TWI525200B - High strength hot rolled steel sheet and manufacturing method thereof - Google Patents

Description

High-strength hot-rolled steel sheet and manufacturing method thereof

The present invention relates to a high-strength hot-rolled steel sheet having high tensile strength (TS) of 980 MPa or more and having high strength, excellent material stability, and weldability, and a method for producing the same.

In recent years, based on the consideration of the Earth's environmental protection, the entire automotive industry has been improving the fuel efficiency of automobiles for the purpose of reducing CO ₂ emissions. Affected by this trend, even for large vehicles such as trucks, the demand for improved fuel efficiency is also rising. In order to improve fuel efficiency, it is effective to reduce the weight of the automobile body due to the use of a thinner member. In addition, in order to ensure the safety of the passengers during the impact, it is also required to strengthen the car body to improve the impact safety of the car body. Based on this point of view, as a material for automotive components, high-strength hot-rolled steel sheets that can achieve both weight reduction and safety have been adopted, and their usage is increasing year by year.

On the other hand, as the strength of the steel sheet increases, the material stability in the longitudinal direction of the steel sheet tends to deteriorate. If the material is unstable, when the steel plate is punched into a member of a predetermined shape, computer-aided design (CAE: Computer Assisted Engineering) prediction accuracy will The deterioration, the control of the amount of spring back becomes difficult, and the dimensional accuracy of the member deteriorates. Further, as the strength of the steel sheet increases, the weldability tends to deteriorate. In general, since the alloy concentration of the high-strength steel sheet is high, there are many problems in the weldability, and in particular, in the heat-affected zone (HAZ portion) at the time of welding, the adverse effect due to softening is relatively easy to appear. come out. In automobile manufacturing lines, there are many cases where the automobile components are joined to each other by spot welding or arc welding. In this case, if the heat-affected zone is softened as described above, various problems such as insufficient strength in the heat-affected zone are caused.

For the above reasons, even for tensile strength High-strength steel sheets of 980 MPa or more are also required to have good material stability and weldability.

In the past, various materials have been reviewed for the material stability or weldability of high-strength hot-rolled steel sheets. For example, the following techniques have been disclosed.

The technique disclosed in Patent Document 1 is to divide its composition into % by mass, including C: 0.01% to less than 0.08%, Si: 0.06 to 2.0%, Mn: 0.96 to 3.0%, P≦0.10%, S≦0.01%, Al: 0.005 to 0.3%, N≦0.01% Ti: 0.01~0.20%, and meets Mn-Si>0.9%, and 0.75≦(C%/12)/(Ti%/48+Nb%/93+Mo%/96+V%/51-N% /14-S%/32) The condition of ≦1.25, the rest is steel composed of Fe and unavoidable impurities. When hot rolling is performed, the refining temperature is set at 900 °C or higher, and 400 to 600. °C temperature The coiling was carried out to produce a hot rolled steel sheet having excellent tensile strength of 540 MPa or more.

The technique disclosed in Patent Document 2 is to divide its composition into % by mass, including C: 0.05 to 0.12%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.030% or less, S: 0.01% or less, Al: 0.005 to 0.1%, and N: 0.01% or less. Ti: 0.030~0.080%, the rest is composed of Fe and unavoidable impurities. The structure is a structure containing more than 70% of the polygonal ferrite iron, and will be present in crystallizations below 20 nm in size. The amount of Ti is set to a value of Ti* (Ti*=[Ti]-48÷14×[N], [Ti], and [N] are the compositional compositions (% by mass) of Ti and N of the steel sheet, respectively). 50% or more, a technique of producing a high-strength hot-rolled steel sheet having excellent strength uniformity in a steel strip roll and having excellent strength uniformity.

The technique disclosed in Patent Document 3 is to divide its composition into % by mass, including C: more than 0.030% to less than 0.10%, Si: 0.35 to 0.80%, Mn: 1.7 to 3.2%, P: 0.001 to 0.02%, S: 0.0001 to 0.006%, and Al: 0.060% or less. N: 0.0001~0.0070%, Ti: 0.01~0.055%, Nb: 0.012~0.055%, Mo: 0.07~0.55%, B: 0.0005~0.0040%, the rest is composed of iron and inevitable impurities, and will be The X-ray intensity ratio of the {110} plane which is parallel to the plate surface of the plate thickness of 1/8 of the steel plate is set to 1.0 or more, and is formed so that the ratio of the drop is 0.68 or more and less than 0.92, and the maximum tensile strength is obtained. (TS) is a technique of a hot-rolled steel sheet having an excellent weldability and ductility of 780 MPa or more. Further, in Patent Document 3, it is also described that: by reducing C When the content of the six elements of Si, Mn, Ti, Nb, Mo, and B is simultaneously added within a predetermined range, the drop ratio can be appropriately increased to achieve that the shape freezeability at the time of press forming is not deteriorated. To the extent, good weldability is obtained at the same time.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2006-213957

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2009-185361

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-105361

However, the techniques disclosed in Patent Documents 1 and 2 cannot ensure sufficient strength, and a hot-rolled steel sheet having a tensile strength of 980 MPa or more cannot be obtained. Further, the techniques disclosed in Patent Documents 1 and 2 do not examine the weldability of the hot-rolled steel sheet, and if the hot-rolled steel sheet is welded, the heat-affected portion may be softened.

The technique disclosed in Patent Document 3 can improve the weldability of the hot-rolled steel sheet, but it is not always possible to obtain a hot-rolled steel sheet having a tensile strength of 980 MPa or more. Further, in the technique disclosed in Patent Document 3, in addition to Si and Mn, it is necessary to simultaneously add Ti: 0.01 to 0.055%, Nb: 0.012 to 0.055%, Mo: 0.07 to 0.55%, and B: 0.0005 to 0.0040%. Therefore, it is disadvantageous in terms of composition. In addition, although in patent In Document 3, it is described that the main phase of the hot-rolled steel sheet is suitably made of toughened iron or toughened ferrite iron, but as will be described later, Mo is an element which makes the formation of toughened iron unstable. Therefore, in the technique disclosed in Patent Document 3, since the amount of Mo added is 0.07% or more, a hot-rolled steel sheet excellent in material stability cannot be obtained.

As described above, according to the prior art, it is difficult to obtain: A high-strength hot-rolled steel sheet having good quality stability and weldability and a tensile strength of 980 MPa or more.

The present invention has been developed in view of these circumstances, and an object thereof is to provide a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent material stability and weldability.

In order to solve the above problems, the present inventors have made an effort to review various factors that affect the strength, material stability, and weldability of hot-rolled steel sheets.

As a method for improving the strength of the steel sheet, it is conceivable to adopt a method of using a low-temperature metamorphic phase such as toughened iron or granulated iron in the mother phase structure of the hot-rolled steel sheet. However, although the Ma Tian loose iron has higher strength than the toughened iron, it lacks toughness. Therefore, if a steel sheet having the main structure of the granulated iron as the main structure is applied to the automobile member, various problems such as a decrease in the ability of the member to absorb the impact energy are caused.

Therefore, the present inventors focused on toughened iron which is excellent in balance between strength and toughness, and is capable of maintaining both toughness and maintenance. The strength of the hot-rolled steel sheet as the main phase of iron can be reviewed for various factors such as excellent weldability and material stability.

In general, as the C content in the steel becomes higher, the weldability of the steel deteriorates. On the other hand, C is a solid solution strengthening element, and if the C content is decreased, there is a problem that the strength of steel is insufficient. Further, if the C content is reduced, it is easy to form the ferrite iron of the soft structure, and there is still a problem that the strength of the steel is insufficient.

In response to the above problems, the inventors first targeted How can it be achieved: the C content which has an adverse effect on weldability can be limited to the minimum required, and the toughened iron can be used as the main phase of the hot rolled steel sheet, and the tensile strength reaches 980 MPa or more. The program was reviewed. As a result, it has been found that, even if the content of C which contributes to the increase in strength of the steel sheet is reduced, heat of tensile strength of 980 MPa or more can be obtained as long as the precipitation of the iron phase of the ferrite is suppressed. Rolled steel plate. In addition, all of the elements which can effectively suppress the nucleation of the iron phase of the ferrite are investigated, and it is found that it is more effective if a hot-rolled steel sheet in which Ti and V are added in combination is used. Then, after further reviewing the results, I finally got a kind of idea that by controlling the C content below the quantitative amount and optimizing the contents of Ti and V, it is possible to obtain: good weldability and fertilizer. A hot-rolled steel sheet having a structure in which precipitation of a granular iron phase is suppressed, and a tensile strength is 980 MPa or more of a tough iron intermediate phase.

Next, the inventors of the present invention reviewed the material stability of a hot-rolled steel sheet having a toughened iron as a main phase. And after tune After the investigation, it was found that the cause of the deterioration of the material stability was caused by the variation in the coiling temperature at the time of producing the hot-rolled steel sheet. In addition, it was also confirmed that the range of the coiling temperature for obtaining the toughened iron phase is affected by the cooling at the run-out-table, and thus reaches the field of migration boiling, and because Arrived in the field of migration boiling, it is very difficult to stabilize the coiling temperature.

Therefore, the inventors of the present invention are directed to: for reducing materials Quality review of the susceptibility of coiling temperature changes. As a result, it has been found that by adding an appropriate amount of Si to the steel material of the hot-rolled steel sheet, the transformation temperature of the ductile iron (Bs point) can be increased, and the temperature range in which the toughened iron metamorphosis can be increased can be expanded. The practice is effective. In addition, it has also been found that the weldability of the hot-rolled steel sheet is also improved by the addition of Si.

In general, for low temperature metamorphosis of toughened iron and the like When the high-strength steel sheet of the main phase is welded, sometimes the HAZ portion is softened due to the influence of the welding heat history, and the desired strength cannot be obtained. In order to solve such a problem, if the Si content of the steel sheet is corrected, the solid solution strengthening energy of Si can be utilized, and the softening amount in the HAZ portion can be greatly reduced. As described above, the inventors of the present invention have obtained a so-called "high-strength hot-rolled steel sheet having a toughened iron main phase excellent in weldability and material stability by correcting the Si content of the hot-rolled steel sheet. Transcendence.

The present invention has been developed based on the above-mentioned novelty, The gist of the following is as follows.

[1] A high-strength hot-rolled steel sheet having a composition of C: 0.09% or more and 0.17% or less, Si: more than 1.0% and 1.6% or less, and Mn: 1.5% or more and 2.5% or less, and P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.0080% or less, Ti: 0.09% or more and 0.14% or less, V: 0.05% or more and 0.25% or less, and the contents of Cr, Ni, and Mo are respectively given. It is limited to 0.06% or less (including 0%), and the rest is composed of Fe and unavoidable impurities, and has an area ratio of a toughened iron phase of 80% or more and a Fe deposition amount of 0.50% or less. The tensile strength is 980 MPa or more.

The high-strength hot-rolled steel sheet of [2] is in [1] except In addition to the components, B: 0.0001% or more and 0.005% or less in terms of mass%.

[3] High-strength hot-rolled steel sheets, in [1] or [2], except In addition to the above-mentioned composition, one or two selected from the group consisting of Ca: 0.0001% or more and 0.005% or less and rare earth metal (REM): 0.0001% or more and 0.005% or less by mass%.

[4] A method for manufacturing a high-strength hot-rolled steel sheet is to use steel When the material is heated, and after hot rolling, cooling is performed, and coiling is performed to form a hot-rolled steel sheet, the steel material is composed of C: 0.09% or more and 0.17% or less, and Si: more than 1.0. % and 1.6% or less, Mn: 1.5% or more and 2.5% or less, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.0080% or less, and Ti: 0.09% or more and 0.14% or less, V: 0.05% or more and 0.25% or less, and the contents of Cr, Ni, and Mo are respectively limited to 0.06% or less (including 0%), the rest is made up of Fe and unavoidable impurities, The heating temperature of the heating is set to be 1150° C. or higher and 1350° C. or lower, and the final refining rolling temperature of the hot rolling is set to 850° C. or higher, and the cooling is performed within 3 seconds after the final refining rolling of the hot rolling is completed. The initial cooling rate of the cooling is set to 15 ° C /sec or more, and the coiling temperature of the winding is set to 350 ° C or more and 550 ° C or less.

The method for producing a high-strength hot-rolled steel sheet according to [5] is, in addition to the above composition, in addition to the above composition, B: 0.0001% or more and 0.005% or less.

[6] The method for producing a high-strength hot-rolled steel sheet according to [6], wherein, in addition to the above composition, in addition to the above composition, the content of Ca: 0.0001% or more and 0.005% or less, rare earth metal (REM): One or two selected from 0.0001% or more and 0.005% or less.

According to the present invention, a high-strength hot-rolled steel sheet having good toughness and a tensile strength of 980 MPa or more can be obtained. Further, since the high-strength hot-rolled steel sheet of the present invention is excellent in material stability and weldability, it is very suitable for use in construction members and structural members for automobiles. Further, the high-strength hot-rolled steel sheet according to the present invention has the above-described excellent characteristics, can expand the use of the high-strength hot-rolled steel sheet, and has an industrial applicability.

Hereinafter, the present invention will be specifically described.

First, the reason for limiting the composition of the hot-rolled steel sheet of the present invention will be explained. In addition, the following is used to indicate the % of the component, and if not specified, it means: mass% (mass%).

C: 0.09% or more and 0.17% or less

C has the effect of promoting the formation of a toughened iron phase. Further, C is a solid solution strengthening element and also has an effect of increasing the strength of the toughened iron phase. In order to obtain a hot-rolled steel sheet having a tensile strength of 980 MPa, the C content must be set to 0.09% or more. On the other hand, when the C content exceeds 0.17%, the problem that the weldability is deteriorated due to an increase in the hardness difference in the HAZ portion tends to be conspicuous. In the field where the cooling rate near the molten metal portion is large, the granulated iron structure is formed, and the hardness of the granulated iron structure increases as the C content increases. If the content of C is more than 0.17%, the hardness difference between the field in which the granulated iron structure is hardened and the field close to the base material (in other words, the softening field in which the cooling rate is small) becomes large. Therefore, in the softening field, the welded joint is liable to break, and the tensile strength at the welded joint is deteriorated. Therefore, the C content is set to be 0.09% or more and 0.17% or less. A more desirable C content is more than 0.10% and less than 0.16%.

Si: more than 1.0% and less than 1.6%

Si is a coarse oxide or snow that can inhibit the toughness of steel. Ming iron, and it is also an element that contributes to solid solution strengthening. Further, Si has a function of increasing the toughening iron transformation start temperature (Bs point) in the cooling and coiling step after the completion of the hot rolling after the hot rolling steel sheet is produced, thereby expanding the generation of the toughened iron. The metamorphic temperature range is also an effective element for improving the material stability of hot rolled steel sheets. Further, Si has an effect of suppressing softening of the HAZ portion of the steel and is an important element for improving the weldability of the hot-rolled steel sheet.

As described above, Si is an extremely important element in the present invention. In order to obtain the above effects, the Si content must be set to more than 1.0%. On the other hand, if the Si content exceeds 1.6%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to exceed 1.0% and 1.6% or less. A more preferable Si content is 1.1% or more and 1.5% or less.

Mn: 1.5% or more and 2.5% or less

Mn has an effect of suppressing the nucleation of the iron phase of the ferrite, and is an important element for improving the strength of the hot rolled steel sheet. In order to obtain a desired metal structure to be described later, it is necessary to set the Mn content to 1.5% or more. On the other hand, if the Mn content exceeds 2.5%, the castability of the steel is remarkably deteriorated, and the productivity of the hot-rolled steel sheet is drastically lowered. Therefore, the Mn content is set to 1.5% or more and 2.5% or less. A more preferable Mn content is 1.6% or more and 2.4% or less.

In addition, based on the coiling temperature used to obtain the toughened iron phase Degree sensitivity passivation, in order to improve the material stability of hot-rolled steel sheet, it is best to adjust the content of C, Si, Mn to meet the following (1) is appropriate.

15≦-198×[%C]+220×[%Si]-80×[%Mn]≦150...(1)

[%C], [%Si], and [%Mn] in the formula (1) are the contents (% by mass) of C, Si, and Mn, respectively. The middle coefficient of the formula (1) is an element showing C, Si, and Mn, and in the cooling and coiling process after the hot-rolling is completed in the production of the hot-rolled steel sheet, the toughened iron phase can be obtained. The extent of the effect of temperature. Further, the smaller the value of the middle side of the formula (1), the narrower the temperature range in which the toughened iron phase can be obtained, and the material stability of the hot-rolled steel sheet is lowered.

In the invention of the material stability of the hot-rolled steel sheet which is required to have a toughened iron as a main phase, it is preferable to set the value of the middle side of the formula (1) to 15 or more. On the other hand, if the value of the middle side of the formula (1) is excessively large, the ferrite-grained iron phase precipitates and the strength of the hot-rolled steel sheet deteriorates. It is preferable to set the value of the middle side of the formula (1) to 150 or less in order to suppress the precipitation of the iron phase of the ferrite which causes the decrease in strength. Further, it is more preferable to set the value of the middle side of the formula (1) to 30 or more and 140 or less.

P: 0.03% or less

P is a starting point for segregation at the grain boundary and at the grain boundary when the steel is processed. Therefore, P is a harmful element which deteriorates the workability of the hot-rolled steel sheet, so it is preferable to reduce the content as much as possible. In the present invention, in order to avoid the above problem, the P content is limited to 0.03% or less. Better is 0.02% the following.

S: 0.005% or less

S is present in the steel in the form of inclusions such as MnS. Such inclusions are stretched during hot rolling in the production of hot rolled steel sheets. The inclusions that are stretched in this way will become a starting point for cracking during processing, and thus have an adverse effect on the workability of the hot rolled steel sheet. In the present invention, it is preferable to reduce the S content as much as possible, and therefore it is set to 0.005% or less. More preferably, it is below 0.003%.

Al: 0.08% or less

Al is an element having a function as a deoxidizer. In order to obtain such an effect, it is preferred to set the Al content to 0.02% or more. On the other hand, Al forms an oxide or the like and forms a starting point for forming pores when the flexing process is performed. Therefore, if the Al content exceeds 0.08%, the adverse effect on the flexibility of the hot-rolled steel sheet tends to be conspicuous. Therefore, the Al content is set to 0.08% or less. More preferably, it is below 0.06%.

N: 0.0080% or less

N is a combination of Ti and Ti in the stage of steel making and continuous casting to form TiN. The coarse TiN easily turns into a ferrite core. In addition, once TiN is formed, the solid solution Ti is reduced, so that the hardenability of the steel is lowered, and it becomes difficult to form a ferrite-iron phase, and it is difficult to obtain a hot-rolled steel sheet having a tensile strength of 980 MPa or more. Therefore, in the present invention In the middle, it is necessary to suppress the formation of coarse TiN, and the N content is limited to 0.0080% or less. More preferably, it is below 0.0070%. However, if N is excessively reduced, the effect of suppressing the growth of crystal grains by TiN is lost, and the crystal grains are coarsened to lower the toughness of the steel sheet. Therefore, it is preferred that the N content be set to 0.0020% or more.

Ti: 0.09% or more and 0.14% or less

When Ti is present in a hot-rolled steel sheet in a solid solution state, it suppresses nucleation of the iron phase of the ferrite grain, and is an element which contributes substantially to the high strength of the hot-rolled steel sheet. To achieve this effect, the Ti content must be set to 0.09% or more. On the other hand, if the Ti content is excessively excessive, the coarse TiC cannot be melted by heating the steel preform (steel material) at the time of producing the hot-rolled steel sheet. The coarse TiC easily forms a stress concentration portion, and the toughness or workability of the hot rolled steel sheet is lowered. Therefore, the Ti content is set to be 0.14% or less. A more preferable Ti content is 0.10% or more and 0.13% or less.

V: 0.05% or more and 0.25% or less

V is one of the important elements in the present invention. V is similar to Ti and has an effect of suppressing nucleation of the ferrite-grained iron phase in a solid solution state. The present invention is characterized by containing Si in an appropriate amount. If the ferrite-forming iron element is Si, the ferrite-grain metamorphosis will begin to metamorphose during the cooling process on the run-out-table, and thus the desired toughened iron structure cannot be obtained. . In response to this problem, according to the present invention As a result of review by humans, it was confirmed that the method of adding Ti and V in combination can ensure the desired hardenability even when Si is contained in a predetermined amount. Further, it has been confirmed that, in addition to the effect of improving the hardenability, if an appropriate amount of V is contained, the plate crystal structure of the toughened iron becomes fine. If you want to achieve these effects, you must set the V content to 0.05% or more. Further, the V content is preferably 0.08% or more, more preferably 0.1% or more. On the other hand, if the V content exceeds 0.25%, the toughness of the hot-rolled steel sheet is remarkably lowered. Therefore, the V content is limited to 0.25% or less. Further, it is more preferable to set the V content to 0.23% or less.

Cr: 0% or more and 0.06% or less, Ni: 0% or more and 0.06% or less, and Mo: 0% or more and 0.06% or less

Cr, Ni, and Mo are elements for lowering the temperature at which the toughened iron starts in the cooling and coiling step after the hot rolling is performed in the production of the hot rolled steel sheet. Therefore, if the content of Cr, Ni, and Mo is too high, the cooling on the output roller will reach the field of migration boiling, and as a result, the coiling temperature becomes unstable, and the material stability of the hot-rolled steel sheet deteriorates. For this reason, it is preferable to reduce the contents of Cr, Ni, and Mo as much as possible, but the content thereof can be allowed to reach 0.06%, and therefore, the upper limit content is set to 0.06%. Further, the content of these elements is preferably set to 0.04% or less, or may be reduced to impurities. In addition, it is more preferable that the content of each of these elements is set to 0%. Further, the total content of these elements is preferably set to 0.1% or less.

The above is the basic composition of the hot rolled steel sheet of the present invention. In addition to the basic components described above, the following elements may also be included.

B: 0.0001% or more and 0.005% or less

B is easy to segregate in the grain boundary, and has the effect of suppressing the deformation of the iron from the Worthfield iron, and is an element that can improve the material stability of the hot rolled steel sheet. If it is desired to obtain such an effect, the B content is preferably set to 0.0001% or more. On the other hand, when the B content exceeds 0.005%, the above effect tends to be saturated. Therefore, the B content is preferably 0.005% or less, more preferably 0.0003% or more, and still more preferably 0.003% or less.

Further, the hot-rolled steel sheet of the present invention may contain the following elements.

One or two selected from the group consisting of Ca: 0.0001% or more and 0.005% or less and rare earth metal (REM): 0.0001% or more and 0.005% or less

Ca, a rare earth metal (a rare earth metal is Sc, Y, and a lanthanoid element from atomic number 57 to 71) is used to control the form of inclusions in steel, and is effective for suppressing pore formation from inclusions. element. In order to obtain such an effect, it is preferable to contain one or more selected from the group consisting of Ca and rare earth metals, and the content of these elements is preferably set to 0.0001% or more. On the other hand, if the content of these elements exceeds 0.005%, the above effect tends to be saturated, so that the content of Ca and the rare earth metal is preferably set to 0.005% or less. Further, it is more preferable to set the Ca content to 0.0003% or more. Further, it is more preferable to set the Ca content to 0.002% or less. Further, it is more preferable to set the rare earth metal content to 0.0003% or more. Further, it is more preferable to set the rare earth metal content to 0.002% or less. this When both Ca and a rare earth metal are contained at the same time, it is preferable to set the total content of Ca and the rare earth metal to 0.0002% or more. Moreover, in this case, it is preferable to set the total content of Ca and the rare earth metal to 0.006% or less. Further, in this case, it is more preferable to set the total content of Ca and the rare earth metal to 0.0003% or more. Moreover, in this case, it is more preferable to set the total content of Ca and the rare earth metal to 0.002% or less.

Further, in the hot-rolled steel sheet according to the present invention, the components other than the above are Fe and unavoidable impurities.

Examples of unavoidable impurities include Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir. Ru, Os, Tc, Re, Ta, Be, Sr, Sb, Cu, Sn, Mg, etc., and the total content of these impurities is preferably 0.1% or less.

Next, the reason why the structure of the hot-rolled steel sheet of the present invention is limited will be explained.

Area ratio of toughened iron phase: 80% or more

The hot-rolled steel sheet according to the present invention is a structure mainly composed of a toughened iron phase excellent in balance between strength and toughness. The toughened iron phase in the present invention is targeted to the upper toughened iron, the lower toughened iron, and the toughened ferrite.

When the area ratio of the toughened iron phase is less than 80%, not only the hot-rolled steel sheet having a tensile strength of 980 MPa or more cannot be obtained, but the variation in the distribution of the material also becomes large. Therefore, the area ratio of the toughened iron phase is set to 80% or more. Better is more than 85%, for special requirements material stability In the case of a hot rolled steel sheet, it is better to set it at 90% or more.

The hot rolled steel sheet of the present invention is a tough iron single phase group Weaving is appropriate, so the area ratio of the toughened iron phase is 100%.

In the case of a structure other than the toughened iron phase, the structure may be a ferrite phase iron phase, a granulated iron phase, and a residual Worth iron phase. However, the material stability of the hot-rolled steel sheet deteriorates if there is a state in which the iron phase of the Ma Tian or the residual iron phase of the Vostian exists. Therefore, the area ratio of the iron phase of the Matian and the iron phase of the remaining Worthfield is preferably 7% or less, and more preferably 5% or less.

Fe precipitation: 0.50% or less

The hardness (strength) of the toughened iron phase will decrease drastically as the amount of solid solution C decreases. Therefore, in the hot-rolled steel sheet having the toughened iron as the main phase, if the tensile strength is desired to be 980 MPa or more, the amount of solid solution C must be ensured to be more than one amount. In the case of a hot-rolled steel sheet having a toughened iron as a main phase, C in a non-solid solution state is mainly precipitated as a stellite. Therefore, as long as the precipitation of the fermented iron is suppressed, a sufficient amount of solid solution C can be secured, and the desired strength of the hot-rolled steel sheet can be obtained.

The amount of precipitation of Felding iron (Fe ₃ C) can be determined by analyzing the amount of precipitation of Fe. When the Fe deposition amount is more than 0.50% by mass%, the precipitation amount of the fermented iron is increased, and the amount of the solid solution C is insufficient, and as a result, the desired strength of the hot-rolled steel sheet cannot be obtained. Therefore, the amount of precipitation of Fe is limited to 0.50% or less. More preferably, it is 0.40% or less.

In addition, the hardness (strength) of the toughened iron phase is also toughened. The effect of the average platelet spacing of the iron plate crystal. The toughened iron plate crystal has a rectangular shape and can be defined as the length of the long side and the length of the short side. When the length of the short side is regarded as the slab spacing, when the average slab spacing exceeds 400 nm, the material stability of the hot rolled steel sheet tends to deteriorate. Therefore, it is preferred to set the average platelet spacing of the toughened iron phase to 400 nm or less. It has been explained above that V is an effect of making the plate crystals of the toughened iron phase fine. If it is desired to stably obtain a structure in which the average platelet spacing of the toughened iron phase is 400 nm or less, it is preferred to set the V content to 0.1% or more.

In the above manner, the composition of the hot rolled steel sheet is divided into groups. The weaving is optimized (optimized), whereby a high-strength hot-rolled steel sheet having excellent weldability and material stability and having a tensile strength of 980 MPa or more can be obtained. Further, the thickness of the hot-rolled steel sheet according to the present invention is not particularly limited, but is preferably 1.6 mm or more and 10 mm or less.

Next, a method of producing the hot rolled steel sheet of the present invention will be described.

In the present invention, the steel material (steel blank) having the above composition is heated, and after hot rolling, cooling is performed, and coiling is performed to prepare a hot-rolled steel sheet. In this process, the heating temperature of the heating is set to be 1150 ° C or more and 1350 ° C or less, and the final refining rolling temperature of the hot inter-roll rolling is set to 850 ° C or higher, and the cooling is performed at the end of hot rolling. The polishing is started within 3 seconds after the completion of the refining, and the average cooling rate of the cooling is set to 15° C./sec or more, and the winding temperature immediately before entering the winding step is set to 350° C. or higher and 550° C. or lower. .

In the present invention, the melting side of the steel is not particularly limited The method can be carried out by a known melting method such as a converter or an electric furnace. Further, the vacuum degassing furnace can also be used for secondary refining. After that, in consideration of the problem of productivity or quality, it is preferable to use a continuous casting method to produce a steel preform (steel material), but it is also possible to use a block-block rolling method, a thin steel blank continuous casting method, or the like. A known casting method is used to produce a steel embryo. Further, TiN is mainly crystallized during continuous casting, and as described above, coarsened TiN causes a decrease in strength of the hot rolled steel sheet. In order to suppress the coarsening of TiN, it is preferable to set the casting speed during continuous casting to 1.0 m/min or more to suppress the growth of TiN particles, and it is preferable to control the size of TiN to 5 μm or less.

Heating temperature of steel material: 1150 ° C or more and 1350 ° C or less

The steel material obtained by the above method is to be subjected to hot rolling, but in the present invention, before the hot rolling, the steel material must be heated to a substantially homogeneous state. Iron phase to melt coarse carbides. If the heating temperature of the steel material is lower than 1150 ° C, the coarse carbide does not melt, so the amount of solid solution C is reduced, and the strength of the finally obtained hot-rolled steel sheet is remarkably lowered. On the other hand, when the heating temperature exceeds 1,350 ° C, the amount of scale formed is increased, and the scale is bitten into the steel sheet, which deteriorates the surface properties of the steel sheet.

For the above reasons, the heating temperature of the steel material is set to be 1150 ° C or higher and 1350 ° C or lower. More preferably, it is 1150 ° C or more and 1320 ° C or less. However, when hot rolling is performed on the steel material, casting If the steel material is in a temperature range of 1150 ° C or more and 1350 ° C or less, or if the carbide of the steel material is completely melted, it is not necessary to heat the steel material, and it can be directly sent for rolling. Although the holding time of the steel material in the above heating temperature state is not particularly limited, if the holding time is too long, a decarburized layer will be formed on the surface of the steel material, which may reduce fatigue resistance or loss of scale. The resulting drop in yield is a concern. Therefore, the holding time of the steel material in the above heating temperature state is preferably less than 3,600 seconds, more preferably 2,400 seconds or less. Further, the above holding time is a holding time in a temperature range in which the growth of the scale tends to be 1200 ° C or more.

After heating the steel material to reach the above heating temperature, Hot rolling. The hot rolling is usually a combination of rough rolling and final refining rolling, but the conditions for the rough rolling are not particularly limited. Further, for example, in the case of casting a steel blank (steel material) by a thin steel blank continuous casting method, rough rolling can be omitted.

Final refining rolling temperature of hot rolling: above 850 °C

When the rolling is carried out at a temperature lower than 850 ° C, the rolling load is remarkably increased in the steel of the present invention, which may become difficult to manufacture or even impossible to manufacture. In addition, if the rolling is performed at a temperature lower than 850 ° C, the iron phase of the Worth is excessively processed. Therefore, the metamorphosis of "Worstian iron → fat iron metamorphosis" will be refined. The cooling process after the rolling is performed, and thus the desired structure cannot be obtained. For the above reasons, the final refining rolling temperature is set at 850 ° C. on. More preferably, the final refining rolling temperature is set to 870 ° C or higher. Further, it is preferred to set the final finishing rolling temperature to 960 ° C or lower.

Time from the end of the final refining roll to the start of forced cooling: within 3 seconds

The time from the end of the final refining rolling to the start of the forced cooling, if it has been more than 3 seconds, the Worthite iron → fat iron metamorphosis will start, and the desired structure cannot be obtained. In addition, when the steel sheet after the completion of the final refining rolling is held for a long period of time in a high temperature state, carbides are formed by precipitation due to deformation, and the amount of solid solution C which contributes to the increase in strength of the steel sheet is reduced. Therefore, in the present invention, for the purpose of suppressing the deformation of the ferrite and iron, or for the purpose of suppressing the precipitation due to the deformation, it is necessary to promptly start the forced cooling immediately after the end of the hot rolling, and the final refining is completed. After that, forced cooling is started at least within 3 seconds. Better is less than 2 seconds.

Average cooling rate: 15 ° C / sec or more

After the final refining roll is finished, in order to suppress the start of the Worthite iron→fertilizer iron deformation, it is necessary to perform cooling as quickly as possible to reach the coiling temperature. If the average cooling rate of the forced cooling after the final refining rolling is less than 15 ° C / sec, the ferrite iron phase is formed and the desired structure cannot be obtained. Therefore, the average cooling rate after completion of the final refining rolling is set to 15 ° C /sec or more. More preferably, it is 30 ° C / sec or more. However, especially in a steel plate having a plate thickness of 3.2 mm or less, if the cooling rate is too large, it will be difficult. The temperature at which the cooling is stopped is controlled, and the material stability of the hot-rolled steel sheet is deteriorated. Therefore, the average cooling rate is preferably set to 150 ° C / sec or less.

Coiling temperature: 350 ° C or more and 550 ° C or less

In the steel of the present invention, a suitable coiling temperature for obtaining a toughened iron phase is 350 ° C or more and 550 ° C or less. If the coiling temperature is lower than 350 ° C, the granulated iron phase or the residual Worth iron phase will be formed, and the material stability of the hot rolled steel sheet will be deteriorated. On the other hand, if the coiling temperature is higher than 550 ° C, the ferrite-grained iron is metamorphosed, so that a hot-rolled steel sheet having a tensile strength of 980 MPa or more cannot be obtained. For the above reasons, the coiling temperature is set to be 350 ° C or more and 550 ° C or less. A better coiling temperature range is from 350 ° C to 500 ° C. Further, the temperature at which the forced cooling is stopped is preferably set to 350 ° C or more and 550 ° C or less, similarly to the coiling temperature, and is preferably set to 350 ° C or more and 500 ° C or less.

[Examples]

For the steel material having a thickness of 250 mm having the composition shown in Table 1, hot rolling was performed under the hot rolling conditions shown in Table 2 to prepare a hot rolled steel sheet having a thickness of 2.0 to 8.0 mm. Further, the average cooling rate disclosed in Table 2 is the average cooling rate from the final refining rolling temperature to the cooling stop temperature.

The microstructure of the obtained hot-rolled steel sheet was observed, and the area ratio of the toughened iron phase and the average platelet spacing of the tough iron plate crystal were determined. Further, the obtained hot-rolled steel sheet was subjected to extraction residue analysis to determine the Fe deposition amount. Further, the obtained hot-rolled steel sheets were subjected to a tensile test and a welding test, and their mechanical properties (strength, material stability, and the like) and weldability were evaluated. The method of tissue observation, extraction of residue analysis, and various tests is as follows.

(1) Organizational observation Area ratio of toughened iron phase

For the center portion of the plate thickness in the cross section parallel to the rolling direction of the obtained hot-rolled steel sheet, the metal structure revealed by etching with a 5% nitric acid etching solution is magnified by a scanning optical microscope. After the magnification, 10 fields of view were taken. The toughened iron phase has a structure in which a corrosion stain or a fermented iron is observed in the grain. The area ratio of the toughened iron phase is separated by image analysis: the toughened iron phase and the phase other than the toughened iron phase (fertilizer iron phase, 麻田散铁), according to the toughened iron phase relative to the observation field The area ratio is obtained.

Average plate spacing

A sample was produced by a thin film method from the center of the thickness of the obtained hot-rolled steel sheet, and observed by a transmission electron microscope (magnification: 135,000 times), and 50 or more of the samples were toughened for each sample. Iron plate crystal The tissue was measured for the interlaminar spacing, and the average value of the obtained plate crystal spacing was taken as the average plate crystal spacing.

(2) Extraction of residue analysis

A sample was taken from the center of the thickness of the prepared hot-rolled steel sheet, and about 0.2 g of the sample was placed in a 10% AA-based electrolyte (10 vol% acetamidine-1 mass% tetramethylammonium chloride-methanol). Constant current electrolysis was performed at a current density of 20 mA/cm ² . Next, the electrolytic solution after the constant current electrolysis was collected by filtration using a filter paper having a mesh size of 0.2 μm, and the amount of Fe contained in the trap was quantitatively analyzed by an ICP emission spectrometer. The amount of Fe (mass ratio) was determined from the mass of the sample after electrolysis and the amount of Fe contained in the trap.

(3) Tensile test Strength characteristics

From the obtained hot-rolled steel sheet, a Japanese Industrial Standard JIS No. 5 tensile test piece in which the stretching direction is perpendicular to the rolling direction was produced, and it was carried out three times in accordance with Japanese Industrial Standard JIS Z 2241 (2011). In the tensile test, the averaged drop strength (YS), tensile strength (TS), and full tensile length (El) were determined. The crosshead speed of the tensile test (test speed of the tensile test) was set to 10 mm/min. In addition, the strength of the fall is taken as its falling point or 0.2%.

Material stability (distribution deviation value of tensile strength)

From the obtained hot-rolled steel sheets, a total of 153 pieces of Japanese Industrial Standard JIS No. 5 tensile test pieces in which the stretching direction was perpendicular to the rolling direction were produced. Specifically, for each hot-rolled steel sheet (hot-rolled steel coil) after coiling, the position in the longitudinal direction of 51 points (the position where the longitudinal direction of the steel strip is 50-divided, and the long side of the steel strip are specified) The position of the front end of the direction and the position of the trailing end in the longitudinal direction of the steel strip roll, and then the three JIS No. 5 stretches are taken from the center of the width direction of the steel sheet for each longitudinal direction position in each longitudinal direction. Test piece.

Each of the hot-rolled steel sheets was subjected to a tensile test according to the Japanese Industrial Standard JIS Z 2241 (2011) using 153 tensile test pieces, and the tensile strength of all the tensile test pieces (153 in total) was determined. Standard deviation (distribution deviation value of tensile strength).

(4) Welding test

The obtained hot-rolled steel sheet was subjected to arc welding to prepare a welded sample. The welded sample was obtained by welding together the long sides (300 mm) of the sample having a width of 200 mm and a length of 300 mm taken from the same hot-rolled steel sheet. Further, when a sample having a width of 200 mm and a length of 300 mm is used, the longitudinal direction of the sample is kept in line with the rolling direction. The welding conditions are plate gap: 1 mm, welding current: 180 A, welding voltage: 20 V, welding rod: MG-50 made of Kobe steel (welding rod diameter: 1.2 mm), welding speed: 80 cm/min, shielding gas: CO ₂ (80%) Under the condition of +Ar (20%), the welding is performed.

Next, three JIS No. 5 stretches were produced from each welded sample. In the test piece, according to the regulation of JIS Z 2241 (2011), the tensile test was carried out under the same tensile conditions as those of the tensile test carried out in the evaluation of the "strength characteristics" of the "(3) tensile test". The JIS No. 5 tensile test piece produced was such that the welding bead of the welded sample was exactly transverse to the width of the test piece in the central portion between the test piece evaluation points. Further, the crosshead speed of the tensile test was set to 10 mm/min.

The maximum load at the time of fracture of the test piece was measured by a tensile test, and the value of dividing the maximum load by the thickness of the plate was also determined in order to eliminate the influence of the thickness. Also, the fracture position of the welded sample is confirmed.

The above results are shown in Table 3. In Table 3, pull The tensile strength is 980 MPa or more, the distribution deviation of tensile strength (standard deviation of tensile strength) is 40 MPa or less, and the value obtained by dividing the maximum load of the tensile test of the welded specimen by the thickness of the sheet is 22 kN/mm or more, and the welding sample The fracture in the tensile test is a case where the base material is broken, and the steel material of the material sought by the present invention is evaluated as good "○". On the other hand, if any of the above conditions fails to be met, it is rated as bad "X".

Each of the hot rolled steel sheets of the present invention has tensile strength TS It is 980 MPa or more, and is excellent in material stability and weldability. On the other hand, in the hot-rolled steel sheet of the comparative example which falls outside the range of this invention, it is not unable to obtain a predetermined high strength, and the favorable material stability and weldability were not acquired.

Claims (6)

A high-strength hot-rolled steel sheet having a composition of C: 0.09% or more and 0.17% or less, Si: more than 1.0% and 1.6% or less, Mn: 1.5% or more and 2.5% or less, and P: 0.03% or less. , S: 0.005% or less, Al: 0.08% or less, N: 0.0080% or less, Ti: 0.09% or more and 0.13% or less, V: 0.05% or more and 0.25% or less, and the contents of Cr, Ni, and Mo are respectively limited to 0.06. % or less (including 0%), the rest is composed of Fe and unavoidable impurities, and has an area ratio of a toughened iron phase of 80% or more and a Fe deposition amount of 0.50% or less, and the tensile strength is More than 980MPa. The high-strength hot-rolled steel sheet according to the first aspect of the invention is in addition to the above-mentioned components, and further contains, by mass%, B: 0.0001% or more and 0.005% or less. The high-strength hot-rolled steel sheet according to the first or second aspect of the invention, in addition to the above-mentioned composition, contains, in mass%, from 0.0001% to 0.005%, and rare earth metal (REM): 0.0001. One or two selected from the group of 0.005% or more. A method for manufacturing a high-strength hot-rolled steel sheet by performing steel material After heating, after performing hot rolling, cooling and coiling to form a hot-rolled steel sheet, the steel material is composed of C: 0.09% or more and 0.17% or less and Si: more than 1.0% by mass%. 1.6% or less, Mn: 1.5% or more and 2.5% or less, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.0080% or less, Ti: 0.09% or more and 0.13% or less, V: 0.05% The content of Cr, Ni, and Mo is limited to 0.06% or less (including 0%), and the balance is composed of Fe and unavoidable impurities, and the heating temperature of the heating is set at 1150 ° C. The temperature of the final refining rolling of the hot rolling is set to 850 ° C or more and 930 ° C or less at a temperature of 1350 ° C or less, and the cooling is started within 3 seconds after the completion of the final refining rolling of hot rolling, and the cooling is performed. The average cooling rate is set to 15 ° C /sec or more, and the coiling temperature of the winding is set to 350 ° C or more and 550 ° C or less. In the method for producing a high-strength hot-rolled steel sheet according to the fourth aspect of the invention, in addition to the composition, B: 0.0001% or more and 0.005% or less by mass%. High-strength hot-rolled steel as described in claim 4 or 5 In addition to the above-mentioned composition, the method of producing a sheet contains one or two selected from the group consisting of Ca: 0.0001% or more and 0.005% or less, and rare earth metal (REM): 0.0001% or more and 0.005% or less. Kind.