JP2009173959A - High-strength steel sheet and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new low alloy, high strength steel sheet having both of the high strength and good workability, and a producing method therefor. <P>SOLUTION: After annealing a rolled steel having specific chemical compositions to the temperature range of A1 to (Ac3+20)°C, this steel sheet is slowly cooled to the temperature range of A1 to (A1-100)°C at a cooling rate of ≤10°C/sec and successively, and after rapidly cooling to near 500°C at a rate of ≥60°C/sec, successively, this steel sheet is cooled to ≤250°C at a cooling rate of 10 to 30°C/sec without keeping it at near 400°C, and thus, the high strength and excellent ductility with the low alloy composition at the same level as a composite structural steel sheet can be imparted to the steel sheet at the same time. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The invention according to the claims relates to a high-strength steel sheet having high workability while having high tensile strength and a method for producing the same.

  The recent demand for high-strength steel sheets with excellent workability will be described by taking the case of automobiles as an example. From the viewpoint of global environmental conservation, it is essential to reduce the amount of exhaust gas such as CO2 in the automobile field. For that purpose, further weight reduction of the automobile body becomes indispensable. In order to reduce the weight of the car body, it is necessary to increase the strength of steel plates used in automobiles and reduce the thickness. At the same time, passengers must ensure the safety of passengers. For this purpose, it is necessary to further increase the strength of the steel sheet.

  As a method for increasing the strength, solid solution strengthening, precipitation strengthening, crystal grain refinement and the like are basic methods. In order to ensure weldability while saving resources and reducing manufacturing costs, it is extremely important to reduce the amount of alloy addition as much as possible. Therefore, it is impossible to produce a steel sheet that requires extremely high strength by only applying a strengthening mechanism that requires a large amount of alloy addition such as solid solution strengthening or precipitation strengthening. Even if a strengthening mechanism by grain refinement is applied, the crystal grain size that can be produced commercially is 1 to 3 μm, so even if the strength can be increased to some extent, the ductility can be improved to meet the increase in strength. Cannot be expected.

When the strength of the steel plate increases, the workability deteriorates, and at the same time, the dimensional accuracy also deteriorates due to the spring back after press forming. It is difficult to apply high-strength steel sheets by cold working methods such as ordinary press forming.
In the hot pressing method, since the hot pressing is performed, the amount of spring back is extremely small and the shape freezing property is good. And the part with very high intensity | strength can be provided with high precision by the hardening effect in the case of a press. However, it is necessary to heat the steel plate before pressing, and an operation to drop the scale is necessary after pressing. Therefore, the work efficiency is very poor. Furthermore, since the mold contacts the heated steel plate, the short life of the mold is also a drawback, which increases the manufacturing cost.
The steel sheet after hot pressing has a small elongation value, and when the member is deformed, it breaks even if it is slightly deformed. Therefore, it is evaluated that the impact absorbing ability is small. Therefore, it is very difficult to use a hot press part as an important safety part for an automobile or the like.

  One method of improving the dimensional accuracy of a molded product by cold pressing is a method of suppressing the occurrence of springback by providing a bead on the molded product. However, this processing method requires extremely high workability for the steel sheet.

Generally, when the strength of a steel plate is increased, ductility is reduced and workability is reduced. Conventional techniques for increasing the ductility of high-strength steel sheets include so-called dual phase steel sheets composed of ferrite and martensite structures, and TRIP (Transformation Induced Plasticity) steel sheets composed of ferrite and retained austenite structures.
Examples of composite structure steel sheets are shown in Patent Documents 1 and 2. In such a steel sheet, hard martensite is finely dispersed in the ferrite, but this hard martensite causes a large work hardening at the time of deformation, and brings high strength and ductility to the steel sheet.
Examples of the TRIP steel sheet are shown in Patent Documents 3 and 4. This type of steel sheet containing retained austenite has very good ductility and formability due to transformation induced plasticity, depending on its amount and stability to deformation.

Furthermore, the steel sheet described in Patent Document 5 has both high martensite and retained austenite mixed in ferrite, and has a high strength similar to that of a composite steel sheet and an excellent ductility similar to that of a TRIP steel sheet even with a low alloy composition. It was developed as a steel sheet with properties.
JP-A-56-133423 JP 2005-230896 JP JP 62-182225 A JP 2005-76078 JP 2007-231399 A

  As a conventional technique for improving the elongation characteristics in cold working while having high strength, the composite structure steel plate and the TRIP steel plate can be mentioned.

  With a composite structure steel plate, high strength can be obtained even with a relatively low alloy addition amount, and at the same time, good uniform elongation characteristics can be obtained by work hardening. Among them, the steel sheets of Patent Document 1 and Patent Document 2 are further improved in strength and elongation characteristics by finely dispersing martensite or bainite in the ferrite matrix. However, in both documents, the amount of retained austenite is scarce or the amount is insufficiently controlled, so the elongation properties of TRIP steel sheets are not obtained, and the elongation properties are high enough to withstand the processing of parts with complex shapes. Not obtained.

The TRIP steel plate proposed in Patent Document 3 and the TRIP steel plate described in Patent Document 4 in which retained austenite is finely dispersed have high strength, high ductility, and high deep drawability. is there. Therefore, application to a member that requires high workability with a complicated shape and requires high strength is directed.
However, in the annealing cooling process, in order to stabilize austenite, it is necessary to maintain a low temperature near 400 ° C. just above the Ms point at which martensitic transformation starts. Furthermore, due to the low temperature retention, there is a decisive disadvantage that the steel sheet strength is low for the carbon content and the alloy addition amount compared to the composite structure steel plate. The application of TRIP steel sheets was expected in the strength range of 780MPa or more where good elongation characteristics of the steel sheets were difficult to obtain. However, if the same strength level is to be ensured, the required amount of carbon and the amount of alloy addition increase, so that good spot bondability cannot be ensured. For this reason, this steel sheet is not widely used at present.

Therefore, it is desired to develop a steel sheet having a high ductility such as a TRIP steel sheet while having a high strength with a low alloy addition amount like a composite structure steel sheet. Patent Document 5 is a development example of a steel plate having these conflicting characteristics. This steel sheet has a martensite or lower bainite structure as the structure before annealing, and then the temperature range of A1 to (Ac3 + 20) ("~" ranges from the previous value to the subsequent value. The same applies to the following), and the structure in which martensite and retained austenite are mixed is formed in the ferrite by annealing rapidly to near room temperature.
Normally, in such an annealing heat cycle, austenite generated during annealing transforms into martensite during the cooling process, and the amount of austenite remaining is very small. However, when the austenite formed at the time of annealing is finely dispersed to about 1 μm, the strain energy generated by transformation to martensite is increased under cooling, and the austenite remains difficult to be transformed.
In the case of the martensite or lower bainite structure as the structure before annealing, the austenite generated during the annealing is finely dispersed in a lath shape, and austenite remains even when rapidly cooled to near room temperature.
However, it has been found that the retained austenite obtained in this way has low metastability, and therefore there are many variations in elongation characteristics.

Therefore, the inventors of the present application investigated a method for increasing the metastability of retained austenite on the premise of a three-phase structure of ferrite, martensite, and retained austenite in order to obtain high steel sheet strength even with a low alloy.
In order to obtain metastability of retained austenite, it is effective to hold at around 400 ° C in the cooling process of final annealing. However, if holding at around 400 ° C, the amount of martensite is extremely reduced. , The strength decreases. Therefore, the inventors examined a manufacturing process for continuously cooling to near room temperature.
Through such a process, we have developed a new low-alloy / high-strength steel sheet that has both high strength and good workability, and its manufacturing method.

  As a result of intensive research, the inventors have annealed a rolled steel sheet having a specific chemical component to a temperature range of A1 to (Ac3 + 20) ° C, and then 10 ° C to a temperature range of A1 to (A1-100) ° C. Slow cooling at a cooling rate of less than / sec., Followed by rapid cooling to around 500 ° C at a cooling rate of more than 60 ° C / sec, then 10 to 30 ° C / sec. It was invented that by cooling at a cooling rate of 2 mm, it was possible to simultaneously impart high strength and excellent ductility to the steel sheet with a low alloy composition comparable to that of a composite structure steel sheet. The high-strength steel sheet produced by such a method was found to contain martensite and metastable residual austenite in the ferrite matrix.

The high-strength steel sheet described in the claims contains martensite at a volume ratio of 20% or more in the grains and grain boundaries of the ferrite having an average grain size of 10 μm or less, and the average (in the grains and grain boundaries of the ferrite). It is a high-strength steel sheet having a particle size of 3 μm or less and a retained austenite having a carbon concentration of 0.9% or more by mass% and a volume ratio of 5% or more.
Since martensite is present in a volume ratio of 20% or more, it has high strength even if the amount of alloy addition is small. Further, since the retained austenite has a carbon concentration of 0.9% or more by mass%, it can exist metastable at room temperature and can induce transformation due to deformation. Furthermore, since the volume ratio is 5% or more and effectively finely dispersed, good work hardening can be continuously obtained and good elongation characteristics can be obtained.

Regarding the above-described high-strength steel plates, in particular, C: 0.05 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 1.50%, Cu: It is desirable to contain 0.01 to 1.0%, Ni: 0.01 to 1.0%, Mo: 0.01 to 1.0%, or further contain any one or both of Ti: 0.02 to 0.22% and Nb: 0.02 to 0.10%. In each case, the balance is Fe and inevitable impurities.
If such an appropriate kind and amount of chemical components are included, it is easy to obtain a high-strength steel sheet having the above-described structure and exhibiting desirable mechanical properties by performing an appropriate heat treatment.

  Furthermore, C: 0.05 to 0.25%, Si: 0.01 to 0.30%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 0.20%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% , Mo: 0.01 to 1.0%, or even Ti: 0.02 to 0.22%, Nb: 0.02 to 0.10% in the composition range containing either one or both, by performing an appropriate heat treatment, High-strength steel sheet suitable for In each case, the balance is Fe and inevitable impurities. The action of each component will be described later.

As the high-strength steel plate, one having the above-described structure and having a product TS × EL of tensile strength TS (MPa) and elongation value EL (%) of 18000 (MPa ·%) or more is preferable.
This is because such a steel sheet has the above-described structure and has both high strength and good elongation characteristics.

The manufacturing method of the high strength steel sheet according to the claim
1) By mass%: C: 0.05-0.25%, Si: 0.01-2.0%, Mn: 0.3-2.5%, Cr: 0.01-3.0%, Al: 0.01-1.50%, Cu: 0.01-1.0%, Ni: 0.01 Steel material containing -1.0%, Mo: 0.01-1.0%, or further containing either one or both of Ti: 0.02-0.22%, Nb: 0.02-0.10%,
2) Or, in mass%: C: 0.05 to 0.25%, Si: 0.01 to 0.30%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 0.20%, Cu: 0.01 to 1.0%, Ni : Steel containing 0.01 to 1.0%, Mo: 0.01 to 1.0%, or further containing Ti: 0.02 to 0.22%, Nb: 0.02 to 0.10%, or both,
3) Hot-rolled (to a thin plate) by a hot rolling mill with multiple stands, completed rolling at Ar3 or higher, and then started cooling from a temperature higher than Ar3, in a temperature range of 300 ° C or higher and 600 ° C or lower. Winding
4) As is or after cold rolling
5) After heating to a temperature of A1 or higher and (Ac3 + 20) ° C or lower and holding at that temperature
6) Slow cooling at a cooling rate of 10 ° C / sec or less to a temperature range of A1 to (A1-100) ° C
7) Then rapidly cool to 400-550 ° C at a cooling rate of 60 ° C / sec or more
8) It is further characterized by cooling at a cooling rate of 10-30 ° C./sec to 250 ° C. or less.
If it does in this way, said high-strength steel plate can be obtained as mentioned later.
Here, A3 indicates the temperature at which the ferrite phase precipitates or disappears in the austenite phase, Ar3 indicates the transformation point temperature during the cooling process, and Ac3 indicates the transformation point temperature during the heating process. A1 represents the transformation temperature at which the austenite phase precipitates or disappears in the ferrite phase.
In the TRIP steel sheet manufacturing method, since the holding is performed at around 400 ° C., abundant martensite cannot be obtained, and the cooling method (described above) shown in Patent Document 5 has a certain amount of residual. Even if austenite is obtained, its carbon concentration is not stable at 0.9% or more, and a desirable structure and preferable mechanical properties associated therewith are not necessarily obtained. Such a problem is solved by the manufacturing method described in the present claims.

  The high-strength steel sheet described in the claims is a low alloy, but contains a large amount of martensite, so that a very high strength is obtained and abundant retained austenite is contained. It exhibits good processing characteristics that can withstand

  According to the manufacturing method described in the claims, the above-described high-strength steel plate can be manufactured smoothly.

Hereinafter, embodiments of a thin steel plate used for a processed part that requires excellent workability while having a tensile strength of about 600 to 1500 MPa and a manufacturing method thereof will be described.
As a component system of the steel sheet, C: 0.05 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 1.50%, Cu: 0.01 to 1.0% in mass% , Ni: 0.01-1.0%, Mo: 0.01-1.0%, or further containing Ti: 0.02-0.22%, Nb: 0.02-0.10% or both,
Or, in mass% C: 0.05 to 0.25%, Si: 0.01 to 0.30%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 0.20%, Cu: 0.01 to 1.0%, Ni: 0.01 -1.0%, Mo: 0.01-1.0% is included, or further, Ti: 0.02-0.22%, Nb: 0.02-0.10%, or a composition containing either one or both is used as a reference.
In addition, the thin steel plate described here is a steel plate having a thickness of about 4 mm or less. The steel sheet to be produced can be used mainly for parts that require high workability and strength, such as automobiles, home appliances, and electronic equipment products. In addition, it can be applied as a material for steel pipes.

First, the components of the steel sheet will be described.
As carbon (C), an amount in the range of 0.05 to 0.25% is required. If it is less than 0.05%, a steel plate strength of about 600 MPa or more cannot be obtained, so a carbon content of 0.05% or more is necessary. On the other hand, when the carbon content is 0.25% or more, the welded portion is excessively hardened and easily breaks from the welded portion. This is a limitation in use for thin steel plates, so an upper limit was set for the carbon content. And it has been found that when the carbon content is 0.05 to 0.25%, a composite structure in accordance with the gist of the present invention can be obtained.

The amount of silicon (Si) is in the range of 0.01 to 2.0%, or 0.01 to 0.30%. Silicon is utilized for improving the strength by solid solution strengthening and stabilizing the retained austenite, and the retained austenite becomes more stable when it is contained in a larger amount. Therefore, also in the above manufacturing method, the metastability of retained austenite increases as the amount of silicon increases. However, in the production method of the present invention, retained austenite having high metastability can be obtained even if the amount of silicon is small. Therefore, even in the range of 0.01 to 0.30%, a composite structure that meets the gist of the present invention can be obtained.
If the amount of silicon is 0.01% or more, the composite structure and material characteristics of the present invention can be obtained. Therefore, the lower limit was made 0.01%. When the silicon content is 2.0% or more, silicon scale is likely to be generated on the surface of the steel sheet, and good surface properties cannot be obtained as a thin steel sheet. Therefore, even when a large amount of silicon is contained, the upper limit is set to 2.0%.

Manganese (Mn) content is in the range of 0.3-2.5%. When the manganese content is 0.3% or less, the hardenability is insufficient and the composite structure of the present invention cannot be obtained. As a result, the strength and ductility of the steel sheet are lowered, so the manganese content is 0.3% or more.
On the other hand, if the amount of manganese exceeds 2.5%, the strength becomes too high and the manufacturing cost increases, so the upper limit was made 2.5%.

  Chromium (Cr) is an element that improves hardenability like manganese, and can improve the strength of steel, but if it exceeds 3.0%, the strength becomes too high and the manufacturing cost also increases, so the upper limit is 3.0. %.

Aluminum (Al) is in the range of 0.01 to 1.50%, or 0.01 to 0.20%. Aluminum, like silicon, increases the amount of retained austenite and at the same time promotes stabilization. Therefore, also in the manufacturing method described above, the metastability of retained austenite increases as the amount of aluminum increases. However, in the production method of the present invention, retained austenite having high metastability can be obtained even if the amount of aluminum is small. Therefore, even in the range of 0.01 to 0.20%, a composite structure that meets the gist of the present invention can be obtained.
When the amount of aluminum is 1.50% or more, it becomes easy to cause clogging of the casting nozzle in the continuous casting process, and it becomes extremely difficult to manufacture the slab by continuous casting. Therefore, the upper limit of the aluminum content is 1.50%.

  Copper (Cu) improves the strength of the steel by solid solution strengthening, but if it is intentionally added beyond the unavoidable addition in production, the upper limit is set to 1.0%.

  Nickel (Ni) can increase the amount of retained austenite as well as silicon and aluminum while improving the strength of the steel by solid solution strengthening. However, the content is inevitably added in production, and if it is intentionally added, the cost increases, so the upper limit was made 1.0%.

  Molybdenum (Mo) is an element that enhances hardenability like manganese, and can improve the strength of steel. However, if it is intentionally added, it increases costs if it is intentionally added in addition to inevitable manufacturing. Therefore, the upper limit was made 1.0%.

Titanium (Ti) has an effect of suppressing recrystallization and crystal grain growth in the final annealing process as well as refinement of crystal grains in the hot rolling process.
Since the annealing temperature range in the final process is high, recrystallization or crystal grain growth occurs in a steel sheet having extremely fine crystal grains or a steel sheet subjected to cold rolling. If recrystallization or crystal grain growth occurs easily, the fine structure created in the hot rolling process is eliminated. Titanium is an effective element that suppresses recrystallization and grain boundary movement. If the amount is in the range of 0.02 to 0.22%, a dense structure can be maintained even after annealing. However, since the effect does not increase much even if it exceeds 0.22%, the upper limit is 0.22%.

  Niobium (Nb) also has the effect of suppressing recrystallization and crystal grain growth, similar to titanium. In order to obtain a dense structure and maintain the structure after annealing, an amount of niobium in the range of 0.02 to 0.10% is required. When it is 0.02% or less, the effect of suppressing recrystallization and crystal grain growth is lost. In addition, even if the amount of niobium increases beyond 0.10%, the effect does not increase so much. Moreover, since the crack will generate | occur | produce in a slab at the time of continuous casting if the quantity increases, the upper limit was made into 0.10%.

  Phosphorus (P) and sulfur (S) are not necessary components in the steel of the present invention, but are inevitably contained in production. Since any element significantly impairs spot weldability, it is basically desired to reduce it as much as possible.

The slab adjusted to the above-described reference component is either hot-rolled after reheating or hot-rolled immediately after casting.
In performing hot rolling, hot rolling is performed by a rolling mill having a plurality of stands.
It is important to complete the rolling in the austenite region of Ar3 or higher. When the rolling completion temperature is lower than the austenite region, a non-uniform structure is formed and the material is significantly inhibited. Therefore, the temperature at which hot rolling is completed is Ar3 or higher.

  After completion of hot rolling, it is necessary to start cooling from Ar3 or higher, perform cooling at a rate of Δ40 ° C / sec or higher, continuously cool to a temperature range of 300 ° C or higher and 600 ° C or lower, and then wind up. When the cooling start temperature is lower than Ar3, ferrite transformation partially occurs, and when the cooling rate is lower than Δ40 ° C./sec, the same phenomenon occurs and a uniform structure cannot be obtained. If the structure is non-uniform after completion of hot rolling, the structure after annealing tends to be non-uniform, and the material deteriorates.

  A thin steel plate having a thickness of about 4 mm or less is obtained by the above hot rolling or cold rolling after that.

In the final annealing, it is heated in the range of A1 to (Ac3 + 20) ° C, and is kept isothermal for 1 to 120 seconds. After forming ferrite and austenite, it is 10 ° C / sec to the temperature range of A1 to (A1-100) ° C. Slow cooling at the following cooling rate, then rapid cooling to 400-550 ° C at a cooling rate of 60 ° C / sec or more, and further, cooling at 10-30 ° C / sec to 250 ° C or less without holding at around 400 ° C Cool at speed.
At this time, in order to obtain high strength with a low alloy composition, it is desirable to avoid cooling stop as much as possible to 250 ° C. or lower while performing stepwise cooling. However, after rapid cooling, it is necessary to change the cooling rate at around 500 ° C., and temperature measurement is required for control. In order to measure temperature, it is necessary to temporarily stop cooling, but it is necessary to make the stop time within 5 seconds.

Regarding the temperature control during annealing, the meaning of each temperature will be described below.
The soaking temperature after heating is in the range of A1 to (Ac3 + 20) ° C.
The purpose is to make the structure during soaking into two phases of ferrite and austenite.
At temperatures below A1, austenite, the source of martensite, is not formed after cooling, while at temperatures above (Ac3 + 20), it becomes the martensite main phase after cooling, the soft ferrite phase is insufficient, and the ductility is low. Extremely low.

In the first stage cooling after completion of soaking, it is necessary to gradually cool from the soaking temperature to a temperature range of A1 to (A1-100) ° C. at a cooling rate of 10 ° C./sec or less.
In the case of continuous cooling at a rate of 10 ° C./sec or less to a temperature below the A1 point, the austenite phase gradually transforms into a ferrite phase, and the phenomenon of carbon transfer to untransformed austenite occurs.
In the case of cooling only to A1 or more, the carbon concentration of untransformed austenite is insufficient, and it is difficult to obtain retained austenite after completion of cooling. In the case of cooling to (A1-100) or lower, the strength is lowered and the untransformed austenite phase is transformed into pearlite, and the target structure of the present application cannot be obtained.

In the second stage cooling, it is necessary to cool gradually to A1 to (A1-100) ° C. and then rapidly cool to 400 to 550 ° C. at a cooling rate of 60 ° C./sec or more.
This rapid cooling is to suppress pearlite decomposition of untransformed austenite.
When the cooling rate is 60 ° C./sec or less, untransformed austenite undergoes pearlite transformation and the tensile properties deteriorate, so the temperature is set to 60 ° C./sec or more. The rapid cooling end point temperature is 400 to 550 ° C., and the effect is obtained, but the one with 500 ° C. improves the characteristics most. Therefore, it is preferable to control the second-stage cooling end point temperature at 500 ° C.

In the third stage cooling, it is important not to maintain at around 400 ° C., but to cool at a rate of 10 to 30 ° C./sec from the rapid cooling end temperature of around 500 ° C. to 250 ° C. or less.
In this temperature range, a part of untransformed austenite is transformed into martensite, and the remainder becomes retained austenite. By continuously cooling at a cooling rate of 10 to 30 ° C./sec, concentration of carbon to untransformed austenite is further promoted, and residual austenite with high metastability is formed. On the other hand, since cooling is not stopped in this temperature range, moderate martensite is generated, and high strength can be obtained.
When the cooling rate is 30 ° C./sec or more, most of the untransformed austenite is transformed into martensite, making it difficult to obtain moderate retained austenite. On the other hand, at a cooling rate of 10 ° C./sec or less, untransformed austenite decomposes with cementite.

When a large amount of silicon or aluminum is contained, an austempering treatment at around 400 ° C. under cooling causes a carbon concentration phenomenon to untransformed austenite, and a highly metastable residual austenite is obtained. However, when the amount of silicon or aluminum is small and the same treatment is performed, untransformed austenite decomposes with cementite, and residual austenite cannot be obtained.
This is the reason why silicon and aluminum are added in appropriate amounts in many high-strength steel sheets. However, silicon deteriorates the surface properties and aluminum deteriorates continuous castability. The element whose amount is to be suppressed.
In the production method of the present application, even if the amount of silicon or aluminum is small (Si: 0.01 to 0.30%, Al: 0.01 to 0.20%), retained austenite with high metastability is obtained, and an appropriate amount of martensite is included. Therefore, a great feature is that high strength can be obtained.

The structure of the high-strength steel sheet manufactured on the basis of the above-mentioned conditions includes martensite having a volume ratio of 20% or more in the grains and grain boundaries of ferrite having an average grain diameter of 10 μm or less, and the average grain size is Residual austenite having a carbon concentration of 3% or less and a carbon concentration of 0.9% or more is contained by 5% or more by volume.
This coexistence of hard martensite and retained austenite produces properties with high strength characteristics even in low alloys similar to composite steel plates, and also produces the same elongation and processing characteristics as TRIP steel plates.

  FIG. 1 shows the concept of temperature history in hot rolling, cold rolling and annealing in the manufacturing process of the embodiment of the present invention, where the horizontal axis represents time and the vertical axis represents temperature. In the figure, a, b, and c indicate that a hot rolling rough rolling process, a finish rolling process, and a winding process are performed, respectively. Although d shows a cold rolling process, the said process is not necessarily implemented. Furthermore, e, f, g, and h show the soaking process in the annealing process, and the cooling process in the first stage, the second stage, and the third stage, respectively.

  2 (a) and 2 (b) show the material characteristics of a steel sheet that was subjected to final annealing without performing cold rolling after completion of hot rolling in the manufacturing process shown in FIG. The components of the test material are as shown in Table 1. FIG. 2 (a) shows the test results of sample A, and FIG.

The annealing conditions were soaked at (Ac3-10) ° C., gradually cooled to 680 ° C. at 10 ° C./sec, rapidly cooled to 500 ° C. at 125 ° C./sec, and then cooled in the third stage. FIG. 2 shows the relationship between the cooling rate of the third stage (horizontal axis) and the steel material properties (tensile strength TS, elongation value t.EL, product TS × t.EL) after final annealing. After cooling to the second stage, the result of cooling to 400 ° C at 20 ° C / sec and then austempering at 400 ° C for 60 seconds is the same as the third stage cooling rate "1" on the horizontal axis. Arranged in the figure.
Fig. 2 (a) shows a specimen containing 1.50% silicon. When the cooling rate of the third stage decreases, TS decreases and t.EL tends to increase, but for TS x t.EL In both cases, a high value of 20,000 or more is obtained. In particular, the peak is observed when the third stage cooling rate is 10 ° C / sec.
FIG. 2 (b) shows a test material of 0.20% silicon, and a tendency different from that in the case of 1.50% is recognized. As the cooling rate in the third stage decreases, TS tends to decrease in the same manner, but t.EL shows a peak at 20 ° C./sec, and t.EL also decreases as the cooling rate decreases. As a result, TS × t.EL also shows a peak at 20 ° C./sec.

FIG. 3 shows the relationship between the cooling rate (horizontal axis) of the third stage of the test specimen and the morphology of retained austenite (volume fraction Vf [γ], carbon concentration C [γ] of retained austenite).
FIG. 3A shows a test material containing 1.50% silicon, and shows a tendency that both Vf [γ] and C [γ] increase when the cooling rate in the third stage decreases. All show sufficient carbon concentration of 0.9% or more, and the amount of retained austenite of 5.0% or more is obtained.
On the other hand, FIG. 3B shows a sample material of silicon 0.20%, and a tendency different from the case of 1.50% is recognized as in the case of the tensile properties of FIG. Vf [γ] shows a peak at 20 ° C./sec as the cooling rate of the third stage decreases. C [γ] tends to increase as the cooling rate of the third stage decreases, but there is no difference between 20 ° C / sec and 1 ° C / sec (400 ° C for 60 seconds austempering). .
The tendency of Vf [γ] obtained here is the same as the tendency shown by TS × t.EL in FIG. 2, and it is explained that the improvement of tensile properties is caused by transformation-induced plasticity of retained austenite. The

In addition, the measurement of the retained austenite in FIG. 3 was calculated | required by the X ray diffraction method using the K alpha ray of Cu. After the surface electrolytic polishing finish at the 1 / 2t thickness, the integrated strength of the (200) (220) (311) face of the austenite phase and the (200) (211) face of the ferrite phase is measured and calculated from the respective combinations. The average value of residual austenite volume fraction was used.
The carbon concentration in the retained austenite is also calculated by X-ray diffraction, using the diffraction angle indicating the peak intensity of the (111) (200) (220) (311) plane of the austenite phase to obtain the lattice constant of the austenite phase. did.

FIG. 4 shows an SEM observation photograph of a typical cross-sectional structure of the steel of the present invention. Martensite and retained austenite (white portions in the photograph) are present in the ferrite grains and at the grain boundaries, and some exhibiting a lath shape are also observed.
The volume ratio of the white part was measured using commercially available image analysis software, and was 43% in volume ratio. Further, since the volume fraction of retained austenite by X-ray diffraction was 7.8%, the volume fraction of martensite was obtained as the difference, and about 35% was obtained.

Examples of the invention will be described below.
Molten steel having chemical components shown in Table 2 was used as a slab (rolled material) by a continuous casting method or a forging method. Subsequently, these slabs were reheated and hot-rolled to obtain hot-rolled steel sheets.

  In steel types A, B, C, D, and E adjusted to the above-mentioned reference components, carbon, silicon, manganese, chromium, and aluminum are 0.05 to 0.20%, 0.01 to 1.20%, 0.30 to 2.10%, and 0.01 to 3.00%, respectively. , Adjusted in the range of 0.02 to 0.40%. Steel types A, B, and D were added with titanium or niobium, and their amounts were blended up to 0.12% and 0.06%, respectively.

  Steel type F, which is a comparative example, has carbon, silicon, chromium, and titanium amounts within the reference range, but the Mn amount is high and out of the reference range.

About hot rolling, cold rolling, and annealing conditions, each steel type of an Example and a comparative example was processed on the conditions shown in Table 3. When cold rolling is not performed, the steel sheet is hot-rolled to a thickness of 1.4 to 1.8 mm.For cold rolling, the hot-rolling is finished with a thickness of 3.0 to 4.0 mm, and after cold rolling, the thickness is 1.4 to 1.8 mm. mm.
The numbers underlined in Table 3 are outside the standard conditions. For steel type F (No. 15), the chemical composition deviates from the standard, so the steel type indication is underlined.

In hot rolling, the final rolling reduction was 15% or more in all conditions, finish rolling was finished with Ar3 or more, and cooling was started within 5 seconds. The coiling temperature was controlled at a temperature within the standard except for the condition No. 12 in Table 3.
About cold rolling, the rolling reduction was described about what was implemented.

For continuous annealing, the soaking temperature was first set to around (Ac3-10) ° C. However, No. 6 in Table 3 shows an example in which the temperature was higher than the prescribed (Ac3 + 20) ° C.
Subsequently, in the first stage cooling, it was gradually cooled to a temperature range of A1 to (A1-100) ° C. at a cooling rate of 10 ° C./sec. However, No. 4 in Table 3 shows an example in which rapid cooling was started immediately after the completion of soaking, and No. 3 was gradually cooled to a temperature of (A1-100) ° C. or lower.
In the second stage cooling, the end point temperature was unified with the aim of 500 ° C., and the cooling was performed to that temperature at a cooling rate of 125 ° C./sec. However, No. 11 in Table 3 shows an example of cooling at a cooling rate of 60 ° C./sec or less, which is a reference.
After the completion of the second stage cooling, a cooling stop region of about 1 second was provided for temperature measurement while starting the third stage cooling. However, No. 10 in Table 3 shows an example in which the time is about 10 seconds.
Furthermore, in the third stage cooling, cooling was performed at a cooling rate of 10 to 20 ° C./sec until the end point temperature was around 100 ° C. However, No. 7 and No. 9 in Table 3 show examples in which the cooling rate outside the standard is applied, and No. 13 shows an example in which the end point temperature is out of the standard.

SEM observation of the cross section in the rolling direction on the test material after continuous annealing, optical microscope observation and X-ray diffraction by a special etching method to be described later, main phase structure morphology, volume fraction of retained austenite, carbon concentration and its particle size, martens The site volume ratio was investigated. The results are summarized in Table 3.
The results of the tensile test are also shown in Table 3. In Table 3, TS indicates the tensile strength, and t.EL indicates the total elongation value.

In the example of steel type A (No. 1), all of the steel types have a relatively small amount of carbon, so the amount of martensite and the amount of retained austenite are small and the strength level is low, but relatively good elongation characteristics are exhibited.
Steel type B contains a large amount of silicon and aluminum, which are austenite stabilizing elements, and also contains a relatively large amount of carbon, and is therefore a steel type in which retained austenite can be easily obtained. However, abundant retained austenite can be obtained when the No. 2 standard condition is satisfied, but the first stage of slow cooling such as No. 3 and 4 has not been performed according to the standard. The carbon concentration tends to decrease.
Steel type C has less silicon and aluminum than steel type B, but if it is made according to standard processes and conditions, it has high strength and high ductility.
Steel grade D has less silicon and aluminum. Nevertheless, if it is built under the conditions within the standard, the same high strength and high ductility properties as steel grades B and C are obtained. However, if the cooling conditions are inappropriate as in Nos. 7, 9, 10, and 11, the amount of retained austenite and the carbon concentration are lowered, and a tendency to have good tensile characteristics cannot be obtained. In addition, when the soaking temperature exceeds (Ac3 + 20) ° C as in No. 6, the structure forms with martensite as the main phase and the strength is significantly increased, but the strength / ductility balance is greatly reduced. No. 12 is a case where the coiling temperature in the hot rolling is higher than the standard, and the residual austenite grains are slightly coarsened.
Steel type E is a case where chromium is used as a hardenable element, and if the standard conditions are satisfied, good tensile properties can be obtained. On the other hand, No. 13 is the case where the end point temperature after the third stage cooling is too high, and at this time, untransformed austenite decomposes in cementite, hardly any retained austenite is obtained, and the tensile properties are also poor.
Steel type 15 is a case where manganese is higher than the standard. In this case, even if manufactured under the conditions within the standard, the amount of martensite obtained is large and the strength is remarkably high, but the amount of retained austenite is small and the elongation characteristics are low. .

The volume fraction of retained austenite and the carbon concentration were calculated by the X-ray diffraction method.
The grain size of retained austenite is polished by a special etching method (CAMP-ISIJ, 6 (1993), p.1698) after polishing the cross section in the rolling direction of the steel sheet, and the position in the thickness direction 1/4 is observed with an optical microscope. The white retained austenite grains were analyzed by image analysis and measured.
The volume ratio of martensite is obtained by polishing the cross section in the rolling direction of the steel sheet, performing night etching, observing the position in the thickness direction 1/4 by SEM, and measuring the total area of the low temperature transformation phase etched in white by image analysis The residual austenite volume fraction obtained by the above X-ray diffraction measurement was subtracted and calculated.
Tensile properties (tensile strength TS, elongation value t.EL) were measured by a tensile test using a JIS13B specimen shape.

The high-strength steel sheets of the examples have a high strength of TS600 to 1200 MPa in a low alloy composition, and a good strength / ductility balance of 18000 MPa ·% or more in TS × t.EL can be obtained. Therefore, it is suitable for use as a member for automobile structure.
For example, the center pillar of an automobile requires the tensile strength necessary to prevent deformation at the time of collision as well as the support of the door, as well as bending, drawing workability for press molding, etc., and joining with other body parts It can be said that it is a very preferable steel plate as a member that requires a high level of weldability.

It is a diagram which shows the concept of the temperature history in the hot rolling in the manufacturing process of this embodiment, cold rolling, and annealing. Figures 2 (a) and 2 (b) show the third stage cooling rate (horizontal axis) and steel sheet material properties after final annealing (tensile strength TS, elongation value t.EL, and their product TS x t.EL. FIG. 3 (a) and 3 (b) show the relationship between the cooling rate (horizontal axis) in the third stage and the form of retained austenite (volume ratio Vf [γ] of retained austenite, carbon concentration C [γ]). FIG. It is a typical cross-sectional structure | tissue photograph about the high strength steel plate by this invention.

Claims (6)

  The volume of residual austenite containing martensite at a volume fraction of 20% or more and a mean particle diameter of 3 μm or less and a carbon concentration of 0.9% or more by mass% in and within the ferrite grains having an average grain size of 10 μm or less. A high-strength steel sheet characterized by having a rate of 5% or more.   C: 0.05 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 1.50%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0 %, Mo: 0.01 to 1.0%, or Ti: 0.02 to 0.22%, Nb: 0.02 to 0.10%, or both, or both.   C: 0.05 to 0.25%, Si: 0.01 to 0.30%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 0.20%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0 %, Mo: 0.01 to 1.0%, or Ti: 0.02 to 0.22%, Nb: 0.02 to 0.10%, or both, or both.   The high strength steel sheet according to any one of claims 1 to 3, wherein a product of tensile strength (MPa) and elongation value (%) is 18000 (MPa ·%) or more. C: 0.05 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 1.50%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0 %, Mo: 0.01 to 1.0%, or further containing Ti: 0.02 to 0.22%, Nb: 0.02 to 0.10%, or both steel materials,
Hot rolled by a hot rolling mill having a plurality of stands, and after rolling at Ar3 or higher, starts cooling from a temperature of Ar3 or higher, and winds in a temperature range of 300 ° C or higher and 600 ° C or lower,
As it is or after cold rolling,
1) Heat to A1 or higher and (Ac3 + 20) ° C or lower and hold at that temperature.
2) Slow cooling at a cooling rate of 10 ° C / sec or less to a temperature range of A1 to (A1-100) ° C.
3) Then, rapidly cool to 400-550 ° C at a cooling rate of 60 ° C / sec or more.
4) A method for producing a high-strength steel sheet, further comprising cooling to 250 ° C. or lower at a cooling rate of 10 to 30 ° C./sec. C: 0.05 to 0.25%, Si: 0.01 to 0.30%, Mn: 0.3 to 2.5%, Cr: 0.01 to 3.0%, Al: 0.01 to 0.20%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0 %, Mo: 0.01 to 1.0%, or further containing Ti: 0.02 to 0.22%, Nb: 0.02 to 0.10%, or both steel materials,
Hot rolled by a hot rolling mill having a plurality of stands, and after rolling at Ar3 or higher, starts cooling from a temperature of Ar3 or higher, and winds in a temperature range of 300 ° C or higher and 600 ° C or lower,
As it is or after cold rolling,
1) Heat to A1 or higher and (Ac3 + 20) ° C or lower and hold at that temperature.
2) Slow cooling at a cooling rate of 10 ° C / sec or less to a temperature range of A1 to (A1-100) ° C.
3) Then, rapidly cool to 400-550 ° C at a cooling rate of 60 ° C / sec or more.
4) A method for producing a high-strength steel sheet, further comprising cooling to 250 ° C. or lower at a cooling rate of 10 to 30 ° C./sec.