JPH04289126A - Production of hot rolled steel plate having high workability and high tensile strength and excellent in uniformity of quality - Google Patents

Abstract

PURPOSE:To prevent the deterioration of quality at the head end and the tail end of a coil of hot rolled high tensile strength steel plate by performing coiling at specific cooling velocity in specific temp. range after finish rolling in specific temp. range. CONSTITUTION:A continuously cast slab having a composition consisting of, by weight, 0.02-0.20% C, 0.5-2.0% Si, 0.5-2.0% Mn, 0.5-1.0% Cr, 0.01-0.10% Al, and the balance iron with inevitable impurities is roughed. At the time of exerting temporary coiling and finish rolling, finish rolling is done at 800-900 deg.C. Successively, the resulting plate is cooled rapidly at >=15 deg.C/sec cooling rate and coiled at 350-550 deg.C. By this method, the hot rolled steel plate having high workability and high tensile strength and excellent in uniformity of quality can be produced with high productivity.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet having a two-phase structure of ferrite and martensite and having a uniform material quality in the longitudinal direction of the coil.

[0002] In recent years, as a high-strength steel plate with good workability,
So-called dual-phase steel sheets, which have a two-phase structure of ferrite and martensite, have been put into practical use and are used in structural parts of automobiles where workability is important. This 2
Phase structure There are two methods for producing high tensile strength hot rolled steel sheets: a method of continuous annealing after hot rolling, and a method of leaving the steel sheet as hot rolled.
The latter manufacturing method, which is advantageous in terms of cost, is the mainstream.

0003

[Prior Art] As mentioned above, various methods have been proposed for producing high-strength steel sheets with a dual-phase structure as hot-rolled. According to these methods, the alloying components include C,
It is known to hot roll a steel billet to which a suitable combination of Si, Mn, Cr, etc. have been added, and to control the coiling temperature. However, when these methods are implemented industrially, there is a problem in that the mechanical properties of the coil vary greatly in the length direction and width direction. Furthermore, although adding Mo to the above-mentioned alloy components is effective in homogenizing mechanical properties, Mo is extremely expensive, which has the disadvantage of significantly increasing manufacturing costs.

[0004] In order to solve this problem, the applicant company previously published the usual
We proposed a method for manufacturing dual-phase steel plates with minimal mechanical fluctuations by regulating the heating temperature and finishing temperature of the steel billets and performing controlled cooling using steel billets whose basic components are C, Si, Mn, and Cr. .

By the way, all two-phase microstructure type high-strength hot-rolled steel sheets require strict temperature control during production, and in normal hot rolling, during finish rolling, the temperature at the front and rear ends of the sheet bar is low. In addition, because the waiting time on the hot run table is longer toward the rear end of the sheet bar, there is a considerable temperature difference between the center and the rear end of the sheet bar, and therefore the entire length of the hot rolled sheet is It was difficult to make the material uniform throughout.

In this regard, in Japanese Patent Publication No. 52-45304, the rough-rolled strip is sequentially wound onto a coil, and then the subsequent strip is sequentially wound onto the coil while being unrolled and subjected to finish rolling. , by processing similarly,
A method of rolling a coil is proposed and disclosed. According to this rolling method, not only can the length of the hot run table be shortened, but also the temperature drop at the rolling tip of the sheet bar, that is, the unwinding rear end of the sheet bar coil, can be effectively suppressed while waiting on the hot run table. Therefore, it is considered to be effective at least for improving the material of the rear end of the coil.

[0007]

[Problems to be Solved by the Invention] However, even if the above-mentioned rolling method is used,
In manufacturing the ferrite-martensitic two-phase structure type high-strength hot-rolled steel sheet disclosed in the publication, deterioration of the material at the front and rear ends of the coil was unavoidable.

[0008] The above-mentioned rolling method is a method in which each coil is processed individually. This is because the temperature drop at the tip was still unavoidable.

[0009] In addition, especially when manufacturing the above-mentioned high-tensile hot-rolled steel sheets, it is necessary to rapidly cool the steel sheets to a predetermined temperature range after finish rolling, but the tip part passes through the finish rolling mill and is wrapped around the coiler. On the other hand, at the rear end, the steel plate is not restrained until it passes through the finish rolling mill and is wound up by the coiler, so if it is rapidly cooled, not only the shape will be disturbed, but also the cooling will be uneven. This is also because he was inviting people.

[0010] For this reason, conventionally, the front and rear ends were not subjected to rapid cooling treatment, and as a result, the parts that could not be made into the target material were forced to be discarded. Therefore, productivity was significantly hindered and costs increased.

[0011] The present invention advantageously solves the above problems, and even for ferritic-martensitic dual-phase steel sheets that require strict temperature control after finish rolling, it is possible to appropriately control the temperature over the entire length of the coil. The purpose of the present invention is to propose a method for manufacturing a highly formable, high-tensile hot-rolled steel sheet that can be applied to a coil and that can be made to have a uniform material over the entire length of the coil.

0012

[Means for solving the problem] That is, this invention
: 0.02 wt% or more 0.20 wt% or less, S
i: 0.5wt% or more and 2.0wt% or less, Mn
: 0.5wt% or more and 2.0wt% or less, Cr:
0.5wt% or more and 2.0wt% or less and Al:
Continuously cast steel slabs containing 0.01 wt% or more and 0.10 wt% or less, with the remainder being iron and unavoidable impurities, are roughly rolled to form a sheet bar, and then wound into a coil. Thereafter, finish rolling is started from the winding end at a rolling end temperature of 800°C or higher and 900°C or lower, and finish rolling is performed by sequentially connecting the winding ends of subsequent sheet bar coils to the trailing end. , Following this finish rolling, quenching is performed at a cooling rate of 15° C./S or more,
A method for producing a highly workable, high-strength hot-rolled steel sheet with excellent material uniformity, characterized by winding in a temperature range of 350°C or higher and 550°C or lower,

[0013] Further, in the manufacturing method using the steel billet having the above-mentioned composition, following finish rolling, the steel billet is heated at 600°C or higher for 700°C.
Air cool to a predetermined temperature in the range of 00 °C or less, and then
Rapid cooling at a cooling rate of 15°C/S or more, 350°C or more,
This is a method for producing a highly workable, high-strength hot-rolled steel sheet with excellent uniformity, which is characterized by winding in a temperature range of 550° C. or lower.

The greatest feature of this invention is the hot finish rolling step after rough rolling. Hereinafter, this hot finish rolling process will be specifically explained with reference to the drawings. FIG. 1 schematically shows a hot finish rolling line suitable for carrying out the present invention, and FIG. 2 illustrates actual winding, unwinding and joining procedures. In addition, in the illustration, number 1
is a rough rolling mill, 2 is a sheet bar coiler, 3 is an uncoiler, 4 is a joining device, 5 is a finishing mill, and C1
indicates the first coil, C2 indicates the second coil, and C3 indicates the third coil.

In the present invention, first, the first sheet bar after rough rolling is wound up as a coil C1. The coil C1 is then unwound and the unwound tip is fed to the finishing rolling mill 5, while the second sheet bar is wound up as a coil C2. Next, before finish rolling of coil C1,
The rear end of the coil C1 and the unwinding tip of the coil C2 are joined to achieve continuous finish rolling, and at the same time, 3
Wind up the actual sheet bar as coil C3. From then on,
Finish rolling is performed continuously by repeating the above steps.

[0016] By winding the rough-rolled sheet bar into a coil and then performing finish rolling while unwinding, the leading and trailing ends of the material to be rolled in the rough rolling are reversed in the finish rolling. . For this reason, under the temperature gradient that inevitably occurs from the leading edge of rough rolling to the trailing edge, the material to be rolled is rolled from the trailing edge of rough rolling, where the temperature is lower, in finish rolling. The temperature is made uniform over the entire length of the rolled material. In addition, winding into a coil after rough rolling has the effect of making the temperature uniform, especially in the area where the temperature drops locally at the rolling tip during rough rolling.
After rough rolling, it is reheated by being rolled into a coil,
The temperature becomes uniform and finish rolling is performed. Furthermore, by winding the roughly rolled sheet bar into a coil, it is easy to connect it to the preceding sheet bar, and with this connection, except for the leading end of the first rolled material and the rear end of the final rolled material, In finish rolling, rolling can be carried out without rolling edges, and therefore there is no local temperature drop at the rolling edges in finish rolling.

Therefore, as will be described later, even if the product coil is rapidly cooled down to a predetermined low temperature range after finish rolling, there will be no disturbance in shape or non-uniform cooling, and a uniform material can be obtained over the entire length of the product coil. be. The connected portions before finishing rolling are cut off during winding, and continuous rolling and cooling is achieved by winding up with a separate coiler.

The method of welding the seat bar is not particularly limited, but upset welding, high frequency heating welding, etc. are particularly advantageous. In addition, in the above example, the welding process is performed while the welding device is moved in synchronization with the running of the sheet bar, but in other cases, when the welding process is performed with the welding device stopped, this A looper may be placed between the device and the finish rolling mill.

[0019]

[Operation] In order to ensure excellent workability, this invention
It was developed with the aim of forming a two-phase structure of ferrite and martensite.

Generally, the workability of hot rolled steel sheets is controlled by ductility and yield ratio, and the higher the ductility and the lower the yield ratio, the better the workability. In order to achieve this, it is necessary to reduce the ratio of bainite as much as possible and increase the ratio of ferrite and martensite in the structure that constitutes the steel sheet, that is, to approach the ideal two-phase structure consisting only of ferrite and martensite. This is desirable.

[0021] Therefore, the present invention sets appropriate ranges for the conditions for obtaining such a two-phase structure, including the chemical composition, hot rolling and subsequent cooling conditions, and the like.

The reason for this limitation will be described below. (1) Reason for limiting the chemical composition range C: During the transformation from the γ phase to the α phase, C in the α phase diffuses and moves to the γ phase, which is an important component that increases the hardenability and the strength of martensite. It is. If it is less than 0.02 wt%, this effect will be insufficient, and if it exceeds 0.20 wt%, the martensite fraction will be excessive, resulting in deterioration of workability and weldability. Therefore, its content is set to 0.02 wt% or more and 0.20 wt% or less.

[0023]Si: has a large solid solution strengthening ability and can increase the strength without impairing the yield ratio or strength-elongation balance, and activates the transformation from γ phase to α phase and transforms into γ phase. It has properties that are advantageous for two-phase organization, such as promoting the diffusion of C. To obtain this effect, 0.5 wt.
% or more, but if it exceeds 2.0 wt%, the effect is saturated and surface flaws are likely to occur due to the formation of scale called red scale. Therefore, its content is set to 0.5 wt% or more and 2.0 wt% or less.

Mn: An important component that increases hardenability. If it is less than 0.5 wt%, the effect is insufficient;
When it exceeds 2.0 wt%, it has an adverse effect on weldability and tends to slow down the transformation rate from the γ phase to the α phase, resulting in an excessive increase in the martensite fraction. Therefore, its content should be 0.5 wt% or more and 2.0 wt% or less.

Cr: A component that improves hardenability and is an important component for this invention. It has the effect of increasing the stability of the remaining γ phase without significantly inhibiting the transformation from γ phase to α phase, and facilitates two-phase organization. To obtain this effect, 0.5 wt% or more is required, and 2.0 wt% or more is required.
If it exceeds wt%, there are problems such as an excessive increase in the martensite fraction and an increase in costs. Therefore, its content is 0.5 wt% or more, 2.0
It should be less than wt%.

Al: A useful component as a deoxidizer that cleans steel and improves workability. In order to exhibit this effect, 0.01 wt% or more is required, and if it exceeds 0.10 wt%, the effect is saturated. Therefore, its content is set to 0.01 wt% or more and 0.10 wt% or less.

(2) Reasons for limiting hot rolling treatment conditions In manufacturing the steel sheet of this invention, a slab melted by a conventional method is directly rolled, or once cooled and then reheated in a heating furnace. Hot rolling is performed from then on, but during this hot rolling, it is necessary to limit the finish rolling temperature and the cooling conditions after rolling as follows.

a. If the finish rolling temperature exceeds 900 °C, the formation of bainite in the final structure and the martensite fraction will be too high, leading to deterioration of workability.
If the temperature is lower than 0°C, processed structures will remain, which is not preferable. Therefore, the temperature should be 800°C or higher and 900°C or lower.

b. A cooling (quenching) rate of less than 15° C./S causes pearlite transformation and increases the yield ratio. Therefore, in order to obtain the desired ferrite-martensite two-phase structure,
or more is required.

c. If the coil is wound at a temperature exceeding 500°C, pearlite will be formed in the final structure, and if it is below 350°C, bainite will be mixed. Therefore, its temperature is over 350℃, 50℃
The temperature shall be below 0℃.

d. Air cooling treatment: Rather than immediately performing a quenching treatment after finish rolling, performing a short air cooling treatment before the start of quenching to lower the quenching start temperature promotes ferrite transformation and increases the concentration of C in the retained austenite. This is an effective means for improving workability, since the yield ratio can be reduced by making the steel more flexible. However, in this case, if the cooling start temperature after air cooling exceeds 700°C, there is no effect, and if it is lower than 600°C, pearlite transformation occurs, which is undesirable. Therefore, the temperature should be 600°C or more and less than 700°C.

[0032]

[Example] Example 1 C: 0.05 wt%, Si: 1.0 wt%, Mn melted in a converter
: 1.5 wt%, Cr: 1.0 wt%, Al: 0
．． A continuously cast steel slab having a composition of 0.02 wt % was used as a raw material and hot rolled into a hot rolled plate coil having a plate thickness of 3.5 mm. The hot rolling conditions at this time (center position in the longitudinal direction of the steel strip) are: - Winding temperature of the sheet bar after rough rolling: 1050 °C - Rolling end temperature: 840 °C - Rapid cooling start temperature: 800 °C (immediately after the end of rolling) rapid cooling)
・Cooling rate: 30°C/s ・Product winding temperature: 450°C When continuously rolling according to the method of this invention using the rolling equipment shown in Fig. 1 (compatible example), the sheet bar after rough rolling according to the conventional method Hot-rolled sheet coils were manufactured according to the method disclosed in the above-mentioned Japanese Patent Publication No. 52-45304 (comparative example). The tensile properties were investigated in the longitudinal direction over the entire length. These results are shown in FIG. As is clear from FIG. 3, the hot-rolled sheet coil rolled according to the present invention has a uniform material quality over the entire length of the coil. On the other hand, when the conventional method was followed, a large amount of non-uniform parts were generated at the tip and rear ends of the coil, and the yield was only 90.0%, compared to 99.8% in the present invention. Furthermore, when the method disclosed in Japanese Patent Publication No. 52-45304 is followed, the non-uniformity of the leading and trailing ends of the coil, particularly the rolling leading end of the sheet bar, that is, the unwinding trailing end of the sheet bar coil, is improved. However, the yield was 95.0%, which was far from sufficient.

Example 2 C: 0.05 wt%, Si: 1.0 wt%, Mn melted in a converter
: 1.5 wt%, Cr: 1.0 wt%, Al: 0.
Using a continuously cast steel slab having a composition of 0.02 wt% as a material,
Hot rolling was performed to obtain a hot rolled plate coil having a plate thickness of 3.5 mm.

The hot rolling conditions at this time (center position in the longitudinal direction of the steel strip) were: - Winding temperature of the sheet bar after rough rolling: 1050°C - Rolling end temperature: 850°C - Rapid cooling start temperature: 650°C (rolling Air cooling treatment after completion)・
Cooling rate: 50 °C/s Product winding temperature: 450 °C When continuous rolling was performed according to the method of this invention using the rolling equipment shown in Fig. 1 (conforming example), the sheet bar after rough rolling according to the conventional method Hot-rolled coils were manufactured by rolling without coil winding (conventional example) and by rolling according to the method disclosed in the above-mentioned Japanese Patent Publication No. 52-45304 (comparative example). The tensile properties were investigated with respect to the direction. The results of these surveys are shown in Figure 4.

As is clear from FIG. 4, the hot-rolled sheet coil rolled according to the present invention has a uniform material quality over the entire length of the coil.

On the other hand, when the conventional method is followed, a large amount of non-uniformity occurs at the tip and rear ends of the coil, and the yield is only 92%, compared to 99.5% in the present invention. Ta. Furthermore, when the method disclosed in Japanese Patent Publication No. 52-45304 is followed, the non-uniformity of the leading and trailing ends of the coil, particularly the rolling leading end of the sheet bar, that is, the unwinding rear end of the sheet bar coil, is improved. However, the yield was 95%, which was not sufficient.

[0037]

[Effect of the invention] This invention provides C, Si, Mn, C
Using a steel material whose basic component is R, during hot rolling, the sheet bar after rough rolling is once wound into a coil shape, and while finishing rolling is further performed, the winding end of the subsequent sheet bar coil is rolled at the rear end. This system connects the rolling stock in sequence and rolls it continuously, and also controls the temperature during hot rolling and subsequent cooling.

According to the present invention, finish rolling can be performed at a substantially uniform temperature, and a highly workable, high-tensile strength hot-rolled steel sheet with excellent material uniformity can be manufactured with high productivity.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a hot rolling line suitable for carrying out the present invention.

FIG. 2 is an explanatory diagram of winding, unwinding and joining procedures according to the present invention.

FIG. 3 is a graph showing changes in tensile properties in the longitudinal direction of the coil.

FIG. 4 is a graph showing changes in tensile properties in the longitudinal direction of the coil.

[Explanation of symbols]

1 Rough rolling mill 2 Sheet bar coiler 3 Uncoiler 4　Joining device 5 Finishing rolling machine

Claims (2)

[Claims] Claim 1: C: 0.02 wt% or more, 0.02 wt% or more.
20 wt% or less, Si: 0.5 wt% or more, 2.0 wt% or less, Mn: 0.5 wt%
Above, 2.0 wt% or less, Cr: 0.5
wt% or more, 2.0 wt% or less, and Al:
Continuously cast steel slabs containing 0.01 wt% or more and 0.10 wt% or less, with the remainder being iron and unavoidable impurities, are rough rolled and made into sheet bars.
Once wound into a coil shape, finish rolling is started from the end of the winding at a rolling end temperature of 800°C or higher and 900°C or lower, and the winding end of the subsequent sheet bar coil is sequentially applied to the trailing end. A high quality material with excellent material uniformity characterized by connecting and finishing rolling, followed by rapid cooling at a cooling rate of 15°C/S or more, and winding in a temperature range of 350°C or more and 550°C or less. A method for manufacturing high-strength hot-rolled steel sheets. Claim 2: C: 0.02 wt% or more, 0.02 wt% or more.
20 wt% or less, Si: 0.5 wt% or more, 2.0 wt% or less, Mn: 0.5 wt%
Above, 2.0 wt% or less, Cr: 0.5
wt% or more, 2.0 wt% or less, and Al:
Continuously cast steel slabs containing 0.01 wt% or more and 0.10 wt% or less, with the remainder being iron and unavoidable impurities, are rough rolled and made into sheet bars.
Once wound into a coil shape, finish rolling is started from the end of the winding at a rolling end temperature of 800°C or higher and 900°C or lower, and the winding end of the subsequent sheet bar coil is sequentially applied to the trailing end. Connect and perform finish rolling, and following this finish rolling, air cooling to a predetermined temperature in the range of 600 °C or more and 700 °C or less, followed by rapid cooling at a cooling rate of 15 °C / S or more, and then 350 °C or more and 550 °C A method for producing a highly workable, high-strength hot-rolled steel sheet with excellent uniformity, characterized by winding in the following temperature range.