JP4380349B2 - Method for producing precipitation-strengthened hot-rolled steel sheet with uniform mechanical properties - Google Patents

Description

The present invention relates to a method for producing a precipitation-strengthened hot-rolled steel sheet having a strength level of 340 MPa or higher used for automobiles, home appliances, and industrial machines.

Hot-rolled steel sheets are widely used in automobiles, home appliances, industrial machines and the like. In particular, in recent years, in the field of automobiles, there is a strong need for weight reduction, and the usage ratio of high-strength steel sheets having a strength level of 340 MPa or higher tends to increase. However, a high strength steel plate has a problem that mechanical properties are more sensitive to manufacturing conditions than a mild steel plate, and mechanical properties such as strength are not uniform. If the mechanical properties of the steel sheet become non-uniform, the dimensional accuracy during press forming deteriorates and causes a reduction in product yield. Therefore, a method for producing a high-strength steel sheet with more uniform mechanical properties is desired. .
One of the main strengthening methods for steel sheets is precipitation strengthening. Precipitation strengthening is achieved by adding a small amount of carbide-forming elements such as Nb, Ti, and V to steel, and these fine carbides become an obstacle to the progress of dislocation, and realize high strength at low cost. Among carbide generating elements, Nb is known to have a remarkable effect of improving strength.
The amount of strengthening of the steel sheet by precipitation strengthening is determined by the size of the carbides precipitated in the ferrite during the cooling process after hot rolling and the amount of precipitation. It will be very sensitive. For example, in a conventional cooling method such as laminar cooling, a difference in cooling conditions greatly affects the precipitation state. On the other hand, since there is a limit to the accuracy of temperature control industrially, it has been difficult to obtain a high-strength hot-rolled steel sheet having uniform mechanical properties.
On the other hand, conventionally, as a method for uniforming the mechanical properties of a hot-rolled steel sheet, for example, Patent Document 1 discloses a method for reducing material variations in a hot-rolled coil using rapid cooling after hot rolling. It is disclosed.
JP 2002-69534 A

  However, since the method described in Patent Document 1 uses the refinement of the structure of ferrite crystal grains, pearlite, etc. as a means of material homogenization, the material of the precipitation-strengthened high-strength steel sheet intended by the present invention It cannot be applied to the variation suppression method.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for producing a precipitation-strengthened hot-rolled steel sheet having uniform mechanical properties.

The present inventors diligently studied the precipitation mechanism in order to make the mechanical properties of the precipitation strengthened hot rolled steel sheet uniform. As a result, it is clear that by stabilizing the formation of precipitates, which is the main factor of strength, by ultra-rapid cooling immediately after hot rolling, the dependence of strength on cooling conditions can be reduced and strength variations due to fluctuations in operating conditions can be suppressed. It became. That is, NbC is kept in a supersaturated state in the austenite region, and then the supersaturation is increased by rapid cooling at a temperature of 200 ° C./s or higher from the temperature immediately above Ar 3, and then wound up within a temperature range of 550 ° C. to 650 ° C. Found that it was effective.
The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] By mass%, C: 0.01 to 0.16%, Si: 1.0% or less, Mn: 0.1 to 1.9%, P: 0.05% or less, S: 0.01% or less, Al: 0.01 to 0.1%, Nb: 0.015 to The steel containing 0.060%, N: 0.006% or less , the balance being Fe and inevitable impurities is hot-rolled at a finish rolling finish temperature of Ar3 or higher, and then 200 ° C. within 2 seconds after the hot rolling is finished. The mechanical properties are characterized by cooling to 600 to 750 ° C. at a cooling rate of at least / s, then secondary cooling at a cooling rate of 5 to 50 ° C./second, and winding at a temperature of 550 to 650 ° C. A method for producing a uniform precipitation strengthened hot rolled steel sheet.

[2] In addition to the steel of [1], in terms of mass%, Ti: 0.1% or less, V: 0.05% or less, Cr: 1% or less, Mo: 0.3% or less, Cu: 0.3% or less, Ni: 0.3 % Or less, B: 0.003% or less of one type or two or more types, a method for producing a precipitation-strengthened hot-rolled steel sheet having uniform mechanical properties.

[3] In the above [1] or [2], when winding at a temperature of 550 to 650 ° C, the temperature difference of the coil winding length is 30 ° C or less, and the temperature difference of the entire width is 20 ° C or less. A method for producing a precipitation-strengthened hot-rolled steel sheet having uniform mechanical properties.

In the present invention, the high-strength hot-rolled steel sheet is a hot-rolled steel sheet having a tensile strength (hereinafter referred to as TS) of 340 MPa or more, preferably TS590 MPa or more.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the precipitation strengthening type | mold hot-rolled steel plate with a uniform mechanical property can be provided. Since the precipitation-strengthened hot-rolled steel sheet produced according to the present invention has high strength and excellent uniformity in mechanical properties, a part with high dimensional accuracy can be easily produced by press molding or the like. Furthermore, it contributes to the development of society because it contributes to the improvement of product yield and energy saving by effective use of resources and improvement of manufacturing efficiency.

The present invention defines the following components, further defines hot rolling conditions, specifically, hot rolling is performed at a finishing rolling finishing temperature of Ar3 or higher, and then 200 seconds within 2 seconds after the hot rolling is finished. It is characterized by cooling to 600 to 750 ° C. at a cooling rate of at least ° C./s, then secondary cooling at a cooling rate of 5 to 50 ° C./s, and winding at a temperature of 550 to 650 ° C. This is the most important requirement in the present invention, and it becomes possible to obtain a precipitation-strengthened hot-rolled steel sheet having uniform mechanical properties from the above characteristics.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the chemical components of steel in the present invention are as follows.
C: 0.01 to 0.16%
C is added to ensure the strength of the steel sheet. If it is less than 0.01%, Nb carbide cannot be sufficiently produced, and the strength of 340 MPa or more targeted in the present invention cannot be obtained. On the other hand, when it exceeds 0.16%, the workability deteriorates remarkably. From the above, it is set to 0.01% or more and 0.16% or less. In order to lower the Ar3 and stabilize the transformation behavior and stabilize the NbC precipitation at a low temperature to make the mechanical properties more uniform, it is preferably 0.05% or more. Moreover, when performing spot welding, in order to acquire favorable weldability, it is preferable to set it as 0.1% or less.
Si: 1.0% or less Si is a solid solution strengthening element and is added as necessary. However, if it exceeds 1.0%, the surface properties deteriorate, so 1.0% or less. Moreover, when performing a chemical conversion treatment, it is preferable to set it as 0.5% or less in order to prevent the chemical conversion deterioration by the Si oxide of a steel plate surface layer.
Mn: 0.1-1.9%
Mn improves the toughness of the steel sheet and improves the strength by solid solution strengthening. For that purpose, 0.1% or more is necessary. However, when it exceeds 1.9%, the deterioration of workability becomes remarkable. From the above, it should be 0.1% or more and 1.9% or less. In order to lower the Ar3 and stabilize the transformation behavior and stabilize the NbC precipitation at a low temperature to make the mechanical properties more uniform, it is preferably 0.65% or more. Furthermore, if it exceeds 1.6%, the influence of the cooling conditions after hot rolling on the strength becomes large, and material variations may occur, so 1.6% or less is preferable.
P: 0.05% or less P is added as necessary in order to improve the strength of the steel sheet by solid solution strengthening. However, if it exceeds 0.05%, the joint strength of the welded portion is lowered, so 0.05% or less.
S: 0.01% or less S is contained in steel in a trace amount as an impurity. However, if it exceeds 0.01%, weldability and workability are significantly deteriorated, so 0.01% or less.
Al: 0.01 to 0.1%
Al is added to deoxidize and precipitate N as AlN. However, if it is less than 0.01%, the effects of deoxidation and AlN precipitation are not sufficient. On the other hand, if it exceeds 0.1%, the effect of Al addition is saturated and uneconomical. From the above, it is set to 0.01% or more and 0.1% or less.
Nb: 0.015-0.060%
Nb is an essential element for improving the strength of the steel sheet by precipitation strengthening, and is added in an amount of 0.015% or more. However, if it exceeds 0.060%, the carbides become coarse and the precipitation strengthening ability decreases. From the above, it is 0.015% or more and 0.060% or less.
N: 0.006% or less N is contained in the steel in a trace amount as an impurity. However, if it exceeds 0.006%, it combines with Al or Nb to form a nitride and deteriorate the workability, so the content is made 0.006% or less.
In addition, although the steel plate of this invention has the target effect by said component, the following elements can be added according to a desired characteristic. For example, Ti, V, Cr, Mo, Cu, Ni, and B can be added as needed, for adjusting strength, as one or more. In that case, if each addition amount exceeds 0.1%, 0.05%, 1%, 0.3%, 0.3%, 0.3%, 0.003%, mechanical properties such as ductility deteriorate, so when adding Ti, 0.1% or less, V is 0.05% or less, Cr is 1% or less, Mo is 0.3% or less, Cu is 0.3% or less, Ni is 0.3% or less, and B is 0.003% or less.

  Next, a manufacturing method will be described.

The precipitation-strengthened hot-rolled steel sheet of the present invention, after casting a slab adjusted to the above chemical composition range, directly or by heating, performs hot rolling at a finish rolling finishing temperature of Ar3 or higher, and then finishes hot rolling. Within 2 seconds, cool to 600-750 ° C at a cooling rate of 200 ° C / s or higher, then secondary cool at a cooling rate of 5-50 ° C / s or higher, and wind at a temperature of 550-650 ° C. Is obtained.

(1) Hot rolling process in which the finish rolling finish temperature is Ar3 or higher.
The rolling finish temperature in the finishing mill is set to Ar3 or higher, and the ferrite and pearlite after transformation are refined so that the required strength can be obtained. If it is hot-rolled in less than Ar3, that is, in the ferrite and austenite two-phase regions, the formation of equiaxed ferrite is hindered, and in the subsequent cooling process, the precipitation of carbides in the ferrite becomes unstable, leading to non-uniform mechanical properties. .

(2) A step of primary cooling to 600 to 750 ° C. at a cooling rate of 200 ° C./second or more within 2 seconds after completion of hot rolling.
Primary cooling after hot rolling starts within 2 seconds after finish rolling in order to refine ferrite and pearlite after transformation and obtain excellent workability and high strength. When the start time exceeds 2 seconds, ferrite is generated non-uniformly at the austenite grain boundaries, and subsequent precipitation of carbides in the ferrite becomes unstable, leading to non-uniform mechanical properties. In order to obtain better uniformity, it is desirable to start cooling within 1 second after finish rolling.
When the primary cooling rate is less than 200 ° C./s, non-uniform precipitation of ferrite on the austenite grain boundaries during cooling cannot be suppressed, and the size and amount of precipitated carbides vary, resulting in poor mechanical properties. Causes uniformity. Therefore, the cooling rate is 200 ° C./s or more. Furthermore, in order to further improve the effect of the present invention, the cooling rate is preferably 300 ° C./s or more.
The primary cooling end temperature is 600 ° C to 750 ° C. When the primary cooling end temperature exceeds 750 ° C., the effect on the uniform precipitation of Nb carbide due to the increase in supersaturation becomes insufficient and the strength becomes unstable. On the other hand, if the temperature is lower than 600 ° C., the temperature is too low, so that an appropriate winding temperature cannot be secured, and high strength cannot be obtained.
The primary cooling is preferably performed in a nucleate boiling state where there is no water vapor atmosphere on the surface of the steel sheet to be cooled. As a cooling device therefor, a plurality of injection nozzles are used, and 2000 L / m ² · min or more. It is desirable to use equipment that cools at a water pressure of 0.2 MPa or more.

  (3) A step of performing secondary cooling at a cooling rate of 5 to 50 ° C./s after the primary cooling.

  After the primary cooling, secondary cooling is performed at a cooling rate of 5 to 50 ° C./s. If the secondary cooling rate exceeds 50 ° C./s or less than 5 ° C./s, it is difficult to stably control the coiling temperature, resulting in material fluctuation.

(4) A step of winding at 550 ° C. to 650 ° C. after the secondary cooling.
In order to stabilize the carbide precipitation temperature before and after winding and to precipitate the carbide uniformly and finely, the winding temperature is set to 550 ° C to 650 ° C. If the coiling temperature is less than 550 ° C., the temperature is too low, so that a sufficient amount of carbides cannot be obtained, and sufficient strength cannot be obtained. On the other hand, when the temperature exceeds 650 ° C., the ferrite grain size becomes coarse and NbC coarse precipitates, so that sufficient strength cannot be obtained.

  The hot rolling of the present invention may be performed as it is after continuous casting or may be performed after reheating. In the case of reheating, the heating temperature is set to 1200 ° C or higher and the heating time is set to 120 minutes or longer to sufficiently dissolve Nb in austenite and stabilize the amount of carbides precipitated in the ferrite after hot rolling. It is preferable because it is possible. Moreover, in order to stabilize finish rolling completion temperature, it is preferable that rough rolling completion temperature shall be 950 degreeC or more. Furthermore, in order to make the coiling temperature in the coil longitudinal direction uniform, the acceleration of the rolling speed is preferably within 1.5 mpm / s.

  Further, in order to obtain a more uniform material, it is preferable that the temperature difference of the coil winding length is 30 ° C. or less and the temperature difference of the full width is 20 ° C. or less. The difference in precipitation strengthening due to the temperature difference in the coil can be reduced by making the temperature difference of the coil winding length 30 ° C or less and the temperature difference of the entire width 20 ° C or less. A more uniform material can be obtained.

  Further, a skin pass may be performed for shape correction and material adjustment. In that case, it is desirable that the rolling elongation by skin pass is 0.3% to 2%.

After continuous casting of the slab having the chemical composition and composition shown in Table 1, a hot-rolled steel sheet having a thickness of 3.0 mm was produced under the production conditions shown in Table 2. At this time, the cooling after hot rolling was performed at a water pressure of 3000 MPa at a water pressure of 3000 L / m ² · min using nozzles arranged above and below the steel plate at intervals of 50 mm.

The results obtained are shown in Table 3. The mechanical properties of five JIS No. 5 tensile test specimens were collected from the center of the plate width of Top 5 m, 1/4, center, 3/4, Bottom 5 m in the longitudinal direction for each coil and tested according to JIS Z 2241. In the table of mechanical characteristics, TS and El are values at the central portion in the coil longitudinal direction, and ΔTS and ΔEl are differences between the maximum value and the minimum value of the test values collected in the longitudinal direction.

  From Table 3, it can be seen that all the steel plates produced by the method of the present invention are as small as ΔTS ≦ 30 MPa and ΔEl ≦ 3%, and excellent in mechanical property uniformity. Further, the invention examples B to E in which the temperature difference of the coil full length of the coiling temperature at the time of winding is 30 ° C. or less are lower than ΔTS and ΔEl compared to the invention example A, and a more uniform material can be obtained. You can see that

  On the other hand, since the finishing temperature of the symbol F is lower than Ar3, sufficient equiaxed ferrite cannot be obtained, and the precipitation of carbides is non-uniform, resulting in a large material fluctuation. In the code G, since the cooling start time is long, the ferrite formation is not uniform, the carbide precipitation is non-uniform, and the material variation is large. Since the cooling rate of the code H is low, the cooling is not uniform in the coil. As a result, the precipitation of carbides is not uniform and the material variation is large. In code I, since the coiling temperature is high, coarse precipitation of carbide occurs, and the material variation is large. Since the coiling temperature J is low, a part of the bainite structure is generated and the material variation is large.

Claims (3)

In mass%, C: 0.01 to 0.16%, Si: 1.0% or less, Mn: 0.1 to 1.9%, P: 0.05% or less, S: 0.01% or less, Al: 0.01 to 0.1%, Nb: 0.015 to 0.060%, N: The steel containing 0.006% or less and the balance being Fe and inevitable impurities is hot-rolled at the finishing rolling finishing temperature of Ar3 or higher, and then 200 ° C / s or higher within 2 seconds after the hot rolling is finished. Precipitation with uniform mechanical properties, characterized by cooling to 600 to 750 ° C at a cooling rate of 2 ° C, then secondary cooling at a cooling rate of 5 to 50 ° C / sec and winding at a temperature of 550 to 650 ° C A method for producing a reinforced hot-rolled steel sheet. The steel according to claim 1, further comprising, in mass%, Ti: 0.1% or less, V: 0.05% or less, Cr: 1% or less, Mo: 0.3% or less, Cu: 0.3% or less, Ni: 0.3% or less 2. The method for producing a precipitation-strengthened hot-rolled steel sheet having uniform mechanical properties according to claim 1, wherein B: 0.003% or less is contained. 3. The machine according to claim 1, wherein when winding at a temperature of 550 to 650 ° C., the temperature difference of the coil full length of the winding temperature is 30 ° C. or less and the temperature difference of the full width is 20 ° C. or less. For producing precipitation-strengthened hot-rolled steel sheets with uniform mechanical properties.