JP6212956B2 - High-strength hot-rolled steel sheet excellent in bending workability and wear resistance 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 950 MPa or more excellent in bending workability and wear resistance, 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 higher yield strength and excellent bending workability for the steel sheet that is the material. Furthermore, when used in excavators, it is desired to improve wear resistance from the viewpoint of reducing running costs.

  In Patent Document 1, an appropriate amount of Ti is added, and after hot rolling, a structure mainly composed of fine low-temperature transformation products such as martensite and bainite is formed by winding at 450 ° C. or less, and high strength is obtained. ing.

  In Patent Document 2, finish rolling is finished at the Ar3 transformation point or higher, cooling is started within 2 seconds, the cooling rate is set to over 150 ° C / s, and cooling is stopped at a temperature of 350 ° C or lower. A high-strength hot-rolled steel sheet excellent in bending workability, which has a structure mainly composed of martensite, has been proposed.

  In Patent Document 3, finish rolling is performed so that the cumulative reduction ratio in the recrystallized austenite region is 60% or more and 90% or less, and immediately cooled to a temperature of Ms point + 50 ° C. or less at a cooling rate higher than the martensite formation critical cooling rate. Therefore, a high-strength hot-rolled steel sheet excellent in low-temperature toughness having a structure in which the aspect ratio of prior austenite grains in the cross section in the rolling direction is 3 to 18 has been proposed.

JP-A-7-197186 JP 2003-105546 A JP 2011-52321 A

  The invention described in Patent Document 1 provides a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more excellent in delayed fracture resistance, but Patent Document 1 describes the bending workability and resistance to high-strength hot-rolled steel sheet. There are no studies on improvement of wear.

  The invention described in Patent Document 2 provides a high-strength hot-rolled steel sheet having a yield strength of 980 MPa or more excellent in bending workability. Patent Document 2 relates to improvement of wear resistance of the high-strength hot-rolled steel sheet. There is no examination.

  The invention described in Patent Document 3 provides a high-strength hot-rolled steel sheet having a yield strength of 960 MPa or more, which is excellent in low-temperature toughness, delayed fracture characteristics, and bending workability. There are no studies on improving wear resistance.

  As described above, a high-strength hot-rolled steel sheet having a yield strength of 950 MPa or more excellent in bending workability and wear resistance, which satisfies the properties required for a high-strength hot-rolled steel sheet for structural members of construction equipment. And the manufacturing method is not proposed.

  The present invention has been made in view of the above circumstances, and is suitable for structural members of construction equipment such as crane booms and excavators, and is a high-strength hot-rolled steel sheet having a yield strength of 950 MPa or more and excellent in bending workability and wear resistance. And it aims at providing the manufacturing method.

  The present inventors have studied the improvement of bending workability and wear resistance of a high-strength hot-rolled steel sheet, and obtained the following knowledge. By increasing the cumulative reduction ratio of finish rolling in the non-recrystallized austenite region, strain is accumulated in the austenite, and by forming a martensite structure from austenite with a high dislocation density, the surface hardness is improved and wear resistance is increased. A steel plate with excellent resistance is obtained. However, if the cumulative rolling reduction is only increased, cracks from the prior austenite grain boundaries on the surface are likely to occur, and the local ductility of the surface deteriorates. Thereby, the bending workability of a steel plate becomes inferior. By optimizing the cumulative reduction ratio of finish rolling in the non-recrystallized austenite region, the average aspect ratio of the prior austenite grains from the surface layer to the thickness 1/8 is optimized, and both bending workability and wear resistance are optimized. It has been found that a steel sheet having excellent characteristics can be obtained.

  The cumulative reduction ratio of finish rolling in the non-recrystallized austenite region means rolling at a temperature and time at which austenite recrystallization does not proceed between the stands of the finish rolling stand, and starting cooling after finishing finish rolling at the final stand. Up to this point, rolling is performed at a temperature and time at which recrystallization of austenite does not proceed, and the reduction ratio of each stand is added.

  When a part of austenite recrystallization progresses between the stands, the rolling reduction rate of each stand is increased to compensate for the reduction of processing strain due to recrystallization, and the processing strain equivalent to the cumulative strain as non-recrystallized austenite. May be rolled so that is accumulated.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
(1) C% by mass: 0.05% or more, 0.20% or less,
Si: 0.01% or more, 0.55% 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, 0.10% or less,
N: 0.01% or less,
Nb: 0.005% or more, 0.10% or less,
B: 0.0003% or more, 0.0050% or less,
The balance is a chemical composition composed of Fe and inevitable impurities, 90% or more of the structure is martensite, and the average aspect ratio of the prior austenite grains is 3 or more from the surface layer to the plate thickness 1/8 in the cross section in the rolling direction. A high-strength hot-rolled steel sheet excellent in bending workability and wear resistance, characterized by having a structure of 20 or less.
(2) Further, by mass: Ti: 0.01% or more, 0.12% 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, 1.0% or less,
Cr: 0.02% or more, 1.0% or less,
The high-strength hot-rolled steel sheet excellent in bending workability and wear resistance according to claim 1, comprising at least one of the following.
(3) The steel having the chemical composition described in (1) or (2) above is made into a slab by continuous casting, reheated to a temperature range of 1200 ° C. or higher, and accumulated in the non-recrystallized austenite region after rough rolling. Finish rolling to reduce the reduction ratio Rz to more than 40% and 80% or less in the following formula (1), finish rolling at the Ar3 point or more, cool at an average cooling rate of 15 ° C / s or more, and 200 ° C or less (1) or (2), the method for producing a high-strength hot-rolled steel sheet having excellent bending workability and wear resistance.
Rz = Σ _{i = 1} ⁿ (R _i × (1−t _i × exp (−8000 / (T _i +273)))) (1)
i = 1 to n
Here, n is finish rolling stages, reduction ratio in the pre-recrystallization austenite region in R _i is finish rolling i stage (%), T _i is finishing in finish rolling i-rolling temperature _{(℃), t i (i} = 1 ~n-1)) pass between the time (in seconds until the next stand in finish rolling i stage), t _n is the time from finishing final stand to the start cooling (sec), Rz represents a cumulative rolling reduction (%).

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having a yield strength of 950 MPa or more, which is excellent in bending workability and wear resistance, and a manufacturing method thereof.

  The present invention has a chemical composition as defined in claim 1, 90% or more of the structure is martensite, and the average aspect ratio of the prior austenite grains from the surface layer to the plate thickness 1/8 in the cross section in the rolling direction is By forming a structure of 3 or more and 20 or less, a high-strength hot-rolled steel sheet having a yield strength of 950 MPa or more excellent in bending workability and wear resistance is obtained.

  In the present invention, a steel having a predetermined chemical composition is made into a slab by continuous casting, reheated to a temperature range of 1200 ° C. or higher, and the cumulative reduction ratio Rz in the non-recrystallized austenite region after rough rolling is the above (1). By applying finish rolling to more than 40% and 80% or less in the formula, finishing finish rolling at Ar3 point or higher, cooling at an average cooling rate of 15 ° C / s or higher, and winding in a temperature range of 200 ° C or lower The high-strength hot-rolled steel sheet of the present invention can be manufactured.

  The present invention is 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 0.05% or more. Preferably it is 0.10% or more. However, if the content exceeds 0.20%, the toughness deteriorates, so the upper limit is made 0.20%.

  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 0.55%.

  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 the isotropy of mechanical properties are generated. When severe bending workability 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 toughness is deteriorated, so the upper limit is made 0.10%.

  N is desirably as low as possible, and if it is too large, the formability of the steel sheet is deteriorated. When severe formability is required, it is preferably 0.006% or less.

  Nb has an effect of suppressing recrystallization of prior austenite, can reduce the crystal grain size of the hot-rolled steel sheet, and can improve the strength and bending workability. The effect is obtained when the Nb content is 0.005% or more. On the other hand, if it exceeds 0.10%, the effect is saturated, so the upper limit is made 0.10%.

  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, even if it is added in an amount of more than 0.0050%, the effect is saturated.

  Although not essential for satisfying the required characteristics, it is preferable to add at least one of the following amounts of Ti, Ca, Ni, Mo, and Cr in order to reduce manufacturing variation and further improve toughness.

  Ti, like Nb, has the effect of suppressing austenite recrystallization, has the effect of reducing the crystal grain size of the hot-rolled steel sheet, and improving the strength and bending workability. For this reason, it is effective to add 0.01% or more. However, if the content exceeds 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.

  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 formation of objects and improves bending workability. 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 the hardenability. In order to obtain this effect, it is desirable to add 0.02% or more. However, addition of a large amount increases the slab cracking sensitivity and makes it difficult to handle the slab, 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 sensitivity of the slab and makes it difficult to handle the slab, so the upper limit is made 1.0%.

  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.

  In the hot-rolled steel sheet of the present invention, 90% or more of the structure is martensite. More preferably, it is 95% or more. The higher the martensite fraction, the higher the strength, and the better the workability can be obtained by bringing the steel sheet structure closer to a single phase. Is good.

  A strong correlation has conventionally been recognized between wear resistance and surface hardness. It is important to increase the dislocation density of austenite in order to increase the hardness of the surface and to provide a steel plate with excellent wear resistance.

  Therefore, in finish rolling, a high dislocation density is introduced into the austenite before martensitic transformation by increasing the cumulative reduction ratio in the non-recrystallized austenite region. As the dislocation density in the austenite increases, the surface hardness increases, but since the shape of the austenite becomes flattened, cracking at the grain boundary is likely to occur, and the bending workability is inferior.

  Therefore, the average aspect ratio of the prior austenite grains from the surface layer to the plate thickness 1/8 is important. When the average aspect ratio is 3 or more, a surface layer of 320 or more, which is a sufficient Brinell hardness, is obtained. On the other hand, if the average aspect ratio exceeds 20, cracks are likely to occur from the flat grain boundaries of the prior austenite, and bending workability deteriorates.

  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., Nb is not sufficiently dissolved in the slab, the effect of suppressing recrystallization of austenite by Nb is not obtained, and the cumulative reduction ratio in the non-recrystallized austenite region in finish rolling is ensured. It becomes difficult.

  The heated slab is subjected to rough rolling and further finish rolling. In the present invention, it is necessary to make processed austenite, so finish rolling is performed at Ar3 or higher. If rolling is performed at less than Ar3 point, ferrite transformation starts during finish rolling, so that it is impossible to secure a martensite structure fraction of 90% or more.

  In the present invention, the cumulative rolling reduction in the non-recrystallized austenite region is important in finish rolling after rough rolling. The higher the rolling reduction, the higher the dislocation density introduced into the austenite, the finer the martensite, the higher the strength, the higher the average aspect ratio of the prior austenite grains near the surface layer, and the effect of improving the surface hardness. .

The case where finish rolling has an n-stage stand will be described. Even if rolling is performed at a reduction ratio R _i (%) in the i-th stage (i = 1 to n) stand, the dislocation density introduced by finish rolling is recovered during the elapsed time t _i until the next stand (dislocation density). Therefore, the effect of introducing dislocation due to the reduction is reduced. In the present invention, Nb is contained in an amount of 0.005% or more, and the slab is heated to 1200 ° C. or more, so that Nb is sufficiently dissolved in the slab and the effect of suppressing recrystallization of austenite by Nb is exhibited. Under such conditions, the cumulative rolling reduction rate Rz can be determined as in the following equation (1).
Rz = Σ _{i = 1} ⁿ (R _i × (1−t _i × exp (−8000 / (T _i +273)))) (1)
i = 1 to n
However, n is the number of finish rolling stages, R _i is the reduction ratio (%) in the non-recrystallized austenite region in the finish rolling i stage, T _i is the finish rolling temperature (° C.) in the finish rolling i stage, and t _i (i = 1). ~n-1)) pass between the time (in seconds until the next stand in finish rolling i stage), t _n is the time from finishing final stand to the start cooling (sec), Rz represents a cumulative rolling reduction (%). The above formula (1) is based on a formula representing recovery of a general dislocation density, and the rolling conditions in the examples of the present invention are compared with the observation results of the crystal structure after rolling to optimize the formula coefficients. It was obtained as a result of carrying out.

  By setting the cumulative reduction ratio Rz in the non-recrystallized austenite region to more than 40%, martensite is refined to obtain a high strength of 950 MPa or more in yield strength, and old austenite grains from the surface layer to a thickness of 1/8 An average aspect ratio of 3 or more can be obtained, and a Brinell hardness of 320 or more, which is a surface hardness excellent in wear resistance, can be obtained. If it is 40% or less, not only the yield strength of 950 MPa is not obtained in the tensile test, but also the average aspect ratio of the prior austenite grains becomes less than 3 and the surface hardness is insufficient, so the wear resistance deteriorates.

  When the cumulative reduction ratio in the non-recrystallized austenite region is 80% or less, the average aspect ratio of the prior austenite grains from the surface layer to the plate thickness 1/8 is 20 or less, and cracks occur from the grain boundaries of the flattened prior austenite grains. It is suppressed and deterioration of bending workability is suppressed. When the cumulative reduction ratio in the non-recrystallized austenite region is more than 80%, the average aspect ratio of the prior austenite grains is more than 20, so that the bending workability is inferior.

  In the present invention, since 90% or more of the steel sheet structure needs to be martensite, the average cooling rate from finishing finish rolling to winding after finishing finish rolling at Ar3 point or more is 15 ° C./s or more. And then wound up at a temperature of 200 ° C. or lower. When the average cooling rate is less than 15 ° C./s, quenching becomes insufficient, and a martensite structure of 90% or more cannot be formed.

  Although the cooling start time is not particularly specified, it is desirable to start the cooling within 4 seconds because the dislocation density in the austenite decreases if the holding time is long after finish rolling.

  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. The self-annealed martensite is not included in the martensite described in claim 1.

  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 was heated to a temperature of 1200 ° C. to 1250 ° C., and after rough rolling, continuous seven-stage finish rolling was performed, and winding was performed after cooling within 4 seconds 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, “cumulative rolling reduction” is the cumulative rolling reduction Rz in the non-recrystallized austenite region in the finish rolling determined by the above equation (1), “FT7” is the finish rolling finish temperature, and “cooling rate” is the 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 sheet thus obtained was measured for the martensite fraction, the average aspect ratio of the prior austenite from the surface layer in the cross section in the rolling direction to the sheet thickness 1/8 using an optical microscope, tensile test, Brinell test, A bending test was performed.

  As for the martensite fraction of the steel sheet, the area ratio of the martensite structure was obtained by observing from the surface layer of the steel sheet to ¼ the thickness with a width of 500 μm using an optical microscope.

  As for the average aspect ratio of the prior austenite grains from the surface layer to the thickness 1/8 in the cross section in the rolling direction, the corrosive liquid that reveals the prior austenite grain boundaries (ethanol, 2% picric acid, 1% iron (II) chloride) After etching the cross section in the rolling direction of the steel sheet, the aspect ratio of all the prior austenite grains in the observation region was obtained by observing from the surface layer of the steel sheet to the plate thickness 1/8 with a width of 500 μm, and averaging these Value. Here, the aspect ratio of the prior austenite grains is (aspect ratio) = (major axis in the rolling direction) / (minor axis in the plate thickness direction).

  For the steel sheet tensile test, JIS No. 5 test piece was taken in the direction perpendicular to the rolling direction of the steel sheet (width direction), yield strength: YP (MPa), tensile strength: TS (MPa), elongation: EL (%). evaluated.

  A surface hardness test was performed to evaluate the wear resistance. In the surface hardness test, the hardness HB10 / 3000 of the steel sheet surface was measured from the obtained hot-rolled steel sheet using a Brinell hardness tester based on JIS Z2243. The measurement positions were 5 points selected at random, and the average value of the 5 points was determined as the surface hardness of the steel sheet.

  In the bending test, a JIS No. 1 test piece was sampled from a predetermined position (width ¼ part) of the obtained hot-rolled steel sheet, a 180-degree bending test was performed, and a minimum radius (mm) at which no crack was generated was obtained The limit ratio of bending workability represented by the minimum bending radius / plate thickness was determined. When the limit ratio of bending workability was 4.0 or less, it was evaluated that the bending workability was excellent.

  Table 2 shows the structure fraction, the average aspect ratio of the prior austenite grains from the surface layer to the plate thickness 1/8, and the evaluation results of the material.

  As shown in Table 2, all of the examples of the present invention had a cumulative reduction ratio in the non-recrystallized austenite region of more than 40% and 80% or less, and finished the finish rolling at Ar3 point or higher, and 15 ° C / s or higher. Since it is cooled at an average cooling rate and wound in a temperature range of 200 ° C. or less, 90% or more of the structure is martensite, the average aspect ratio is 3 or more and 20 or less, and the yield strength is 950 MPa or more. The surface Brinell hardness is 320 or more, and the limit ratio of bending workability is 4.0 or less.

  In Comparative Examples 4 and 22, since the cooling rate is less than 15 ° C./s, the martensite structure fraction is less than 90%, and the yield strength does not satisfy 950 MPa or more. Also, the Brinell hardness is less than 320, and the wear resistance is inferior.

  In Comparative Examples 9 and 16, since the cumulative reduction ratio in the non-recrystallized austenite region during finish rolling is less than 40% and the average aspect ratio of the prior austenite grains is less than 3, the yield strength of 950 MPa is satisfied. Not. Also, the Brinell hardness is less than 320, and the wear resistance is inferior.

  On the other hand, in Comparative Example 25, since the cumulative reduction ratio in the non-recrystallized austenite region is more than 80%, the average aspect ratio of the prior austenite grains does not satisfy 20 or less, and the limit ratio of bending workability is also high. It is over 4.0 and is inferior.

  In Comparative Example 12, since the coiling temperature is higher than 200 ° C., the martensite structure fraction is less than 90% and the yield strength does not satisfy 950 MPa. Also, the Brinell hardness is less than 320, and the wear resistance is inferior.

  In Comparative Example 29, the upper limit of the component amount of S is exceeded, and the bending workability is inferior due to the generation of MnS stretching inclusions even when the hot rolling conditions are within the scope of the invention.

  Comparative Example 30 is lower than the component lower limit value of the Nb amount, and the effect of suppressing recrystallization in austenite during finish rolling cannot be obtained, and the cumulative reduction ratio in the non-recrystallized austenite region calculated by the formula (1) exceeds 40%. However, the former austenite was coarsened, the yield strength was not met at 950 MPa or more, and the Brinell hardness and bending workability were inferior. The formula (1) is an equation that is established when the effect of suppressing austenite recrystallization when the Nb content is 0.005% or more and the contained Nb is sufficiently dissolved in the slab. Since Comparative Example 30 deviates from this condition, the effect of the present invention could not be exhibited only by securing the cumulative rolling reduction calculated by equation (1).

Claims (3)

% By mass
C: 0.05% or more, 0.20% or less,
Si: 0.01% or more, 0.55% 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, 0.10% or less,
N: 0.01% or less,
Nb: 0.005% or more, 0.10% or less,
B: 0.0003% or more, 0.0050% or less,
The balance has a chemical composition composed of Fe and inevitable impurities, 90% or more of the structure is martensite, and the average aspect ratio of the prior austenite grains from the surface layer to the plate thickness 1/8 in the cross section in the rolling direction is 3 A high-strength hot-rolled steel sheet excellent in bending workability and wear resistance characterized by having a structure of 20 or less. Furthermore, Ti by mass: 0.01% or more, 0.12% 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, 1.0% or less,
Cr: 0.02% or more, 1.0% or less,
The high-strength hot-rolled steel sheet excellent in bending workability and wear resistance according to claim 1, comprising at least one of the following. The steel having the chemical composition according to claim 1 or claim 2 is made into a slab by continuous casting, reheated to a temperature range of 1200 ° C. or higher, and the cumulative reduction ratio Rz in the non-recrystallized austenite region after rough rolling is obtained. In the following formula (1), finish rolling is performed to be more than 40% and not more than 80%, finish rolling is finished at an Ar3 point or higher, cooled at an average cooling rate of 15 ° C / s or higher, and in a temperature range of 200 ° C or lower. The method for producing a high-strength hot-rolled steel sheet having excellent bending workability and wear resistance according to claim 1 or 2, wherein the steel sheet is wound.
Rz = Σ _{i = 1} ⁿ (R _i × (1−t _i × exp (−8000 / (T _i +273)))) (1)
i = 1 to n
Here, n is finish rolling stages, reduction ratio in the pre-recrystallization austenite region in R _i is finish rolling i stage (%), T _i is finishing in finish rolling i-rolling temperature _{(℃), t i (i} = 1 ~n-1)) pass between the time (in seconds until the next stand in finish rolling i stage), t _n is the time from finishing final stand to the start cooling (sec), Rz represents a cumulative rolling reduction (%).