JP4325865B2 - High-tensile steel plate with excellent workability and its manufacturing method - Google Patents

Description

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

Steel sheets used for press forming in automobiles and industrial machines are required to have both excellent strength and workability, and such required characteristics are becoming increasingly sophisticated. In recent years, TRIP steel sheets have attracted attention in order to meet such demands. The TRIP steel sheet has an austenite structure remaining, and when deformed at a temperature equal to or higher than the martensite transformation start temperature (Ms point), the retained austenite (γ _R ) is transformed into martensite by stress and exhibits a large elongation. Steel plates, for example, TRIP type composite structure steel (PF steel) composed of polygonal ferrite + bainite + retained austenite structure, and TRIP type bainite steel (BF steel) composed of bainitic ferrite + retained austenite + martensite are known. It has been. 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 the ferrite-austenite two-phase region temperature after adjusting the cooling rate after hot rolling and introducing a martensite structure, a bainite structure, etc., and performing cold rolling. To produce residual austenite.
JP 2002-309334 A JP 2002-302734 A JP 2003-73773 A JP 2003-171735 A

  The present invention has been made as part of the above-described improved technology, and its purpose is to provide a technology 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-mentioned problems, the inventors of the present invention have a steel material having a second phase structure (structure containing residual austenite) with a relatively increased Al content in the steel material. Is the amount of carbon in the steel (C), the volume fraction of retained austenite (fγ _R ) in the steel structure, and the carbon concentration (Cγ _R ) in the retained austenite satisfy a predetermined relationship: The strength, total elongation, and stretch flangeability (hole expansion ratio) can be satisfied at a higher level, and the carbon content (C), volume fraction of retained austenite (fγ _R ), and the residual austenite satisfy the relation of the carbon concentration (C gamma _R) is to properly control the reduction ratio of cold rolling carried out prior to the heat treatment for generating a residual austenite (such as 2-phase region heating) Together, we found that to be maintained for a predetermined time at a predetermined temperature range after cold rolling is very effective, 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 structural structure, the second phase structure is a structural structure of retained austenite, the steel sheet,
(1) C: 0.10 to 0.6 mass%, Si: 1.0 mass% or less, Mn: 1.0 to 3 mass%, Al: 0.3 to 2.0 mass%, P: 0.0. Made of steel satisfying 02 mass% or less, S: 0.03 mass% or less,
(2) The volume fraction of retained austenite obtained by the saturation magnetization measurement method is 5 to 40% by area (the entire field of view is 100%), and (3) the amount of carbon in the steel (C: mass%), the residual The relationship between the volume fraction of austenite (fγ _R ) and the carbon concentration (Cγ _R ) in the retained austenite satisfies the relationship of the following formula (I).
(Fγ _R × Cγ _R ) / C ≧ 50 (I)

  The high-strength steel sheet further comprises (a) sulfide control elements such as Ca: 0.003 mass% or less, REM: 0.003% mass or less, (b) Nb: 0.1 mass% or less, Ti: Precipitation strengthening and microstructure refinement elements such as 0.1 mass% or less, V: 0.1 mass% or less, (c) Mo: 2 mass% or less, Ni: 1 mass% or less, Cu: 1 mass% or less, Cr : Residual austenite stabilizing elements such as 2% by mass or less may be contained.

The preferable area ratio of tempered martensite, tempered bainite, and ferrite (the area of the entire photo is 100%) is, for example, as follows when measured with an optical micrograph.
Tempered martensite or tempered bainite: 20 to 90 area%
Ferrite: 0 to 60 area%
The retained austenite preferably contains 60% by area or more of lath-like retained austenite having a major axis / minor axis ratio of 3 or more with respect to the total retained austenite.

In the high-tensile steel sheet of the present invention, even when the tensile strength (TS) is 750 to 1050 MPa, the tensile strength (TS), the total elongation (El), and the hole expansion rate (λ) satisfy the relationship of the following formula. It will be a thing.
TS × El ≧ 22,000, TS × λ ≧ 20,000
[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 above-described high-tensile steel sheet according to the present invention is galvanized on the surface, more specifically, hot-dip galvanized to suppress deterioration of quality by suppressing rusting during storage, transportation or use, in addition to the bare one. More specifically, those subjected to rust prevention by alloying hot dip galvanizing are also included.

  The high-tensile steel plate as described above is, for example, C: 0.10 to 0.6 mass%, Si: 1.0 mass% or less (including 0 mass%), Mn: 1.0 to 3 mass%, Al : Steel made of steel satisfying 0.3 to 2.0% by mass, P: 0.02% by mass or less, S: 0.03% by mass or less, and martensite structure or bainite structure introduced, 30% It can manufacture by hold | maintaining for 10 to 500 second in the temperature range of 450-550 degreeC, after heating to a ferrite-austenite two-phase area | region temperature without performing cold rolling by the following reduction ratios, or cold rolling.

  In addition, by this method, when producing galvanized steel, more specifically, an alloyed hot-dip galvanized steel sheet, the plating treatment is performed after heating in the above two-phase temperature range or holding in the temperature range of 450 to 550 ° C. In addition to heating in the two-phase temperature range or holding in the temperature range of 450 to 550 ° C., the hot dip galvanization and further the alloying heat treatment of the plating layer are performed during the process. It is also possible to do so, whereby a galvanized steel sheet, or even an alloyed heat-treated steel sheet thereof can be obtained efficiently.

  Various steel parts obtained by processing the above-described high-tensile steel plate, its galvanized steel plate, and its alloyed heat-treated steel plate are also included in the technical scope of the present invention.

  According to the present invention, it is possible to provide a second-phase structure (structure containing residual austenite) steel sheet and galvanized steel sheet that can satisfy the strength, total elongation, and stretch flangeability (hole expansion ratio) at a higher level.

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

  The metal structure observed by the optical microscope of the steel sheet according to the present invention has a parent phase structure and a second phase structure dispersed in an island shape with respect to the parent phase structure. According to the optical micrograph, the matrix structure is gray and is composed of at least a tempered martensite structure or a tempered bainite structure. Moreover, in addition to these tempered martensite structure or tempered bainite structure, a ferrite structure may be contained. 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 steel sheet of the present invention having the above-described structure is an important point in balancing strength, total elongation, and stretch flangeability (hole expansion ratio) at a high level. That is, tempered martensite and tempered bainite are characterized by the fact that they have a lath-like crystal grain and high hardness, but have a lower dislocation density and are softer than ordinary martensite and bainite structures. “Tempered bainite” and “martensitic structure and bainite structure” can be distinguished by, for example, observation with a transmission electron microscope (TEM). The presence of “tempered martensite” and “tempered bainite” as the parent phase is an important factor in increasing both the total elongation and stretch flangeability.

As described above, the matrix structure may contain ferrite in addition to the tempered martensite and tempered bainite. This ferrite is precisely a polygonal ferrite, that is, a 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 by optical micrograph, TEM photograph, hardness measurement (the structure can be distinguished by TEM observation, hardness measurement, etc.), the area ratio of tempered martensite, tempered bainite, and ferrite ( As a guideline, the total area of the photo is 100%.
Tempered martensite or tempered bainite: 20 area% or more (for example, 25 area% or more, or 30 area% or more), 90 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 exerting the TRIP (transformation-induced plasticity) effect, and is useful for improving the total elongation. The amount of retained austenite can be measured by a saturation magnetization measurement method, and it is desirable that it is 5% by volume or more (preferably 8% by volume or more, more preferably 10% by volume or more) when the whole is taken as 100%. . However, when the amount of retained austenite increases excessively, the stretch flangeability (hole expansion rate) tends to deteriorate, so the amount of retained austenite is preferably 40% by volume or less (preferably 30% by volume or less, more preferably 20% by volume or less).

  In the conventional TRIP steel sheet, residual austenite having random orientation exists in the prior austenite grain boundary, whereas in the present invention, residual austenite having substantially the same orientation along the block boundary in the same packet is present. It also has the feature that it exists.

  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 the same TRIP (transformation-induced plasticity) effect as the TRIP steel sheet having spherical retained austenite, but also has a remarkable effect of improving stretch flangeability. Because it is. The lath-like retained austenite having a major axis / minor axis ratio of 3 or more is desirably, for example, 60 area% or more, preferably 65 area% or more, and more preferably 70 area% or more with respect to the total retained austenite.

[component]
Next, chemical 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 amount of C is 0.10% or more, preferably 0.13% or more, and more preferably 0.15% or more. However, when C is excessive, not only is the effect saturated, but defects due to center segregation are likely to occur at the casting stage, so the amount of C is 0.6% or less, preferably 0.5% or less, more preferably 0. 4% or less. If the amount of C exceeds 0.3%, the weldability tends to decrease. Therefore, considering the weldability, the amount of C is not more than 0.3%, preferably not more than 0.28%, more preferably not more than 0.8. It is recommended to be 25% or less.

Si: 1.0% or less (including 0%)
In addition to being useful as a solid solution strengthening element, Si is an element effective for suppressing the decomposition of residual austenite and the formation of carbides. In addition to degrading plating properties, etc., it also adversely affects workability (stretch flangeability and total elongation), so it is desirable to keep it at most 1.0%, more preferably 0.8% or less.

Al: 0.3 to 2.0%
Al is an element particularly effective in suppressing the decomposition of retained austenite and the formation of carbides, and is contained in an amount of 0.3% or more, more preferably 0.5% or more. However, if it is too much, hot embrittlement is likely to occur, so 2.0% or less, more preferably 1.8% or less. Almost all of the conventional TRIP steel sheets including the above-mentioned patent documents have an Al content of 0.1% or less, and as far as the inventors know, the Al content is actively increased to 0.3% or more at the example level. There is no TRIP steel sheet. This seems to be because Al was considered to be an oxide inclusion source that adversely affects workability and hot brittleness. However, according to the study by the present inventors, the steel material in which the Al content is increased to a level of 0.3 to 2.0% as described in detail later is high in combination with other component compositions and structure control. It has been clarified that a TRIP steel sheet that exhibits a high value in total elongation and stretch flangeability while maintaining strength is provided.

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, excessive Mn causes cracking of the cast slab, so it is 3% or less, preferably 2.5% or less, more preferably 2.0% or less.

P: 0.02% or less P is an element effective for securing desired retained austenite, and the effect is effectively exhibited by setting it to 0.001% or more, more preferably 0.005% or more. The However, since secondary workability deteriorates when P is excessive, it should be limited to 0.02% or less, preferably 0.015% or less.

S: 0.03% or less S is a harmful element which forms sulfide inclusions such as MnS and causes cracking to deteriorate workability, and 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.

  In addition to the above components, the steel sheet of the present invention may contain the following components.

At least one selected from Ca: 0.003% or less and REM: 0.003% or less These Ca and REM (rare earth elements) both control the form of sulfide in steel and are effective for improving workability. It is an element. Examples of rare earth elements include Sc, Y, and lanthanoids. In order to effectively exhibit the above action, it is recommended to contain 0.0003% or more (particularly 0.0005% or more). However, even if it is added excessively, the effect is saturated and uneconomical. Therefore, it is preferable to suppress it to 0.003% or less (particularly 0.002% or less).

At least one selected from Nb: 0.1% or less, Ti: 0.1% or less, and V: 0.1% or less These Nb, Ti, and V have precipitation strengthening and microstructure refining effects. It is an element useful for increasing the strength. In order to effectively exhibit such an action, it is recommended to contain 0.01% or more (particularly 0.02% or more). However, even if added excessively, the effect is saturated and the economic efficiency is lowered, so that each is made 0.1% or less (preferably 0.08% or less, more preferably 0.05% or less).

At least one selected from Mo: 2% or less, Ni: 1% or less, Cu: 1% or less, and Cr: 2% or less These Mo, Ni, Cu and Cr are useful as steel strengthening elements and It is an effective element effective for stabilizing retained austenite. In order to exhibit such an action effectively, it is preferable to contain 0.05% or more (particularly 0.1% or more). However, even if added excessively, the effect is saturated and uneconomical, so Mo and Cr are each 2% or less (preferably 1% or less, more preferably 0.8% or less), and Ni and Cu are each 1%. Or less (preferably 0.5% or less, more preferably 0.4% or less).

  The remaining components of the steel sheet of the present invention are Fe and inevitable impurities, but may further contain other elements as long as the above-described structural characteristics are satisfied.

As described above, the steel sheet of the present invention is composed of a specific component and a specific structure. As still another characteristic element, the amount of carbon in the steel (C: mass%) and the volume of the retained austenite are described. The relationship between the rate (fγ _R ) and the carbon concentration (Cγ _R ) in the retained austenite satisfies the relationship of the following formula (I): strength, total elongation, and stretch flangeability (hole expansion rate) It is important to further improve the balance of the above.
(Fγ _R × Cγ _R ) / C ≧ 50 (I)

  When the value of the above formula (I) is less than 50, the strength is high, but the total elongation and stretch flangeability are deteriorated so that it can be confirmed also by the examples described later, which is not suitable for the purpose of the present invention. A more preferable value of the formula (I) is 55 or more.

Incidentally, fγ _R indicates the amount of retained austenite, Cγ _R is an index indicating the stability of the retained austenite, and the higher the value of (fγ _R × Cγ _R ), the more stable retained austenite. Exists, and the plasticity induced transformation (TRIP) effect is effectively exhibited. Therefore, it is considered that the fact that this value is relatively large with respect to C and the value of formula (I) is large (50 or more) is an important factor for enhancing the total elongation and stretch flangeability. .

As described above, the steel sheet of the present invention satisfies a specific structure and a specific component, and ensures 50 or more in the value of the above formula (I), whereby strength, total elongation, and stretch flangeability (hole expansion ratio). Will be balanced at an extremely high level. And the steel plate of the present invention that satisfies the above elements combines 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). The tensile strength (TS), total elongation (El), and hole expansion ratio (λ) can also satisfy the relationship of the following formula.
TS × El ≧ 22,000, TS × λ ≧ 20,000
[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 steel sheet satisfying the above specified requirements of the present invention exhibits stable and excellent workability due to its proper component composition and metal structure, and of course, its characteristics are effectively exhibited as a bare steel sheet. Then, the characteristics of the surface-treated steel sheet subjected to, for example, phosphate treatment or the like, or the plated steel sheet subjected to plating treatment such as hot dip galvanizing and further alloying heat treatment are exhibited regretfully.

[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 TRIP steel sheet). And cold rolling at a rolling reduction of 30% or less, or soaking at a two-phase temperature of ferrite-austenite without performing cold rolling, and 10 to 10 in a temperature range of 450 to 550 ° C. It can be manufactured by holding for 500 seconds.

  When a steel sheet having a martensite structure or a bainite structure (including those having a martensite-ferrite structure and a bainite-ferrite structure) is soaked in a two-phase region and then held in a predetermined temperature range for a predetermined time, A second phase (a phase containing residual austenite) different from the phase (tempered martensite, tempered bainite, etc.) can be generated. If cold rolling is performed under appropriate conditions prior to this heat treatment, an appropriate second phase (a phase containing residual austenite) can be formed during the heat treatment, and as a result, the total elongation and stretch flangeability (holes) The spreading rate) can be remarkably improved. The rolling reduction at this time is specifically set to about 0% or more (preferably 5% or more, more preferably 10% or more), 30% or less (preferably 25% or less, more preferably 20% or less). It is good.

  By the way, the reduction also contributes to an increase in the lath-like retained austenite, and the lath-like retained austenite increases as the reduction ratio decreases. In the present invention, since the rolling reduction is defined as described above, it is difficult to drastically change the amount of lath austenite by largely varying the rolling reduction, but when it is desired to increase the lath retained austenite For this, a lower rolling reduction may be selected from the range, and cold rolling may be omitted in some cases.

  In addition, the steel plate in which the martensite structure or the bainite structure is introduced can be obtained according to a conventional method. That is, if the steel sheet heated in the austenite region is rapidly cooled to a temperature below the Ms point, a martensite structure can be introduced. If the steel sheet is rapidly cooled to a temperature above the Ms point and below the Bs point, the bainite structure is transformed. Can be introduced. 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 a pearlite structure is not desirable for the present invention, it is desirable to set a cooling pattern so as to avoid a 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 forming a ferrite structure, the ferrite structure should be stably introduced by monotonic cooling. Therefore, it is preferable to employ a multi-stage cooling method in which the cooling rate is set in a plurality of times. In particular, a method in which cooling is started again after holding at the austenite-ferrite two-phase 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 ( _{Ar 3} -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 below 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 damaged. ), The retained austenite structure is coarsened or decomposes into carbides.

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

  Thereafter, cooling to a temperature of 450 ° C. or higher (preferably 470 ° C. or higher) to 550 ° C. or lower (preferably 530 ° C. or lower) and maintaining in that temperature range reduces the amount of retained austenite by lowering the Ms point of the austenite phase. This is to ensure. The holding time in the temperature range is appropriately set according to the amount of austenite generated at the two-phase region temperature and the set amount of retained austenite in the target TRIP steel sheet, but at least 10 seconds (preferably 50 seconds or more). Should be secured. However, if the holding time is too long, the bainite transformation proceeds and the amount of retained austenite decreases, so it should be suppressed to 500 seconds or less, more preferably 200 seconds or less.

  Considering actual operation, it is easy to perform the heat treatment after cold rolling using a continuous annealing facility. In addition, when galvanizing, for example, hot dip galvanizing, is performed on a cold-rolled sheet, it is possible to perform hot dip galvanization after performing heat treatment under the above-mentioned proper conditions, and then perform alloying heat treatment. However, it is also possible to set the galvanizing conditions or a part of the alloying heat treatment conditions so as to satisfy the heat treatment conditions and perform the heat treatment in the plating step.

  The steel sheet of the present invention and the hot dip galvanized product thus obtained are not only excellent in strength but also excellent in total elongation and stretch flangeability, and therefore 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 the following Table 1 (units in the table are mass%) are vacuum-melted to obtain experimental slabs having a thickness of 20 to 30 mm, and then hot rolling shown in FIG. A (monotonic) cooling pattern or a hot-rolled-two-stage cooling pattern shown in FIG. 2 was used to form a hot-rolled sheet having a thickness of 2.5 mm and further cold-rolled to produce a cold-rolled sheet having a thickness of 2.0 mm. This cold-rolled sheet was heated to a ferrite-austenite two-phase region temperature (830 ° C.), held for 120 seconds, soaked, rapidly cooled to a predetermined temperature, and subjected to heat treatment for a predetermined time to produce a TRIP steel sheet. The meanings of the symbols in FIGS. 1 and 2 are as follows.
SRT: hot rolling heating temperature FDT: hot rolling finishing temperature CR1: first stage cooling rate CTN: holding temperature after first stage cooling CR2: second stage cooling rate CT: coiling temperature Tables 2, 4 and 6 below show the conditions of hot rolling-one-stage or two-stage cooling, the structure of the hot-rolled sheet, the reduction ratio of cold rolling, the soaking temperature, the holding temperature and the holding time. Table 3 below shows the structure of the obtained TRIP steel sheet, the value of the formula (I), the tensile strength (TS), the total elongation (El), the stretch flangeability (hole expansion ratio: λ), and the phosphate treatment property. , 5 and 7.

  Further, among the data in Tables 2 to 7 below, with respect to some samples having different Al contents, the holding temperature and holding time after soaking after hot rolling and cold rolling are calculated by the above formula (I 3 and 4 and the influence of the holding temperature and holding time after soaking on the amount of retained austenite are shown in FIGS.

  In addition, the structure of the hot-rolled sheet and the TRIP steel sheet shown in Tables 2 to 7 was examined as follows. That is, after the steel plate was corroded with a repeller and the structure was identified by observation with a transmission electron microscope (TEM; magnification: 15000 times), each of tempered martensite, tempered bainite, and ferrite was observed based on an optical micrograph (magnification 1,000 times). The area ratio 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 was also measured based on the optical micrograph. On the other hand, the volume fraction of retained austenite is measured by saturation magnetization measurement [Japanese Patent Laid-Open No. 2003-90825 and “R & D Kobe Steel Engineering Reports” Vol. 52, no. 3 (Dec. 2002)], and the C concentration in the retained austenite was measured with an X-ray microanalyzer (XMA) after grinding the steel plate to a thickness of 1/4 and then chemically polishing (ISIJ Int. Vol.33, 1993, No. 7, P.776).

  Tensile strength (TS) and total elongation (El) were measured using JIS No. 5 test piece. Stretch flangeability was a test piece having a diameter of 100 mm and a plate thickness of 2.0 mm, and a hole having a diameter of 10 mm in the center. After punching, the hole was widened on the beam with a 60 ° conical punch and evaluated by measuring the hole expansion rate (λ) at the time of crack penetration (Iron and Steel Federation Standard JFST 1001).

  Moreover, the phosphoric acid treatment property and the Fe concentration in the galvanizing were obtained by the following methods.

[Phosphate treatment]
Each test steel sheet was dipped in a 43 ° C. phosphate treatment solution (trade name “LB-L3020” manufactured by Nihon Parkerizing Co., Ltd.) for 2 minutes, then lifted and dried, and then the surface was observed with SEM (magnification: 2000 times). Then, the adhesion state of phosphate crystals is examined. Further, the test steel sheet subjected to the phosphate treatment was immersed in a solution of [ammonium dichromate 20 g + ammonia water 490 g + water 490 g] for 15 minutes at room temperature, then lifted and dried. Obtain the amount of acid salt attached. From the above test results, phosphatability is evaluated in three stages according to the following criteria.
A: Phosphate crystals adhere to the entire surface without gaps, and the amount of phosphate adhered is 4 g / m ² or more.
○: Phosphate crystals adhere to almost the entire surface, and the phosphate adhesion amount is 3 g / m ² or more and less than 4 g / m ² .
X: A portion where phosphate crystals are not attached to a part of the surface is observed, and the amount of phosphate attached is less than 3 g / m ² .

[Alloyed zinc plating]
Each test steel sheet is immersed in a molten zinc bath and then treated at 550 ° C. for 60 seconds. The plated layer of the obtained alloyed galvanized steel sheet is dissolved with hydrochloric acid, and the Zn concentration in the solution is quantitatively analyzed by ICP to determine the Fe concentration in the alloyed galvanized plate. When the Fe concentration is in the range of 8 to 13%, it is normal, and it is judged that alloying is sufficiently advanced (good), and when it is less than 8%, it is judged as defective.

  As is apparent from FIG. 3, in the conventional steel sheet having an Al content of 0.03% by mass, the value obtained from the formula (I) is almost linearly linear as the holding temperature after soaking increases. However, as specified in the present invention, the steel material having an Al content of more than 0.3% by mass shows a unique tendency that the value of formula (I) shows a peak in the region where the holding temperature is 450 to 550 ° C. Further, from FIG. 4, the value of the formula (I) shows a peak when the holding time is 10 to 500 seconds. And in this way, using the holding temperature and holding time at which a high value can be obtained as the value of formula (I), the strength (TS), total elongation (EL), and hole expansion rate (λ) are stable at a high level. Can be confirmed.

  The tendency confirmed by FIGS. 3 and 4 is substantially the same in the relationship between the retained austenite amount shown in FIGS. 5 and 6 and the holding temperature and holding time, and the present invention uses a steel material having a relatively high Al content. In FIG. 5, it can be seen that by setting the holding temperature to 450 to 550 ° C. and the holding time to 10 to 500 seconds, 5 vol% or more can be secured in the amount of retained austenite.

Experimental example 2
Test steels having the composition shown in the following Table 8 (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-1 shown in FIG. A hot-rolled sheet with a thickness of 2.5 mm was formed by a step (monotonic) cooling pattern, and further cold-rolled to produce a cold-rolled sheet with a thickness of 2.0 mm. This cold-rolled sheet is soaked at 930 ° C. for 120 seconds, and then cooled, isothermally held, and continuously air-cooled in the pattern shown in FIG. 7 to obtain a cold-rolled steel sheet.

Next, as shown in FIG. 7, each cold-rolled steel sheet was held at 840 ° C. for 80 seconds and then dipped and run in a molten zinc bath at 460 ° C. for 15 seconds, and then at a predetermined alloying treatment temperature (T ₀ ). By maintaining the time, an alloyed hot-dip galvanized steel sheet is obtained. The series of processing conditions are shown in Tables 9 and 10.

  For each of the obtained galvanized steel sheets, the metal structure was observed in the same manner as in Experimental Example 1, and the area ratios of tempered martensite (TM), tempered bainite (TB), and phierite (F) were obtained. The ratio of the lath-like retained austenite to the total retained austenite was measured by the method, and the volume fraction of retained austenite and the C concentration in the retained austenite were further measured. The results are collectively shown in Table 11.

  About each galvanized steel plate obtained above, tensile strength (TS), total elongation (El), stretch flangeability (λ) were measured by the same method as described above, and phosphoric acid treatment property and The Fe concentration during galvanization was determined. The results are collectively shown in Table 12.

  8, 9, and 10 show the mechanical properties of each alloyed galvanized steel sheet [tensile strength (TS), total elongation (El) and hole expansion ratio (λ)] based on the results shown in Tables 7 to 11 above. Graph showing the relationship between the alloying heat treatment temperature (FIG. 8), the mechanical property relationship (FIG. 9) as well as the alloying heat treatment time, and the relationship between the alloying heat treatment temperature and the residual γ property (FIG. 10). It is.

  From these figures, it can be considered as follows. That is, when comparing the matrix structure of the cold-rolled steel sheet before hot-dip galvanized steel with ferrite-pearlite and the matrix structure with tempered martensite or tempered bainite, the latter is more preferable than the former. By adopting preferable alloying heat treatment temperature and alloying treatment time, it can be seen that relatively balanced characteristics are exhibited in strength (TS), total elongation (EL), and hole expansion ratio (λ) (Fig. 8, 9). Further, in the residual γ characteristics, when the matrix structure of the cold-rolled steel sheet before hot-dip galvanizing is ferrite-pearlite and the matrix structure is tempered martensite or tempered bainite, the former structure is compared. It can be seen that, when the preferable heat treatment temperature for alloying is employed, a favorable γ characteristic can be obtained.

It is a figure which shows an example of the hot rolling and cooling process employ | adopted in the Example. It is a figure which shows the other hot rolling and cooling process employ | adopted in the Example. FIG. 3 is a graph showing the influence of the holding temperature after soaking on the value of the formula (I). It is a graph which shows the influence which the holding time after soaking has on the value of said Formula (I). It is a graph which shows the influence which the holding temperature after soaking has on the amount of retained austenite of the steel plate obtained. It is a graph which shows the influence which the retention time after soaking has on the amount of retained austenite of the steel plate obtained. It is a pattern diagram which shows the continuous annealing conditions employ | adopted in Experimental example 2, and the continuous hot dip galvanization process conditions. It is a graph which shows the influence which the alloying temperature of hot dip galvanization has on the mechanical characteristic of a plated steel plate. It is a graph which shows the influence which the alloying time of hot dip galvanization has on the mechanical characteristics of a plated steel plate. It is a graph which shows the influence which the alloying temperature of hot dip galvanization has on the residual (gamma) characteristic of a plated steel plate.

Claims (14)

It has a parent phase structure and a second phase structure, the parent phase structure is a steel plate having at least tempered martensite or tempered bainite as a constituent structure, and the second phase structure is a residual austenite as a constituent structure,
(1) C: 0.10 to 0.6 mass%, Si: 1.0 mass% or less, Mn: 1.0 to 3 mass%, Al: 0.3 to 2.0 mass%, P : 0.02% by mass or less, S: 0.03% by mass or less, the balance: steel made of Fe and inevitable impurities ,
(2) The volume fraction of retained austenite obtained by a saturation magnetization measurement method is 5 to 40%,
(3) The relationship between the amount of carbon in steel (C: mass%), the volume fraction of retained austenite (fγ _R ), and the carbon concentration (Cγ _R ) in the retained austenite satisfies the relationship of the following formula (I) A high-tensile steel sheet with excellent workability characterized by
(Fγ _R × Cγ _R ) / C ≧ 50 (I) A steel sheet having a martensite structure or a bainite structure is cold-rolled at a rolling reduction of 30% or less, or heated to a ferrite-austenite two-phase temperature without performing cold rolling, and then 450 to 550 ° C. The high-tensile steel sheet according to claim 1, which is obtained by holding for 10 to 500 seconds in a temperature range of 2. The high-tensile steel plate according to claim 1 or 2 , further comprising Ca: 0.003% by mass or less and / or REM: 0.003% by mass or less. Further Nb: 0.1 wt% or less, Ti: 0.1 wt% or less, and V: in any one of claims 1 to 3 containing at least one selected from the group consisting of 0.1 wt% or less High-strength steel sheet as described. Further Mo: 2 mass% or less, Ni: 1% by mass or less, Cu: 1 mass% or less, and Cr: 2 contains at least one percent by weight is selected from the group consisting of the following claims 1 or 4 The high-tensile steel sheet described in 1. The matrix structure may contain ferrite in addition to the tempered martensite and tempered bainite. When the area ratio of the structure is measured with an optical micrograph, the area ratio of tempered martensite, tempered bainite, and ferrite (photograph) The high-tensile steel sheet according to any one of claims 1 to 5 , wherein the total area is 100%).
Tempered martensite or tempered bainite: 20 to 90 area%
Ferrite: 0 to 60 area% The high-strength steel sheet according to any one of claims 1 to 6 , wherein the retained austenite contains 60% by area or more of lath-like retained austenite having a major axis / minor axis ratio of 3 or more with respect to the total retained austenite. The high-tensile steel sheet according to any one of claims 1 to 7 , wherein the surface is galvanized. The high-tensile steel sheet according to claim 8 , wherein the galvanizing is hot dip galvanizing. The high-tensile steel sheet according to claim 9 , wherein the galvanizing is further subjected to alloying heat treatment.   C: 0.10 to 0.6 mass%, Si: 1.0 mass% or less, Mn: 1.0 to 3 mass%, Al: 0.3 to 2.0 mass%, P: 0.02 mass% Hereinafter, a steel sheet made of steel satisfying S: 0.03 mass% or less and having a martensite structure or a bainite structure introduced is cold-rolled at a rolling reduction of 30% or less, or without performing cold rolling. A method for producing a high-tensile steel sheet excellent in workability, characterized in that after heating to a ferrite-austenite two-phase region temperature, holding at a temperature range of 450 to 550 ° C for 10 to 500 seconds. The method according to claim 11 , wherein a galvanized steel sheet is manufactured by further performing a hot dip galvanizing process or further performing an alloying heat treatment. The galvanized steel sheet according to claim 11 , wherein the galvanized steel sheet is manufactured by performing hot dip galvanizing treatment or further alloying heat treatment including heating at the two-phase region temperature and / or holding at a temperature region of 450 to 550 ° C. Manufacturing method. A steel part obtained by processing the high-tensile steel plate according to any one of claims 1 to 10 .

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