JPH04224635A - Manufacture of high workability hot-rolled high tensile strength steel sheet excellent in uniformity of quality - Google Patents

Abstract

PURPOSE:To obtain a hot-rolled high tensile strength steel sheet having uniform quality and high workability over the whole length of a coil by executing rolling with front tip part and rear end part of a sheet coil in the rough-rolling reversed to a steel slab having the specific composition at the time of executing finish-rolling and succes sively executing the prescribed heat treatment continuously after rolling. CONSTITUTION:The hot-rolling is executed to the steel slab containing 0.05-0.30wt.% C, < 1.0% Si, 1.0-2.5% Mn and 0.005-0.10% Al at 800-1000 deg.C finish controlling temp. after executing the roughrolling. At this time, the first sheet after executing the rough- rolling is wound as a coil C1 and successively, teh coil 1 is recoiled and the recoiled front tip part is supplied to a finishing mill 5 and on the other hand, the second coil C2 is wound. Successively, before completing the finish rolling as the coil C1, the rear end part of coil C1 and the recoiled front tip part of coil C2 are jointed with a joining device 4 to contrive continuation of the finish-rolling and substantial uniformity of working temp. Successively, after cooling the rolled sheet to <=300 deg.C at >=40 deg.C/sec cooling rate, this is reheated to 725-840 deg.C, and after holding for 5-200 sec, this cooled to <=300 deg.C at 3-50 deg.C/sec cooling rare.

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 hot-rolled high-strength steel sheet having uniform material properties in the longitudinal direction of the coil and excellent formability.

0002

[Prior Art] Generally, as the strength of hot-rolled steel sheets increases, such as yield point and tensile strength, the ductility such as total elongation and bending decreases, and the higher the tensile strength of the steel, the less suitable it is for cold working applications. However, in recent years, high-strength steel sheets have come to be used in various fields, and there is a strong demand for the development of hot-rolled high-strength steel sheets with excellent cold workability. Ferrite-martensitic composite structure type hot-rolled high-strength steel sheet is a high-strength steel sheet that was developed against this background.It has material properties such as a low yield ratio and an excellent strength-ductility balance, and is suitable for use in automobile wheels. Currently, 55 to 70 kgf/mm2 class materials are used.

By the way, a wheel disk is a component with a complex shape, and during its manufacturing process, the material undergoes various types of cold working, such as bending, stretching, deep drawing, unbending, and stretch flanging. Since severe processing is applied, the materials used in these products are required to have comprehensive workability that can withstand the various workability described above. Furthermore, it is also necessary for the processed product to have excellent fatigue strength.

However, in conventional hot-rolled high-strength steel sheets with a ferrite-martensitic composite structure, (1) the dimensional accuracy of the product after forming is limited due to the anisotropy of mechanical properties based on the anisotropy of the structure; (2) Similarly, due to the anisotropy of mechanical properties, stretch flangeability and burring workability in the direction perpendicular to rolling are inferior.
3) Bending/unbending workability was inferior to other high-strength steel sheets; (4) Fatigue resistance was inferior to other high-tensile strength steel sheets. In addition, in order to obtain a ferrite-martensitic composite structure as hot-rolled, it is necessary to increase the content of Si, Mn, Cr, P, etc. among the components, which is disadvantageous in terms of manufacturing cost.

[0005] As a solution to the above problem, Japanese Unexamined Patent Application Publication No. 1988-
No. 145965 and Japanese Patent Application Laid-open No. 58-2485 propose a hot-rolled high-strength steel sheet in which the steel structure is made into a ferrite-bainite composite structure by subjecting C-Si-Mn steel to cooling control and coiling temperature control. ing. However, such hot-rolled high-strength steel sheets with a ferrite-bainite composite structure are intended to improve only the drawbacks of the above-mentioned hot-rolled high-strength steel sheets with a ferrite-martensitic composite structure, such as low yield ratio characteristics and elongation. On the contrary, it is inferior to ferrite-martensitic composite steel in terms of properties, stretchability, deep drawing formability, etc., and from the perspective of overall composite formability, high-strength steel sheets are not necessarily good in all aspects. I could not say it.

[0006] In this regard, the applicant company previously published Japanese Patent Application Laid-Open No. 2-1
In order to solve the above problem, Japanese Patent No. 75817 discloses a hot-rolled high-strength steel sheet with a ferrite-martensitic composite structure and a ferrite-bainite composite structure using a steel with a composition similar to that of the general C-Si-Mn system. While retaining the advantages of hot-rolled high-strength steel sheets in terms of material properties,
Furthermore, we have advantageously overcome these drawbacks and proposed a new composite structure type hot-rolled high-strength steel sheet with excellent overall composite formability.

[0007] Incidentally, all of the above-mentioned composite structure type hot-rolled high-strength steel sheets require strict temperature control during production, and in normal hot rolling, during finish rolling, the tip and back of the sheet bar are Because the temperature is forced to drop locally at the ends, and because the waiting time on the hot run table is longer toward the rear end of the seat bar, there is a considerable temperature difference between the center and the rear end of the seat bar. Therefore, it was difficult to make the material uniform over the entire length of the hot-rolled sheet.

In this regard, in Japanese Patent Publication No. 52-45304, a rough-rolled strip is once wound into a coil, and then while being unrolled and subjected to finish rolling, subsequent strips are sequentially wound into a coil, A method was proposed in which coils were continuously rolled by a similar process. According to this rolling method, not only can the length of the hot run table be shortened, but also the temperature drop at the tip of the sheet bar while waiting on the hot run table can be effectively suppressed. This is considered to be effective in improving the material of the return tail end.

[0009]

[Problems to be Solved by the Invention] However, even if the above-mentioned rolling method is used, it is still difficult to produce the ferrite-bainite composite structure hot-rolled high-strength steel sheet disclosed in JP-A-2-175817. Deterioration of the material at the leading and trailing ends could not be avoided. This is because the above-mentioned rolling method is a method in which each coil is processed individually, so apart from eliminating the temperature gradient that inevitably occurs from the tip to the rear end of the sheet bar, at least the temperature at the unwinding tip of the sheet bar is reduced. The descent was still unavoidable. In particular, when producing the above-mentioned hot-rolled high-strength steel sheets, it is necessary to rapidly cool them down to a predetermined low temperature range after finish rolling, and the tip end is heated on one side until it passes through the finish rolling mill and is wound around the coiler. The steel plate is not restrained at the rear end after it passes through the finishing mill until it is wound up by the coiler, so if it is rapidly cooled, it will not only have a disordered shape but also uneven cooling. be. For this reason, in the past, the leading and trailing ends were not subjected to quenching treatment, and as a result, the parts that could not be made of the target material had to be discarded. Therefore, productivity was significantly hindered and costs increased.

[0010] The present invention advantageously solves the above-mentioned problems, and even steel materials that require strict temperature control after finish rolling can be appropriately subjected to a prescribed heat treatment over the entire length of the coil. The purpose of the present invention is to propose a method for manufacturing a highly workable hot-rolled high-strength steel sheet that can be made of uniform material over the entire length of the coil.

[0011]

[Means for solving the problem] That is, this invention
: 0.05 to 0.30wt% (hereinafter simply expressed as %), S
After rough rolling, a steel slab containing i: 1.0% or less, Mn: 1.0 to 2.5%, and Al: 0.005 to 0.10% is subjected to finish rolling at a temperature of:
When performing hot rolling under the conditions of 800 to 1000°C, the sheet bar that has undergone rough rolling is once wound into a coil, and then finish rolling is started from the end of the winding, and the end of the winding of the following sheet bar is placed on the back end. Connect sequentially to perform finish rolling continuously, and following this finish rolling, 4
After cooling to 300℃ or less at a cooling rate of 0℃/s or more, reheat to a temperature range of 725 to 840℃.
This is a method for producing a highly workable hot-rolled high-strength steel sheet with excellent material uniformity, which comprises holding for ~200 seconds and then cooling to 300°C or less at a cooling rate of 3 to 50°C/s.

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 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 C
1 is the first coil, C2 is the second coil, C3
shows 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 end is fed to the finishing mill 5, while the second sheet bar is wound up as a coil C2. Next, before finishing the finish rolling of the coil C1, the tail end of the coil C1 and the unwinding tip of the coil C2 are joined to make the finish rolling continuous, and at the same time, the third sheet bar is connected to the coil C3 2. Wind it up as From then on,
Finish rolling is performed continuously by repeating the above steps.

[0014] 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 rough rolling are reversed in finishing 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. Also, in the above example,
The case where the joining process is performed while the joining device is moved in synchronization with the running of the sheet bar has been shown, but in other cases, when the joining process is performed while the joining device is stopped, the joining device and finishing rolling mill must be connected. You can place a looper between them.

[0017]

[Operation] This invention was developed with the aim of forming a new type of composite structure different from the conventional one in order to ensure excellent composite formability. In other words, while conventional steel has a structure in which massive martensite phases are dispersed in a matrix of polygonal ferrite phase, in the case of this invention steel, the matrix is a tempered martensite phase, and the matrix is composed of tempered martensite. There is a newly formed fine martensite phase in the portion corresponding to the phase lath, and this exhibits a uniformly dispersed composite structure. Such a composite structure steel sheet can only be obtained by hot rolling and cooling to create a structure mainly consisting of martensitic phase, and then reheating in a two-phase region and cooling. The martensite part corresponds to the part where martensite in the old structure was tempered and became ferrite, and the fine martensite part corresponds to the part where the lath part of martensite in the old structure became austenite during reheating in the two-phase region. It becomes martensite again upon cooling. When such a composite structure is formed, properties different from those of conventional composite structure steel are developed.

In addition, the hard phase in conventional composite structure steel is high carbon martensite, but since this high carbon martensite phase has high strength, it has almost no deformability. The phase takes charge, and ferrite undergoes significant work hardening. When the work hardening of the ferrite phase reaches the same level as the strength of high-carbon martensite, the hard phase acts as a void nucleus, causing immediate proliferation of voids and fracture. In contrast, in the steel of this invention, low carbon martensite is the hard phase and tempered martensite is the soft phase, and the soft phase takes charge of deformation in the initial stage of processing, but the hard phase of low carbon martensite is stronger than the high carbon martensite. Since the deformability of martensite is much greater, when the work hardening of the soft phase reaches the same stage as the strength of low carbon martensite, the hard phase also becomes subject to deformation. Therefore, in the subsequent stages, the soft phase and hard phase deform as a unit, and since the hard phase does not act as a void nucleus, the time of fracture deformation is delayed, resulting in high workability. .

As described above, the structure of the steel of the present invention has low yield ratio properties, high elongation properties, good overhang workability, good bending workability, good stretch flangeability, good deep drawability, and This results in material properties that are excellent in overall composite forming processability, such as good repeated bending processability. It should be noted that such a composite structure can only be obtained through the manufacturing process according to the present invention, and can be obtained not only through the conventional hot-rolled manufacturing process but also through the hot-rolled and then reheated manufacturing process. Even if it exists, it cannot be obtained when the structure after hot rolling is a ferrite-pearlite structure or a bainite structure.

[0020] The present invention provides conditions for obtaining such a composite structure, including chemical components, hot rolling conditions,
Appropriate ranges are set for the intermediate structure at the end of hot rolling and cooling, and the subsequent reheating and cooling treatment conditions. The reason for this limitation will be explained below.

(1) Reason for limiting the range of chemical components C: 0.05 to 0.30% C is in order to obtain the necessary strength and to make the final structure a composite structure of tempered martensite and fine martensite. It is an essential element and requires at least 0.05%, but if it exceeds 0.30%, weldability deteriorates, so it is limited to a range of 0.05 to 0.30%.

Si: 1.0% or less Si promotes solid solution strengthening and good composite structure, and effectively contributes to improving the strength-elongation balance, but if the content exceeds 1.0%, red 1. Since scale will occur and cause deterioration of bending workability.
．． The content was set to be 0% or less.

[0023] Mn: 1.0 to 2.5% Mn is C
Similarly, it is a useful element to obtain the necessary strength and desired composite structure, and its content must be at least 1.0%, but if it exceeds 2.5%, weldability deteriorates, so the content is It was limited to a range of 1.0 to 2.5%.

Al: 0.005 to 0.10% Al is a useful element that improves the cleanliness of steel through its deoxidizing effect, but if it is less than 0.005%, the effect of its addition is poor; Even if it is added in excess of 0.00%, the effect will reach saturation and the elongation properties will deteriorate.
The content was set to be in the range of 5 to 0.10%.

[0025] This invention steel basically has C in the above range,
Although it can be manufactured by adjusting the amounts of Si, Mn, and Al, further improvement in material quality can be expected by appropriately adjusting the contents of the elements described below. Its effects and appropriate doses are described below.

[0026] Nb, Ti: Both of these are precipitation-strengthening elements, and when used in appropriate amounts can increase strength without deteriorating weldability. What is the appropriate amount here?
In the case of Nb, it is in the range of 0.005 to 0.10%, and in the case of Ti, it is in the range of 0.01 to 0.20%. The lower limit of the appropriate amount range for any element is because the effect of adding the element is not obtained if the amount is less than the lower limit, and the upper limit is because the effect is saturated even if added in an amount exceeding this limit.

[0027] Cr, Ni, Mo: All of these elements are elements that improve hardenability, and if used in appropriate amounts, increase the martensite ratio during hot rolling and cooling, and improve the lath structure of martensite. Through the action of refining the particles, the final composite structure after the two-phase region reheating-cooling treatment in the next step is improved, and various molding processability is improved. In order to obtain such an effect, 0.
It is necessary to add 10% or more of Cr
It is desirable that Ni+Mo≦0.50%.

[0028] Ca:CaS has the effect of controlling the shape of S-based inclusions by making them into fine spheres, and effectively contributes to improving mechanical properties, particularly stretch flangeability, burring workability, rolling anisotropy, and the like. To obtain such an effect, 0.0
It is desirable to add more than 0.005%, but even if it is added more than 0.030%, the effect not only reaches saturation, but also
On the other hand, it worsens the cleanliness, so when adding 0.0
It is preferably in the range of 0.005 to 0.030%.

P, S: Both of these elements promote segregation, increase non-metallic inclusions, etc. and have an adverse effect on various workability, so it is desirable to use them in as low a range as possible. However, in the case of P, it is permissible within the range of 0.015% or less, and in the case of S, it is permissible within the range of 0.010% or less.

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

i) Finish rolling temperature: 800-10
00℃. This final structure has no anisotropy and is uniform.
This is a necessary requirement to obtain a composite structure having fine martensite. That is, in order to obtain a good composite structure according to the present invention, it is important to make the structure after hot rolling and cooling as close as possible to a uniform, fine martensitic single phase structure. If the temperature is lower than that, the austenite grains become finer and rolling strain is introduced into the austenite grains due to low-temperature rolling, and pro-eutectoid ferrite is likely to be generated from these parts, making it impossible to obtain the desired structure. On the other hand, when the finish rolling temperature exceeds 1000°C, the austenite grains become too coarse, leading to coarsening of the martensite phase.

ii) Cooling conditions after hot rolling: Cooling to 300°C or less at a cooling rate of 40°C/s or more The cooling conditions after hot rolling in this invention are limited as in the case of finish rolling temperature. The structure at a later point in time is determined as a necessary requirement to obtain a uniform, fine martensitic single-phase structure. The reason why the cooling rate is set to 40°C/s or more and the temperature after quenching is set to 300°C or less is to set the martensite ratio after cooling to 50% or more, and if the cooling rate is less than 40°C/s or When the temperature after rapid cooling exceeds 300 °C, the total ratio of pro-eutectoid ferrite, pearlite, baitonite, etc. in the structure after cooling becomes 50% or more,
This is because anisotropy and uniformity deteriorate. Although the upper limit of the cooling rate is not particularly specified, it is 200℃/
Even if the cooling rate exceeds s, the effect reaches saturation, so
It is preferable to set the cooling rate to about 200° C./s or less. The reason why 50% or more martensite is required in the steel structure after hot-rolling and cooling is that if the martensite ratio is less than 50%, new This is because the distribution state of martensite deteriorates, making it difficult to obtain the material properties targeted by the present invention.

(3) Reason for limiting the two-phase region reheating conditions and cooling treatment conditions In order to obtain good workability, the target structure in this invention is a microstructure of tempered martensite and fine martensite, Therefore, it is reheated to a two-phase region and then cooled. Here, the tempered martensite corresponds to the part where martensite after hot rolling is tempered and becomes ferrite.
On the other hand, fine martensite is a portion that has been turned into austenite after heating in the two-phase region and is turned into martensite again when cooled. Therefore, in order to partially austenitize it, it is necessary to hold it in a temperature range of 725°C or higher for at least 5 seconds, and then to turn it into martensite again, it must be heated to 300°C or lower at a cooling rate of 3 to 50°C/s. It is necessary to cool down to If the reheating temperature exceeds 840°C, austenitization will progress too much and the target structure will not be obtained, and if the holding time exceeds 200 seconds, productivity will decrease, so the reheating conditions should be 725°C to 725°C.
The temperature was limited to 840°C for 5 to 200 seconds. Furthermore, the reason why the upper limit of the cooling control temperature range is set to 300° C. is that if the cooling control ends above this temperature, the formation of new martensite becomes insufficient and the desired material cannot be obtained.

[0034]

[Example] C: 0.15%, Si: 0.05%, M
After rough rolling, a steel slab containing n: 1.51% and Al: 0.020%, with the remainder being essentially Fe,
It was hot-rolled under various conditions shown in Table 1 to obtain a hot-rolled sheet with a thickness of 2.6 mm. The steel structure after hot rolling is as shown in the same table. Each hot rolled sheet was then pickled and then reheated in a continuous annealing furnace under the same conditions shown in Table 1. Table 2 also shows the results of examining the tensile properties of the hot-rolled steel sheets thus obtained. Also, in FIG. 3, coil No. 4.No. 1
8 and no. The results of an investigation on changes in the tensile properties of No. 19 in the longitudinal direction of the coil are shown below.

[0035]

[Table 1]

[0036]

[Table 2]

As is clear from Table 2, the steel sheets manufactured according to the present invention have very little difference in material quality in the longitudinal direction. In addition, No. 12, No. No. 16 had a small difference in material quality, but the desired strength or ductility was not achieved.

[0038]

[Effects of the Invention] Thus, according to the present invention, the sheet bar is finish-rolled continuously at a substantially uniform processing temperature, and further cooling and reheating treatments are performed under precise temperature control, thereby achieving uniform material quality. A hot-rolled high-strength steel sheet with excellent workability and excellent properties can be obtained with high productivity.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a hot finish rolling line suitable for use in 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 how the tensile properties change 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 (1)

[Claims] [Claim 1] C: 0.05 to 0.30 wt%, Si
: 1.0 wt% or less, Mn: 1.0 to 2.5 wt% and Al: 0.00
After rough rolling, a steel slab containing 0.5 to 0.10 wt% is subjected to finish rolling at a temperature of:
When performing hot rolling under the conditions of 800 to 1000°C, the sheet bar that has undergone rough rolling is once wound into a coil, and then finish rolling is started from the end of the winding, and the end of the winding of the following sheet bar is placed on the back end. Connect sequentially to perform finish rolling continuously, and following this finish rolling, 4
After cooling to 300℃ or less at a cooling rate of 0℃/s or more, reheat to a temperature range of 725 to 840℃.
A method for producing a highly workable hot-rolled high-strength steel sheet with excellent material uniformity, characterized by holding the steel sheet for ~200 seconds and then cooling it to 300°C or less at a cooling rate of 3 to 50°C/s.