JP2010138449A - Precipitation strengthening type dual phase hot rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having high strength (TS: ≥1,180 MPa) and also having excellent workability. <P>SOLUTION: The hot rolled steel sheet has a composition composed of, by mass, 0.10 to 0.16% C, ≤0.5% Si, 0.5 to 1.8% Mn, ≤0.5% Al and 0.001 to 0.005% N, and further composed of at least one kind selected from 0.14 to 0.2% Ti and 0.25 to 0.40% Nb, and the balance Fe with inevitable impurities, and has a dual phase structure composed of, by volume fraction, 30 to <45% of a tempered bainite phase and a ferrite phase, in which the ferrite phase and tempered bainite phase both contain MX precipitates with an external diameter of <10 nm (wherein: M denotes a metallic element(s); and X denotes carbon or nitrogen). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a precipitation-strengthened hot-rolled steel sheet that achieves both strength and ductility, and in particular, automotive parts that require excellent workability for forming into complex shaped parts while maintaining a tensile strength of 1180 MPa or more. The present invention relates to a hot-rolled steel sheet suitable for use in

As the weight of automobile parts progresses from the standpoint of improving fuel efficiency and saving resources, steel sheets that are the raw material are required to have higher strength. On the other hand, the shape of automobile parts, particularly automobile undercarriage parts, tends to be complicated, and it is essential that the steel plate as the material has excellent workability and shape freezing properties.
Therefore, it is indispensable for steel sheets for automobile parts to have both excellent strength and workability. However, since they are generally in a contradictory relationship, it is not easy to achieve both strength and workability.
For example, although double-phase steel (DP steel) can achieve high elongation (high ductility) by containing ferrite, it has a multi-phase structure, so that it has poor stretch flangeability required when molding automobile undercarriage parts.
On the other hand, bainite single-phase steel containing no ferrite has good stretch flangeability but poor ductility.

As a technique for solving the above problem, a technique is known in which the main phase is a ferrite single phase suitable for ensuring high ductility, and precipitation strengthening by fine precipitates is used as a main strengthening mechanism. For example, in Patent Document 1 and Patent Document 2, in the production of a hot-rolled steel sheet containing appropriate amounts of Ti and Mo, finish rolling is finished in the austenite region immediately above the Ar ₃ transformation point, and the winding temperature is 570 ° C. or more and 700 ° C. By making the following, a precipitate with a fine and thermal stability of less than 10 nm is dispersed in the parent phase, which is almost a ferrite single phase, and the tensile strength exceeds 800 MPa, and further exceeds 900 MPa, strength and ductility or elongation. A method for producing a hot-rolled steel sheet having excellent flangeability is disclosed. According to the above method, it is said that it is possible to provide hot rolled steel sheets of various strength levels with industrial stability by controlling the precipitation strengthening amount with high accuracy. It has been commercialized.
JP 2003-89848 A JP 2002-322539 A

  However, even with the above technique, it is not easy to further increase the strength while ensuring ductility. In particular, if the alloying elements contained in the steel are increased for the purpose of increasing the amount of precipitates, it will be strongly affected by the soaring of alloying elements found in recent years, so it is difficult to produce hot-rolled steel sheets industrially and stably. Become.

On the other hand, for example, in Patent Document 3 and Patent Document 4, in a duplex steel sheet in which a tempered martensite or tempered bainite phase is a main phase and a residual austenite phase is dispersed as a second phase, both high strength and stretch flangeability are achieved. Techniques to be disclosed are disclosed.
However, in these duplex steel sheets, excellent uniform elongation due to the TRIP effect and stretch flangeability of several tens of percent or more can be stably obtained, but the tensile strength is at most about 800 MPa.
JP 2003-73773 A JP 2005-336526 A

  The present invention has been made in view of the above situation, and can be found in conventional ferrite precipitation strengthened hot-rolled steel sheets without containing a large amount of expensive alloy elements by an appropriate combination of structure strengthening and precipitation strengthening. An object of the present invention is to provide a high-strength hot-rolled steel sheet that solves the problems and has both a strength of 1180 MPa or more and excellent ductility.

The inventors of the present invention have other strengthening mechanisms (precipitation strengthening) with regard to a method for appropriately controlling the structure form essential for both strength and workability (particularly ductility and stretch flangeability) for steel types having a tensile strength of 1180 MPa or more. In addition, the inventors studied diligently in consideration of the balance with crystal grain refinement and the like. As a result, when trying to increase the strength by precipitating fine precipitates in the structure with ferrite as the matrix, high carbon and high strength are required to increase the precipitation strengthening ability of carbides due to restrictions on the hot rolled steel sheet manufacturing process. It was found that sufficient strength could not be obtained due to limitations in alloying.
It was also found that it is difficult to achieve a tensile strength of 1180 MPa or more with the bainite single phase.

  For this reason, the present inventors have studied a multiphase structure in which the ferrite phase on which fine precipitates are precipitated is the main phase and the bainite phase is very effective in strengthening the structure, and the second phase is the second phase. As a result, ferrite reinforced with MX precipitates having an outer diameter of less than 10 nm is the main phase, and the bainite phase is present as the second phase in a volume fraction of 30% to less than 45%, and then subjected to tempering treatment. It has been found that the strength and ductility of the steel sheet can be adjusted to a desired range by controlling the phase to an appropriate strength.

Moreover, the method of manufacturing the hot rolled steel plate which has the above structures stably industrially was also examined.
In the conventional method for the tissue of the hot-rolled steel sheet and ferrite + bainite dual phase structure, for example, after completion of the finish rolling at austenite temperature region (A ₃ transformation point or higher), Ms point at a specific cooling rate (Martens A process of cooling to the Bs point (bainite transformation start temperature) and below and winding up is performed. Furthermore, Japanese Patent Application Laid-Open No. 2003-171736 discloses a method of heating the bainite above the A ₁ transformation point in a continuous annealing or plating process. There is a concern that cementite precipitates and coarsens in the bainite phase, which is a two-phase, and that hardness decreases (strength decreases) due to a decrease in dislocation density.

  However, the inventors of the present invention, after finishing rolling in the austenite temperature range, the steel controlled to an appropriate composition range, quickly cooled to a two-phase region of an austenite phase and a ferrite phase, and after holding for a very short time, bainite Winding by cooling to the transformation temperature range, and further tempering for about 3 hours at a lower temperature range (550 to 650 ° C) than before, the phenomenon opposite to the conventional strength reduction, namely strength It was found that an increase effect and a ductility improvement effect can be obtained.

  As an example, for steels containing 0.05 mass% C-0.01 mass% Si-1.4 mass% Mn-0.10 mass% Ti-0.20 mass% Mo as the main component, the steel structure is changed to a ferrite single phase by changing the hot rolling conditions. Ferritic steel, bainite steel and martensitic steel hot-rolled steel sheets having a structure, a bainite single-phase structure and a martensite single-phase structure were prepared. FIG. 1 shows the results of investigating changes in mechanical properties before and after tempering when these were tempered at 620 ° C. for 1 hour.

As is clear from FIG. 1, under the above tempering conditions, there is almost no change in both TS (tensile strength) and EL (breaking elongation) before and after tempering in ferritic steel, whereas in bainite steel and martensitic steel. The effect of increasing the tensile strength TS by tempering is recognized, and the effect is particularly remarkable in bainitic steel. Further, in martensitic steel, there is no change in the EL value before and after tempering, whereas in bainite steel, the EL value is greatly improved by tempering. This phenomenon corresponds to a change in carbide morphology (M ₃ C → MC) in the bainite phase at a temperature of 620 ° C., and in the steel grade system where MC and M ₃ C precipitation compete, It shows that MC precipitation is finally stable under tempering conditions.
The same phenomenon has been confirmed for the steel having the composition of the present invention.

  The reason why the effect of improving the mechanical properties after tempering is confirmed when the structure of the hot-rolled steel sheet is a bainite single-phase structure is that fine carbides in bainite change from cementite to finer MX precipitates. . From the above results, although the ductility of the bainite single-phase structure does not reach that of the ferrite single-phase structure, the bainite single-phase structure is subjected to an appropriate tempering treatment and its mechanical properties are controlled to provide an excellent balance between strength and ductility. It can be seen that the obtained organization is obtained.

  Next, regarding the MX precipitates present in the ferrite phase and the tempered bainite phase, the size and precipitation state of the MX precipitates necessary for obtaining a hot-rolled steel sheet having an excellent strength-ductility balance, and the manufacturing method thereof Was investigated. As described in Patent Document 1 and Patent Document 2 presented above, in the cooling step after finish rolling, the MX precipitates in the ferrite phase are deposited by interfacial precipitation during the austenite → ferrite transformation. It is known that the diameter of less than 10 nm is effective in securing the strength-ductility balance of the ferrite phase.

On the other hand, the usual bainite phase is accompanied by the precipitation of cementite (M ₃ C precipitates including Fe ₃ C, part of Fe substituted with other metal elements). When tempering is performed under various conditions, M ₃ C changes to MC, and the effect of improving strength and ductility can be obtained. Here, as a result of TEM (transmission electron microscope) observation of the bainite single-phase structure after tempering in which the effect of improving strength and ductility was confirmed, the bainite single-phase structure after tempering was It was confirmed that there was a coherently precipitated plate-like precipitate having a certain orientation relationship with the outer diameter, which was less than 10 nm, the same as the outer diameter of the MX precipitate in the ferrite phase.

  Accordingly, a method for producing a hot-rolled steel sheet which has a multiphase structure including a tempered bainite phase and a ferrite phase, and both the ferrite phase and the tempered bainite phase contains MX precipitates having an outer diameter of less than 10 nm was studied. As a result, after finishing rolling in the austenite temperature range, the steel controlled to an appropriate composition range is quickly cooled to the two-phase range of the austenite phase and the ferrite phase, and then cooled to the bainite transformation temperature range after holding for a very short time. Further, it was revealed that a desired multiphase structure can be obtained with good reproducibility by performing tempering treatment for about 3 hours in a lower temperature range (550 to 650 ° C.) than in the past. Further, it was confirmed that the main phase of the hot-rolled steel sheet obtained by the above method was a ferrite phase, and the volume fraction of the tempered bainite phase was 30% or more and less than 45%.

  In the present invention, the MX precipitate is a precipitate having a NaCl-type crystal structure composed of Ti, Nb, V, Mo, W, etc. as the metal element M, C or N as X, or both. The most effective precipitate for the precipitation strengthening mechanism is MC.

This invention is made | formed based on the said knowledge, The summary structure is as follows.
(1) By mass%, C: 0.10% to 0.16%, Si: 0.5% or less, Mn: 0.5% to 1.8%,
Al: 0.5% or less and N: 0.001% or more and 0.005% or less, and Ti: 0.14% or more and 0.2% or less and Nb: 0.25% or more and 0.40% or less, and the balance Fe and inevitable impurities MX with a volume fraction of 30% or more and less than 45% tempered bainite phase and a ferrite phase, both of which have an outer diameter of less than 10 nm. A hot-rolled steel sheet containing precipitates (wherein M represents a metal element and X represents carbon or nitrogen).

  (2) In the hot-rolled steel sheet described in (1) above, at least one selected from V: 0.12% to 0.20%, Mo: 0.25% to 0.40% and W: 0.50% to 0.80% A hot-rolled steel sheet containing the above.

  Appropriate post-heat treatment (tempering) for hot-rolled steel sheets with a multiphase microstructure, in which the ferrite phase on which fine precipitates with excellent ductility are precipitated is the main phase and the bainite phase is the second phase, which is extremely effective in strengthening the structure. By applying the treatment, a hot-rolled steel sheet having a very good strength-ductility balance having a tensile strength of 1180 MPa or more and a breaking elongation of 15% or more while suppressing the amount of alloying elements can be obtained.

  Hereinafter, the reason for limiting the steel composition of the present invention and the reason for limiting the structure fraction will be described. Moreover, about the tempering conditions for obtaining the steel structure of this invention, since it changes with steel composition, hot-rolling conditions, etc., it cannot specify uniquely, However, General suitable conditions are disclosed. Hereinafter, with respect to the steel composition, mass% is simply referred to as “%”.

(Reason for limiting steel composition)
C: 0.10% to 0.16%
C is an essential element for realizing MX precipitation strengthening. In particular, in order to deposit the entire amount of MX precipitate forming elements contained at the same time, it is necessary to contain an equivalent amount or more of C in terms of the atomic ratio with the sum of these MX precipitate forming elements. In the present invention in which ferrite is the main phase and the bainite phase is the second phase, a C content capable of ensuring a wide austenite + ferrite two-phase region is required. However, excessive content increases the amount of undissolved MX precipitates in the slab reheating temperature range and promotes coarsening of the MX precipitates in the two-phase region, so the upper limit of C content is 0.16% . On the other hand, the lower limit of the C content was set to 0.10% from the viewpoint of securing the precipitation of MX in the ferrite region.

Si: 0.5% or less, Al: 0.5% or less
Si and Al are effective as deoxidizing elements during steelmaking, and are preferably contained because they dissolve in ferrite and improve steel strength. However, if the content of Si and Al exceeds 0.5% respectively, the transformation temperature rises and it becomes difficult to ensure the two-phase region temperature after rolling, so the upper limit of the content of Si and Al is 0.5% And

Mn: 0.5% to 1.8%
Mn is effective in improving the strength of the matrix by solid solution strengthening, and is an element indispensable for controlling the transformation point of steel as an austenite stabilizing element. In the present invention, Mn is an extremely important element for securing a two-phase region of an austenite phase and a ferrite phase. However, content 1.8 percent leads to reduction of A ₃ transformation point, MX phase interfacial precipitation during transformation to ferrite from austenite is suppressed, since the precipitation strengthening of the ferrite phase becomes insufficient, the Mn content The upper limit was 1.8%.

N: 0.001% to 0.005%
N is also an element effective in achieving MX precipitation strengthening, but generally has a strong tendency to form stable MN precipitates in the austenite region, and these stable MN precipitates are austenite grains in the slab heating temperature region. It plays an important role in suppressing coarsening. These effects are sufficiently obtained if N is contained in an amount of 0.001% or more as long as it contains MX forming elements such as Ti and Nb. On the other hand, excessive N content fixes a large amount of MX-forming elements as nitrides, and relatively reduces the amount of ultrafine MX precipitates utilized in the present invention. For this reason, the upper limit of the N content is suppressed to about 0.005% or less that can be achieved by a general steelmaking process.

Ti: 0.14% to 0.2% and Nb: 0.25% to 0.40%
Ti is an essential element for achieving precipitation strengthening by the fine precipitates of MX, and forms very stable TiN, particularly in the austenite region, and works effectively in suppressing coarsening of austenite grains. It is essential that the lower limit of the Ti content be equal to or greater than N in terms of atomic ratio depending on the N content. For this reason, about 0.02% is essential for the steel of the present invention in which the upper limit of the N content is 0.005%. Furthermore, in the case of effectively depositing MX mainly composed of carbide after hot rolling, Ti and Nb are overwhelmingly effective elements compared with other MX forming elements, and particularly Ti is cost effective. Is the most dominant element. However, when an equivalent amount of Ti is contained in an atomic ratio with respect to C, coarse TiC is formed in the slab manufacturing stage due to its extremely strong carbide forming ability, and it becomes difficult to form a solution during slab reheating. . In particular, in a steel type having a high C content using a transformation structure such as the steel type of the present invention, it is desirable to contain Ti at an atomic ratio that fixes about 30% of the C content. For these reasons, the range in which the effect of promoting the precipitation of fine MX can be obtained is 0.14% or more and 0.20% or less. Further, Nb can be contained instead of Ti or together with Ti. In this case, the Nb content is set to 0.25% or more and 0.40% or less for the same reason as Ti. When both Ti and Nb are contained, the total content of Ti and Nb is preferably Ti + Nb: 0.2% or more and 0.3% or less.

Furthermore, in addition to the above basic components, the following elements can be contained as required.
V: 0.12% or more and 0.20% or less, Mo: 0.25% or more and 0.40% or less and W: 0.50% or more and 0.80% or less
In addition to Ti or Nb, which is an essential element of the steel of the present invention, the formation of MX precipitates is promoted, and it is also effective for strengthening precipitation due to MX fine precipitates. . These steels, which do not contain Ti or Nb, have a small ability to form MX precipitates by themselves, but are extremely effective as elements for increasing the total amount of MX fine precipitates. These elements are also desirably contained at an atomic ratio necessary to fix 30% of the C content, and each content is, for example, 0.12% or more and 0.20% or less for V, and 0.25% for Mo. If it is more than 0.40% and W, it is 0.50% or more and 0.80% or less.

(Reason for limitation of tempered bainite structure fraction)
Next, the reason why the volume fraction of the tempered bainite phase with respect to ferrite is limited to 30 to 45% will be described. 0.15 mass% C-0.3Si mass% -1.5Mn mass% -0.12Ti mass% -0.18V mass% steel, the intermediate holding temperature after hot rolling changed between 675 ℃ and 780 ℃ The final coiling temperature: 880 ° C, tempering conditions: 600 ° C x 2h, a dual-phase hot-rolled steel sheet was produced, and a specimen in the L direction (rolling direction) was prepared from this hot-rolled steel sheet to obtain mechanical properties TS and fracture. The elongation EL was measured. Regarding the volume fraction of the bainite phase after tempering, a polished sample of L section (section obtained by cutting the sample parallel to the rolling direction) was etched with 3% nitric acid-methanol solution, and 3000 times SEM (scanning electron microscope) structure The average value of the area ratio of the tempered bainite phase was determined for the five fields of view of the photograph and was converted from this area ratio. As a result, as shown in FIG. 2, the tempered bainite phase in the hot-rolled steel sheet satisfying TS ≧ 1180 MPa and EL ≧ 15% as TS increases with increasing volume fraction of the tempered bainite phase. The volume fraction of the phase was found to be 30-45%. Therefore, the tempered bainite structure fraction of the hot-rolled steel sheet in the present invention is limited to 30 to 45% in volume fraction.

(Reason for limitation of MX precipitates in both tempered bainite phase and ferrite phase)
In the present invention, it is important to precipitate both MX precipitates having an outer diameter of less than 10 nm in both the ferrite phase and the tempered bainite phase. As described above, the main phase is a ferrite phase, and for the strengthening of the ferrite phase, it is preferable to finely disperse MX precipitates mainly composed of carbides. This state can be realized by the phase interface precipitation phenomenon accompanying the γ → α transformation in the cooling step after finishing rolling. On the other hand, the bainite phase of the second phase contains M ₃ C as it is, and sufficient strength and elongation cannot be achieved as it is. However, by appropriately tempering the bainite phase at a low temperature, all or part of the precipitated M ₃ C changes to finer MX precipitates, and the strength and elongation are improved for the first time. Further, regarding the hot rolled steel sheet of FIG. 2 satisfying TS ≧ 1180 MPa and EL ≧ 15%, the precipitates contained in the tempered bainite phase were observed by TEM, and MX precipitates having an outer diameter of less than 10 nm were confirmed. For this reason, in the present invention, it is important that MX precipitates having an outer diameter of less than 10 nm exist in both the main phase ferrite and the second phase bainite constituting the steel.

  Note that the MX precipitate utilized in the present invention has a NaCl-type crystal structure and generally precipitates in a Baker-Nutting orientation relationship with respect to the ferrite or bainite phase. Thus, at the initial stage of deposition, it is not spherical but has a plate-like or disk-like form with a maximum thickness of about 2 nm. For this reason, in order to specify the size of the precipitate, the disc diameter or the plate-like diagonal length is defined as the outer diameter of the precipitate.

  Examples of the production method (tempering conditions) for obtaining the structure of the steel of the present invention include the following methods. Steel having the above-mentioned composition is melted into steel slabs (including ingots, slabs, and thin slabs). After hot rolling is completed in the austenite single-phase region, it is rapidly cooled to the ferrite + austenite two-phase region. Then, after holding for a very short time, cool to the bainite transformation temperature range. Then, after reheating and holding in the tempering temperature range for adjusting the strength of the bainite structure, it is made into a hot-rolled steel sheet by air cooling. Moreover, after ending the hot rolling in the austenite + ferrite two-phase region and quickly cooling to the bainite transformation temperature region, a tempering treatment may be performed.

(Slab heating conditions)
When a steel slab such as a steel slab is subjected to hot rolling after being reheated to a predetermined temperature (slab heating temperature) after cooling, or immediately before the steel slab becomes lower than the predetermined temperature, the steel slab is subjected to hot rolling immediately. Any of them may be applied. Further, before the steel slab is completely cooled, it may be hot-rolled by heating to the predetermined temperature for a short time. The slab heating temperature is preferably about 1100 to 1200 ° C. in order to re-dissolve the carbide (or to prevent precipitation growth). In order not to leave undissolved Ti-based carbides, it is desirable that the holding time at this temperature be 30 minutes or longer.

(Finish rolling finish temperature)
Optimization of the finish rolling end temperature is important for securing elongation and stretch flangeability and reducing the rolling load. In the present invention, it is desirable to set the temperature to 700 ° C to 900 ° C. If it is less than 700 ° C, it becomes substantially a ferrite single phase, and the structure strengthening by the second phase cannot be utilized. Further, during hot rolling, MX tends to coarsen due to strain-induced precipitation of MX, and the precipitation strengthening of ferrite grains becomes insufficient. On the other hand, as the rolling temperature rises, the rolling load decreases and the workability is stabilized. Moreover, by finishing the hot rolling in the austenite single phase region or the austenite + ferrite two-phase region, the fine dispersion of the MX precipitate is realized by the phase interface precipitation during the transformation from the austenite phase to the ferrite phase. Furthermore, after finish rolling, the untransformed austenite region undergoes bainite transformation by rapidly cooling to the bainite transformation temperature region. However, since it is not easy to secure a high finish rolling temperature in a general production line, it is desirable that the upper limit temperature is set to 900 ° C., which is low even in an austenite single phase region.

(Cooling conditions after finishing rolling)
In the present invention, it is essential to finely disperse MX precipitates in the main phase ferrite structure and to secure a bainite phase for obtaining the second phase of the present invention. Therefore, when rolling is completed in the austenite single-phase region, air-cooled to the austenite + ferrite two-phase region, held for a very short time, then cooled to the Ms point or more and the Bs point or less at a sufficient cooling rate, and the untransformed austenite region Must be the bainite phase. On the other hand, when rolling is completed in the austenite + ferrite two-phase region, it is important to quickly cool to a bainite phase. In consideration of actual manufacturability, it is desirable to obtain a desired two-phase structure by changing the cooling rate by controlling the amount of water after the hot finish rolling is completed in the austenite single phase region.

(Tempering conditions)
In order to realize a double phase structure of main phase ferrite + tempered bainite phase, it is necessary to temper the steel sheet subcooled below the Bs point after finishing rolling under appropriate conditions. It is desirable to set it within 650 ℃ × 3h. When the tempering temperature is less than 600 ° C., the precipitation of cementite takes priority in the bainite structure, and it is difficult to form an essential requirement for exhibiting the effects of the present invention, that is, the formation of MX precipitates to be consistently precipitated in the tempered bainite phase. On the other hand, when the tempering temperature exceeds 650 ° C., fine MX precipitates in the ferrite phase become coarse and cause a decrease in strength. Further, although the tempering time depends on the tempering temperature, even when the tempering temperature is the lower limit temperature of 600 ° C., maintaining the tempering temperature for 3 hours or more causes a significant reduction in production efficiency. The tempering process may be performed by either an online winding process or batch annealing after winding.

  The hot-rolled steel sheet of the present invention includes those subjected to surface treatment or surface coating treatment. In particular, the hot-rolled steel sheet of the present invention is suitable as a base steel sheet for a surface-treated steel sheet on which a hot dip galvanized coating film is formed. That is, since the hot-rolled steel sheet of the present invention has good workability, good workability can be maintained even if a hot-dip galvanized coating film is formed. Here, the hot dip galvanized plating is hot dip plating mainly composed of zinc and zinc (containing approximately 80% by mass or more of zinc), and also contains alloy elements such as Al and Cr in addition to zinc. Including. Moreover, the thing with which the hot dip zinc system plating was performed and the thing which performed the alloying process after plating are included.

  A steel slab having the composition shown in Table 1 is heated to 1100 to 1250 ° C., subjected to rolling to a finish rolling finish temperature of 800 ° C. and a plate thickness of 3.5 mm by a hot rolling process, and then air-cooled to 730 ° C. After holding for 5 s, it was rapidly cooled to 380 ° C. Thereafter, the steel sheet was inserted into a heating furnace at 600 ° C. to 650 ° C., held for 1 to 3 hours, and then taken out from the heating furnace and air-cooled.

  The obtained steel sheet was pickled, and the L cross-sectional structure was observed with an optical microscope and a precipitate was observed with a TEM. For TEM observation, a sample was taken from a position of 1/4 of the thickness of the steel sheet, a thin film sample was prepared by electrolytic polishing, and the precipitation of MX precipitates in ferrite and tempered bainite was observed. The volume fraction of the bainite phase was converted from the area ratio of the bainite phase obtained from the above SEM structure photograph.

  Regarding the mechanical properties of the steel sheet, a tensile test piece in the rolling direction was prepared in accordance with the provisions of JIS Z 2201 plate test piece No. 13A, and the tensile strength and elongation at break were measured by a JIS Z 2241 tensile test method. The presence or absence of fine MX precipitates (MX precipitates having an outer diameter of less than 10 nm) in the ferrite phase and the tempered bainite phase was confirmed by TEM observation. Specifically, thin film samples were prepared from each sample by electropolishing, and the presence or absence of fine MX precipitates was confirmed by observation from the [001] zone axis incidence of the ferrite phase and tempered bainite phase.

  Table 2 shows the results. Regarding the observation results of the fine MX precipitates in Table 2, the case where 80% or more of the precipitates present in the observed field of view are less than 10 nm in outer diameter was indicated as “◯”.

  As shown in Table 2, the steel of the present invention in which the tempered bainite phase fraction and the outer diameter of the MX precipitate are within the proper ranges have a tensile strength of 1180 MPa or more and a breaking elongation of 15% or more, and an excellent balance between strength and ductility. Have good mechanical properties.

  In particular, from the viewpoint of weight reduction and production cost reduction, while it is desired to stably manufacture a high-strength and high-formability steel sheet, the precipitation-strengthened double-phase hot-rolled steel sheet provided by the present invention is particularly useful in the automotive field and the like. It can be used as a material for members that require formability as well as strength.

(A) And (b) is a figure which shows the change of the tensile strength and elongation of a ferritic steel, a bainite steel, and a martensitic steel before and after a tempering process, respectively. FIG. 4 is a diagram showing the influence of the volume fraction of the tempered bainite phase relative to the ferrite phase, which is the main phase, on the tensile strength TS and elongation EL of steel. It is a figure which shows the thermomechanical process of this invention steel.

Claims (2)

% By mass
C: 0.10% to 0.16%,
Si: 0.5% or less,
Mn: 0.5% to 1.8%,
Al: 0.5% or less and
N: 0.001% or more and 0.005% or less,
Tempering: Ti: 0.14% or more and 0.2% or less and Nb: 0.25% or more and 0.40% or less, with the balance consisting of Fe and inevitable impurities, with a volume fraction of 30% or more and less than 45% MX precipitates that have a multiphase structure including a bainite phase and a ferrite phase, and both the ferrite phase and the tempered bainite phase have an outer diameter of less than 10 nm (where M is a metal element, and X is carbon or nitrogen) A hot-rolled steel sheet comprising: The hot rolled steel sheet according to claim 1, further comprising:
V: 0.12% to 0.20%,
Mo: 0.25% to 0.40% and
W: A hot-rolled steel sheet containing at least one selected from 0.50% to 0.80%.

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