KR20200128159A - High strength steel plate and high strength galvanized steel plate - Google Patents

Abstract

The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition composition, contains 93% by volume or more of martensite, and 2% by volume or less in total of ferrite, pearlite and bainite, Retained austenite is 7% by volume or less, and in the image obtained by observing the metal structure with a scanning electron microscope, the number of laths in martensite measured by a cutting method with a total length of 300 μm is 240 or more, and the tensile strength is It is characterized in that it is 1470 MPa or more.

Description

High strength steel plate and high strength galvanized steel plate

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 structural members of automobiles are required to have higher strength in order to realize improved fuel economy. In addition, when a high-strength steel sheet is applied to a structural member of an automobile, from the viewpoint of collision safety, the high-strength steel sheet is required to have high shock absorption energy.

A high tensile strength TS (Tensile Strength) of a high-strength steel sheet, and it is known that the higher the 0.2% proof stress σ _{0 .2} or a yield point UYP (Upper Yield Point), increasing the impact absorption energy. From this point of view, it is required that a steel sheet applied to a structural member of an automobile has a tensile strength TS of 1470 MPa or more, and a 0.2% proof strength or an upper yield point UYP of 1000 MPa or more. Hereinafter, the tensile strength TS may be abbreviated as "tensile strength", and 0.2% proof strength or upper yield point UYP as "yield strength".

Among the above required characteristics, as a technique for improving the tensile strength of a high-strength steel sheet, for example, a technique such as Patent Document 1 has been proposed. In this patent document 1, the respective fractions of auto-tempered martensite, ferrite, bainite, and retained austenite are controlled, and the size and number of precipitates of iron-based carbides in the auto-tempered martensite are regulated, so that tensile strength and It is disclosed that processability can be improved.

However, in this technique, only the tensile strength and workability are examined, and the yield strength is not considered. In addition, in this technique, the yield strength is measured after 0.3% temper rolling. Although the yield strength can be increased by temper rolling, in the case of an ultra-high strength steel sheet of 1470 MPa or more, a sufficient elongation rate may not necessarily be ensured by temper rolling in some cases.

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

Japanese Patent No. 5365216

High-strength steel sheet according to an aspect of the present invention,

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% and less than 0.015%,

S: more than 0% and less than 0.0050%,

Al: 0.015 to 0.060%,

Cr: 0.20 to 0.80%,

Ti: 0.015 to 0.080%,

B: 0.0010 to 0.0040%

Each containing, the balance being iron and unavoidable 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,

In the image obtained by observing the metal structure with a scanning electron microscope, the number of laths in martensite measured by a cutting method of 300 μm in total length is 240 or more, and

It is characterized in that the tensile strength is 1470 MPa or more.

1 is a schematic diagram showing a heat pattern in an annealing process.
2 is an explanatory view when measuring the number of laths by a cutting method.
3 is a schematic diagram showing a heat pattern in heat treatment 1 in an example.
4 is a schematic diagram showing a heat pattern in heat treatment 2 in an example.
5 is a schematic diagram showing a heat pattern in heat treatment 3 in an example.
Fig. 6 is a microscopic photograph for a drawing showing an example of a structure in the high-strength steel sheet of the present embodiment.

In order to provide a high-strength steel sheet having a tensile strength of 1470 MPa or more and having a high yield strength, the inventors of the present invention have determined the amount of bainite, martensite, and retained austenite, as well as lath, which is a substructure of bainite and martensite. Paying attention to it, it has been reviewed carefully.

As a result, the chemical composition composition of the steel sheet, the volume fraction of martensite, the volume fraction of bainite, etc. (including ferrite and pearlite), the volume fraction of retained austenite, and a scanning electron microscope (SEM: Scanning Electron Microscope) In the image observed by the method (hereinafter, sometimes referred to as ``SEM phase''), the number of laths in martensite measured by a cutting method of 300 μm in total length is defined as described below, respectively, to achieve the above object. The present invention was completed by discovering this and further researching based on the knowledge. On the other hand, when referred to as "high strength" hereinafter, it is used for the purpose of "the strength level at which the tensile strength is 1470 MPa or more".

Rath is a substructure of martensite. The structure of martensite is multi-layered, so in one old austenite grain, there are a plurality of packets, which are a set of particles having the same front wall surface, and inside each packet, there are blocks, which are parallel band-shaped regions. In addition, each block has a set of laths, which are martensite crystals containing high-density transitions with almost the same crystal orientation.

The number of laths in martensite (hereinafter sometimes referred to as ``the number of laths per 300 μm total length'') measured by the cutting method in a total length of 300 μm as defined in the present invention is the thickness of the steel sheet subjected to nital corrosion 1 In part /4, the cross section parallel to the rolling direction was photographed at 3000 times with a field emission scanning electron microscope (FE-SEM), and the total length was measured by a cutting method of 300 μm.

The present inventors considered that lath in martensite affects the yield strength and tensile strength, and repeated intensive studies. As a result, it was found that satisfying the requirements described later for the number of laths per 300 μm in total length is important to achieve both high yield strength and tensile strength. Hereinafter, an embodiment of the present invention will be described in detail.

[Number of laths per 300μm total length: 240 or more]

In the high-strength steel sheet of this embodiment, it is necessary that the number of laths per 300 μm in total length be 240 or more. When the number of laths is less than 240, the yield strength or tensile strength decreases. The reason is not necessarily clear, but you can think of it as follows. First, in addition to having the effect of increasing the yield strength by interfering with the motion of the lath and the lath, in the chemical composition system of this embodiment, iron-based carbides such as fine cementite and residual austenite in the form of a film are present at the boundary of the lath. Thus, there is a possibility that it becomes an additional barrier to the motion of the dislocation. From the above, it is considered that the more lath per predetermined length, the higher the yield strength and the tensile strength are. The lower limit of the number of laths is preferably 245 or more, and more preferably 250 or more. About the upper limit of the number of laths, it is approximately 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 embodiment. By setting this martensite to 93% by volume or more with respect to the entire metal structure, the yield strength and tensile strength can be increased. When the amount of martensite is less than 93% by volume, other soft tissues start plastic deformation under low stress, and the yield strength decreases. The lower limit of martensite is preferably 94% by volume or more, and more preferably 95% by volume or more. The upper limit of martensite is generally 99% by volume or less. Martensite includes tempered martensite and self-annealed martensite, but when excessively tempered, the number of laths per 300 μm in total length is less than 240, so it is not included in the martensite targeted in the present embodiment.

[Ferrite, pearlite and bainite: 2% by volume or less in total]

Compared with martensite which is a base structure, these structures are soft, and when these structures increase, these structures themselves start plastic deformation at low stress, and yield strength and tensile strength are lowered. From this viewpoint, ferrite, pearlite, and bainite need to be 2% by volume or less in total with respect to the entire metal structure. The upper limit of these structures is preferably 1.5% by volume or less, and more preferably 1.0% by volume or less. The lower limit of bainite may be 0% by volume. In the following, unless otherwise specified, ferrite, pearlite and bainite are represented by "bainite".

[Residual austenite: 7% by volume or less]

About the retained austenite in the metal structure, it is necessary to set it as 7 volume% or less with respect to the whole metal structure. A small amount of film-like retained austenite present at the lath boundary may have an effect of increasing tensile strength and yield strength by suppressing the movement of dislocations. However, since retained austenite itself is softer than a martensite structure, even if it is in the form of a film, if excessively present, the yield strength and tensile strength are also lowered. From this point of view, the retained austenite needs to be 7 vol% or less. The upper limit of retained austenite is preferably 6% by volume or less, and 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 this embodiment, in addition to defining the number of laths, martensite volume fraction, bainite volume fraction, and retained austenite volume fraction as described above, it is necessary to appropriately define the chemical composition of the steel sheet. . The reason for setting these ranges is as follows. In addition, "%" in the following chemical component composition all means "mass%".

(C: 0.200 to 0.280%)

C is an element necessary to secure the strength of the steel sheet. If the amount of C is insufficient, the tensile strength of the steel sheet decreases. Therefore, the amount of C is made 0.200% or more. The lower limit of the amount of C is preferably 0.205% or more, and more preferably 0.210% or more. However, when C is added excessively, the volume ratio of retained austenite increases more than 7% by volume, and there is a fear that a decrease in yield strength may be caused. 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 even 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. In addition, it is considered that there is an effect in securing fine lath by suppressing excessive tempering of martensite. In order to effectively exhibit such an effect, the amount of Si needs to be 0.40% or more. The lower limit of the amount of Si is preferably 0.50% or more, and more preferably 0.60% or more. It is more preferably 0.70% or more, and even more preferably 0.80% or more. However, when the amount of Si becomes excessive, the volume ratio of retained austenite increases, and there is a fear of causing a decrease in yield strength. Therefore, the upper limit of the amount of Si is set to 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 the increase in strength of the steel sheet, and is necessary in order to suppress the formation of ferrite and bainite to obtain a target martensite-based structure. In order to effectively exhibit such an effect, the amount of Mn needs to be 2.00% or more. The lower limit of the amount of Mn is preferably 2.05% or more, and more preferably 2.10% or more. However, when the Mn amount becomes excessive, there is a concern that slab breakage, an increase in cold rolling load, and the like may occur. Therefore, the upper limit of the amount of Mn is set to 3.00% or less. The upper limit of the amount of Mn is preferably 2.90% or less, and 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% and less than 0.015%)

P is an element that is inevitably included, and is an element that segregates at grain boundaries and promotes grain boundary embrittlement. Therefore, it is recommended to reduce it as much as possible in order to avoid fracture during processing. 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, and more preferably 0.010% or less. On the other hand, P is an impurity that is unavoidably mixed in steel, and it is impossible for industrial production to make the amount 0%.

(S: more than 0% and less than 0.0050%)

Like P, S is also an element that is inevitably contained, and it is recommended that the amount of S be reduced as much as possible in order to generate inclusions and avoid fractures during processing. Therefore, the S amount is set to 0.0050% or less. The upper limit of the amount of S is preferably 0.0040% or less, and more preferably 0.0030% or less. On the other hand, S is an impurity that is unavoidably mixed in steel, so it is impossible for industrial production to make the amount 0%.

(Al: 0.015 to 0.060%)

Al is an element that acts as a deoxidizing agent. In order to effectively exhibit such an action, the amount of Al needs to be 0.015% or more. The lower limit of the amount of Al is preferably 0.025% or more, and more preferably 0.030% or more. However, when the amount of Al becomes excessive, a large amount of inclusions such as alumina are generated in the steel sheet, and there is a fear of causing fracture during processing. Therefore, the upper limit of the amount of Al is set to 0.060% or less. The upper limit of the amount of Al is preferably 0.055% or less, and more preferably 0.050% or less.

(Cr: 0.20 to 0.80%)

Cr is necessary in order to suppress the formation of ferrite and bainite, and to obtain a target martensite-based structure. In addition, it is thought that it has the effect of making the lath finer by suppressing excessive tempering of martensite. In order to effectively exhibit such an effect, the amount of Cr needs to be 0.20% or more. The lower limit of the amount of Cr is preferably 0.25% or more, and more preferably 0.30% or more. However, when the amount of Cr becomes excessive, when hot-dip galvanizing or alloying hot-dip galvanizing is formed on the surface of the steel sheet, non-plating may occur. Therefore, the upper limit of the amount of Cr was made 0.80% or less. The upper limit of the amount of Cr is preferably 0.75% or less, and more preferably 0.70% or less.

(Ti: 0.015 to 0.080%)

Ti is an element that improves the strength of a steel sheet by forming carbides or nitrides. In addition, it is an element that is effective in effectively exhibiting the effect of improving hardenability by B described later. That is, Ti reduces N in the steel by forming nitride, and as a result, the formation of nitride B is suppressed, and B becomes a solid solution, so that the effect of improving the hardenability by B can be effectively exhibited. In this way, Ti contributes to increase in strength of the steel sheet by improving the hardenability. In order to effectively exhibit such an effect, the amount of Ti needs to be 0.015% or more. The lower limit of the amount of Ti is preferably 0.018% or more, and more preferably 0.020% or more.

However, when the amount of Ti becomes excessive, Ti carbide or Ti nitride becomes excessive, which may cause cracking during processing. Therefore, the upper limit of the amount of Ti is set to 0.080% or less. The upper limit of the amount of Ti 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 an effect of suppressing the formation of ferrite or bainite by improving hardenability. This is an element that contributes to the increase in strength of the steel sheet. In order to effectively exhibit such an effect, the amount of B needs to be 0.0010% or more. The lower limit of the amount of B is preferably 0.0012% or more, and more preferably 0.0014% or more. However, when the amount of B becomes excessive, the effect is saturated and the cost only increases, so the amount of B is set to 0.0040% or less. The upper limit of the amount of B is preferably 0.0030% or less.

The basic component of the high-strength steel sheet of this embodiment is as described above, and the balance is substantially iron. However, it is naturally permissible to contain impurities inevitably mixed in due to circumstances such as raw materials, materials, and manufacturing facilities. As such inevitable impurities, other than the above-described P and S, for example, N, O, etc. are included, and these are preferably in the following ranges.

(N: 0.0100% or less)

N is inevitably present as an impurity element, and may cause cracking during processing. From this point of view, the amount of N is preferably 0.0100% or less, more preferably 0.0060% or less, and still more preferably 0.0050% or less. The smaller the amount of N is, the more preferable it is, but it is difficult in industrial production to set it as 0%.

(O: 0.0020% or less)

O inevitably exists as an impurity element, and may cause cracking during processing. From this point of view, the amount of O is preferably 0.0020% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. The smaller the amount of O is, the more preferable it is, but it is difficult in industrial production to set it as 0%.

In the high-strength steel sheet of this embodiment, if necessary, elements such as Cu, Ni, Cr, Mo, V, Nb, and Ca may be contained in the ranges shown below, and the characteristics of the steel sheet may be further improved depending on the type of the contained element. Improves. These elements can be contained individually or in combination as appropriate in the ranges shown below, respectively.

(Cu: more than 0% and less than 0.30%)

Cu is an element effective in improving the corrosion resistance of the steel sheet, and may be contained as necessary. The effect increases as the content increases, but in order to effectively exhibit the above effect, the amount of Cu is preferably set to 0.03% or more, more preferably 0.05% or more. 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% and less than 0.30%)

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

(Mo: more than 0% and less than 0.30%)

Mo is an element contributing to the increase in strength of the steel sheet, and may be contained as necessary. The effect increases as the content increases, but in order to effectively exhibit the above effect, the amount of Mo is preferably 0.03% or more, and more preferably 0.05% or more. However, when the Mo amount becomes excessive, the effect becomes saturated and the cost increases. Therefore, the upper limit of the amount of Mo is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.20% or less.

(V: more than 0% and less than 0.30%)

V is an element that contributes to the increase in strength of the steel sheet, and may be contained as necessary. The effect increases as the content increases, but in order to effectively exhibit the above effect, the amount of V is preferably 0.05% or more, more preferably 0.010% or more. However, when the amount of V becomes excessive, the effect is saturated and the cost increases. Therefore, the upper limit of the amount of V is preferably 0.30% or less, more preferably 0.25% or less, further preferably 0.20% or less, and even more preferably 0.15% or less.

(Nb: more than 0% and less than 0.040%)

Nb is an element that contributes to increase the strength of the steel sheet, and may be contained as necessary. The effect increases as the content increases, but in order to effectively exhibit the above effect, the amount of Nb is preferably 0.003% or more, and more preferably 0.005% or more. However, when the amount of Nb becomes excessive, the bendability deteriorates. Therefore, the upper limit of the amount of Nb is preferably 0.040% or less, more preferably 0.035% or less, and still more preferably 0.030% or less.

(Ca: more than 0% and less than 0.0050%)

Ca is an element effective in spheroidizing sulfides in steel to increase bendability, and may be contained as necessary. The effect increases as the content increases, but in order to effectively exhibit the above effect, the Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more. However, when the Ca amount 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 of manufacturing the high-strength steel sheet of the present embodiment will be described.

The high-strength steel sheet of this embodiment that satisfies the above requirements can be produced by appropriately controlling the annealing step after cold rolling in each step of hot rolling, cold rolling, and annealing (heating, cracking, and cooling). Hereinafter, conditions for manufacturing the high-strength steel sheet of the present embodiment are described in the order of hot rolling, cold rolling, and subsequent annealing.

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

[Hot rolling conditions]

If the heating temperature before hot rolling is low, there is a concern that carbides such as TiC are difficult to be dissolved in austenite. Therefore, the heating temperature before hot rolling is preferably set to 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 less, and more preferably 1300°C or less.

When the finish rolling temperature of hot rolling is low, deformation resistance during rolling increases, and there is a fear that operation becomes difficult. Therefore, it is preferable that the finish rolling temperature is 850°C or higher. The finish rolling temperature is more preferably 870°C or higher. However, if the finish rolling temperature is too high, there is a concern that scratches due to scale may occur. Therefore, the upper limit of the finish rolling temperature is preferably 980°C or less, and more preferably 950°C or less.

In view of productivity, the average cooling rate of hot rolling from finish rolling to winding is preferably 10°C/sec or more, and more preferably 20°C/sec or more. On the other hand, when the average cooling rate becomes too fast, it may harden and subsequent cold rolling becomes difficult. Therefore, the average cooling rate is preferably 100°C/sec or less, and more preferably 50°C/sec or less.

[Hot-rolling coiling temperature: 620°C or higher]

When the hot-rolling take-up temperature is less than 620°C, the strength of the hot-rolled steel sheet increases, and there is a concern that it becomes difficult to reduce by cold rolling. Therefore, the coiling temperature at the time of 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, when the coiling temperature at the time of hot rolling becomes too high, the scale becomes thick and pickling property deteriorates. Therefore, the coiling temperature is preferably 750°C or less, and more preferably 700°C or less.

[Rolling rate during cold rolling: 10% or more and 70% or less]

The hot-rolled steel sheet is pickled in order to remove scale and is subjected to cold rolling. When the rolling rate (same as the "reduction rate") at the time of cold rolling is less than 10%, it becomes difficult to ensure a predetermined sheet thickness tolerance. In order to obtain a steel sheet having a predetermined thickness, it is necessary to make the sheet thickness thin in the hot rolling step, and when thinning in the hot rolling step, the length of the steel sheet is lengthened, so pickling takes time and productivity decreases. Therefore, it is preferable that the rolling rate at the time of cold rolling is 10% or more. It is more preferably 20% or more, and still more preferably 25% or more. On the other hand, if the rolling rate during cold rolling exceeds 70%, the possibility of cracking during cold rolling increases. Therefore, it is preferable that the upper limit of the rolling rate at the time of cold rolling is 70% or less. It is more preferably 65% or less, and still more preferably 60% or less.

In order to obtain the high-strength steel sheet of this embodiment, it is recommended to appropriately control the annealing step after cold rolling. In this annealing step, the following (a) a soaking step at 900°C or higher after heating, (b) a first cooling step from 900°C to 540°C performed following the step (a), (c) the above The second cooling step from 540°C to 440°C, which is performed following the step (b), (d) the third cooling step from 440°C to 280 to 230°C, (e) from 230°C to 50°C or less It basically includes a fourth cooling process. By manufacturing including such a process, the high-strength steel sheet of this embodiment is obtained.

In addition, the high-strength steel sheet of this embodiment includes those having a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet on the surface thereof, but in the case of manufacturing these galvanized steel sheets, the temperature from 540°C to 440°C in the above (c) In the second cooling step, immersion treatment in molten zinc and subsequent heat treatment for alloying zinc and iron may be performed in combination.

The heat pattern of the annealing step including each step of the above (a) to (e) is shown in the schematic diagram of Fig. 1, and will be described in more detail below.

(a) Cracking process at 900°C or higher after heating

It is heated to 900°C or higher, and maintained at 900°C or higher for 20 seconds or longer. When the soaking temperature is less than 900°C, there is a possibility that soft ferrite reducing yield strength and tensile strength may be produced. Therefore, the lower limit of the temperature is set to 900°C or higher. It is preferably 905°C or higher, and more preferably 910°C or higher. On the other hand, although the upper limit of the soaking temperature is not particularly provided, it is preferably 1000°C or less because productivity is deteriorated. More preferably, it is 980 degreeC or less, More preferably, it is 960 degreeC or less.

Further, even if the soaking temperature is 900°C or higher, if the holding time at 900°C or higher is less than 10 seconds, there is a possibility that ferrite is generated. Therefore, the holding time at 900°C or higher is 10 seconds or longer. It is preferably 15 seconds or more, and more preferably 20 seconds or more. On the other hand, the upper limit of the holding time is not particularly set, but since productivity deteriorates, it is preferably 200 seconds or less, and more preferably 100 seconds or less.

(b) the first cooling process from 900℃ to 540℃

The average cooling rate in the first cooling step from 900°C to 540°C is 10°C/sec or more and 50°C/sec or less. When this average cooling rate is less than 10°C/sec, the possibility of ferrite generation increases, and it becomes difficult to secure desired yield strength and tensile strength. Therefore, the average cooling rate needs to be 10°C/sec or more, preferably 11°C/sec or more, and more preferably 12°C/sec or more. On the other hand, when the average cooling rate exceeds 50° C./sec, it becomes difficult to control the temperature of the steel sheet, and equipment cost increases. Therefore, the upper limit of the average cooling rate needs to be 50°C/sec or less, preferably 40°C/sec or less, and more preferably 30°C/sec or less.

(c) 2nd cooling process from 540℃ to 440℃

The average cooling rate up to the cooling stop temperature in the second cooling step of 540°C or less needs to be 0.5°C/sec or more. When the average cooling rate in the second cooling step is less than 0.5°C/sec, an increase in bainite is concerned. Therefore, the average cooling rate is set at 0.5°C/sec or more. Preferably it is 0.8 degreeC/second or more. On the other hand, the upper limit of the average cooling rate is not particularly set, but since it is necessary to remarkably increase the capability of the facility, it is preferably 50° C./sec or less. More preferably, it is 40 degreeC/second or less, More preferably, it is 30 degreeC/second or less.

The cooling stop temperature in the second cooling step needs to be 440°C or higher. When 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 440°C or higher. Preferably it is 445 degreeC or more, More preferably, it is 450 degreeC or more.

On the other hand, in Fig. 1, three types of cooling patterns in the first cooling step are shown, but 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, in this second cooling step, the average cooling rate including the immersion treatment in the plating bath → alloying heat treatment needs to satisfy the above conditions. The steel sheet temperature before immersion in the plating bath is preferably in the range of more than 440°C to 480°C.

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

(d) 3rd cooling process from 440℃ to 280~230℃

The average cooling rate to the cooling stop temperature in the 3rd cooling process needs to be 5.0°C/sec or more. If the average cooling rate in the third cooling step is less than 5.0°C/sec, an increase in bainite is concerned. In addition, even if the formation of bainite is suppressed, the distribution of carbon from martensite generated after passing through the Ms point to retained austenite proceeds, thereby stabilizing, thereby reducing the amount of transformation into martensite. As a result, since it becomes easy to contain retained austenite exceeding 7%, the average cooling rate is set at 5.0°C/sec or more.

The Ms point is the temperature at which martensite starts transformation, and based on the following formula (I) described in "Steel Materials" (published by the Metallurgical Society of Japan, p. 45), it is simple from the chemical composition composition of the steel sheet. It can be obtained by On the other hand, [] in the following formula (I) represents the content (mass%) of each element, and the element not contained in the steel sheet is calculated as 0%.

Ms point (℃)=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/sec or more, and more preferably 20°C/sec or more. The upper limit of the average cooling rate at this time is not particularly set, but in order to excessively speed up the average cooling rate, it is necessary to remarkably increase the facility capacity, and thus it is preferably 50° C./second or less. More preferably, it is 40 degreeC/second or less, More preferably, it is 30 degreeC/second or less.

The cooling stop temperature in the third cooling step needs to be 230°C or more and 280°C or less. When the cooling stop temperature in the third cooling step is less than 230°C, there is a possibility that the magnetic tempering of martensite becomes excessive, the number of laths in the martensite decreases, and the tensile strength decreases. Therefore, the lower limit of the cooling stop temperature in the third cooling step is made 230°C or higher. Preferably it is 240 degreeC or more, More preferably, it is 250 degreeC or more.

On the other hand, if the cooling stop temperature in the third cooling step exceeds 280°C, there is a possibility that bainite increases, resulting in a decrease in yield strength and tensile strength. Therefore, the upper limit of the cooling stop temperature in the third cooling step is 280°C or less. It is preferably 275°C or less, and more preferably 270°C or less.

(e) 4th cooling process from 230℃ to 50℃ or less

In the 4th cooling process which is performed continuously after the said 3rd cooling process, the average cooling rate from 230 degreeC to 50 degreeC or less cooling stop temperature is preferably 3.0 degrees C/sec or less. On the other hand, 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 is not considered.

It is considered that the presence of a film-like austenite in an appropriate amount at the lath boundary increases the effect as a barrier for dislocation movement, and is preferable for securing yield strength and tensile strength. When the average cooling rate in the fourth cooling step is greater than 3.0°C/sec, the retained austenite is less than 1% by volume, and the effect as a barrier for dislocation movement becomes difficult to exhibit. Therefore, the average cooling rate is 3.0°C/sec or less. It is preferably 2.5°C/sec or less, and more preferably 2.0°C/sec or less. On the other hand, the lower limit of the average cooling rate at this time is not particularly provided, but since productivity deteriorates, it is preferably 0.05°C/sec or more. More preferably, it is 0.10 degreeC/second or more.

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

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

The tensile strength in the high-strength steel sheet of this embodiment is preferably 1500 MPa or more, and more preferably 1550 MPa or more. The higher the tensile strength is, the upper limit thereof is not particularly limited, but is usually about 1800 MPa. In addition, the yield strength is preferably 1020 MPa or more, and more preferably 1040 MPa or more. It is preferable that the yield strength is also high, and the upper limit thereof is not particularly limited, but is usually about 1400 MPa.

The high-strength steel sheet of this embodiment has sufficiently high yield strength and tensile strength even without temper rolling, but it is also possible to achieve a higher yield strength by performing temper rolling.

A hot dip galvanized layer (GI: Hot Dip-Galvanized) or an alloyed hot dip galvanized layer (GA: Alloyed Hot Dip-Galvanized) may be provided on the surface of the high-strength steel sheet of the present embodiment. That is, a high-strength hot-dip galvanized steel sheet and a high-strength alloyed hot-dip 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 are also included in the present invention. The type of the zinc plating layer at this time is not particularly limited, and the plating layer may contain an alloying element. In addition, the galvanized layer is coated on one side or both sides of the steel sheet.

As described above, the present specification discloses various aspects of technology, but the main technology among them is summarized below.

The high-strength steel sheet according to an aspect of the present invention is, by mass%, C: 0.200 to 0.280%, Si: 0.40 to 1.50% or less, Mn: 2.00 to 3.00%, P: more than 0% and 0.015% or less, S: 0% More than 0.0050%, Al: 0.015 to 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 unavoidable impurities,

With respect to the entire metal structure, martensite was 93% by volume or more, ferrite, pearlite, and bainite were 2% by volume or less in total, and the residual austenite was 7% by volume or less, and the metal structure was examined under a scanning electron microscope. In the observed image, the number of laths in martensite 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.

With the above configuration, it is possible to realize a high-strength steel sheet having a yield strength of 1000 MPa or more at a high strength level having a tensile strength of 1470 MPa or more.

The high-strength steel sheet, if necessary, in addition, by 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% Hereinafter, it is also useful to contain at least one selected from the group consisting of Nb: more than 0% and 0.040% or less, and Ca: more than 0% and 0.0050% or less, and the characteristics of the high-strength cold rolled steel sheet are further improved depending on the type of element contained. Improves.

A high-strength galvanized steel sheet according to another aspect of the present invention is characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the high-strength steel sheet as described above.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and it is also possible to carry out changes in a range that may be suitable for the purpose of the earlier and later periods, and they are all It is contained in the technical scope of the present invention.

Example

An experimental slab of the chemical composition (steel grade: steel A, B, C) shown in Table 1 was prepared. The slab was heated to 1250°C, and hot rolling was performed to a thickness of 2.8 mm to 3.1 mm. The finish rolling temperature at this time was set to 900°C, the average cooling rate from finish rolling to coiling in hot rolling was set to 20°C/sec, and the coiling temperature was set to 650°C, and hot rolling was performed. After pickling the obtained hot-rolled steel sheet, surface grinding or cold rolling was combined to reduce the thickness to a thickness of 1.4 mm to 2.6 mm. At this time, the cold rolling rate (rolling rate during cold rolling) of a certain steel type is within the range of 10% to 60%. In Table 1, the column of "-" means that it is not added, and the column of "<" means that it is less than the measurement limit. In addition, P, S, N, and O are unavoidable impurities as described above, and the values indicated in the columns of P, S, N, and O refer to amounts inevitably included. In addition, the balance includes iron and unavoidable impurities other than the unavoidable impurities shown above.

Thereafter, the obtained cold-rolled steel sheet was annealed by heat treatment (heat treatment 1 to 3) of the heat pattern shown in FIGS. 3 to 5. Specifically, heat treatments 1 to 3 were performed for steel types A and B. For other steel types C, heat treatment 1 was performed.

Detailed data in the heat treatment shown in Figs. 3 to 5 are shown in Tables 2 to 4 below. That is, the heat pattern shown in Fig. 3 is based on the data shown in Table 2 (heat treatment 1), and the heat pattern shown in Fig. 4 is based on the data shown in Table 3 (heat treatment 2), and The heat pattern is based on the data shown in Table 4 below (heat treatment 3). On the other hand, "s" shown in Figs. 3 to 5 means "second". In addition, in Tables 2 to 4, corresponding steps [(a) to (e)] of Fig. 1 are shown.

In 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 measured positions sequentially showing values (set temperature, cooling rate) corresponding to Figs. 3 to 5, but in Figs. 3 to 5, step positions shown in Tables 2 to 4 Are partially omitted. In addition, in Tables 2-4, the cooling rate indicated by minus indicates that it is a heating rate (heating rate).

On the other hand, in Tables 2 to 4, the average cooling rate in the temperature range specified in the above steps (a) to (c) is not specified, but these values are based on the data in Tables 2 to 4 Can be calculated. For example, in Table 2, when the passing time at which the steel sheet temperature becomes 900°C ("Total time" shown in Table 2; the same hereinafter) is calculated, it becomes "130 seconds", and the average cooling from 900°C to 540°C The speed [average cooling rate in the above step (b)] is 12.9°C/sec [≒(900°C-540°C)/(158 sec-130 sec)].

In addition, in Table 2, when the passage time at which the steel sheet temperature becomes 440°C is calculated, it becomes "252 seconds", and the average cooling rate from 540°C to 440°C [average cooling rate in the above step (c)] is 1.06°C /Sec[≒(540℃-440℃)/(252sec-158sec)]. Similarly, calculating the average cooling rate from 440°C to 280°C [average cooling rate in step (d) above] is 20.0°C/sec [=(440°C-280°C)/(260 sec-252 sec) ].

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

[The fraction of each structure in the metal structure]

In this example, the fractions of martensite, bainite, and retained austenite present in the 1/4 part of the sheet thickness of the steel sheet were measured as follows. According to the manufacturing method of the present embodiment, the possibility of the presence of structures other than the above (for example, ferrite or pearlite) in each region is extremely low, and thus the structures other than the above are not measured. Therefore, in 1/4 part of the sheet thickness of the steel sheet, the total of martensite, bainite, and retained austenite was calculated to be 100% by volume.

[Volume fraction of residual austenite]

As for the retained austenite, a test piece of 1.4 mm × 20 mm × 20 mm is cut out from the annealing steel sheet, ground to 1/4 part of the sheet thickness, chemically polished, and then retained austenite by X-ray diffraction (hereinafter, The volume fraction of "remaining γ") was measured (ISIJ Int. Vol. 33. (1993), No. 7, P. 776). As the measuring device, a two-dimensional micro-part X-ray diffraction device "RINT-PAPIDII" (trade name: manufactured by Rigaku Corporation) is used, and the measuring surface is near 1/4 part of the plate thickness. Co was used as the target, and the number of measurements was performed once for each test.

[Volume fraction of martensite and bainite]

Bainite and martensite were measured by the scattering method as follows. First, a 1.4mm×20mm×20mm test piece was cut out from the steel plate, a cross-section parallel to the rolling direction was polished, and nital corrosion was performed, and then the structure of 1/4 part of the plate thickness was subjected to FE-SEM (Field Emission Scanning). Electron Microscope) photographed (magnification 3000 times). Observation was performed on the FE-SEM image using a grid of 0.3 μm intervals, based on the color of the particles, etc., bainite and martensite were distinguished, and each volume fraction was measured. As for the measurement point, the structure at the point where the lattice intersects at a right angle was classified, 100 points were irradiated, and the fraction was calculated. Measurement was performed for each one field of view.

In detail, in the SEM photograph after nital corrosion, the structure seen as black is bainite, and the rest is martensite. Fig. 6 (picture substitute photo) shows an example of a metal structure showing bainite and martensite.

As described in detail above, in this example, since retained austenite and other structures (bainite, martensite) are measured by different methods, the sum of these structures is necessarily 100% by volume. Can't. Therefore, in determining the volume fractions of bainite and martensite, adjustment was made so that the total of the entire structure was 100% by volume. Specifically, to the value obtained by subtracting the fraction of retained austenite measured by the X-ray diffraction method from 100% by volume, each fraction of bainite and martensite measured by the dot counting method is proportionally distributed and converted, The volume fractions of bainite and martensite were determined.

[Number of laths per 300μm total length]

The number of laths with a total length of 300 μm measured is a cross section parallel to the rolling direction in a 1/4 part of the sheet thickness of a steel plate subjected to nital corrosion, photographed at 3000 times in FE-SEM, and the total length of 300 μm was cut by a cutting method. It was measured. The cutting method is usually a method of measuring the particle diameter (JIS G 0551:2013), but in this example, it was applied as a method of measuring the number of laths. Specifically, on the FE-SEM image, a line having a total length of 300 μm was drawn, and the number of lines passing through the lath (the number of intersecting points) was measured. The ras is an SEM image of a steel plate subjected to nital corrosion at a magnification of 3000 times in FE-SEM, and a region of 1 μm or more in a white portion. The state when the number of laths is measured by the cutting method is schematically shown in Fig. 2 (Fig. 2(a), Fig. 2(b)).

[Tensile characteristics]

For tensile strength TS, 0.2% proof strength σ _0.2 , a JIS No. 5 test piece (plate-shaped test piece) was taken so that the direction perpendicular to the rolling direction in the plane parallel to the rolling surface of cold rolling became the longer side of the test piece, It was tested according to JIS Z 2241:2011.

About the pass criterion, 1470 MPa or more for tensile strength TS and 1000 MPa or more for yield strength (0.2% proof stress σ _0.2 ) were set as pass.

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

From this result, it can consider 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) satisfying the chemical composition composition specified in the present invention. In this example, the percentage of each tissue of the metal structure, and general is the number of Las properly adjusted per 300μm, a yield strength of at least (0.2% proof stress σ _{0. 2)} is 1000MPa, a tensile strength of TS by more than 1470MPa, the acceptance criteria You can see that you are satisfied with

In comparison, test No. 1 to 3, 5, and 6 are comparative examples that do not satisfy any of the requirements stipulated in the present invention, and thus do not satisfy certain characteristics of the steel sheet.

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

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

Test No. Fig. 3 is an example of manufacturing under inappropriate heat treatment conditions (heat treatment 3 shown in Fig. 5) using a steel class that does not satisfy the chemical composition composition specified in the present invention (steel class A in Table 1). In this example, a steel grade that does not contain Cr is used, and the cooling stop temperature in the third cooling step [the step (d) shown in Fig. 1] is set to 100°C (step 9 in Table 4). Tensile strength TS decreased as the number of laths per 300 μm was decreased.

On the other hand, test No. For 5 and 6, a steel type satisfying the chemical composition composition specified in the present invention (steel type B in Table 1) is used, but the heat treatment conditions are outside the appropriate range (heat treatment 2 shown in Fig. 4, heat treatment 3 shown in Fig. 5). ), the desired characteristics are not obtained.

Specifically, test No. In 5, since the average cooling rate in the third cooling step [the step (d) shown in Fig. 1] is not set to 5.0°C/sec or more (steps 6 to 13 in Table 4), there are many retained austenite. Thus, the yield strength and tensile strength TS decreased.

Test No. 6 is an example in which the cooling stop temperature in the third cooling process [the process of (d) shown in Fig. 1] is set to 100°C (step 9 in Table 4), so that the tensile strength TS by reducing the number of laths per 300 μm in total length Has deteriorated.

This application is based on Japanese Patent Application No. 2018-58189 filed on March 26, 2018 and Japanese Patent Application No. 2019-008594 filed on January 22, 2019, the contents of which are incorporated herein.

In order to express the present invention, the present invention has been described appropriately and sufficiently through the embodiments while referring to specific examples and drawings in the foregoing, but those skilled in the art can easily change and/or improve the above-described embodiments. Be aware. Accordingly, it is construed that the modified form or the improved form is encompassed within the scope of the claim unless the modified form or improved form carried out by a person skilled in the art is at a level that deviates from the scope of the claim described in the claims.

The present invention has wide industrial applicability in the technical field related to steel sheets, galvanized steel sheets, their manufacturing methods, and structural parts such as automobiles.

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% and less than 0.015%,
S: more than 0% and less than 0.0050%,
Al: 0.015 to 0.060%,
Cr: 0.20 to 0.80%,
Ti: 0.015 to 0.080%, and
B: 0.0010 to 0.0040%
Each containing, the balance being iron and unavoidable 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,
In the image obtained by observing the metal structure with a scanning electron microscope, the number of laths in martensite measured by a cutting method of 300 μm in total length is 240 or more, and
High strength steel sheet, characterized in that the tensile strength is 1470 MPa or more. The method of claim 1,
In addition, by 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% The high-strength steel sheet containing at least one selected from the group consisting of the following, 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.

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