radial direction from the outer peripheral surface of the coil toward the inner peripheral surface, toward the outer peripheral surface from the inner peripheral surface of the coil. The absolute value of the value obtained by subtracting TS at the position (Cold point) (CP) where 1/3 of the coil thickness is included. ΔEL is a radial direction from the inner peripheral surface of the coil toward the outer peripheral surface from the EL at a position (Hot point) (HP) 5 mm along the radial direction from the outer peripheral surface of the coil toward the inner peripheral surface. Is the absolute value of the value obtained by subtracting EL at the position (Cold point) (CP) where the coil thickness is 1/3.
Here, the HP and CP will be described in more detail with reference to the drawings.
FIG. 1 is a perspective view showing a coil with a part cut away. A coil 1 shown in FIG. 1 is formed of a cold-rolled steel sheet wound in a cylindrical shape. The coil 1 has an outer peripheral surface 2 and an inner peripheral surface 3 that are formed by a rolled cold-rolled steel plate.
Hot point (HP) is a position 5 mm along the radial direction r from the outer peripheral surface 2 to the inner peripheral surface 3 of the coil 1.
Cold point (CP) is a position where 1/3 of the coil thickness t is entered along the radial direction r from the inner peripheral surface 3 to the outer peripheral surface 2 of the coil 1.
Both HP and CP are the center positions of the plate width W of the coil 1.

The present invention was completed after further studies based on the above findings.
Below, after demonstrating the raw material hot-rolled steel plate of this invention first, the manufacturing method is demonstrated.

[Material hot-rolled steel sheet]
The material hot-rolled steel sheet of the present invention is, in mass%, C: more than 0.050% and 0.200% or less, Si: 0.01% or more and less than 0.50%, Mn: more than 1.90% and 2.60%. Less than P, 0.100% or less, S: 0.0200% or less, Al: 2.000% or less, and N: 0.0100% or less, and Ti: 0.005% or more and 0.000% or less. 100% or less, and Nb: containing at least one selected from the group consisting of 0.005% or more and 0.100% or less, with the remaining component composition consisting of Fe and inevitable impurities,
The steel structure has an area ratio of 70% or more for bainite, 0% to less than 20% for polygonal ferrite, and 0.5% to 10% for pearlite.

(Component composition)
Below, the component composition of the raw hot-rolled steel sheet of the present invention will be described first. Unless otherwise specified, “%” in the component composition means “mass%”.

<C: more than 0.050% and 0.200% or less>
C is an element necessary for increasing the strength when martensite is generated to form a cold-rolled steel sheet.
When the amount of C is 0.050% or less, the amount of ferrite when a cold-rolled steel sheet is formed increases, it is difficult to secure a desired amount of martensite, and a desired strength cannot be obtained.
On the other hand, if C is added excessively over 0.200%, the welded portion and the heat-affected zone are markedly hardened when a cold-rolled steel sheet is formed, and the mechanical properties of the welded portion are deteriorated. And arc weldability also deteriorate.
Therefore, the C content is in the range of more than 0.050% and 0.200% or less. The amount of C is preferably in the range of 0.065% or more and 0.150% or less because the effect of the present invention is more excellent.

<Si: 0.01% or more and less than 0.50%>
Si is an element effective for ensuring good ductility because it improves the work hardening ability of ferrite when a cold-rolled steel sheet is formed.
However, if the Si content is less than 0.01%, the effect of addition becomes poor, so the lower limit is made 0.01%.
On the other hand, excessive addition of 0.50% or more of Si lowers the chemical conversion property when it is made into a cold-rolled steel sheet, and a temper color is generated. The reason is that Si is very easy to oxidize and forms a film of Si oxide (SiO ₂ ) on the surface of the steel sheet. This oxide inhibits the formation reaction of the chemical conversion film during the chemical conversion treatment, and the chemical conversion film is formed. It is presumed that a micro area (scaling) that is not generated occurs. Further, red scale and the like are generated in the raw hot-rolled steel sheet, which causes deterioration of the surface properties when the cold-rolled steel sheet is formed.
Therefore, the Si amount is set to a range of 0.01% or more and less than 0.50%. The amount of Si is preferably in the range of 0.10% or more and 0.40% or less because the effect of the present invention is more excellent.

<Mn: more than 1.90% and less than 2.60>
Mn is an element that stabilizes austenite.
When the amount of Mn is 1.90% or less, in the cooling process after hot rolling, ferrite transformation and pearlite transformation proceed excessively, and a structure mainly composed of bainite cannot be obtained in the raw hot-rolled steel sheet. The material stability becomes insufficient.
Also, when holding for a long time such as batch annealing with BAF in the two-phase region of ferrite and austenite, Mn is concentrated in austenite, and even when the cooling rate is extremely slow, ferrite transformation and bainite transformation hardly occur. In the cold-rolled steel sheet, a desired martensite amount can be secured, and such an effect is recognized when the Mn amount of the steel exceeds 1.90%. Thereby, it becomes possible to stably manufacture a cold-rolled steel sheet having a tensile strength (TS) of 590 MPa or more.
On the other hand, an excessive addition of Mn amount of 2.60% or more generates a large amount of martensite when a cold-rolled steel sheet is formed, and the desired ductility cannot be obtained.
Therefore, the amount of Mn is made a range of more than 1.90% and less than 2.60%. The amount of Mn is preferably 2.00% or more and less than 2.60%, more preferably less than 2.50%, for the reason that the effect of the present invention is more excellent.

<P: 0.100% or less>
P is an element that has a solid solution strengthening action and can be added according to a desired strength. Further, it is an element effective for promoting the ferrite transformation and forming a composite structure when a cold rolled steel sheet is formed. In order to obtain such an effect, the P content is preferably 0.001% or more.
On the other hand, if the amount of P exceeds 0.100%, it becomes difficult to perform laser welding and flash butt welding of the coil joint in each manufacturing process. Furthermore, the spot weldability when using a cold-rolled steel sheet is also significantly deteriorated. Therefore, the P amount is set to a range of 0.100% or less.
The amount of P is preferably in the range of 0.001% or more and 0.040% or less because the effect of the present invention is more excellent.

<S: 0.0200% or less>
S segregates at the grain boundaries, embrittles the steel during hot working, and lowers local deformability when it is present as a sulfide to form a cold-rolled steel sheet. On the other hand, if the amount of S exceeds 0.0200%, it becomes difficult to perform laser welding and flash butt welding of the coil joint in each manufacturing process. Furthermore, the spot weldability when using a cold-rolled steel sheet is also significantly deteriorated. Therefore, the S amount is 0.0200% or less.
The amount of S is preferably in the range of 0.0001% or more and 0.0100% or less, and more preferably in the range of 0.0001% or more and 0.0050% or less, because the effect of the present invention is more excellent. .

<Al: 2.000% or less>
Al is an element effective for expanding the two-phase region of ferrite and austenite and reducing the annealing temperature dependency, that is, the material stability when making a cold-rolled steel sheet. Al also acts as a deoxidizer and is an effective element for the cleanliness of steel. However, if the Al content is less than 0.005%, the effect of addition is poor, so the lower limit is preferably 0.005%.
On the other hand, if Al is added in excess of 2.000%, the risk of steel piece cracking during continuous casting increases, and the productivity decreases. Therefore, when adding Al, the amount is made into the range of 2.000% or less.
The amount of Al is preferably in the range of 0.005% or more and 2.000% or less, and more preferably in the range of 0.010% or more and 1.500% or less, because the effect of the present invention is more excellent. .

<N: 0.0100% or less>
N is an element that degrades the aging resistance when a cold-rolled steel sheet is formed. In particular, when the amount of N exceeds 0.0100%, deterioration of aging resistance becomes remarkable. Therefore, the N amount is set to a range of 0.0100% or less.
The amount of N is preferably in the range of 0.0005% or more, more preferably in the range of 0.0010% or more and 0.0070% or less, because the effect of the present invention is more excellent.

  The component composition of the material hot-rolled steel sheet of the present invention is at least one selected from the group consisting of Ti: 0.005% to 0.100% and Nb: 0.005% to 0.100%. Containing.

<Ti: 0.005% or more and 0.100% or less>
Ti forms precipitates with C, S, and N, suppresses coarsening of recrystallized grains, and contributes effectively to improvement of tensile strength (TS) when a cold-rolled steel sheet is formed. Moreover, when B is added, since N is precipitated as TiN, precipitation of BN is suppressed, and the effect of B described later is effectively expressed. Moreover, ductility at high temperature is improved, and castability in continuous casting is improved. The effect is obtained by adding 0.005% or more of Ti. However, when the Ti content exceeds 0.100%, the carbonitride content is remarkably increased and the ductility is lowered.
Therefore, when Ti is added, the Ti amount is in the range of 0.005% to 0.100%. The amount of Ti is preferably in the range of 0.020% or more and 0.090% or less because the effect of the present invention is more excellent.

<Nb: 0.005% or more and 0.100% or less>
Nb forms fine precipitates at the time of hot rolling or annealing, suppresses the coarsening of recrystallized grains, and contributes effectively to the improvement of TS when a cold-rolled steel sheet is formed. The effect is acquired by adding Nb 0.005% or more. However, when the amount of Nb exceeds 0.100%, the amount of carbonitrides increases remarkably, and the ductility when made into a cold-rolled steel sheet decreases. In addition, the cost increases.
Therefore, when Nb is added, the Nb content is in the range of 0.005% to 0.100%. The amount of Nb is preferably in the range of 0.010% or more and 0.080% or less because the effect of the present invention is more excellent.

  The component composition of the material hot-rolled steel sheet of the present invention can further contain at least one selected from the following group A to group E in mass%.

<[Group A] Ni: 0.01% or more and 1.00% or less, Cu: 0.005% or more and 1.000% or less, Cr: 0.01% or more and less than 0.5%, and Mo: 0.0. At least one selected from the group consisting of 005% and 0.500%>
Ni is an element that stabilizes retained austenite and is effective in securing good ductility when made into a cold-rolled steel sheet, and further increases the strength when made into a cold-rolled steel sheet by solid solution strengthening. From the viewpoint of obtaining this addition effect, the Ni content is preferably 0.01% or more. On the other hand, if the Ni content exceeds 1.00%, the area ratio of hard martensite may be excessive. It also becomes a factor of cost increase. For this reason, when adding Ni, the amount of Ni is preferably 0.01% or more and 1.00% or less.
Cu is an element effective for increasing the strength when a cold-rolled steel sheet is formed. From the viewpoint of obtaining this addition effect, the amount of Cu is preferably 0.005% or more. On the other hand, when the amount of Cu exceeds 1.000%, a problem of recyclability occurs. For this reason, when adding Cu, the amount of Cu is preferably 0.005% or more and 1.000% or less.
Cr and Mo are elements that contribute to improving the hardenability while increasing the strength of the cold-rolled steel sheet. From the viewpoint of obtaining this addition effect, the Cr content is preferably 0.01% or more, and the Mo content is preferably 0.005% or more. Further, excessive addition of Cr and Mo causes an increase in cost. For this reason, the Cr content is preferably less than 0.5% and the Mo content is preferably 0.500% or less.

<[Group B] V: at least one selected from the group consisting of 0.005% to 0.100% and W: 0.005% to 0.100%>
V and W are elements that can be added as necessary because they are effective in increasing the strength when formed into a cold-rolled steel sheet by precipitation strengthening. From the viewpoint of obtaining this addition effect, the V amount is preferably 0.005% or more, and the W amount is preferably 0.005% or more. On the other hand, when these elements are added excessively, the amount of carbonitride is remarkably increased, and the ductility of the cold-rolled steel sheet may be lowered. In addition, the cost increases.
Therefore, when adding at least one selected from the group consisting of V and W, the V amount is preferably 0.005% or more and 0.100% or less, and the W amount is 0.005% or more and 0.100% or less. preferable.

<[Group C] selected from the group consisting of Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0050% At least one kind>
Ca, Mg, and REM (Rare Earth Metal) are effective elements for spheroidizing the shape of sulfide and improving the adverse effect of sulfide on local ductility. In order to obtain this effect, 0.0005% or more must be added. On the other hand, if each of Ca, Mg and REM is added excessively, inclusions and the like are increased, which may cause surface and internal defects.
Therefore, when adding Ca, Mg and REM, the amount is preferably in the range of 0.0005% or more and 0.0050% or less.

<[Group D] Sb: 0.002% to 0.200%, Sn: 0.002% to 0.010%, and Ta: 0.001% to 0.010% At least one kind>
Sn and Sb are elements that can be added as necessary from the viewpoint of suppressing decarburization in the thickness region of about several tens of μm of the steel sheet surface layer caused by nitriding or oxidizing the steel sheet surface. By suppressing such nitriding and oxidation, the amount of martensite on the steel sheet surface can be prevented from decreasing, so Sn and Sb are effective elements for ensuring strength and material stability. On the other hand, when Sn and Sb are added excessively, the toughness may be lowered. For this reason, when adding at least one selected from the group consisting of Sn and Sb, the Sb content is preferably 0.002% or more and 0.200% or less, and the Sn content is preferably 0.002% or more and 0.010%. .
Ta, like Ti and Nb, generates alloy carbide and alloy carbonitride and contributes to high strength. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta) (C, N), thereby suppressing the coarsening of precipitates and strengthening precipitation. It is thought that there is an effect that stabilizes the contribution to the strength improvement by. From the viewpoint of obtaining the precipitate stabilization effect described above, the Ta amount is preferably 0.001% or more. On the other hand, when Ta is added excessively, the effect of addition is saturated and the cost also increases. For this reason, when adding Ta, the amount of Ta is preferably 0.001% or more and 0.010% or less.

<[Group E] B: 0.0001% or more and 0.0050% or less>
B has an action of suppressing the formation and growth of ferrite from the austenite grain boundary, and can be added as necessary because it can flexibly control the structure. For example, there is an effect of suppressing ferrite transformation and pearlite transformation during primary cooling after hot rolling. Furthermore, it suppresses ferrite transformation and bainite transformation during cooling after holding in batch annealing. From the viewpoint of obtaining such an effect of addition, the B content is preferably 0.0001% or more.
On the other hand, if the amount of B exceeds 0.0050%, formability (ductility, deep drawability, hole expansibility) may be deteriorated.
For this reason, when adding B, B amount is preferably 0.0001% or more and 0.0050% or less, and more preferably 0.0005% or more and 0.0030% or less.

<Remainder>
In the above component composition, the balance other than the above components is Fe and inevitable impurities.

[Steel structure]
Next, the steel structure (microstructure) of the hot-rolled steel sheet according to the present invention will be described.

<Area ratio of bainite: 70% or more>
As above-mentioned, in order to ensure material stability when using a cold-rolled steel sheet and to secure TS of 590 MPa or more, the area ratio of bainite is set to 70% or more in the material hot-rolled steel sheet of the present invention.
When the area ratio of bainite is more than 70%, when cold-rolled steel sheet is used, it becomes difficult to secure TS of 590 MPa or more in the above-described Cold point (CP), and it is possible to ensure material stability when cold-rolled steel sheet is used. It becomes difficult.
The area ratio of bainite is preferably in the range of 80% or more because the effect of the present invention is more excellent. The upper limit of the area ratio of bainite is not particularly limited and is 100%.

<Area ratio of polygonal ferrite: 0% or more and less than 20%>
In order to ensure material stability when a cold-rolled steel sheet is used and to secure TS of 590 MPa or more, the area ratio of polygonal ferrite is set to 0% or more and less than 20% in the material hot-rolled steel sheet of the present invention.
When the area ratio of polygonal ferrite is 20% or more, when cold-rolled steel sheet is used, it is difficult to secure a TS of 590 MPa or more in the above-mentioned Cold point (CP), and the material stability when cold-rolled steel sheet is obtained. It becomes difficult to secure.
The area ratio of polygonal ferrite is preferably in the range of 0% or more and 15% or less because the effect of the present invention is more excellent.

<Perlite area ratio: 0.5% to 10%>
In order to ensure material stability when a cold-rolled steel sheet is used, and to ensure a TS of 590 MPa or more, the pearlite area ratio is set to 0.5% or more and 10% or less in the material hot-rolled steel sheet of the present invention.
When the area ratio of pearlite is more than 10%, when cold-rolled steel sheet is used, it is difficult to secure TS of 590 MPa or more in the above-mentioned Cold point (CP), and it is possible to ensure material stability when cold-rolled steel sheet is used. It becomes difficult.
The area ratio of pearlite is preferably in the range of 0.5% or more and 5% or less because the effect of the present invention is more excellent.

Here, the area ratios of bainite, polygonal ferrite, and pearlite can be obtained as follows.
That is, after the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is polished, it is corroded with 3% by volume nital. 10 position observations at a magnification of 2000 times using a SEM (scanning electron microscope) for the 1/4 position of the corroded steel sheet (position corresponding to 1/4 of the thickness in the depth direction from the steel sheet surface). And obtain a tissue image. Using the obtained tissue image, the area ratio of each structure (bainite, polygonal ferrite, pearlite) is calculated for 10 visual fields by Image-Pro of Media Cybernetics, and the values are averaged. In the above structure image, bainite has a gray structure (underground structure) containing carbide and subgrain boundaries inside, polygonal ferrite has a gray structure not containing carbide, and pearlite has a layered structure of cementite and ferrite. Identified by

  In addition to bainite, polygonal ferrite and pearlite, the microstructure of the material hot-rolled steel sheet of the present invention includes carbides such as cementite, non-recrystallized ferrite, martensite, residual austenite, ferrite other than polygonal ferrite (acicular shape). Ferrite etc.) may be included. These structures may be included as long as the total area ratio is within a range of 10% or less, and the effects of the present invention are not impaired.

[Thickness]
The plate thickness of the material hot-rolled steel sheet of the present invention is not particularly limited, and is, for example, 1.5 mm or more and 6.0 mm or less.

[Production method of hot-rolled steel sheet]
Next, the manufacturing method of the raw material hot-rolled steel sheet of the present invention (hereinafter also simply referred to as “the manufacturing method of the present invention”) will be described.

The manufacturing method of the present invention is a method for manufacturing the above-described hot-rolled steel sheet of the present invention.
More specifically, the production method of the present invention comprises:
By subjecting the steel slab having the above-described composition to hot rolling at a finish rolling exit temperature of 800 ° C. or higher and 1000 ° C. or lower, a hot rolled steel plate is obtained,
The hot rolled steel sheet is cooled at an average cooling rate of 20 ° C./second to 120 ° C./second in the temperature range from the finish rolling exit temperature to 650 ° C., and the temperature range from 650 ° C. to the following winding temperature is set. Cool at an average cooling rate of 5 ° C / second or more and 40 ° C / second or less,
This is a method for obtaining the above-described hot-rolled steel sheet of the present invention by winding the hot-rolled steel sheet after cooling at a winding temperature of 400 ° C. or higher and 600 ° C. or lower.

  Hereinafter, each condition in the manufacturing method of this invention is demonstrated in detail.

[Steel slab]
In the production method of the present invention, a steel slab having the above-described component composition is used.
Steel slabs are usually heated. The heating temperature of the steel slab is preferably 1100 ° C. or higher and 1300 ° C. or lower because the effect of the present invention is more excellent.
Precipitates present in the heating stage of the steel slab exist as coarse precipitates in the finally obtained steel sheet and do not contribute to the strength. Therefore, the Ti and Nb-based precipitates precipitated during casting are redissolved. Better. When the heating temperature of the steel slab is less than 1100 ° C., it is difficult to sufficiently dissolve the carbide, and there is a possibility that it may lead to troubles during hot rolling due to an increase in rolling load. For this reason, the heating temperature of the steel slab is preferably 1100 ° C. or higher. From the viewpoint of scaling off defects such as bubbles and segregation on the surface of the slab, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, the heating temperature of the steel slab is preferably 1100 ° C. or higher.
On the other hand, when the heating temperature of the steel slab is higher than 1300 ° C., the scale loss may increase as the oxidation amount increases. For this reason, the heating temperature of the steel slab is preferably 1300 ° C. or lower.

  The steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation, but can also be manufactured by an ingot-making method or a thin slab casting method. After manufacturing a steel slab, the conventional method of once cooling to room temperature and heating again after that can also be used. After manufacturing the steel slab, do not cool to room temperature, insert it into a heating furnace as it is, or perform energy saving processes such as direct feed rolling and direct rolling that immediately roll after keeping a little heat Applicable. The steel slab is made into a sheet bar by rough rolling under normal conditions. When the heating temperature is lowered, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing troubles during hot rolling.

[Finishing temperature at finish rolling of hot rolling: 800 ° C or higher and 1000 ° C or lower]
As described above, a hot-rolled steel sheet is obtained by subjecting a steel slab (heated steel slab) to hot rolling (rough rolling and hot rolling including finish rolling).
At this time, if the finish rolling exit temperature exceeds 1000 ° C., the amount of oxide (scale) generated increases rapidly, the interface between the base iron and the oxide becomes rough, the surface of the steel plate after pickling and cold rolling. The quality tends to deteriorate, and the chemical conversion processability when cold-rolled steel sheet is reduced. Furthermore, when the finish rolling exit temperature exceeds 1000 ° C., the crystal grain size of the resulting hot-rolled steel sheet becomes excessively coarse. As a result, the area ratio of bainite in the raw hot-rolled steel sheet is less than 70%, the area ratio of polygonal ferrite is 20% or more, and it is difficult to ensure material stability when the obtained hot-rolled steel sheet is a cold-rolled steel sheet. It becomes. Moreover, it becomes difficult to make TS 590 MPa or more when the raw material hot-rolled steel sheet obtained is a cold-rolled steel sheet. On the other hand, when the finish rolling outlet temperature is less than 800 ° C., the rolling load increases, the rolling load increases, or the rolling reduction in a state where austenite is not recrystallized becomes high. As a result, an abnormal structure of the obtained material hot-rolled steel sheet develops, the area ratio of bainite becomes less than 70%, and it becomes difficult to ensure material stability when the obtained material hot-rolled steel sheet is a cold-rolled steel sheet. .
Therefore, the finish rolling exit temperature of hot rolling is set to a range of 800 ° C. or higher and 1000 ° C. or lower. The finish rolling exit temperature of hot rolling is preferably in the range of 850 ° C. or more and 950 ° C. or less because the effect of the present invention is more excellent.

  During hot rolling, rough rolled sheets may be joined together and finish rolled continuously. The rough rolled plate may be wound up once. In order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. The coefficient of friction during lubrication rolling is preferably 0.10 or more and 0.25 or less because the effect of the present invention is more excellent.

[Average cooling rate of primary cooling (cooling in the temperature range from finish rolling delivery temperature to 650 ° C.): 20 ° C./second or more and 120 ° C./second or less]
Next, primary cooling is performed with respect to the obtained hot-rolled steel sheet.
When the average cooling rate of primary cooling is less than 20 ° C./second, ferrite transformation and pearlite transformation proceed excessively, the area ratio of bainite in the resulting hot-rolled steel sheet is less than 70%, and the area ratio of polygonal ferrite is 20 %, The area ratio of pearlite exceeds 10%, and when cold-rolled steel sheet is used, it is difficult to secure TS of 590 MPa or more in the above-described Cold point (CP). As a result, it is difficult to ensure material stability.
On the other hand, when the average cooling rate exceeds 120 ° C./second, a shape failure of the steel sheet occurs.
Therefore, the average cooling rate of the primary cooling is set to a range of 20 ° C./second or more and 120 ° C./second or less. The average cooling rate of the primary cooling is preferably in the range of 25 ° C./second or more and 100 ° C./second or less because the effect of the present invention is more excellent.

[Average cooling rate of secondary cooling (cooling in the temperature range from 650 ° C. to winding temperature): 5 ° C./second or more and 40 ° C./second or less]
After the primary cooling, secondary cooling is further performed.
When the average cooling rate of the secondary cooling is less than 5 ° C./second, the ferrite transformation and pearlite transformation proceeds excessively, the area ratio of bainite of the resulting hot-rolled steel sheet is less than 70%, the area ratio of polygonal ferrite Is 20% or more, and the area ratio of pearlite is more than 10%. When a cold-rolled steel sheet is used, it is difficult to secure TS of 590 MPa or more in the above-described Cold point (CP). As a result, it is difficult to ensure material stability.
On the other hand, when the average cooling rate of the secondary cooling exceeds 40 ° C./second, a shape failure of the steel sheet occurs.
Therefore, the average cooling rate of the secondary cooling is set to a range of 5 ° C./second or more and 40 ° C./second or less. The average cooling rate of the secondary cooling is preferably in the range of 5 ° C./second or more and 35 ° C./second or less because the effect of the present invention is more excellent.

[Winding temperature: 400 ° C or higher and 600 ° C or lower]
After the secondary cooling, it is wound up last. Thus, the raw material hot-rolled steel sheet of the present invention described above is obtained.
When the coiling temperature exceeds 600 ° C., the ferrite transformation and pearlite transformation proceeds excessively, the area ratio of bainite of the resulting hot-rolled steel sheet is less than 70%, the area ratio of polygonal ferrite is 20% or more, When the area ratio exceeds 10% and a cold-rolled steel sheet is formed, it is difficult to secure TS of 590 MPa or more in the above-described Cold point (CP). As a result, it is difficult to ensure material stability.
On the other hand, when the coiling temperature is less than 400 ° C., the resulting hot-rolled steel sheet has a structure mainly composed of martensite, the strength is significantly increased, the rolling load in cold rolling is increased, and the steel sheet shape is poor. May occur.
Therefore, the coiling temperature after hot rolling is set to a range of 400 ° C. or higher and 600 ° C. or lower. The winding temperature is preferably in the range of 450 ° C. or more and 600 ° C. or less because the effect of the present invention is more excellent.

[Usage]
The material hot-rolled steel sheet of the present invention described above is suitable for manufacturing a high-strength cold-rolled steel sheet using batch annealing.
When manufacturing high-strength cold-rolled steel sheets using batch annealing, the material hot-rolled steel sheets of the present invention are subjected to pickling, cold rolling, batch annealing and temper rolling to obtain high-strength cold-rolled steel sheets. Is preferred.
In addition, since pickling can remove the oxide (scale) on the surface of the steel sheet, it is important for ensuring good chemical conversion treatment and plating quality of the high-strength cold-rolled steel sheet as a final product. Moreover, pickling may be performed once, or pickling may be performed in a plurality of times. Pickling may be performed according to a conventional method.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these.

[Manufacture of hot-rolled steel sheet]
Steel having the composition shown in the following Table 1 (the balance consisting of Fe and inevitable impurities) was melted in a converter, and a steel slab was obtained by a continuous casting method.
The obtained steel slab was hot-rolled under the conditions shown in Table 2 below to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was cooled (primary cooling and secondary cooling) and wound under the conditions shown in the same table. In this way, a hot-rolled steel sheet (coil) was obtained.
About the obtained raw material hot-rolled steel sheet, the steel structure (microstructure) was investigated by the method mentioned above. The results are shown in the same table.

[Manufacture of cold-rolled steel sheet]
The obtained material hot-rolled steel sheet was pickled and then cold-rolled under conditions of a reduction rate of 56.3%. Thereafter, degreasing was performed in an electrolytic cleaning line, and further, batch annealing was performed so that the above-described Hot point (HP) and Cold point (CP) in the coil satisfy the following temperature conditions.
(Temperature conditions)
-Hot point (HP): Hold | maintained at 700 degreeC for 15 hours-Cold point (CP): Hold | maintained at 680 degreeC for 10 hours Then, temper rolling was performed on the conditions of elongation rate 1.9%. In this way, a cold rolled steel sheet was obtained.

[Evaluation]
The following evaluation was performed about the obtained cold-rolled steel plate.

<Tensile test>
From the obtained cold-rolled steel sheet, a JIS No. 5 test piece was collected so that the tensile direction was a direction perpendicular to the rolling direction of the steel sheet. Using the collected specimens, a tensile test was performed in accordance with JIS Z 2241 (2011), and TS (tensile strength) and EL (total elongation) were measured.
Here, if TS ≧ 590 MPa, it can be said that the strength is high.
Further, it can be said that the ductility is good if EL ≧ 26% in the TS: 590 MPa class and EL ≧ 21% in the TS: 780 MPa class.
TS: 590 MPa class means that TS is 590 MPa or more and less than 780 MPa, and TS: 780 MPa class means that TS is 780 MPa or more and less than 900 MPa.
Furthermore, TS variation ΔTS between Hot point (HP) and Cold point (CP) and EL variation ΔEL between HP and CP were determined from the following equations. If ΔTS ≦ 60 MPa and ΔEL ≦ 4%, it can be said that the variation in TS and EL in the coil is small and the material stability is good.
ΔTS = | TS (HP) −TS (CP) |
ΔEL = | EL (HP) −EL (CP) |
Here, in the above formula, TS (HP) represents the tensile strength at HP, and TS (CP) represents the tensile strength at CP.
In the above formula, EL (HP) represents the total elongation in HP, and EL (CP) represents the total elongation in CP.

<Chemical conversion processability>
The obtained cold-rolled steel sheet was subjected to chemical conversion treatment by the following method using a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nihon Parkerizing Co., Ltd., and the chemical conversion treatment property was evaluated. .
First, the obtained cold-rolled steel sheet was degreased using a degreasing liquid fine cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., then washed with water, and then a surface conditioning liquid preparen Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. Was used for surface adjustment for 30 seconds. The surface-adjusted cold-rolled steel sheet was immersed in a chemical conversion treatment solution (Palbond L3080) at 43 ° C. for 120 seconds, then washed with water and dried with warm air. Thus, the cold-rolled steel sheet was subjected to chemical conversion treatment.
About the surface of the cold-rolled steel plate after the chemical conversion treatment, 5 fields of view were randomly observed using a SEM (scanning electron microscope) at a magnification of 500 times. The area ratio [%] of the region (skee) where the chemical conversion film was not generated was obtained by image processing, and the following evaluation was performed based on the obtained area ratio.
Score 5: 5% or less Score 4: More than 5% but 10% or less Score 3: More than 10% and 25% or less Score 2: More than 25% and 40% or less Score 1: More than 40% Chemical conversion processability if score 4 or score 5 Is good. Among these, a rating of 5 is preferable.

In the above Tables 1 to 3, the underlined portion indicates outside the scope of the present invention.
Further, in Table 1 above, “−” indicates that the amount is an inevitable impurity level, and is specifically as follows.
Ti: less than 0.005%, Nb: less than 0.005% [Group A] Ni: less than 0.01%, Cu: less than 0.005%, Cr: less than 0.01%, Mo: less than 0.005% [Group B] V: less than 0.005%, W: less than 0.005% [Group C] Ca: less than 0.0005%, Mg: less than 0.0005%, REM: less than 0.0005% [Group D] Sb: less than 0.002%, Sn: less than 0.002%, Ta: less than 0.001% [Group E] B: less than 0.0001% In Table 2, B is bainite, and PF is polygonal ferrite. , P is pearlite, θ is carbide (TiC, NbC, cementite, etc.), F ′ is unrecrystallized ferrite, M is martensite, and RA is retained austenite.

[Contrast between inventive example and comparative example]
As can be seen from Tables 1 to 3, the material hot-rolled steel sheet of the example of the present invention having a specific component composition and a specific steel structure has high strength and excellent material stability and chemical conversion treatment when made into a cold-rolled steel sheet. showed that.

On the other hand, no. No. 2 hot-rolled steel sheet, finish rolling exit temperature is less than 800 ° C. No. 3 material hot rolled steel sheet, No. 3 having an average cooling rate of primary cooling of less than 20 ° C./second. No. 4 hot-rolled steel sheet, No. 4 whose average cooling rate of secondary cooling is less than 5 ° C./second. No. 5 material hot-rolled steel sheet and No. 5 with a winding temperature exceeding 600 ° C. The material hot-rolled steel sheet No. 6 had an area ratio of bainite of less than 70%, and the material stability when it was a cold-rolled steel sheet was insufficient. Moreover, the strength (particularly the strength at CP) when cold-rolled steel sheet was used was insufficient.
In addition, No. of finish rolling delivery temperature exceeding 1000 ° C. About the raw material hot-rolled steel sheet of 2, the chemical conversion property when it was made into the cold-rolled steel sheet was also inadequate. As described above, it is presumed that the amount of oxide (scale) generated increases rapidly, the interface between the base iron and the oxide becomes rough, and the surface quality of the steel sheet after pickling and cold rolling deteriorates.
No. C content is 0.050% or less. No. 16 material hot-rolled steel sheet had insufficient strength when made into a cold-rolled steel sheet.
In addition, No. having a Si content of 0.50 or more. No. 17 material hot-rolled steel sheet was insufficient in chemical conversion treatment.
Moreover, No. whose Mn content is 1.90 or less. The 18 material hot-rolled steel sheet had insufficient strength. In addition, the area ratio of bainite was less than 70%, and the material stability was insufficient.
Moreover, No. containing neither Ti nor Nb. The 19 material hot-rolled steel sheet had insufficient strength.

[Contrast of Examples of the Invention]
No. 1, no. 7-15 and no. From the comparison of 33 to 36 (contrast of embodiments not containing the A to E groups), the area ratio of polygonal ferrite was No. 1.5 or more. 1, no. 7-13, no. 15 and no. 33-36 showed the more excellent chemical conversion treatment property.
No. 1 and No. 7 and no. No. 8 containing Ti (contrast between modes not containing groups A to E and having similar component compositions of C, Si, Mn, P, S, Al, and N). 1 and no. 8 showed better ductility. Among them, No. containing both Ti and Nb. 8 showed better ductility.
No. 7 and no. From the comparison with 15 (contrast between embodiments containing no A to E groups and containing C, Si, Mn, P, S, Al, N and Nb), the area ratio of pearlite is 1.2% No. No. 7 showed better material stability.

[Change of reduction ratio]
Except for changing the rolling reduction of the cold rolling to 30.0 to 65.0%, the cold rolled steel sheet was manufactured and evaluated according to the same procedure as the manufacturing of the cold rolled steel sheet described above. Similar results were obtained.

[Changes to batch annealing conditions]
When the cold-rolled steel sheet was manufactured and evaluated according to the same procedure as that of the cold-rolled steel sheet described above except that the batch annealing conditions were changed as follows, the same results as in Table 3 were obtained.
・ Hot point (HP): Hold at 680 to 730 ° C. for 5 to 30 hours ・ Cold point (CP): Hold at 660 to 710 ° C. for 2 to 20 hours

[Change in growth rate]
Except for changing the elongation rate of temper rolling to 0.7%, when the cold-rolled steel sheet was manufactured and evaluated according to the same procedure as the cold-rolled steel sheet described above, the same results as in Table 3 above were obtained. It was.

  According to the material hot-rolled steel sheet of the present invention, it is possible to produce a high-strength cold-rolled steel sheet having excellent material stability having a tensile strength (TS) of 590 MPa or more. The high-strength cold-rolled steel sheet produced from the material hot-rolled steel sheet according to the present invention can improve fuel efficiency by reducing the weight of the vehicle body when applied to, for example, a skeleton member or an energy absorbing member of an automobile. The utility value is very large.

1: Coil 2: Outer peripheral surface 3: Inner peripheral surface CP: Cold Point
HP: Hot Point
r: Radial direction of coil t: Coil thickness W: Coil plate width

Claims (3)

In mass%, C: more than 0.050% and less than 0.200%, Si: 0.01% or more and less than 0.50%, Mn: more than 1.90% and less than 2.60%, P: 0.100% or less , S: 0.0200% or less, Al: 2.000% or less, and N: 0.0100% or less, and Ti: 0.005% or more and 0.100% or less, and Nb: 0 A composition comprising at least one selected from the group consisting of 0.005% or more and 0.100% or less, with the balance being Fe and inevitable impurities;
A material hot-rolling material for high-strength cold-rolled steel sheets having an area ratio of 70% or more of bainite, 0 to 20% of polygonal ferrite, and a steel structure of pearlite of 0.5% to 10%. steel sheet. The material hot-rolled steel sheet for high-strength cold-rolled steel sheets according to claim 1, wherein the component composition further contains at least one selected from the following groups A to E in mass%.
[Group A] Ni: 0.01% to 1.00%, Cu: 0.005% to 1.000%, Cr: 0.01% to less than 0.5%, and Mo: 0.005 %: At least one selected from the group consisting of 0.5% to 0.500% [Group B] V: 0.005% to 0.100% and W: 0.005% to 0.100% [Group C] Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less At least one selected from the group consisting of [Group D] Sb: 0.002% to 0.200%, Sn: 0.002% to 0.010%, and Ta: 0.001% to 0.000. Selected from the group consisting of 010% or less Even without one [E group] B: 0.0001% or more 0.0050% or less A hot-rolled steel sheet is obtained by subjecting the steel slab having the composition according to claim 1 or 2 to hot rolling at a finish rolling exit temperature of 800 ° C. or higher and 1000 ° C. or lower,
The hot rolled steel sheet is cooled at an average cooling rate of 20 ° C./second to 120 ° C./second in a temperature range from the finish rolling outlet temperature to 650 ° C., and a temperature range from 650 ° C. to the following winding temperature is set. Cool at an average cooling rate of 5 ° C / second or more and 40 ° C / second or less,
The high-strength hot-rolled steel sheet for high-strength cold-rolled steel sheet according to claim 1 or 2 is obtained by winding the hot-rolled steel sheet after cooling at a coiling temperature of 400 ° C or higher and 600 ° C or lower. The manufacturing method of the raw hot-rolled steel sheet for cold-rolled steel sheets.