JP7134106B2 - High-strength steel sheet and high-strength galvanized steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength steel sheet and a high-strength galvanized steel sheet having a galvanized layer on the surface of the high-strength steel sheet.

Steel sheets used for automobile structural members are required to have higher strength in order to improve fuel efficiency. Further, when high-strength steel sheets are applied to automobile structural members, the high-strength steel sheets are required to have high impact absorption energy from the viewpoint of collision safety.

It is known that the higher the tensile strength TS (Tensile Strength) of a high-strength steel sheet and the higher the 0.2% proof stress σ _0.2 or the upper yield point UYP (Upper Yield Point), the higher the impact absorption energy. For these reasons, steel sheets that are applied to automobile structural members are required to have a tensile strength TS of 1470 MPa or more and a 0.2% proof stress or an upper yield point UYP of 1000 MPa or more. Hereinafter, the tensile strength TS may be abbreviated as "tensile strength", and the 0.2% yield strength or upper yield point UYP as "yield strength".

As a technique for improving the tensile strength of a high-strength steel sheet among the properties required as described above, for example, a technique such as that disclosed in Patent Document 1 has been proposed. In this patent document 1, by controlling the fractions of autotempered martensite, ferrite, bainite, and retained austenite, and specifying the size and number of precipitates of iron-based carbides in the autotempered martensite, tensile It is disclosed that strength and workability can be improved.

However, in this technique, only tensile strength and workability are considered, and yield strength is not considered. Also, in this technique, the yield strength is measured after 0.3% temper rolling. The yield strength can be increased by temper rolling, but in the case of ultra-high-strength steel sheets of 1470 MPa or more, it may not always be possible to secure a sufficient elongation by temper rolling.

Japanese Patent No. 5365216

The present invention has been made in view of the circumstances as described above, and an object of the present invention is to provide a high-strength steel sheet having a yield strength of 1000 MPa or more at a high strength level of a tensile strength of 1470 MPa or more, and such a high-strength steel sheet. An object of the present invention is to provide a high-strength galvanized steel sheet having a galvanized layer on the surface of the steel sheet.

The high-strength steel sheet of the present invention, which can solve the above problems,
in % by mass,
C: 0.200 to 0.280%,
Si: 0.40 to 1.50% or less,
Mn: 2.00-3.00%,
P: more than 0%, 0.015% or less,
S: more than 0%, 0.0050% or less,
Al: 0.015-0.060%,
Cr: 0.20 to 0.80%,
Ti: 0.015 to 0.080%,
B: 0.0010 to 0.0040%,
respectively, the balance being iron and inevitable impurities,
With respect to the entire metal structure, martensite is 93% by volume or more, ferrite, pearlite and bainite are 2% by volume or less in total, and retained austenite is 7% by volume or less,
Further, in an image obtained by observing the metal structure with a scanning electron microscope, the number of laths in martensite measured by a cutting method for a total length of 300 μm is 240 or more, and the tensile strength is 1470 MPa or more. do.

If necessary, the high-strength steel sheet of the present invention further has, in mass%, Cu: more than 0%, 0.30% or less, Ni: more than 0%, 0.30% or less, Mo: more than 0%, 0.30 % or less, V: more than 0%, 0.30% or less, Nb: more than 0%, 0.040% or less, and Ca: more than 0%, 0.0050% or less. It is also useful to add elements to further improve the properties of the high-strength cold-rolled steel sheet according to the types of elements contained.

The present invention also includes a high-strength galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the high-strength steel sheet as described above.

The high-strength steel sheet of the present invention is configured as described above, and can achieve a high-strength steel sheet having a yield strength of 1000 MPa or more at a high strength level of a tensile strength of 1470 MPa or more.

FIG. 1 is a schematic diagram showing a heat pattern in the annealing process. FIG. 2 is an explanatory diagram for measuring the number of laths by the cutting method. FIG. 3 is a schematic diagram showing a heat pattern in heat treatment 1. FIG. FIG. 4 is a schematic diagram showing a heat pattern in heat treatment 2. As shown in FIG. FIG. 5 is a schematic diagram showing a heat pattern in heat treatment 3. As shown in FIG. FIG. 6 is a drawing-substitute micrograph showing an example of the structure of the high-strength steel sheet of the present invention.

In order to provide a high-strength steel sheet having a tensile strength of 1470 MPa or more and a high yield strength, the present inventors determined the amount of bainite, martensite and retained austenite, and the lath, which is the substructure of bainite and martensite. We have focused on and conducted extensive investigations.

As a result, the chemical composition of the steel sheet, the volume fraction of martensite, the volume fraction of bainite (including ferrite and pearlite), the volume fraction of retained austenite, and the image observed with a scanning electron microscope (SEM) (hereinafter sometimes referred to as "SEM image"), if the number of laths in martensite measured by a cutting method with a total length of 300 µm is defined as described above, the above object can be achieved. We found that and completed the present invention. In addition, when it is called "high strength" below, it is used in the meaning of "strength level whose tensile strength is 1470 MPa or more."

Lath is the substructure of martensite. The structure of martensite is multi-layered, and within one prior austenite grain, there are multiple packets, which are aggregates of grains with the same habit plane, and each packet has parallel belt-like regions. There are certain blocks, and in each block there is a collection of laths which are martensite crystals containing a high density of dislocations with approximately the same crystal orientation.

The number of laths in martensite measured by a cutting method for a total length of 300 μm (hereinafter sometimes referred to as “the number of laths per 300 μm total length”) defined in the present invention is the thickness of the steel plate subjected to nital corrosion. In 1/4 part, the cross section parallel to the rolling direction was photographed at 3000 times with FE-SEM (Field Emission Scanning Electron Microscope), and the total length of 300 μm was measured by the cutting method.

The present inventors thought that laths in martensite affect the yield strength and tensile strength, and conducted extensive research. As a result, it was found that satisfying the later-described requirements for the number of laths per 300 μm total length is important for achieving both high yield strength and high tensile strength. The requirements defined in the present invention will be described in order.

[Number of laths per total length of 300 μm: 240 or more]
In the high-strength steel sheet of the present invention, it is necessary that the number of laths per 300 μm in total length is 240 or more. When the number of laths is less than 240, the yield strength or tensile strength is lowered. Although the reason for this has not necessarily been clarified, it was possible to think as follows. First, in addition to the fact that the boundary between laths hinders the movement of dislocations and has the effect of increasing the yield strength, in the chemical composition system of the present invention, fine iron-based carbides such as cementite and film-like retained austenite are present in the laths. It exists at the boundary and can be an additional barrier to dislocation motion. From the above, it is considered that the yield strength and tensile strength increase when the number of laths per predetermined length increases. The lower limit of the number of laths is preferably 245 or more, more preferably 250 or more. The upper limit of the number of laths is generally 600 or less.

[Martensite: 93% by volume or more]
Martensite in the metal structure is the base structure of the high-strength steel sheet of the present invention, and by making it 93% by volume or more with respect to the entire metal structure, the yield strength and tensile strength are increased. When martensite is less than 93% by volume, other soft structures start plastic deformation at low stress, resulting in low yield strength. The lower limit of martensite is preferably 94% by volume or more, more preferably 95% by volume or more. The upper limit of martensite is approximately 99% by volume or less. Martensite includes tempered martensite and self-annealed martensite, but if tempered excessively, the number of laths per 300 μm in total length becomes less than 240, so it is not included in the martensite targeted by the present invention. do not have.

[Ferrite, pearlite and bainite: 2% by volume or less in total]
These structures are softer than martensite, which is the base structure, and when these structures increase, these structures themselves start plastic deformation at low stress, resulting in lower yield strength and tensile strength. . From this point of view, the total content of ferrite, pearlite and bainite should be 2% by volume or less with respect to the entire metal structure. The upper limit of these structures is preferably 1.5% by volume or less, more preferably 1.0% by volume or less. The lower limit of bainite may be 0% by volume. Hereinafter, unless otherwise specified, ferrite, pearlite and bainite are represented by "bainite".

[Retained austenite: 7% by volume or less]
Retained austenite in the metal structure should be 7% by volume or less with respect to the entire metal structure. A small amount of film-like retained austenite existing at the lath boundaries may have the effect of increasing the tensile strength and yield strength by suppressing the movement of dislocations, but retained austenite itself is softer than the martensitic structure. Therefore, even in the form of a film, both the yield strength and the tensile strength decrease when it is present in excess. From this point of view, the retained austenite should be 7% by volume or less. The upper limit of retained austenite is preferably 6% by volume or less, more preferably 5% by volume or less. The lower limit of retained austenite is generally 1% by volume or more.

In the high-strength steel sheet of the present invention, in addition to specifying the number of laths, the martensite volume fraction, the bainite volume fraction, and the retained austenite volume fraction as described above, it is also necessary to appropriately define the chemical composition of the steel sheet. be. The reasons for setting these ranges are as follows. "%" in the following chemical composition means "% by mass".

(C: 0.200-0.280%)
C is an element necessary for ensuring the strength of the steel sheet. When the amount of C is insufficient, the tensile strength of the steel sheet is lowered. Therefore, the amount of C is made 0.200% or more. The lower limit of the C content is preferably 0.205% or more, more preferably 0.210% or more. However, when C is added excessively, the volume fraction of retained austenite increases to more than 7% by volume, which may lead to a decrease in yield strength. Therefore, the upper limit of the amount of C is made 0.280% or less. The upper limit of the amount of C is preferably 0.270% or less, more preferably 0.260% or less. It is more preferably 0.250% or less, and still more preferably 0.240% or less.

(Si: 0.40 to 1.50%)
Si is known as a solid-solution strengthening element, and is an element that effectively acts to improve tensile strength while suppressing a decrease in ductility. It is also considered effective in suppressing excessive tempering of martensite and securing fine laths. In order to effectively exhibit such effects, the amount of Si must be 0.40% or more. The lower limit of the Si content is preferably 0.50% or more, more preferably 0.60% or more. It is more preferably 0.70% or more, and still more preferably 0.80% or more. However, if the amount of Si becomes excessive, the volume fraction of retained austenite increases, which may lead to a decrease in yield strength. Therefore, the upper limit of the amount of Si is made 1.50% or less. The upper limit of the amount of Si is preferably 1.40% or less, more preferably 1.30% or less.

(Mn: 2.00-3.00%)
Mn is an element that contributes to increasing the strength of the steel sheet, and is necessary for suppressing the formation of ferrite and bainite and achieving the targeted martensite-based structure. In order to effectively exhibit such effects, the Mn content must be 2.00% or more. The lower limit of the Mn amount is preferably 2.05% or more, more preferably 2.10% or more. However, an excessive amount of Mn may cause slab breakage, an increase in cold rolling load, and the like. Therefore, the upper limit of the amount of Mn is made 3.00% or less. The upper limit of the amount of Mn is preferably 2.90% or less, more preferably 2.80% or less. It is more preferably 2.70% or less, and even more preferably 2.60% or less.

(P: more than 0%, 0.015% or less)
P is an element that is unavoidably contained, and is an element that segregates at grain boundaries and promotes grain boundary embrittlement. Therefore, the amount of P is set to 0.015% or less. The upper limit of the amount of P is preferably 0.013% or less, more preferably 0.010% or less. Incidentally, P is an impurity that is unavoidably mixed in steel, and it is impossible to reduce its amount to 0% in terms of industrial production.

(S: more than 0%, 0.0050% or less)
Like P, S is an element that is unavoidably contained, and it is recommended to reduce the amount of S as much as possible in order to avoid inclusions and breakage during working. Therefore, the amount of S is set to 0.0050% or less. The upper limit of the amount of S is preferably 0.0040% or less, more preferably 0.0030% or less. Incidentally, S is an impurity that is unavoidably mixed in steel, and it is impossible in terms of industrial production to reduce its amount to 0%.

(Al: 0.015-0.060%)
Al is an element that acts as a deoxidizing agent. In order to effectively exhibit these effects, the Al content must be 0.015% or more. The lower limit of the Al content is preferably 0.025% or more, more preferably 0.030% or more. However, if the amount of Al becomes excessive, many inclusions such as alumina are formed in the steel sheet, which may cause breakage during working. Therefore, the upper limit of the amount of Al is made 0.060% or less. The upper limit of the Al content is preferably 0.055% or less, more preferably 0.050% or less.

(Cr: 0.20-0.80%)
Cr is necessary to suppress the formation of ferrite and bainite and to obtain the targeted martensite-based structure. In addition, it is considered to have the effect of suppressing excessive tempering of martensite and making laths finer. In order to effectively exhibit such effects, the Cr content must be 0.20% or more. The lower limit of the Cr content is preferably 0.25% or more, more preferably 0.30% or more. However, if the amount of Cr is excessive, non-coating may occur when hot-dip galvanizing or alloying hot-dip galvanizing is formed on the surface of the steel sheet. Therefore, the upper limit of the amount of Cr is made 0.80% or less. The upper limit of the Cr content is preferably 0.75% or less, more preferably 0.70% or less.

(Ti: 0.015 to 0.080%)
Ti is an element that forms carbides and nitrides to improve the strength of the steel sheet. In addition, it is an effective element for effectively exhibiting the hardenability improvement effect of B, which will be described later. That is, Ti reduces N in the steel by forming nitrides, as a result of which the formation of B nitrides is suppressed, B becomes a solid solution, and the hardenability improvement effect of B can be effectively exhibited. In this way, Ti improves the hardenability, thereby contributing to the increase in the strength of the steel sheet. In order to effectively exhibit such effects, the amount of Ti should be 0.015% or more. The lower limit of the Ti amount is preferably 0.018% or more, more preferably 0.020% or more.

However, when the amount of Ti becomes excessive, Ti carbides and Ti nitrides become excessive, which may cause cracks during working. Therefore, the upper limit of the amount of Ti is made 0.080% or less. The upper limit of the Ti amount is preferably 0.070% or less, more preferably 0.060% or less, and still more preferably 0.050% or less. Even more preferably, it is 0.040% or less.

(B: 0.0010 to 0.0040%)
B has the effect of improving hardenability and suppressing the formation of ferrite and bainite. As a result, it is an element that contributes to increasing the strength of the steel sheet. In order to effectively exhibit such effects, the amount of B should be 0.0010% or more. The lower limit of the B content is preferably 0.0012% or more, more preferably 0.0014% or more. However, if the amount of B becomes excessive, the effect is saturated and the cost only increases, so the amount of B is made 0.0040% or less. The upper limit of the amount of B is preferably 0.0030% or less.

The basic components of the high-strength steel sheet of the present invention are as described above, and the balance is substantially iron. However, it is naturally permissible for steel to contain impurities that are unavoidably brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. Such unavoidable impurities include, for example, N, O, etc. in addition to P and S described above, and it is preferable that each of these is within the following range.

(N: 0.0100% or less)
N is inevitably present as an impurity element and may cause cracks during processing. For these reasons, the N content is preferably 0.0100% or less, more preferably 0.0060% or less, and even more preferably 0.0050% or less. The smaller the amount of N, the better, but it is difficult in terms of industrial production to make it 0%.

(O: 0.0020% or less)
O is inevitably present as an impurity element and may cause cracks during processing. For these reasons, the O content is preferably 0.0020% or less, more preferably 0.0015% or less, and even more preferably 0.0010% or less. The smaller the amount of O, the better, but it is difficult in terms of industrial production to make it 0%.

The high-strength steel sheet of the present invention may contain elements such as Cu, Ni, Cr, Mo, V, Nb, and Ca within the ranges shown below, if necessary. The properties of the steel sheet are further improved. These elements can be contained singly or in appropriate combination within the ranges shown below.

(Cu: more than 0%, 0.30% or less)
Cu is an element effective in improving the corrosion resistance of the steel sheet, and may be contained as necessary. Although the effect increases as the content increases, the Cu content is preferably 0.03% or more, more preferably 0.05% or more, in order to effectively exhibit the above effect. However, when the amount of Cu becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the amount of Cu is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.15% or less.

(Ni: more than 0%, 0.30% or less)
Ni is an element effective for improving the corrosion resistance of the steel sheet, and may be contained as necessary. Although the effect increases as the Ni content increases, the Ni content is preferably 0.03% or more, more preferably 0.05% or more, in order to effectively exhibit the above effect. However, when the amount of Ni becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the Ni content is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.15% or less.

(Mo: more than 0%, 0.30% or less)
Mo is an element that contributes to increasing the strength of the steel sheet, and may be contained as necessary. Although the effect increases as the content increases, the Mo content is preferably 0.03% or more, more preferably 0.05% or more, in order to effectively exhibit the above effect. However, if the amount of Mo becomes excessive, the effect will be saturated and the cost will increase. Therefore, the upper limit of Mo content is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.20% or less.

(V: more than 0%, 0.30% or less)
V is an element that contributes to increasing the strength of the steel sheet, and may be contained as necessary. Although the effect increases as the content increases, the V content is preferably 0.05% or more, more preferably 0.010% or more, in order to effectively exhibit the above effect. However, if the amount of V becomes excessive, the effect saturates and the cost increases. Therefore, the upper limit of the V amount is preferably 0.30% or less, more preferably 0.25% or less, still more preferably 0.20% or less, and even more preferably 0.15% or less. is.

(Nb: more than 0%, 0.040% or less)
Nb is an element that contributes to increasing the strength of the steel sheet, and may be contained as necessary. Although the effect increases as the content increases, the Nb content is preferably 0.003% or more, more preferably 0.005% or more, in order to effectively exhibit the above effect. However, an excessive amount of Nb degrades bendability. Therefore, the upper limit of the Nb content is preferably 0.040% or less, more preferably 0.035% or less, and still more preferably 0.030% or less.

(Ca: more than 0%, 0.0050% or less)
Ca is an element effective in spheroidizing sulfides in steel and improving bendability, and may be contained as necessary. Although the effect increases as the Ca content increases, the Ca content is preferably 0.0005% or more, more preferably 0.0010% or more, in order to effectively exhibit the above effects. However, when the amount of Ca becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the amount of Ca is preferably 0.0050% or less, more preferably 0.0030% or less, and still more preferably 0.0025% or less.

Next, a method for producing the high-strength steel sheet of the present invention will be described.

The high-strength steel sheet of the present invention, which satisfies the above requirements, can be obtained by appropriately controlling each step of hot rolling, cold rolling and annealing (heating, soaking and cooling), especially the annealing step after cold rolling. can be manufactured. Hereinafter, the conditions for producing the high-strength steel sheet of the present invention will be described in the order of hot rolling, cold rolling, and subsequent annealing.

The conditions for hot rolling are, for example, as follows.

[Hot rolling conditions]
If the heating temperature before hot rolling is low, it may be difficult for carbides such as TiC to form a solid solution in austenite. Therefore, the heating temperature before hot rolling is preferably 1200° C. or higher. This heating temperature is more preferably 1250° C. or higher. However, if the heating temperature before hot rolling becomes too high, the cost increases. Therefore, the upper limit of the heating temperature before hot rolling is preferably 1350°C or lower, more preferably 1300°C or lower.

If the finish rolling temperature of hot rolling is low, the deformation resistance during rolling increases, which may make operation difficult. Therefore, the finish rolling temperature is preferably 850° C. or higher. The finish rolling temperature is more preferably 870° C. or higher. However, if the finish rolling temperature is too high, scratches due to scale may occur. Therefore, the upper limit of the finish rolling temperature is preferably 980°C or lower, more preferably 950°C or lower.

Considering productivity, the average cooling rate from finish rolling to coiling in hot rolling is preferably 10° C./second or more, more preferably 20° C./second or more. On the other hand, if the average cooling rate is too high, the steel may be hardened, making subsequent cold rolling difficult. Therefore, the average cooling rate is preferably 100° C./second or less, more preferably 50° C./second or less.

[Hot rolling coiling temperature: 620°C or higher]
If the hot-rolling coiling temperature is less than 620°C, the strength of the hot-rolled steel sheet increases, and it may become difficult to reduce the steel sheet by cold rolling. Therefore, the coiling temperature during hot rolling is preferably 620° C. or higher, more preferably 630° C. or higher, and still more preferably 640° C. or higher. On the other hand, if the coiling temperature during hot rolling becomes too high, the scale becomes thicker and the pickling property deteriorates. Therefore, the winding temperature is preferably 750° C. or lower, more preferably 700° C. or lower.

[Reduction during cold rolling: 10% or more and 70% or less]
The hot-rolled steel sheet is pickled for scale removal and subjected to cold rolling. If the rolling reduction during cold rolling (synonymous with "rolling reduction") is less than 10%, it becomes difficult to ensure a predetermined thickness tolerance. In order to obtain a steel plate of a specified thickness, the thickness must be reduced in the hot rolling process. do. Therefore, the rolling reduction during cold rolling is preferably 10% or more. It is more preferably 20% or more, still more preferably 25% or more. On the other hand, if the rolling reduction during cold rolling exceeds 70%, the possibility of cracking during cold rolling increases. Therefore, the upper limit of the rolling reduction during cold rolling is preferably 70% or less. It is more preferably 65% or less, still more preferably 60% or less.

In order to obtain the high-strength steel sheet of the present invention, it is recommended to appropriately control the annealing process after cold rolling. In this annealing step, the following (a) soaking step at 900 ° C. or higher after heating, (b) a first cooling step from 900 ° C. to 540 ° C. following the step (a), ( c) a second cooling step from 540° C. to 440° C. following the step (b); (d) a third cooling step from 440° C. to 280-230° C.; (e) from 230° C. a fourth cooling step to 50° C. or below. The high-strength steel sheet of the present invention can be obtained by manufacturing including these steps.

The high-strength steel sheet of the present invention also includes those having hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets on their surfaces. In the second cooling step from to 440° C., the immersion treatment in molten zinc and the subsequent heat treatment for alloying zinc and iron may be performed together.

The heat pattern of the annealing process including the above steps (a) to (e) is shown in the schematic diagram of FIG.

(a) Soaking process at 900° C. or higher after heating Heat to 900° C. or higher and hold at 900° C. or higher for 20 seconds or longer. If the soaking temperature is less than 900°C, soft ferrite that reduces the yield strength and tensile strength may be produced. Therefore, the lower limit of the temperature is made 900° C. or higher. It is preferably 905° C. or higher, more preferably 910° C. or higher. Although the upper limit of the soaking temperature is not particularly set, it is preferably 1000° C. or less in order to deteriorate the productivity. It is more preferably 980° C. or lower, still more preferably 960° C. or lower.

Even if the soaking temperature is 900° C. or higher, if the holding time at 900° C. or higher is less than 10 seconds, ferrite may be generated. Therefore, the holding time at 900° C. or higher is set to 10 seconds or longer. It is preferably 15 seconds or more, more preferably 20 seconds or more. The upper limit of the holding time is not particularly set, but it is preferably 200 seconds or less, more preferably 100 seconds or less, because productivity deteriorates.

(b) First cooling step from 900°C to 540°C The average cooling rate in the first cooling step from 900°C to 540°C is 10°C/second or more and 50°C/second or less. If this average cooling rate is less than 10° C./sec, the possibility of ferrite formation increases, making it difficult to ensure desired yield strength and tensile strength. Therefore, the average cooling rate should be 10° C./second or more, preferably 11° C./second or more, and more preferably 12° C./second or more. On the other hand, if the average cooling rate exceeds 50° C./second, it becomes difficult to control the steel sheet temperature, and the facility cost increases. Therefore, the upper limit of the average cooling rate should be 50° C./second or less, preferably 40° C./second or less, and more preferably 30° C./second or less.

(c) Second cooling step from 540°C to 440°C The average cooling rate to the cooling stop temperature in the second cooling step below 540°C must be 0.5°C/second or more. If the average cooling rate in the second cooling step is less than 0.5°C/sec, there is concern that bainite will increase. Therefore, the average cooling rate is set to 0.5° C./second or more. It is preferably 0.8° C./second or more. Although the upper limit of the average cooling rate is not particularly set, it is preferably 50° C./second or less because it is necessary to remarkably increase the capacity of the equipment. It is more preferably 40° C./second or less, still more preferably 30° C./second or less.

The cooling stop temperature in the second cooling step should be 440° C. or higher. If the cooling stop temperature in the second cooling step is less than 440°C, the yield strength and tensile strength decrease due to an increase in bainite. Therefore, the lower limit of the cooling stop temperature in the second cooling step is set to 440° C. or higher. It is preferably 445° C. or higher, more preferably 450° C. or higher.

Although three types of cooling patterns in the first cooling step are shown in FIG. 1, this indicates that any cooling pattern may be used as long as the above average cooling rate can be secured. . In short, if the temperature range from 540° C. to 440° C. is passed within 200 seconds, an average cooling rate of 0.5° C./second or more can be secured.

In the case of hot-dip galvanizing, the average cooling rate including immersion treatment in the plating bath→alloying heat treatment in the second cooling step must satisfy the above conditions. The temperature of the steel sheet before immersion in the plating bath is preferably in the range of over 440° C. to 480° C. or less.

After the immersion treatment in the molten zinc, if necessary, a heat treatment for alloying zinc and iron is performed. In this alloying heat treatment, it is necessary to set the temperature (alloying heat treatment temperature) to 440° C. or higher and 540° C. or lower in order to ensure plating performance. If this temperature is less than 440° C., zinc plating and diffusion of iron are insufficient, and an alloyed hot-dip galvanized layer cannot be formed. Therefore, the lower limit of the alloying heat treatment temperature is set to 440° C. or higher. It is preferably 445° C. or higher, more preferably 450° C. or higher. On the other hand, if the alloying heat treatment temperature exceeds 540 ° C., the possibility of ferrite formation increases, and in addition to the decrease in tensile strength, the diffusion of iron into zinc becomes excessive, resulting in an alloy that is brittle and easily peeled off. It becomes a hot-dip galvanized layer, and there is a high possibility that the plating will peel off during press molding or the like.

(d) Third cooling step from 440° C. to 280-230° C. The average cooling rate to the cooling stop temperature in the third cooling step should be 5.0° C./second or more. If the average cooling rate in the third cooling step is less than 5.0°C/sec, there is concern that bainite will increase. Moreover, even if the formation of bainite is suppressed, the distribution of carbon from the martensite formed after passing through the Ms point to the retained austenite progresses, which stabilizes the martensite and reduces the amount transformed into martensite. As a result, more than 7% of retained austenite tends to be contained, so the average cooling rate is set to 5.0° C./second or more.

The Ms point is the temperature at which martensite starts to transform, and based on the following formula (I) described in "Iron and Steel Materials" (published by the Japan Institute of Metals, p.45), from the chemical composition of the steel sheet can be obtained easily. [ ] in the following formula (I) indicates the content (% by mass) of each element, and the element not contained in the steel sheet is calculated as 0%.
Ms point (°C) = 550 - 361 [C] - 39 [Mn] - 35 [V] - 20 [Cr] -
17 [Ni]-10 [Cu]-5 ([Mo] + [W]) + 15 [Co] + 30 [Al] (I)

The average cooling rate is preferably 15.0° C./second or higher, more preferably 20° C./second or higher. Although the upper limit of the average cooling rate at this time is not particularly set, it is preferably 50° C./sec or less because it is necessary to remarkably increase the facility capacity in order to increase the average cooling rate excessively. It is more preferably 40° C./second or less, still more preferably 30° C./second or less.

The cooling stop temperature in the third cooling step should be 230° C. or higher and 280° C. or lower. If the cooling stop temperature in the third cooling step is less than 230°C, martensite self-tempers excessively, and the number of laths in martensite decreases, possibly resulting in a decrease in tensile strength. Therefore, the lower limit of the cooling stop temperature in the third cooling step is set to 230° C. or higher. It is preferably 240° C. or higher, more preferably 250° C. or higher.

On the other hand, if the cooling stop temperature in the third cooling step exceeds 280°C, bainite will increase, which may lead to a decrease in yield strength and tensile strength. Therefore, the upper limit of the cooling stop temperature in the third cooling step is set to 280° C. or less. It is preferably 275° C. or lower, more preferably 270° C. or lower.

(e) Fourth cooling step from 230°C to 50°C or less In the fourth cooling step that follows the third cooling step, the average cooling rate from 230°C to the cooling stop temperature of 50°C or less is 3. 0° C./second or less is preferable. When the cooling stop temperature in the third cooling step is higher than 230°C, the average cooling rate from the cooling stop temperature in the third cooling step to 230°C does not matter.

The presence of an appropriate amount of film-like austenite at the lath boundaries enhances the effect of dislocation migration barriers, and is considered preferable for ensuring yield strength and tensile strength. If the average cooling rate in the fourth cooling step is higher than 3.0° C./sec, the retained austenite will be less than 1% by volume, and the effect as a dislocation migration barrier will be less likely to be exhibited. Therefore, the average cooling rate is set to 3.0° C./sec or less. It is preferably 2.5° C./second or less, more preferably 2.0° C./second or less. Although the lower limit of the average cooling rate at this time is not particularly set, it is preferably 0.05° C./second or more because the productivity deteriorates. More preferably, it is 0.10° C./second or more.

The high-strength steel sheet of the present invention is not limited to those obtained by the above manufacturing method. The high-strength steel sheet of the present invention may be obtained by other manufacturing methods as long as it satisfies the requirements specified in the present invention.

In the high-strength steel sheet of the present invention, the chemical composition is adjusted as described above, and martensite: 93% by volume or more, bainite: 2% by volume or less, and retained austenite: 7% by volume or less with respect to the entire metal structure. And, in the SEM image of the metal structure, the number of laths measured by a cutting method with a total length of 300 μm is 240 or more, and the tensile strength is 1470 MPa or more. Such a high-strength steel sheet has a tensile strength of 1470 MPa or more and a yield strength of 1000 MPa or more.

The tensile strength of the high-strength steel sheet of the present invention is preferably 1500 MPa or more, more preferably 1550 MPa or more. The higher the tensile strength, the better, and although the upper limit is not particularly limited, it is usually about 1800 MPa. Yield strength is preferably 1020 MPa or more, more preferably 1040 MPa or more. The higher the yield strength, the better, and although the upper limit is not particularly limited, it is usually about 1400 MPa.

The high-strength steel sheet of the present invention has sufficiently high yield strength and tensile strength without temper rolling, but it is possible to achieve even higher yield strength by performing temper rolling.

The high-strength steel sheet of the present invention includes those having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the steel sheet, but the type of the galvanized layer at this time is not particularly limited. It may contain an alloying element. Also, the galvanized layer is coated on one side or both sides of the steel sheet.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples, and it is also possible to implement it by adding changes within the scope that can conform to the gist of the above and below. All of them are included in the technical scope of the present invention.

Experimental slabs having chemical compositions (steel types: steels A, B, and C) shown in Table 1 below were produced. The slab was heated to 1250° C. and hot rolled to a plate thickness of 2.8 mm to 3.1 mm. The hot rolling was performed at a finish rolling temperature of 900°C, an average cooling rate of 20°C/sec from finish rolling to coiling, and a coiling temperature of 650°C. After pickling the obtained hot-rolled steel sheet, the thickness was reduced to 1.4 mm to 2.6 mm by combining surface grinding or cold rolling. At this time, the cold rolling rate (rolling rate at the time of cold rolling) of any kind of steel is in the range of 10% to 60%. In Table 1, columns with "-" indicate that no addition was made, and columns with "<" indicate that the amount was below the limit of measurement. Moreover, P, S, N, and O are unavoidable impurities as described above, and the values shown in the columns of P, S, N, and O mean amounts unavoidably contained. In addition, the balance includes iron and inevitable impurities other than the inevitable impurities shown above.

After that, the obtained cold-rolled steel sheets were annealed by heat treatments (heat treatments 1 to 3) whose heat patterns are shown in FIGS. Specifically, steel types A and B were subjected to heat treatments 1 to 3. Other steel type C was subjected to heat treatment 1.

Detailed data for the heat treatments shown in FIGS. 3-5 are shown in Tables 2-4 below. That is, the heat pattern shown in FIG. 3 is based on the data shown in Table 2 below (heat treatment 1), and the heat pattern shown in FIG. 4 is based on the data shown in Table 3 below ( Heat treatment 2), the heat pattern shown in FIG. 5 is based on the data shown in Table 4 below (heat treatment 3). Note that "s" shown in FIGS. 3 to 5 means "seconds". Tables 2-4 also show the corresponding steps [(a)-(e)] of FIG.

In the heat treatments 1 to 3 shown in FIGS. 3 to 5, hot-dip galvanizing treatment and alloying heat treatment are not performed in the second cooling step [step (c) shown in FIG. 1]. The "steps" shown in Tables 2 to 4 below indicate actual measurement positions that sequentially indicate numerical values (set temperature, cooling rate) corresponding to FIGS. The step positions shown in are partially omitted. In addition, the cooling rate indicated by minus in Tables 2 to 4 indicates the heating rate (heating rate).

In Tables 2 to 4, there are places where the average cooling rate in the temperature range specified in the above steps (a) to (c) is not specified, but these values are shown in the data in Tables 2 to 4. can be calculated based on [ The average cooling rate in the step (b)] is 12.9° C./second [≈(900° C.-540° C.)/(158 seconds-130 seconds)].

Also, if the passage time for the steel plate temperature to reach 440°C is calculated in Table 2, it is "252 seconds", and the average cooling rate from 540°C to 440°C [the average cooling rate in the step (c)] is 1. 06° C./second [≈(540° C.−440° C.)/(252 seconds−158 seconds)]. Similarly, if the average cooling rate from 440 ° C. to 280 ° C. [average cooling rate in the step (d)] is calculated, it is 20.0 ° C./sec [= (440 ° C.-280 ° C.) / (260 seconds −252 seconds)].

For each steel sheet thus obtained, the volume fraction of martensite, the volume fraction of bainite, the volume fraction of retained austenite, the number of laths per 300 μm in total length, and tensile properties were measured according to the following procedures.

[Fraction of each structure in the metal structure]
In this example, the fractions of martensite, bainite, and retained austenite existing in 1/4 part of the plate thickness of the steel plate were measured as follows. According to the manufacturing method of the present example, the possibility that structures other than the above (for example, ferrite and pearlite) exist in each region is extremely low, so structures other than the above were not measured. Therefore, it was calculated so that the total of martensite, bainite, and retained austenite was 100% by volume in 1/4 part of the plate thickness of the steel plate.

[Volume fraction of retained austenite]
Retained austenite is obtained by cutting out a test piece of 1.4 mm × 20 mm × 20 mm from the steel plate after annealing, grinding it to 1/4 part of the plate thickness, chemically polishing it, and then using an X-ray diffraction method to obtain retained austenite (hereinafter referred to as described as "retained γ") was measured (ISIJ Int. Vol. 33. (1993), No. 7, P. 776). A two-dimensional X-ray diffractometer "RINT-PAPID II" (trade name: manufactured by Rigaku Corporation) is used as the measuring device, and the measuring surface is about 1/4 part of the plate thickness. Co was used as the target, and one measurement was performed for each test.

[Volume ratio of martensite and bainite]
Bainite and martensite were measured by the point counting method as follows. First, a test piece of 1.4 mm × 20 mm × 20 mm was cut out from the above steel plate, the cross section parallel to the rolling direction was polished, and nital corrosion was applied. Observation was made with a Field Emission Scanning Electron Microscope photograph (magnification: 3000 times). Observation was carried out using a grid of 0.3 μm intervals on the FE-SEM image, bainite and martensite were distinguished based on the color of the grains, and the respective volume fractions were measured. As for the measurement points, the tissue at the point where the grid intersects at right angles was separated, 100 points were investigated, and the fraction was calculated. Measurement was performed for each field of view.

Specifically, in the SEM photograph after nital corrosion, the structure that looks black is bainite, and the remaining portion is martensite. FIG. 6 (photograph substituting for a drawing) shows an example of a metal structure showing bainite and martensite.

As described in detail above, in this example, retained austenite and other structures (bainite, martensite) are measured by different methods, so the total of these structures is not necessarily 100% by volume. Not exclusively. Therefore, when determining the volume fractions of bainite and martensite, adjustments were made so that the total of all structures would be 100% by volume. Specifically, the fraction of bainite and martensite measured by the point counting method is proportionally distributed to the value obtained by subtracting the fraction of retained austenite measured by the X-ray diffraction method from 100% by volume. Finally, the volume fractions of bainite and martensite were determined.

[Number of laths per total length of 300 μm]
The number of laths with a total length of 300 μm was measured by photographing a cross section parallel to the rolling direction at 1/4 part of the plate thickness of the steel plate subjected to nital corrosion at a magnification of 3000 with an FE-SEM, and cutting the total length of 300 μm. Measured. The cutting method is usually a method for measuring particle size (JIS G 0551:2013), but in this example it was applied as a method for measuring the number of laths. Specifically, a line with a total length of 300 μm was drawn on the FE-SEM image, and the number of times the line passed over the lath (the number of crossing points) was measured. A lath is defined as an area of 1 μm or more in a white portion in a FE-SEM image of a steel plate subjected to nital corrosion taken at a magnification of 3000 times. FIG. 2 [FIGS. 2(a) and 2(b)] schematically shows the state when the number of laths is measured by the cutting method.

[Tensile properties]
For tensile strength TS and 0.2% yield strength σ _0.2 , JIS No. 5 test piece (plate-shaped test piece ) was collected and tested according to JIS Z 2241:2011.

Regarding acceptance criteria, a tensile strength TS of 1470 MPa or more and a yield strength (0.2% proof stress σ _0.2 ) of 1000 MPa or more were considered acceptable.

These results are shown in Table 5 below together with applicable steel grades (steel grades A, B, and C in Table 1) and heat treatment conditions (heat treatments 1 to 3).

From this result, it can be considered as follows. Test no. 4 and 7 are examples manufactured under appropriate heat treatment conditions (heat treatment 1 shown in FIG. 3) using steel grades (steel grades B and C in Table 1) that satisfy the chemical composition specified in the present invention. In this example, the fraction of each structure in the metal structure and the number of laths per 300 μm in total length are appropriately adjusted, the yield strength (0.2% proof stress σ _0.2 ) is 1000 MPa or more, and the tensile strength TS is 1470 MPa or more, it can be seen that the acceptance criteria are satisfied.

On the other hand, Test No. 1 to 3, 5, and 6 are comparative examples that do not satisfy any of the requirements defined in the present invention, and do not satisfy any of the properties of the steel sheet.

Specifically, Test No. 1 is an example using a steel type (steel type A in Table 1) which is manufactured under appropriate heat treatment conditions (heat treatment 1 shown in FIG. 3) but does not satisfy the chemical composition specified in the present invention. In this example, since a Cr-free steel type was used, the bainite was excessive and the number of laths per 300 μm in total length was reduced, resulting in a decrease in yield strength.

Test no. No. 2 is an example manufactured under inappropriate heat treatment conditions (heat treatment 2 shown in FIG. 4) using a steel type (steel A in Table 1) that does not satisfy the chemical composition specified in the present invention. In this example, a steel type containing no Cr is used, and the average cooling rate in the third cooling step [step (d) shown in FIG. 1] is not 5.0° C./sec or more ( Steps 6 to 13) in Table 3, the amount of retained austenite increased, and the yield strength and tensile strength TS decreased.

Test no. 3 is an example in which a steel type (steel type A in Table 1) that does not satisfy the chemical composition specified in the present invention was used and manufactured under inappropriate heat treatment conditions (heat treatment 3 shown in FIG. 5). In this example, a steel type containing no Cr is used, and the cooling stop temperature in the third cooling step [step (d) shown in FIG. 1] is set to 100 ° C. (step 9 in Table 4). As the number of laths per 300 μm in total length decreased, the tensile strength TS decreased.

On the other hand, Test No. 5 and 6 use a steel grade (steel grade B in Table 1) that satisfies the chemical composition specified in the present invention, but the heat treatment conditions are out of the appropriate range (heat treatment 2 shown in FIG. 5), the desired properties were not obtained.

Specifically, Test No. 5 is an example where the average cooling rate in the third cooling step [step (d) shown in FIG. 1] is not 5.0° C./sec or more (steps 6 to 13 in Table 4), The amount of retained austenite increased, and the yield strength and tensile strength TS decreased.

Test no. 6 is an example in which the cooling stop temperature in the third cooling step [step (d) shown in FIG. As a result, the tensile strength TS decreased.

Claims (3)

in % by mass,
C: 0.200 to 0.280%,
Si: 0.40 to 1.50% or less,
Mn: 2.00-3.00%,
P: more than 0%, 0.015% or less,
S: more than 0%, 0.0050% or less,
Al: 0.015-0.060%,
Cr: 0.20 to 0.80%,
Ti: 0.015 to 0.080%, and B: 0.0010 to 0.0040%,
respectively, the balance being iron and inevitable impurities,
With respect to the entire metal structure, martensite is 93% by volume or more, ferrite, pearlite and bainite are 2% by volume or less in total, and retained austenite is 7% by volume or less,
Further, in an image obtained by observing the metal structure with a scanning electron microscope, the number of laths in martensite measured by a cutting method for a total length of 300 μm is 240 or more, and the tensile strength is 1470 MPa or more. high-strength steel plate. Furthermore, in mass %, Cu: more than 0% and 0.30% or less, Ni: more than 0% and 0.30% or less, Mo: more than 0% and 0.30% or less, V: more than 0% and 0.30%. The high-strength steel sheet according to claim 1, containing one or more selected from the group consisting of 30% or less, Nb: more than 0% and 0.040% or less, and Ca: more than 0% and 0.0050% or less. A high-strength galvanized steel sheet comprising a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the high-strength steel sheet according to claim 1 or 2.

Images