JP2005076078A - High tensile strength steel sheet having excellent workability, and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To establish a technique capable of satisfying the balance of strength, total elongation, and stretch flanging properties (hole stretchability) on higher levels. <P>SOLUTION: The steel sheet has a base phase structure and a second phase structure. The base phase structure is at last composed of tempered martensite or tempered bainite, and the second phase structure is composed of residual austenite. The ratio of the residual austenite is 5 to 40 vol.%. Further, when the maximum distance between the second phase structures confronted with the base phase structure interposed is measured based on an optical photomicrograph, the maximum distance is ≤8 μm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength steel sheet excellent in workability (stretch flangeability and total elongation), and relates to an improved technique for a TRIP (Transformation Induced Plasticity) steel sheet.

Steel sheets used for press forming in automobiles, industrial machines and the like are required to have both excellent strength and workability, and such required characteristics have been increasing in recent years. In recent years, TRIP steel sheets have attracted attention in order to meet such expectations. The TRIP steel sheet has a retained austenite structure, and when deformed at a temperature equal to or higher than the martensite transformation start temperature (Ms point), the retained austenite (γ _R ) is induced and transformed into martensite by stress, resulting in a large elongation. For example, TRIP type composite structure steel (PF steel) composed of polygonal ferrite + bainite + residual austenite structure, and TRIP type bainite steel (BF steel) composed of bainetic ferrite + residual austenite + martensite. Are known. However, although PF steel is inferior in stretch flangeability, BF steel has the disadvantage that although it is excellent in stretch flangeability, the elongation is small.

In view of this, various studies have been made to provide a steel sheet having excellent formability such as stretch flangeability (hole expandability) while maintaining excellent strength / elongation balance due to retained austenite. For example, in Patent Documents 1 to 4, steel sheets having tempered martensite and tempered bainite as the parent phase structure and retained austenite as the second phase structure are excellent in strength, elongation, and stretch flangeability. Has been introduced. For example, these steel sheets are cooled in a specific pattern from a ferrite-austenite two-phase region temperature after adjusting the cooling rate after hot rolling to introduce a martensite structure, a bainite structure, etc., and performing cold rolling. To produce retained austenite.
JP 2002-309334 A JP 2002-302734 A JP 2003-73773 A JP 2003-171735 A

  The present invention has been made paying attention to the circumstances as described above, and its purpose is a technique that can satisfy the balance of strength, total elongation, and stretch flangeability (hole expansion ratio) at a higher level. Is to establish.

  As a result of intensive studies to solve the above problems, the present inventors have found that if the distance between the second phase structures (structures including residual austenite) is further reduced, the strength, total elongation, and stretch flangeability (holes) In order to satisfy the spreading ratio at a higher level and to further reduce the distance between the second phase structures, a cold rolling process performed prior to heat treatment (such as two-phase region heating) to generate retained austenite. The inventors have found that it is extremely effective to strictly control the rolling reduction, and have completed the present invention.

That is, the high-tensile steel sheet excellent in workability according to the present invention has a matrix structure and a second-phase structure, and the matrix structure contains at least tempered martensite or tempered bainite and, if necessary, ferrite. The second phase structure is composed of retained austenite. And this steel plate is (1) C: 0.10-0.6 mass%, Si + Al: 0.1-2 mass%, Mn: 1.0-3 mass%, P: 0.02 mass% or less (0 Mass%) and S: 0.03 mass% or less (excluding 0 mass%),
(2) The retained austenite is 5 to 40% by volume,
(3) The gist of the present invention is that when the maximum distance between the second phase structures facing each other with the matrix structure interposed therebetween is measured based on an optical micrograph, the maximum distance is 8 μm or less.

  The high-tensile steel sheet is further controlled in the form of sulfides such as (1) Ca: 0.003% by mass or less (excluding 0% by mass), REM: 0.003% by mass or less (not including 0% by mass). Element, (2) Nb: 0.1% by mass or less (excluding 0% by mass), Ti: 0.1% by mass or less (not including 0% by mass), V: 0.1% by mass or less (0% by mass) (3) Mo: 2% by mass or less (not including 0% by mass), Ni: 1% by mass or less (not including 0% by mass), Cu: Residual austenite stabilizing elements such as 1% by mass or less (not including 0% by mass) and Cr: 2% by mass or less (not including 0% by mass) may be contained.

  The area ratios of tempered martensite, tempered bainite, and ferrite (the area of the entire photo is 100%) are as follows, for example, when measured with an optical micrograph.

Tempered martensite or tempered bainite: 20 to 85 area%
Ferrite: 0 to 60 area%
It is recommended that the retained austenite contains at least 60 area% of lath-like retained austenite having a major axis / minor axis ratio of 3 or more with respect to the total retained austenite.

  According to the high-tensile steel plate of the present invention, even when the tensile strength is 750 to 1050 MPa, the tensile strength TS, the total elongation El, and the hole expansion ratio λ are related to the following formulas (1) and (2). Can be satisfied.

TS × El ≧ 24000 (1)
TS × λ ≧ 20000 (2)
[Wherein, TS represents the measurement result of tensile strength (unit: MPa), El represents the measurement result of total elongation (unit:%), and λ represents the measurement result of hole expansion ratio (unit:%)]
The high-tensile steel plate as described above is, for example, C: 0.10 to 0.6% by mass, Si + Al: 0.1 to 2% by mass, Mn: 1.0 to 3% by mass, P: 0.02% by mass. The steel sheet containing the following (not including 0% by mass) and S: 0.03% by mass or less (not including 0% by mass) and having the martensite structure or bainite structure introduced is reduced by 12 to 28. %, Then heated to the ferrite-austenite two-phase region temperature, and then rapidly cooled to a temperature not lower than the Ms point and not higher than the Bs point while avoiding the ferrite transformation, pearlite transformation, and bainite transformation. Can be produced by increasing the C concentration of the austenite and decreasing the Ms point of the austenite phase.

  The present invention also includes steel parts obtained by processing the high-tensile steel plate.

  According to the present invention, since the distance between the second phase structures (structures including residual austenite) is extremely close, the strength, total elongation, and stretch flangeability (hole expansion ratio) can be satisfied at a higher level.

[Organization]
The steel sheet of the present invention is characterized by its structure and components. First, the organization that most characterizes the present invention will be described.

  In the steel sheet of the present invention, when the structure is observed with an optical microscope, a matrix structure and a second phase structure dispersed in an island shape with respect to the matrix structure are observed. The matrix structure is optical. The micrograph shows gray and is composed of at least a tempered martensite structure or a tempered bainite structure. In addition to these tempered martensite structure or tempered bainite structure, the ferrite structure may also be a constituent structure of the matrix structure. is there. On the other hand, the second phase structure (island-like structure) is white in the optical micrograph and is composed of retained austenite. A black portion composed of cementite may be observed, and the black portion is also included in the second phase structure in that it is dispersed in an island shape.

  The fact that the steel sheet of the present invention has the above structure is an important point in order to balance the strength, total elongation, and stretch flangeability (hole expansion ratio) at a high level. In other words, tempered martensite and tempered bainite are characterized by the fact that the crystal grains are lath-like and the hardness is high, but the dislocation density is small and soft compared to the normal martensite structure and bainite structure. “Tempered martensite and tempered bainite” and “martensitic structure and bainite structure” can be distinguished by, for example, observation with a transmission electron microscope (TEM). The presence of these “tempered martensite” and “tempered bainite” as the parent phase is important in terms of enhancing both the total elongation and stretch flangeability. As described above, the matrix structure may contain ferrite in addition to the tempered tempered martensite and tempered bainite. The ferrite precisely means polygonal ferrite, that is, ferrite having a low dislocation density. When ferrite is included, stretch flangeability can be further enhanced. For example, when the area ratio of the structure is measured with an optical micrograph (the structure can be distinguished by TEM observation, hardness measurement, etc.), the area ratio of tempered martensite, tempered bainite, and ferrite (the area of the entire photograph is 100%) is as follows.

Tempered martensite or tempered bainite: 20 area% or more (for example, 25 area% or more, or 30 area% or more) 85 area% or less (for example, 65 area% or less, or 50 area% or less)
Ferrite: 0 area% or more (for example, 10 area% or more, or 15 area% or more) 60 area% or less (for example, 50 area% or less, or 40 area% or less)
Residual austenite is an essential structure for exhibiting the TRIP (transformation-induced plasticity) effect, and is useful for improving the total elongation. The amount of retained austenite can be examined by a saturation magnetization measurement method, and it is necessary to be 5% by volume or more (preferably 8% by volume or more, more preferably 10% by volume or more). However, when the amount of retained austenite becomes large, stretch flangeability (hole expansion rate) deteriorates. Accordingly, the residual austenite needs to be 40% by volume or less (preferably 30% by volume or less, more preferably 20% by volume or less).

  Meanwhile, the retained austenite in the conventional TRIP steel plate has retained austenite with random orientation in the prior austenite grain boundaries, whereas the retained austenite of the present invention is substantially the same along the block boundary in the same packet. There is also a feature that residual austenite having an orientation exists.

  It is possible to balance the strength, total elongation, and stretch flangeability (hole expansion ratio) at a high level by using the above structure, but in the present invention, the second phase structures are brought close to each other. , Strength, total elongation, and stretch flangeability (hole expansion ratio) are satisfied at a higher level. FIG. 1 shows the maximum distance between the second phases (more precisely, the maximum distance between the second phase structures facing each other across the matrix structure. For example, in the partially enlarged schematic diagram of the optical micrograph shown in FIG. FIG. 2 is a graph showing the relationship between the maximum distance between the second phases and stretch flangeability (hole expansion rate). As is apparent from these graphs, as the maximum distance between the second phases becomes shorter, both the total elongation and stretch flangeability (hole expansion ratio) are improved. Therefore, in the present invention, the maximum distance between the second phases is 8 μm or less, preferably 6 μm or less, more preferably 5 μm or less, and the total elongation and stretch flangeability (hole expansion rate) have been further improved.

  The parent phase and the second phase are preferably formed with a structure substantially as described above, but other structures inevitably remain in the manufacturing process (pearlite, the parent phase is a tempered martensite structure. Tempered bainite structure in the case, tempered martensite structure in the case where the matrix is a tempered bainite structure) and the inclusion of precipitates is not excluded.

  In the steel sheet of the present invention, it is desirable that the retained austenite has a lath-like (needle-like) form. The TRIP steel sheet having lath-like retained austenite not only has an equivalent TRIP (transformation-induced plasticity) effect, but also has a noticeable effect of improving stretch flangeability as compared with the TRIP steel sheet having spherical retained austenite. Because. The ratio of the retained austenite having a major axis / minor axis ratio of 3 or more can be estimated from the area ratio of the white portion having a major axis / minor axis ratio of 3 or more in the optical micrograph, and the total retained austenite (total For example, 60% by area or more, preferably 65% by area or more, and more preferably 70% by area or more with respect to the white portion) is recommended.

[component]
Next, the components of the steel sheet of the present invention will be described. Hereinafter, all units of chemical components mean mass%.

C: 0.10 to 0.6%
C is an essential element for securing high strength and retaining retained austenite. Specifically, it is an important element for dissolving sufficient C in the austenite phase and leaving the desired austenite phase at room temperature, and is useful for increasing the balance between strength and stretch flangeability. The C content is 0.10% or more, preferably 0.13% or more, and more preferably 0.15% or more. On the other hand, when C is excessive, not only the effect is saturated, but also defects due to center segregation are likely to occur at the casting stage. Accordingly, the C content is 0.6% or less, preferably 0.5% or less, and more preferably 0.4% or less. In addition, when the amount of C exceeds 0.3%, the weldability is lowered. Therefore, considering the weldability, it is recommended that the C content be 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

Si + Al: 0.1 to 2%
Si and Al are effective elements for suppressing the decomposition of retained austenite and the formation of carbides. In particular, Si is also useful as a solid solution strengthening element. Accordingly, Si and Al are added in a total amount of 0.1% or more, preferably 0.3% or more, more preferably 0.5% or more, particularly 1.0% or more. However, even if 2% or more in total is added, not only the effect is saturated and the economic efficiency is lowered, but also hot brittleness is easily caused. Accordingly, the total content of Si and Al is 2% or less, preferably 1.8% or less.

Mn: 1.0 to 3%
Mn is an element effective for stabilizing austenite and securing a retained austenite of a predetermined amount or more. Therefore, Mn is 1.0% or more, preferably 1.2% or more, more preferably 1.3% or more. On the other hand, if Mn is excessive, it will cause slab cracking. Therefore, Mn is 3% or less, preferably 2.5% or less, more preferably 2.0% or less.

P: 0.02% or less (excluding 0%)
P is an element effective for securing desired retained austenite. Therefore, it is recommended that P be essential (over 0%), 0.001% or more, preferably 0.005% or more. However, when P is excessive, secondary workability deteriorates. Therefore, P is 0.02% or less, preferably 0.015% or less.

S: 0.03% or less (excluding 0%)
Since S forms sulfide inclusions such as MnS and becomes a starting point of cracking and deteriorates workability, it is desirable to reduce it as much as possible. Therefore, S is 0.03% or less, preferably 0.01% or less, more preferably 0.005% or less. Note that it is difficult to set S to 0%, and S is about 0% (usually 0.001% or more).

  The steel sheet of the present invention may contain the following components in addition to the essential components.

At least one selected from Ca: 0.003% or less (excluding 0%) and REM: 0.003% or less (not including 0%), these Ca and REM (rare earth elements) are all in steel. It is an element that controls the form of sulfides and is effective in improving workability. Examples of the rare earth element include Sc, Y, and lanthanoid. In order to effectively exhibit the above action, it is recommended that the content be 0.0003% or more (particularly 0.0005% or more). However, even if added excessively, the effect is saturated and the economic efficiency is lowered, so that the content is 0.003% or less (particularly 0.002% or less).

At least selected from Nb: 0.1% or less (not including 0%), Ti: 0.1% or less (not including 0%), and V: 0.1% or less (not including 0%) One kind of these Nb, Ti, and V has an effect of precipitation strengthening and structure refinement, and is an element useful for increasing the strength. In order to effectively exhibit such an action, it is recommended that the content be 0.01% or more (particularly 0.02% or more). However, even if added excessively, the effect is saturated and the economic efficiency is lowered. Therefore, the content is 0.1% or less (preferably 0.08% or less, more preferably 0.05% or less).

Mo: 2% or less (not including 0%), Ni: 1% or less (not including 0%), Cu: 1% or less (not including 0%), and Cr: 2% or less (including 0%) Absent)
These No, Ni, Cu, and Cr are useful elements for strengthening steel and are effective elements for stabilizing the retained austenite and securing a predetermined amount or more. In order to effectively exhibit such an action, it is recommended that the content be 0.05% or more (particularly 0.1% or more). However, even if added in excess, the effect is saturated and the economic efficiency is lowered, so Mo and Cr are each 2% or less (preferably 1% or less, more preferably 0.8% or less), Ni and Cu are each 1 % Or less (preferably 0.5% or less, more preferably 0.4% or less).

  The steel plate of the present invention may further contain other elements as long as the above-described structural characteristics can be maintained, while the balance may be Fe and inevitable impurities.

  Since the steel sheet of the present invention is formed with a specific component and a specific structure as described above, more specifically, since the distance between the second phase structures (structures including residual austenite) is short, the strength, Stretch and stretch flangeability (hole expansion ratio) are balanced at an extremely excellent level. For example, the steel sheet of the present invention can achieve both excellent total elongation and stretch flangeability (hole expansion ratio) even when the tensile strength is 750 to 1050 MPa (that is, about 780 MPa class to about 980 MPa class). For example, according to the steel sheet of the present invention, the tensile strength TS, the total elongation El, and the hole expansion ratio λ can satisfy the relationship of the following formula (1) and the following formula (2).

TS × El ≧ 24000 (1)
TS × λ ≧ 20000 (2)
[Wherein, TS represents the measurement result of tensile strength (unit: MPa), El represents the measurement result of total elongation (unit:%), and λ represents the measurement result of hole expansion ratio (unit:%)]
According to the particularly excellent steel sheet of the present invention, the left side (TS × El) of the above formula (1) can be set to 24500 or more (particularly 25000 or more), and the left side (TS × λ) of the above formula (2). ) May be 38000 or more (particularly 40000 or more).

[Production method]
The above-described TRIP steel sheet of the present invention is a steel sheet in which a martensite structure (non-tempered martensite structure; quenched martensite structure) or a bainite structure (untempered bainite structure) is introduced (component composition is a TRIP steel sheet). Is cold-rolled at a reduction ratio of 12 to 28%, and then heated to a ferrite-austenite two-phase temperature, and then subjected to ferrite transformation, pearlite transformation, and bainite transformation to a temperature not lower than Ms and not higher than Bs. It can be manufactured by quenching while avoiding, increasing the C concentration of the austenite phase in the temperature range and lowering the Ms point of the austenite phase. When a steel sheet having a martensite structure or a bainite structure (including those having a martensite-ferrite structure or a bainite-ferrite structure) is heated to a two-phase region and cooled in a predetermined pattern, the parent phase (tempered martensite) , Tempered bainite, and the like) can be generated in a second phase (a phase containing residual austenite). And if it cold-rolls on suitable conditions prior to this heat processing, the 2nd phase (phase containing a retained austenite) produced | generated at the time of heat processing can be adjoined. For example, when cold-rolling a steel sheet composed of a martensite-ferrite structure or a bainite-ferrite structure, even if the rolling reduction is too low or too high, the distance between the second phases becomes long and the total elongation or stretch flangeability (hole Although the improvement of the expansion ratio is insufficient, the distance between the second phases can be shortened by controlling to an appropriate reduction ratio, and the total elongation and stretch flangeability (hole expansion ratio) can be remarkably improved. In addition, even when a steel sheet having a martensite structure or a bainite structure, which is substantially free of a ferrite structure, is cold-rolled, the effect of increasing the distance between the second phases when the rolling reduction is low (total elongation and Although there is a tendency to alleviate the decrease in stretch flangeability), the effect of increasing the distance between the second phases when the rolling reduction is high (total elongation and decrease in stretch flangeability) tends to be amplified and is also appropriate. It is necessary to set the reduction ratio. Specifically, the rolling reduction is set within a range of about 12% or more (preferably 15% or more, more preferably 18% or more) and 28% or less (preferably 25% or less, more preferably 23% or less). .

  4 shows an optical micrograph when the reduction ratio is 0%, FIG. 5 shows an optical micrograph when the reduction ratio is 20%, and FIG. 6 shows an optical micrograph when the reduction ratio is 60%. As is clear from these micrographs, the distance between the second phases is long regardless of whether the rolling reduction is 0% or 60%, while when the rolling reduction is 20%, The distance is shortened.

  As is apparent from FIGS. 4 to 6, the rolling reduction contributes to an increase in lath-like retained austenite. The lath-like retained austenite increases as the rolling reduction decreases. In the present invention, since the rolling reduction is defined in a narrow range from the viewpoint of shortening the distance between the second phases, it is difficult to vary the amount of lath austenite drastically by greatly varying the rolling reduction. When it is desired to increase the state of retained austenite, a lower reduction ratio may be selected from the range.

  Further, FIG. 7 shows an optical micrograph when a steel sheet having a ferrite-pearlite structure is heated to a two-phase region as described above and cooled in a predetermined pattern, not a steel sheet in which a martensite structure or a bainite structure is introduced. As is apparent from this photograph, when the initial structure (before cold rolling or before two-phase region heating) is ferrite-pearlite, the distance between the second phases increases and the lath-like retained austenite decreases. Therefore, at the beginning, as described above, a martensite structure or a bainite structure needs to be introduced.

  In addition, the steel plate in which the martensite structure or the bainite structure is introduced can be obtained according to a conventional method. In other words, a martensite structure can be introduced by rapidly cooling a steel sheet heated to the austenite region to a temperature below the Ms point, and a bainite structure is introduced by quenching to a temperature above the Ms point and below the Bs point and then isothermally transforming. can do. The ferrite structure can be introduced by setting a cooling pattern so as to pass through the ferrite transformation region in the continuous cooling transformation curve (CCT curve). Since the pearlite structure is not desirable for the present invention, the cooling pattern is set so as to avoid the pearlite transformation region. By the way, for the purpose of generating a martensite structure or a bainite structure, a method of rapidly cooling to a predetermined temperature is simple, but when generating a ferrite structure, the ferrite structure should be stably introduced by monotonic cooling. Therefore, it is recommended to adopt a multi-stage cooling method in which the cooling rate is set in multiple times, and in particular, a method in which cooling is started again after holding at the austenite-ferrite two-phase region temperature is recommended. . In any case of adopting any of the above cooling patterns, it is recommended that the cooling rate be, for example, 10 ° C./second or more (preferably 20 ° C./second or more).

In consideration of actual operation, it is efficient to introduce a martensite structure or a bainite structure in the cooling process after hot rolling. In this case, the hot finishing temperature (FDT) is set to about (A _r3 -50) ° C., and after cooling with the various cooling patterns described above, the temperature below the Ms point (when a martensite structure is introduced), or the Ms point It is recommended to wind at a temperature equal to or higher than the Bs point (when a bainite structure is introduced). In addition, the start temperature (SRT) of hot rolling can be selected from the range which can maintain the said finishing temperature, for example, is about 1000-1300 degreeC.

  The heat treatment method after cold rolling will be described in more detail as follows.

The reason for heating to the ferrite-austenite two-phase region temperature (A ₁ point or more and A ₃ point or less) is to generate an austenite structure while leaving a martensite structure and a bainite structure. The heating time of the two-phase region temperature can be appropriately selected according to the set amounts of tempered martensite, tempered bainite, and retained austenite in the target TRIP steel sheet, and varies depending on the heating temperature and the subsequent cooling rate. Therefore, it is difficult to define uniformly, but for example, a range of 10 seconds or more (preferably 20 seconds or more, more preferably 30 seconds or more), 600 seconds or less (preferably 500 seconds or less, more preferably 400 seconds or less). You can choose from. If the heating time is too short, the residual austenite structure is insufficient, and if the heating time is too long, the tempered martensite structure and the tempered bainite structure are insufficient (or the lath-like structure that is characteristic of tempered martensite and tempered bainite is impaired). At the same time, coarsening of the retained austenite structure or formation of carbides due to decomposition of the retained austenite structure is likely to occur.

  The rapid cooling from the two-phase temperature is to avoid ferrite transformation, pearlite transformation, and bainite transformation. Specifically, it is fast enough to avoid the Fs line, Ps line, Bs line, etc. in the CCT curve. The cooling is performed (for example, at a speed of 3 ° C./second or more, preferably 5 ° C./second or more).

  After cooling to a temperature not lower than the Ms point and not higher than the Bs point [for example, a temperature of 300 ° C. or higher (preferably 350 ° C. or higher) 480 ° C. or lower (preferably 450 ° C. or lower)], the C concentration of the austenite phase is increased in the temperature range. This is because the amount of retained austenite is secured by lowering the Ms point of the austenite phase. The holding time in the temperature range can be appropriately set according to the amount of austenite generated at the two-phase range temperature and the set amount of retained austenite in the target TRIP steel sheet, but it is difficult to prescribe uniformly. For example, it is 10 seconds or more (preferably 50 seconds or more). If the holding time is too long, the bainite transformation proceeds and the amount of retained austenite decreases. Therefore, it is recommended that the holding time be controlled to 300 seconds or less, preferably 200 seconds or less.

  Considering actual operation, it is easy to perform the heat treatment after cold rolling using a continuous annealing facility. When plating a cold-rolled plate, the plating conditions may be set so as to satisfy the heat treatment conditions, and the heat treatment may be performed in the plating step.

  The steel sheet of the present invention is not only excellent in strength but also excellent in total elongation and stretch flangeability, and can be easily processed. Therefore, high strength steel parts can be provided.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental example 1
Test steels having the composition shown in Table 1 below (units in the table are mass%) are vacuum-melted to obtain a slab for experiment having a thickness of 20 to 30 mm, and then hot rolling shown in FIG. -1 stage (monotonic) cooling pattern or hot rolling shown in FIG. 8 (b)-2 stage cooling pattern to form a hot rolled sheet having a thickness of 2.0 to 2.5 mm, and further cold rolling to a sheet thickness of 2 A cold rolled sheet of 0.0 mm was produced. This cold-rolled sheet was heated to a ferrite-austenite two-phase region temperature (800 ° C.), held for 120 seconds, soaked, rapidly cooled to a temperature of 400 ° C., and held for 120 seconds to produce a TRIP steel sheet. The meanings of the symbols in FIGS. 8A and 8B are as follows.
SRT: hot rolling heating temperature FDT: hot rolling finishing temperature CR1: first stage cooling rate CTN: first stage cooling temperature CR2: second stage cooling speed CT: winding temperature Tables 2 to 4 below show the conditions of hot rolling-one-stage or two-stage cooling, the structure of the hot-rolled sheet, and the reduction ratio of cold rolling. Tables 2 to 4 below also show the structure, tensile strength (TS), total elongation (El), stretch flangeability (hole expansion ratio: λ) of the obtained TRIP steel sheet. In addition, in Tables 2 to 4 below, a TRIP steel sheet having a tensile strength (TS) of 790 to 810 MPa is extracted, and the relationship between the maximum distance between the second phases and the total elongation is shown in FIG. FIG. 2 shows the relationship between the maximum distance between the second phases and the hole expansion ratio λ.

  In addition, the structure | tissue of the hot rolled sheet and TRIP steel plate shown to the said Tables 2-4 was investigated as follows. That is, the steel plate is repeller-corroded and the structure is identified by observation with a transmission electron microscope (TEM; magnification: 15000 times), and then the areas of tempered martensite, tempered bainite, and ferrite based on an optical micrograph (magnification: 1000 times). The rate was calculated. The ratio of lath-like retained austenite (residual austenite having a major axis / minor axis ratio of 3 or more) to the total retained austenite and the maximum distance between the second phases were also measured based on the optical micrograph. On the other hand, the volume fraction of retained austenite was measured by a saturation magnetization measurement method [Japanese Patent Laid-Open No. 2003-90825, R & D Kobe Steel Engineering Reports / Vol. 52, no. 3 (Dec. 2002)].

  Tensile strength (TS) and total elongation (El) were measured using a JIS No. 5 test piece.

  For stretch flangeability, a test piece with a length of 70 mm x width 70 mm x thickness 2.0 mm was prepared, a hole with a diameter of 10 mm was punched out in the center, and then the hole was expanded on the beam with a 60 ° conical punch. Evaluation was made by measuring the hole expansion rate (λ) at the time of penetration (Iron and Steel Federation Standard JFST 1001).

  As apparent from FIGS. 2 and 1, the shorter the distance between the second phases, the better the hole expansion ratio λ and the total elongation El.

  As is clear from Table 2, if the hot-rolled structure is ferrite-pearlite, the maximum distance between the second phases becomes long (Experiment No. 9). In order to shorten the maximum distance between the second phases, it is effective to set the hot-rolled structure to martensite, ferrite-martensite, bainite, or ferrite-bainite (Experiment Nos. 1 to 8). When the quenching end temperature CT is set low (200 ° C. in this example), a martensite structure can be generated (Experimental Examples 1-4), and when the quenching end temperature CT is set high (400 ° C. in this example) Can produce a bainite structure (Experiment Nos. 5 to 8). Furthermore, when rapid cooling is performed in two stages, the ferrite structure can be easily mixed (Experiment Nos. 2 to 4 and 6 to 8).

  Furthermore, as is clear from Table 3, the maximum distance between the second phases becomes long even if the rolling reduction of the cold rolling is too low or too high (Experiment Nos. 10 to 11 and 13 to 14). In order to shorten the maximum distance between the second phases, it is important to appropriately control the cold rolling reduction ratio (about 20%) (Experiment No. 12).

  In addition, as is clear from Table 4, in order to shorten the maximum distance between the second phases, it is important to set the steel components appropriately. That is, steel type No. with too little C content. No. 1 (Experiment No. 15), and steel grade No. with too little Mn content. In the example using No. 6 (Experiment No. 20), the maximum distance between the second phases is large, whereas the steel type No. with an appropriate component design is used. In the examples using 2 to 5 and 7 to 9, the maximum distance between the second phases is shortened (Experiment Nos. 16 to 19 and 21 to 23). In these examples, the component design, the hot-rolled structure, and the cold rolling reduction ratio are appropriate, so that the balance of tensile strength (TS), total elongation (El), and hole expansion ratio (λ) was excellent. TRIP steel sheet can be manufactured.

FIG. 1 is a graph showing the relationship between the maximum distance between the second phases and the total elongation. FIG. 2 is a graph showing the relationship between the maximum distance between the second phases and the hole expansion ratio λ. FIG. 3 is a partially enlarged schematic view of an optical micrograph for explaining the maximum distance between the second phases. FIG. 4 is an optical micrograph when the rolling reduction is 0%. FIG. 5 is an optical micrograph when the rolling reduction is 20%. FIG. 6 is an optical micrograph when the rolling reduction is 60%. FIG. 7 is an optical micrograph when a steel sheet having a ferrite-pearlite structure is heated in two phases. Fig.8 (a) is a conceptual diagram which shows an example of the hot rolling process of an Example. FIG.8 (b) is a conceptual diagram which shows the other example of the hot rolling process of an Example.

Claims (9)

A steel plate having a parent phase structure and a second phase structure, wherein the parent phase structure includes at least tempered martensite or tempered bainite; and the second phase structure includes residual austenite. ,
(1) C: 0.10 to 0.6 mass%, Si + Al: 0.1 to 2 mass%, Mn: 1.0 to 3 mass%, P: 0.02 mass% or less (0 mass%) And S: 0.03% by mass or less (excluding 0% by mass),
(2) The retained austenite is 5 to 40% by volume,
(3) When the maximum distance between the second phase structures facing each other with the matrix structure interposed therebetween is measured based on an optical micrograph, the maximum distance is 8 μm or less, and the processability is excellent. High tensile steel plate.   Furthermore, it contains at least one selected from Ca: 0.003% by mass or less (excluding 0% by mass) and REM: 0.003% by mass or less (not including 0% by mass). High tensile steel plate.   Furthermore, Nb: 0.1% by mass or less (not including 0% by mass), Ti: 0.1% by mass or less (not including 0% by mass), and V: 0.1% by mass or less (including 0% by mass) The high-tensile steel plate according to claim 1 or 2, which contains at least one selected from (No).   Furthermore, Mo: 2% by mass or less (not including 0% by mass), Ni: 1% by mass or less (not including 0% by mass), Cu: 1% by mass or less (not including 0% by mass), and Cr: 2 The high-tensile steel sheet according to any one of claims 1 to 3, comprising at least one selected from mass% or less (not including 0 mass%). The matrix structure may contain ferrite in addition to the tempered martensite and tempered bainite,
The area ratio of the tempered martensite, tempered bainite, and ferrite when the area ratio of the structure is measured with an optical microscope photograph (the area of the entire photograph is 100%) is as follows. A high-strength steel sheet according to any one of the above.
Tempered martensite or tempered bainite: 20 to 85 area%
Ferrite: 0 to 60 area%   The high-tensile steel sheet according to any one of claims 1 to 5, wherein the retained austenite includes lath-like retained austenite having a major axis / minor axis ratio of 3 or more with respect to 60% by area or more of the total retained austenite. The tensile strength is 750 to 1050 MPa,
The high-tensile steel sheet according to any one of claims 1 to 6, wherein the tensile strength TS, the total elongation El, and the hole expansion ratio λ satisfy a relationship represented by the following formula (1) and the following formula (2).
TS × El ≧ 24000 (1)
TS × λ ≧ 20000 (2)
[Wherein, TS represents the measurement result of tensile strength (unit: MPa), El represents the measurement result of total elongation (unit:%), and λ represents the measurement result of hole expansion ratio (unit:%)]   C: 0.10 to 0.6 mass%, Si + Al: 0.1 to 2 mass%, Mn: 1.0 to 3 mass%, P: 0.02 mass% or less (excluding 0 mass%), and S: A steel sheet containing 0.03% by mass or less (excluding 0% by mass) and having a martensite structure or a bainite structure introduced is cold-rolled at a reduction ratio of 12 to 28%, and then ferrite-austenite After heating to a two-phase region temperature, quenching is performed while avoiding ferrite transformation, pearlite transformation, and bainite transformation to a temperature not lower than the Ms point and not higher than the Bs point. A method for producing a high-strength steel sheet excellent in workability characterized by lowering the strength.   A steel part obtained by processing the high-tensile steel plate according to claim 1.