JP5895772B2 - High-strength hot-rolled steel sheet with excellent appearance and excellent isotropic toughness and yield strength and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a yield strength of 900 MPa or more, which has excellent appearance and isotropic toughness and yield strength, and a method for producing the same.

  Construction crane booms tend to be longer. Therefore, in order to reduce the weight of the boom itself and increase the lifting and carrying capacity, there is a high demand for a steel sheet that has a higher yield point and is excellent in bending workability and toughness for the steel sheet that is the material. Furthermore, in order to ensure the member performance at a high level, it is desired to reduce the anisotropy of bending workability, toughness, and yield strength. Moreover, since the boom of a crane is used with the hot-rolled scale remaining, it is desired that the steel plate has an excellent appearance from the viewpoint of surface properties for improving dimensional accuracy and vehicle appearance.

  Patent Document 1 discloses a technique for improving the C bending performance by increasing the strength by precipitation of TiC in ferrite by addition of Ti, and changing from MnS of A-based inclusions to TiS of C-based inclusions. .

  Patent Document 2 discloses a method for manufacturing a steel sheet that is excellent in isotropic workability and toughness by adding an appropriate amount of Ti, and a technique that reduces material fluctuations in the steel sheet.

Japanese Examined Patent Publication No. 3-65425 JP 2010-156016 A

  The invention described in Patent Document 1 aims to increase the strength by precipitation of TiC in ferrite by adding Ti. Continuous hot rolling provided with a rough rolling mill, a finishing rolling mill in which a plurality of finishing rolling mills are arranged in series, a cooling device (hereinafter referred to as an ROT cooling device) provided on a run-out table, a winder, etc. The apparatus discloses continuous finish rolling with a high rolling rate of 900 ° C. or less. In this case, the crystal grains of the structure become finer and high toughness can be obtained, but ferrite and bainite show strong orientation accumulation, and the anisotropy of yield strength and toughness is very strong.

  Although the invention described in Patent Document 2 makes it possible to improve the isotropy of bending workability and toughness of the steel sheet, the absolute value of the absorbed energy is reduced by the addition of Si, and the absolute value of the absorbed energy is I found it inferior. In addition, as disclosed in Patent Document 1, it is aimed to make the structure finer and tougher by performing continuous finish rolling with a high reduction rate of 900 ° C. or lower. By continuously finishing and rolling at a low temperature, a large amount of Ti-based precipitates are precipitated, the elongation of MnS is prevented, and the anisotropy is reduced by the martensite structure. However, in order to perform continuous finish rolling at a low temperature, it is necessary to reduce the inlet side temperature of the first finish rolling mill of the continuous rolling mill row of the continuous hot rolling apparatus to less than 1000 ° C. as much as possible from the viewpoint of productivity. It has been newly found that martensite is low in strength relative to the amount of C added in the steel and that toughness deterioration occurs due to brittle cracks starting from TiN. Moreover, since Si is added, Si becomes a scale and becomes a pattern, which causes the appearance to deteriorate.

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a yield strength of 900 MPa or more, which is excellent in appearance and excellent in toughness and isotropic yield strength, and a method for producing the same.

  In order to improve the appearance of the hot-rolled steel sheet, the present inventors are effective to reduce the amount of Si, to achieve both high yield strength and toughness, and to reduce the anisotropy of yield strength and toughness. It was found that it is effective to make the steel sheet structure martensite and to make the upper limit of the TiC precipitation amount 0.05% or less. Further, the A-based inclusions shown in Patent Documents 1 and 2 are involved not only in toughness and bending workability but also in the anisotropy of yield strength, and the A-based inclusions are 0.010% clean. It was found that a steel sheet excellent in isotropy can be produced when the temperature is less than 1 degree. As the means, as described in Patent Documents 1 and 2, suppression of A-based inclusions by addition of Ti is effective. However, as described in Patent Document 1, Ti precipitates as TiC during finish rolling, so that austenite The solid solution C is decreased, and the strength of martensite in the steel sheet structure is decreased. The present inventors analyzed the relationship between hot rolling conditions and Ti precipitates in detail, and the effect of the inlet temperature of the first finishing mill in the finishing mill line of the continuous hot rolling apparatus on the TiC precipitation amount and steel plate strength. It was found that by controlling the amount of TiC within an appropriate range, it is possible to achieve both suppression of A-based inclusions and steel plate strength.

  The present invention has been made based on the above findings, and provides a high-strength hot-rolled steel sheet having a yield strength of 900 MPa or more and excellent in appearance and isotropic toughness and yield strength, and a method for producing the same.

That is, the gist of the present invention is as follows.
(1) By mass% C: 0.04% or more, 0.15% or less,
Si: 0.01% or more, 0.25% or less,
Mn: 0.1% or more, 2.5% or less,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005% or more, 0.05% or less,
N: 0.01% or less,
Ti: 0.08% or more, 0.12% or less B: 0.0003% or more, 0.0050% or less,
Remaining: having a chemical composition composed of Fe and inevitable impurities, 90% or more of the structure is martensite, TiC precipitation is 0.05% or less, and cleanliness of A-based inclusions defined in JISG0202 There Ri der 0.010% or less, the absorbed energy in the Charpy test ratio in the rolling direction and the width direction at least 0.6, 1.2 or less, more tensile ratio in the rolling direction and the width direction in the yield strength of the test A high-strength hot-rolled steel sheet having an excellent appearance characterized by being 0.8 or more and 1.2 or less and having a yield strength of 900 MPa or more, and excellent isotropic toughness and yield strength.
(2) Further, by mass% Nb: 0.01% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0030% or less Ni: 0.02% or more, 3.0% or less,
Mo: 0.02% or more, 0.5% or less,
Cr: 0.02% or more, 1.0% or less,
A high-strength hot-rolled steel sheet having excellent appearance and excellent isotropic toughness and yield strength, according to claim 1.
(3) The continuous casting slab having the chemical composition described in the above (1) or (2) is reheated, and finished continuously by rough rolling and a finishing rolling mill in which a plurality of finishing rolling mills are arranged in series. After rolling, when cooling and winding, the reheating temperature is set to 1200 ° C. or higher, the entrance temperature of the finishing mill in the first stage of the finishing rolling mill row is set to 1000 ° C. or higher and 1100 ° C. or lower, and the final rolling mill row is finished. As described in (1) or (2) above , the exit temperature of the step finishing mill is Ar3 point or higher, cooled at an average cooling rate of 15 ° C / second or higher, and wound at 200 ° C or lower . A method for producing a high-strength hot-rolled steel sheet with excellent appearance and isotropic toughness and yield strength.

  The present invention relates to a high strength hot rolled steel sheet having a yield strength of 900 MPa or more, containing a predetermined component, 90% or more of the structure is martensite, the precipitation amount of TiC is within an appropriate range, and M is caused by MnS. By controlling the cleanliness of system inclusions to 0.010% or less, the surface appearance is excellent, the steel sheet structure is highly uniform, and the toughness and yield strength isotropic are high strength over 900 MPa. A hot-rolled steel sheet can be provided. Specifically, the absorbed energy in the Charpy test specified in JISZ 2242 is a ratio of the rolling direction to the width direction of 0.6 or more and 1.2 or less, and the yield strength is a ratio of the rolling direction to the width direction of 0.8. As mentioned above, 1.2 or less can be satisfied.

  In addition, the addition amount of Si and Ti is optimized, the entrance temperature of the finishing mill in the first stage of the finishing mill row is 1000 ° C. or more and 1100 ° C. or less, and the exit temperature of the finishing mill in the last stage of the finishing mill train Is made at an Ar3 point or higher, cooled at an average cooling rate of 15 ° C./second or higher, and wound at a low temperature of 200 ° C. or lower, so that the uniformity of the steel sheet structure is high, the precipitation amount of TiC is in an appropriate range, and MnS is caused. The A-type inclusions are suppressed, and the predetermined metal structure can be obtained.

  The present invention relates to a high strength hot rolled steel sheet having a yield strength of 900 MPa or more, containing a predetermined component, 90% or more of the structure is martensite, the precipitation amount of TiC is within an appropriate range, and M is caused by MnS. By suppressing the cleanliness of the system inclusions to 0.010% or less, the steel sheet has excellent surface appearance, high uniformity of the steel sheet structure, and isotropically improved toughness and yield strength isotropy. Is.

  In addition, the addition amount of Si and Ti is optimized, the entrance temperature of the finishing mill in the first stage of the finishing mill row is 1000 ° C. or more and 1100 ° C. or less, and the exit temperature of the finishing mill in the last stage of the finishing mill train Is made at an Ar3 point or higher, cooled at an average cooling rate of 15 ° C./second or higher, and wound at a low temperature of 200 ° C. or lower, so that the uniformity of the steel sheet structure is high, the precipitation amount of TiC is in an appropriate range, and MnS is caused. The A-type inclusions are suppressed, and the predetermined metal structure can be obtained.

The individual constituent requirements of the present invention will be described in detail below.
First, the reasons for limiting the components of the present invention will be described.

  C is an important element that determines the strength of the present invention. In order to obtain the desired strength, it is necessary to contain 0.04% or more. Preferably it is 0.06% or more. However, if the content exceeds 0.15%, the toughness deteriorates, so the upper limit is made 0.15%.

  Si is an element necessary for preliminary deoxidation and is effective for increasing the strength as a solid solution strengthening element. The addition amount necessary for the preliminary deoxidation is 0.01% or more. However, if added excessively, the surface appearance is impaired, so the upper limit is made 0.25%.

  Mn is effective for increasing the strength as a hardenability and solid solution strengthening element. In order to obtain the desired strength, 0.1% or more is necessary. If added excessively, MnS harmful to toughness isotropic properties is generated, so the upper limit is made 2.5% or less.

  P is preferably as low as possible, and if contained in excess of 0.1%, workability and weldability are adversely affected and fatigue characteristics are also reduced.

  S is desirably as low as possible, and if it is too large, inclusions such as MnS that are harmful to toughness isotropic properties are generated. When severe low temperature toughness is required, the content is preferably 0.006% or less.

  Since Al is an element necessary for deoxidation of molten steel, it is necessary to contain 0.005% or more in order to obtain the effect. However, when added excessively, alumina precipitated in a cluster form is generated and the toughness is deteriorated, so the upper limit is made 0.05%.

  N forms Ti and precipitates at a higher temperature than S, and not only reduces Ti effective for fixing S but also forms coarse Ti nitrides and deteriorates toughness. Therefore, it should be reduced as much as possible, but it is acceptable if it is 0.01% or less.

  Ti, like S, is an element that affects the isotropic toughness in the present invention. Precipitation of TiS can render drawing inclusions such as MnS harmless, improve low temperature toughness, and improve isotropic properties. Therefore, it is necessary to add 0.01% or more, but even if it contains more than 0.12%, the effect is not only saturated but also the alloy cost is increased. Therefore, the Ti content is 0.01% or more and 0.12% or less.

  By adding B, the grain boundary strength can be increased and the toughness can be improved. When the B content is 0.0003% or more, an effect of improving toughness is obtained. On the other hand, since the effect is saturated even if it is added more than 0.0050%, the upper limit is made 0.0050% or less.

  Although not essential for satisfying the required characteristics, it is preferable to add the following elements in order to reduce manufacturing variability and further improve toughness.

  Nb can reduce the crystal grain size of the hot-rolled steel sheet and further increase toughness. The effect is obtained when the Nb content is 0.01% or more. On the other hand, if it exceeds 0.10%, the effect is saturated, so the upper limit is made 0.10%.

  Ca is a suitable element for refining the structure by dispersing many fine oxides in the deoxidation of molten steel, and fixing the S in the steel as spherical CaS for desulfurization of the molten steel. It is an element that suppresses the generation of objects and improves hole expansibility. These effects are obtained when the addition amount is 0.0005%, but since saturation occurs at 0.0030%, the Ca content is set to 0.0005% or more and 0.0030% or less.

  Ni is an effective element for improving low temperature toughness. In order to obtain this effect, it is desirable to add 0.02% or more. However, the addition of a large amount delays the ferrite transformation, so the upper limit is made 3.0%.

  Mo is an element effective for improving the hardenability. In order to obtain this effect, addition of 0.02% or more is desirable. However, the addition of a large amount increases the cracking susceptibility of the slab and makes it difficult to handle the slab, so the upper limit is made 0.5%.

  Cr is an effective element for improving hardenability. In order to obtain this effect, addition of 0.02% or more is necessary. However, the addition of a large amount decreases the ductility, so the upper limit is made 1.0%.

  Next, the crystal structure of the steel sheet of the present invention will be described.

  The steel sheet of the present invention must have a microstructure of 90% or more martensite, a TiC precipitation amount of 0.05% or less, and a cleanness of A-based inclusions defined in JIS G0202 of 0.010% or less. It is.

  When 90% or more of the structure is martensite, the crystal orientation is randomized, and the material anisotropy due to the structure of the tensile test or the Charpy test can be made harmless.

  The A-based inclusions in the steel sheet are obtained by extending a soft inclusion such as MnS in the rolling direction by hot rolling. These inclusions become the starting point of cracking when the notch direction of the Charpy specimen is tested in the rolling direction or when a tensile test is performed in the rolling direction, and there is a difference in the material when the test is taken in a direction perpendicular to the rolling direction. .

  If the cleanliness of the A-based inclusions is 0.010% or less, material anisotropy due to stretched inclusions is suppressed, and the ratio of the absorbed energy in the Charpy test in the rolling direction to the width direction is 0.6 or more. The ratio of the rolling direction to the width direction is 0.8 or more and 1.2 or less in the yield strength of the tensile test at 2 or less, and the material isotropy can be improved.

  It was newly found that the cleanliness of the A-based inclusion has a correlation with the amount of TiC deposited. There are two effects of TiC. One is that MnS precipitates with TiC as a nucleus, and therefore, there is an effect of finely dispersing MnS by dispersing fine TiC in the steel. Accordingly, there is an effect of reducing coarse stretch inclusions that are harmful to the anisotropy of the material. Second, since hard TiC and MnS are precipitated together, there is also an effect of making it difficult to stretch during rolling. In the present invention, the cleanliness of the A-based inclusions can be made 0.010% or less by depositing TiC.

  However, when TiC is excessively precipitated, the amount of C effective for the strength of martensite decreases, so the strength decreases. Therefore, the amount of TiC deposited for suppressing the stretch inclusions and ensuring the yield strength of 900 MPa is 0.02% or more and 0.05% or less.

  Next, a manufacturing method will be described.

  When hot rolling a continuous cast slab having the chemical composition of the present invention (hereinafter referred to as slab), the slab is first heated to 1200 ° C. or higher. When the slab is heated below 1200 ° C., TiC is not sufficiently dissolved in the slab, and C required for the strength of martensite is insufficient.

  The heated slab is subjected to rough rolling, and further, finish rolling is continuously performed by a finishing rolling mill row in which a plurality of finishing rolling mills are arranged in series. In the present invention, in order to ensure a yield strength of 900 MPa, the inlet side temperature of the first finishing mill in the finishing mill row is set to 1000 ° C. or higher and 1100 ° C. or lower, and the outlet side of the final rolling mill in the final rolling mill row is set. Finish rolling is performed at a temperature of Ar3 or higher.

  If the inlet temperature of the first finishing mill is less than 1000 ° C., TiC causes strain-induced precipitation, and TiC is excessively precipitated, so that the yield strength of 900 MPa is not satisfied. On the other hand, if the inlet side temperature of the first-stage finish rolling mill is higher than 1100 ° C., the amount of TiC deposited becomes insufficient, and the effect of suppressing stretching due to composite precipitation with MnS cannot be obtained sufficiently. In order to effectively utilize the strength of martensite with respect to the amount of C in the slab, it is preferable that TiC is small, but in order to make the cleanness of A-based inclusions 0.010% or less, 0.02 % Or more must be deposited. In order to minimize the decrease in the strength of martensite due to the precipitation of TiC, the upper limit needs to be 0.05%. If the inlet temperature of the first finishing mill is 1000 ° C. or higher and 1100 ° C. or lower, the TiC precipitation amount can be 0.02% or higher and 0.05% or lower.

  The reason for performing finish rolling with the exit side temperature of the finishing mill in the final stage of the finishing rolling mill set at Ar3 point or higher is that when hot rolling is performed in a two-phase region below Ar3 point, ferrite is generated during rolling, This is because the martensite fraction decreases. On the other hand, the upper limit of the exit side temperature of the finishing mill in the final stage of the finishing mill row need not be determined in order to obtain the effect of the present invention, but in order to prevent significant deterioration of the absolute value of toughness, It is desirable that the temperature is not higher than ° C.

  In the present invention, since 90% or more of the steel sheet structure needs to be martensite, after finish rolling, the steel sheet is cooled at an average cooling rate of 15 ° C./second or more and wound at a temperature of 200 ° C. or less. When the average cooling rate is less than 15 ° C./second, quenching becomes insufficient and a martensite structure cannot be formed. Moreover, when it winds up at the temperature over 200 degreeC, since a bainite will produce | generate or the self-annealing of a martensite will occur, yield strength is insufficient.

  Steel containing the components shown in Table 1 was melted in a converter and formed into a slab having a thickness of 230 mm by continuous casting. Thereafter, the slab is reheated to a temperature of 1200 ° C. to 1250 ° C., and a continuous heat provided with a roughing mill, a finishing mill row in which six finishing rolling mills are arranged in series, an ROT cooling device, a winder, and the like. Rough rolling and finish rolling were performed with a hot rolling device, and winding was performed after ROT cooling to produce a hot-rolled steel sheet. Table 2 shows the steel type symbols used, the hot rolling conditions, and the steel plate thickness. In Table 2, “FT0” is the inlet temperature of the finishing mill in the first stage of the finishing mill row, “FT6” is the outlet temperature of the finishing mill in the last stage of the finishing mill row, and “cooling rate” is finish rolling. The average cooling rate from the end of the process to the winding up, “winding temperature”, is the temperature taken up after the end of cooling.

  The steel plate thus obtained was measured for the martensite fraction and A-based inclusions using an optical microscope, and further measured for TiC precipitation, tensile test, Charpy test, bending workability evaluation test, appearance observation. Went.

  About the martensite fraction of the steel plate, the area ratio was obtained with a field of view of 500 × 500 μm using an optical microscope, and the cleanliness of the A-based inclusion was obtained by the method specified in JISG0202.

  The amount of TiC deposited was obtained by dissolving the steel material with an electrolytic solution and performing extraction residue analysis with a 0.1 μm mesh to determine the mass of TiC. Mass% was calculated by dividing this by the mass of the matrix dissolved in the extraction residue analysis.

  For the steel sheet tensile test, JIS No. 5 test pieces were taken in the rolling direction (L direction) and width direction (C direction) of the steel sheet, yield strength: YP (MPa), tensile strength: TS (MPa), elongation: EL (%) Was evaluated.

  The absorbed energy was measured with a 5.0 mm sub-size V-notch test piece specified in JISZ2242, and a Charpy impact test was conducted at −20 ° C. with the notch in the rolling direction (L direction) and the width direction (C direction) of the steel sheet. It was.

  Bending workability is a JIS No. 1 test piece. The test piece is taken from the rolling direction (L direction) and the width direction (C direction), and the bending radius is 4 times the plate thickness x 4 times. The cracked material was designated as “x”.

  For the evaluation of the appearance of the steel sheet, the hot rolled coil was cut in the longitudinal direction by 500 mm in the longitudinal direction and the area ratio of the scale pattern was measured. The case where the scale pattern was 10% or less of the whole was designated as “◯”. On the other hand, a scale pattern exceeding 10% was designated as “x”.

  Table 3 shows the evaluation results of the structure fraction, TiC precipitation, cleanliness and material of A-based inclusions, and appearance.

  As shown in Table 3, all of the inventive examples have a martensite fraction of 90% or more, a TiC precipitation amount of 0.05% or less, and a cleanness of the A-based inclusions of 0.010% or less. As a result, the absorbed energy ratio in the rolling direction and the width direction is 0.6 or more and 1.2 or less, and the yield strength satisfies the ratio of 0.8 or more and 1.2 or less. Is. The appearance is also “◯”.

  In Comparative Examples 4 and 22, since the cooling rate is less than 15 ° C./second, the martensite fraction is less than 90%, and the yield strength in the L direction does not satisfy 900 MPa. Also, the isotropic of absorbed energy and yield strength and the C-direction bending workability are inferior. In Comparative Examples 9 and 25, since FT0 is less than 1000 ° C., the TiC precipitation amount exceeds 0.05%, the effective C amount in martensite is low, and the yield strength of 900 MPa is not satisfied. In Comparative Example 12, since the coiling temperature is higher than 200 ° C., the martensite fraction is less than 90% and does not satisfy the yield strength of 900 MPa. Also, the isotropic of absorbed energy and yield strength and the C direction bending workability are inferior. In Comparative Example 16, since FT0 exceeds 1100 ° C., the amount of TiC deposited is less than 0.02%, the cleanliness of the A-based inclusion exceeds 0.010%, and the isotropic of absorbed energy and the C-direction bending workability are It is inferior. Since Comparative Example 29 exceeds the component upper limit of the S amount and the cleanliness of the A-based inclusion exceeds 0.010%, the isotropic of absorbed energy even if the hot rolling condition is within the scope of the invention, C direction bending workability is inferior. In Comparative Example 30, the upper limit of the Si content is exceeded, and there are many scale patterns in the appearance evaluation, which is “x”.

Claims (3)

In mass% C: 0.04% or more, 0.15% or less,
Si: 0.01% or more, 0.25% or less,
Mn: 0.1% or more, 2.5% or less,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.005% or more, 0.05% or less,
N: 0.01% or less,
Ti: 0.08% or more, 0.12% or less B: 0.0003% or more, 0.0050% or less,
Remaining: having a chemical composition composed of Fe and inevitable impurities, 90% or more of the structure is martensite, TiC precipitation is 0.05% or less, and cleanliness of A-based inclusions defined in JISG0202 There Ri der 0.010% or less, the absorbed energy in the Charpy test ratio in the rolling direction and the width direction at least 0.6, 1.2 or less, more tensile ratio in the rolling direction and the width direction in the yield strength of the test A high-strength hot-rolled steel sheet having an excellent appearance characterized by being 0.8 or more and 1.2 or less and having a yield strength of 900 MPa or more, and excellent isotropic toughness and yield strength. Furthermore, by mass% Nb: 0.01% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0030% or less Ni: 0.02% or more, 3.0% or less,
Mo: 0.02% or more, 0.5% or less,
Cr: 0.02% or more, 1.0% or less,
A high-strength hot-rolled steel sheet having excellent appearance and excellent isotropic toughness and yield strength, according to claim 1. The continuous casting slab having the chemical composition according to claim 1 or 2 is re-heated, rough-rolled, continuously finish-rolled in a finishing mill line in which a plurality of rolling mills are arranged in series, cooled, and wound. When performing the removal, the reheating temperature is set to 1200 ° C. or higher, the inlet temperature of the first finishing mill in the finishing mill row is set to 1000 ° C. or higher and 1100 ° C. or lower, and the outlet temperature of the final finishing mill is set at Ar 3 points. It is made into the above, It cools with the average cooling rate of 15 degrees C / sec or more, and it winds up at 200 degrees C or less, It is excellent in the external appearance of Claim 1 or Claim 2, and isotropic of toughness and yield strength A method for producing excellent high-strength hot-rolled steel sheets.