JP2013040380A - High-strength hot-rolled steel sheet having both formability and fatigue characteristic in base material and weld heat affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel sheet having excellent formability and improved fatigue characteristics not only in the base material but also in an HAZ (heat affected zone).SOLUTION: The high-strength hot-rolled steel sheet includes a component composition comprising, in mass%, C: 0.05 to 0.20%, Si: not more than 2.0%, Mn: 1.0 to 2.5%, Al: 0.001 to 0.10%, and V: 0.0005 to 0.10%, and further comprising Ti: 0.02 to 0.20% and/or Nb: 0.02 to 0.20% in a manner of satisfying the equation: C-12×(V/51+Ti/48+Nb/93)>0.03, with the balance being iron and inevitable impurities; and has a structure comprising, in an area ratio relative to the entire structure, ferrite: 50 to 90%, bainite: 10 to 50%, and the sum of martensite and retained austenite: less than 10%, wherein the mean particle diameter of a precipitated carbide present in the ferrite is smaller than 6 nm and the total content of V, Ti and Nb constituting the precipitated carbide is 0.02% or more.

Description

  The present invention relates to a high-strength hot-rolled steel sheet used for parts that require strength, workability, and fatigue characteristics, such as automobile undercarriages and frame parts.

  In recent years, thin steel plates used for automobile parts have been increased in strength in order to realize collision safety and fuel efficiency improvement. The strength of automobile undercarriage parts and frame parts has been increased, but in order to reduce the weight of parts, it is necessary to improve the fatigue strength as well as the static strength. Moreover, since it is processed into a complicated shape, compatibility with workability (ductility, stretch flangeability) is required.

  In order to improve workability, it is effective to use DP steel consisting of two types of structures with a large strength ratio. Furthermore, as a method for improving the fatigue properties of DP steel, the ferrite part, which has low strength and is likely to cause stress concentration, is strengthened. It is known to be effective. For example, Patent Document 1 describes that in a DP steel composed of a main phase ferrite precipitated and strengthened with a carbide of Ti or Nb and a hard second phase, the average ferrite grain size of the surface layer portion up to 20 μm is set to 5 μm or less. Patent Document 2 describes that, in DP steel in which the second phase is martensite, acicular ferrite, and retained austenite, precipitation-strengthening of pro-eutectoid ferrite improves strength-workability-fatigue properties. ing.

  The hot-rolled steel sheets described in Patent Documents 1 and 2 have a retention and residence time of around 700 to 800 ° C., a dispersion of Ti and Nb carbides in the ferrite, and precipitation strengthening of the main phase ferrite. ing. In this hot-rolled steel sheet, it is considered that precipitates that are finely dispersed and precipitated by holding and staying in the above-mentioned temperature range for a short period of time become obstacles to repeated movement of dislocations and improve fatigue properties. . However, in the prior art, it cannot be said that a sufficient fatigue property improving effect is obtained.

  Accordingly, the present inventors have conducted extensive research and development on precipitation strengthening of ferrite in DP steel for the purpose of further improving the fatigue characteristics of DP steel. As a result, when ferrite is strengthened with precipitates such as Ti, Nb, and V in DP steel, the retention and residence time in the above temperature range is lengthened, and the precipitate is moderately coarsened, resulting in high fatigue. It was found that a characteristic improvement effect can be obtained. Based on this knowledge, the following high-strength hot-rolled steel sheet was completed, and a patent application was already filed (see Patent Document 3).

The high-strength cold-rolled steel sheet proposed by the present inventors in Patent Document 3 (hereinafter referred to as “prior invention steel sheet”)
In mass%, C: more than 0.01%, 0.30% or less, Si: 0.1% or more, 2.0% or less, Mn: 0.1% or more, 2.5% or less, V: 0.01% or more, 0.15% or less, Nb: 0.02% or more, 0.30% or less, Ti: 0.01% or more, 0.15% or less, or one or more of the following conditional expressions (1) is included so that the balance is Fe and inevitable impurities, the ferrite fraction is 50% or more and 95% or less, and the hard second phase fraction consisting of martensite + retained austenite is 5% or more and 50%. The average grain size r of the precipitate having the following structure and formed in ferrite is 6 nm or more, and the average grain size r and the precipitate fraction f represented by the following formula (2) are expressed by the following conditional formula ( It is a high-strength hot-rolled steel sheet excellent in strength-elongation balance and fatigue characteristics satisfying 3).
C-12 × (V / 51 + Nb / 93 + Ti / 48) ≧ 0.01 (1)
f = (2.08Ti + 1.69V + 1.14Nb) / 100 (2)
r / f ≦ 13000 (3)
Here, the element symbol in the above formulas (1) and (2) means mass% of the element.

  The prior invention steel plate is excellent in workability and fatigue characteristics, but the processed automobile parts are often used by welding to the vehicle body or other members, etc. It is known that the weld heat affected zone (hereinafter also referred to as “HAZ”) has a lower fatigue strength than the base metal. For this reason, when using automobile parts by welding, it is not sufficient to simply improve the fatigue characteristics of the base metal, and it is important to improve the fatigue characteristics of the HAZ. The prior invention steel sheet exhibits an excellent effect in improving the fatigue characteristics of the base material, but there is room for improvement in the HAZ fatigue characteristics.

  On the other hand, as a method of improving the fatigue properties of HAZ of hot-rolled steel sheet, by preheating the weld line to 350 to 500 ° C. during welding, the residual austenite is included in HAZ. A welding method for improving fatigue characteristics is disclosed (see Patent Document 4). However, this method requires a preheating work before welding, and has a problem inferior in workability of welding work.

JP-A-9-137249 JP-A-11-189842 JP 2007-321201 A Japanese Patent Laid-Open No. 9-66763

  The present invention has been made paying attention to the above circumstances, and its purpose is to provide a steel-strength hot-rolled steel sheet that is excellent in formability (workability) and can improve fatigue characteristics not only in the base material but also in HAZ. There is to do.

  In this invention steel plate, it replaced with DP steel and decided to employ | adopt ferrite + bainite steel which is excellent with the balance of intensity | strength (TS) -elongation (EL) -stretch flangeability ((lambda)). And like the said prior invention steel plate, this prior invention steel plate improves the fatigue characteristics of the base material by strengthening the base material structure by making a predetermined amount of precipitated carbides such as V, Ti and Nb present in the ferrite. Contrary to this, by refining the precipitated carbides, V and C derived from V carbides (VC) are dissolved in the matrix at the time of heating by welding, thereby suppressing the austenite grains from being refined, It enhances hardenability, suppresses the formation of ferrite during cooling after welding and promotes the formation of bainite, and at the same time increases the amount of solute C of the bainite, thereby improving the strength of the bainite itself. It is also possible to improve the fatigue strength.

% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.20%,
Si: 2.0% or less,
Mn: 1.0 to 2.5%
Al: 0.001 to 0.10%,
V: 0.0005 to 0.10%
Including,
Ti: 0.02 to 0.20%, and / or
Nb: 0.02 to 0.20%
To satisfy the following formula 1,
The balance has a component composition consisting of iron and inevitable impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Ferrite: 50-90%
Bainite: 10-50%
Martensite + retained austenite: having a structure of less than 10%,
The average particle size of precipitated carbides present in the ferrite is less than 6 nm,
High-strength hot rolling that combines formability and fatigue properties of the base metal and weld heat affected zone, characterized in that the total content of V, Ti and Nb constituting the precipitated carbide is 0.02% or more. It is a steel plate.
Formula 1 C-12 × (V / 51 + Ti / 48 + Nb / 93)> 0.03
However, the element symbol in a formula means the mass% of the said element.

The invention described in claim 2
The high-strength hot-rolled steel sheet having both formability and fatigue characteristics of the base metal and the weld heat-affected zone according to claim 1, wherein the average grain size of the bainite is more than 5 µm.

The invention according to claim 3
Ingredient composition further
Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: A high-strength hot-rolled steel sheet having both formability and fatigue characteristics of the base metal and the weld heat-affected zone according to claim 1 or 2, comprising one or more of 0.01 to 1.0%. It is.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet that is excellent in fatigue characteristics of both the base material and the HAZ while ensuring formability.

It is a figure explaining the process of an Example.

  As described above, the present inventors, based on ferrite + bainite steel strengthened with ferrite by precipitated carbide, are measures for improving HAZ fatigue properties while ensuring the formability and fatigue properties of the base material. Has been studied.

  Here, HAZ is formed in the vicinity of the weld metal, but the form of the structure is classified into three regions of a coarse grain region, a fine grain region, a two-phase region, or a tempering region in order from the side close to the weld metal. The And in conventional steel, it is known that the characteristics of each region of the HAZ generally exhibit the following behavior. That is, in the coarse grain region, austenite grains become coarse during heating by welding, so that they become martensite or bainite and generally have high strength during cooling after welding. On the other hand, in the fine-grained region, austenite grains become finer when heated by welding, so ferrite and upper bainite are likely to be formed during cooling after welding, and the strength decreases and becomes the starting point of fatigue failure . In the two-phase region or tempering region, the strength is reduced by tempering and the fatigue strength is also reduced.

  Therefore, the present inventors first considered dispersing fine precipitated carbides in ferrite as a first measure for improving the fatigue characteristics of HAZ. Thereby, the ferrite is strengthened in the two-phase region or the tempering region, and works in the direction of improving the fatigue characteristics. However, in the coarse and fine grained regions, the austenite grains are refined by the pinning action of the precipitated carbide, the formation of ferrite and upper bainite is promoted, the amount of martensite is insufficient, and the precipitated carbide fixes carbon. Therefore, since the amount of solute C in martensite decreases, it works in the direction of deteriorating the fatigue characteristics.

  The above-described prior art (Ti + Nb) -added steel will be described more specifically as an example. In (Ti + Nb) -added steel, the austenite grains are coarse during hot rolling and the transformation start point is on the long time side, but since the cooling rate during hot rolling is small, ferrite transformation can be promoted, and ferrite + bainite is formed. Is possible. However, at the time of heating by welding, the austenite grains become fine due to the pinning action of (Ti, Nb) C and the Ti and Nb fix C in the regions corresponding to the coarse and fine regions of HAZ. Therefore, the amount of dissolved C in the austenite decreases, and the transformation start point shifts to the short time side. Therefore, when cooling after welding, ferrite transformation and upper bainite transformation are likely to occur even if the cooling rate is large. . Further, even if martensite is formed by cutting the bainite nose, the fatigue properties cannot be ensured because the martensite strength proportional to the amount of dissolved C is lowered.

  Therefore, it was determined that the fatigue properties of HAZ cannot be reliably and sufficiently improved by simply dispersing fine precipitated carbides in ferrite.

  Therefore, as a second measure for reliably and sufficiently improving the fatigue characteristics of HAZ, the present inventors partially dissolved V carbide (VC) having a low melting point during precipitation heating. Thus, it has been considered that both the precipitation strengthening of the base material and the hardenability of the coarse and fine grain regions of the HAZ can be achieved.

  More specifically, with respect to the (Ti + Nb) -added steel, by replacing a part of (Ti + Nb) with V, while maintaining the base metal structure as it is while following the structure formation behavior in hot rolling, the HAZ It was thought that the fatigue characteristics of can be improved using the following mechanism.

  That is, in the regions corresponding to the coarse and fine grain regions of HAZ, a part of VC in [Ti, Nb, V] C, which is a precipitated carbide, is partially dissolved during heating by welding, and the austenite grains are refined. The action is reduced. Also, the hardenability is enhanced by the solid solution of V and C in austenite, and the transformation start point shifts to the long time side, so the formation of ferrite and upper bainite during cooling after welding is suppressed, and the formation of martensite. The amount is secured. Furthermore, the strength of martensite itself is improved by increasing the amount of dissolved C. As described above, the fatigue characteristics of the coarse and fine grain regions of the HAZ are improved by improving the strength of the martensite itself and ensuring the amount of formation.

  In order to realize reliable and sufficient improvement of HAZ fatigue characteristics by the above mechanism, it is necessary to further promote the solid solution of VC in the precipitated carbide, and for this purpose, the precipitated carbide is smaller than a predetermined size. It is necessary to make it finer.

  Based on the above thinking, further studies such as confirmation experiments were carried out, and the present invention was completed.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on ferrite + bainite steel. In particular, the average grain size of precipitated carbides present in ferrite was limited to 6 nm or more in the steel sheet of the prior invention. On the other hand, the present invention steel plate is different in that it is limited to less than 6 nm.

<Ferrite: 50 to 90%, Bainite: 10 to 50%, Martensite + retained austenite: less than 10%>
If the ferrite is less than 50% or the bainite exceeds 50%, the elongation EL cannot be secured by connecting the bainite, while the ferrite exceeds 90% or the bainite is less than 5%. The tensile strength TS and stretch flangeability λ cannot be secured. Preferably, ferrite: 60 to 80%, bainite: 20 to 40%.
As a structure other than the main phase ferrite and bainite, martensite + residual austenite (MA) is preferably less than 10%. This is because the balance of strength-elongation-stretch flangeability decreases due to the presence of a harder structure.

<Average particle size of precipitated carbides present in ferrite: less than 6 nm>
This is because, by making the precipitated carbides finer, the solid solution of VC in the precipitated carbides is promoted, thereby realizing a sure and sufficient improvement of the HAZ fatigue characteristics by the above mechanism. Preferably, it is 5 nm or less.
In the steel sheet of the prior invention, this value was defined to be 6 nm or more to improve the fatigue characteristics of the base material.In the steel sheet of the present invention, the degree of improvement in the fatigue strength of the base material was sacrificed, By improving the fatigue properties of HAZ, the fatigue strength of the base material and HAZ can both be improved in a well-balanced manner.

<Total content of Ti, Nb and V constituting the precipitated carbide: 0.02% or more>
It defines the total amount of carbide alloy elements that contribute to precipitation strengthening. The degree of precipitation strengthening is said to be proportional to f / r (where f is the fraction of precipitated carbide and r is the grain size of precipitated carbide), so fatigue can be increased by increasing this parameter corresponding to the fraction of precipitated carbide f. Strength is improved. Preferably, it is 0.03% or more, more preferably 0.05% or more.

<Average particle size of bainite: more than 5 μm>
It is desirable to coarsen the average grain size of bainite to more than 5 μm, so that the balance of strength-elongation-stretch flangeability of the base material is sacrificed slightly, but HAZ is a bainite region in which carbides are not precipitated. By increasing the size, the austenite grains are coarsened during heating during welding and the hardenability is enhanced, thereby suppressing the formation of ferrite and upper bainite and improving the fatigue characteristics. More preferably, it is 8 μm or more.

[Measurement method of area ratio of each phase, average particle size of precipitated carbide existing in ferrite, total content of Ti, Nb and V constituting precipitated carbide, and average particle size of bainite]
Here, each measuring method of the area ratio of each phase, the average particle diameter of the precipitated carbide existing in the ferrite, the total content of Ti, Nb and V constituting the precipitated carbide, and the average particle diameter of bainite will be described. .

  Regarding the area ratio of each phase of the microstructure in the steel sheet, each test steel sheet was subjected to nital corrosion, photographed with five fields of view with a scanning electron microscope (SEM; magnification 1000 times), ferrite, bainite, pearlite, and martensite + residual Each ratio of austenite was determined by point calculation.

  For the average particle size of the precipitated carbides present in the ferrite, the precipitate was extracted by the extraction replica method, and the ferrite region was observed and photographed with a transmission electron microscope at a magnification of 150,000 × 1 μm × 1 μm. The area of each particle was obtained by image analysis of precipitates observed in the circle (diameter equivalent to 2 nm or more), the circle equivalent diameter was obtained from the area, the average value was calculated, and the average particle size was obtained.

  The total content of Ti, Nb and V constituting the precipitated carbide was determined by an extraction residue analysis method. After grinding the front and back surfaces of the steel sheet by 0.2 mm each, the sample was immersed in an AA (acetylacetone) -based electrolytic solution for electrolysis. After completion of electrolysis, the precipitate on the sample surface was ultrasonically peeled in methanol. The electrolytic solution and the ultrasonic peeling solution after electrolysis were suction filtered to collect the residue (precipitate). The filter used was a membrane filter (pore size: 0.1 μm) made of polycarbonate. The residue was ashed by heating with a filter, and the residue was heated again by adding an alkaline solvent to melt the residue. Next, an acid and water were added to dissolve the melt, and then water was added to make a constant volume, which was used as an analysis solution. After measuring the amounts of V, Nb, and Ti in the analysis solution using an ICP emission spectrometer, the measurement results and electrolytic mass (mass difference before and after electrolysis) are used to determine the Ti, Nb, and V constituting the precipitate in the sample. The total content was calculated.

  The average grain size of bainite is defined as one grain in the entire area of bainite surrounded by ferrite in the SEM photograph after nital corrosion, and the area of the area is measured by image analysis to obtain the equivalent circle diameter. Obtained by conversion.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.05 to 0.20%
C is a strengthening element, and as the amount of C increases, the area ratio of ferrite decreases. If it is less than 0.05%, the required strength cannot be obtained, and if it exceeds 0.20%, the area ratio of bainite becomes too large to secure the TS-EL-λ balance. Preferably, it is 0.06 to 0.15%.

Si: 2.0% or less Si, as a solid solution strengthening element of ferrite, contributes to the improvement of TS-EL-λ balance and also contributes to the improvement of fatigue characteristics. However, if it exceeds 2.0%, ferrite is strengthened too much and ELl is lowered. Preferably it is 0.5 to 1.7%.

Mn: 1.0 to 2.5%
Mn is added as a deoxidizing element and contributes to the improvement of the TS-EL-λ balance by solid solution strengthening. However, if it is less than 1.0%, deoxidation is insufficient and the TS-EL-λ balance is deteriorated, and if it exceeds 2.0%, the hardenability becomes too high and the area ratio of ferrite is lowered. Preferably it is 1.2 to 2.0%.

Al: 0.001 to 0.10%,
Al has an effect of improving the TS-EL balance by solid solution strengthening, and is added as necessary. However, if the value is less than the lower limit, the effect cannot be obtained, and if the value exceeds the upper limit, segregation at the grain boundary promotes grain boundary fracture and lowers the TS-EL-λ balance.

V: 0.0005 to 0.10%,
The fatigue characteristics of the base material are improved by forming fine carbides in ferrite together with the following Ti and Nb. Also, in HAZ, by solid solution during heating by welding, it suppresses the refinement of austenite grains, and by increasing the amount of solute C and solute V, the hardenability of HAZ is improved and the strength is increased. It also improves the fatigue properties of HAZ. Therefore, V is an essential additive element. Preferably it is 0.002 to 0.08%.

Ti: 0.02 to 0.20%, and / or
Nb: 0.02 to 0.20%
Ti and Nb, like V, improve the fatigue properties of the base material by forming fine carbides in the ferrite. However, if it is less than the lower limit, the effect of precipitation strengthening is insufficient, and even if the content exceeds the upper limit, the effect of improving the characteristics cannot be obtained. Ti and Nb are selective additive elements unlike V, and either one or both are added and used. Preferably each is 0.03 to 0.15%.

C-12 × (V / 51 + Ti / 48 + Nb / 93)> 0.03 Formula (1)
This equation means that the amount of free C that is not fixed by V, Nb, and Ti is left over 0.03%. Free C contributes to securing the required area ratio of bainite. The calculated value (referred to as component parameter) on the left side is preferably 0.05% or more. In addition, the element symbol in a formula means the mass% of the said element.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and unavoidable impurities. Examples of the unavoidable impurities include P, S, N, O, etc. The following acceptable components can be added within the range not impairing the above-mentioned action.

Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: One or more of 0.01 to 1.0% These elements have the effect of suppressing the formation of structures other than martensite and retained austenite by increasing the hardenability of steel, and are necessary. Added accordingly. However, if it is less than the lower limit, the effect cannot be obtained, and if it exceeds the upper limit, the ferrite becomes brittle and the TS-EL-λ balance is lowered. More preferably, it is 0.1 to 0.8% respectively.

  Next, the preferable manufacturing method for obtaining the said steel plate of this invention is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
The steel sheet of the present invention is manufactured by heating a steel material satisfying the above component composition, followed by hot rolling including finish rolling, rapid cooling after hot rolling, slow cooling after quenching stop, rapid cooling after slow cooling, and winding. To do.

[heating]
Heating before hot rolling is performed at a temperature not lower than the solution temperature of TiC and not higher than 1300 ° C. in order to solidify Ti having the highest solution temperature among the carbides. The solution temperature T (K) of TiC is expressed by the formula: T = -9260 / [log (C · Ti) -4.68] (where C and Ti in the formula are the contents of each element in the steel ( Mass%))). By this heating, an austenite single phase is obtained, and V, Ti, and Nb are dissolved in austenite. If the heating temperature is lower than the solution temperature of TiC, at least Ti cannot be dissolved in austenite, and coarse carbides are formed, so that the effect of improving fatigue characteristics cannot be obtained. On the other hand, temperatures exceeding 1300 ° C. are difficult to operate. The minimum with a preferable heating temperature is 1100 degreeC, and a more preferable minimum is 1000 degreeC.

[Hot rolling]
Hot rolling is performed so that the finish rolling temperature is 880 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation occurs at a high temperature and the precipitated carbides in the ferrite are coarsened, so that a certain finish rolling temperature is required. The finish rolling temperature is more preferably 900 ° C. or higher in order to coarsen austenite grains and increase the grain size of bainite. The upper limit of the finish rolling temperature is set to 1000 ° C. because it is difficult to secure the temperature.

[Rapid cooling after hot rolling]
After completion of the finish rolling, quenching is performed at a cooling rate (first quenching rate) of 20 ° C./s or more within 5 s, and quenching is stopped at a temperature of 580 ° C. or more and less than 670 ° C. (quenching stop temperature). This is because the precipitation carbide formed in the ferrite is refined by lowering the starting temperature of the ferrite transformation. When the cooling rate (first quenching rate) is less than 20 ° C./s, pearlite transformation is promoted, or when the quenching stop temperature is less than 580 ° C., pearlite transformation or bainite transformation is promoted, both of which are ferrites having a predetermined phase fraction. It becomes difficult to obtain bainite steel, and the balance of strength-elongation-stretch flangeability is lowered. On the other hand, when the quenching stop temperature is 670 ° C. or higher, the precipitated carbides in the ferrite are coarsened, and the HAZ fatigue characteristics cannot be ensured. The quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° C.

[Slow cooling after rapid cooling stop]
After the rapid cooling stop, it is slowly cooled for 5 to 20 seconds at a cooling rate (slow cooling rate) of 10 ° C./s or less by cooling or air cooling. Thus, the precipitated carbide in the ferrite is appropriately refined while sufficiently progressing the formation of the ferrite. When the cooling rate exceeds 10 ° C./s or the slow cooling time is less than 5 s, the amount of ferrite formed is insufficient. On the other hand, if the slow cooling time exceeds 20 s, the precipitated carbide does not become coarse, and the HAZ fatigue characteristics cannot be ensured.

[Rapid cooling after slow cooling, winding]
After the slow cooling, it is rapidly cooled again at a cooling rate of 20 ° C./s or higher (second rapid cooling rate), and wound up at a temperature exceeding 300 ° C. and not exceeding 450 ° C. This is because the balance of strength-elongation-stretch flangeability is improved by making the remainder a bainite-based structure. When the cooling rate (second quenching rate) is less than 20 ° C./s or the coiling temperature exceeds 450 ° C., pearlite is formed, whereas when it is less than 300 ° C., a lot of martensite and retained austenite are formed, and the strength-elongation -The balance of stretch flangeability decreases.

  In order to confirm the effect of the present invention, the influence on the mechanical properties of the base metal and the HAZ was investigated for high-strength hot-rolled steel sheets produced by varying the composition of components and hot-rolling conditions. Test steels having respective component compositions shown in Table 1 below were vacuum-melted to prepare test materials having a plate thickness of 30 mm. This test material was hot-rolled under the process shown in FIG. 1 and the conditions shown in Table 2 below to produce a hot-rolled steel sheet. More specifically, after holding at the heating temperature HT for 30 min, finish rolling was performed at the finish rolling temperature FDT, and the finished plate thickness was 3 mm. After the finish rolling, the steel sheet was cooled to the quenching stop temperature Tm at the first quenching rate RCR1 and allowed to cool for a cooling time (slow cooling time) tm. The cooling rate (slow cooling rate) MCR during cooling was 10 ° C./s or less. Then, it cooled to coiling temperature CT with 2nd rapid cooling speed | rate RCR2, hold | maintained for 30 minutes, and cooled in the furnace.

  With respect to the hot-rolled steel sheet (equivalent to the base material) thus obtained, the area ratio of each phase and the precipitation carbides present in the ferrite were measured by the measurement method described in the above [Mode for carrying out the invention]. The average particle size, the total content of Ti, Nb and V constituting the precipitated carbide, and the average particle size of bainite were measured.

  Further, after grinding the front and back surfaces of the hot-rolled steel plate corresponding to the base material into a plate sample having a thickness of 2 mm, a tensile test is performed in accordance with JISZ2241, and the tensile strength (TS) and elongation ( EL) was measured.

  In addition, after grinding the front and back surfaces of a hot-rolled steel plate corresponding to the above-mentioned base material into a plate sample having a thickness of 2 mm, a hole expansion test was performed in accordance with the iron standard JFST001, and the hole expansion rate was measured. Is the stretch flangeability (λ) of the base material.

  Further, the front and back surfaces of the hot-rolled steel sheet corresponding to the base material are ground by 0.2 mm each, and then an SN curve is created by a plane bending test described in JIS Z 2275 to determine the fatigue limit. It was defined as fatigue strength. Further, the fatigue limit ratio (FL / TS) of the base material was calculated from the fatigue strength (FL) of the base material and the tensile strength (TS).

  Next, in order to simulate the HAZ fine grain region, the hot-rolled steel sheet corresponding to the base material is heated to 950 ° C. at a temperature increase rate of 30 ° C./s with a heat treatment simulator, and then immediately cooled to room temperature at a cooling rate of 30 ° C./s. The material was cooled to a fine grain region simulation material.

  In addition, in order to simulate the tempering zone of HAZ, a hot-rolled steel sheet corresponding to the above base material is heated to 700 ° C. at a heating rate of 30 ° C./s with a heat treatment simulator, and then immediately cooled to room temperature at a cooling rate of 30 ° C./s. It was cooled and used as a simulated tempering zone.

And, for these fine grained region simulating material and tempered zone simulated material, were subjected to fatigue test in the same manner as the hot-rolled steel sheet corresponding the base material, since the fatigue limit does not exist, non with 2 × 10 ⁶ times The time strength at which breakage occurred was defined as fatigue strength.

  These measurement results are shown in Table 3.

  As shown in these tables, steel No. which is the steel sheet of the present invention. 1 to 6, 11, 15 to 17, 20, 23 to 26 are all manufactured using the steel grades satisfying the range of the composition of the present invention under the recommended hot rolling conditions. All the requirements were satisfied, and a high-strength hot-rolled steel sheet having both the base material and HAZ fatigue characteristics was obtained while ensuring the balance of strength-elongation-stretch flangeability of the base material.

  On the other hand, steel No. which is a comparative steel. 7 to 10 and 12 to 14 all use steel grades that do not satisfy the requirements of the component composition defined in the present invention, and are manufactured under the recommended hot rolling conditions, but the strength of the base material-stretch-stretch flangeability And at least one of the fatigue properties of the base material and the HAZ is inferior.

  Moreover, steel No. which is another comparative steel. Although 18, 19, 21, and 22 all used steel grades that satisfy the range of the component composition of the present invention, they were manufactured under conditions that deviated from the recommended hot rolling conditions, and as a result, satisfied the requirements of the structure of the present invention. Furthermore, the balance of strength-elongation-stretch flangeability of the base material and the fatigue characteristics of the base material and HAZ are also inferior.

Claims (3)

% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.20%,
Si: 2.0% or less,
Mn: 1.0 to 2.5%
Al: 0.001 to 0.10%,
V: 0.0005 to 0.10%
Including,
Ti: 0.02 to 0.20%, and / or
Nb: 0.02 to 0.20%
To satisfy the following formula 1,
The balance has a component composition consisting of iron and inevitable impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Ferrite: 50-90%
Bainite: 10-50%
Martensite + retained austenite: having a structure of less than 10%,
The average particle size of precipitated carbides present in the ferrite is less than 6 nm,
High-strength hot rolling that combines formability and fatigue properties of the base metal and weld heat affected zone, characterized in that the total content of V, Ti and Nb constituting the precipitated carbide is 0.02% or more. steel sheet.
Formula 1 C-12 × (V / 51 + Ti / 48 + Nb / 93)> 0.03
However, the element symbol in a formula means the mass% of the said element.   The high-strength hot-rolled steel sheet having both formability and fatigue characteristics of the base metal and the weld heat-affected zone according to claim 1, wherein the average grain size of the bainite is more than 5 µm. Ingredient composition further
Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: A high-strength hot-rolled steel sheet having both formability and fatigue characteristics of the base metal and the weld heat-affected zone according to claim 1 or 2, comprising one or more of 0.01 to 1.0%. .