JP5821929B2 - High-strength hot-rolled steel sheet with excellent material stability and weldability and method for producing the same - Google Patents

Description

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

In recent years, from the viewpoint of global environmental conservation, the automobile industry as a whole has been directed to improving automobile fuel efficiency with the aim of reducing CO ₂ emissions. In response to this trend, demands for improving fuel consumption are increasing even for large vehicles such as trucks. In order to improve fuel efficiency, it is effective to reduce the weight of automobile bodies by reducing the thickness of the members used. In addition, in order to ensure the safety of passengers in the event of a collision, it is also required to strengthen the automobile body and improve the collision safety of the automobile body. From such a viewpoint, a high-strength hot-rolled steel sheet capable of achieving both weight reduction and safety is used as a material for automobile members, and the amount of use is increasing year by year.

  On the other hand, as the strength of the steel plate increases, the material stability in the longitudinal direction of the steel plate tends to deteriorate. If the material is unstable, the accuracy of prediction by CAE (Computer Assisted Engineering) decreases when the steel sheet is pressed into a member of a predetermined shape, and the control of the springback amount becomes difficult, and the dimensional accuracy of the member deteriorates. . In addition, as the strength of the steel plate increases, the weldability also tends to deteriorate. High strength steel sheets generally have a high alloy concentration, so that weldability often becomes a problem, and in particular, the adverse effects of softening in the heat affected zone (HAZ zone) in welding tend to be obvious. In many cases, automobile members are joined to each other by spot welding or arc welding. However, when the heat-affected zone is softened as described above, various problems such as insufficient strength at the heat-affected zone are caused.

For the above reasons, it is desired that the steel sheet having a high tensile strength of 980 MPa or more has good material stability and weldability.
So far, various studies have been made on material stability and weldability of high-strength hot-rolled steel sheets. For example, the following techniques have been disclosed.

  Patent Document 1 includes mass%, 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%, including Ti: 0.01-0.20%, satisfying Mn-Si> 0.9%, and 0.75≤ (C% / 12) / (Ti% / 48 + Nb% / 93 + Mo% / 96 + V% / 51-N % / 14−S% / 32) ≦ 1.25, and when hot-rolling steel consisting of Fe and unavoidable impurities in the balance, the finishing temperature is 900 ° C. or higher and the steel is wound at 400 to 600 ° C. Thus, a technology for producing a hot-rolled steel sheet having excellent material uniformity with a tensile strength of 540 MPa or more is disclosed.

  In Patent Document 2, in mass%, 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%, N : 0.01% or less, Ti: 0.030 to 0.080%, with the balance consisting of Fe and inevitable impurities, with a structure containing polygonal ferrite in a fraction of 70% or more, and in precipitates with a size of less than 20 nm The amount of Ti present in Ti * (Ti * = [Ti] −48 ÷ 14 × [N], where [Ti] and [N] are the Ti and N component compositions (mass%) of the steel sheet, respectively)) A technique for making a high-strength hot-rolled steel sheet having excellent strength uniformity with small strength variation within the coil by making it 50% or more is disclosed.

  Patent Document 3 includes mass%, 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%, Al: Contains 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 balance being iron and inevitable impurities When the X-ray intensity ratio of the {110} plane parallel to the plate surface in the plate thickness 1/8 layer of the steel sheet is 1.0 or more, the yield ratio is 0.68 or more and less than 0.92, and the maximum tensile strength ( (TS) is 780 MPa or more, and a technology for producing a hot-rolled steel sheet having excellent weldability and ductility has been disclosed. Patent Document 3 discloses that the C content is reduced, and six kinds of elements of Si, Mn, Ti, Nb, Mo, and B are simultaneously added within a predetermined range, thereby freezing the shape during pressing. It is described that a good weldability can be obtained while at the same time increasing the yield ratio to such an extent that does not deteriorate.

JP 2006-213957 A JP 2009-185361 A JP-A-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, in the techniques disclosed in Patent Documents 1 and 2, the weldability of the hot-rolled steel sheet is not studied, and when the hot-rolled steel sheet is welded, the heat-affected zone may be softened.

  With the technique disclosed in Patent Document 3, the weldability of the hot-rolled steel sheet can be improved, but a hot-rolled steel sheet having a tensile strength of 980 MPa or more cannot always be obtained. In addition, in the technique disclosed in Patent Document 3, in addition to Si and Mn, Ti: 0.01 to 0.055%, Nb: 0.012 to 0.055%, Mo: 0.07 to 0.55%, B: 0.0005 to 0.0040% are added simultaneously. Is a disadvantage in terms of cost. Furthermore, Patent Document 3 describes that bainite or bainitic ferrite is suitable as the main phase of the hot-rolled steel sheet. However, as will be described later, Mo is an element that makes bainite formation unstable. Therefore, in the technique disclosed in Patent Document 3, since Mo is added by 0.07% or more, a hot-rolled steel sheet having excellent material stability cannot be obtained.

As described above, in the prior art, it has been difficult to obtain a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more with good material stability and weldability.
The present invention has been made in view of such 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 in material stability and weldability.

In order to solve the above problems, the present inventors have intensively studied various factors that affect the strength, material stability, and weldability of hot-rolled steel sheets.
As a technique for improving the steel sheet strength, a technique can be considered in which the matrix structure of the hot-rolled steel sheet is a low-temperature transformation phase such as bainite or martensite. However, martensite has higher strength than bainite, but has poor toughness. Therefore, when a steel sheet mainly composed of martensite is applied to an automobile member, various troubles such as a reduction in impact energy absorption ability of the member are caused.

Therefore, the present inventors pay attention to bainite having an excellent strength-toughness balance, and intensively study various factors that achieve both weldability and material stability while maintaining the strength of a hot-rolled steel sheet mainly composed of bainite. did.
In general, the weldability of steel decreases as the C content of the steel increases. On the other hand, C is a solid solution strengthening element. If the C content is reduced, insufficient strength of steel becomes a problem. Further, if the C content is reduced, ferrite, which is a soft structure, is likely to be generated, so that insufficient strength of the steel is also a problem.

  Based on the above problems, the present inventors first made a means to set the main phase of the hot-rolled steel sheet to bainite and the tensile strength to be 980 MPa or more while limiting the C content that adversely affects weldability to the minimum necessary. Was examined. As a result, even if the content of C contributing to high strength of the steel sheet is reduced, a hot-rolled steel sheet with a tensile strength of 980 MPa or more can be obtained if precipitation of the ferrite phase can be suppressed. I found it. In addition, as a result of exhaustive investigation of elements effective for suppressing nucleation of the ferrite phase, it has been found that it is effective to use a hot-rolled steel sheet in which Ti and V are added in combination. As a result of further investigation, the C content is suppressed to a predetermined amount or less, and by optimizing the Ti and V contents, the weldability is good and the precipitation of the ferrite phase is suppressed. It was found that a hot rolled steel sheet with a bainite main phase having a tensile strength of 980 MPa or more can be obtained.

  Next, the present inventors examined the material stability of a hot-rolled steel sheet containing bainite as a main phase. And after the investigation, it was clarified that the cause of the deterioration of the material stability was the fluctuation of the coiling temperature during the production of the hot-rolled steel sheet. Also, it is very difficult to stabilize the coiling temperature because the coiling temperature range for obtaining the bainite phase reaches the transition boiling region by cooling at the run-out table, and because it reaches the transition boiling region. That was also confirmed at the same time.

  Therefore, the present inventors examined means for reducing the sensitivity of the material to fluctuations in the winding temperature. As a result, we obtained the knowledge that it is effective to increase the bainite transformation start temperature (Bs point) and widen the temperature range causing bainite transformation by adding an appropriate amount of Si to the steel material of the hot-rolled steel sheet. It was. Moreover, the knowledge that the weldability of a hot-rolled steel sheet is also improved by the addition of Si was obtained.

  Generally, when a high-strength steel plate having a low-temperature transformation phase such as bainite as a main phase is welded, the HAZ part may be softened due to the welding heat history, and a desired strength may not be obtained. In response to such problems, when the Si content of the steel sheet is optimized, the softening amount in the HAZ part can be greatly reduced by the solid solution strengthening ability of Si. As described above, the present inventors have obtained the knowledge that by optimizing the Si content of the hot-rolled steel sheet, a high-strength hot-rolled steel sheet having a bainite main phase excellent in weldability and material stability can be obtained. It was.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.09% to 0.17%, Si: more than 1.0% to 1.6%, Mn: 1.5% to 2.5%, P: 0.03% or less, S: 0.005% or less, Al: 0.08% In the following, 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 are contained so as to satisfy the following formula (1) , and Cr, Ni and Mo contents are 0.06% respectively. The composition is limited to the following (including 0%), the balance is composed of Fe and inevitable impurities, the area ratio of the bainite phase is 80% or more, and the amount of Fe precipitation is 0.50% or less. Material stability and weldability characterized by a tensile strength of 980 MPa or more, a standard deviation of tensile strength of 40 MPa or less, a maximum load / sheet thickness of a weld sample tensile test of 22 kN / mm or more and a base metal fracture High strength hot rolled steel sheet with excellent resistance.
Record
15 ≦ −198 × [% C] + 220 × [% Si] −80 × [% Mn] ≦ 150 (1)
Here, [% C], [% Si], and [% Mn] are the contents (mass%) of C, Si, and Mn, respectively.

[2] A high-strength hot-rolled steel sheet excellent in material stability and weldability, characterized in that, in addition to the above-mentioned composition, B: 0.0001% to 0.005% in addition to the above composition.

[3] In [1] or [2], in addition to the above composition, one or two selected from Ca: 0.0001% to 0.005% and REM: 0.0001% to 0.005% in mass% A high-strength hot-rolled steel sheet excellent in material stability and weldability, characterized by containing.

[4] When the steel material is heated and hot-rolled, and then cooled, wound, and made into a hot-rolled steel sheet, the steel material is, in mass%, C: 0.09% to 0.17%, Si: Over 1.0%, 1.6% or less, Mn: 1.5% or more, 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, 0.14% or less, V : 0.05% or more and 0.25% or less are contained so as to satisfy the following formula (1), the contents of Cr, Ni and Mo are limited to 0.06% or less (including 0%) respectively, the balance being Fe and inevitable impurities The heating temperature of the heating is 1150 ° C. or higher and 1350 ° C. or lower, the finish rolling temperature of the hot rolling is 850 ° C. or higher, and the cooling is started within 3 s after finishing the hot rolling finish rolling, the average cooling rate of cooling and 15 ° C. / s or more, the coiling temperature of the winding and 350 ° C. or higher 550 ° C. or less, 80% or more the area ratio of the bainite phase It has a structure in which the amount of Fe precipitation is 0.50% or less, the tensile strength is 980 MPa or more, the standard deviation of tensile strength is 40 MPa or less, the maximum load / sheet thickness of the weld sample tensile test is 22 kN / mm or more, and A method for producing a high-strength hot-rolled steel sheet excellent in material stability and weldability, characterized in that the hot-rolled steel sheet is a base metal fracture .
Record
15 ≦ −198 × [% C] + 220 × [% Si] −80 × [% Mn] ≦ 150 (1)
Here, [% C], [% Si], and [% Mn] are the contents (mass%) of C, Si, and Mn, respectively.

[5] In [4], in addition to the above composition, B: 0.0001% or more and 0.005% or less by mass% is further included. Production of high strength hot rolled steel sheet excellent in material stability and weldability Method.

[6] In [4] or [5], in addition to the above composition, one or two selected from Ca: 0.0001% to 0.005% and REM: 0.0001% to 0.005% in mass% A method for producing a high-strength hot-rolled steel sheet excellent in material stability and weldability.

  According to the present invention, a high-strength hot-rolled steel sheet having a high toughness and a tensile strength of 980 MPa or more can be obtained. Moreover, since the high-strength hot-rolled steel sheet according to the present invention is excellent in material stability and weldability, it is suitable for use as a structural member for buildings or automobiles. Furthermore, since the high-strength hot-rolled steel sheet according to the present invention has the excellent characteristics as described above, further application development of the high-strength hot-rolled steel sheet is possible, and there is a remarkable industrial effect.

The present invention will be specifically described below.
First, the reasons for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% (mass%) unless there is particular notice.

C: 0.09% or more and 0.17% or less
C has an effect of promoting the formation of a bainite phase. C is a solid solution strengthening element and has an effect of increasing the strength of the bainite phase. In order to obtain a hot-rolled steel sheet having a tensile strength of 980 MPa, the C content needs to be 0.09% or more. On the other hand, when the C content exceeds 0.17%, a problem of deterioration in weldability due to an increase in hardness difference in the HAZ portion becomes obvious. In the region where the cooling rate close to the molten metal portion is large, a martensite structure is formed. When the amount of C exceeds 0.17%, the hardness difference between the region that becomes a martensite structure and hardens, and the region that is close to the base material, that is, the region that softens with a low cooling rate, increases. For this reason, the welded joint is easily broken in the softened region, and the tensile strength of the welded joint is reduced. Therefore, the C content is 0.09% or more and 0.17% or less. Preferably, it is more than 0.10% and not more than 0.16%.

Si: more than 1.0% and less than 1.6%
Si is an element that suppresses coarse oxides and cementite that inhibit the toughness of steel and contributes to solid solution strengthening. In addition, when manufacturing hot-rolled steel sheet, Si increases the bainite transformation start temperature (Bs point) in the cooling and winding process after the hot rolling, and has the effect of expanding the temperature range that causes bainite transformation. Therefore, it is also an element effective for improving the material stability of the hot-rolled steel sheet. Furthermore, Si has an effect of suppressing softening in the HAZ part of steel, and is an important element for improving the weldability of hot-rolled steel sheets.

  As described above, Si is an extremely important element in the present invention, and in order to obtain the above effect, the Si content needs to be more than 1.0%. On the other hand, when the Si content exceeds 1.6%, the toughness of the hot-rolled steel sheet decreases. Therefore, Si content shall be more than 1.0% and 1.6% or less. Preferably they are 1.1% or more and 1.5% or less.

Mn: 1.5% to 2.5%
Mn is an important element that has an effect of suppressing nucleation of the ferrite phase and contributes to an increase in strength of the hot-rolled steel sheet. In order to obtain a desired metal structure to be described later, the Mn content needs to be 1.5% or more. On the other hand, when the Mn content exceeds 2.5%, the castability of the steel is remarkably lowered, and the productivity of the hot-rolled steel sheet is greatly lowered. Therefore, the Mn content is 1.5% or more and 2.5% or less. Preferably they are 1.6% or more and 2.4% or less.

From the viewpoint of slowing the coiling temperature sensitivity for obtaining the bainite phase and improving the material stability of the hot-rolled steel sheet, the contents of C, Si, and Mn are adjusted to satisfy the following formula (1): It is desirable to do.
15 ≦ −198 × [% C] + 220 × [% Si] −80 × [% Mn] ≦ 150 (1)

  In the formula (1), [% C], [% Si] and [% Mn] are the contents (mass%) of C, Si and Mn, respectively. The coefficient in the middle of equation (1) is the degree of influence of each element of C, Si, and Mn on the temperature at which a bainite phase is obtained in the cooling and winding process after hot rolling when manufacturing a hot-rolled steel sheet. Is shown. In addition, the smaller the value of the middle side of the equation (1), the narrower the temperature range at which the bainite phase is obtained, and the material stability of the hot-rolled steel sheet decreases.

  In the present invention where the material stability of the hot-rolled steel sheet containing bainite as the main phase is required, the value of the middle side of the formula (1) is preferably 15 or more. On the other hand, if the value of the middle side of the formula (1) becomes excessively large, the ferrite phase may precipitate and the strength of the hot-rolled steel sheet may be reduced. In order to suppress the precipitation of the ferrite phase, which causes a decrease in strength, it is preferable to set the value of the middle side of the formula (1) to 150 or less. More preferably, it is 30 or more and 140 or less.

P: 0.03% or less
P segregates at the grain boundaries and becomes the starting point of grain boundary cracking during steel processing. Thus, since P is a harmful element that deteriorates the workability of the hot-rolled steel sheet, it is preferable to reduce its content as much as possible. In the present invention, the P content is limited to 0.03% or less in order to avoid the above problems. Preferably it is 0.02% or less.

S: 0.005% or less
S exists as an inclusion such as MnS in steel. This inclusion extends during hot rolling when producing a hot-rolled steel sheet. The inclusions thus extended adversely affect the workability of the hot-rolled steel sheet because it becomes a starting point of cracking during processing. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and it is 0.005% or less. Preferably it is 0.003% or less.

Al: 0.08% or less
Al is an element that acts as a deoxidizer. In order to obtain such an effect, the Al content is preferably 0.02% or more. On the other hand, Al forms an oxide or the like and becomes a starting point of voids during bending, so if the Al content exceeds 0.08%, an adverse effect on the bendability of the hot-rolled steel sheet becomes obvious. Therefore, the Al content is 0.08% or less. Preferably it is 0.06% or less.

N: 0.0080% or less
N combines with Ti at the stage of steelmaking and continuous casting to form TiN. Coarse TiN tends to be a ferrite nucleation site. In addition, when TiN is formed, solid solution Ti decreases, so the hardenability of the steel decreases and a ferrite phase is easily generated, and it is difficult to obtain a hot rolled steel sheet with a tensile strength of 980 MPa or more. Become. Therefore, in the present invention, it is necessary to suppress the formation of coarse TiN, and the N content is limited to 0.0080% or less. Preferably it is 0.0070% or less. However, if N is reduced too much, the effect of suppressing the growth of crystal grains by TiN is lost, the crystal grains become coarse and the toughness of the steel sheet decreases. For this reason, it is preferable that N content shall be 0.0020% or more.

Ti: 0.09% to 0.14%
Ti, when present in the hot-rolled steel sheet in a solid solution state, is an element that suppresses nucleation of the ferrite phase and substantially contributes to increasing the strength of the hot-rolled steel sheet. In order to obtain such an effect, the Ti content needs to be 0.09% or more. On the other hand, when the Ti content is excessively high, coarse TiC cannot be dissolved in the heating stage of the slab (steel material) when manufacturing a hot-rolled steel sheet. Coarse TiC tends to be a stress concentration part, which reduces the toughness and workability of hot-rolled steel sheets. Therefore, the Ti content is 0.14% or less. Preferably it 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, like Ti, has an effect of suppressing nucleation of the ferrite phase in a solid solution state. One feature of the present invention is that Si is contained in an appropriate amount. However, when Si, which is a ferrite-forming element, is contained, ferrite transformation starts during cooling on the runout table, and the desired bainite structure is obtained. It can no longer be obtained. As a result of investigations by the present inventors, it has been confirmed that desired hardenability can be ensured even when a predetermined amount of Si is contained by adding Ti and V in combination. It was. In addition to the effect of improving hardenability, it was also confirmed that when an appropriate amount of V is contained, the lath structure of bainite is refined. In order to obtain these effects, the V content needs to be 0.05% or more. On the other hand, if the V content exceeds 0.25%, the toughness of the hot-rolled steel sheet is significantly reduced. Therefore, the V content is limited to 0.25% or less. Preferably it is 0.08% or more and 0.23% or less.

Cr: 0% to 0.06%, Ni: 0% to 0.06%, Mo: 0% to 0.06%
Cr, Ni, and Mo are all elements that lower the bainite start temperature in the cooling / winding step after the hot rolling when manufacturing a hot-rolled steel sheet. Therefore, when the content of Cr, Ni, and Mo increases, cooling at the runout table reaches the transition boiling region, and the coiling temperature becomes unstable, resulting in deterioration of the material stability of the hot rolled steel sheet. For these reasons, it is desirable to reduce the contents of Cr, Ni, and Mo as much as possible. However, each of up to 0.06% is acceptable, so the upper limit was set to 0.06%. Further, the content of these elements is preferably 0.04% or less, and may be reduced to the impurity level. Furthermore, the content of these elements is more preferably 0%. The total content of these elements is preferably 0.1% or less.

The above is the basic composition of the hot-rolled steel sheet of the present invention, but in addition to the basic composition described above, the following elements may be further contained.
B: 0.0001% to 0.005%
B is an element that easily segregates at grain boundaries, has an effect of suppressing the start of austenite → ferrite transformation, and enhances the material stability of the hot-rolled steel sheet. In order to obtain such an effect, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.005%, the above effect is saturated, so 0.005% or less is preferable. More preferably, it is 0.0003% or more and 0.003% or less.

Moreover, the hot-rolled steel sheet of the present invention may contain the following elements.
Ca: 0.0001% or more and 0.005% or less, REM: One or two selected from 0.0001% or more and 0.005% or less
Ca and REM (REM: Sc, Y and lanthanoid elements with atomic numbers 57 to 71) are effective elements for controlling the form of inclusions in steel and suppressing the generation of voids generated from inclusions. . In order to obtain such an effect, it is preferable to contain one or more selected from Ca and REM, and the content of these elements is preferably 0.0001% or more. On the other hand, since the above effects are saturated when the content of these elements exceeds 0.005%, the content of Ca and REM is preferably 0.005% or less. Further, it is more preferable to set Ca: 0.0003% to 0.002% and REM: 0.0003% to 0.002%. Further, when both Ca and REM are contained, the total content of Ca and REM is preferably 0.0002% or more and 0.006% or less, and more preferably 0.0003% or more and 0.002% or less.

In the hot rolled steel sheet of the present invention, components other than those described above are Fe and inevitable impurities.
Inevitable impurities include, for example, 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 and the like, and the content of these is preferably 0.1% or less in total.

Next, the reason for limiting the structure of the hot-rolled steel sheet of the present invention will be described.
Area ratio of bainite phase: 80% or more The hot-rolled steel sheet of the present invention has a bainite phase that is excellent in strength-toughness balance as a main structure. The bainite phase in the present invention is intended for upper bainite, lower bainite, and bainitic ferrite.

  When the area ratio of the bainite phase is less than 80%, a hot-rolled steel sheet having a tensile strength of 980 MPa or more cannot be obtained, and the material variation increases. Therefore, the area ratio of the bainite phase is 80% or more. It is preferably over 85%, and more preferably 90% or more in the case of a hot-rolled steel sheet that particularly requires material stability.

Since it is preferable that the hot-rolled steel sheet of the present invention has a bainite single-phase structure, the case where the area ratio of the bainite phase is 100% is included.
When a structure other than the bainite phase is contained, examples of the structure include a ferrite phase, a martensite phase, and a retained austenite phase. However, when the martensite phase and the retained austenite phase are present, the material stability of the hot-rolled steel sheet is lowered. Therefore, the total area ratio of the martensite phase and the retained austenite phase is preferably 7% or less, and more preferably 5% or less.

Fe precipitation amount: 0.50% or less The hardness (strength) of the bainite phase greatly decreases as the amount of dissolved C decreases. Therefore, in order to obtain a tensile strength of 980 MPa or more in a hot-rolled steel sheet having bainite as the main phase, it is necessary to ensure a solid solution C amount of a certain amount or more. In the case of a hot-rolled steel sheet containing bainite as a main phase, C that is not in a solid solution state is mainly precipitated as cementite. Therefore, if the precipitation of cementite is suppressed, a sufficient amount of solute C can be ensured, and the desired hot-rolled steel sheet strength can be obtained.

The precipitation amount of cementite (Fe ₃ C) can be determined by analyzing the precipitation amount of Fe. And when the amount of Fe precipitation exceeds 0.50% by mass%, the amount of cementite precipitation increases and the amount of solute C becomes insufficient. As a result, the desired hot-rolled steel sheet strength cannot be obtained. Therefore, the Fe precipitation amount is limited to 0.50% or less. Preferably it is 0.40% or less.

  The hardness (strength) of the bainite phase is also affected by the average lath interval of the bainite lath. The bainite lath has a rectangular shape, and the long side length and the short side length can be defined. When the short side length is the lath interval, when the average lath interval exceeds 400 nm, the material stability of the hot-rolled steel sheet tends to deteriorate. Accordingly, the average lath interval of the bainite phase is preferably 400 nm or less. As described above, V has the effect of reducing the lath of the bainite phase. In order to stably obtain a structure having an average lath interval of the bainite phase of 400 nm or less, the V content is preferably 0.1% or more.

  As described above, by optimizing the composition and structure of the hot-rolled steel sheet, a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more excellent in weldability and material stability can be obtained. The thickness of the hot-rolled steel sheet of the present invention is not particularly limited, but is preferably 1.6 mm or more and 10 mm or less.

Next, the manufacturing method of this invention hot rolled sheet steel is demonstrated.
In the present invention, a steel material (steel slab) having the above composition is heated and hot-rolled, and then cooled, wound, and made into a hot-rolled steel sheet. At this time, the heating temperature of the heating is 1150 ° C. or more and 1350 ° C. or less, the hot rolling finish rolling temperature is 850 ° C. or more, the cooling is started within 3 s after finishing the hot rolling finish rolling, and the cooling The average cooling rate is 15 ° C./s or more, and the winding temperature immediately before the winding step is 350 ° C. or more and 550 ° C. or less.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, the slab (steel material) is preferably formed by a continuous casting method from the viewpoint of productivity and quality, but the slab may be formed by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. . TiN precipitates mainly during continuous casting. However, as described above, coarsened TiN causes a reduction in the strength of the hot-rolled steel sheet. In order to suppress the coarsening of TiN, it is preferable to suppress the TiN particle growth by setting the casting speed during continuous casting to 1.0 m / min or more and to reduce the TiN size to 5 μm or less.

Heating temperature of the steel material: 1150 ° C. or higher and 1350 ° C. or lower The steel material obtained as described above is hot-rolled. In the present invention, the steel material is heated prior to hot rolling to be substantially homogeneous austenite. As a phase, it is necessary to dissolve coarse carbides. When the heating temperature of the steel material is below 1150 ° C., coarse carbides do not dissolve, so the amount of solute C decreases, and the strength of the finally obtained hot-rolled steel sheet significantly decreases. On the other hand, when the heating temperature exceeds 1350 ° C., the scale bites and deteriorates the surface properties of the steel sheet.

  For the above reasons, the heating temperature of the steel material is set to 1150 ° C or higher and 1350 ° C or lower. Preferably they are 1150 degreeC or more and 1320 degrees C or less. However, when hot rolling the steel material, if the steel material after casting is in the temperature range of 1150 ° C or higher and 1350 ° C or lower, or if the carbide of the steel material is dissolved, the steel material is heated. Direct rolling may be performed without any problem. The holding time of the steel material at the above heating temperature is not particularly limited. However, if the holding time becomes too long, a decarburized layer is formed on the surface of the steel material, and there is a concern about a decrease in fatigue resistance and a decrease in yield due to scale loss. . Therefore, the holding time of the steel material at the heating temperature is preferably less than 3600 s, and more preferably 2400 s or less. The holding time is a holding time in a temperature range of 1200 ° C. or higher at which scale growth becomes remarkable.

  After the steel material is heated to the above heating temperature, hot rolling is performed. Hot rolling usually consists of rough rolling and finish rolling, but the rough rolling conditions are not particularly limited. For example, when casting a slab (steel material) by a thin slab continuous casting method, rough rolling may be omitted.

Hot rolling finish rolling temperature: 850 ℃ or more
When rolling at a temperature lower than 850 ° C., the rolling load of the steel of the present invention is remarkably increased, making it difficult or impossible to manufacture. Further, when rolling at a temperature lower than 850 ° C., the austenite phase is excessively processed, so that the austenite → ferrite transformation proceeds in the cooling process after finish rolling, and a desired structure cannot be obtained. For the above reasons, the finish rolling temperature is 850 ° C. or higher. Preferably, it is 870 degreeC or more and 960 degrees C or less.

Time to start forced cooling after finishing rolling: within 3s If more than 3 seconds elapse after finishing rolling to start forced cooling, austenite → ferrite transformation starts and the desired The organization cannot be obtained. Moreover, if the steel plate after finish rolling is kept in a high temperature state for a long time, carbides are generated by strain-induced precipitation, and the amount of solute C that contributes to increasing the strength of the steel plate is reduced. Therefore, in the present invention, for the purpose of suppressing ferrite transformation and the purpose of suppressing strain-induced precipitation, it is necessary to start forced cooling immediately after completion of hot rolling, and forced cooling within at least 3 seconds after finishing rolling. To start. Preferably it is within 2 s.

Average cooling rate: 15 ° C./s or more After the finish rolling, in order to suppress the start of austenite → ferrite transformation, it is necessary to cool to the coiling temperature as quickly as possible. When the average cooling rate of forced cooling after finish rolling is less than 15 ° C./s, a ferrite phase is generated and a desired structure cannot be obtained. Therefore, the average cooling rate after finish rolling is 15 ° C./s or more. Preferably it is 30 ° C./s or more. However, particularly in a steel sheet having a thickness of 3.2 mm or less, if the cooling rate is excessively increased, it becomes difficult to control the cooling stop temperature, and the material stability of the hot-rolled steel sheet is lowered. Therefore, the average cooling rate is preferably 150 ° C./s or less.

Winding temperature: 350 ° C. or higher and 550 ° C. or lower In the steel of the present invention, an appropriate winding temperature for obtaining a bainite phase is 350 ° C. or higher and 550 ° C. or lower. When the coiling temperature is lower than 350 ° C., a martensite phase and a retained austenite phase are generated, and the material stability of the hot-rolled steel sheet is lowered. On the other hand, when the coiling temperature exceeds 550 ° C., the ferrite transformation proceeds, 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 350 ° C. or higher and 550 ° C. or lower. A preferable winding temperature range is 350 ° C. or more and 500 ° C. or less. The temperature at which forced cooling is stopped is preferably 350 ° C. or higher and 550 ° C. or lower, and more preferably 350 ° C. or higher and 500 ° C. or lower, like the coiling temperature.

  A steel material having a thickness of 250 mm having the composition shown in Table 1 was hot-rolled under the hot rolling conditions shown in Table 2 to obtain a hot-rolled steel plate having a thickness of 2.0 to 8.0 mm. In addition, the average cooling rate described in Table 2 is an average cooling rate from the finish rolling temperature to the cooling stop temperature.

  The obtained hot-rolled steel sheet was observed for structure, and the area ratio of the bainite phase and the average lath spacing of the bainite lath were obtained. Moreover, the extraction residue analysis was performed about the obtained hot-rolled steel plate, and the amount of Fe precipitation was calculated | required. Furthermore, the obtained hot-rolled steel sheet was subjected to a tensile test and a welding test to evaluate mechanical properties (strength, material stability, etc.) and weldability. The methods of tissue observation, extraction residue analysis, and various tests were as follows.

(1) Microstructure observation The area ratio of the bainite phase In the obtained hot-rolled steel sheet, the corrosion appearing structure with 5% nital was magnified 1000 times with a scanning optical microscope at the central part of the cross section parallel to the rolling direction. I took 10 fields of view. The bainite phase is a structure having a form in which corrosion marks and cementite are observed in the grains. The area ratio of the bainite phase was determined by separating the bainite phase and the phases other than the bainite phase (ferrite phase, martensite phase, etc.) by image analysis and calculating the area ratio of the bainite phase with respect to the observation field.

Average lath spacing Samples were prepared from the center of the thickness of the obtained hot-rolled steel sheet by a thin film method, and observed with a transmission electron microscope (magnification: 135000 times). The interval was measured, and the average value of the obtained lath intervals was defined as the average lath interval.

(2) Extraction residue analysis A sample was taken from the center of the thickness of the obtained hot-rolled steel sheet and about 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol) 0.2 g was subjected to constant current electrolysis at a current density of 20 mA / cm ² . Next, the electrolytic solution after constant-current electrolysis was collected by filtration with a 0.2 μm filter, and the amount of Fe contained in the collected product was quantified with an ICP emission spectrometer. The amount of precipitated Fe (mass ratio) was determined from the electrolyzed sample mass and the amount of Fe contained in the collected material.

(3) Tensile test Strength characteristics From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece with the tensile direction perpendicular to the rolling direction was prepared, and a tensile test in accordance with the provisions of JIS Z 2241 (2011) was performed three times. The average yield strength (YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed in the tensile test was 10 mm / min. The yield strength was the yield point or 0.2% proof stress.

Material stability (variation in tensile strength)
From the obtained hot-rolled steel sheet, a total of 153 JIS No. 5 tensile test pieces whose tensile direction was perpendicular to the rolling direction were produced. Specifically, for each hot-rolled steel sheet (hot-rolled coil) after winding, 51 longitudinal positions (positions obtained by dividing the coil longitudinal direction into 50 equal parts, the tip position in the coil longitudinal direction, and the tail in the coil longitudinal direction) The edge position was specified, and at each longitudinal position, three JIS No. 5 tensile test specimens were collected from the central portion in the plate width direction for each longitudinal position.
For each hot-rolled steel sheet, 153 tensile test specimens were used to conduct a tensile test in accordance with the provisions of JIS Z 2241 (2011), and the standard deviation (tensile strength) of all tensile test specimens (total of 153 specimens) Variation).

(4) Welding test Arc welding was performed using the obtained hot-rolled steel sheet to produce a welding sample. The weld sample was prepared by butt-welding sides of a length (300 mm) of a sample having a width of 200 mm and a length of 300 mm taken from the same hot-rolled steel sheet. A sample having a width of 200 mm and a length of 300 mm was collected so that the length direction of the sample coincided with the rolling direction. Welding conditions are: plate gap: 1mm, welding current: 180A, welding voltage: 20V, welding wire: Kobe Steel MG-50 (wire diameter: 1.2mm), welding speed: 80cm / min, shielding gas: CO ₂ (80 %) + Ar (20%) butt welding.
Next, three JIS No. 5 tensile test pieces were prepared from each welded sample, and when evaluating the “strength characteristics” of the “(3) tensile test” in accordance with the provisions of JIS Z 2241 (2011). A tensile test was performed under the same tensile conditions as the tensile test performed. The JIS No. 5 tensile test piece was prepared so that the weld bead portion of the weld sample crossed in the width direction of the test piece at the center of the distance between the test piece scores. The crosshead speed in the tensile test was 10 mm / min.
The maximum load until breakage was measured by a tensile test, and a value obtained by dividing the maximum load by the plate thickness in order to eliminate the influence of the plate thickness was obtained. Moreover, the fracture position of the weld sample was confirmed.

  The above results are shown in Table 3. In Table 3, the tensile strength is 980 MPa or more, the variation in tensile strength (standard deviation of tensile strength) is 40 MPa or less, and the value obtained by dividing the maximum load of the weld sample tensile test by the plate thickness is 22 kN / mm or more. In addition, when the fracture in the weld sample tensile test was a base material fracture, the evaluation was good as “◯” as the material required in the present invention. On the other hand, if any one of the above conditions is not satisfied, the evaluation is “bad”.

  All of the hot-rolled steel sheets of the present invention have a tensile strength TS: 980 MPa or more, and are excellent in material stability and weldability. On the other hand, the hot-rolled steel sheet of the comparative example outside the scope of the present invention does not have a predetermined high strength, or does not have good material stability and weldability.

Claims (6)

% By mass
C: 0.09% or more and 0.17% or less, Si: more than 1.0%, 1.6% or less,
Mn: 1.5% to 2.5%, 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 are contained so as to satisfy the following formula (1), the contents of Cr, Ni and Mo are limited to 0.06% or less (including 0%) respectively, and the balance is Fe and inevitable It has a composition consisting of impurities, has an area ratio of bainite phase of 80% or more, has a structure with Fe precipitation of 0.50% or less, tensile strength of 980 MPa or more, and standard deviation of tensile strength of 40 MPa or less A high-strength hot-rolled steel sheet with excellent material stability and weldability, characterized in that the maximum load / thickness of the weld sample tensile test is 22 kN / mm or more and the base material is fractured .
Record
15 ≦ −198 × [% C] + 220 × [% Si] −80 × [% Mn] ≦ 150 (1)
Here, [% C], [% Si], and [% Mn] are the contents (mass%) of C, Si, and Mn, respectively.   The high-strength hot-rolled steel sheet having excellent material stability and weldability according to claim 1, further comprising B: 0.0001% to 0.005% by mass% in addition to the composition.   3. In addition to the composition, the composition further contains one or two kinds selected from Ca: 0.0001% to 0.005% and REM: 0.0001% to 0.005% by mass%. A high-strength hot-rolled steel sheet with excellent material stability and weldability as described in 1. After heating the steel material, applying hot rolling, cooling, winding, and hot rolling steel sheet,
The steel material in mass%,
C: 0.09% or more and 0.17% or less, Si: more than 1.0%, 1.6% or less,
Mn: 1.5% to 2.5%, 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 are contained so as to satisfy the following formula (1), the contents of Cr, Ni and Mo are limited to 0.06% or less (including 0%) respectively, and the balance is Fe and inevitable The composition is composed of impurities, the heating temperature of the heating is 1150 ° C. or more and 1350 ° C. or less, the finish rolling temperature of the hot rolling is 850 ° C. or more, and the cooling is started within 3 s after finishing the hot rolling finish rolling. The average cooling rate of the cooling is 15 ° C./s or more, the winding temperature of the winding is 350 ° C. or more and 550 ° C. or less , the area ratio of the bainite phase is 80% or more, and the Fe precipitation amount is 0.50% or less. A hot-rolled steel sheet with a tensile strength of 980 MPa or more, a standard deviation of tensile strength of 40 MPa or less, a maximum load / thickness of the weld sample tensile test of 22 kN / mm or more, and a base metal fracture. A method for producing a high-strength hot-rolled steel sheet having excellent material stability and weldability.
Record
15 ≦ −198 × [% C] + 220 × [% Si] −80 × [% Mn] ≦ 150 (1)
Here, [% C], [% Si], and [% Mn] are the contents (mass%) of C, Si, and Mn, respectively.   The method for producing a high-strength hot-rolled steel sheet having excellent material stability and weldability according to claim 4, further comprising, in addition to the composition, B: 0.0001% to 0.005% by mass.   6. In addition to the composition, it further contains one or two selected from Ca: 0.0001% to 0.005% and REM: 0.0001% to 0.005% by mass%. A method for producing a high-strength hot-rolled steel sheet having excellent material stability and weldability as described in 1.