JP6984320B2 - Nickel-containing steel sheet for low temperature with excellent toughness and its manufacturing method - Google Patents

Description

The present invention relates to a nickel-containing steel sheet for low temperature having excellent toughness and a method for producing the same. Steel sheets manufactured by this method can be used for general welded structures such as shipbuilding, bridges, construction, marine structures, pressure vessels, tanks, and line pipes, but fracture toughness at a low temperature of about -253 ° C is particularly required. It is effective for use in low temperature tanks.

With the use of hydrogen as energy, infrastructure development has become an important issue. Austenitic stainless steel or the like is used as a material for a tank that liquefies and stores hydrogen. However, in order to increase the size of the tank, a ferrite-based low-temperature steel that can easily increase its strength is required. However, ferrite-based low-temperature steels cause low-temperature brittleness, and overcoming this is the key to practical use. For example, it is desirable that the minimum value of Charpy impact absorption energy at -253 ° C of a material that has been heat-treated at 200 ° C for 1 hr after applying a strain of 6% is 150 J or more. At current levels, achieving this is not always easy.
Inclusions may be involved in the low value of the Charpy impact absorption energy of ferrite low temperature nickel steel at -253 ° C, which occurs with a very low probability. In steel slabs manufactured by continuous casting, inclusions of several μm remain without floating separation, but at normal cleanliness, such independent inclusions are Charpy at -253 ° C. The effect on shock absorption energy is minor. However, when a cluster of several μm inclusions is aggregated and coalesced, the Charpy impact absorption energy at −253 ° C. of the material heat-treated at 200 ° C. for 1 hr after applying a strain of 6% may decrease to 150 J or less. be. The main inclusion is alumina (Al ₂ O ₃ ). Alumina clusters are a common event in the steelmaking process and are difficult to improve in essence.

There is cross rolling as a method of reducing the harm caused by inclusions, for example, extension inclusions such as MnS. Cross-rolling is hot rolling that creates the shape of a steel sheet. Of the rolling that is normally performed only in the longitudinal direction of the steel sheet, some rolling is performed in the width direction of the steel sheet, and the inclusions are MnS. In the case of, since the elongation of MnS in the longitudinal direction of the steel sheet is suppressed, the Charpy impact absorption energy is improved in the Charpy test using the test piece so that the longitudinal direction of the test piece is parallel to the rolling width direction.

For example, in Patent Document 1 and Patent Document 2, bending workability and low-temperature toughness are improved by performing widthwise rolling in the unrecrystallized temperature range when performing cross rolling. However, when rolling in the width direction in the unrecrystallized temperature range, rolling in the unrecrystallized region is performed while the austenite grain size before rolling is large, and the toughness may become unstable. Cannot achieve the purpose of. Further, Patent Document 3 defines a rolling reduction ratio between widthwise rolling and longitudinal rolling when cross-rolling is performed, so that a steel sheet having high isotropic properties is obtained. Although this method is effective for controlling inclusions, it is not possible to reduce the average effective crystal grain size, which is the factor that most affects the Charpy impact absorption energy, only by specifying the reduction ratio. Cannot be achieved.
That is, the current technology cannot provide a nickel-containing steel sheet for low temperature with excellent toughness.

Patent No. 4897125 Japanese Unexamined Patent Publication No. 2005-226080 Japanese Unexamined Patent Publication No. 2002-161341

The present invention is to provide a nickel-containing steel sheet for low temperature having excellent toughness and a method for producing the same.

The present invention provides a nickel-containing steel sheet for low temperature having excellent toughness and a method for producing the same, and the gist thereof is as follows.
(1) Steel is by mass%, C: 0.02% or more and 0.12% or less, Si: 0.02% or more and 0.35% or less, Mn: 0.10% or more and 1.50% or less, P. : 0.0010% or more and 0.0100% or less, S: 0.0001% or more and 0.0035% or less, Ni: 10.0% or more and 15.0% or less, Al: 0.002% or more and 0.090% or less , N: 0.0001% or more and 0.0070% or less, TO: 0.0001% or more and 0.0030% or less, and the balance is a steel composition consisting of Fe and unavoidable impurities, which is equivalent to the old austenite circle. A nickel-containing steel sheet having a particle size of 20 μm or less, an effective crystal grain size of 7.0 μm or less, and a tensile strength of 690 MPa or more and 900 MPa or less.

(2) Further, in terms of mass%, Cu: 0.01% or more and 2.00% or less, Cr: 0.01% or more and 5.00% or less, Mo: 0.01% or more and 1.00% or less, B: 0 .0002% or more and 0.0500% or less, Nb: 0.001% or more and 0.050% or less, Ti: 0.001% or more and 0.050% or less, V: 0.001% or more and 0.050% or less, Ca : 0.0003% or more and 0.0300% or less, Mg: 0.0003% or more and 0.0300% or less, REM: 0.0003% or more and 0.0300% or less, and the balance is Fe. The nickel-containing steel plate according to (1) above, which has a steel composition composed of unavoidable impurities.

(3) In terms of mass%, C: 0.02% or more and 0.12% or less, Si: 0.02% or more and 0.35% or less, Mn: 0.10% or more and 1.50% or less, P: 0. 0010% or more and 0.0100% or less, S: 0.0001% or more and 0.0035% or less, Ni: 10.0% or more and 15.0% or less, Al: 0.002% or more and 0.090% or less, N: The following is a slab or steel piece having a steel composition containing 0.0001% or more and 0.0070% or less, TO: 0.0001% or more and 0.0030% or less, and the balance is Fe and unavoidable impurities. A method for producing a nickel-containing steel sheet, which comprises the steps (A) to (C) to obtain a steel sheet having an old austenite circle-equivalent particle size of 20 μm or less and an effective crystal grain size of 7.0 μm or less. ..
(A) The slab or steel slab is rolled into a steel sheet having a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, a rolling reduction ratio of 4 or higher, and a finish rolling 1-pass front entry side temperature of 600 ° C. or higher and 850 ° C. or lower. None, then hot rolling process with air cooling;
(B) The steel plate obtained by the hot rolling step is heated at a temperature rise rate of 0.3 ° C./s or more in a temperature range of 550 ° C. or higher and 700 ° C. or lower to a temperature range of 680 ° C. or higher and 830 ° C. or lower. Quenching step of holding for 5 minutes or more and 100 minutes or less and then quenching; and (C) Tempering step of holding the steel plate obtained by the quenching step in a temperature range of 500 ° C. or more and 660 ° C. or less for 5 minutes or more and 100 minutes or less. ..

(4) In terms of mass%, C: 0.02% or more and 0.12% or less, Si: 0.02% or more and 0.35% or less, Mn: 0.10% or more and 1.50% or less, P: 0. 0010% or more and 0.0100% or less, S: 0.0001% or more and 0.0035% or less, Ni: 10.0% or more and 15.0% or less, Al: 0.002% or more and 0.090% or less, N: The following is a slab or steel piece having a steel composition containing 0.0001% or more and 0.0070% or less, TO: 0.0001% or more and 0.0030% or less, and the balance being Fe and unavoidable impurities. A method for producing a nickel-containing steel sheet, which comprises the steps (A) to (C) to obtain a steel sheet having an old austenite circle-equivalent particle size of 20 μm or less and an effective crystal grain size of 7.0 μm or less. ..
(A) The slab or steel slab is rolled into a steel sheet having a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, a rolling reduction ratio of 4 or higher, and a finish rolling 1-pass front entry side temperature of 600 ° C. or higher and 900 ° C. or lower. None, then hot rolling process with water cooling at a cooling rate of 200 ° C / s or less;
(B) The steel plate obtained by the hot rolling step is heated at a temperature rise rate of 0.3 ° C./s or more in a temperature range of 550 ° C. or higher and 700 ° C. or lower to a temperature range of 680 ° C. or higher and 830 ° C. or lower. Quenching step of holding for 5 minutes or more and 100 minutes or less and then quenching; and (C) Tempering step of holding the steel plate obtained by the quenching step in a temperature range of 500 ° C. or more and 660 ° C. or less for 5 minutes or more and 100 minutes or less. ..

(5) In terms of mass%, C: 0.02% or more and 0.12% or less, Si: 0.02% or more and 0.35% or less, Mn: 0.10% or more and 1.50% or less, P: 0. 0010% or more and 0.0100% or less, S: 0.0001% or more and 0.0035% or less, Ni: 10.0% or more and 15.0% or less, Al: 0.002% or more and 0.090% or less, N: The following is a slab or steel piece having a steel composition containing 0.0001% or more and 0.0070% or less, TO: 0.0001% or more and 0.0030% or less, and the balance being Fe and unavoidable impurities. A method for producing a nickel-containing steel sheet, which comprises the steps (A) to (C) to obtain a steel sheet having an old austenite circle-equivalent particle size of 20 μm or less and an effective crystal grain size of 7.0 μm or less. ..
(A) The slab or steel slab is rolled into a steel sheet having a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, a rolling reduction ratio of 4 or higher, and a finish rolling 1-pass front entry side temperature of 600 ° C. or higher and 900 ° C. or lower. None, and then a hot rolling step of water cooling with a water cooling stop temperature of 150 ° C or higher and 550 ° C or lower;
(B) The steel plate obtained by the hot rolling step is heated at a temperature rise rate of 0.3 ° C./s or more in a temperature range of 550 ° C. or higher and 700 ° C. or lower to a temperature range of 680 ° C. or higher and 830 ° C. or lower. Quenching step of holding for 5 minutes or more and 100 minutes or less and then quenching; and (C) Tempering step of holding the steel plate obtained by the quenching step in a temperature range of 500 ° C. or more and 660 ° C. or less for 5 minutes or more and 100 minutes or less. ..

(6) The above-mentioned (3) to (5), wherein an intermediate heat treatment having a heating temperature range of 580 ° C. or higher and 720 ° C. or lower and a holding time of 5 minutes or longer and 100 minutes or shorter is performed between quenching and tempering. ). The method for manufacturing a nickel-containing steel sheet according to any one of.

(7) Further, in terms of mass%, Cu: 0.01% or more and 2.00% or less, Cr: 0.01% or more and 5.00% or less, Mo: 0.01% or more and 1.00% or less, B: 0 .0002% or more and 0.0500% or less, Nb: 0.001% or more and 0.050% or less, Ti: 0.001% or more and 0.050% or less, V: 0.001% or more and 0.050% or less, Ca : 0.0003% or more and 0.0300% or less, Mg: 0.0003% or more and 0.0300% or less, REM: 0.0003% or more and 0.0300% or less, and the balance is Fe. The method for producing a nickel-containing steel sheet according to any one of (3) to (6) above, wherein the steel composition is composed of unavoidable impurities.

According to the present invention, even when alumina clusters are inevitably present, it is possible to provide a nickel-containing steel sheet for low temperature with excellent toughness and a manufacturing method thereof by devising a manufacturing method in the thick plate process, which is industrially possible. It can be said that this is a highly valuable invention.

It is a graph which shows the relationship between the reduction ratio and the temperature before finishing 1 pass on the particle size corresponding to the old austenite circle in the air cooling after hot rolling. It is a graph which shows the relationship between the reduction ratio and the temperature before finishing 1 pass on the particle size equivalent to the old austenite circle in water cooling after hot rolling. It is a graph which shows the influence of the temperature rise rate at the time of quenching on the particle size equivalent to the old austenite circle. It is a graph which shows the relationship between the average effective crystal grain size and toughness after strain aging. It is a graph which shows the relationship between the particle size equivalent to the old austenite circle and the average effective crystal grain size.

The present invention will be described in detail. The inventor avoids the decrease in toughness caused by alumina clusters in the steel sheet containing nickel for low temperature and having a Ni content of more than 10.0% and 15.0% or less in the process after hot rolling instead of the steelmaking process. Or, I eagerly examined whether it could be recovered. As a result, it was found that the old austenite is significantly refined by slightly increasing the heating rate of 600 ° C. or higher and 750 ° C. or lower when the temperature of quenching is raised after appropriate hot rolling. Since the miniaturization of the particle size equivalent to the old austenite circle leads to the miniaturization of the final structure, that is, the structure mainly composed of tempered martensite, the toughness can be significantly improved.

In the present invention, the combination of the two production methods is important in order to significantly reduce the particle size equivalent to the old austenite circle. The first point is to properly control the conditions of hot rolling performed before quenching, and the second point is to appropriately control the temperature rising conditions during quenching after quenching. ..
In the present invention, the hot rolling step (A step), the quenching step (B step), and the tempering step (C. The first step A, that is, the conditions of hot rolling performed before quenching will be described. The conditions for hot rolling differ between the case of hot rolling and then air cooling and the case of hot rolling and then water cooling.

(1) When air-cooled after hot rolling;
First, a case of air cooling after hot rolling will be described. A slab or steel slab containing more than 10.0% and 15.0% or less of Ni is heated at a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, then hot-rolled, and then air-cooled. Hot rolling is performed at a rolling reduction ratio of 4 or more, and the temperature before finishing 1 pass is set to 600 ° C. or higher and 850 ° C. or lower. Here, the rolling ratio is a value obtained by dividing the thickness of the steel piece before rolling by the thickness of the steel plate after rolling. The temperature before finishing 1 pass refers to the temperature of the surface of the steel sheet measured immediately before the final 1 pass of rolling, for example, within 5 seconds. In FIG. 1, an experiment was conducted in which the rolling reduction ratio and the temperature before finishing 1 pass were variously changed in the case of air cooling after rolling, and the old austenite grain size and the finishing 1 pass at the time of quenching and heating were estimated using the steel plate after heat treatment. The relationship between the pre-temperature is shown. When the temperature one pass before finishing is more than 850 ° C., the structure at the time of cooling to room temperature by air cooling is coarse, so that the particle size equivalent to the old austenite circle becomes large. Further, if the temperature one pass before finishing is less than 600 ° C., hot rolling cannot be performed because the deformation resistance is large. Further, when the reduction ratio is less than 4, the structure after air cooling becomes coarse, so that the particle size equivalent to the old austenite circle becomes large. Here, the particle size equivalent to the old austenite circle is defined by polishing the surface parallel to the plane formed by the longitudinal direction and the thickness direction of the optical microscope sample collected from the center of the plate thickness of the final product steel plate after heat treatment. Further, it refers to a circle-equivalent diameter calculated from a particle size number measured by the method described in JISG0551 after the old austenite grain boundaries at the time of quenching and heating are revealed by a corrosive liquid such as picric acid.

(2) When water-cooled after hot rolling;
Next, a case of hot rolling and then water cooling will be described. A slab or steel slab containing more than 10.0% and 15.0% or less of Ni is heated at a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, then hot-rolled, and then water-cooled. Hot rolling is performed at a rolling reduction ratio of 4 or more, and the temperature before finishing 1 pass is set to 600 ° C. or higher and 900 ° C. or lower. In the case of water cooling after hot rolling, the same miniaturization effect can be obtained when the upper limit of the temperature one pass before finishing is 50 ° C. higher than that of air cooling by lowering the transformation temperature. In FIG. 2, in the case of water cooling after hot rolling, experiments were performed in which the rolling ratio and the temperature before finishing 1 pass were variously changed, and the grain size equivalent to the old austenite circle and the finishing 1 pass were estimated using the heat-treated steel sheet. The relationship between the pre-temperature is shown. When the temperature one pass before finishing is more than 900 ° C., the structure at the time of cooling to room temperature by water cooling is coarse, so that the particle size equivalent to the old austenite circle becomes large. Further, if the temperature one pass before finishing is less than 600 ° C., hot rolling cannot be performed because the deformation resistance is large. Further, when the reduction ratio is less than 4, the structure after water cooling becomes coarse, so that the particle size equivalent to the old austenite circle becomes large.

Temperature rise rate during quenching;
Next, the B step, that is, the quenching step will be described.
As for the heating rate during heating during quenching, by setting the average heating rate of 600 ° C or higher and 750 ° C or lower to 0.3 ° C / s or higher, the particle size equivalent to the old austenite circle can be significantly reduced. can. Here, the average temperature rise rate is, in this case, a value obtained by dividing the temperature difference of 150 ° C. between 600 ° C. and 750 ° C. by the time required for temperature rise during this period. FIG. 3 shows the relationship between the particle size equivalent to the old austenite circle and the average temperature rise rate of 600 ° C. or higher and 750 ° C. or lower, which was estimated using the steel plate after heat treatment by conducting experiments in which the heating rate of quenching was variously changed. .. When the heating rate at the time of quenching is 0.3 ° C / s or more, the particle size equivalent to the old austenite circle becomes small.

Here, in order to clarify the temperature section in which the temperature rise rate is increased, the old austenite circle when the standard temperature rise is performed with the temperature rise rate of 200 ° C. or higher and the quenching heating temperature or lower set to 0.1 ° C./s. Only in three conditions, that is, 200 ° C or more and less than 600 ° C, in which the equivalent particle size is increased to 2 ° C / s only in a specific temperature range and the temperature rise rate in other temperature ranges is 0.1 ° C / s. The old austenite circle at the time of temperature rise under the condition that the temperature was set to 2 ° C / s, the condition was set to 2 ° C / s only at 600 ° C or higher and 750 ° C, and the condition was set to 2 ° C / s only under the heating temperature of 750 ° C. Compared with the equivalent particle size. As a result, as shown in Table 1, the particle size equivalent to that of the old austenite circle was remarkably fine under the condition that only the temperature of 600 ° C or higher and 750 ° C or lower was set to 2 ° C / s and the other temperature sections were set to 0.1 ° C / s. The change was seen. For this reason, it is necessary to increase the temperature rise rate of 600 ° C. or higher and 750 ° C. or lower in order to reduce the particle size equivalent to the old austenite circle by increasing the temperature rise rate.

By refining the average effective crystal grain size, the toughness after strain aging is improved. FIG. 4 shows the relationship between the toughness after strain aging and the average effective crystal grain size. In the steel sheet of the present invention, from the viewpoint of high safety, the absorbed energy of the Charpy test at a test temperature of 253 ° C., which was carried out by collecting a test piece after applying a strain of 6% and then heat-treating at 200 ° C. for 1 hour. Is often required in terms of specifications, and the maximum value is 150J, so 150J or more was accepted. In this case, the average effective crystal grain size needs to be 7.0 μm or less. When the particle size equivalent to the old austenite circle becomes finer, the final structure also becomes finer. FIG. 5 shows the relationship between the average effective crystal grain size after quenching and tempering and the grain size equivalent to the old austenite circle estimated using the steel sheet after quenching and tempering. Here, the average effective crystal grain size is the chemical polishing or electrolytic polishing for mechanical polishing and strain removal, or polishing with colloidal silica or the like for a sample collected from the central portion of the plate thickness of a steel plate that has been completely heat-treated. The diameter is equivalent to a circle calculated by defining an interface having an orientation difference of 15 ° or more as a grain boundary from the data measured by EBSD (Electron Backscatter Diffraction Pattern).

As shown in FIG. 4, the average effective crystal grain size required to achieve an absorption energy of 150 J in the Charpy test at a test temperature of -253 ° C is a maximum of 7.0 μm. Therefore, in FIG. 5, the average effective crystal grain is shown. A diameter of 7.0 μm or less was regarded as acceptable. In this case, the particle size of the old austenite at the time of quenching and heating needs to be 20 μm or less.

The range of alloying elements of steel sheets is specified below.
Since C is an element essential for ensuring strength, the amount added thereof should be 0.02% or more. However, on the other hand, an increase in the amount of C causes a decrease in toughness, so the upper limit is set to 0.12%.

Since Si is an element essential for ensuring strength, the amount of Si added should be 0.02% or more. However, on the other hand, the addition of Si exceeding 0.35% causes deterioration of toughness and weldability, so the upper limit is set to 0.35%.

Mn is an element effective for increasing strength, and it is necessary to add at least 0.10% or more, but conversely, if it is added in excess of 1.50%, tempering embrittlement sensitivity increases and toughness decreases. .. Therefore, the amount of Mn added is defined as 0.10% or more and 1.50% or less.

If P is less than 0.0010%, the productivity is significantly reduced due to an increase in the refining load, which is not preferable. If it exceeds 0.0100%, the toughness decreases due to temper embrittlement. Therefore, the amount of P added is defined as 0.0010% or more and 0.0100% or less.

If S is less than 0.0001%, the productivity is significantly reduced due to an increase in the refining load, which is not preferable. If it exceeds 0.0035%, the toughness decreases. Therefore, the amount of S added is defined as 0.0001% or more and 0.0035% or less.

Ni needs to be added at least 10.0% or more in order to secure toughness at the lower limit. Moreover, if it exceeds 15.0%, the manufacturing cost will increase significantly. Therefore, the amount of Ni added is specified to be more than 10.0% and 15.0% or less.

Al is an element effective for deoxidation, and it is necessary to add at least 0.002% or more, but conversely, if it is added in excess of 0.090%, the toughness decreases through the formation of alumina clusters through the reoxidation of molten steel. do. Therefore, the amount of Al added is defined as 0.002% or more and 0.090% or less.

The lower limit of N is not particularly specified due to the material characteristics, but if it is less than 0.0001%, the productivity is significantly reduced due to the increase in the refining load, so it is set to 0.0001% or more. However, if the addition exceeds 0.0070%, the toughness decreases, so the upper limit is 0.0070%. Therefore, the amount of N added is defined as 0.0001% or more and 0.0070% or less.

TO is the total amount of oxygen in the components, and the lower limit is not specified in terms of material characteristics, but if it is less than 0.0001%, the productivity will be significantly reduced due to the increase in refining load, so 0.0001% or more. And. If it is added in excess of 0.0030%, the toughness decreases through the formation of alumina clusters, so the upper limit is 0.0030%. Therefore, the amount of TO added is set to 0.0001% or more and 0.0030% or less.

In the present invention, the following elements can be added as those that further affect the strength.
Cu needs to be added at least 0.01% or more in order to secure the strength, but if it exceeds 2.00%, the toughness decreases. Therefore, the amount of Cu added is defined as 0.01% or more and 2.00% or less.

Cr is an element that ensures hardenability and affects strength, and it is necessary to add at least 0.01% or more, but conversely, if it is added in excess of 5.00%, toughness and weldability will be improved. descend. Therefore, the amount of Cr added is defined as 0.01% or more and 5.00% or less.

Mo is an element effective for ensuring strength and reducing tempering embrittlement, and it is necessary to add at least 0.01%, but conversely, if it is added in excess of 1.00%, toughness and weldability deteriorate. .. Therefore, the amount of Mo added is defined as 0.01% or more and 1.00% or less.

B is an element that is effective in improving hardenability and affects strength. If it is less than 0.0002%, the effect is small, and if it is added more than 0.0500%, the toughness is lowered. Therefore, the amount of B added is defined as 0.0002% or more and 0.0500% or less.

Nb is an element effective for ensuring strength. Additions of less than 0.001% have a small effect, and additions of more than 0.050% result in a decrease in toughness. Therefore, the amount of Nb added is defined as 0.001% or more and 0.050% or less.

Ti is an element effective for ensuring strength. Additions of less than 0.001% have a small effect, and additions of more than 0.050% result in a decrease in toughness. Therefore, the amount of Ti added is defined as 0.001% or more and 0.050% or less.

V is an element effective for ensuring strength. Additions of less than 0.001% have a small effect, and additions of more than 0.050% result in a decrease in toughness. Therefore, the amount of V added is defined as 0.001% or more and 0.050% or less.

Ca is an element that affects the crystal grain size, affects the strength, and is an effective element for preventing nozzle blockage. Additions of less than 0.0003% have a small effect, and additions of more than 0.0300% result in a decrease in toughness. Therefore, the amount of Ca added is defined as 0.0003% or more and 0.0300% or less.

Mg is an element that affects strength and is effective in improving toughness. Additions of less than 0.0003% have a small effect, and additions of more than 0.0300% result in a decrease in toughness. Therefore, the amount of Mg added is specified to be 0.0003% or more and 0.0300% or less.

REM is an element that affects strength and is effective in improving toughness. Additions of less than 0.0003% have a small effect, and additions of more than 0.0300% result in a decrease in toughness. Therefore, the amount of REM added is specified to be 0.0003% or more and 0.0300% or less.

In manufacturing steel sheets and welding materials, Zn, Sn, Sb, etc., which can be mixed as unavoidable impurities eluted from the furnace material, etc. during the raw materials used including additive alloys or melting, are also less than 0.002%. If it is mixed with the above, the effect of the present invention is not impaired.

Next, the method for manufacturing the steel sheet of the present invention will be described.
The steel sheet is manufactured by a method of hot rolling a slab manufactured by continuous casting by the above method, but in addition to the above, the following generally carried out to miniaturize a structure mainly composed of martensite. Conditions are also needed. In step A of the hot rolling process, when the heating temperature of the steel pieces exceeds 1300 ° C, the structure mainly composed of martensite after transformation becomes coarse due to the grain growth of austenite, and when the temperature is lower than 850 ° C, hot rolling is difficult. Therefore, the temperature is set to 850 ° C or higher and 1300 ° C or lower.
Hot rolling is performed at a reduction ratio of 4 or more as described in FIGS. 1 and 2 above, and the temperature before finishing 1 pass is set to 600 ° C or higher and 850 ° C or lower when the subsequent cooling is air cooling, and when water cooling. Is 600 ° C. or higher and 900 ° C. or lower.

When water cooling is performed after hot rolling, it is preferable that the cooling rate during water cooling is 200 ° C / s or less because the equipment cost increases if the cooling rate during water cooling exceeds 200 ° C / s. When water cooling is performed after rolling, water cooling can be stopped halfway in addition to water cooling to room temperature. If the water cooling stop temperature is less than 150 ° C, the temperature becomes uneven and the material varies, and if the water cooling stop temperature exceeds 550 ° C, the structure becomes coarse. Therefore, the water cooling stop temperature should be 150 ° C or higher and 550 ° C or lower. Is also preferable.

After the hot rolling step, that is, after the A step, the B step, which is a quenching step, is performed. The rate of temperature rise of 600 ° C. or higher and 750 ° C. or lower during quenching is 0.3 ° C./s or higher as described in FIG. If the maximum heating temperature during quenching is less than 680 ° C, untransformed structure remains and the toughness decreases, and if it exceeds 830 ° C, the old austenite during quench heating becomes coarse and the toughness decreases. Therefore, the maximum heating temperature at the time of quenching is set to 680 ° C. or higher and 830 ° C. or lower.
When the holding time during quenching and heating is 5 minutes or less, the material becomes non-uniform, and when it is 100 minutes or more, the structure becomes coarse and the toughness decreases. Therefore, the holding time during quenching and heating is set to 5 minutes or more and 100 minutes or less.

If necessary, an intermediate heat treatment can be performed between quenching and tempering. When the heating temperature of the intermediate heat treatment is less than 580 ° C, the toughness decreases, and when it exceeds 720 ° C, the effect of improving the toughness by stabilizing austenite by the intermediate heat treatment is hardly obtained. Therefore, the heating temperature of the intermediate heat treatment is 580 ° C or higher and 720 ° C. The following is preferable.

If the retention time of the intermediate heat treatment is less than 5 minutes, the reverse transformation hardly progresses, and the effect of improving the toughness by stabilizing the old austenite during quenching and heating is hardly obtained. The retention time of the intermediate heat treatment is set to 5 minutes or more and 100 minutes or less because the toughness is lowered due to destabilization.

The C step, which is a tempering step, is preferably carried out at 500 ° C. or higher and 660 ° C. or lower because the toughness is lowered due to tempering embrittlement at a temperature lower than 500 ° C. and the toughness is lowered at a temperature higher than 660 ° C. Further, the tempering holding time is preferably 5 minutes or more and 100 minutes or less because the toughness at which a sufficient effect can be obtained decreases when the tempering time is less than 5 minutes and the productivity decreases when the tempering time is 100 minutes or more.
The tensile strength of the steel of the present invention is in the range of the tensile strength of 690 MPa or more and 900 MPa or less required in the art.

Tensile tests and Charpy impact tests were carried out on steel sheets having a thickness of 15 and 50 mm manufactured under various chemical components and manufacturing conditions. Table 2-1 shows the chemical composition of the steel sheet, the particle size equivalent to the old austenite circle, the average effective crystal grain size, and the plate thickness, and Table 2-2 shows the evaluation results of hot rolling conditions, heat treatment conditions, and mechanical properties. The holding time at the maximum heating temperature for quenching, intermediate heat treatment, and tempering was 20 minutes for a plate thickness of 15 mm and 40 minutes for a plate thickness of 50 mm.

The tensile test was performed based on the metal material tensile test method described in JIS Z 2241. The test piece was collected so that the longitudinal direction of the test piece was perpendicular to the rolling direction at a portion that entered the inside from the surface of the steel sheet by 1/4 of the plate thickness. Two tests were performed at room temperature. Two tests were conducted at room temperature, and the average tensile strength was 690 MPa or more and 900 MPa or less.

In the Charpy impact test, a full-size test piece of a 2 mm V notch test piece was applied from the surface of the steel sheet by applying a strain of 6% at room temperature in advance and then heat-treated at 200 ° C. for 1 hr. At the inside, the test piece was sampled so that the longitudinal direction of the test piece was perpendicular to the rolling direction and the line connecting the front edges of the notches was parallel to the plate thickness direction. Three tests were conducted at a test temperature of -253 ° C, and the average value of the three was 150 J or more.

As shown in Examples 1 to 30, a steel sheet having excellent tensile strength and toughness was obtained by manufacturing the steel sheet by the components and the manufacturing method specified in the present invention.
From the above examples, it is clear that the steel sheets of Invention Examples Examples 1 to 30, which are the steel materials manufactured by the present invention, are steel sheets having excellent tensile strength and toughness.

Claims (7)

Steel is by mass%,
C: 0.02% or more and 0.12% or less,
Si: 0.02% or more and 0.35% or less,
Mn: 0.10% or more and 1.50% or less,
P: 0.0010% or more and 0.0100% or less,
S: 0.0001% or more and 0.0035% or less,
Ni: More than 10.0% and less than 15.0%,
Al: 0.002% or more and 0.090% or less,
N: 0.0001% or more and 0.0070% or less,
TO: Steel composition containing 0.0001% or more and 0.0030% or less, the balance being Fe and unavoidable impurities, the particle size equivalent to the old austenite circle is 20 μm or less, and the effective crystal grain size is 7. A nickel-containing steel sheet having a tensile strength of 690 MPa or more and 900 MPa or less, which is 0.0 μm or less. In addition, by mass%,
Cu: 0.01% or more and 2.00% or less,
Cr: 0.01% or more and 5.00% or less,
Mo: 0.01% or more and 1.00% or less,
B: 0.0002% or more and 0.0500% or less,
Nb: 0.001% or more and 0.050% or less,
Ti: 0.001% or more and 0.050% or less,
V: 0.001% or more and 0.050% or less,
Ca: 0.0003% or more and 0.0300% or less,
Mg: 0.0003% or more and 0.0300% or less,
REM: The nickel-containing steel sheet according to claim 1, wherein the steel composition contains any one or more of 0.0003% or more and 0.0300% or less, and the balance is composed of Fe and unavoidable impurities. .. By mass%,
C: 0.02% or more and 0.12% or less,
Si: 0.02% or more and 0.35% or less,
Mn: 0.10% or more and 1.50% or less,
P: 0.0010% or more and 0.0100% or less,
S: 0.0001% or more and 0.0035% or less,
Ni: More than 10.0% and less than 15.0%,
Al: 0.002% or more and 0.090% or less,
N: 0.0001% or more and 0.0070% or less,
TO: A slab or steel piece containing 0.0001% or more and 0.0030% or less and having a steel composition in which the balance is Fe and unavoidable impurities is subjected to the following steps (A) to (C). A method for producing a nickel-containing steel sheet, which comprises a steel sheet having a particle size equivalent to the old austenite circle of 20 μm or less and an effective crystal grain size of 7.0 μm or less.
(A) The slab or steel slab is rolled into a steel sheet having a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, a rolling reduction ratio of 4 or higher, and a finish rolling 1-pass front entry side temperature of 600 ° C. or higher and 850 ° C. or lower. None, then hot rolling process with air cooling;
(B) The steel plate obtained by the hot rolling step is heated at a temperature rise rate of 0.3 ° C./s or more in a temperature range of 550 ° C. or higher and 700 ° C. or lower to a temperature range of 680 ° C. or higher and 830 ° C. or lower. Quenching step of holding for 5 minutes or more and 100 minutes or less and then quenching; and (C) Tempering step of holding the steel plate obtained by the quenching step in a temperature range of 500 ° C. or more and 660 ° C. or less for 5 minutes or more and 100 minutes or less. .. By mass%,
C: 0.02% or more and 0.12% or less,
Si: 0.02% or more and 0.35% or less,
Mn: 0.10% or more and 1.50% or less,
P: 0.0010% or more and 0.0100% or less,
S: 0.0001% or more and 0.0035% or less,
Ni: More than 10.0% and less than 15.0%,
Al: 0.002% or more and 0.090% or less,
N: 0.0001% or more and 0.0070% or less,
TO: A slab or steel piece containing 0.0001% or more and 0.0030% or less and having a steel composition in which the balance is Fe and unavoidable impurities is subjected to the following steps (A) to (C). A method for producing a nickel-containing steel sheet, which comprises a steel sheet having a particle size equivalent to the old austenite circle of 20 μm or less and an effective crystal grain size of 7.0 μm or less.
(A) The slab or steel slab is rolled into a steel sheet having a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, a rolling reduction ratio of 4 or higher, and a finish rolling 1-pass front entry side temperature of 600 ° C. or higher and 900 ° C. or lower. None, then hot rolling process with water cooling at a cooling rate of 200 ° C / s or less;
(B) The steel plate obtained by the hot rolling step is heated at a temperature rise rate of 0.3 ° C./s or more in a temperature range of 550 ° C. or higher and 700 ° C. or lower to a temperature range of 680 ° C. or higher and 830 ° C. or lower. Quenching step of holding for 5 minutes or more and 100 minutes or less and then quenching; and (C) Tempering step of holding the steel plate obtained by the quenching step in a temperature range of 500 ° C. or more and 660 ° C. or less for 5 minutes or more and 100 minutes or less. .. By mass%,
C: 0.02% or more and 0.12% or less,
Si: 0.02% or more and 0.35% or less,
Mn: 0.10% or more and 1.50% or less,
P: 0.0010% or more and 0.0100% or less,
S: 0.0001% or more and 0.0035% or less,
Ni: More than 10.0% and less than 15.0%,
Al: 0.002% or more and 0.090% or less,
N: 0.0001% or more and 0.0070% or less,
TO: A slab or steel piece containing 0.0001% or more and 0.0030% or less and having a steel composition in which the balance is Fe and unavoidable impurities is subjected to the following steps (A) to (C). A method for producing a nickel-containing steel sheet, which comprises a steel sheet having a particle size equivalent to the old austenite circle of 20 μm or less and an effective crystal grain size of 7.0 μm or less.
(A) The slab or steel slab is rolled into a steel sheet having a heating temperature of 850 ° C. or higher and 1300 ° C. or lower, a rolling reduction ratio of 4 or higher, and a finish rolling 1-pass front entry side temperature of 600 ° C. or higher and 900 ° C. or lower. None, and then a hot rolling step of water cooling with a water cooling stop temperature of 150 ° C or higher and 550 ° C or lower;
(B) The steel plate obtained by the hot rolling step is heated at a temperature rise rate of 0.3 ° C./s or more in a temperature range of 550 ° C. or higher and 700 ° C. or lower to a temperature range of 680 ° C. or higher and 830 ° C. or lower. Quenching step of holding for 5 minutes or more and 100 minutes or less and then quenching; and (C) Tempering step of holding the steel plate obtained by the quenching step in a temperature range of 500 ° C. or more and 660 ° C. or less for 5 minutes or more and 100 minutes or less. .. Any one of claims 3 to 5, wherein an intermediate heat treatment having a heating temperature range of 580 ° C. or higher and 720 ° C. or lower and a holding time of 5 minutes or longer and 100 minutes or shorter is performed between quenching and tempering. The method for manufacturing a nickel-containing steel sheet according to. In addition, by mass%,
Cu: 0.01% or more and 2.00% or less,
Cr: 0.01% or more and 5.00% or less,
Mo: 0.01% or more and 1.00% or less,
B: 0.0002% or more and 0.0500% or less,
Nb: 0.001% or more and 0.050% or less,
Ti: 0.001% or more and 0.050% or less,
V: 0.001% or more and 0.050% or less,
Ca: 0.0003% or more and 0.0300% or less,
Mg: 0.0003% or more and 0.0300% or less,
REM: Any one of claims 3 to 6, wherein the steel composition contains at least one of 0.0003% or more and 0.0300% or less, and the balance is Fe and unavoidable impurities. The method for manufacturing a nickel-containing steel sheet according to.

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