KR102416075B1 - High-strength steel sheet and high-strength galvanized steel sheet - Google Patents

Abstract

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

Description

High-strength steel sheet and high-strength galvanized steel sheet

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 fuel efficiency improvement. In addition, when high-strength steel sheet is applied to structural members of automobiles, from the viewpoint of collision safety, high-strength steel sheet is required to have high impact absorption energy.

It is known that the higher the tensile strength TS (Tensile Strength) of the high-strength steel sheet, and the higher the 0.2% yield strength σ _0.2 or the higher _UYP (Upper Yield Point), the higher the impact absorption energy. From this point of view, the steel sheet applied to structural members of automobiles is required to have a tensile strength TS of 1470 MPa or more and a 0.2% yield strength or upper yield point UYP of 1000 MPa or more. Hereinafter, the tensile strength TS may be abbreviated as "tensile strength", and 0.2% yield strength or upper yield point UYP may be abbreviated as "yield strength", respectively.

Among the above required characteristics, as a technique for improving the tensile strength of a high-strength steel sheet, for example, a technique similar to that of Patent Document 1 is proposed. In this patent document 1, by controlling each fraction of autotempered martensite, ferrite, bainite, and retained austenite, and stipulating the size and number of precipitation of iron-based carbides in autotempered martensite, the 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, sufficient elongation cannot necessarily be secured by temper rolling in some cases.

The present invention has been made in view of the above circumstances, and the object thereof 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 high-strength steel sheet is to provide

Japanese Patent No. 5365216 Publication

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

in mass %,

C: 0.200 to 0.280%;

Si: 0.40 to 1.50% or less;

Mn: 2.00 to 3.00%;

P: greater than 0% and less than or equal to 0.015%;

S: greater than 0% and less than or equal to 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 remainder 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 observed with the scanning electron microscope, the number of laths in the martensite measured with a total length of 300 µm by a cutting method is 240 or more, and

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

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the heat pattern of an annealing process.
It is explanatory drawing at the time of measuring the number of laths by the cutting method.
It is a schematic diagram which shows the heat pattern in the heat processing 1 in an Example.
It is a schematic diagram which shows the heat pattern in the heat processing 2 in an Example.
It is a schematic diagram which shows the heat pattern in the heat processing 3 in an Example.
6 is a micrograph for use in drawings showing an example of a structure in the high-strength steel sheet of the present embodiment.

The present inventors, in order to provide a high-strength steel sheet having a tensile strength of 1470 MPa or more and also having a high yield strength, the amount of bainite, martensite and retained austenite, in addition to lath, which is a substructure of bainite and martensite It has been carefully reviewed and paid attention to.

As a result, the chemical composition of the steel sheet, the volume fraction of martensite, the volume fraction of bainite and the like (including ferrite and pearlite), the volume fraction of retained austenite, and a scanning electron microscope (SEM) In the observed image (hereinafter, sometimes referred to as "SEM image"), if the number of laths in martensite measured with a cutting method of 300 µm in total length is specified as described later, the above object is achieved This invention was completed by discovering the thing and repeating further research based on the said knowledge. In addition, when calling "high strength" below, it uses for the meaning of "strength level with a tensile strength of 1470 MPa or more".

Las is a substructure of martensite. The structure of martensite is multi-layered, and in one old austenite grain, a plurality of packets, which are aggregation of particles having the same regular wall surface, exist, and in each packet, blocks that are parallel band-shaped regions exist. , in addition, in each block, there is a set of laths, which are martensite crystals with high-density transitions in 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 a cutting method for a total length of 300 µm as defined in the present invention is 1 of the thickness of the steel sheet subjected to nital corrosion In the /4 part, a cross section parallel to the rolling direction is photographed at 3000 times with an FE-SEM (Field Emission Scanning Electron Microscope), and a total length of 300 μm is measured by a cutting method.

The inventors of the present invention considered that lath in martensite had an influence on yield strength and tensile strength, and intensive studies were repeated. As a result, it was found that it is important to achieve both high yield strength and tensile strength to satisfy the requirements described later for the number of laths per 300 µm of total length. EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is 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 micrometers in total is 240 or more. When the number of laths is less than 240, the yield strength or tensile strength decreases. The reason for this is not necessarily clear, but it can be thought of as follows. First, in addition to the effect of increasing the yield strength by preventing the movement of dislocations at the boundary between lath and lath, in the chemical composition system of this embodiment, iron-based carbides such as fine cementite and film-like retained austenite exist at the boundary of lath. Therefore, there is the possibility of becoming an additional barrier to the movement of dislocations. From the above, it is thought that the yield strength and tensile strength also become high when there are many laths per predetermined length. 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 600 or less in general.

[Martensite: 93% by volume or more]

Martensite in the metal structure is the matrix structure of the high-strength steel sheet of the present embodiment. By making this martensite into 93 volume% or more with respect to the whole metal structure, yield strength and tensile strength can be made high. When the amount of martensite is less than 93% by volume, other soft tissues start plastically deforming at a low stress, and the yield strength is lowered. The lower limit of martensite is preferably 94 vol% or more, and more preferably 95 vol% or more. The upper limit of martensite is generally 99% by volume or less. Although martensite includes tempered martensite and magnetic annealed martensite, when excessively tempered, the number of laths per 300 µm of total length will be less than 240, so it is not included in the target martensite in this embodiment.

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

Compared to martensite as a matrix structure, these tissues are soft, and when these tissues increase, these tissues themselves initiate plastic deformation at low stress, resulting in lower yield strength and tensile strength. From this point of view, it is necessary to make the total amount of ferrite, pearlite and bainite 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, and more preferably 1.0% by volume or less. The lower limit of the 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 a 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 retained austenite present at the lath boundary may have the effect of increasing the tensile strength or yield strength by suppressing the movement of dislocations. However, since retained austenite itself is soft compared to the martensite structure, if it exists in excess even in a film form, the yield strength and tensile strength also decrease. From this point of view, the retained austenite needs to be 7% by volume or less. The upper limit of retained austenite becomes like this. Preferably it is 6 volume% or less, More preferably, it is 5 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 prescribing the number of laths, martensite volume ratio, bainite volume ratio, and retained austenite volume ratio as described above, it is also necessary to appropriately define the chemical composition of the steel sheet. . The reasons for setting these ranges are as follows. In addition, "%" in the following chemical component composition means all "mass %".

(C: 0.200 to 0.280%)

C is an element necessary to secure the strength of the steel sheet. When 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 will increase more than 7 volume%, and there exists a possibility of causing the fall of 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, and more preferably 0.260% or less. More preferably, it is 0.250% or less, More preferably, it is 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. Moreover, it is thought that it is effective in suppressing excessive tempering of martensite and ensuring a fine lath. 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. More preferably, it is 0.70% or more, More preferably, it is 0.80% or more. However, when the amount of Si becomes excessive, the volume fraction of retained austenite increases, and there is a possibility that the yield strength may be lowered. Therefore, the upper limit of the amount of Si is made into 1.50% or less. The upper limit of the amount of Si becomes like this. Preferably it is 1.40 % or less, More preferably, it is 1.30 % or less.

(Mn: 2.00 to 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 and to set it as 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 amount of Mn becomes excessive, there is a risk of causing slab breakage, increase in cold rolling load, and the like. 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. More preferably, it is 2.70% or less, Even more preferably, it is 2.60% or less.

(P: more than 0% and less than or equal to 0.015%)

P is an element that is unavoidably included, and is an element that segregates at grain boundaries and promotes grain boundary embrittlement. Therefore, the P amount is made 0.015% or less. The upper limit of the amount of P becomes like this. Preferably it is 0.013 % or less, More preferably, it is 0.010 % or less. On the other hand, since P is an impurity that is unavoidably mixed in steel, it is impossible for industrial production to set the amount to 0%.

(S: more than 0% and less than or equal to 0.0050%)

S is also an element that is unavoidably contained in the same way as P, and in order to generate inclusions and avoid breakage during processing, it is recommended to reduce the amount of S as much as possible. Therefore, the amount of S is made 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, since S is an impurity that is unavoidably mixed in steel, it is impossible for industrial production to set the amount to 0%.

(Al: 0.015 to 0.060%)

Al is an element that acts as a deoxidizer. In order to effectively exhibit such an effect, the Al content 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, many inclusions, such as alumina, are produced|generated in a steel plate, and there exists a possibility of causing a fracture|rupture at the time of processing. Therefore, the upper limit of the amount of Al is made 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 set it as a target martensite-based structure. Moreover, it is thought that it has the effect of suppressing excessive tempering of martensite and making lath fine. 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, if the amount of Cr becomes excessive, non-plating may occur when hot-dip galvanizing or alloyed hot-dip galvanizing is formed on the surface of a steel sheet. Therefore, the upper limit of the amount of Cr was made into 0.80 % or less. The upper limit of the amount of Cr becomes like this. Preferably it is 0.75 % or less, More preferably, it is 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. Moreover, it is an effective element also in exhibiting effectively the hardenability improvement effect by B mentioned later. That is, Ti reduces N in steel by forming a nitride, and as a result, formation of B nitride is suppressed, B becomes a solid solution state, and the hardenability improvement effect by B can be exhibited effectively. In this way, Ti contributes to strengthening the steel sheet by improving the hardenability. In order to effectively exhibit such an effect, the Ti amount 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 cracks during processing. Therefore, the upper limit of the Ti amount is made 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 improves hardenability and has the effect of suppressing the formation of ferrite or bainite. Thereby, it is an element contributing to high strength of a steel plate. 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 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 embodiment are as described above, and the remainder is substantially iron. However, it is of course permissible for the steel to contain impurities that are unavoidably mixed depending on the conditions of raw materials, materials, manufacturing facilities, etc. As such unavoidable impurities, other than P and S mentioned above, for example, N, O, etc. are contained, and it is preferable that these are each in the following ranges.

(N: 0.0100% or less)

N is unavoidably present as an impurity element, and may cause cracks during processing. From this point, it is preferable that the amount of N is 0.0100 % or less, More preferably, it is 0.0060 % or less, More preferably, it is 0.0050 % or less. Although it is so preferable that there is little N amount, it is difficult to set it as 0% from the viewpoint of industrial production.

(O: 0.0020% or less)

O is unavoidably present as an impurity element, and may cause cracks 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. Although it is so preferable that there is little O amount, it is difficult to set it as 0% from the viewpoint of industrial production.

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

(Cu: greater than 0% and less than or equal to 0.30%)

Cu is an element effective for the improvement of the corrosion resistance of a steel plate, and you may contain it as needed. Although the effect increases as content increases, in order to exhibit the said effect effectively, it is preferable that Cu content sets it as 0.03 % or more, More preferably, it is 0.05 % or more. However, when the amount of Cu becomes excessive, the effect is saturated and the cost increases. Therefore, it is preferable that the upper limit of the amount of Cu is 0.30 % or less, More preferably, it is 0.20 % or less, More preferably, it is 0.15 % or less.

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

Ni is an element effective for the improvement of the corrosion resistance of a steel plate, and you may contain it as needed. Although the effect increases as content increases, in order to exhibit the said effect effectively, it is preferable that Ni content shall be 0.03 % or more, More preferably, it is 0.05 % or more. However, when the amount of Ni becomes excessive, while the effect is saturated, cost increases. Therefore, it is preferable that the upper limit of the amount of Ni is 0.30 % or less, More preferably, it is 0.20 % or less, More preferably, it is 0.15 % or less.

(Mo: greater than 0% and less than or equal to 0.30%)

Mo is an element contributing to increase in strength of the steel sheet, and may be contained as necessary. Although the effect increases as content increases, in order to exhibit the said effect effectively, it is preferable that Mo amount shall be 0.03 % or more, More preferably, it is 0.05 % or more. However, when the amount of Mo becomes excessive, while the effect is saturated, cost increases. Therefore, it is preferable that the upper limit of the amount of Mo is 0.30 % or less, More preferably, it is 0.25 % or less, More preferably, it is 0.20 % or less.

(V: greater than 0% and less than or equal to 0.30%)

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

(Nb: greater than 0% and less than or equal to 0.040%)

Nb is an element contributing to increase in strength of the steel sheet, and may be contained as necessary. Although the effect increases as content increases, in order to exhibit the said effect effectively, it is preferable to make Nb content into 0.003 % or more, More preferably, it is 0.005 % or more. However, when the amount of Nb becomes excessive, bendability is deteriorated. Therefore, it is preferable that the upper limit of the amount of Nb is 0.040 % or less, More preferably, it is 0.035 % or less, More preferably, it is 0.030 % or less.

(Ca: greater than 0% and less than or equal to 0.0050%)

Ca is an element effective for spheroidizing the sulfide in steel and improving bendability, and may be contained as needed. Although the effect increases as content increases, in order to exhibit the said effect effectively, it is preferable that the amount of Ca shall be 0.0005 % or more, More preferably, it is 0.0010 % or more. However, when the amount of Ca becomes excessive, the effect is saturated and the cost increases. Therefore, it is preferable that the upper limit of the amount of Ca is 0.0050 % or less, More preferably, it is 0.0030 % or less, More preferably, it is 0.0025 % or less.

Next, the method of manufacturing the high strength steel plate of this embodiment is demonstrated.

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

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

[Hot Rolling Conditions]

When the heating temperature before hot rolling is low, there exists a possibility that carbides, such as TiC, may become difficult to solid-solve in austenite. Therefore, it is preferable that the heating temperature before hot rolling shall be 1200 degreeC or more. This heating temperature becomes like this. More preferably, it is 1250 degreeC or more. However, when the heating temperature before hot rolling becomes too high, cost will rise. Therefore, it is preferable that the upper limit of the heating temperature before hot rolling is 1350 degrees C or less, More preferably, it is 1300 degrees C or less.

When the finish rolling temperature of hot rolling is low, the deformation resistance at the time of rolling may become large, and there exists a possibility that operation may become difficult. Therefore, the finish rolling temperature is preferably 850°C or higher. The finish rolling temperature is more preferably 870°C or higher. However, when the finish-rolling temperature becomes too high, there is a possibility 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.

The average cooling rate from finish rolling to winding in hot rolling is preferably 10°C/sec or more, more preferably 20°C/sec or more, in consideration of productivity. On the other hand, when an average cooling rate becomes fast too much, it may harden and subsequent cold rolling may become difficult. Therefore, it is preferable that an average cooling rate shall be 100 degrees C/sec or less, More preferably, it is 50 degrees C/sec or less.

[Hot rolling coiling temperature: 620℃ or higher]

When the hot-rolling coiling temperature is less than 620°C, the strength of the hot-rolled steel sheet increases, and there is a fear that it becomes difficult to roll down by cold rolling. Therefore, it is preferable that the coiling temperature at the time of hot rolling shall be 620 degreeC or more, More preferably, it is 630 degreeC or more, More preferably, it is 640 degreeC or more. On the other hand, when the coiling temperature at the time of hot rolling becomes high too much, a scale will thicken and pickling property will deteriorate. Therefore, the coiling temperature is preferably 750°C or less, and more preferably 700°C or less.

[Rolling rate at the time of cold rolling: 10% or more and 70% or less]

A hot-rolled steel sheet is pickled for descaling, and is used for cold rolling. When the rolling ratio (synonymous with "reduction ratio") at the time of cold rolling is less than 10%, it becomes difficult to ensure a predetermined plate|board thickness tolerance. In order to obtain a steel sheet of a predetermined thickness, the sheet thickness must be thinned in the hot rolling process, and when the sheet thickness is thinned in the hot rolling process, the length of the steel sheet becomes long, so pickling takes time and productivity is reduced. Therefore, it is preferable that the rolling ratio at the time of cold rolling shall be 10 % or more. More preferably, it is 20% or more, More preferably, it is 25% or more. On the other hand, when the rolling ratio at the time of cold rolling exceeds 70 %, the possibility that a crack will generate|occur|produce at the time of cold rolling increases. Therefore, it is preferable that the upper limit of the rolling ratio at the time of cold rolling is 70 % or less. More preferably, it is 65 % or less, More preferably, it is 60 % or less.

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

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. A 2nd cooling process WHEREIN: What is necessary is just to perform the immersion treatment to molten zinc and the subsequent heat treatment for alloying zinc and iron together.

The heat pattern of the annealing process including each process of said (a)-(e) is demonstrated more concretely below, showing in the schematic diagram of FIG.

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

Heat to 900 degreeC or more, and hold|maintain at 900 degreeC or more for 20 second or more. When the soaking temperature is less than 900°C, there is a possibility that soft ferrite that reduces yield strength or tensile strength is produced. Therefore, the lower limit of the said temperature shall be 900 degreeC or more. Preferably it is 905 degreeC or more, More preferably, it is 910 degreeC or more. On the other hand, although the upper limit in particular of a soaking temperature is not provided, in order to deteriorate productivity, it is preferable that it is 1000 degrees C or less. More preferably, it is 980 degrees C or less, More preferably, it is 960 degrees C or less.

In addition, even if the soaking temperature is set to 900°C or higher, if the holding time at 900°C or higher is less than 10 seconds, ferrite may be formed. Therefore, the holding time at 900°C or higher is 10 seconds or longer. Preferably it is 15 second or more, More preferably, it is 20 second or more. On the other hand, although the upper limit in particular of holding time is not provided, since productivity deteriorates, Preferably it is 200 second or less, More preferably, it is 100 second or less.

(b) the first cooling step from 900°C to 540°C

The average cooling rate in the 1st cooling process from 900 degreeC to 540 degreeC shall be 10 degrees C/sec or more and 50 degrees C/sec or less. When the average cooling rate is less than 10° C./sec, the possibility that ferrite will be formed increases, making it difficult to secure the desired yield strength and tensile strength. Therefore, the said average cooling rate needs to be 10 degrees C/sec or more, Preferably it is 11 degrees C/sec or more, More preferably, it is 12 degrees C/sec or more. On the other hand, when the said average cooling rate exceeds 50 degreeC/sec, it becomes difficult to control the steel plate temperature, and equipment cost increases. Therefore, the upper limit of the said average cooling rate needs to be 50 degrees C/sec or less, Preferably it is 40 degrees C/sec or less, More preferably, it is 30 degrees C/sec or less.

(c) a second cooling step from 540°C to 440°C

The average cooling rate to the cooling stop temperature in the second cooling step of 540°C or lower 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, there is concern about an increase in bainite. Therefore, the said average cooling rate shall be 0.5 degreeC/sec or more. Preferably it is 0.8 degreeC/sec or more. On the other hand, although the upper limit in particular of an average cooling rate is not provided, Since it is necessary to raise the capability of an installation remarkably, 50 degrees C/sec or less is preferable. More preferably, it is 40 degrees C/sec or less, More preferably, it is 30 degrees C/sec or less.

The cooling stop temperature in the second cooling step needs to be 440°C or higher. When the cooling stop temperature in a 2nd cooling process will be less than 440 degreeC, yield strength and tensile strength will fall by the increase of bainite. Therefore, the minimum of the cooling stop temperature in a 2nd cooling process shall be 440 degreeC or more. Preferably it is 445 degreeC or more, More preferably, it is 450 degreeC or more.

In addition, in said FIG. 1, although three types of cooling patterns in a 1st cooling process were shown, this has shown that any cooling pattern may be sufficient as long as the said average cooling rate can be ensured. In other words, when the temperature range from 540° C. to 440° C. is passed within 200 seconds, an average cooling rate of 0.5° C./sec or more can be secured.

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

After the immersion treatment in molten zinc, if necessary, an alloying heat treatment of zinc and iron is performed. In this alloying heat treatment, in order to ensure the performance of plating, it is necessary to make temperature (alloying heat processing temperature) into 440 degreeC or more and 540 degrees C or less. If this temperature is less than 440°C, diffusion of galvanizing and iron becomes insufficient, and an alloying hot-dip galvanizing 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 in addition to a decrease in tensile strength, the diffusion of iron into zinc becomes excessive, and the alloying hot-dip galvanized layer is brittle and easy to peel off. This increases the possibility that the plating is peeled off during press molding or the like.

(d) the third cooling step from 440°C to 280 to 230°C

The average cooling rate to the cooling stop temperature in the third cooling step needs to be 5.0°C/sec or more. When 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 when the formation of bainite is suppressed, the carbon distribution from martensite generated after passing through the Ms point to retained austenite is stabilized and the amount of transformation to martensite is reduced. As a result, since it becomes easy to contain retained austenite exceeding 7 %, the said average cooling rate shall be 5.0 degreeC/sec or more.

The Ms point is the temperature at which martensite starts to transform, and based on the following formula (I) described in "Steel Materials" (published by the Japanese Metallurgical Society, p. 45), it is simple from the chemical composition of the steel sheet. can be obtained with In addition, [ ] 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. Although the upper limit in particular of the average cooling rate at this time is not provided, in order to make an average cooling rate too fast, since it is necessary to raise installation capacity remarkably, it is preferable that it is 50 degrees C/sec or less. More preferably, it is 40 degrees C/sec or less, More preferably, it is 30 degrees C/sec or less.

It is necessary to make the cooling stop temperature in a 3rd cooling process 230 degreeC or more and 280 degrees C or less. When the cooling stop temperature in the third cooling step is lower than 230°C, the self-tempering of the martensite becomes excessive, the number of laths in the martensite decreases, and the tensile strength may decrease. Therefore, the lower limit of the cooling stop temperature in the third cooling step is 230°C or higher. Preferably it is 240 degreeC or more, More preferably, it is 250 degreeC or more.

On the other hand, when the cooling stop temperature in the 3rd cooling process exceeds 280 degreeC, bainite may increase and the fall of yield strength and tensile strength may be caused. Therefore, the upper limit of the cooling stop temperature in the 3rd cooling process is made into 280 degrees C or less. Preferably it is 275 degrees C or less, More preferably, it is 270 degrees C or less.

(e) the fourth cooling process from 230°C to 50°C or less

In the 4th cooling process performed continuously after the said 3rd cooling process, as for the average cooling rate from 230 degreeC to the cooling stop temperature of 50 degrees C or less, 3.0 degrees C/sec or less is preferable. 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 counted.

The presence of an appropriate amount of film-form austenite at the lath boundary increases the effect as a barrier to dislocation movement, and is considered preferable in order to ensure yield strength and tensile strength. When the average cooling rate in the fourth cooling step is higher than 3.0°C/sec, the retained austenite becomes less than 1% by volume, making it difficult to exert the effect as a barrier to movement of dislocations. Therefore, the said average cooling rate shall be 3.0 degreeC/sec or less. Preferably it is 2.5 degrees C/sec or less, More preferably, it is 2.0 degrees C/sec or less. On the other hand, although the minimum in particular of the average cooling rate at this time is not provided, since productivity deteriorates, it is preferable that it is 0.05 degreeC/sec or more. More preferably, it is 0.10 degreeC/sec or more.

The high strength steel sheet of this embodiment is not limited to what was obtained by said manufacturing method. The high strength steel sheet of this embodiment may be obtained by another manufacturing method as long as the structural requirements prescribed|regulated by this invention are satisfy|filled.

In the high-strength steel sheet of this embodiment, while adjusting the chemical composition as described above, 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, on the SEM image of the metal structure, the number of laths measured by a cutting method of 300 μm in total length is 240 or more, and the 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 becomes like this. Preferably it is 1500 MPa or more, More preferably, it is 1550 MPa or more. A higher tensile strength is better, and although the upper limit is not specifically limited, Usually, it is about 1800 MPa. Moreover, yield strength becomes like this. Preferably it is 1020 MPa or more, More preferably, it is 1040 MPa or more. The higher yield strength is also good, and although the upper limit is not specifically limited, Usually, it is about 1400 MPa.

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

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

Although this specification discloses the technique of various aspects as mentioned above, the main technique among them is put together below.

High-strength steel sheet according to an aspect of the present invention, in 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% Exceeding 0.0050% or less, Al: 0.015 to 0.060%, Cr: 0.20 to 0.80%, Ti: 0.015 to 0.080%, B: 0.0010 to 0.0040%, 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, and the metal structure is analyzed by a scanning electron microscope. The observed image WHEREIN: The number of laths in the martensite measured by the cutting method of 300 micrometers in total length is 240 or more, and tensile strength is 1470 MPa or more, It is characterized by the above-mentioned.

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, further, 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% Below, 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 properties of the high-strength cold-rolled steel sheet are further improved depending on the type of element contained. is improved

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 may be implemented with modifications within a range suitable for the purpose of the first period and the latter period, and they are all It is included in the technical scope of the present invention.

Example

Experimental slabs having the chemical composition shown in Table 1 below (steel types: steels A, B, and C) were prepared. The slab was heated to 1250°C and hot-rolled to plate thickness: 2.8 mm to 3.1 mm. The finish rolling temperature at this time was 900 degreeC, the average cooling rate from finish rolling of hot rolling to coiling|winding was 20 degreeC/sec, and the coiling temperature was 650 degreeC, 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 sheet thickness: 1.4 mm - 2.6 mm. At this time, the cold rolling rate (rolling rate at the time of cold rolling) of a certain steel type is in 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, respectively. In addition, 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 unavoidably included amounts. In addition, the balance contains iron and unavoidable impurities other than the unavoidable impurities shown above.

Then, about the obtained cold-rolled steel sheet, it annealed by the heat treatment (heat treatment 1-3) of the heat pattern shown in FIGS. Specifically, heat treatment 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 below (heat treatment 1), the heat pattern shown in Fig. 4 is based on the data shown in Table 3 below (heat treatment 2), and the heat pattern shown in Fig. 5 is The heat pattern is based on the data shown in Table 4 below (Heat Treatment 3). On the other hand, "s" shown in FIGS. 3-5 means "second". In addition, in Tables 2-4, the corresponding process [(a)-(e)] of FIG. 1 is shown.

In the heat treatments 1 to 3 shown in Figs. 3-5, hot-dip galvanizing treatment and alloying heat treatment are not performed in the second cooling step (step (c) shown in Fig. 1). "Step" shown in Tables 2 to 4 below indicates the actual measurement positions sequentially showing numerical values (set temperature, cooling rate) corresponding to FIGS. 3 to 5, but in FIGS. 3 to 5, the step positions shown in Tables 2 to 4 are partially omitted. In addition, in Tables 2-4, the cooling rate indicated by a minus indicates that it is a heating rate (temperature increase rate).

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

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

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 the tensile properties were measured according to the following procedure.

[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 this Example, in each area|region, since the possibility that the structure|tissue (for example, ferrite or pearlite) other than the above exists is extremely low, the structure|tissue other than the above is not measured. Then, it calculated so that the sum total of martensite, bainite, and retained austenite might become 100 volume% at 1/4 part of plate|board thickness of a steel plate.

[volume fraction of residual austenite]

Retained austenite is obtained by cutting out a test piece of 1.4 mm × 20 mm × 20 mm from the steel sheet after the annealing, grinding to 1/4 part of the sheet thickness, chemical polishing, and X-ray diffraction method to obtain retained austenite (hereinafter, "Residual 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) was used, and the measuring surface was around 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 ratio of martensite and bainite]

Bainite and martensite were measured by the point dispersion method as follows. First, a test piece of 1.4 mm × 20 mm × 20 mm is cut out from the steel plate, a cross section parallel to the rolling direction is polished, and nital corrosion is performed, and then the structure of 1/4 part of the plate thickness is obtained by FE-SEM (Field Emission Scanning) Electron Microscope) was observed with a photograph (magnification 3000 times). Observation was performed on the FE-SEM image using a grating at intervals of 0.3 µm, bainite and martensite were distinguished based on the color of the particles, and the respective volume fractions were measured. As for the measurement point, the structure in the point where a grid|lattice cross|intersects at right angles was classified, 100 points|pieces were investigated, and the fraction was computed. The measurement was performed for each one view.

In detail, in the SEM photograph after nital corrosion, the texture that appears black is bainite, and the remaining part is martensite. An example of a metal structure showing bainite and martensite is shown in FIG. 6 (a photograph substituted for a drawing).

As described above in detail, in this Example, since retained austenite and other structures (bainite and martensite) are measured by different methods, the total of these structures is necessarily 100% by volume. can't Then, in determining each volume fraction of bainite and martensite, it adjusted so that the sum total of all structures|tissues might become 100 volume%. Specifically, each fraction of bainite and martensite measured by the point method is proportionally distributed to the numerical value obtained by subtracting the fraction of retained austenite measured by the X-ray diffraction method from 100% by volume and converted, and finally to determine the respective volume fractions of bainite and martensite.

[Number of Lass per 300μm Total Length]

For the number of laths measuring 300 μm in total length, the cross section parallel to the rolling direction in 1/4 of the sheet thickness of the steel sheet subjected to nital corrosion was photographed at 3000 times with FE-SEM, and the total length of 300 μm was taken by the cutting method. it is measured Although the cutting method is a method of measuring the particle size normally (JIS G 0551:2013), 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 the lines passing through the lath (the number of intersecting points) was measured. Lars is an SEM image photographed at a magnification of 3000 times in FE-SEM of a steel sheet subjected to nital corrosion, and the region was 1 µm or more in the white part. The state at the time of measuring the number of laths by the cutting method is schematically shown in FIG.2 (FIG.2(a), FIG.2(b)).

[Tensile properties]

For tensile strength TS, 0.2% yield strength σ _0.2 , JIS No. 5 test piece (plate-shaped test piece) is taken so that the direction perpendicular to the rolling direction in the face parallel to the rolling face of cold rolling becomes the longer side of the test piece, Tested according to JIS Z 2241:2011.

As for the pass criteria, 1470 MPa or more for the tensile strength TS and 1000 MPa or more for the yield strength (0.2% yield strength σ _0.2 ) were regarded 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 prescribed in the present invention. In this example, the fraction of each structure in the metal structure and the number of laths per 300 µm total length are appropriately adjusted, the yield strength (0.2% yield strength σ 0.2 ₎ is 1000 MPa or _{more ,} and the tensile strength TS is 1470 MPa or more, the acceptance criteria It can be seen that the .

In contrast, test No. 1 to 3, 5 and 6 are comparative examples that do not satisfy any of the requirements prescribed by the present invention, and thus do not satisfy any characteristic of the steel sheet.

Specifically, test No. 1 is an example using a steel type (steel type A in Table 1) which was manufactured under appropriate heat treatment conditions (heat treatment 1 shown in FIG. 3) but does not satisfy the chemical composition prescribed by the present invention. In this example, since a Cr-free steel type was used, bainite became excessive, and the number of laths per 300 micrometers of total length also decreased, so that the yield strength fell.

test no. 2 shows an example of manufacturing 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 prescribed in the present invention. In this example, a Cr-free steel type 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 (Table 3). Steps 6 to 13), retained austenite increased, and yield strength and tensile strength TS decreased.

test no. 3 shows an example of manufacturing under inappropriate heat treatment conditions (heat treatment 3 shown in FIG. 5) using a steel type (steel type A in Table 1) that does not satisfy the chemical composition prescribed in the present invention. In this example, a Cr-free steel type is used, and the cooling stop temperature in the third cooling step (step (d) shown in FIG. 1 ) is 100° C. (Step 9 in Table 4). As the number of laths per 300 µm decreased, the tensile strength TS decreased.

On the other hand, test No. 5 and 6 use the steel grade (steel grade B in Table 1) satisfying the chemical composition prescribed in the present invention, 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 properties are not obtained.

Specifically, test No. In 5, the average cooling rate in the third cooling step (step (d) shown in FIG. 1 ) is an example in which the average cooling rate is not 5.0° C./sec or more (steps 6 to 13 in Table 4), and there is a large amount of retained austenite. As a result, 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. 1) is 100°C (step 9 in Table 4), and the number of laths per 300 μm in total length decreases, resulting in tensile strength TS has been lowered

This application is based on Japanese Patent Application 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 by reference.

In order to express the present invention, in the foregoing, the present invention has been adequately and sufficiently described through embodiments with reference to specific examples and drawings. have to recognize Accordingly, as long as the modified form or improved form implemented by those skilled in the art does not deviate from the scope of the claims described in the claims, it is construed that the modified form or the improved form is encompassed within the scope of the claims.

INDUSTRIAL APPLICABILITY The present invention has broad industrial applicability in the technical fields related to steel sheets, galvanized steel sheets, manufacturing methods thereof, and structural parts for automobiles and the like.

Claims (3)

in mass %,
C: 0.200 to 0.280%;
Si: 0.40 to 1.50%,
Mn: 2.00 to 3.00%;
P: greater than 0% and less than or equal to 0.015%;
S: greater than 0% and less than or equal to 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 remainder 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 1% by volume or more and 7% by volume or less,
In the image observed with a scanning electron microscope, the number of laths in martensite measured with a total length of 300 μm by a cutting method is 240 or more,
A high-strength steel sheet having a tensile strength of 1470 MPa or more and a yield strength of 1000 MPa or more. The method of claim 1,
Further, in mass %, Cu: greater than 0% and less than or equal to 0.30%, Ni: greater than 0% and less than or equal to 0.30%, Mo: greater than 0% and less than or equal to 0.30%, V: greater than 0% and less than or equal to 0.30%, Nb: greater than 0% and less than or equal to 0.040% and Ca: High strength steel sheet containing at least one selected from the group consisting of more than 0% and 0.0050% or less. A high-strength galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on a surface of the high-strength steel sheet according to claim 1 or 2.

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