JP5630003B2 - High-strength steel sheet having a tensile strength of 1500 MPa or more and a method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a tensile strength of 1500 MPa or more and a method for producing the same, which are used for automobile steel sheets and the like.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, there has been a demand for improved fuel efficiency of automobiles. In addition, in order to ensure the safety of passengers in the event of a collision, there is an increasing demand for improved safety centered on the collision characteristics of automobile bodies.

  In order to meet the demands for weight reduction and high strength of the car body at the same time, it is effective to reduce the weight by increasing the strength of the component materials and reducing the plate thickness as long as there is no problem with rigidity. Recently, high-strength steel of 1180 MPa class has also started to be used for automobile parts. Furthermore, in recent years, there is an increasing demand for higher-strength steel plates exceeding 1500 MPa. However, with such a high strength material exceeding 1500 MPa, ductility, stretchability and the like are lowered and it is difficult to form by cold pressing. For this reason, molding by hot pressing or molding mainly by bending is often applied. Furthermore, high strength steel plates exceeding 1500 MPa are beginning to be concerned about the problem of delayed fracture, which was not previously a problem with thin steel plates.

  On the other hand, a steel wire material called piano wire is well known as an ultra-high strength material. This is an ultra-high-strength steel wire of 4000MPa class or higher that is obtained by strongly processing eutectoid steel, which is a full pearlite structure, by wire drawing. In the field of wire rods, various studies on the structure have been made so far.

  For example, Patent Document 1 specifies chemical components, suppresses the formation of pro-eutectoid cementite by heat treatment for obtaining a pearlite structure called patenting before wire drawing, and average lamella spacing of pearlite is 0.15 μm or less. We have proposed a high-strength steel wire rod with excellent wire drawing workability by making it finer.

  In addition, as a report of trying to increase the strength of eutectoid steel by rolling rather than wire drawing, Non-Patent Document 1 evaluated the mechanical properties of cold rolled sheets using eutectoid steel. Has been reported.

JP 2003-193195 A

Wantang Fu, T. Furuhara, and T. Maki: ISIJ International, 44 (2004) 1, p.171

  As listed in Patent Document 1, many studies have been made on wire materials for the strong processing of eutectoid steel. However, little consideration has been given to rolling. This is because the processing by drawing like a wire basically results in a uniform compression process in the circumferential cross-section direction, whereas the processing by rolling makes the deformation in the cross-section direction complicated, so that This is because there are many cases where rolling is not possible due to the occurrence of cracks or internal cracks.

  For example, in the component system shown in Non-Patent Document 1, workability during cold rolling becomes a problem, cracks are likely to occur during rolling, and it is expected that sample preparation is difficult.

  In view of such circumstances, the present invention provides a high-strength steel sheet having a tensile strength of 1500 MPa or more used for automobile steel sheets and the like, and excellent in bendability and delayed fracture resistance, and a method for producing the same. With the goal.

In order to solve the above-mentioned problems, the inventors conducted extensive research and investigation from the viewpoints of the microstructure, chemical composition, and manufacturing method of the steel sheet.
As a result, the main structure is a structure in which ferrite and carbide form a layer, and the aspect ratio of the carbide is 10 or more, and the layered structure in which the layer interval is 50 nm or less is 65% or more by volume ratio with respect to the entire structure. Succeeded in obtaining a steel plate to be used. The steel sheet thus obtained was found to be a steel sheet having a high strength with a tensile strength of 1500 MPa or more.
Further, the carbide in the layered structure composed of ferrite and carbide is an expanded carbide having an aspect ratio of 10 or more, and the fraction of carbide having an angle within 25 ° with respect to the rolling direction is 75% in area ratio. When it was above, it discovered that it was excellent also in the bendability and delayed fracture resistance in addition to the said characteristic.
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] Component composition is mass%, C: 0.3-1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005-0.1%, S: 0.05% or less , Al: 0.005 to 0.1%, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the main phase structure is composed of ferrite and carbide, and the aspect ratio of carbide is 10 A high-strength steel sheet having a tensile strength of 1500 MPa or more, wherein the layered structure having a layer spacing of 50 nm or less is 65% or more by volume ratio with respect to the entire structure.
[2] In the above [1], the fraction of the carbide forming a layer with the ferrite having an aspect ratio of 10 or more and an angle within 25 ° with respect to the rolling direction is 75% in area ratio. A high-strength steel sheet with a tensile strength of 1500 MPa or more, characterized by the above.
[3] In mass%, C: 0.3 to 1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005 to 0.1%, S: 0.05% or less, Al: 0.005 ~ 0.1%, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the pearlite structure is the main phase, the ferrite structure in the remaining structure is 20% or less in volume ratio to the whole structure (Including 0%), and having a structure in which the lamella spacing of the pearlite structure is 500 nm or less, and performing cold rolling at a rolling rate of 60% or more on a steel sheet having a Vickers hardness of HV200 or more A method for producing high-strength steel sheets with a characteristic tensile strength of 1500 MPa or more.
[4] In mass%, C: 0.3 to 1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005 to 0.1%, S: 0.05% or less, Al: 0.005 Steel slab containing ~ 0.1%, N: 0.01% or less, with the balance consisting of Fe and inevitable impurities, heated to 1100 ° C or higher, then finish rolling exit temperature: hot at 850 ° C or higher After rolling, cooling is performed at a cooling rate of 15 ° C./s or more, winding is performed at a winding temperature of 550 to 650 ° C., and then cold rolling is performed at a rolling rate of 60% or more. A method for manufacturing high-strength steel sheets with a strength of 1500 MPa or more.
[5] In mass%, C: 0.3 to 1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005 to 0.1%, S: 0.05% or less, Al: 0.005 ~ 0.1%, N: 0.01% or less, hot-rolled sheet having a composition comprising Fe and unavoidable impurities as the balance is heated at a heating temperature of 820 ° C or higher, and a cooling rate of 15 ° C / s or higher. Cooling to 550 ° C to 650 ° C, holding at 550 ° C to 650 ° C, then cooling to room temperature, then cold rolling at a rolling rate of 60% or higher, a high tensile strength of 1500 MPa or more A method for producing a strength steel plate.
[6] A method for producing a high-strength steel sheet having a tensile strength of 1500 MPa or more, wherein the cold rolling is performed at a rolling rate of 75% or more in any one of [3] to [5].
[7] In any one of the above [3] to [6], after the cold rolling, a hot dip galvanizing treatment or an alloying hot dip galvanizing treatment is further performed. A method for producing a strength steel plate.
In addition, in this specification, all% which shows the component of steel is mass%. The high-strength steel sheet of the present invention is a cold-rolled steel sheet having a tensile strength of 1500 MPa or more and a steel sheet in which a plating film mainly composed of zinc is formed on the steel sheet (for example, an electrogalvanized steel sheet, hot-dip zinc System-plated steel sheet, galvannealed steel sheet).

  According to the present invention, a high-strength steel sheet having a tensile strength of 1500 MPa or more and excellent in bendability and delayed fracture resistance can be obtained. The high-strength steel plate obtained by the present invention is suitably used as a steel plate for automobiles because it has high strength while maintaining sufficient basic performance as a material for automobile parts.

It is a figure which shows how to obtain | require an organization.

Hereinafter, the present invention will be described in detail.
First, the relationship between chemical components (component composition) and microstructure and mechanical properties was investigated in detail.
The conventional pearlite steel sheet has a low strength compared to other low-temperature transformation phases because of the lamellar structure of ferrite and cementite. Therefore, a method for increasing the strength of pearlite steel sheets without containing special elements was studied. As a result, it has been found that high strength can be achieved by making the structure a ferrite and carbide layer (hereinafter also referred to as a layered structure) that is strongly processed by cold rolling. And the steel sheet having the above structure is a component composition utilizing the solid solution strengthening of Si and Mn as a steel sheet before cold rolling, the main phase is a pearlite structure, the ferrite structure is 20% or less, and the pearlite structure It has also been found that a steel sheet having a structure with a lamellar spacing of 500 nm or less and a Vickers hardness of HV200 or more can be obtained by cold rolling.
Conventionally, when cold-rolling a pearlite steel sheet, workability has become a problem, and cracks tend to occur during rolling, making it difficult to prepare samples. On the other hand, in this invention, the steel plate before a cold rolling was made into the steel plate which consists of a specific component and a specific structure, and succeeded in obtaining the high strength steel plate derived from a pearlite structure | tissue by a strong process. This is a feature in the present invention and an important requirement.

  The steel structure has a main phase structure in which ferrite and carbide form a layer, and the carbide has an aspect ratio of 10 or more and a layered structure in which the interval between the layers is 50 nm or less with respect to the entire structure. The volume ratio is 65% or more. With such a structure, a high-strength steel plate of 1500 MPa or more can be obtained.

  Even if it does not contain special elements, it is not clear why the cold rolling process has become possible, but by dissolving Si and Mn in the ferrite, the ferrite is solid-solution strengthened and the void It is conceivable that the occurrence is suppressed and the cold workability is improved.

  As described above, the present invention provides a high strength in a steel sheet having a pearlite structure that could not be obtained stably in the past, as a layered structure composed of a chemical component and a structure, that is, ferrite and carbide, and a layered structure. This was achieved by controlling the aspect ratio and layer spacing of carbides in the structure, and the present invention was completed.

  Next, the improvement of the bendability and delayed fracture resistance of the high-strength steel plate of 1500 MPa or higher was studied. In such a case, among the carbides forming a layer with ferrite, the aspect ratio is 10 or more and the fraction of carbides having an angle of 25 ° or less with respect to the rolling direction is set to 75 MPa or more by area ratio, and 1500 MPa or more. Although it was a high-strength steel sheet, it was found that it was excellent in bendability and delayed fracture resistance. This is considered to be because the carbides extended in the rolling direction reinforce the bending direction like a fiber structure.

  From the above, preferably, the fraction of the carbide forming a layer with ferrite has an aspect ratio of 10 or more and an angle within 25 ° with respect to the rolling direction is 75% or more in terms of area ratio.

  Hereinafter, in carrying out the present invention, the systematic limitation range and the reason for limitation will be described.

Main phase structure: Layered structure composed of ferrite and carbide As described above, a steel sheet having a predetermined pearlite structure is cold-rolled to form a structure in which ferrite and carbide extend in layers in the rolling direction. As a result, it was confirmed that the strength not found in the conventional pearlite structure can be achieved.
At this time, the layered structure extends substantially parallel to the rolling direction, and the extending direction is generally at an angle of 40 ° or less with the rolling direction.
This layered structure is generally derived from a pearlite structure before cold rolling. That is, the layered structure is obtained by cold working based on the pearlite structure before cold rolling. Therefore, the layered structure ratio with respect to the entire structure is determined by the fraction of the pearlite structure before cold rolling. If the pearlite structure before cold rolling is 75% or more of the entire structure, the layered structure is also 75% or more. The main phase structure of the high-strength steel sheet of the present invention is formed. If the pearlite structure before cold rolling is 80% or more of the entire structure, the layered structure is also 80% or more. Form the main phase structure.
In order to obtain the desired strength (tensile strength of 1500 MPa or more), the layered structure needs to be as follows.
A layered structure in which the carbide aspect ratio is 10 or more and the interval between the layers is 50 nm or less is set to 65% or more by volume ratio with respect to the entire structure.
In a structure in which the carbide aspect ratio is small, that is, a structure in which a large number of granular carbides exist, the interface between the carbide and the ferrite in contact with the carbides is considered to be the starting point of voids, and the desired characteristics cannot be obtained. When the aspect ratio is less than 10, the carbide is not expanded, and voids are generated from the interface with the ferrite in contact with the carbide, leading to a decrease in ductility. Therefore, the aspect ratio of carbide is 10 or more.
Further, the finer the layer spacing, the more effective the strength rises. In order to obtain the desired tensile strength, the layer spacing is set to 50 nm or less. If the layer spacing exceeds 50 nm, the target strength (tensile strength of 1500 MPa or more) cannot be obtained.
From the above, it is preferable that the carbide in the layered structure has an aspect ratio of 10 or more and a layer interval between the carbide and ferrite: 50 nm or less. If such a layered structure has a volume ratio of 65% or more with respect to the entire structure, a high strength of 1500 MPa or more can be obtained. Therefore, in the present invention, the aspect ratio of the carbide is 10 or more and the gap between the layers is The layered structure having a thickness of 50 nm or less is set to 65% or more by volume ratio with respect to the entire structure.
In addition, the above structure is obtained by etching a cross section parallel to the rolling direction with nital, taking 5 or more fields of view with a scanning electron microscope (SEM) at a magnification of 5,000 times or more, and measuring it by a method such as image analysis as follows. be able to. Here, the layer spacing means the average distance between the center points in the thickness direction of the adjacent ferrite layers and carbide layers. The average distance is, for example, one ferrite layer and one carbide layer as a set of layers, and in the structure observation, several sets of layers are cut by a line segment of a predetermined length in the direction perpendicular to the layer extending direction. What is necessary is just to measure by measuring. Note that a layer that is not completely cut by the line segment at both ends of the line segment is not measured.
That is, it is calculated by layer interval = line segment length / (number of pairs cut by line segment × 2).
Further, the volume ratio of the layered structure having a layer spacing of 50 nm or less and a carbide aspect ratio of 10 or more can be obtained as follows.

  That is, in a layered structure in which a line segment perpendicular to the rolling direction is drawn in a SEM photograph (cross section in the rolling direction) of 5,000 times or more and the line segment is cut, a layer having a carbide aspect ratio of 10 or more and a layer interval of 50 nm or less The fraction (line segment length) of the portion having the structure is obtained. The said fraction is calculated | required about 5 visual fields or more, and the average value is made into a volume ratio. An example of a line segment is shown in FIG.

Furthermore, in order to obtain a high-strength steel sheet having a tensile strength of 1500 MPa or more and excellent in bendability and delayed fracture resistance, the structure must be as follows.
Among the carbides forming a layer with ferrite, the fraction of carbides with an aspect ratio of 10 or more and an angle of 25 ° or less with respect to the rolling direction is 75% or more in area ratio. When bending, the ferrite / cementite interface is voided. It tends to be the starting point of Therefore, assuming that the rolling direction is the longitudinal direction of the bending sample, in the structure in which the carbides extend in the rolling direction, the carbide whose longitudinal direction is within 25 ° with respect to the bending direction (longitudinal direction of the bending sample) is 75% or more. Being present improves bending workability and also improves delayed fracture resistance. More preferably, it is 80% or more.
Further, the extension direction of such carbide can be measured by the following method. Sheet thickness cross section (rolling direction cross section) parallel to the rolling direction is nital etched and taken with a scanning microscope (SEM) over 5 fields of view at 3,000 times or more and measured as follows by image analysis and other methods. be able to. Of carbides layered with ferrite, for carbides with an aspect ratio of 10 or more, draw a straight line parallel to the major axis of the carbide, and the angle between the straight line and the rolling direction is within 25 °. The fraction is obtained by area ratio. The above-mentioned fraction is obtained for 5 fields of view or more, and the average value is the fraction of carbides forming a layer with ferrite and having an aspect ratio of 10 or more and an angle of 25 ° or less with respect to the rolling direction. .

Next, the limitation range and reason for limitation of the chemical component (component composition) will be described.
C: 0.3-1.0%
C is an indispensable component for ensuring the strength, and it is necessary to contain 0.3% or more in order to generate a pearlite structure necessary for obtaining the target strength (tensile strength of 1500 MPa or more). If the amount of C is less than 0.3%, it is difficult to obtain a sufficient amount of pearlite structure. Preferably it is 0.4% or more. On the other hand, the content of C exceeding 1.0% causes a decrease in ductility and decreases workability. From the above, the C content is 0.3% or more and 1.0% or less. Preferably, it is 0.4% or more and 1.0% or less.

Si: 2.5% or less
Since Si is an element effective for solid solution strengthening, it is preferable to contain 0.01% or more. However, when the amount of Si exceeds 2.5%, surface defects called red scale occur during hot rolling. In addition, when hot dip galvanizing (including alloying) is performed, the wettability of plating is deteriorated to cause uneven plating and the surface appearance is deteriorated. Therefore, the Si content is 2.5% or less. Further, it is preferably 1.5% or less from the viewpoint of plating processability, and more preferably 0.5% or less from the viewpoint of improving surface properties and plating properties.

Mn: 2.5% or less
Mn is preferably contained in an amount of 0.01% or more from the viewpoint of solid solution strengthening. However, when the amount of Mn exceeds 2.5%, segregation occurs and workability deteriorates. Therefore, the Mn content is 2.5% or less.

Si + Mn: 1.0% or more
In order to utilize the solid solution strengthening of Si and Mn, Si and Mn should be 1.0% or more in total. If it is less than 1.0%, the effect is small, cracks occur during cold working, and the desired structure cannot be obtained.

P: 0.005-0.1%
P is an element effective for solid solution strengthening. However, if the amount of P is less than 0.005%, the effect does not appear, so 0.005% or more, preferably 0.01% or more is contained. On the other hand, an excessive P content exceeding 0.1% causes P to segregate at the grain boundaries, causing embrittlement and deteriorating impact resistance. Further, when the alloyed hot-dip galvanized steel sheet is used, the alloying speed is greatly delayed. Therefore, the P content is 0.1% or less.

S: 0.05% or less
S is an inclusion such as MnS, which causes deterioration in impact resistance and cracks along the metal flow of the weld. Therefore, S should be as low as possible, but 0.05% or less from the viewpoint of manufacturing cost.

Al: 0.005-0.1%
Al is useful as a solid solution strengthening element and deoxidation element of steel. Moreover, it has the effect | action which fixes the solid solution N which exists as an impurity and improves normal temperature aging resistance. In order to exert such an effect, the Al content is 0.005% or more. On the other hand, if the Al content exceeds 0.1%, a high alloy cost is incurred and further causes surface defects. From the above, the Al content is 0.005% to 0.1%.

N: 0.01% or less
N is an element that degrades room temperature aging resistance. Further, when the amount of N increases, a large amount of Ti or Al needs to be added to fix the solid solution N. Therefore, it is preferable to reduce as much as possible from these points. However, since it is acceptable up to about 0.01%, the N amount is set to 0.01% or less.

  The balance is Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is desirable to reduce it to 0.003% or less. In the present invention, Cu, W, V, Zr, Sn, and Sb may be contained in a range of 0.1% or less as trace elements that do not impair the effects of the present invention.

Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention is adjusted to the above chemical component range, has a pearlite structure as a main phase, has a ferrite structure of 20% or less, and has a structure in which the lamella spacing of the pearlite structure is 500 nm or less, and has a Vickers hardness Is obtained by subjecting a steel plate of HV200 or more to cold rolling at a rolling rate of 60% or more (preferably 75% or more).
In order to obtain the above-described layered structure of ferrite and carbide after cold rolling, the structure before cold rolling has a pearlite structure as the main phase, and the ferrite structure in the remaining structure is 20% or less by volume with respect to the entire structure. It is necessary to. Note that the pearlite structure as the main phase means that the volume ratio of the pearlite structure to the entire structure is 75% or more.
As the fraction of the ferrite structure increases, the cold workability improves, but in order to ensure the strength after cold rolling, it is necessary to make it 20% or less (including 0%). That is, in order to obtain the layered structure after cold rolling, it is preferably made of a pearlite structure, but may contain 20% or less of a ferrite structure. When the ferrite structure is more than 20%, a strength of 1500 MPa or more cannot be obtained even when cold rolling with a rolling rate of 60% or more is performed. In addition to the pearlite structure and the ferrite structure, carbides, martensite, and the like may inevitably exist. However, if the volume ratio is 5% or less, there is no problem in obtaining the effects of the present invention.
Furthermore, the pearlite lamella spacing needs to be 500 nm or less. If it is larger than 500 nm, transverse cracks occur during subsequent cold rolling, and sufficient cold workability cannot be obtained. Moreover, since the structure after rolling becomes coarse, the strength cannot be ensured. Preferably it is 300 nm or less.
Here, the lamella spacing of pearlite means the average distance between the center points in the thickness direction of the adjacent ferrite layers and carbide layers constituting the lamellae. The average distance is, for example, one ferrite layer and one carbide layer as a set of layers, and the number of layers is determined by a line segment of a predetermined length in a direction perpendicular to the layer extending direction in the structure observation. What is necessary is just to measure by measuring whether it is cut | disconnected. Note that a layer that is not completely cut by the line segment at both ends of the line segment is not measured.
That is, lamella spacing = line segment length / (number of pairs cut by line segment × 2).
Note that the above structure can be measured by a method such as image analysis by etching a cross section parallel to the rolling direction with nital and taking three or more fields of view at 3,000 times or more using a scanning microscope (SEM). In addition, since ferrite and carbide are alternately arranged in a simple manner, (number of sets cut by line segment) may be obtained as (number of carbides).
Furthermore, the Vickers hardness needs to be HV200 or more. If the Vickers hardness is less than HV200, the strength increase due to cold rolling is small and the desired strength cannot be obtained.

And the steel plate which has the said structure before rolling can be manufactured with the following two methods.
1) A steel sheet having the above-described structure before cold rolling is produced by defining a hot rolling process.
The steel slab adjusted to the above chemical composition range is heated to 1100 ° C or higher, then hot rolled to finish rolling exit temperature: 850 ° C or higher, and then cooled at a cooling rate of 15 ° C / s or higher. , Winding temperature: It is set as a hot-rolled sheet at 550-650 degreeC. Next, cold rolling is performed at a rolling rate of 60% or more.

  When the slab heating temperature is less than 1100 ° C., the rolling load increases and the risk of trouble occurring during hot rolling increases. Therefore, the slab heating temperature is set to 1100 ° C. or higher. Note that the upper limit of the slab heating temperature is preferably 1300 ° C. in view of an increase in scale loss accompanying an increase in oxidized weight.

  The steel slab heated under the above conditions is subjected to hot rolling for rough rolling and finish rolling. Here, the steel slab is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed under normal conditions.

  Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet. At this time, the finish rolling outlet temperature (hereinafter also referred to as FT) is set to 850 ° C. or higher. When FT is less than 850 ° C., the load during hot rolling becomes high. Therefore, FT is 850 ° C or higher.

  After finish rolling, the cooling rate to the coil winding temperature (hereinafter sometimes referred to as CT) is 15 ° C./s or more. If the cooling rate is less than 15 ° C / s, pearlite transformation proceeds at CT or higher, resulting in pearlite with a large lamellar spacing, and the desired properties cannot be obtained even by cold rolling. Therefore, the cooling rate is 15 ° C./s or more.

  The coil winding temperature (CT) is 550 ° C to 650 ° C. When the temperature is higher than 650 ° C., the lamella spacing of pearlite becomes coarse and granular carbides are generated, resulting in a decrease in workability during cold working. Further, at a temperature lower than 550 ° C., the progress of the pearlite transformation is slow, and the transformation may not be completed. Therefore, CT is 550 ° C or higher and 650 ° C or lower.

The steel sheet obtained as described above is cold-rolled at a rolling rate of 60% or more to obtain a desired strength (tensile strength of 1500 MPa or more). If the rolling rate is less than 60%, it may be difficult to obtain a tensile strength of 1500 MPa or more. In the present invention, the rolling rate means the reduction rate and is defined by the following equation.
Rolling ratio (%) = (t ₀ −t) / t ₀ × 100
t ₀ : Initial sheet thickness (mm), t: Finished sheet thickness (sheet thickness after rolling) (mm)
2) A steel sheet having the pre-rolling structure is manufactured by heat-treating the hot-rolled sheet.
As another method, the steel adjusted to the above chemical composition range is subjected to hot rolling, and the obtained hot rolled sheet is heated at a heating temperature of 820 ° C. or more, and a cooling rate of 15 ° C. / After cooling to 550 ° C. to 650 ° C. over s, hold at 550 ° C. to 650 ° C. and then cool to room temperature. Next, cold rolling is performed at a rolling rate of 60% or more.

  The conditions for hot rolling for rough rolling and finish rolling do not have to be specified in particular, and are performed under normal conditions to obtain hot rolled sheets.

  Heat the hot-rolled sheet to 820 ° C or higher. If it is less than 820 ° C., the ferrite phase (ferrite structure) is large and the intended strength cannot be obtained. In order to obtain a stable and uniform structure, the temperature is preferably 850 ° C. or higher.

  Then, the pearlite structure is obtained by cooling to 550 ° C. to 650 ° C. at a cooling rate of 15 ° C./s or more capable of suppressing the pearlite transformation, and holding in the temperature range of 550 ° C. to 650 ° C. When the cooling rate is less than 15 ° C./s, pearlite transformation proceeds at a holding temperature or higher, resulting in pearlite having a large lamellar spacing, and the desired properties cannot be obtained even by cold rolling. With regard to the pearlite transformation, when the holding temperature is higher than 650 ° C., the lamella spacing of the pearlite becomes coarse and granular carbides are generated, resulting in a decrease in workability during cold working. Further, it is considered that when the holding temperature is lower than 550 ° C., the progress of the pearlite transformation is slow and the transformation is not completed. The holding time may be held for a time during which the pearlite transformation sufficiently proceeds, and is preferably 15 seconds or longer.

Next, cold rolling is performed after cooling to room temperature. Cooling to room temperature is not specified. Cold rolling is performed at a rolling rate of 60% or more to obtain the desired strength (tensile strength of 1500 MPa or more). If the rolling rate is less than 60%, it may be difficult to obtain a tensile strength of 1500 MPa or more.
Furthermore, when producing a high-strength steel sheet having a tensile strength of 1500 MPa or more and excellent in bendability and delayed fracture resistance, the rolling ratio is set to 75% or more. When the cold rolling rate is less than 75%, a specified amount of carbides extending in the rolling direction cannot be secured, and excellent bendability cannot be obtained. More preferably, it is 80% or more.

As described above, a high-strength steel sheet having a tensile strength of 1500 MPa or more and excellent bendability and delayed fracture resistance can be obtained. Further, after the cold rolling, a hot dip galvanizing treatment or an alloying hot dip galvanizing treatment can be performed. Moreover, according to a conventional method, electroplating processes, such as electrogalvanization, can also be performed.
When hot dip galvanizing is performed, the steel sheet is infiltrated into the plating bath at a bath temperature of 420 to 480 ° C., and the amount of adhesion is adjusted by gas wiping or the like.
Furthermore, when performing an alloying process, it is desirable to process at 480-550 degreeC or less. If the temperature exceeds 550 ° C., the processed structure by cold rolling starts recrystallization, and the target characteristics and structure may not be obtained. Also, the powdering property is deteriorated. Alloying does not proceed at temperatures below 480 ° C.
Further, the plating adhesion amount is preferably ²⁰ to 150 g / m ² per side. If it is less than 20 g / m ^2, the corrosion resistance deteriorates. If it exceeds 150 g / m ^2, the cost will increase and the corrosion resistance will be saturated.
The alloying degree is preferably 7 to 15%. If it is less than 7%, uneven alloying occurs and the appearance is deteriorated, so-called ζ phase is generated, and the slidability is deteriorated. If it exceeds 15%, a large amount of hard and brittle Γ phase is formed and the plating adhesion deteriorates.

  Steel Nos. A to G having the composition shown in Table 1 were melted in a converter and made into a slab by a continuous casting method. Next, these slabs were heated to 1250 ° C., and then finish-rolled at the finish rolling exit temperature shown in Table 2, and then cooled at a cooling rate of 15 ° C./s, and the coiling temperature ranged from 550 to 650 ° C. Winding and steel plates No. 1-8 were obtained.

  In addition, after heating the above slab to 1250 ° C and hot rolling at a finish rolling temperature of 880 ° C and a coiling temperature of 600 ° C, heating at 1000 ° C for 1 hour, It cooled and hold | maintained on the conditions shown, and cooled to room temperature, and obtained steel plate No. 9-17. The holding time was 60 s.

  The steel sheet obtained as described above was measured for pearlite volume fraction, ferrite volume fraction, pearlite lamella spacing, and Vickers hardness as a steel sheet structure observation before cold rolling. In addition, the volume ratio of each structure | tissue measured each area ratio and made this the volume ratio.

  Subsequently, the steel plates No. 1 to 17 are cold-rolled at the cold rolling rates shown in Tables 2 and 3, and the cold workability is evaluated. Observation and investigation of tensile properties, bending workability and delayed fracture resistance were conducted. Details of each survey method are as follows.

  Moreover, about one part, after cold rolling, the alloying hot dip galvanization process was performed and it was set as the plating process steel plate. The plating process was performed in a plating bath having a bath temperature of 463 ° C., and an alloying process was performed at 500 ° C.

Microstructure observation before cold rolling The ferrite fraction is obtained by taking a specimen from each steel sheet before cold rolling, etching the sheet thickness section (L section) parallel to the rolling direction, and performing a scanning electron microscope (SEM). ), And taken three or more fields of view at 1,000 times, and measured by techniques such as image analysis.
In addition, the pearlite lamella spacing (S ₀ ) was obtained by the following equation in accordance with the above method using SEM, imaging 3 fields or more at 3,000 times or more. Here, since ferrite and carbide are alternately arranged, (number of sets cut by line segment) was used as the number of carbides (n).
S ₀ = L / 2 (1)
L: Average intercept interval divided by the number of carbides n in the arbitrary length l Note that the arbitrary length l is a length satisfying n ≧ 20.

Vickers hardness
Measurement was performed in accordance with JIS Z 2244. The test force was 9.8 N (1 kgf).

Cold workability Cold workability was evaluated by determining whether the rolling rate that can be rolled without causing lateral cracks was the critical rolling rate, and whether rolling was possible without causing cracks of 60% or more. In other words, a cold rolling rate of 60% or more and rolling without transverse cracks was evaluated as ◯.

The aspect ratio of carbide in the lamellar structure composed of ferrite and iron-based carbide after cold working is taken from the steel sheet after each cold rolling, and the thickness section (L section) parallel to the rolling direction is taken. Nital etching was performed, and using a scanning electron microscope (SEM), images of 5 fields or more were taken at 5,000 times or more, and measured by an image analysis technique.
The aspect ratio is defined as “longer diameter (length (maximum diameter)) / short diameter (thickness (minimum diameter)) of carbides”, and the detailed measurement is omitted for those whose ratio is clearly 10 times or more. The layer spacing and the volume ratio of the layered structure were determined by the above methods. The length of the line segment was set so that the number of layers N ≧ 20.
Further, the angle between the major axis of the carbide satisfying an aspect ratio of 10 or more and the rolling direction was measured as follows. A plate thickness section (L section) parallel to the rolling direction was subjected to nital etching, and five or more fields of view were photographed with a scanning microscope (SEM) at 3,000 times or more, and measured by an image analysis technique. A straight line is drawn parallel to the major axis of the carbide having an aspect ratio of 10 or more, and for the carbide in which the straight line is within 25 ° with respect to the rolling direction, the fraction (area ratio) with respect to the carbide forming a layer with ferrite is obtained, and the average The value was determined.

Tensile properties JIS No. 5 tensile specimens were sampled from each cold-rolled annealed sheet in the 90 ° direction (C direction) with respect to the rolling direction, and the crosshead speed was 10 mm / min in accordance with the provisions of JIS Z 2241. A tensile test was performed to determine tensile strength (TS (MPa)), yield strength (YS (MPa)), and total elongation (El (%)).

Bending workability Bending workability was evaluated by taking a test piece (width 30 mm, length 100 mm, thickness 1 mm) having a longitudinal direction at 0 ° (L direction) with respect to the rolling direction, and evaluating it by a V bending test. The case where the bending radius that can be formed without cracking was 4 times or less the plate thickness (R / t ≦ 4, R: bending radius (mm), t: plate thickness (mm)) was judged to be good.

Delayed fracture resistance The delayed fracture resistance is obtained by taking a specimen (width 30mm, length 100mm, thickness 1mm) in the 0 ° direction (L direction) with respect to the rolling direction, and bolting after U bending (R = 10mm). The fastened sample was immersed in hydrochloric acid having pH = 3, and an undestructed sample was judged good for 48 hours or more.

  The results obtained as described above are shown in Table 2 and Table 3 together with the conditions.

  From Table 2 and Table 3, it can be seen that in the present invention examples, high strength steel sheets having a tensile strength of 1500 MPa or higher are obtained while maintaining sufficient basic performance. It can also be seen that the bendability and delayed fracture resistance are further improved by setting the structure having an angle of 25 ° or less to the rolling direction to 75% or more.

  The steel plate of the present invention can be suitably used for parts such as various automobiles that require high strength, centering on the outer plate of the automobile.

Claims (7)

  Ingredient composition is mass%, C: 0.3-1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005-0.1%, S: 0.05% or less, Al: 0.005-0.1%, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the main phase structure is a layer of ferrite and carbide, and the carbide aspect ratio is 10 or more, A high-strength steel sheet having a tensile strength of 1500 MPa or more, wherein a layered structure having a space between the layers of 50 nm or less is 65% or more by volume ratio to the whole structure.   Further, among the carbides forming a layer with ferrite, the fraction of carbides having an aspect ratio of 10 or more and an angle of 25 ° or less with respect to the rolling direction is 75% or more by area ratio. Item 1. A high-strength steel sheet having a tensile strength of 1500 MPa or more as described in item 1. It is a manufacturing method of the high strength steel plate according to claim 1 or 2,
In mass%, C: 0.3-1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005-0.1%, S: 0.05% or less, Al: 0.005-0.1% , N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the pearlite structure is the main phase, and the ferrite structure in the balance structure is 20% or less (0%) The pearlite structure has a lamella spacing of 500 nm or less, and a steel sheet having a Vickers hardness of HV200 or more is subjected to cold rolling at a rolling rate of 60% or more. A method for producing high-strength steel sheets with a tensile strength of 1500 MPa or more. It is a manufacturing method of the high strength steel plate according to claim 1 or 2,
In mass%, C: 0.3-1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005-0.1%, S: 0.05% or less, Al: 0.005-0.1% , N: 0.01% or less, with the balance being a steel slab having a composition composed of Fe and inevitable impurities, heated to 1100 ° C or higher, and then subjected to hot rolling at a finish rolling exit temperature of 850 ° C or higher. After cooling at a cooling rate of 15 ° C / s or more, winding at a winding temperature of 550 to 650 ° C, and then cold rolling at a rolling rate of 60% or more. A manufacturing method of high strength steel plate of 1500MPa or more. It is a manufacturing method of the high strength steel plate according to claim 1 or 2,
In mass%, C: 0.3-1.0%, Si: 2.5% or less, Mn: 2.5% or less, Si + Mn: 1.0% or more, P: 0.005-0.1%, S: 0.05% or less, Al: 0.005-0.1% , N: 0.01% or less, the remainder of the hot-rolled sheet having a component composition consisting of Fe and inevitable impurities is heated at a heating temperature of 820 ° C. or more, and a cooling rate of 550 ° C. to 15 ° C./s or more. After cooling to 650 ° C., holding at 550 ° C. to 650 ° C., cooling to room temperature, and then cold rolling at a rolling rate of 60% or more of a high strength steel plate having a tensile strength of 1500 MPa or more Production method.   The method for producing a high-strength steel sheet having a tensile strength of 1500 MPa or more according to any one of claims 3 to 5, wherein the cold rolling is performed at a rolling rate of 75% or more.   After the cold rolling, a hot dip galvanizing treatment or an alloying hot dip galvanizing treatment is further performed. The high strength steel sheet having a tensile strength of 1500 MPa or more according to any one of claims 3 to 6 Production method.