JP5194522B2 - High-strength, high-workability hot-rolled steel sheet excellent in workability of the friction stir welding method and its production - Google Patents

Description

  The present invention relates to a high-strength, high-workability hot-rolled steel sheet excellent in workability of the friction stir welding method and a method for producing the same, and particularly to a hot-rolled steel sheet suitable for use in an automotive steel sheet, In addition to imparting high strength, when the friction stir welding method is applied to the steel sheet, it is intended to improve its workability.

  As a friction stir welding method, a tool made of a material substantially harder than a workpiece is used in Patent Document 1, and this tool is inserted into a workpiece joint and moved while rotating the tool. There has been proposed a method in which a friction stir zone is formed by heat and plastic flow generated between a workpiece and a workpiece, and the workpiece is continuously joined in the longitudinal direction. The joining method disclosed in Patent Document 1 is referred to as a friction welding method, a friction joining method, a friction stir welding method, a friction stir welding method, or the like. In the present specification, these are collectively referred to as a friction stir welding method. Shall.

  Also, in Patent Document 2, the tool pin is pressed against a predetermined joining point of the objects to be joined while rotating the tool, and the object around the pin is heated and softened by frictional heat to insert the pin. Then, the objects to be joined in the vicinity of the joining point are agitated with a rotating pin, and the objects to be joined are integrated at the joining point, and then the joining tool is joined by pulling out the joining tool along the axis with a moving motor. A joining method characterized by spot joining at a point is disclosed. In this specification, this joining method is called a friction stir spot joining method. In this friction stir spot welding method, as in the friction stir welding method, the friction stir zone is formed by the frictional heat and plastic flow generated between the pin part of the rotating tool and the object to be joined. There is much in common in the phenomenon.

  The friction stir welding method disclosed in Patent Document 1 can be joined by moving the tool while rotating the joining member in a fixed state. For this reason, there is an advantage that even a member that is substantially infinitely long in the welding direction can be continuously solid-phase bonded in the longitudinal direction. Moreover, since it is a solid-phase joining using the plastic flow of the metal by the frictional heat of a rotating tool and a joining member, there exists an advantage that it can join, without melt | dissolving a junction part. Furthermore, since the heating temperature is low, not only the deformation after bonding is small, but also the bonding portion is not melted, and there are many advantages such as fewer defects.

  The friction stir welding method and the friction stir welding method described above are widely used in the fields of aircraft, ships, railway vehicles, automobiles and the like as joining methods of low melting point metal materials typified by aluminum alloys. The reason for this is that these low melting point metal materials are difficult to obtain satisfactory properties at the joints by the conventional arc welding method or resistance spot welding method in terms of cost and efficiency. This is because when spot bonding is applied, productivity is improved and a high-quality bonded portion can be obtained.

  On the other hand, the friction stir welding method and the friction stir spot welding method are applied to steel materials that are mainly applied as materials for structures such as buildings, ships, heavy machinery, pipelines, and automobiles. Less popular than metal materials.

  That is, in the friction stir welding method and the friction stir spot welding method, the principle is that the material of the rotary tool is substantially harder than the workpiece, but in the case of joining a low melting metal material as the material of the rotary tool. Inexpensive tool steel can be used, but in the case of joining steel materials, as disclosed in Patent Document 3 and Patent Document 4, it is possible to use high-quality materials such as polycrystalline boron nitride (PCBN) and polycrystalline diamond. This is because wear-resistant materials are currently used, and the production technology and cost of such high wear resistance greatly influence the spread of the friction stir welding method and the friction stir spot welding method.

  Furthermore, since the load on the rotating tool made of this highly wear-resistant material is excessive, the joining conditions related to efficiency such as the plate thickness and joining speed that can be joined are greatly limited, and replacement due to tool wear or breakage is limited. Construction efficiency is not good due to work.

  As the joining method, the less constrained the joining conditions and the higher the construction efficiency, the better in practical use, and the ease of use in practical use is generally expressed as workability, but the friction stir welding method and the friction stir spot joining method. However, the workability of these steel materials is not sufficient in comparison with arc welding or resistance spot welding methods widely used for joining steel materials.

  In order to solve the above problem, the inventors previously described, in Patent Document 5, “a low alloy structural steel, a temperature range width that becomes a ferrite single phase in an equilibrium state of 600 ° C. or higher, an austenite phase, Proposed a friction stir welding method consisting of a low alloy structural steel for friction stir welding, characterized in that the total temperature range of the two phases of the ferrite phase is 200 ° C. or higher.

  With friction stir welding, as described above, a rotating tool made of a material harder than the workpiece is inserted into the workpiece joint and moved while rotating, and the softening by the frictional heat with the rotating tool and the softened portion rotate. In this joining method, the durability of the rotating tool depends on the hardness of the rotating tool, the wear resistance and the deformation of the workpiece at the ultimate temperature of the joint. Determined by relative relationship with resistance.

When low-alloy welded structural steel is friction stir welded by the friction stir welding method or the friction stir spot joining method, the ultimate temperature of the joint is 900 ° C. or higher, but the high wear resistance disclosed in Patent Documents 3 and 4 above When using a rotary tool made of a porous material, the deformation resistance of low alloy welded structural steel tends to be excessive in relation to the hardness and wear resistance of the rotary tool in the ultimate temperature range of the joint. It was necessary to limit the joining conditions such as the joining speed and the thickness of the steel sheet to ensure the durability of the rotating tool.
As a result, although the frequency of replacement work due to wear and breakage of the tool can be suppressed, since the joining time becomes long, the construction efficiency cannot be directly improved. That is, improvement in workability was not obtained.

  The invention disclosed in Patent Document 5 advantageously solves the above-described problem. For low alloy structural steels, the temperature range of the ferrite single phase in the equilibrium state of 600 ° C. or higher, the austenite phase, and the ferrite phase are 2 By setting the total temperature range width to be at least 200 ° C, the deformation resistance of the steel material against the hardness and wear resistance of the rotating tool can be effectively reduced even in the ultimate temperature range of the joint of 900 ° C or higher. In addition, it is possible to relieve the joining conditions when carrying out the friction stir welding, and to significantly improve the workability.

  On the other hand, various studies have been made on high strength hot-rolled steel sheets having high workability as steel sheets for automobiles. For example, Patent Documents 6 to 9 disclose Transformation Induced Plasticity steel (hereinafter referred to as TRIP steel) containing 5% or more of retained austenite and the remainder having a structure mainly composed of pro-eutectoid ferrite. There has been little research on automotive steel plates for friction stir welding.

  That is, Patent Documents 6 to 9 disclose high-strength hot-rolled steel sheets having ductility suitable for use as automobile steel sheets. In these Patent Documents 6 to 9, friction stir intended in the present invention is disclosed. No consideration is given to the application to the joining method and the friction stir spot joining method, and in the component composition disclosed in the above patent document, the temperature range width that becomes a ferrite single phase in an equilibrium state of 600 ° C. or higher and The condition that the sum of the temperature range widths of the two phases of the austenite phase and the ferrite phase was 200 ° C. or higher was not always satisfied.

JP 7-505090 Gazette Japanese Patent No. 3429475 Special table 2003-532542 gazette Special Table 2003-532543 Japanese Patent Application No. 2006-202909 Specification JP 63-4017 A Japanese Patent No. 2559272 Japanese Patent No. 3412157 Japanese Patent No. 3400351

The present invention has been developed in view of the above-described present situation, and relates to the improved invention of Patent Document 5 mentioned above.
That is, the present invention naturally has the ductility and strength required as a steel plate for automobiles, and it is possible to obtain excellent workability when the friction stir welding method and the friction stir spot welding method are applied. The object is to propose a high-strength hot-rolled steel sheet together with its advantageous production method.

As a result of intensive studies to overcome the above problems, the inventors have obtained the following knowledge.
First, as for workability when the friction stir welding method is applied, as described in Patent Document 5, the steel material is expanded by expanding the ferrite single-phase region and the austenite-ferrite two-phase region in the vicinity of the ultimate temperature of the joint. It was reconfirmed that it is advantageous to reduce the deformation resistance.
In addition, in order to obtain the high ductility and high strength required for automotive steel sheets, it is effective to have an appropriate amount of retained austenite as the second phase in proeutectoid ferrite, which is the main phase of steel. Got.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. Steel composition is mass% C: 0.05-0.40%,
Si: 4.0% or less,
Mn: 0.5-3.0% and
Al: containing 4.0% or less, and (Si + Al) satisfies ≧ 0.5%, the balance being Fe and unavoidable impurities, the residual austenite by 5% or more as the second phase in the pro-eutectoid ferrite main phase Friction stir welding method characterized in that the sum of the temperature range width that becomes a ferrite single phase and the temperature range width that becomes two phases of an austenite phase and a ferrite phase in an equilibrium state of 600 ° C or higher is 200 ° C or higher High strength and high ductility hot-rolled steel sheet with excellent workability.

2. In the above 1, the steel plate is further in mass%,
Cr: 0.2-2.0%
Nb: 0.003-0.10%,
V: 0.003-0.10% and
Ti: 0.003-0.10%
A high-strength, high-ductility hot-rolled steel sheet excellent in workability of the friction stir welding method, comprising one or more selected from among them.

3. Steel composition is mass% C: 0.05-0.40%,
Si: 4.0% or less,
Mn: 0.5-3.0% and
Al: containing 4.0% or less, and (Si + Al) satisfies ≧ 0.5%, the balance being a steel slab having the composition of Fe and unavoidable impurities, after heating, completion of the finish rolling at a temperature range of nine hundred eighty to seven hundred and fifty ° C. After that, it is cooled to a temperature range of 500 to 300 ° C. at a cooling rate of 20 ° C./s or more, and then wound up. A method of manufacturing a steel sheet.

4 . Oite above 3, the steel slab further by mass%,
Cr: 0.2-2.0%
Nb: 0.003-0.10%,
V: 0.003-0.10% and
Ti: 0.003-0.10%
A method for producing a high-strength, high-ductility hot-rolled steel sheet excellent in workability of the friction stir welding method, comprising one or more selected from among them.

  In accordance with the present invention, by expanding the ferrite single-phase region and the austenite-ferrite two-phase region in the vicinity of the ultimate temperature of the joint, the deformation resistance of the steel material in the friction stir welding is greatly reduced, resulting in the durability of the rotating tool. And the limitation of bonding conditions such as bonding speed is eased. In addition, this reduces the frequency of replacement work due to wear and breakage of the tool and shortens the joining time, thus improving the construction efficiency. Furthermore, by forming a microstructure in which 5% or more of retained austenite is present as the second phase in the pro-eutectoid ferrite main phase, it is possible to obtain high strength along with high ductility due to the TRIP effect. It is possible to obtain a high-strength, high-ductility hot-rolled steel sheet that satisfies the high TS × El value.

The elucidation process of the present invention will be described below.
Conventional welding structural steel, for example, low alloy welding structural steel, at equilibrium, 2 next phase of the austenite phase and ferrite phase in A ₁ point in the vicinity of 730 ° C., from around 1450 ° C. A _3-point around 900 ° C. A Austenite single phase up to ₄ points. The temperatures at points A ₁ , A ₃ and A ₄ vary somewhat depending on the amount of alloy.

  Now, the crystal structure of the austenite phase is a face-centered cubic lattice, and there are 12 slip systems based on the combination of dislocation slip direction <110> and slip surface {111}. On the other hand, the ferrite phase of the body-centered cubic lattice has 48 slip systems from the slip direction <111> and the slip surfaces {110}, {112}, {123}. Therefore, since the ferrite phase has more slip systems than the austenite phase, there is less interference between dislocations in the process of dislocation slipping during plastic deformation. That is, since there is little work hardening, the deformation resistance at the time of friction stir welding becomes low.

Therefore, the higher the ratio of the ferrite phase to the austenite phase near the ultimate temperature of the joint, the lower the deformation resistance at the time of friction stir welding, especially the temperature range width that becomes a ferrite single phase in an equilibrium state of 600 ° C. or higher. It was found that the target steel material can be obtained by adjusting the components so that the total temperature range width of the two phases of the austenite phase and the ferrite phase becomes large. In particular, the effect was remarkable when the temperature range width of the ferrite single phase and the temperature range width of the two phases of the austenite phase and the ferrite phase were 200 ° C. or more.
The reason why the base temperature in the equilibrium state to be determined is 600 ° C. is that when the steel is friction stir welded, the temperature of the joint becomes approximately 600 ° C. or higher.

In order to expand the temperature range of the ferrite single phase and the temperature range of the austenite-ferrite two phases in an equilibrium state of 600 ° C or higher, it is effective to add ferrite stabilizing elements such as Si and Al. It is thought that.
The reason for this is that these ferrite stabilizing elements form a γ loop in the phase diagram of Fe and each element.

  For example, FIG. 1 is a Fe—Al equilibrium diagram of a system containing C: 0.1% and Mn: 1.5% by mass%, but austenite (indicated by γ in the figure) single-phase region as the Al content increases As the amount of Al decreases to 1.2% or more, the austenite single phase region is not recognized. As seen in the figure, the closed austenite single phase region formed by adding a ferrite stabilizing element such as Al is called a γ loop. In addition to Al, ferrite stabilizing elements such as Si are known to have the same tendency.

Therefore, the inventors have a basic composition of Fe-0.11% C-1.5% Mn-0.013% P-0.002% S in mass%, and the above-mentioned various ferrite stabilizing elements are contained in various proportions. The total temperature range of the temperature range of the ferrite single phase and the austenite-ferrite two phase was investigated, and the size of the total temperature range and the tensile strength of the steel material at 900 ° C. were investigated.
The obtained results are shown in FIG.

As shown in the figure, the tensile strength of the steel material decreases as the total temperature range of the temperature range of the ferrite single phase and the temperature range of the austenite-ferrite two phases increases. This tendency is remarkable when the total temperature range of the temperature range of the ferrite single phase and the temperature range of the austenite-ferrite two phase is 200 ° C. or more.
Therefore, in the present invention, the total of the temperature range width that becomes the ferrite single phase and the temperature range width that becomes the two phases of the austenite phase and the ferrite phase in the equilibrium state of 600 ° C. or higher is defined as 200 ° C. or higher.

  As described above, the tensile strength of the steel material is set to 200 ° C or more by adding the temperature range width of the ferrite single phase and the temperature range width of the two phases of the austenite phase and the ferrite phase in an equilibrium state of 600 ° C or higher. That is, the deformation resistance decreases.

  Moreover, in order to ensure good ductility and high strength as a steel plate for automobiles, it is considered effective to obtain a TRIP effect in the presence of an appropriate amount of retained austenite as the second phase.

Then, the component composition which can achieve various requirements as mentioned above was examined next.
As a result, the following preferred component composition range was found. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.05-0.40%
C is an element that not only contributes effectively to strengthening of steel but is also effective in obtaining retained austenite. On the other hand, since C is an austenite stabilizing element, excessive addition leads to expansion of the austenite single phase region in an equilibrium state of 600 ° C. or higher, and the temperature range width of the ferrite single phase, the austenite phase and the ferrite phase 2 Reduce the total temperature range width to be a phase. Here, if the C content is less than 0.05%, the effect in obtaining retained austenite in the steel is poor, whereas if it exceeds 0.40%, the temperature range width and austenite phase that becomes a ferrite single phase in an equilibrium state of 600 ° C. or higher. In order to reduce the total temperature range width of the two phases, and the ferrite phase, the C content is limited to a range of 0.05 to 0.40%.

Si: 4.0% or less
Si is a ferrite stabilizing element that forms a γ loop, and is intended to increase the sum of the temperature range width of the ferrite single phase and the temperature range width of the two phases of the austenite phase and ferrite phase in an equilibrium state of 600 ° C or higher. Is a useful element. It is also an essential element for the formation of retained austenite. However, if the content exceeds 4.0%, the above effect is saturated, and the toughness is reduced, and further, the scale properties are reduced to cause a problem in surface quality. Therefore, the Si content is limited to 4.0% or less.

Mn: 0.5-3.0%
Mn is not only useful as a steel strengthening element, but is also an effective element for obtaining retained austenite. On the other hand, since it is an austenite stabilizing element, excessive addition leads to expansion of the austenite single phase region in an equilibrium state of 600 ° C. or higher, resulting in a temperature range width that becomes a ferrite single phase, an austenite phase, and a ferrite phase. Reduce the total temperature range. Here, when the Si content is less than 0.5%, the effect of obtaining retained austenite in the steel is poor, while when it exceeds 3.0%, the temperature range width that becomes a ferrite single phase in an equilibrium state of 600 ° C. or higher and the austenite phase In order to reduce the total temperature range width of the two phases of the ferrite phase, the amount of Mn was limited to a range of 0.5 to 3.0%.

Al: 4.0% or less
Al, like Si, is a ferrite stabilizing element that forms a γ loop, and is the total of the temperature range width that becomes a ferrite single phase and the temperature range width that becomes two phases of an austenite phase and a ferrite phase in an equilibrium state of 600 ° C or higher. It is an element that is useful for increasing the size and is an element indispensable for the formation of retained austenite. However, if the content exceeds 4.0%, the above effects are saturated and the toughness is reduced, so the Al content is limited to 4.0% or less.

(Si + Al): 0.5% or more As described above, both Si and Al are ferrite stabilizing elements that form a γ loop, and the temperature range width, austenite phase, and ferrite that become a ferrite single phase in an equilibrium state of 600 ° C or higher This is an important element for increasing the total temperature range width of the two phases. It is also an essential element for the production of retained austenite. However, if the total amount of Si and Al is less than 0.5%, the contribution to the two effects described above is poor, so that Si and Al are included in a total amount of 0.5% or more.
A particularly preferable range is (Si + Al): 1.0 to 3.0%.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cr: 0.2-2.0%
Cr has the effect of suppressing the formation of bainite in the second phase, which has an adverse effect on impact resistance, and promoting the growth of acicular ferrite. However, if the content is less than 0.2%, the effect of addition is poor. On the other hand, if the content exceeds 2.0%, the above effect is saturated, and coarse Cr carbide is produced and the ductility is adversely affected, so Cr is 0.2 to 2.0%. It was limited to the range.

Nb, V, Ti: 0.003-0.10%
Nb, V and Ti are all elements that precipitate as carbonitrides in ferrite and are effective in further increasing the strength of steel sheets. Therefore, they are added as necessary, but if the content is less than 0.003%, The addition effect is poor. On the other hand, if it exceeds 0.10%, the above effect is saturated and the ductility is adversely affected. Therefore, Nb, V, and Ti are limited to the range of 0.003 to 0.10%, respectively.

  The balance is Fe and inevitable impurities. Typical examples of inevitable impurities include P and S. Since both P and S are elements that promote central segregation, it is desirable to reduce them as much as possible. Preferably, P should be limited to 0.050% or less and S to 0.0050% or less.

As mentioned above, although the suitable component composition range of this invention was demonstrated, it is not enough for this invention to adjust a component structure to said range, and it is important to adjust a steel structure to the following ranges.
That is, 5% or more of retained austenite is present as the second phase in the pro-eutectoid ferrite main phase.
Here, the main phase means 60 to 90% pro-eutectoid ferrite in the area ratio of the entire steel structure. The second phase is composed of acicular ferrite, bainite, martensite, retained austenite, etc., but in the present invention, it is important that 5% or more of retained austenite is present in the volume ratio of the entire steel. is there. This is because if the amount of retained austenite in the entire steel is less than 5%, a sufficient TRIP effect cannot be obtained, and high strength and high ductility cannot be achieved at the same time. On the other hand, if the amount of retained austenite becomes too large, the retained austenite becomes difficult to transform into martensite during deformation. It is desirable to make it about 30% or less of the whole.

Next, the production method of the present invention will be specifically described.
In the present invention, in order to improve the ductility of the steel sheet, it is necessary to generate retained austenite. For this purpose, it is desired that the austenite be stabilized by C enrichment or the like.
And, by performing finish rolling in the temperature range of 980 to 750 ° C, the structure is refined, ferrite that is advantageous for improving ductility is generated, and the concentration of C is promoted in austenite, contributing to the austenite residue. Let me. At this time, if the finish rolling temperature exceeds 980 ° C., the structure becomes coarse, and ductility is hindered due to the delay of ferrite transformation. On the other hand, at a temperature lower than 750 ° C., the ferrite processed structure remains and the ductility is impaired. Accordingly, the finish rolling needs to be completed in a temperature range of 980 to 750 ° C.

After the above finish rolling, the cooling rate to the coiling temperature of 500 to 300 ° C. is 20 ° C./s or more from the viewpoint of avoiding the formation of pearlite which is disadvantageous for the remaining austenite and promoting the refinement of the structure. (Invention pattern)

In addition, after the above finish rolling, the steel sheet is once cooled to a temperature range of 800 to 600 ° C. at a cooling rate of 20 ° C./s or more and removed at an isothermal holding treatment for 1 to 40 seconds or at a cooling rate of 20 ° C./s or less. It is good also as a process which cools at the cooling rate of 20 degrees C / s or more to the coiling temperature of 500-300 degreeC after giving a cold process (reference pattern) .

  On the other hand, when the coiling temperature exceeds 500 ° C., not only bainite proceeds excessively after coiling, but also pearlite is generated, and 5% or more of retained austenite cannot be obtained. On the other hand, if it is less than 300 ° C., the martensitic transformation proceeds more than necessary, and the required retained austenite cannot be obtained. Therefore, the winding temperature was limited to a range of 500 to 300 ° C.

Example 1
Steel slabs with various composition shown in Table 1 were heated and rough-rolled, then finish-rolled, cooled and wound under the various conditions shown in Table 2 and hot rolled with a thickness of 3.2 mm. A steel plate was used. Among the slabs shown in Table 1, symbols S1 to S5 satisfy the requirements of the present invention, and the temperature range width that becomes a ferrite single phase and the temperature range width that becomes two phases of an austenite phase and a ferrite phase in an equilibrium state of 600 ° C. or higher. Although the total is 200 ° C. or higher, the symbols S6 to S10 do not satisfy the requirements of the present invention, and in particular, the symbols S6 to S8 are the temperature range width, austenite phase and ferrite phase that become a ferrite single phase in an equilibrium state of 600 ° C. or higher. This is an example in which the total temperature range width of the two phases is lower than 200 ° C.

Among the manufacturing conditions such as finish rolling, cooling, and winding shown in Table 2, conditions A to D belong to the pattern I (invention pattern) shown in FIG. 3, and among these, A and B satisfy the limiting requirements of the invention pattern. Satisfying, C and D do not satisfy the limiting requirements of the pattern of the present invention. On the other hand, the conditions E to I belong to the pattern II (reference pattern) shown in FIG. 3, among which E and F satisfy the limitation requirement of the reference pattern , and G, H, and I do not satisfy the limitation requirement of the reference pattern. is there.

Test specimens were taken from the position in the width direction 1/4 of the hot-rolled steel sheet obtained by combining the slabs having the composition shown in Table 1 and the manufacturing conditions shown in Table 2, and X-ray tests were performed on the surface in the thickness direction 1/4. Was used to measure the amount of retained austenite. In addition, a JIS No. 5 tensile specimen was taken from a position in the width direction 1/4 of the steel sheet, and a tensile test was conducted under the condition of a strain rate of 2 × 10 ⁻² / s to investigate the mechanical properties.

Table 3 shows the results of examining the retained austenite amount (residual γ amount), yield strength (YS), tensile strength (TS), total elongation (El), TS × El balance, and TRIP effect of the obtained steel sheet. Show.
Symbols T1 to T10 indicate that the composition of the slab shown in Table 1 satisfies the requirements of the present invention, and the manufacturing conditions shown in Table 2 satisfy the limiting requirements of the pattern of the present invention or the reference pattern . On the other hand, symbols T11 to T15 are those in which the component composition of the slab satisfies the requirements of the present invention, but the manufacturing conditions do not satisfy the limiting requirements of the pattern of the present invention or the reference pattern . Further, symbol T16~T20 is production conditions but meet the limitation requirements of the present invention the pattern or reference pattern, in which component composition of the slab does not meet the requirements of the present invention.
The TRIP effect is good when the residual austenite is 5% or more and TS x El is 17000 MPa ·% or more (O), while the residual austenite is less than 5% or TS x El is 17000 MPa ·%. If not, it was judged as bad (x).

As shown in Table 3, the component composition of the slab satisfies the requirements of the present invention, and any manufacturing conditions steel symbol T1~T10 satisfying the limiting requirements of the present invention the pattern or reference pattern, TS, El, and Not only was the TS × El balance excellent, but the TRIP effect was also good.

Subsequently, the steel plates having the characteristics shown in Table 3 were subjected to friction stir welding under the same joining conditions. The joint butting surfaces were joined by one pass on one side according to the surface condition of the degree of milling with a so-called I-shaped groove with no angle. In addition, a tool made of polycrystalline boron nitride (PCBN) was used as a rotating tool, and at the time of joining, the joint was shielded with argon gas to prevent surface oxidation.
Table 4 shows the joining conditions.
The tool advance angle shown in Table 4 is as shown in FIG.

FIG. 5 shows the shape of the rotating tool used. At the time of joining, the load applied to the tool was measured. A load applied to the rotating tool in the same direction as the joining direction was defined as an X load, a load applied in a direction perpendicular to the joining direction as a Y load, and a load applied in the same direction as the axial direction of the tool as a Z load.
FIG. 6 illustrates the X, Y, and Z load directions.

  Table 5 shows the load applied to the rotating tool when each steel plate is joined. The FSW workability shown in Table 5 is good when the X load is 2.6 kN or less, the Y load is 1.0 kN or less, and the Z load is 20.0 kN or less. The X load is 2.6 kN, the Y load is 1.0 kN, and the Z load. Was considered defective when either exceeded 20.0 kN. It can be considered that any load in the X, Y, and Z directions applied to the tool is reduced, that is, the deformation resistance is reduced.

As shown in Table 5, when friction stir welding was performed under the same joining conditions, the component composition of the slab shown in Table 1 satisfied the requirements of the present invention, and the manufacturing conditions shown in Table 2 were the pattern of the present invention or reference. All of the invention examples satisfying the pattern limitation requirements had good TRIP effect and FSW workability.
In contrast, none Comparative Example either component composition of the slab shown in Table 1 does not meet the requirements of the present invention, or the production conditions shown in Table 2 does not satisfy the limited requirement of the present invention the pattern or reference pattern, TRIP The effect and / or FSW workability were poor.

Example 2
A steel slab having the composition shown in slab symbol S5 in Table 1 as a basic component and various additive elements added in the proportions shown in Table 6 is finish-rolled, cooled and wound under the production conditions shown in symbol B in Table 2. Thus, a hot rolled steel sheet having a thickness of 3.2 mm was obtained.
The hot-rolled steel sheet thus obtained was subjected to a tensile test at a strain rate of 2 × 10 ⁻² / s in the same manner as in Example 1 to investigate the yield point, tensile strength, and elongation. Furthermore, in order to evaluate impact resistance, the dynamic n value at the time of high-speed tensile deformation was investigated. The dynamic n value was defined as the instantaneous n value at an elongation of 10% when the tensile deformation was performed under the condition of strain rate: 2 × 10 ³ / s. If this dynamic n value is high, more energy can be absorbed during high-speed deformation, and it is considered that the impact resistance is high.
The results obtained are shown in Table 7.

  As shown in Table 7, all of Invention Examples 11 to 14 that satisfy the requirements of the present invention were excellent in TS, El and TS × El balance, and also had a good dynamic n value.

Subsequently, the steel plate obtained as described above was used as a workpiece, and friction stir welding was performed in the same manner as in Example 1. The load applied to the rotating tool at the time of joining was measured in the same manner as in Example 1.
Table 8 shows the obtained results.

As shown in Table 8, in each of the inventive examples 11 to 14 that satisfy the requirements of the present invention, the X load was 2.49 kN or less, the Y load was 0.81 kN or less, and the Z load was 17.89 kN or less.
Thereby, it turns out that the invention example has reduced the load applied to a rotary tool, ie, a deformation resistance.

It is a Fe-Al equilibrium diagram of a system containing C: 0.1% and Mn: 1.5%. It is a graph which shows the relationship between the total temperature range width | variety of the temperature range width | variety used as a ferrite single phase and an austenite-ferrite 2 phase, and the tensile strength in 900 degreeC. It is the graph which showed the manufacture log | history of the hot rolled sheet steel in the Example. It is a schematic diagram which shows the concrete friction stir welding procedure in an Example. It is a figure which shows the dimension and shape of the rotation tool in an Example. It is a figure which shows the direction of the X, Y, Z load concerning a rotary tool at the time of the friction stir welding in an Example.

Claims (4)

Steel composition is mass% C: 0.05-0.40%,
Si: 4.0% or less,
Mn: 0.5-3.0% and
Al: containing 4.0% or less, and (Si + Al) satisfies ≧ 0.5%, the balance being Fe and unavoidable impurities, the residual austenite by 5% or more as the second phase in the pro-eutectoid ferrite main phase Friction stir welding method characterized in that the sum of the temperature range width that becomes a ferrite single phase and the temperature range width that becomes two phases of an austenite phase and a ferrite phase in an equilibrium state of 600 ° C or higher is 200 ° C or higher High strength and high ductility hot-rolled steel sheet with excellent workability. In Claim 1, the steel sheet is further in mass%,
Cr: 0.2-2.0%
Nb: 0.003-0.10%,
V: 0.003-0.10% and
Ti: 0.003-0.10%
A high-strength, high-ductility hot-rolled steel sheet excellent in workability of the friction stir welding method, comprising one or more selected from among them. Steel composition is mass% C: 0.05-0.40%,
Si: 4.0% or less,
Mn: 0.5-3.0% and
Al: containing 4.0% or less, and (Si + Al) satisfies ≧ 0.5%, the balance being a steel slab having the composition of Fe and unavoidable impurities, after heating, completion of the finish rolling at a temperature range of nine hundred eighty to seven hundred and fifty ° C. After that, it is cooled to a temperature range of 500 to 300 ° C. at a cooling rate of 20 ° C./s or more, and then wound up. A method of manufacturing a steel sheet. Oite to claim 3, steel slab further by mass%,
Cr: 0.2-2.0%
Nb: 0.003-0.10%,
V: 0.003-0.10% and
Ti: 0.003-0.10%
A method for producing a high-strength, high-ductility hot-rolled steel sheet excellent in workability of the friction stir welding method, comprising one or more selected from among them.