CN110100030B

CN110100030B - Ultrahigh-strength hot-rolled steel sheet having excellent bending workability and method for producing same

Info

Publication number: CN110100030B
Application number: CN201780080291.1A
Authority: CN
Inventors: 孔钟判; 郑济淑; 高永柱
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-23
Filing date: 2017-12-15
Publication date: 2021-04-20
Anticipated expiration: 2037-12-15
Also published as: KR102209592B1; CN110100030A; WO2018117552A1; KR20190095459A

Abstract

One aspect of the present invention relates to an ultra high strength hot rolled steel sheet excellent in bending workability, comprising in wt%: c: 0.18 to 0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01 to 0.05%, Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001 to 0.01% and the balance Fe and unavoidable impurities, and the microstructure comprises, in terms of area fraction, at least 90% of martensite, 4 to 10% of self-tempered martensite and at most 5% of retained austenite.

Description

Ultrahigh-strength hot-rolled steel sheet having excellent bending workability and method for producing same

Technical Field

The present invention relates to an ultra high strength hot rolled steel sheet having excellent bending workability and a method for manufacturing the same.

Background

According to the stability of collision between vehicle body and passenger and CO₂Since the ultrahigh strength and ultra light weight of a vehicle body are required worldwide due to strict requirements such as environmental regulations, ultrahigh strength steel sheets of 1.0Gpa or more are actively being developed.

Most of the ultra high strength hot rolled steel sheets used for bumper reinforcements and door impact beams and the like as most of vehicle body reinforcements are required to have high strength while having excellent bending workability and shape quality for Roll Forming (Roll Forming).

In order to satisfy such physical properties, steel sheets for vehicle structural members are basically composed of a combination of Ferrite (Ferrite), bainite (Baintie), Martensite (Martensite), and Tempered Martensite (Tempered Martensite) phases, and are applied in two-Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, composite Phase (Complex Phase) steels, Martensite-strengthened ultra-high strength (MART) steels, and the like, according to the composition ratio of these phases.

The steel is mainly applied to parts requiring high energy absorption performance in a vehicle collision, such as members, pillars, bumper reinforcements, side members, and the like, and is processed by roll forming, and thus is required to have a tensile strength of 1.0Gpa or more and a high elongation.

On the other hand, many studies and developments have been made to provide ultra-high strength steels having tensile strengths of 1.2GPa or more for use as vehicle body reinforcing material members, and typical examples thereof are patent documents 1 to 5.

Patent document 1 discloses a method for manufacturing an ultra-high strength cold rolled steel sheet having a tensile strength of 1.2GPa level for a vehicle bumper reinforcement material, the method including the steps of: homogenizing aluminum killed steel at 1050-1300 ℃, performing hot finish rolling at 850-950 ℃ above the Ar3 transformation point, and performing hot rolling coiling at 550-650 ℃, wherein the aluminum killed steel is added with C: 0.15 to 0.20%, Si: 0.3-0.8%, Mn: 1.8-2.5%, Al: 0.02 to 0.06%, Mo: 0.1 to 0.4%, Nb: 0.03-0.06%, S: 0.02% or less, P: 0.02% or less, N: 0.005% or less, and comprises a finish rolling of elements that are inevitably contained in the manufacturing process of the steel; cold rolling the steel plate at a cold reduction rate of 30-80%, and then continuously annealing at a temperature of A3 or above; and slowly cooling the steel plate to 600-700 ℃ for the first time, then rapidly cooling the steel plate to 350-300 ℃ for the second time at a cooling speed of-10 to-50 ℃/second, and then slowly cooling the steel plate at the temperature of 350-250 ℃ and keeping the temperature for more than 1 minute.

Patent document 2 discloses a method for producing a cold-rolled steel sheet, which comprises, in wt%: c: 0.05 to 0.20%, Si: 2.5% or less, Mn: the steel with the content of less than 3.0 percent, impurities and a small amount of alloy elements is added with Cr: 0.3% or less, Mo: 0.3% or less, Ni: 0.3% or more, to manufacture a cold-rolled steel sheet having a strength of 1180 to 1400MPa, a bending/twisting of the steel sheet of 10mm or less, and a good shape. In addition, it also discloses: the steel sheet is rapidly cooled at a high temperature using a continuous annealing heat treatment apparatus, then subjected to a general water cooling (quenching) by performing an overaging treatment at a temperature range of 150 to 200 ℃, and then subjected to a tempering (tempering) treatment to improve a sheet shape defect (deformation of the steel sheet in a width direction).

Patent document 3 discloses a method for manufacturing an ultra-high strength cold rolled steel sheet having a tensile strength of 1470MPa grade, which comprises the following steps in weight%: c: 0.1 to 0.6%, Si: 1.0-3.0%, Mn: 1.0-3.5%, Al: 1.5% or less and Cr: 0.003-2.0% of a cold-rolled steel sheet is heated to Ac 3-Ac 3+50 ℃, then cooled at a cooling rate of 3 ℃/sec or more, and then kept at a constant temperature in a range of (Ms-100 ℃) to Bs (bainite start temperature), so that the phase fraction of retained austenite before processing of the cold-rolled steel sheet is 10% or more, the austenite grains have a length of a minor axis of 1 μm or more and an average axial ratio (major axis/minor axis) of 5 or more, and have hydrogen embrittlement resistance.

Patent document 4 discloses a method for producing a cold-rolled steel sheet having a tensile strength of 1.5GPa, which includes the following continuous annealing steps: heating a cold-rolled slab to a temperature range of [ (Ac3-90 ℃) to (Ac3 +/-15 ℃) at a heating rate of 1-5 ℃/s, firstly cooling to a temperature range of 500-750 ℃ at a cooling rate of 1-3 ℃/s, secondly cooling to a temperature range of [ (Ms-120) -460 ℃) at a cooling rate of 3-50 ℃/s, keeping the temperature constant for 6-500 seconds, or slowly cooling at a cooling rate of 1 ℃/s or less, wherein the slab comprises the following components in percentage by weight: c: 0.10 to 0.27%, Si: 0.001 to 1.0%, Mn: 2.3-3.5%, Al: 1.0% or less (except 0%), Cr: 2% or less (except 0%), P: 0.02% or less (except 0%), S: 0.01% or less (except 0%), N: 0.01% or less (except 0%), B: 0.005% or less (except 0%), Ti: 0.004-0.03%, Mo: 0.02% or less (except 0%), Nb: 0.05% or less (excluding 0%), and the balance of Fe and other unavoidable impurities.

However, in the case according to patent documents 1 to 4, since a cold rolling and Annealing Line (CAL) process is required after hot rolling, the manufacturing cost is sharply increased, and there is a problem that tensile strength is relatively low for application to a bumper or a reinforcing material for a vehicle, which is currently commercially used.

In addition, patent document 5 discloses a manufacturing method for obtaining an ultra high strength having a tensile strength of 1.8GPa by hot press forming a cold-rolled steel sheet including, in wt%: c: 0.26-0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, Ti with the content satisfying 3.42N + 0.001-3.42N +0.5, and Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less and Al: 1% or less of one or two or more, and includes, as the case may be: b: 0.01% or less, Nb: 1.0% or less, Mo: 1.0% or less, Ca: 0.001 to 0.005% of one or more kinds.

According to patent document 5, although an ultra high strength of 1.8GPa in tensile strength can be secured, the cold-rolled steel sheet requires a further Hot Press Forming step (Hot Press Forming), and thus the manufacturing cost further increases.

Therefore, it is required to develop an ultra high strength hot rolled steel sheet and a method for manufacturing the same, which can replace the conventional ultra high strength cold rolled steel sheet and hot formed steel, and can secure higher tensile strength and remarkably reduce manufacturing costs.

(Prior art document)

(patent document 1) Korean laid-open patent publication No. 2004-0057777

(patent document 2) Japanese laid-open patent publication No. 2007-100114

(patent document 3) Korean laid-open patent publication No. 2008-73763

(patent document 4) Korean laid-open patent publication No. 2013-0069699

(patent document 5) Japanese laid-open patent publication No. 2008-0111549

Disclosure of Invention

Technical problem to be solved

An object of one aspect of the present invention is to provide a 1.8 GPa-grade ultra-high strength hot rolled steel sheet having excellent bending workability only by a hot rolling process by using an endless continuous rolling mode in a continuous casting-rolling direct connection process, and a method for manufacturing the same.

On the other hand, the technical problem to be solved by the present invention is not limited to the above. The technical problems to be solved by the present invention can be understood by the whole content of the present specification, and additional technical problems of the present invention can be easily understood by those of ordinary skill in the art to which the present invention pertains.

(II) technical scheme

In addition, another aspect of the present invention relates to a method for manufacturing an ultra high strength hot rolled steel sheet having excellent bending workability, the method comprising the steps of: continuously casting molten steel into a thin slab of 60-120 mm, wherein the molten steel comprises the following components in percentage by weight: c: 0.18 to 0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01 to 0.05%, Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001-0.01% and the balance of Fe and inevitable impurities; roughly rolling the heated sheet billet to enable the temperature of the edge part of the strip plate at the rough rolling outlet side to reach 850-1000 ℃, thereby obtaining the strip plate; performing finish rolling on the strip plate in the temperature range of Ar3+ 10-Ar 3+100 ℃ to obtain a hot-rolled steel plate; air cooling the hot rolled steel plate for 1-3 seconds, then cooling at a cooling speed of more than 200 ℃/second, and rolling at a temperature below Mf-50 ℃; and placing the rolled hot rolled steel sheet between two other rolled hot rolled steel sheets, and the steps are continuously performed.

Furthermore, the described solutions do not list all the features of the present invention. The various features of the invention, together with its advantages and effects, may be understood in further detail with reference to the following detailed description.

(III) advantageous effects

According to the present invention, it is possible to provide a 1.8 GPa-grade ultra-high strength hot rolled steel sheet having excellent bending workability only by a hot rolling process by using an endless continuous rolling mode in a continuous casting-rolling direct connection process, which can not only replace the ultra-high strength hot rolled steel sheet and a hot formed steel but also secure higher tensile strength and can remarkably reduce manufacturing costs, and a method for manufacturing the same.

Drawings

Fig. 1 is a photograph of a rolled slab of invention example 3.

FIG. 2 is a photograph showing the surface of the PO material of invention example 3.

FIG. 3 is a photograph of a structure by a Transmission Electron Microscope (TEM) in inventive example 3, wherein (a) is a photograph at 20000 times and (b) is a photograph at 100000 times as magnified for [ X ] portion in (a).

Fig. 4 is a schematic diagram of a process using endless continuous rolling mode in a continuous casting-rolling direct connection process.

Best mode for carrying out the invention

Preferred embodiments of the present invention will be described below. However, the embodiment of the present invention may be modified into other various embodiments, and the scope of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The inventors of the present invention have recognized that ultra-high strength steel having tensile strength of the order of 1.5GPa or more, which is used as a member for reinforcing a vehicle body, can be manufactured only by a hot rolling process, and can replace conventional cold rolled steel sheets and hot-pressed steel sheets, so that the manufacturing cost can be significantly reduced, and have conducted intensive studies in order to manufacture a hot rolled steel sheet of the order of 1.8GPa only by a hot rolling process.

As a result, it was confirmed that a endless continuous rolling mode is used in the continuous casting-rolling direct connection process and the composition and manufacturing process are precisely controlled, so that a high strength hot rolled steel sheet having a tensile strength of 1.8GPa grade can be manufactured, and thus the present invention has been completed.

Ultrahigh-strength hot-rolled steel sheet having excellent bending workability

Hereinafter, an ultra high strength hot rolled steel sheet excellent in bending workability according to an aspect of the present invention will be described in detail.

An ultra high strength hot rolled steel sheet excellent in bending workability according to one aspect of the invention comprises, in wt%: c: 0.18 to 0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01 to 0.05%, Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001 to 0.01%, and the balance Fe and inevitable impurities,

the fine structure contains, in terms of area fraction, at least 90% of martensite, 4-10% of self-tempered martensite, and at most 5% of retained austenite.

First, the alloy composition of the present invention will be described in detail. Hereinafter, unless otherwise specified, the unit of the content of each element represents wt%.

C：0.18～0.28％

Carbon (C) is an important element for improving strength by changing the structure to martensite when rapidly cooling after hot rolling.

When the C content is less than 0.18%, the strength of martensite itself is low, and it may be difficult to secure the strength required in the present invention. On the other hand, if the C content exceeds 0.28%, the weldability and the strength excessively increase, and the bending workability decreases. Therefore, the C content is preferably 0.18 to 0.28%.

Further, a more preferable lower limit of the C content may be 0.20%, and a still more preferable lower limit may be 0.21%. Further, a more preferable upper limit of the C content may be 0.27%, and a still more preferable upper limit may be 0.26%.

Mn：1.2～2.2％

Manganese (Mn) suppresses the formation of ferrite and improves austenite stability, so that a low-temperature transformation phase is easily formed, thereby increasing strength.

When the Mn content is less than 1.2%, it may be difficult to secure the strength required in the present invention. On the other hand, when the Mn content exceeds 2.2%, segregation zones are formed inside and/or outside the continuous cast slab and the hot rolled steel sheet, the generation and propagation of cracks are induced, so that the final quality of the steel sheet is degraded, and weldability and/or bending workability may be deteriorated. Therefore, the Mn content is preferably 1.2 to 2.2%.

Further, a more preferable lower limit of the Mn content may be 1.30%, and a still more preferable lower limit may be 1.4%. Further, a more preferable upper limit of the Mn content may be 2.1%, and a still more preferable upper limit may be 2.0%.

Si：0.1～0.5％

Silicon (Si) is an effective element that can secure strength without reducing ductility of the steel sheet. In addition, Si is an element that promotes the formation of ferrite and promotes the enrichment of C into non-transformed austenite to promote the formation of martensite.

When the Si content is less than 0.1%, it is difficult to sufficiently secure the above effect. On the other hand, when the Si content exceeds 0.5%, red scale is generated on the surface of the steel sheet and its trace remains on the surface of the steel sheet after pickling, so that the surface quality may be degraded. Therefore, the Si content is preferably 0.1 to 0.5%.

P：0.005～0.05％

Phosphorus (P) is an element having an effect of reinforcing steel.

When the P content is less than 0.005%, it is difficult to secure the effect. On the other hand, when the P content exceeds 0.05%, segregation may occur in grain boundaries and/or interphase grain boundaries to induce brittleness. Therefore, the content of P is preferably limited to 0.005 to 0.05%.

S: less than 0.01%

Sulfur (S) is an impurity, and S forms MnS nonmetallic inclusions in steel and segregates when continuous casting solidifies, thereby possibly inducing high-temperature cracks. Therefore, the S content is limited to as low a level as possible, preferably to 0.01% or less. However, when the S content is limited to 0%, an excessively high cost is required, and therefore, 0% may be excluded.

Al：0.01～0.05％

Aluminum (Al) functions to suppress the formation of carbides to increase the ductility of steel.

When the Al content is less than 0.01%, the above effect is insufficient. On the other hand, when the Al content exceeds 0.05%, a large amount of AlN precipitates are formed, high-temperature ductility is reduced, so that the edge quality of a cast slab or a lath may be reduced, and the Al is concentrated on the surface of a steel sheet to deteriorate the plating property. Therefore, the Al content is preferably 0.01 to 0.05%.

Ti：0.01～0.10％

Titanium (Ti) is an element that increases the strength of steel as a precipitate and a nitride-forming element. Ti is an element that forms TiN near the solidification temperature to remove solid N, thereby reducing the amount of precipitates such as AlN and the like, preventing the high-temperature ductility from being reduced, and reducing the susceptibility to Edge (Edge) cracking.

When the Ti content is less than 0.01%, too many fine AlN and/or BN precipitates are precipitated, resulting in a reduction in ductility of the cast slab, thereby degrading slab quality. On the other hand, when the Ti content exceeds 0.10%, coarse TiN precipitates are formed, so that it is difficult to expect the effect of grain refinement and the production cost is increased. Therefore, the Ti content is preferably 0.01 to 0.10%.

B：0.001～0.0045％

Boron (B) is an element that increases the hardenability of steel.

If the B content is less than 0.001%, the above effect is insufficient, and if the B content exceeds 0.0045%, the austenite recrystallization temperature is increased and weldability is deteriorated. In addition, excessive precipitates such as BN are precipitated, resulting in a decrease in high-temperature ductility, and thus the edge quality of the slab and/or Bar plate (Bar plate) may be deteriorated. Therefore, the B content is preferably 0.001 to 0.0045%.

In addition, a more preferable lower limit of the B content may be 0.0015%, and a more preferable upper limit may be 0.004%.

N：0.001～0.01％

Nitrogen (N) is an austenite stabilizing and nitride forming element.

When the N content is less than 0.001%, the above effect is insufficient. On the other hand, when the N content exceeds 0.01%, excessive nitrides are precipitated, resulting in a reduction in high-temperature ductility, and thus the edge quality of the slab and/or Bar plate (Bar plate) may be deteriorated. In addition, the size of the precipitates may become coarse, and the strength may be lowered. Therefore, the N content is preferably 0.001 to 0.01%.

The remaining component of the present invention is iron (Fe). However, in a general manufacturing process, undesirable impurities may be inevitably mixed from raw materials or an ambient environment, and thus, the impurities cannot be excluded. These impurities are well known to those skilled in the general manufacturing process and therefore not specifically mentioned in their entirety in this specification.

In this case, in addition to the above alloy composition, the alloy may further contain, in wt%: 0.001-0.05%, Cr: 0.5 to 1.0%, Mo: 0.001-0.05% and Sb: 0.005-0.02% of one or more.

Nb：0.001～0.05％

Niobium (Nb) is an element effective for increasing the strength of the steel sheet and for reducing the grain size.

When the Nb content is less than 0.001%, it may be difficult to secure the above effects, and when the Nb content exceeds 0.05%, too much NbC, (Ti, Nb) CN, etc. are formed, thereby possibly inducing low-temperature brittleness of the continuous cast slab. Therefore, the Nb content is preferably 0.001 to 0.05%.

Cr：0.5～1.0％

Chromium (Cr) acts to solid solution strengthen the steel and helps form martensite by delaying the transformation of the bainite phase on cooling.

When the Cr content is less than 0.5%, the above effect is insufficient. On the other hand, when the Cr content exceeds 1.0%, the manufacturing cost increases and the ductility of the steel sheet decreases. Therefore, the Cr content is preferably 0.5 to 1.0%.

Mo：0.001～0.05％

Molybdenum (Mo) is an effective component for strengthening yield strength by solid solution strengthening and forming fine precipitates, and improving impact toughness and bending workability by grain boundary strengthening.

When the Mo content is less than 0.001%, the above-described effect is difficult to obtain, and when the Mo content exceeds 0.05%, the effect is saturated, and ductility may be reduced. Therefore, the Mo content is preferably 0.001 to 0.05%.

Sb：0.005～0.02％

Antimony (Sb) is an element that functions to suppress the generation of hot-rolled scale defects.

When the Sb content is less than 0.005%, it is difficult to secure the above effect, and when the Sb content exceeds 0.02%, the manufacturing cost increases and the workability becomes poor, and Sb as a low melting point element may cause problems such as edge cracking. Therefore, the Sb content is preferably 0.005 to 0.02%.

In addition, in addition to the alloy composition, one or more of Cu, Ni, Sn, and Pb are contained as residual elements, and the total thereof may be 0.2 wt% or less. The residual elements are impurity elements derived from scraps used as raw materials in a steel making process, and when the sum thereof exceeds 0.2%, surface cracks of a thin slab may be generated and the surface quality of the hot rolled steel sheet may be reduced.

In addition, Ceq defined by the following relational expression 1 of the C, Si, Mn, P and S may be 0.25 to 0.45.

Relation 1: ceq ═ C + Si/30+ Mn/20+2P +3S

(in the relational expression 1, each symbol of the element represents a value representing the content of each element in% by weight.)

The above-mentioned relational expression 1 is a composition relational expression for securing weldability of a steel sheet, and in the present invention, excellent resistance spot weldability can be secured and excellent mechanical physical properties can be imparted to a welded portion by controlling the Ceq (carbon equivalent) value to 0.25 to 0.45.

When Ceq is less than 0.25, hardenability is low, so that it is difficult to secure a desired tensile strength. On the other hand, when Ceq exceeds 0.45, weldability may be reduced, and it may be difficult to obtain physical properties of a perfect weld. Therefore, it is preferable to control the composition so that Ceq satisfies the range of 0.25 to 0.45.

The microstructure of the present invention will be described in detail below.

The microstructure of the present invention contains, in terms of area fraction, 90% or more of martensite, 4 to 10% of self-tempered martensite, and 5% or less of retained austenite.

Martensite is used to ensure the ultra-high strength, and when the martensite fraction is less than 90%, it may be difficult to ensure the desired tensile strength.

When the self-tempered martensite is less than 4%, the bending workability may be deteriorated, and when the self-tempered martensite exceeds 10%, it may be difficult to secure a desired tensile strength.

On the other hand, retained austenite is a phase unstable at normal temperature and may be transformed into martensite at the time of working to improve strength and workability, but when the retained austenite exceeds 5%, the amount of transformation into martensite at the time of working is large, so that deformation of the product due to volume expansion and unevenness in hardness may be caused. On the other hand, even when the retained austenite content is 0%, the ultrahigh strength and bending workability required in the present invention can be ensured, and therefore, the lower limit thereof is not particularly limited.

In this case, the martensite lath (lath) minor axis pitch may be 5 μm or less.

The martensite lath (lath) short axis pitch may affect the strength and bending workability, and when the martensite lath (lath) short axis pitch exceeds 5 μm, it may be difficult to secure the required strength and bending workability.

The fine structure of the present invention may further contain ferrite in an amount of 4.5% by area fraction or less.

When the area fraction of ferrite exceeds 4.5%, it may be difficult to secure tensile strength, and therefore, the area fraction of ferrite is preferably 4.5% or less. On the other hand, even when ferrite is 0%, the ultra high strength and bending workability required in the present invention can be secured, and therefore, the lower limit of the area fraction of ferrite is not particularly limited.

On the other hand, the tensile strength of the hot rolled steel sheet according to the invention may be 1.8Gpa or more, and the bending workability may be 3 or less.

The bending workability (R/t) is a value obtained by dividing the minimum bending radius (R) at which no crack occurs after the 90 ° bending test by the thickness (t) of the steel sheet, and when the bending workability is 3 or less, it can be suitably applied to the manufacture of a bumper reinforcement material, a door impact beam, and the like, which are vehicle body reinforcement materials. More preferably, the bending workability may be 2.5 or less.

Further, the thickness of the hot-rolled steel sheet according to the present invention may be 2.0mm or less.

In addition, the thickness variation (Crown) in the width direction of the hot-rolled steel sheet according to the invention may be 40 μm or less. Wherein the thickness deviation (Crown) in the width direction indicates the difference between the thickness at the edge 25mm and the thickness of the central portion.

Method for producing ultrahigh-strength hot-rolled steel sheet having excellent bending workability

Next, a method for producing an ultra high strength hot rolled steel sheet excellent in bending workability according to another aspect of the present invention will be described in detail.

The method for manufacturing an ultra high strength hot rolled steel sheet excellent in bending workability according to another aspect of the present invention comprises the steps of: continuously casting molten steel meeting the alloy composition into a thin slab with the thickness of 60-120 mm; rough rolling the heated sheet billet to enable the edge part of the strip plate at the rough rolling outlet side to reach 850-1000 ℃, thereby obtaining the strip plate; finish rolling the strip plate at the temperature range of Ar3+ 10-Ar 3+100 ℃ to obtain a hot rolled steel plate; air cooling the hot rolled steel plate for 1-3 seconds, then cooling at a cooling speed of more than 200 ℃/second, and rolling at a temperature below Mf-50 ℃; and placing the rolled hot rolled steel sheet between two other rolled hot rolled steel sheets, and the steps are continuously performed.

The continuous execution of the steps represents the utilization of a headless continuous rolling mode in the continuous casting-rolling direct connection process.

A so-called manufacturing process using a thin slab (small-sized steel plant process), which is a new steel manufacturing process that has recently been receiving attention, is a continuous casting-rolling direct connection process, and this process has a small temperature variation in the width direction and the length direction of a strip and enables the production of a transformation structure steel having a good material variation in terms of process characteristics, and therefore has received attention as a potential process.

The continuous casting-rolling direct connection process has a traditional batch type and a newly developed endless continuous rolling mode.

In the case of the batch mode, in order to compensate for the difference between the continuous casting speed and the rolling speed, the finish rolling is performed after being wound in a coil box before the finish rolling mill, and thus, there may occur problems such as a reduction in scale peelability, a reduction in surface quality, and a plate fracture when a steel plate having a thickness of 3.0mm or less is produced.

In the case of the endless continuous rolling mode, there is no process of winding before finish rolling unlike the batch mode, and thus although the problems occurring in the batch mode are solved, in order to compensate for the difference between the continuous casting speed and the rolling speed, more precise control is required.

Fig. 4 is a diagram showing an example of a process using the endless continuous rolling mode in the continuous casting to finish rolling direct connection process. A thin slab a having a thickness of 50 to 150mm is manufactured in the continuous caster 100, and since there is no coil box between the roughing mill 400 and the finishing mill 600, a steel sheet can be continuously rolled, so that the passing property is excellent, the risk of sheet breakage is very low, and a thin product having a thickness of 3.0mm or less can be produced. A Roughing Mill Scale Breaker (RSB) 300 is provided before the Roughing Mill 400 and a Roughing Mill Scale Breaker (FSB) 500 is provided before the Finishing Mill 600, so that Scale is easily removed and a Pickled Oiled (Pickled & Oiled, PO) material having excellent surface quality can be produced when the hot-rolled steel plate is Pickled in a subsequent process. In addition, in the finish rolling step, since the rolling speed difference in one steel sheet is 5% or less, isothermal constant speed rolling can be realized, temperature variation in the width and length directions of the steel sheet is remarkably reduced, cooling can be precisely controlled in a Run Out Table (ROT) 600, and a steel sheet having excellent material variation can be manufactured.

The respective steps will be described in detail below.

Step of continuous casting

And continuously casting the molten steel meeting the alloy composition into a thin slab with the thickness of 60-120 mm.

When the thickness of the thin slab exceeds 120mm, high-speed casting is difficult, and the rolling load is increased at the time of rough rolling, and when the thickness of the thin slab is less than 60mm, the temperature of the cast slab is sharply decreased, so that it is difficult to form a uniform structure. Although a heating device may be additionally provided to solve the problem, it increases the production cost, and therefore is preferably excluded as much as possible. Therefore, the thickness of the thin slab is preferably 60 to 120 mm.

At this time, the continuous casting step may be performed using mold flux having basicity of 1.0 or more. The basicity is expressed as the ratio of CaO (%)/SiO 2 (%).

Generally, high-strength steel contains a large amount of components added to ensure high strength, and has very high sensitivity to linear cracking. Therefore, when the mold flux having the basicity of less than 1.0 is used, the heat transfer amount is high, and the slab surface is forcibly cooled, so that the sensitivity of generating linear cracks becomes high, and therefore, it is preferable to use the mold flux having the basicity of 1.0 or more.

In addition, the casting speed of the continuous casting can be 4-8 mpm.

The reason why the casting speed is set to 4mpm or more is that high-speed casting and rolling processes are constructed in connection, and a casting speed of a predetermined or more is required in order to secure a target termination temperature. Further, when the casting speed is slow, there is a risk of occurrence of segregation in the cast slab, and when the segregation occurs, it is difficult to secure strength and workability, and there is an increased risk of occurrence of material variation in the width direction or the length direction. When the casting speed exceeds 8mpm, the work success rate may be lowered due to the unstable molten steel level.

Rough rolling step

And roughly rolling the heated thin slab to enable the edge part of the strip at the rough rolling outlet side to reach 850-1000 ℃, thereby obtaining the strip.

When the temperature of the edge portion of the strip on the rough rolling exit side is lower than 850 ℃, a large amount of AlN precipitates and the like are generated, so that high temperature ductility is reduced, and the susceptibility to the generation of edge cracks may become very high. On the other hand, when the temperature of the edge portion of the strip on the rough rolling outlet side exceeds 1000 ℃, the temperature of the center portion of the thin slab becomes excessively high, a large amount of acid-water type scale is generated, and the surface quality after pickling may be deteriorated.

At this time, the rough rolling may be performed such that the surface temperature of the thin slab reaches 1000 to 1200 ℃ at the rough rolling inlet side.

When the surface temperature of the sheet bar is lower than 1000 c at the rough rolling inlet side, the rough rolling load increases, and cracks may be generated at the strip edge portion during the rough rolling, in which case edge portion defects of the hot rolled steel sheet may be caused. On the other hand, when the surface temperature exceeds 1200 ℃, hot rolled scale remains, and thus hot rolled surface quality may be degraded.

At this time, the method may further include the steps of: before the rough rolling, spraying cooling water to the heated sheet bar at a pressure of 100-200 bar or more to remove oxide skin; and after the rough rolling, enabling the strip plates to pass through a first row and a second row in sequence to remove oxide skin, wherein the first row sprays cooling water at a pressure of 50-250 bar, and the second row sprays cooling water at a pressure of 100-300 bar.

For example, before rough rolling, cooling water of 40 ℃ or less may be sprayed from rough Mill Scale Breaker (hereinafter, referred to as "RSB") nozzles at a pressure of 100 to 200bar to the heated thin slabs to remove surface Scale so that the thickness of the Scale becomes 200 μm or less, and after rough rolling, before finish rolling, the surface of the slabs may be removed so that the thickness of the Scale becomes 20 μm or less using first and second rows of nozzles of finish Mill Scale Breaker (hereinafter, referred to as "FSB").

When the pressure of the cooling water before rough rolling is less than 100bar, a large amount of acid water type scale and the like remain on the surface of the thin slab, so that the surface quality after acid washing may be deteriorated, and when the pressure of the cooling water before rough rolling exceeds 200bar, the edge temperature of the strip plate is sharply reduced, and the risk of edge crack generation is high.

When the pressures of the first and second rows of nozzles after rough rolling are less than 50 and 100, respectively, scale cannot be sufficiently removed, and a large amount of spindle-shaped and scale-shaped scale is generated on the surface of the finish-rolled steel sheet, so that the surface quality may be deteriorated after pickling. On the other hand, when the pressure of the first row of nozzles exceeds 250bar, or the pressure of the second row of nozzles exceeds 300bar, the finish rolling temperature becomes too low to obtain an effective austenite fraction, so that it may be difficult to secure a desired tensile strength.

Further, since it is difficult to sufficiently remove the scale only by the nozzles of the first row, and spindle-shaped scale, which is a fatal defect for the product, may be generated at the time of finish rolling, it is preferable to remove the scale by using the nozzles of the first row and the second row as described above.

Step of Hot Rolling

The strip is finish rolled at a temperature ranging from Ar3+10 ℃ to Ar3+100 ℃ to obtain a hot rolled steel sheet.

When the finish rolling temperature is lower than Ar3+10 ℃, the load of the rolls at the time of hot rolling is greatly increased, the energy consumption is increased, and the operation speed is slowed down, and when a widthwise temperature deviation occurs, the local decrease is below Ar3, and pro-eutectoid ferrite may occur, so that the desired amount of martensite may not be sufficiently obtained after cooling.

In addition, when the finish rolling is performed at a temperature exceeding Ar3+100 ℃, the crystal grains are coarse and high strength cannot be obtained, and the cooling rate needs to be further increased in order to obtain sufficient martensite.

Cooling and winding step

And air-cooling the hot-rolled steel sheet for 1-3 seconds, cooling at a cooling rate of 200 ℃/second or more, and rolling at Mf-50 ℃ or less.

The reason why the slab is air-cooled for 1 to 3 seconds after the finish rolling is to remove the internal residual stress of the slab generated during the finish rolling and to refine and cool the martensite laths to a temperature of Ar3 or higher. When the air cooling time is less than 1 second, the residual stress inside the slab generated at the finish rolling cannot be removed, and the shape may be deteriorated at the time of winding. On the other hand, when the air-cooling time exceeds 3 seconds, pro-eutectoid ferrite may be formed, and thus sufficient martensite may not be ensured after completion of cooling.

When the cooling rate after the air cooling is less than 200 ℃/sec, a sufficient martensite structure may not be secured due to the ferrite and bainite noses (Nose). In addition, when the rolling temperature exceeds Mf-50 ℃, it is difficult to obtain a martensite structure, and the martensite structure obtained by cooling may be self-tempered (Auto Tempering) to form a large amount of self-tempered martensite, so that it may be difficult to obtain a desired tensile strength.

Carrying step

And placing the rolled hot rolled steel plate between the other two or more rolled hot rolled steel plates.

When leveling (Skin Pass) is performed in a state where a large amount of residual water exists on the surface and inside of a hot rolled steel sheet wound at a low temperature, a fatal press-in defect may be generated on the surface of a slab due to adhesion between the residual water and the scale.

When the rolled hot rolled steel sheet is placed between other two or more rolled hot rolled steel sheets, residual water can be removed, and self-tempered martensite can be ensured due to self-tempering of a part of martensite.

At this time, the method may further include the steps of: and carrying out flat rolling on the loaded hot rolled steel plate at 50-150 ℃.

The temper rolling temperature is limited to 50-150 ℃ because the shape of the hot rolled steel sheet is easily corrected by the warm rolling effect.

In another aspect, the method may further comprise the steps of: the hot-rolled steel sheet placed on the steel sheet was Pickled to obtain a Pickled and Oiled (PO) material. In general, the treatment method used in the hot rolling pickling process is not particularly limited, since it can be used.

Detailed Description

The present invention will be described in more detail below with reference to examples. However, the following examples are only examples for illustrating the present invention in more detail, and do not limit the scope of the claims of the present invention. This is because the scope of the right of the present invention is determined by the contents recited in the claims and reasonably derived therefrom.

(examples)

For molten steels having the composition shown in the following table 1, hot rolled steel sheets having a thickness of 1.4mm were obtained in an endless continuous rolling mode in a continuous casting-rolling direct connection process (slab thickness: 93mm, casting speed: 5.8mpm) using the manufacturing conditions described in the following table 2, and then temper rolling was performed at 100 ℃ to manufacture hot rolled steel sheets.

The hot rolled steel sheet was subjected to an acid pickling process to obtain a PO material, and then the microstructure, the mechanical physical properties, whether cracks were generated, the thickness deviation (Crown) in the width direction, whether press-in defects of the PO material were generated, and the surface quality of the PO material were measured and evaluated, and are described in table 3 below.

For the fine structure, the area ratios of martensite (M), self-tempered martensite (AT), and ferrite (F) were observed and measured using a Scanning Electron Microscope (SEM) and a Transmission Electron Microscope (TEM). For Retained Austenite (RA), the area ratio was measured using an Electron Back Scatter Diffraction (EBSD) device. In addition, the values obtained by measuring the minor axis pitch of martensite laths using a photograph taken with a transmission electron microscope and averaging the measured values are described in the following table 3.

The Yield Strength (YS), Tensile Strength (TS) and Yield Ratio (YR) as mechanical physical properties are values measured by using a test piece of JIS5 No. with reference to a 90 ° direction with respect to the rolling direction at a width W/4.

The bending workability was evaluated by dividing the bending radius (R) by the steel sheet thickness (t) to 2.5 and determining whether or not cracks occurred after the 90 ° bending test, and the presence or absence of cracks is shown in table 3 below.

Whether or not the linear crack and the edge crack are generated is confirmed for the first time by the naked eye in the lath and the rolled sheet, and then confirmed for the second time using a Surface Defect Detector (SDD) device as a Surface Defect Detector (Detector).

For the thickness deviation (Crown) in the width direction, the difference between the thickness at the edge 25mm and the thickness of the central portion was measured and described. The thickness deviation acceptable standard may be 40 μm or less.

And for judging whether the press-in defect of the PO material is generated or not, the first confirmation is carried out on the rolled plate by naked eyes, and then the second confirmation is carried out by utilizing the SDD.

Evaluation criteria for PO material surface quality are as follows.

O: the average deviation of the gloss in the width direction is 10% or less

And (delta): the average deviation of the gloss in the width direction is 10-20%

X: the average deviation of the gloss in the width direction is more than 20 percent

On the other hand, Ar3 in Table 2 indicates the ferrite transformation start temperature, Mf indicates the martensite transformation end temperature, and Ar3 and Mf are values calculated using the common thermodynamic software JmatPro V-9.

[ Table 1]

In table 1, Ceq is C + Si/30+ Mn/20+2P +3S, and each element symbol is a value representing the content of each element in wt%.

[ Table 2]

In said table 2, the Roughing Mill Scale Breaker (RSB) is the cooling water spray pressure before the Roughing Mill, and the Finishing Mill Scale Breaker (FSB) is the cooling water spray pressure after the Roughing Mill.

Whether or not the rolled hot-rolled steel sheet is placed means whether or not the rolled hot-rolled steel sheet is placed between two rolled steel sheets.

[ Table 3]

In table 3, M represents Martensite (Martensite), AT represents self-Tempered Martensite (Auto-Tempered Martensite), RA represents Retained austenite (Retained austenite), and F represents Ferrite (Ferrite) structure.

All of invention examples 1 to 6 satisfying the conditions proposed in the present invention satisfied tensile strength of 1.8GPa or more and bending workability of 2.5 or less.

Moreover, comparing invention examples 1 to 3 with invention examples 4 to 6, invention examples 1 to 3 satisfying the cooling water injection pressure before and after rough rolling, which is further proposed in the present invention, can ensure both bending workability and ultrahigh strength and can ensure excellent surface quality

FIG. 1 is a photograph of a surface of a PO material obtained by pickling a hot-rolled steel sheet of invention example 3, and it can be confirmed that the rolled shape and the surface quality are excellent.

FIG. 3 is a photograph of a structure by a Transmission Electron Microscope (TEM) in inventive example 3, wherein (a) is a photograph at 20000 times and (b) is a photograph at 100000 times as magnified for [ X ] portion in (a). From the results, it is understood that martensite laths of 1 μm or less finely develop well, and that fine needle-shaped cementite of 100nm or less (lower end photograph arrow) is present in the coarse laths, and that self-tempered martensite is present.

Comparative example 1 did not satisfy the basicity condition proposed in the present invention, thereby generating linear cracks due to strong cooling of the surface.

Comparative examples 2 to 4 did not satisfy the cooling conditions proposed in the present invention, and thus did not satisfy the required material.

Comparative example 5 did not satisfy the finish rolling temperature proposed in the present invention, and thus did not satisfy the desired material quality.

Comparative example 6 is an example in which the mounting proposed in the present invention was not performed, and the surface quality was deteriorated because bending cracks were generated due to a small amount of self-tempered martensite, and press-in defects were generated at the time of temper rolling due to the presence of residual water inside the strip.

In comparative examples 7 and 8, since the edge temperature of the strip plate was low, AlN, BN, etc. were excessively precipitated, and the high temperature ductility was lowered, thereby causing edge cracks.

In comparative example 9, the carbon content exceeded 0.28%, and the bending workability was deteriorated.

Comparative examples 10 and 11 did not satisfy the components proposed in the present invention, and thus tensile strength was deteriorated.

Comparative examples 12 to 15 did not satisfy the contents of Al, Ti, B and N used for controlling edge cracking in the present invention, and thus edge cracking was generated and the surface quality was deteriorated.

In addition, it was confirmed that comparative examples 2 to 5, 10 to 12 and 15, which contained more than 4.5 area% of ferrite, had poor tensile strength.

Although the present invention has been described with reference to the embodiments, it will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims.

Description of the reference numerals

a: and (3) slab b: rolled plate

100: the continuous casting machine 200: heating device

300: rough rolling Scale cleaner (Roughing Mill Scale Breaker, RSB)

400: roughing mill

500: finishing Mill Scale Breaker (FSB)

600: finishing mill 700: run-out table

800: high-speed cutting machine 900: rolling machine

Claims

1. An ultra-high strength hot rolled steel sheet having excellent bending workability,

the hot rolled steel sheet is composed of, in wt%: 0.18 to 0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01 to 0.05%, Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001 to 0.01%, and the balance Fe and unavoidable impurities, wherein the microstructure comprises 90% or more of martensite, 4 to 10% of self-tempered martensite, and 5% or less of retained austenite in terms of area fraction, and the hot-rolled steel sheet does not cause linear cracks and edge cracks.

2. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the hot rolled steel sheet further comprises, in wt%: nb: 0.001-0.05%, Cr: 0.5 to 1.0%, Mo: 0.001-0.05% and Sb: 0.005-0.02% of one or more.

3. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the hot-rolled steel sheet contains one or more of Cu, Ni, Sn and Pb as residual elements, and the total of the residual elements is 0.2 wt% or less.

4. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

in the hot rolled steel sheet, Ceq defined by the following relation 1 is 0.25 to 0.45,

relation 1:

Ceq＝C+Si/30+Mn/20+2P+3S

in the relational expression 1, each element symbol is a value representing the content of each element in weight%.

5. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the martensite has a minor axis pitch of 5 μm or less.

6. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the fine structure further contains 4.5% or less of ferrite by area fraction.

7. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the hot-rolled steel sheet has a tensile strength of 1.8GPa or more and a bending workability of 3 or less,

the bending workability represents a value obtained by dividing the minimum bending radius (R) at which cracks do not occur after a bending test of 90 ° by the thickness (t) of the steel sheet.

8. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the thickness of the hot rolled steel plate is less than 2.0 mm.

9. The ultra high strength hot rolled steel sheet excellent in bending workability according to claim 1,

the thickness variation of the hot-rolled steel sheet in the width direction is 40 [ mu ] m or less.

10. A method for manufacturing an ultra high strength hot rolled steel sheet having excellent bending workability, comprising the steps of:

continuously casting molten steel into a thin slab of 60-120 mm, wherein the molten steel is prepared from the following components in percentage by weight: 0.18 to 0.28%, Mn: 1.2-2.2%, Si: 0.1-0.5%, P: 0.005-0.05%, S: 0.01% or less, Al: 0.01 to 0.05%, Ti: 0.01-0.10%, B: 0.001-0.0045%, N: 0.001-0.01%, and the balance of Fe and inevitable impurities;

roughly rolling the heated sheet billet to enable the temperature of the edge part of the strip plate at the rough rolling outlet side to reach 850-1000 ℃, thereby obtaining the strip plate;

performing finish rolling on the strip plate in the temperature range of Ar3+ 10-Ar 3+100 ℃ to obtain a hot-rolled steel plate;

air cooling the hot rolled steel plate for 1-3 seconds, then cooling at a cooling speed of more than 200 ℃/second, and rolling at a temperature below Mf-50 ℃; and

the rolled hot rolled steel plate is placed between the other two rolled hot rolled steel plates,

wherein the continuous casting step is performed using mold flux having basicity of 1.0 or more, the rough rolling is performed such that the surface temperature of the sheet bar at the rough rolling inlet side reaches 1000 to 1200 ℃, and each step is continuously performed.

11. The method for producing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10,

the casting speed of the continuous casting is 4-8 mpm.

12. The method for manufacturing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10, further comprising the steps of:

before the rough rolling, spraying cooling water to the heated sheet bar at a pressure of 100-200 bar or more to remove oxide skin;

and after the rough rolling, enabling the strip plates to pass through a first row and a second row in sequence to remove oxide skin, wherein the first row sprays cooling water at a pressure of 50-250 bar, and the second row sprays cooling water at a pressure of 100-300 bar.

13. The method for manufacturing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10, further comprising the steps of:

and carrying out flat rolling on the loaded hot-rolled steel plate at the temperature of 50-150 ℃.

14. The method for manufacturing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10, further comprising the steps of:

and performing acid pickling treatment on the loaded hot rolled steel plate to obtain an acid-pickled and oiled steel material, namely a PO material.

15. The method for producing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10,

the molten steel further includes, in weight%: nb: 0.001-0.05%, Cr: 0.5 to 1.0%, Mo: 0.001-0.05% and Sb: 0.005-0.02% of one or more.

16. The method for producing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10,

the molten steel contains one or more of Cu, Ni, Sn and Pb as residual elements, and the total of the residual elements is 0.2 wt% or less.

17. The method for producing an ultra high strength hot rolled steel sheet excellent in bending workability according to claim 10,

in the molten steel, Ceq defined by the following relation 1 is 0.25 to 0.45,

relation 1:

Ceq＝C+Si/30+Mn/20+2P+3S